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A Study of the Movement of Moisture in and from Concrete at Elevated and Non-Uniform Temperatures.
Chapman, D. A
Download date: 30. Jun. 2018
A STUDY OF TIlE MOVEMENT OF
MOISTURE IN AND FROM CONCRETE
AT ELEVTED AND NON-UNIFORM TEMPERTURES.
A Thesis submitted to
The University of London
for the Degree of Ph.D.
in the
Faculty of Engineering
by
David Andrew Chapman.
King's College January, 1976.
(L.LO:uI
fl ¶_'
ABSTR 'CT.
Many of the properties of concrete are
directly dependent upon the state of water inside
the concrete. An investigation into the mcvement
of moisture in and from Limestone Aggregate
concrete specimens at elevated and non-uniform tam-
peratures has been carried out.
Sealed specimens at elevated temperatures
have been allowed to lose moisture in a controlled
manner. From the variation of pore pressure
with weight los4', new definitions of water in
concrete are proposed. These are called Active,
Passive and Bound water.
Other specimena, sealed on all sides
except one, which was left open to the atmosphere,
were tested under non-uniform temperatures. The
time dependent variation in the pore prssures,
water contents, transverse and longitudinal s hri* k
-age values due to the migration of fluid through
the concrete were monitored in different specimens.
After heating had ceased, samples were removed from
various specimens. Permeability measurements
and Adsorption ts'ts were performed. Other samples
were viewed under a Scanning Electron Microscope.
The movement of moisture in and from the specimens
appears dependent upon the temperature values and
the concrete coefficient of permeability, which
increases as moisture is lost. Water accumulates
in the central regions and the combined effect
of excess moisture and elevated temperature enhances
hydration, and this is reflected in the microstructure
observed. Both the transverse and longitudinal
shrinkage behaviour of concrete is directly propor-
tional to the water content results, with shrinkage
increasing as moisture is lost.
The venting of specimens at various
positions along their length in order to dissipate
the pore pressures developed was investigated.
This causes an increased loss of moisture that may
be detrimental to the efficiency of concrete acting
as a biological shield.
CKNOWL1:DGEt1ENTS.
The author is indebted to many people whom
he would like to thank for their help and encouragement
during the execution of the research work described
in this thesis.
Firstly, to Dr. G. L. England, Reader in
Engineering Mechanics at King's College, without whose
suggestions, guidance, patience, interest and encourage-
ment this research work would never have been undertaken
or completed.
To Mr. A. W. Blake, Chief Technician in the
Department of Civil Engineering and all his technical
staff for their assistance and suggestions in the
experimental aspects of the research.
To Mr. Frank Josey and Mr. Jim Luck, former
technicians in the Concrete Laboratory at King's
College. Their willingness to assist, their wit
and humour under any conditions will always remain
in the author's memory.
To Mr. . Moore, Chief Technician in the
Department of Crystallography at Birkbeck College,
who assisted the author in his studies with the
Scanning Electron Microscope.
To Mr. R. Hunt, senior technician in the
Department of' Civil Engineering at King's College,
who developed and printed all the photogra.hs in
this thesis.
Finally, to Mrs. G. Rowsell for typing this
thesis.
VOLUME ONL - INDEX.
Page No.
Title Page1.
Abstract ii.
Acknowledgements iv.
Index V.Conversion of' British Units to S.I. Units xv.
CHAPTER ONE - INTRODUCTION AND LACKGROUND
TO THESIS. 1.
CHAPTER TWO - REVIEW OF LITER%TURE
9..
I. Introduction
11.
II. Cement Chemistry
(a) Introduction
12.(b) Setting of cement at
normal temperatures
13.(c) Setting of cement at
eleva ted tempera tures 17.
III. Cement Physics
(a) Introduction
21.(b) The Physical Structure
of Cement Paste
31.Cc) Definition of the types
of water in cementpaste and their physicalproperties
32.
IV. Movement of Moisture in andfrom Concrete.
(a) Introduction
38.(b) Diffusion theory 39.(c) Concrete permeability
41.(d) Pore Pressures in
concrete
48.(e) Drying and Moisti&re
movement in concrete
52.under Thermal Gradients
(f) Methods of monitoringmoisture movement in andfrom concrete
58.
VI. Limestone Aggregate Concrete
(a) Introduction(b) The Chemical and Physical
Structure of DolomiticLimes tone
(c) Effect of curing tempera-ture on the hydrationproducts of LimestoneAggregate concrete
(d) Cement/Aggregate reactionand the loss of strengthat elevated temperatures.
V. The Engineering Propertiesof Concrete.
(a) Introduction(b) Concrete Strength(c) Modulus of Elasticity(d) Poisson's Ratio(e) Thermal Expansion of
Concrete(f) Heat Flow Properties
of Concrete(.g) Carbonation(h) Shrinkage(i) Creep
CHAPTER THREE - INSTRUMENTATION.
Abs tract
List of Contents
I. Experimen tal Programme
II. Measurement of Pore Pressurein Concrete
(a) General Principles(b) Transducer Details and
Calibrations.(c) Porous plate details(d) Pressure Attachment
details and measurementprocedure.
III. Temperature Measurement
(a) Thermocouple design(b) Temperature meter
Page NO.
64.65.66.67.
68.
70.72.73.76.
78.
78.
80.
85.
85.
90.
91.
92.94.
97.
98.
102.103.
Page ?o.
IV. Measurement of ShrinkageStrain.
(a) Introduction 105.(b) Measurement of horizontal
shrinkage strain
106.(c) Measurement of Vertical
shrinkage strain
107.
V. Measurement of EvaporableWater Content.
(a) Introduction 108.(b) Moisture Meter details
110.(c) Moisture Meter calibra- 112.
tions.
VI. The Weighing of Test Specimens
(a) Introduction
121.(b) Procedure for weighing
Migration and venting test
123.series specimens
VII. The Measurement of various tyof water in Migration specimensat the end of testing.
(a) Introduction 127.(b) Evaporable water content
128.(c) Non—evaporable water
129.content
(d) ssessment of the loss ofwater in specimen
131.
CHAPTER POUR - EXPERIMENTS AND EXPERIMENTLTECHNIQUES.
Abstract
132.
List of Contents 134.
I. Introduction. 136.
II. The Concrete Mix. 137.
III. The Release Test Series.
(a) Introduction
140.(b) Specimen Details
140.(c) Casting, sealing and
curing of specimens
142.(d) Assembly of specimens
in oven
143.(e) Testing procedure
144.
183.
185.
186.
Page No
146.
149.
152.
154.157.159.
161.
164.
166.
168.171.172.173.
176.
IV. The Migration and VentingTest Series.
(a) Manufacture, preparationand general details ofthe specimens
(b) Casting of the specimensand Introduction ofInstrumentation.
(c) Curing of specimens andcompletion of assembly
(d) heating and insulation(e) . Testing procedure(f) Determination of Final
water distributions.
V. The Pore Pressure/Shrinkage Test
(a) Introduction(b) Design of mould for concrete
specimen(c) Casting and Curing
procedure.(d) The casting and construc-
tion of the sealing jacket(e) heating of specimen(f) Testing Procedure(g) Casting of subsidiary tests(h) Testing precedure for
subsidiary tests.
VI. The Preparation and Examinationof the Electron MicroscopeSpecimens.
(a) Introduction(b) Extraction of sample from
concrete Specimens(c) Coating of sample for the
Stereoscan microscope(d) Procedure employed in
examining specimens in theStereoscan microscope.
CIRPTER FIVE - RESULTS OF TIlE RELEASE TEST
EXPERIMENTS.
•thstract
List of Contents
I. introduction
177.
178.
179.
180.
204.
218.
231.
240.
243.
246.
249.
252.
ldC ro.
II.
III.
Iv.
Test procedure and controlspecimens
Presentation of' Results.
(a) Control Specimens(b) Results of test specimens
Discussion of Results.
(a) Control specimens(b) Idealised model of a
sealed porous materialsubjected to a ReleaseTest
(c) Comparison of the ideali-sed model and the actualconcrete sp ecimens
(d) Examination of experi-mental results
(e) The 103 C specimennomal i. es
188.
192.194.
196.
198.
V. Scanning Electron MicroscopePho tographs.
(a) Introduction(b) Dummy specimen results(c) 105 C release test results(d) 150°C release test results(e) 175°C release test results
0(f) 200 C heated specimenresults
(g) Comparisons and conclusionsfrom scanning electronmicroscope photographs.
VI. Conclusions.
CHAPTER SIX - RESULTS OP MIGR'TI0N SERIES
TEST SJECIMENS.
A b st r act.
List of Contents.
I. Introduction.
II. Presentation of Results.
(a) Introduction(b) Water Content results(c) Weight Loss results
234.235.236.237.238.239.
256.256.260.
Pae ?o.
(d) Pore Pressure results 263.(e) Permeability test results
264.(f) .\bsorption test results 268.
'II. Discussion of results.
(a) Introduction 273.(b) Water Content Results 274.Cc) Weight Loss results 295.(d) Pore Pressure results 299.(e) Permeability test results 306.(F) Absorption test results 313.
IV. The Scanning Electron ticro-scope Photographs.
(a) Introduction(b) Listing of Photoraphs(c) Discussion of results
i. Introductionii. Photographs from
spec. l - basetemperature 105°C
iii. Photographs fromspec. 132 - basetemperature 125°C
iv. Photographs fromspec. C3 - basetemperature 150°C
v. Photographs fromspec. D4 - basetemperature 175°C
vi. Photographs fromspec. ES - basetemperature 200 C
vii. Photographs fromcontrol specimensL12 and M13 - Lab.temperature.
viii. Comparisons of theMicros t r u c t u r e sseen in the "Dry"regions
ix. Comparisons of theMicros t r U C t u r esseen in the "Semi-dry" regions
x. Comparisons of theMicros tructuresseen in the "normal"regions.
xi. Comparisons of theMicros t r u C t u r e sseen in the "Evap-ouration dry" regions
319.320.
345.
346.
349.
352.
355.
358.
360.
362.
365.
367.
370.
Page ?o.
xii. Comparisons of theMicros t. r u c t u r e sseen in the "wet"regions. 372.
xiii. \ggregate/cementreaction. 375•
xiv. Effect o MoistureMigration on theMicros truc ture ofconcrete. 376.
xv. Comparison of theMicrostructuresseen in the "dry"regions with thosefrom the ReleaseTest specimens. 381.
xvi. Comparison of theMicrostructuresseen in the "semi-dry" regions withthose from the ReleaseTest specimens. 383.
xvii. Comparisons betweenthe Microstrixcturesseen in the "normal"and "wet" regions withthe hydrati3n speimenscured at 17 C, 40 C0and (.0 C under water. 384.
xviii. Scanning Electron Micro-scope photographs of thepermeability specimens. 386.
xix. Scanning Electron Micro-scope photographs d?"Equivalent Sec tions"for the bsorption testsamples. 389.
(d) Conclusions from ScanningElectron Microscope 394.Pho tographs.
V. Conclusions. 398.
Cll%I'TLR SEVEN - RESULTS OF TUE VENTING SERIES
TEST SPECIMENS.
"bs tract. 404.
List of Contents. 406.
I. Introdu c t i: c!:. 408.
"age ?o.
11. Presentation of Results.
(a) Presentation of results ofspecimens vented at thestart of heating at variousBase temperatures.
i. Water content resultsii. Weight loss results
iii.. Pore Pressure results
(b) Presentation of results ofspecimens vented at varioustimes of heating at thesame base temperature of105°C.
i. Water Content resultsii. Weight Loss results
j.jj. Pore Pressure results
III. Discussion of Results.
(a) Introduction(b) Discussion of Results of
specimens vented at thestart of heating atvarious base temperatures.
i. Water content resultsii. Weight Loss results
iii. Pore pressure results
(c) Discussion of' results ofspecimens vented at varioustimes of heating at thesame base temperature of150°C.
i. Water content resultsii. Weight Loss results
iii. Pore Pressure results
IV. Comparison of the Behaviour ofVenting Test Specimens vented atthe start of Heating and Migrationspecimens at the same nominal basetemperatures.
(a) Introduction(b) 105°C Base temperature
specimens(c) 125°C Base temperature
specimens-o
(d) 1DO C Base Temperaturespecimens
412.413.415.
416.417.418.
418.
419.425.426.
430.436.437.
441.
441.
446.
447.
453.455.
439.
463.
464.
465.
Page No.
(e) 175°C Base Temperaturespecimens0
(F) 200 C Base Temperaturespecimens
(g) Conclusions
V. Conclusions.
CIIAPTIiR El GIlT - HYDRTI0N EXPERIMENTS.
Abs tract.
List of Contents.
I. Introduction.
II. Listing of the Scanning ElectronMicroscope Photographs.
III. Discussion of results.
(a) Introduction(b) Effect of Curing Temperature(c) Effect of Aggregate Type(d) Effect of ggregateJCement
ratio and sand content.
449.
469.
477.
479.483.
485.
IV. Conclusions. 487.
ChAPTER NINE - PORE PIthSSURE/sIIRINKAGE EXPERIMENT
A b St r act. 490.
List of Contents 492.
I. Introduction. 493.
I'. Presentation of results.
(a) Subsidiary Experiment todetermine the variation ofThermal Expansion withEvaporable water content
496.(U) Subsidiary Experiment to
assess the effectivenessof the seal used in thePore pressure/Shrinkagespecimen - specimen 1.9. 497.
(c) Pore Pressure/Shrinkagespecimen. 498.
Page ro,
111. Discussion of' results.
(a) Variation of ThermalExpansion with evaporablewater content experiment 500.
(b) Specimen 1.9: the sub-sidiary experiment toassess the effectivenessof the seal used in themain specimen 502.
(c) Pore Pressure/Shrinkagespecimen 505.
(d) Assessment of the Sealbehaviour 510.
(e) The behaviour of Shrinkagewith changing moisturecontent 515.
(f) The overall behaviour ofwater content, shrinkageand Pore Pressure in a 521Migration specimen
IV. Conclusion. 525.
CI1PTER TEN - GENE . L CONCLUSIONS. 529.
BIL3LIOGR,p1Iy.
CONVERSION OF COMMON BRITISH UNITS
TO EQUIVALENT VALUES IN SI. UNITS
1 in.
1 ft.
1 lb.
2204 lbs.
1 lbf.
1 lb/in3
1 lb/in2
25.4 mm
0.305 m
= 0.454 kg
= 1 tonne
= 4.448 N
0.0277 g/mm3
6895 N/rn2
-I •'T' flA IL
I NTi&)JJUCTIer
Li CIUUILo TO TIlL TULSI.
C li i TL L - I fCiiLiCTI ciic'ui 'ru T; 1. TIALS1 .
The use of concrete as a primary containment
material in nuclear reactors has been e8tabliahed for over
a decade. The magnox type of reactor was the fir8t type
used in a commercial power 8tation in the United Kingdom
when Calder Hall was opened in 1956. This reactor was
contained in a steel pressure vessel enclosed by a mass
concrete biological shield. The subsequent development
of the Advanced Gas-cooled Reactor permitted the use of
higher working temperatures, demanding a more advanced
container.
In the early nuclear reactors, limitations on
reactor size and working pressures were dictated by the
site-fabricated steel pressure vessels. These were
removed by the adoption of the prestressed concrete pressure
vessel. This type of structure permits an integral reactor
design, with the entire gas circuit, including boilers
contained within a single pressure envelope. At the
same time enhanced saeety, and resultant economy are
achieved, and the provision of a separate biological shield
obviated. The first reactors to employ this integrated
design in the United Kingdom were constructed at Oldbury.
the early nineteen sixties. The design of these, and
later ones constructed at Wylfa and I ungeness are described
in detail by Taylor (1). Many others have since been
built throughout the world using the same design principle.
The shape of these vessels has varied, spherical and
cylindrical ones being used. The internal dimensions
have varied up to a maximum size in the order of 100 ft.
while the wall thicknesses can be up to 2Oft. The
internal working pressures are in the range 500-600 1b/in9.
gauge.
3
A common feature of concrete pressure vessels
is the provision of a steel liner membrane on the inner
surface. This prevents leakage of gas through the
concrete, acts as a thermal shield for the concrete and
reduces the neutron dosage emitted from the reactor. The
harmful radiation emitted is in the form of neutron and
gamma rays. Neutron rays are slowed down and absorbed
by 'light' elements, and in concrete hydrogen atoms act
in this manner. The capturing of the neutrons by the
hydrogen atoms produces some beat in the form of secondary
gamma rays, which together with the original gamma rays
are absorbed by the 'heavy' elements (e.g. Carbon, Sulphur,
Iron) in concrete. The tlückness of concrete is sufficient
to reduce any harmful radiation to a safe level, but it
is worth noting that from the view point of radiation
protection, the highest possible moisture content spread
evenly over the cross-section of the container and
constant, or just slightly changing with time would be
desirable.
Due to the chemical reaction inside the reactors
and the neutron bombardment, non-linear thermal gradients
are set up in the walls of the Prestressed Concrete Pressure
vessel. Up to the present day, the concrete temperatures
have been maintained at acceptable maximum levels,
generally in the 60°C - 80°C region by a system of
multi-layer stainless steel foil insulation, the foils
being typically 0.004 ins, thick (2), attached to the
inner surface of th steel liner and water-cooling tubes
welded to the external surface of the liner.
Since the Energy crisis of 1973, attention has
been refocused on the development of nuclear power, with
lb
Special reference to the efficiency of the present systems
in use, and where cost economies can be applied. System
efficiency can be increased by raising the working tempera-
tures and pressures inside the reactor. Cost economies
can be effected by a reduction in the volume of the
costly cooling and insulation systems. Both these actions
would of course increase the maximum temperature in the
concrete. Billig (3) claims that the detrimental effects
on the physical properties of concrete due to high
temperatures are tolerable for constant temperature
conditi)ons up to about 250°C, but under temperature cycling
conditions, these detrimental effects are accentuated.
The properties of concrete which are post impor-
tant to the concrete vessel designer have been li8ted in
several publications (2,4), and are dealt with in the
review of literature. However, many of the properties
of concrete at high terperatures are influenced by the
choice of aggregate used. Crispino (5) has described a
research programme sponsored by Euroatom to develop a
concrete capable of being used at high temperature.
At present a concrete mix of modified Low Heat
Portland Cement with limestone, coarse and fine aggregate
with a water/cement ratio of 0.425 is favoured by the
U.S.A.E. Waterways Authority for use at high temperatures
in concrete pressure vessels (6), while a Mix of Sulphate
Resisting Portland Cement, j" crushed limestone and
quartzite sand with a water/cement ratio of 0.47 was
developed for the Oldbury concrete pressure vessels (7).
This was one of the many reasons why Limestone aggregate
was chosen for the practical work described later in this
thes is.
5
Tht. majority of the properties ofconcrete
important in pressure vessel design, and discussed
later, are related to the amount of water in the concrete
and the changes in moisture content that take place with
the passage of time. During the past decade a series
of experimental studies (8,9,10) have been carried out
at King's College, University of London into moisture
migration in conrete specimen under a thermal gradient
and its effect upon various physical and engineering
properties of the concrete. This work forms the back-
ground to the investigation described in this thesis,
and is referred to throughout.
Ross, Iliston and England (8) found that on a
simulated thick-wall section sealed at the hot face and
- 0subjected to a thermal gradient of 12 C/ft. drying had
only taken place in the foot of concrete adjacent to
each face after heatinghad been applied for a year.
Sharp (10) later investigated this in more detail with
sealed cylindrical specimens open at one end to atmos-
phere. These specimens were 4-1/lb' t diameter and of
length 2, 3, 4 and 10 ft. One specimen of each length
was tested at a base temperature of 80, 105, 135 and
lbO°C respectively for nearly two years. It was found
that the penetration of the drying-front at the hot end
was temperature related. The experimental evidence also
suggested that at temperatures below 100°C, drying was
not likely to be a very important factor in a reactor
pressure vessel. It was also found that depending upon
the length of the moisture path, excess water was present
in the intermediate regions, as shown in Figure 1:1.
Similar studies have since been reported by }Iornby (11)
and MacDonald (12), neither of whom found these areas of
increased water content above the original mixing water
content. However, the latest study by Hundt and
Schiinmelwitz (13) under similar conditions with a hot
end temperature of 80°C showed an area between 60cm and
80cm from the heating plite with a 8mall increase in
moisture content over the moisture content before heating.
Parkinson (9) investigated water movement and any accom-
panying shrinkage in a concrete subjected to a thermal
gradient. Specimens were sealed along the sides and
heated end, and thermally insulated. Measurements of
water content and shrinkage were made with time in the
specimens whose lengths ranged from 10 inches to 5 ft. and whose
hot end temperature ranged from 60 0C to 95°C, shrinkage
accompanied drying and Plate 1:1 shows a pictorial
distribution of the transverse shrinkage in a 5 ft.
specimen (of Parkinson's) heated for 6 years with a base
temperature of' 80°C. This specimen was started by
Parkinson (9), who took readings for 4 years. The
present author pursued this for a further two years until
it was six years old. Figures 1:2, 1:3, and 1:4 show
the actual temperature distribution, strain distribution
and evaporable water content distribution measured at the
end of the six years' heating. Some of Parkinson's
specimens were sealed at the cold end so that evaporation
was prevented. Here, measurements of moisture content
showed an accumulation of water which was accompanied
by "swelling". This "swelling" phenomena was also
reported by England and Ross (14) in a series of laminated
specimens under a thermal gradient and sealed to prevent
moisture from the sides. A diagram of the specimen and
results recorded are shown in Figure 1:5.
Sharp (10) measured pore pressures inside sealed
concrete specimens that developed as a result of the entrained
air and rapid increase of saturated vapour pressure with
increase of temperature. The magnitude of the pressures
developed were found to be dependent only upon the
temperature and the volume of capillary water. A typical
relationship between temperature and pressure is shown in
Figure l:b. A similar specimen to those used to establish
the relationship between the temperature and pore pressures
developed was heated to 150°C and thereafter allowed to
lose weight by periodically opening a release valve;
Figure 1:7 shows the results obtained. The flat position
of the curve corresponds to the surated vapour pressure
at 150°C.
Sealed cylindrical specimens 3-i" in diameter and
2 ft. long, heated at one end to temperatures of 140°C and
175°C were also tested by Sharp (10). Pore pressures
in these specimens were monitored by recording the
pressures inside porous inclusions which were positioned
in the concrete at the time of casting. Also embedded
in the concrete were thermocouples and moisture meters in
order to monitor the variations in moisture contents and
temperatures throughout the tests. It was observed that
the pore pressures dissipate slowly in the hot regions
as water migrates towards the cooler parts of the specimens,
and that some indication of the changing water content
throughout the specimens could be obtained by a comparison
of the recorded pressures and tho8e calculated on the basis
or entrained air and saturated water vapour. Typical
distributions of gauge pressure and evaporable water at
vious ages is shown in Figurel:8.
In this thesis, the behaviour of the pore
pressure in sealed limestone concrete specimens that
are periodically allowed to .rse weight at various elevated
temperatures is investigated. The variation of pore
pressure and moisture content with time at various points
in five and ten foot long sealed cylindrical specimens of
limestone concrete at hot end temperatures between 105°C
and 200 0C is also investigated. Similar five foot
specimens at various hot end temperatures were "vented",
at various locations of the jacket in relation to the
heated end, to atmosphere 1 at various times during heating
in order to assess the effect upon the moisture migration
rate and characteristice. An investigation into
behaviour of transverse and longitudinal shrinkage,
moisture content, and pore pressure in a sealed cylindrical
specimen of limestone concrete with a hot end temperature
of 150°C is described. lastly, a description of the
investigation and micrographs are given for samples taken
from several of the specimens mentioned above at the end
of heating.
CIItPTER TWO - REVIEW OF LITER.TURE.
I. Introduction
II. Cement Chemistry
(a) Introduction
(b) Setting of cement at normal temperatures
(c) Setting of cement at elevated temperatures
III. Cement Phics
(a) Introduction
(b) The Physical structure of cement paste
(c) Definition of the types of water in cement
paste and their physical properties.
IV. Movement of Moisture in and from concrete
(a) Introduction
(b) Diffusion Theory
(c) Concrete Permeability
(d) Pore Pressures in concrete
(e) Drying and Moisture Movement in concrete
under Thermal Gradients
(f) Methods of monitoring moisture movement in
and from concrete.
V. The Engineering Properties of concrete
(a) Introduction
(b) Concrete strength
(c) Modulus of elasticity
(d) Poisson's Ratio
(e) Thermal Expansion of Concrete
(fl Heat flow Properties of concrete(g) Carbonation
(Ii) Shrinkage
(i) Creep
VI. Limestone Aggregate Concrete
(a) Introduction
(b) The Chemical and Physical structure
of Dolomitic Limestone
(c) Effect of curing temperature on the
hydration products of Limestone
Aggregate Concrete.
(d) Cement/aggregate reaction and the loss
of strength at elevated temperatures.
CHAPTER TWO - REVIEW OF LITERATURE.
2-I ThTRODUCTION
The use of cementing materials is very old.
The ancient Egyptians, the ancient Greeks and the
Rornans all used cementing materials in their buildings,
some of which are still standing to-day. Most of
these cementing materials were some foim of calcined
limestone with lime, water, sand and crushed rock added
to create concrete.
Joseph Aspdin, who in 1824 took out a patent
for the manufacture of "Portland Cement" is regarded as
the father of the present day worldwide cement manufac-
turing industry. Aspdin prepared his cement by beating
a mixture of clay and limestone in a furnace until
carbon dioxide had been given off. This process was
later refined by Isaac Johnson in 1845, who burnt a
mixture of clay and chalk until a clinker had been formed.
This was at a much higher temperature than in the Aspdin
process, and is still the basic process used today.
The manufacture of cement in Britain alone is over twenty
million tonnes per annum and cement is regarded as one
of the essential materials for any country.
The compound formed by mixing cement, aggregate
and water together is known as concrete. When first
mixed together, these materials form a viscous material.
However, a slow chemical reaction takes place between
the cement and water. The result of this reaction is a
solid matrix known as cement paste. This cement paste
forms around the aggregate particles and eventually
binds them together to form the solid material known
as concrete.
Although concrete is perhaps the most used
building material in the world today, the chemical and
physical make-up of the cement paste is very complex
and, even today, is not by any means fully understood.
However, since the beginning of the l9bO's, advances
in physical and chemical techniques has enabled the
advancement of knowledge into the physics and chemistry
of cement paste and its interaction with the aggregate
particles in concrete.
In this review of literature, advances in the
understanding of the Physics and Chemistry of cement
paste and concrete are covered, with special reference
to the use of a scanning electron microscope. The
theories and factors influencing the movement of
moisture in and from concrete are reviewed. Measure-.
ment of pore pressures in concrete and previous work done
on moisture migration under thermal gradients is
reviewed and the methods of monitoring the moisture
movement are listed. The engineering properties of
concrete both at normal and elevated temperatures that
are important to the prestressed concrete pressure
vessel designer are listed, with special attention paid
to the properties of limestone aggregate concrete.
Lastly, the physical and chemical structure of limestone
aggregate is examined in detail, and some previous
work with limestone aggregate concrete is reviewed.
2-11 CEMENT CHEMISTRY.
(a) Introduction.
Several types of cement exist and they can
be divided into two nlain groups, Portland cements and
non-Portland cements. All the Portland cements are
similar in form, with vaying proportions of the compounds
contained in Portland cement to give the various types
of cement (e.g. Rapid Hardening Portland Cement,
Sulphate Resisting Portland cement). The most
commonly used Non-Portland cement was High-Alumina
cement, but the use of this has now ceased for structu-
ral purposes following severa.J. collases of buildings
containing High-Alumina cement. It is still, however,
used in refactory concrete. All the experimental
work described in this thesis was perfomed using Normal
Portland Cement and so the discussion will be confined
to this type of cement only.
(b) Setting of cement at normal temperatures (15,lb,17).
There are four major constituents of Normal
Portland cement. These are tricalcium silicate
(3CaO.SiO2 written C3S), Dicalcium Silicate (2CaO.Si02
written C2 S), tetracalcium aluminoferrite (4CaO.Al203.Fe203
written C4AF and tricalcium aluminate (3CaO.Al203
written C3A)
In reality these constituents are not
pure Corn[)ounds, but may be represented by the che,nicai..
formulae given. Typicni. proportions of t1ee main con-
stituents, obtained by Brunauer and Copeland (18) using
X-ray diffraction analysis on several samples of Normal
Portland cement are shown in Table 2:1.
tL _
TA1LE 2:1
CONSTITUENT PROPORTION 0,
TRICALCIUM SILICATE C3S 53
DICALCIUM SILICATE C2S 24
TRICALCIUM ALUMINATE C3A 8
TETRACALCIUM ALUMINOFERRITE C4AF 8
TOTAL 93
On the addition of water, Portland cement
reacts immediately to form new compounds, commonly known
as hydration products (i.e. a hydration product is one
formed by the addition of water). These reaction
products manifest themselves in an immediate increase
in the viscosity of the paste, and prevent the reactants
coming together freely, thus slowing down the rate of
reaction. As the hydration process continues the
compounds that collectively make up cement disappear and
are replaced by their hydration products. Even so,
a year after mixing there still remains some free water
and unhydrated cement, and the rate of reaction is so
slow to be only significant over a period of years. A
diagrammaticrepresentation of the hydration process is
shown in Figure 2:1.
The rates of reaction between the various major
constituents and the water added varies considerably, and
the various compounds produced play differing r6les in
the life of concrete. These reactions can be written
as follows:-
Is.
2(3CaO.SiO2) + 61120 2Si02.H20
+
(TRICALCItJM SILICATE) + (WATER) = (TOBERMORITE GEL) +
3Ca(OH)2
(CALCIUM HYDROXIDE)
2(2CaO.Si0 2 ) + 41120 = 3CaO.2SiO2 .3H20 +
(DICALCILJM SILICATE) + (WATER) (TOBERMORITE GEL)+
Ca(011)2
(CLCIUM HYDROXIDE)
4CaO.a1203.Fe203+ 1120 + 2Ca(011)
(TETRACALCI UM ALUM- + (WATE1) + (CALCIUM HYDROXIDE)INOFERRI TE)
= 6CaO.Al203.Fe203.12H20
= (CALCIUM AL[JM1NOFERRITE HYDRATE)
3CaO.Al203 + 121120 + Ca(OH)2
(TRICALCIUM ALUMINATE)+ (WATER) + (CALCIUM HYDROXIDE)
3CaO.Al203.Ca(OH)2.121120
(TETRACALCIUM ALUMINATE HYDRATE)
The most important reactions are between the C 2 S and C3S
constituents and the water. As can be seen, these both
form Calcium Hydroxide and the same hydrated Calcium
Silicate, 3CaO.2SiO2.3H20.
This Calcium Silicate Hydrate is called
Tobermori.to Gel because the crystal structure resembles
that of the natural mineral tobermorite, and is the main
cementing component of concrete, being primarily respon-
sible for the setting, hardening, strength and dimensional
stabiiity of concrete. The reaction between the C3S
component and water is comparatively quick and complete
after a short time. The C2 S constituent, however,
reacts much slower and it is this material which remains
incompletely hydrated even after many years. By using
X—ray diffraction analysis, Brunauer and Copeland (18)
were able to show that Toberniorite gel makes up about
I'
50 per cent by weight of a fully hydrated cement and
that Calcium bydroxide made up 25 per cent by weight.
Some of the Calcium Hydroxide saturates the excess water,
the remainder being in the form of small crystals or
amorphous particles. By removing calcium hydroxide in
the reactions between C2S and C3 S and water and 80
preventing the calcium hydroxide from saturating the
solution the setting of the cement can be inhibited.
The reaction between the tetracalciurn Alumino-
ferrite (C4AP) and the saturated solutiónof calcium
hydroxide gives a compound called calcium aluminoferrite
hydrate which resembles a class of natural minerals called
hydrogarnets. This reaction, although faster than that
between C2 S and water, is considerably slower than those
between C3 S and C3 A and the free water, as can be seen from
Figure 2:2. This compound Calcium aluminoferrite
is believed to further hydrate into tricalcium aluminate
hydrate and an amorphous phase (16) C 4AF contributes
little to the strength of the paste, but acts as a flux
during the manufacture of cement. The reaction between
the trjcalcjum aluminate constituent and water is the
fastest reaction. The proportion of tricalcium aluminate
in normal portland cement ha8 a large influence on the
rate of heat release and hardening rates in the setting
of the cement. A large proportion of tricalcium aluminate
in cement leads to a "quick" oi' "flash set". To Counter-
act this gypsum is added to the clinker as a "retarder"
to slow down the hydration of the C 3 A constituent.
This gypsum reacts with the C 3A to form Calcium Monoulfo-
alumina te.
3Ca).Al 203+ 101120 + CaSO4.2H20
(TRICALCIUM ALUMINATE) + (WATER) + (GYPSUM)
3CaO. Al203 . CaSO4. 121120
(CALCIUM MONOSULFOALUMINATE)
Many workers have attempted to study the chemical
reactions that occur in the setting of cement paste.
Bogue and Lerch (19) observed that the products of
hydration of cement are chemically the same as the
products of hydration of the individual compounds under
similar conditions. However, these products can interact
with other compounds in the system. The advent of X-ray
diffraction analysis, electron microscopy and electron
probe microanalysis has now enabled research workers to
study the chemistry of the setting of cement paste in
far greater detail and depth than was previously possible
with pure chemical analysis.
A full account of the chemistry ofcement
hydration is out of place here. However, a knowledge
of the more important factors in the formation of set
cement paste is a necessary preliminary to the understan-
ding of the chemical and physical effects occurring in
concrete at elevated temperatures.
(c) Setting of cement at elevated temperatures.
As can be expected, the rate of hydration of
Portland cement is increased by raising the temperature
of the cement and water. This, of course, is wellknown
by the Pre-cast and Ready-mix Concrete industries, where
both steam curing for efficient use of shuttering and
accelerated curing for an early indication of strength are
widely used. However, the curing temperature has an
important effect upon the products of hydration, as
several of the hydration compounds produced at normal
teilperatures are unstable at elevated temperatures.
Imi.ach (20), in a detailed study of the chemistry of
0 0various cement pastes and concretes cured at 25 C, 90 C
and 140°C using X-ray diffraction analysis, managed to
identify several different hydration products in all
her specimens. A Normal Portland cement paste cured
at 25°C had main hydration products of Calcium Hydroxide
and Ettringite (Ca 3 Al2O7 .3CaSO4 .32H20). The mainproducts of hydration of the specimens cured at 90°C
were Calcium Hydroxide, tobermorite and traces of
hexagonal aluminate. It was noticeable that no
ettringite was present in the specimen cured at 90°C,
nor was it detected in the specimen cured at 140°C,
whose main hydration products were Calcium Hydroxide,
tobermorite and Qc.-C2 S, a form of dicalcium silicatethat occurs at higher temperatures. No aluminate was
present in this specimen. Pastes made with pure
tricalcjum silicate were also examined. The higher
the curing temperature, the more the proportion of
Calcium Hydroxide and less the proportion of C3S wasdetected. Specimens made with siliceous material were
found to have a higher degree of hydration than corres-
ponding ones made with pure cement alone or with lime-
stone aggregate, indicating that siliceous aggregates
may react with the cement paste. The main products
of hydration were o-C2 S hydrate and tobermorite, these
being more abundant at the higher curing terperatures.
0 0Calcium Hydroxide was absent in the 90 C and 140 C
specimens except in one made with C 3 S and Quartzite
sand. however, it was present in all specimens cured
at these temperatures with limestone aggregate or no
aggregate at all. The results obtained with limestone
aggregate are reviewed in a later part of this chapter.
Imlach's work shows that not only does the temperature
of curing have an important effect u.,on the setting of
the cement, but that when aggregate 18 present, the minerals
contained in the aggregate also have a greater effect
upon the products of hydration at elevated curing
temperatures.
Verbeck and Helmuth (21) have also looked at
the hydration of cement at various temperatures. They
found that although elevated temperatures up to 100°C
enhanced hydration and also early strength, long term
hydration was retarded at elevated temperatures, and
hence ultimate strength was also reduced. The reason
they put forward for thia phenomenon was that elevated
temperatures caused a high concentration of hydration
products in the zone around a cement grain. These
hydration products were not allowed to diffuse and
precipitate uniformly tfroughout the specimen.
It is interesting to find that no work appears
to have been done on the chemical effects caused by
raising the temperature of mature concrete. It is
probable that the chemical and physical equilibria are
disturbed by a rise in temperature, but that any changes
are not enough to cause chemical in8tability in the
mature concrete. This is borne out by the fact that
concrete is often subjected to continuous elevated
temperatures in such places as boiler houses with no
detrimental effects. This has also been borne out
by many tests carried out at King's College over the
past fifteen years. Many samples (22,23) have been
subjected to elevated temperatures, none of which
collapsed due to chemical instability of the mature
concrete. However, it is worth noting bere that a
loss in strength with the elevation in temperature of'
mature limestone aggregate concrete has been reported
(9); whether this is due to a chemical reaction or
some physical factor will be reviewed and dealt with
later.
2-111 CEMENT PHYSICS.
(a) Introduction.
Hardened cement paste is a heterogeneous
mixture of components as can be seen from the chemistry
of cement paste reviewed in the previous section.
Cement paste is a material of colloidal size and is
frequently described as a crystalline gel. This is
a contradictory physical description, which when one
examines the cement paste structure, one finds is
clearly justified.
In this section, work on the determination of
the physical structure of cement paste is reviewed.
The definitions of the types of water and their deter-
minations and properties by various workers is also
reviewed. These types of water can affect the engin-
eering properties of the mature cicrete as well as
the moisture migration characteristics in mature
concrete at elevated temperatures.
(b) The_Piysical Structure_of çment_Pasçe
Early studies into the physical structure
of cement paste were severly hadicapped owing to the
techniques available at the time. However, workers
(19) in 1934 were aware that cement paste was probably
made up of cement gel, unhiydrated cement and capillary
pores. The limitations of the light microscope in
resolution and magnification made visual observations
of the structure of cement paste useless until the
development of the electron microscope.
Radczlwski, MUller and Eitel (24) in 1939
appear to be the first to use electron microscopy in
the study of cement. They investigated the hydration
of lime, and compared pictures of Calcium Hydroxide
obtained with picture8 of' the products of cement
hydration. This work established that electron micro-
scopy was a useful tool for investigating the hydration
products of Portland cement.
All the early electron microscope examinations
of specimens were carried Out using the technique of
dispersing a powdered specimen on to a substrate.
This method also enables one to obtain electron
diffraction patterns which is useful in the positive
identification of minerals present (25). In 1943
Sliepcevich, Gildart and Katz (26) studied the products
of pure synthetic silicates and aluminates and of
commercial Portland cements. This study revealed two
forms 0f Calcium Hydroxidespheres and fibrous masses.
Calcium Silicate hydrate occurred as rhombic slabs and
amorphous spherulites, while the Calcium aluminate
hydrates appeared to form thin hexagonal plates and
rhombic slabs and needles. These rhombic slabs were
probably calcite formed due to heavy carbon dioxide
Contamination. Boutet (27) in 1950 showed that the
hydration products of cement were clusters of micro—
crystals formed into chains, the particles remaining
firmly connected at their points.
In the 1950's and early 1960's many workers
u8ed the di8persion of a powdered specimen method in
electron microscopy to study set Portland cement.
A detailed reivew of these studies is given by Midgley (28
These studies from powder mounts suggested several impor-
tant features of cement paste. Firstly, it appeared
that Calcium Silicate hydrate occurs in three main
types:
(i) Fibrous material, the fibres up to irm long
and a few broad, and appear to Consist of
bundles of smaller fibres or of rolled øoils.
(ii) Irregular masses of small distorted plates a
few hundred across and a few tens of thick.
(iii) Crumpled foils.
Midgley (29) has shown that the relative
proportions of the platey and fibrous hydrates depend
on the water/cement ratio. At low values (0.25) the
platey form predominates whereas the fibrous material
is more abundant in pastes with a water/cement ratio
of 1.0.
Secondly, that Calcium hydroxide appears to
occur in two forms; as a plate-like structure and
also as small spheroidal growths. Thirdly, that
calcium Aluminate hydrates appear as thin hexagonal
plates. Fourthly, several workers showed that
ettringite, a complex calcium sulphaoluminate hydrate,
Ca 3Al207 .3CaSo4 .32H20, was lath-like and occurred as
long and rather coarse rods or splines, sometimes
with well-developed end faces.
Typical "powder mount" method micrographs can
be seen in Plates 1:1, 1:2 and 1:3. The advantages
of these micrographs are that the instrument gives high
resolution and so detail is much more easily seen.
However, they have the disadvantage that they only give
a two-dimensional view of the hydration products. A
slight improvement to give a three-dimensional picture of
cement paste was the development of the Replica technique
for use in the transmission electron microscope. This
technique produces a thin film, transparent to an electron
beam, which replicates the surface of set cement paste.
The replication of the specimens is tedious and very
liable to artifacts, but a good resolution is obtained
from the instrument. The micrographs obtained using
this method look rather like aerial photographs of a
mountain range. Gille (30) appears to be the first
person to use the replica method for the examination of
set Portland cements. The replica method was used by
several workers between 1958 and 1968, and many electron
micrographs of cement surfaces were published. It was
the innovation of the scanning electron microscope with
its large depth of focus, that made available a suitable
instrument for studying the three-dimensional arrangement
of the fracture surface of a cement paste specimen.
The only disadvantages of the scanning electron micro-
scope are that electron diffraction is not possible (nor
is it with the replica technique), and that the instru-
ment has only a resolution of about 200 against 10
for the transmission microscope.
In the scanning electron microscope, a very
narrow electron beam scans the surface of the specimen
under investigation. The electrons which are scattered
by the specimen are collected by a detector and the
amplified signal controls the t*ightness of a spot on
a Cathode-ray tube. The electron beam is made to scan
the surface in a raster and the spot on the Cathode-ray
tube follows the same raster and so forms an image of
the specimen. The preparation of the specimen forthe
scanning electron microscope is much easier than in the
"powder mount" or replica techniques using the trans-
mission electron microscope. The actual specimen
preparation method is described in the chapter on
"experiments and Experimental Techniques".
The first results of the examination of a frac-
tured surface of a hardened cement paste was the
publication of several scanning electron microscope
photographs by Cbatterji and Jeffery in 1966 (31).
These were photographs of samples of Ordinary Portland
cement paste with a water/cement ratio of 0.6 cured
under a thin suspension of Portland cement in water for
1, 3 and 14 days. The hydration products in the 1 day
sample consisted of hexagonal crystals, between 6ni
and lOrm in length and needles of about item in length.
They considered the former to be either calcium hydroxide
or calcium monosulpho-aluminate and the latter to be
either ettringite or calcium silicate hydrate. The
3 day specimen exhibited two types of needles. The
larger type (4rm to Srm in length and O.25rm thick)
occurred individually and were thought to be ettringite.
The smaller type (only about item in length) occurred in
clusters. The 14 day specimen was similar to the 3 day,
although more fine needles (up to l.5rm in length)
were present. The larger type of needles was thought
to be ettringite and the smaller ones Calcium Silicate
hydrate.
Subsequent studies by Chatterji and Jeffery (32)
in 1967 and by Moore, Chatterji and Jeft'ery (33) in 1968
on a sample of C3S with a water/cement ratio of 0.6
cured for 7 day8 and 1 year revealed needles that they
considered to be calcium silicate hydrate. These
needles were less that lrm in length after 7 days,
but were up to Srm in length and in clusters rather
like tropical sea anemones after 1 year. All the
needles of both types form an entangled mass of three
dimensions.
Diamond (34) in studies on cement paste using
a scanning electron microscope concluded that the
situation was much more intricate than Chatterji and
Jeffery had found. Areas of elongated particles were
observed, but in addilion there were also areas com-
prising only more or less equant "gel-bodies" and areas
in which both types of morphology occurred. The
elongated particles ranged up to lOrm in length,
although the gel-bodies appeared to be rounded lumps
of less than lrm in diameter.
These "gel-bodies" were similar to the
photographs of Calcium Silicate hydrate prepared by
shaking lime and colloidal silica at 25°C for 34 days
obtained by Speakman and Ma)umdar (35). They also
produced good photomicrographs showing fibrous and
plate-like formations and also large (over lOrm
length) well formed crystals of Calcium Hydroxide.
Mills (36,37) used a scanning electron micro-
scope to examine specimens of set cement paste and
concrete that had been subjected to creep and shrinkage
tests for 100 days. Fibrous and amorphous materials
similar to those previously described were observed
but, in addition, "well-formed rosettes" of petal-
shaped crystals were found. These well-formed flowers,
as named by Mills, were present in the shrinkage specimens
of vaying water/cement ratios. however, in the specimens
subjected to sustained loading, these "flowers" only
occurred in bleeding cavities. However, the walls of
cracks were covered with apparently amorphous material
which Mills thought to be collapsed flowers. He
concluded that the previous shrinkage and strain history
was respon8ible for the different appearances of hydrate
structure observed. It must be noted that in all
these specimens of Mills', the calcium hydroxide had
been etched with distilled water. Hence a direct
comparison with any of the work on the fractured surfaces
not treated this way does not apply because it cannot
be assumed that only calcium hydroxide is etched.
In a recent study at the Building Research Establish-
merit (38), a scanning electron microscope was used to
examine the microstructure of concrete with special
reference among several, other studies to the cement/
aggregate interface and the effects of sulphate attack.
Photomicrographs of the interface between the hydrated
cement and sand heavy aggregate gave no indication8 of
reaction across the boundary between the two phases in
either of the specimens examined. In a specimen of
concrete that had been subjected to long-term exposure
by sulphate-containing solutions, micrographs revealed
large, well-formed crystals of ettringite in a previously
empty void. This is consistent with the internal
expansive forces generated by sulphate attack on concrete
that ultimately cause cracking and collapse. Also
published in this paper (38) are two micrographs of
the fracture surface of a typical Portland cement clinker.
These show that even with the large depth of field of
the irrument, it is still not possible to characterise
individual phases in a direct manner, and one has to
depend on known morphological features of these clinker
consttuents for their identification. These two
micrographs are, however, a useful standard reference.
In her investigation mentioned earlier,
Imlach (20) looked at the microstructure of several
specimens with a scanning electron microscope. The
micrographa obtained with Ordinary Portland cement and
limestone aggregate will be reviewed later, but it is
worth mentioning the others obtained here.
Samples of' ordinary Portland cement paste with
0 0a water/cement ratio of 0.3 and cured at 25 C and 140 C
were viewed after one year. Both contained regions of
hexagonal platey crystals and "gel-like" material. In
general, the paste cured at 140°C appeared to have
possibly more structure than that cured at 25°C.
Specimens of C 3 S paste with a water/cement ratio of 0.3
_o 0 0and cured at 23 C, 90 C and 140 C were viewed after
1 year. The paste cured at 25°C showed hexagonal
platey crystals, which was almost certainly Calcium
Hydroxide and "gel-like" material. The proportion of
"gel-like" material increased with curing temperature.
However, unlike the specimen of Moore, Chatterji and
Jeffery (33) no needle-shaped material was observed.
The sample of Ordinary Portland Cement and Thames Ballast
aggregate with a water/cement ratio of 0.3 cured at 25°C
was of great interest. Platey crystals, lath-like
rods and needles and a honey-combed structure were all
observed. The play crystals and lath-like structures
were similar to those seen by Chatterji and Jeffery (31)
and were almost certainly Calcium H'droxide and ettringite.
The ettringite rods were 2Orm to 3Orm in length. The
"honey-combed" material seemed to consist of intergrown
spherulitic aggregates of fibrous material, and is
probably calcium silicate hydrate. A reaction rim on
aggregate particles was observed and it appeared likely
that the cement had reacted with the Thames Ballast.
Another interestin, comparison made by Imlach was two
samples of Ordinary Portland Cement with a water/cement
ratio of 0.3 cured at 25°C, one with an aggregate of
Quartz lulO and the other with an aggregate of Quartz
0001. Both these samples were viewed after 18 months'
curing. With the Quartz 1010, a lot of platey material
was observed, with also a large amount of "cauliflower-
like" material present, and bridging material of
"coral-like" strands. A reaction between the cement
and Quartz 1010 seemed to exist. The sample made
with Quartz 0001 also showed a reaction between the
cement and quartz, although different to the one des-
cribed with quartz 1010. Much less "cauliflower-like"
material existed, and it seemed slightly different. A
lot more platey cry8tal and fibrous material was present.
Lastly, samples of ordinary portland cement and calcite
0 0with a water/cement ratio of 0.3 cured at 90 C and 140 C
were viewed after 20 months. Both samples showed
"rod-like" structures growing into the cement from the
calcite; "gel-like" material was also present.
Although the scanning electron microscope now
allows a clear three-dimensional visual examination
of the solid products of hydration, due to the high
vacuum under which theecimen is examined, an
assessment and distribution of the types of water
existing in the specimens is not possible. A18o,
because of the small area being viewed at any one time,
micrographe can only be taken as a guide to the whole
microstructure of the cement paste.
The structure of the cement gel has a signifi-
cant effect upon the drying behavour of concrete. This
is due to its high porosity, that both Powers (39, 40)
and Neville (1t) report as 28%. The gel pores are
usually filled with water and their size has been estima-
ted by Powers (39, 40) and several others to have a size
from 18 to 20 , which corresponds to six times the
diameter of a water molecule. The relationship between
porosites and the water/cement ratio has been looked at
by Schimmelwitz and Hundt (41). They conclude that
the water/cement ratio within certain limits seems to
have no effect on the kind of the growing hydration
products as long as drying has not progressed too far.
This of course is a contradiction to the results of
Midgley (29) mentioned earlier.
The high porosity of the gel is regarded by
most workers to have little influence on the permeability
of the concrete, as the pores are so small. It is the
capillary porosity, which Powers (39, 40) has given to
usually be between 30 and 40 per cent, that is regarded
as important when considering concrete permeability. The
capillary pores cariiot be viewed directly, but their
size has been estimated from vapour pressure measurements
to be of the order of S x 10 ' ins (42), which is over a
thousand times larger than the gel pores. The capillary
pores vary in shape and form an interconnected system
that is randomly distributed in the hardened Concrete.
If no drying has taken place from the concrete the
capillary pores are mainly filled with water, but also
contain water vapour and air.
As the volume of hydration products increases
with time, the capillary pores become blocked. The
actual time for this to occur depends upon the water/
Cement ratio and degree of fineness of the cement (16).
For an extremely fine cement the maximum
water/cement ratio could be as high as 1.0, below which
capillaries cease to be continuous. Conversely, for a
coarse cement this could be at a water/cement ratio
below 0.7. It is very important to eliminate continuous
capillaries in concrete. The quality of the cement
paste and hence concrete is determined by the relative
volumes of cement gel, unhydrated cement and capillaries.
(c) Definition of the types of water in cement paste
and their physical properties.
Water in cement paste is known to be in three
physical states. These are:-
(i) Water of crystallisation, or "solid" water
chemically bound to the hydrate crystals.
(ii) Free "liquid" water saturated with calcium
hydroxide in the capillary pore spaces in the
paste.
(iii) Adsorbed water on the walls or within the lattice
of the crystals of hydrated calcium silicate.
A precise definition of the adsorbed phase
cannot be given as the degree of adsorption varies from
the highly adsorbed molecules adsorbed directly onto or
with the gel crystals, to the molecules several molecular
diameters from the crystals which have only a slight
degree of imposed orientation. The mechanism is one of
electiostatic attraction since the water molecules act
as dipoles and adopt a particular orientation in any
imposed electrostatic field.
There are no techniques available for deter-
mining how water is distributed between the different
physical states listed above, if the water in concrete
is to be classified, arbitrary definitions must be
formulated and several attempts to do this have been made.
Powers and Brownyard (43) were the first to arbitarily
divide water in concrete into evaporable and non-
evaporable types. The evaporable water was defined by
them as that which is lost on drting to constant weight
at 23°C in a vacuum dessiccator with anhydrous magnesium
perchlorate. The non-evaporable water was defined a
that which is retained by the concrete when subjected to
the above drying technique, but is lost in ignition at
1000°C. Copelaiid and Hayes (44) later modified the
method of determining the evaporable water, which they
claimed was an improvement on the method given by Powers
and Brownyard, as it was quicker, more reliable and gave
a value of non-evaporable water more nearly equal to the
chemically combined water. The cement paste is dried at
the vapour pressureof water over ice at 79°C which is
equal to a pressure of 0.5 x lO mm of mercury, to
determine the evaporable water content, and the non-
evaporable water content is then found by ignition as
before. Later, Hilsdorf (45) defined evaporable water
as that water which is given up when concrete is dried to
constant weight at 105°C, which he states is approximately
equal to the amount given up when the concrete is
dried at 20°C and zero relative vapour pressure. He
defined the non-evaporable water as before. It is this
definition of evaporable water that has been adopted in
the practical work later described ii thia thesis. The
advantages of this definition over those.involving drying
in a desiccator are that no elaborate apparatus is
necessary and that fairly large specimens can be dried to
3k—.
constant weight so that weighing errors are kept to a.
minimum. The disadvantage is that the rise in tempera-
ture can cause an increase in rate ot' hydration so that
some water can become chemically combined. However, as
most specimens in this thesis were over a year old when
evaporable water content measurements were made, errors
due to this phenomena were very small.
Basically, the non .-evaporable water is approx-
imately equal to the chemically bound water or water of
hydration. The evaporable water is approximately equal
to the gel water (defined by Ieville (Th) as the water
which fills the gel pores, assuming that the porosity
of the gel is 28%) plus the capillary water.
The physical properties of the different types
of water held in cement paste has been the subject of
several detailed investigations by many workers. The
first, and probably most comprehensive, was by Powers
and Brownyard (43), who published their findings in a
paper of nine parts. Of these nine parts, those
numbered two, five and eight dealt specifically with the
physical properties of the various types of water in
cement paste.
In parttwo, which was entitled "Studies of
Water Fixation", they first put forward the idea of
evaporable and non-evaporable water and defined them
in the manner previously described. They then go on
to study the relationship between the relative vapour
pressure and amount of evaporable water in concrete.
They did this by a series of Sorption curves for
concrete at various vapour pressures. The results
indicated that as the relative vapour pressure increases,
the evaporable water content increases. They also
found that the total water held at any vapour pressure
increases as the length of curing time increases, as
does the amount of non-evaporable water. The Sorption
curves also showed that with humidities below 40%, the
amount of evaporable water was proportional to the
amount of non-evaporable water. At saturation, however,
as the amount of non-evaporable water increased, the
amount of evaporable water decreased. From these
results, they concluded the evaporable water existed in
two forms; some occupying the small pores between the
particles of hydrated cement, and which they called "gel"
water. The rest occupies the larger capillary pores and
they called it capillary water. At the humidities below
407o, all the evaporable water occupies the gel spaces.
Above 40% humidity, the gel pores become full andwater
collects in tIcapillary pores.
In part five, entitled "Studies of Hardened
Paste by means of Specific volume measurements", they
divided the total water present in saturated specimens
into two categories; that which bad a specific volume
less than unity and that which had a specific volume
equal to unity. All the water with a specific volume
less than unity they called "compressed water", and
this comprised of the non-evaporable water and a part
of the evaporaDle water. They calculated that the
specific volume of gel water is about 0.90 and that of
capillary water 1.0. The combined value of specific
volumes of the non-evaporable water and the gel water
was 0.8k They also concluded in this part that the
that the maximum amount of non-evaporable water that
can exist in concrete is approximately 50% of the mix-
ing water.
In part eight, entitled "The freezing of
water in Hardened cement paste", they stated that no
freezing occurred in saturated samples until they
were upercooled -5°C to -12°C. Further freezing
caused more ice to form, The ice was believed to form
in the capillary space out8ide the gel, and the authors
thought it was unlikely that the gel water froze in
place, although at low temperatures it contributed to
the ice. Powers (4b) in a later study concluded that
the amount of non-evaporable water found in hardened
cement paste depends on the curing conditions and the
fineness of cement. Lie suggested that the weight of
non-evaporable water was about 26% the weight of cement
for completely hydrated normal Portland cement.
Hilsdorf (45) proposed a value of 24%, Powers (4Q) in
a 1er paper adjusted this value to 227% and it is
now generally accepted (16) to be 23%. Powers in
this study (4b) also states that hydration ceases when
the capillary pores are filled, which can occur where
the original weight of water is less than 39% of the
weight of cement; where the capillary pores contain
rio water, which is know as self-desiccation, or when all
the cement has hydrated. Copeland (47) obtained the
value of the specific voluiieof evaporable water in
cement paste by measuring the apparent specific volume
of the total water at various non-evaporable water contents.
}Ie found that the specific volume of evaporable water
compared with free water W88 0.99 by ex*rapolating theWn1
graph obtained to 'Wt - 0. Using the same curve andWn1
extrapolating to 'Ut - 1, he found that the specific
volume of non-evaporable water was 0.74. Neville (16)
reports a value of 0.74o.
This work on the physical properties of water
can be summarised in a series of mathematical formulae.
If Un and Wc are the weights of non-evaporable and
cement respectively, and h is the fraction of cement
hydrated, then-
?4ii - 0.23hWc .. .. .. .. .. (1)
Taking as the apparent density of the non-evaporable
water ande as the density of cement, the volume oe
the solids of hydration, V 01 is given by:-
v.. ..solec en
For normal portland cement e 3'2e and adopting a
value of 0.75 for the apparent specific volume of non-
evaporable water compared with free water so that
-- '• e then substituting in equn. (2)
for p ,p and Uc 'n n
hWV - 0.49 c .. . .aol -
ew
The porosity of the gel is assumed to be 28% (39, 40).
Therefore:-
V1 - 0.28Vgel aol
Substituting for 'sol from equation (3) and re-
arranging
bWV0.19 c .. .. .. (4)gel -
Assuming that the density of the gel and capillary water
are approximately equal to that of free water, then
the weight of gel water is given by:-
14 O.19bW 0.83W .. (5)gel C n
If the ratio of total mixing water to cement byWtI
weight is 'Wc and if no water has escaped, the ght
of capillary water 14 is given by:-
WtW - 14 - W - U - ( 'Wc - o.42h) 14cap t gel n c .. .. (6)
These equations are used in the practical work to calculate
the various weights of gel and capillary water of the
specimens tested in:the Release Test Series.
2- IV. MOVEMENT OF MOISTURE IN AND FROM CONCRETE.
(a) Introduction.
Water exists in concrete in both liquid and
vapour forms. During the early life of a prestressed
concrete pressure vessel, movement of moisture
inside the thick walls is insignificant; however, it
has been showu that drying occurs near the surfaces (12).
This drying is at ambient temperature, while later,
when the reactor has been commissioned, a thermal
gradient exists inside the wall and there is both
movement of moisture within the walls and loss from the
external surface.
The mechanisms that cause the migration of
moisture and the factors that influence its rate at
both ambient and elevated temperatures are reviewed in
this section, as are the methods available for monitering
moisture movement in a concrete structure.
(b) Diffusion Theory.
There have been several attempts by research
workers to apply the laws of diffusion to moisture
movement and loss in concrete. The transfer of
fluid matter from one region to another within a porous
solid may be caused by either a definite force acting
on the fluid or by the random motions of the molecules
of fluid. The existence of a concentration gradient
within the fluid, however it has been 8et up, will
result in diffusion. The movement of moisture may be
considered to be caused by a "force" derived from the
difference in concentration of the fluid. Diffusion
can be expressed in the form of Pick's Law, and this
for uniaxial diffusion is:-
2k.dc
dx2Where c is the concentration of free water, k is the
coefficient of diffusion, t is the time elapsed after
the start of the diffusion process, and x is the distance
from the exposed face. The coefficient of diffusion
increases greatly with temperature, as shown in
Figure 2:3; however, it is fairly constant at elevated
temperatures (4). The value of the coefficient of
diffusion is quite small and decreases with moisture loss
at normal terperatures. Thermal gradients may increase
130.
the diffusion coefficient, particularly at high tempera-
tures where considerably greater internal water
pressures exiut in the concrete. The diffusion
coefficient also varies with the type of cement, mix
proportions of the concrete and the degree of hydration.
Pjh1aavaara (46) who has done a lot of work
on the drying of concrete found that the rate of diffusion
in concrete was in agreement with Pick's Law where the
concentration of fluid and the concentration gradient were
low, and all the moving fluid was vapour. However, he
found when liquid was moving through the concrete or
the concentration gradient was high, Fick's Law was not
obeyed.
Hughes, Lowe and Walker (49, 51) in two papers
report a series of drying tests on concrete and cement
paste spheres of various diameters. These spheres
were dried in a stream of air and the rate of water loss
found by weighing. Attempts were made to fit the
experimental data to Pick's Law of Diffusion.
In the first paper (49) they found that at
constant temperatures between 5o°C and 95°C, a single
constant diffusion coefficient could be used to
describe the drying at any particular temperature,
provided that the surface drying conditions were adequate.
Prom this they concluded that either one process of
drying predominates at these temperatures, or simply
that the much higher value of the diffusion coefficient
at the higher temperatures prevents any small concentra.-
tion dependance being noticed.
Ii'.
The second paper (51) describes drying tests
made at 30°C and 70% Relative humidity on the same type
of specimens. In these tests it was found that the
diffusion coefficient was dependent upon the concentra-
tion of the evaporable water present.
From the work reported, applying the laws of
diffusion to migration of moisture through concrete, it
appears these are only obeyed when the majority of fluid
moving is either gas or vapour. When the migrating
fluid is in liquid form, then other factors become
important, and these arementioned in later sections.
(c) Concrete Permeability.
The flow of water through concrete is funda-
nientally similar to flow through any porous body (16).
However, the permeability of concrete is not just a
simple function of porosity, but depends upon the pore
size, distribution and continuity throughout the concrete.
In the development of the pore 8ystenl, the water/cement
ratio is the influencing parameter. As mentioned in
the section on the physical structure of cement paste,
two types of pores - gel and capillary - exist in
hardened cement paste. The volume of gel pores
decreases as the degree of maturity of the paste rises,
and hence the total porosity of the paste is reduced.
The volume of capillary pores is related to the water/
cement ratio. 4 high water/cement ratio leads to a
large volume of pores which are at the beginning filled
with water, and become drier as maturing progresses, and
can produce a highly conductive system with respect to
the transport of moisture. Figure 2:4 shows the parts
by volume of the individual cement paste components in
relation to the water/cement ratio for the highest
possible degree of maturity which can be reached (13).
It is obvious that, provided hydration has practically
come to an end, and the water/cement ratio lies within
the range of ordinarily used values, the capillary
water only fills a small part of the volume of the
paste compared with the other components.
Powers and Brownyard(43) in Part 7 of their
paper, entitled "Permeability and Absorptivity" suggest
that the flow at' water through hardened cement paste
obeys D'Arcy's Law, which states that the volumetric
rate of flow through a porous medium d
is directly
proportional to the product of the area of flow (A)
and hudraulic gradient (bH) divided by the length of
the path of flow (L) and is written:-
1- Ak 1i
dt L
where is the coefficient of permeability and depend8
on the properties of the porous medium and the
permeating fluid.
This expression can be amended to give an
expression for the coefficient of permeability Ic
which only depends upon the properties of the porous
medium. This is written:-
dt fl L
where fl is the viscosity of the permeating liquid, g
is the acceleration due to gravity, p is the pressure
drop across the medium and k is the coefficient of
permeability with units of area.
Powers (52) gives the permeability of cement
—14gel as about 7 x 10 cm/Gec. Because cement paste
consists of capillary pores as well as gel, and water
can flow more easily through the capillary pores than
the gel pores, the permeability of cement paste as a
whole is 20 to 100 times more permeable than the gel
itself. The permeability of cement paste also varies
with both water/cement ratio and progress of hydration.
Table 2:2 shows the reduction in permeability of cement
paste of water/cement ratio o.7 with the progress of
hydration (53), while Figure 2:5 shows the relationship
between permeability and water/cement ratio for mature
cement paste (53).
TABLE 2:2.
Just as the permeability of hardened cement
paste as a whole is 20 to 100 times more permeable than
the gel itself, 80 jS the permeability of the concrete
1*•
generally much higher than that of hardened cement
paste itself. This is due to several factors. One
of these is that the bond between the hardened cement
paste is never perfect (54), and these bond discontinuities
bring about local capillary systems, which are inter-
connected by capillaries in the cement paste as well
when the water/cement ratio is high. Another factor
is that aggregates have fissures and pores in them that
permit the flow of fluid through them. These pores,
although few, are irly large and lead to quite a high
permeability. Table 2:3 (52) lists the permeability
of some common rocks, and alongside th? figures is
a list of water/cement ratios of mature paste of the
same permeability.
TABLE 2:3.
Water! cementCoefficient of
ratio of matureType of rock permeability
cm/secpaste of samepermeability.
Dense trap 2.47 x IO2 0.38
Quartz diorit 8.24 x io_2 0.42
Marble 2.39 x lO" 0.48
larble 5.77 x lO0
Granite 5.35 x l0 0.70
Sandstone 1.23 x io8 o.71
Granite i.so x io8
It is interesting to note that the permeability
of granite is of the same order as that of cement paste
having a water/cement ratio of 0.7, a value which is
145
generally regarded as too high for good quality concrete.
The different values of permeability fcr the same rock is
due to the fact that the D'Arcy coefficient of permea-
bility (k 1 ) is proportional to the square of the
apparent porosity (55). Although a rock is classified
into a certain type, its apparent porosity can be very
different, depending upon its geographical location.
Table 2:4 shows the porosity of some common rocks and
the variations found (5b).
TABLE 2:4.
Rock Group Porosity %
Gritstone 0 - 48.0
Quartzite 1.9 - 15.1
Limestone 0 - 37.b
Granite 0.4 -
Apart from being, a controlling factor in the
rate at which moisbre migration takes place in concrete
at both normal and elevated temperatures, the coefficient
of permeability of concrete is itself affected by
elevated temperatures and thermal cycling. Although
there appears to be no published work on the effect of
elevated temperatures and thermal gradients on the
coefficient of permeability, several conclusions can
be drawn from other investigations performed on concrete
under these conditions.
k4,
When the temperature of concrete is raised, it
will expend, although the expansion depends on the mix
and hygral btate at the time of the temperature change.
The two constituents of concrete, the cement paste and
the aggregate have dissimilar thermal coefficients,
and this can lead to the formation of microcracks between
the cement paste and aggregate, and hence increase the
permeability. Thie dit'ference in the coefficients of
thermal expansion is enhanced by an increase in tempera-
ture (5) so this increase in permeability with temperature
would be to the power of two or three for a linear
increase in temperature.
When concrete is under a thermal gradient, the
effect is a little more complicated and depends very
much on the moisture condition at the point considered.
In a hot zone, on the application of the temperature,
both the cement paste and aggregate will expand as
described above and hence the permeability would increase.
However, Liecause of the increase in temperature, the
internal pore pressures would be increased, and as the
temperatures are not uniform throughout, pressure and
temperature gradients would exist inside the mass,
and movement of' moisture away from the hot zones to
the cooler ones takes place. The first water to be
lost in the hot zone is "free" or capillary water, and
the removal of this causes little or no shrinkage of
te cement paste (16), and hence very little effect on
the permeability would be expected. However, as loss
of moisture from the hot zones continues, shrinkage of
the gel occurs. This will increase the permeability,
until the shrinkage stops (when all the water that can
be driven off at that temperature has been lost) and
the permeability will then cease increasing. A
further increase in permeability may occur in very hot
zones due to a change in the hydration products
(breakdown, etc). This is difficult to forecast as
the actual hydration products vary with different curing
conditions and different cements, and so each situation
is unique.
In tbe cooler zones, into which the moisture
migrates, the situation is a little more difficult to
forecast. The inflow of moisture, mainly in the form of
water will cause a slight swelling of the paste. This
would have the effect of a small decrease in the permea-
bility. Ingland and Skipper (57) suggest that the
movement of water from the hot zones produces a local
leaching effect and suLmequent deposition of hydration
products in the cooler regions; this would have the
effect of decreasing the permeability in the cooler
regions. There is, as yet, no experimental evidence
to support this.
Thermal cycling has the effect of decreasing
the bond between the cenient paste and the aggregate
(58, t) and this of course will increase the permeability.
However, it is the first thermal cycle that causes most
damage, and the increase in permeability will diminish
as the number of thermal cycles increases.
1
(d) Pore Pressures in_Concrçte.
Concrete is a complex mixture, containing
amongst other constituents liquid water, water vapour
and entrained air. The total pore pressure inside
concrete depends upon the partial pressures of the
water vapour and air, and when the temperature rises,
both these pressures would be expected to increase.
If the concrete is sealed, or the mass is so large that
it is effectively sealed, so that the contents of the
pores cannot escape, the temperature rise should produce
a significant increase in the total pore pressure.
Additionally, if the concrete is under a thermal gradient,
a pressure gradient similar in shape to the temperature
gradient will be set up, owing to the different pore
pressures set up inside the concrete. Sharp and
England (59) found that the migration of water in
0concrete at temperatures over 100 C is seriously
affected by this pressure gradient.
The first person to measure pore pressures
inside a sealed specimen of concrete and report this
in the literature appears to be I3remer (t0). His
apparatus consisted of a sealed block of concrete in
which was embedded a thermocouple and two mercury-filled
pressure measuring pipes which were connected to
manometers. One of the measuring pipes was perforated
so that the pressure in the mercury was equal to the
pressure in the concrete, while the other was sealed.
Bremer foud that the pressures recorded in the capil-
laries varies between about 40% and 60% of the
saturated pressure of steam for various temperatures.
His results are shown in Figure 2:6.
Sharp (10) , as mentioned in tie introduction
to this thesis, measured pore pressures in two
separate test series of sealed concrete specimens. In
the first series, small cylindrical specimens 4" long
and 4-i" diameter, sealed with a mild steel sealing
jacket consisting of a tube and two end plates, were
heated in an oven, and readings of temperature and
pove pressure recorded. Each specimen contained two
thermocouples and a porous inclusion in the form of a
small square of sintered bronze plate, which was linked
by small bore copper tubing to a perspex U-tube filled
with mercury via a valve. The pore pressure was
measured by opening the valve and adjusting the mercury
levels in the perspex U-tube back to its datum level by
the application of a back pressure, equal to the pore
pressure on the other side of the U-tube. Specimens
of various ages from 2 days to 260 days were tested,
the temperature of the specimen being elevated from
room temperature to 150 0C. Many of the specimens
underwent several temperature cycles in order to
investigate whether the pore pressures were affected
by the previous temperature history of the specimen.
Specimens were also tested and then re-tested several
months later to investigate what effect this ageing had
on the pore pressures. Sharp concluded from the
results that the pore pressure depended on the tempera-
ture, and that the value was greater than the saturated
vapour pressure of steam for any given temperature,
6o
provided tie specimen remained sealed from casting.
(These values are much higher than those found by
Bremer). The age of the concrete affected the degree
of hydration and so had a small effect on the pore
pressure values; however, the previous temperature
history had no appreciable effect on the pore pressures
developed. Sharp managed to develop a method of
predicting the pore pressures in8ide his specimens
mathematically. This method took into account the
relative volumes of capillary water and air in the
concrete, but it was found that if the volumes of non—
evaporable water and gel water and their expansion were
taken into account, the pore pressures predicted were
higher than those measured experimentally.
A similar specimen was heated to a constant
temperature of 150°C and fluid allowed to escape
periodically. The pore pressure was measured when
conditions in the specimen were steady after each release
of fluid. The amount of fluid allowed to escape was
also monitored by weighing. The results obtained from
this test are given in Figure 2:7. Sharp separated
the variation in pore pressure with weight loss into
three areas. The first part of the curve, when the
pressure is dropping with weight loss, corresponds to
the loss of the entrained air. The flat portion of
the curve, which he found was the same value of pressure
as the saturated vapour pressure of steam at 150°C, was
maintained while the gel pores in the concrete were full
of water. However, when the gel pores began to empty,
5%
the pressure curve begins to fall away rapidly and becomes
zero when all the evaporable water has escaped from
the concrete.
The second series of tests in which pore
pressure was measured by Sharp was a set of eight sealed
concrete specimens 3-finches diameter and 2ft. long.
Each specimen was sealed with a steel jacket which
also acted as a mould during casting. This sealing
jacket consisted of a 3/lb in. thick tube, to one end
of which was welded a " plate. The specimens were
sealed at the top by a steel plate held into position by
tie rods which passed down the outside of the jacket.
Copper tubing was fixed to this lid so that pressures
could be measured at the top of the specimen. Some of
the specimens, however, were left open to atmosphere,
and this was done by not connecting he copper tube at
the top to a U-tube, but leaving it to atmosphere.
Each specimen contained 13 thermocouples, six or seven
porous inclusions each connected to a perspex U-tube, and
six electrical resistance meters. The base was heated
by electrical heaters bolted to the base, and the base
and sides were thermally insulated so that a thermal
gradient was imposed on each specimen. Base tempera-
tures varied from 130°C to 175°C, and specimezis were
tested at various ages from 28 to 200 days, for durations
ranging from 110 to 4O days. It was found that the
temperature gradient produced a pressure gradient in
these specimens, and cau8ed uniaxial migration of fluid
along the specimens. These pore pressures dissipated
slowly in the hot regions as water migrated to the
ca.
cooler parts of the specimen. The behaviour of the pore
pressures in the specimens completely sealed and those
allowed to remain at atmospheric pressure were similar
in the early stages of heating. However, in the
specimens completely sealed at the top, the pressure
rose rapidly until it was higher than at points lower
down the specimen, and an accumulation of water was
detected. This did not occur in those specimens allowed
to remain at atmospheric pressure, although liquid
collected on the surface of the concrete.
From the behaviour of the pressures in these
specimens, Sharp managed to develop a numerical
step-by-step method of analysis to predict the migration
behaviour in a concrete specimen. This took into account
that the migrating fluid consisted of liquid water,
water vapour, gaseous air and disolved air, and that
these migrate according to the laws of permeability and
diffusion.
(e) Drying and Moisture Movement in Concrete under
Thermal gradients.
Diffusion theory, concrete permeability and
the pore pressures developed in concrete all influence
to a varying degree the moisture movement and its rate
in concrete under a thermal gradient. The predominant
factors actually influencing the moisture movement
depend upon the temperatures involved and the state in
which the fluid is migrating. Over the past few years,
engineers and prestressed concrete pressure vessel
designers have become interested in the moisture movement
53
in concrete under thermal gradients and several investi-
gations have been undertaken and reported, and attempts
made to apply various theories to the experimental
results obtained.
McDonald (12), tested a pie-shaped specimen
designed to simulate part of a prestresesed concrete
pressure vessel. The lateral surfaces were sealed
and thermally insulated, one of the two ends being
left open and the other closed after casting until the
heat was applied. After casting, the temperature,
shrinkage and moisture distributions were monitored
for 17 months while the specimen matured. The concrete
mix was a modified low heat Portland cement with crushed
fine and coarse limestone aggregate and a water/cement
ratio of 0.425. The temperature was measured using
iron-constantan thermocouples, the moisture content
using a surface back scatter nuclear gauge and the
shrinkage using Carison strain meters. After 17 months,
a temperature gradient of 44°C was applied using eight
independent resistance wire heating elements wrapped
around the specimen. The results were reported in
two sections, the effects of the concrete hydration and
effects of the thermal gradient.
The temperature of the concrete rose to a
maximum 30 hours after casting. The maximum rise in
temperature was recorded in the midsection of the
specimen, with a temperature recorded of 76°C, giving
a rise of 52°C. The minimum rise was 31°C near the
open end. The temperatures stabilised near room
ci'
temperature about 60 days after casting. The strain
readings showed similar patterns to the temperature
distributions. The maximum concrete strains recorded
during the hydration cycle was 500 microetrains near
the midsection, while the minimum recorded was 300
microstrains near the ends. The moisture content, as
indicated by the nuclear surface moisture gauges was
fairly constant in all the sections except at the two
ends, where a decrease was detected. This decrease
was greater at the open end. This drying of concrete.
detected by McDonald at the open end is interesting.
Powers (40) states that the rate at which water dries
out of concrete depends on the relative humidity in the
concrete, and that the capillary water dries out first
even if the capillary pores are not interconnected.
lie explained this by the theory of hydrostatic tension
in that as the relative humidity drops, air comes out
of solution in the capillary pores and the water is
pushed through the gel towards the surface of the concrete
where it is evaporated into the atmosphere. At
relative humidities above 45%, Powers estimates that
all the gel pores are full and some capillary water is
present; below this value there is no capillary water
and the gel pores begin to empty.
As a result of the application of the temper-.
ature gradient, McDonald reports that the concrete
temperatures within the specimen continued to increase,
but at a decreasing rate until 43 days after the tem-
perature gradient was applied, no changes in the local
temperatures were observed and hence a steady temperature
distribution existed. No significant changes in
moisture content were observed after the heat was
applied. The 8train variations followed the tempera-
ture variations. lie concluded that after 127 days of
heating, the specimen remained in an essentially
steady-state strain condition.
Poitevin (el) reports a study of moisture
migration under sustained temperature gradients on
concrete cylinders 80mm in diameter and 300mm long.
Three types of specimen were tested; completely sealed
specimens left at room temperatures; partially sealed
specimens left at room temperatures and partially sealed
and laterally insulated specimens subjected to a
longitudinal temperature gradient of 20°C. All the
specimens were sub)ected to a 3 month curing phase,
and then left for periods of up to 9 months under their
specific thermal gradients. An interesting point is
that the non-evaporable water content of the sealed
specimens measured after 9 months was only 0.30 per
unit wt. of mix water, while that of the partially sealed
specimen exhibited a value of 0.36. A conclusion was
drawn that the hydration process was slowed down in the
concrete in the sealed condition. The water distribu-
tions were monitored bymeasuring the electrical
conductivity of stainless steel electrodes embedded in
the concrete, while the longitudinal shrinkage by
comparators. The rates of moisture loss were found to
be influenced by the sealing conditions and the temperature
5'.
gradient. The lowest loss was for the sealed epecimex
at room temperature, while the highest loss was for the
partially sealed specimens under the thermal gradient.
The water distribution in sealed specimens was fairly
constant throughout, as it was in the partially
sealed specimens at room temperature. Moisture
migration away from the higher temperature zone was
detected in the partially sealed specimens under a
thermal gradient. The moisture that had been lost
was mainly evaporable water. The longitudinal
shrinkage strains in all the specimens were linearly
related to the corresponding moisture losses.
Bertero and Polivka (6) investigated the
influence of freely allowing moisture to escape during h€utizg
on sample8 of limestone concrete cylinders, sealed in
copper jackets with strain gauges embedded in them.
These were raised to a temperature of 300°F. The
moisture was allowed to escape by puncturing the
sealing jacket. When the moisture, which was in the
form of steam, was allowed to escape, a sudden
contraction was noted. This was indicated by the
transverse strain readings obtained.
England and Ross (14), England and Sharp (59)
and Sharp (10), in a series of papers and publications
report on migration tests in which pore pressure,
water distributions and shrinkage distribution in both
sealed and partially sealed concrete specimens under
a thermal gradient are monitored. These are reviewed
and dealt with in other sections. It is worth noting
s2.
here, though, that they conclude at temperatures in
0excess of 100 C significant pore pressures develop and
become major factors affecting the movement of moisture.
Hundt and Schimmelwitz (13) measured the heat
and moi8ture distributions on beams of dimensions 40cm x
40cm x 240cm. These beams were sealed vapour—tight on
the long sides and also the heated front side, and provided
with thermal insulation on all sides except one open
end. The open end was exposed to a climate of 20°C and
50% Relative Humidity, while the other end was kept at
a temperature of 80°C. The heat and moisture transport
was regarded as one—dimensional in the direction of the
beam axis. The temperature distribution throughout
the specimens was monitored using thermocouples, while
the moisture distribution was measured using a neutron
probe that was moveable up the central longitudinal axis
of the beams in a glass tube. After 80 days' heating,
the biggest change8 in moisture content were noticed in
the area adjoining the heated face. The area of high
moisture loss extended to about 30 cm. from the heated face.
The adjoining area between 30cs and 200cm. from the heated
face showed an approximately constant moisture distribu-
tion. However, between 60cm. and 80cm. from the heated
face, an area showing a small increase in moisture
content was noticeable. This is similar to the results
of Sharp (10), although the heating period in this case
is not nearly 80 long. The last 40 cms. of the beams
to the open end showed a slight decrease in moisture
content, probably due to evapourationa]. drying. Similar
long term tests are planned by Hundt and Schimmelwitz,
alongside a further test programme dealing with the
change of heat conductivity in relation to the maturing
process and drying of concrete.
It is obvious that investigations into the
moisture movement in concrete under thermal gradients
are still continuing, and it is hoped that the experi-
mental results of similar investigations described later
in this thesis will contibute to a further understanding
of the subject.
(C) Methods of monitoring moisture movement in and
from concrete.
In any moisture migration tests, an assessment
of the way in which the moisture is distributed through
the concrete structure during the test is very important,
as is the moisture loss from the structure. Several
methods have been used to determine the moisture
distribution in a migration specimen and the amount of
moisture lost by non—destructive methods and these are
reviewed in this section.
(i) DIRECT WEIGHING: Weighing of a specimen while
moisture movement is in progress will give a quantitative
method of assessing the exact amount of moisture 108t at
any one time. However, weighing gives no indication of
the distribution of the various types of water in
concrete, and is only really possible in small scale
laboratory specimens. The weighing of a large structure
such as a concrete pressure vessel is impossible in
order to detect and monitor moisture movement from it.
(ii) HUMIDITY GAUGES: Several relative humidity
gauges have been developed for monitoring moisture in
concrete struCture8. The best known of' these is
described in a paper by Abrams and Monfore (62) and
called a Monforc Gauge. This gauge consists of a
one-inch piece of Dacron thread which changes in length
with change in humidity. This thread is connected to a
2-inch length of fine resistance wire and causes this to
change in length as the humidity alters, and hence the
resistance of the wire. The humidity is determined by
measuring the change in resistance by a sensitive
Wheatetone Bridge. The disadvantages of this gauge
were that each time it was used it needed to be calibrated
against known humidities and corrected for temperatures.
Humidity gauges do not, in general, give the quantitative
amount of evaporable water present in the concrete, but
are useful for indicating whether drying has taken place,
especially at ambient temperatures.
(iii) ELECFROMAGNETIC AND RADIOACTIVE ATTENUATION
METHODS: Various methods have been developed and used
utilising the fact that certain radioactive emissions and
radio waves of various frequencies are attenuated to a
varying degree by the watermolecules or certain "light"
elements that exist in concrete.
One of these methods utilises the fact that
gamma rays emitted from a radioactive source are
attenuated, and that the attenuation is a function of the
mass of the material through which the rays are passed.
However, using this method to indicate moisture distribu-
tions in concrete pre-supposes that the only mass change
occurring is loss of water. This is probably a reasonable
assumption. A big disadvantage of this method is that
as the mass change in concrete is small, a high radio-
active source is needed, requiring safe and very
expensive shielding.
Another method using radioactive emission is '
fast neutron attenuation. This utilises the fact that
neutrons are slowed down by elastic collisions with
light atoms, namely hydrogen, in Concrete. If one
assumes that hydrogen is the only "light" element
present, then the attenuation can be used to measure the'
total hydrogen content, which can be regarded as a
function of the total water content (both evaporable and
non—evaporable water). This is a useful sy8tem, and
the apparatus can be calibrated in a theoretical manner
that holds good for concrete, provided hydrogen is the
only light element present. The drawback is the
radioactive shielding required with the fast neutron
souree. A form of this method was used by Hundt and
Schimmelwitz (13), although in their paper they do not
give precise detail8.
Another method that can be used is Proton
Magnetic Resonance. This is described in a paper by
Blame (3). This method uses the fact that when an
atomic nucleus is subjected to an intense magnetic field
and a radio frequency, there will be a value of frequency
for the particular field strength at which magnetic
resonance occurs. The frequency varies with the size
of the nucleus, and when tuned to the hydrogen frequency
is known as proton magnetic resonance as a hydrogen
nucicus consists of one proton. This method is quite
61.
useful as it can detect water in various phases in
concrete. The disadvantage of the method is that it
can only be used on small specimens as the magnetic
field required is so intense.
A method that has been used with considerable
success in several investigations (9,64), is that of
microwave attenuation. A radio wave in the microwave
frequency region suffers attenuation when it is passed
into water, the attentuation being a function of the amount
of water present in a material such as concrete. In
dry concrete the attentuation is almost zero. By
varying the wavelength used, ditferent phases of water
can be detected in a quantitative way, although lengthy
calibration is necessary in order to ensure accurate
results. Attenuation is also atected by temperature, the at-
tenuationin water decreasing as temperature increases,
being zero at 100°C (9). The disadvantages of this
method are that microwaves are reflected when metals
are present, and so cannot be used in reinforced or
prestressed structures, or those encased in steel, and
that access all around the specimen is required in order
to take measurements. This method of determining
moisture distribution was used by Parkinson (9) very
successfully in concrete specifliens under a thermal
gradient.
(iv) ELECTRICAL MEASUREMENTS: These can be either in
the form of' direct measurement of the resistance or
capacitance of' a piece of concrete, or the measurement
of the electrical resi8tanCe of an absorbent block
'a.
embedded in the concrete.
The electrical re8istance of concrete is
mainly a function of the amount of saturated calcium
hydroxide solution present in its capillary system,
although at low water contents the resistance of the
aggregate and cement paste have some influence. The
reeistance is generally measured by embedding electrodes
in concrete and passing a current between them and
measuring the voltage. The resistance is then the
voltage divided by the current. Problems arise
because of variations in contact resistance between the
electrodes and the concrete and that it is is not
possible to know whether the current flows in a straight
line between the electrodes. The variation between
electrical resistance and temperature is also very
complex and an alternating current must be used to
eliminate electrolytic effects inside the concrete.
Parkinson (9) used electrical resistance measurements
as a secondary method to microwave attenuation for deter-
mining the moisture dietributions in his migration
specimens.
The measurement of electrical capacity is an
alternative to resistance as a way of measuring water
content, as water has a high dielectric constant.
However, problems are met using this method due to the
variable dissolved salt content in the water and with
contact resistance in the same way as the direct measure-
ment of electrical resistance.
In order to overcome some of the difficulties
inherent in the electrical resistance or capacitance
measurement methods, the absorbent block method or
moisture meter has been developed. These moisture
meters are generally in the form of two electrodes,
usually cast into a mortar, plaster or fibreglass block.
These moisture meters use the principle that the
resistance, measured between the two electrodes, varies
with the water content of the surrounding mortar, plaster
or fibreglass. The water content of the material
surrounding the electrodes generally has a relationship
with the sur!ounding concrete, and this has to be deter-
mined by calibration. The relationship between
electrical resistance of the meter and temperature is
generally very complex and calibrations are necessary.
Ageing effects of the meter also generally require taking
into account.
A paper published by the Building Research
Station (65)
describes the development of a moisture
meter for measuring the moisture content of porous
building materials. Thie consisted of two electrodes,
one straight and the other a helix, set in a matrix
of plaster. A similar type of meter was later devel-
oped by Lee and Bryden-Smith (66), who cast the
electrodes in mortar. These meters were then auto-
claved in steam in order to stabilise the structure of
the mortar and reduce the ageing effects. Similar
meters were used by Sharp (10) and in the experimental
work described in this thesis. A full aceunt of the
calibrations performed on these meters is described
later, in Chapter Three.
"U
2-V THE ENGINEERING PROPERTIES OP CONCRETE.
(a) Introduction.
The engineering properties of concrete,
especially those that must be taken into account by a
prestressed concrete pressure vessel designer, have
been well summarised by Browne (4). Since many of the
properties of the hardened concrete depend upon the
mix proportions and type of aggregate selected,
especially when elevated temperatures are involved,
much care has been taken into finalising the actual
mix proportions and type of aggregate to be used.
Often, designing a mix to meet workability and placing
requirements will have an adverse effect upon such
properties as strength, creep and shrinkage. Other
factors that have to be borne in mind in the selection
of materials and design of the mix proportions are
that the construction period of a structure such as a
concrete pressure vessel can be as long as 3 years (b7)
and that the service life of the vessel can be over
30 years. This means that variations in water/cement
ratios, cement contents, aggregate gradings, curing
conditions, etc. must be taken into account, and that
the long-term properties of the concrete are as impor-
tant as the early age properties.
The tendency has been, especially with the
pressure vessels constructed in the United States, to
use crushed limestone for the aggregate. The mix
specially designed by the United States Army Engineers
and Waterways Experimental Station coneists of Modified
'5..
Low Heat Portland cement and crushed coarse and fine
limestone aggregate with a water/cement ratio of 0.425
(12, 6). Limestone aggregate was selected because of
its low thermal expansion.
In this section the more important engineering
properties of concrete, both at atmospheric and elevated
temperatures, are reviewed, with special reference to
limestone aggregate, which was also used in the practical
work de8cribed later in this thesis.
(b) Concrete Strength.
The concrete strength depends upon the mix
proportions, although the prime influence is the water/
cement ratio as indicated in Pigure 2:8. Strength
increases with age, and also can be affected by curing
conditions. Piblajavaara (68) reports the acceleration
of the hydration process at temperatures of 40 0C to 50°C
in sealed non-mature concrete, that increases strength.
However, higher temperatures and early drying cause a
decrease in strength, lie also reports that the strength
of a dry, fully-mature cement stone has a considerably
higher strength than that of a similar wet one.
Parkinson (9), Campbell-Allen and Desai (58) and
Hannant (b9, 70) all report loss of strengths of sealed
limestone concretes of up to 40% when heated to elevated
temperatures around 100°C. However, Ilannant (69) showed
that when water was allowed to evapourate during tbe
test, no strength reduction in the limestone concrete was
recorded, especially when the temperature was over 1000C.
'Ip
Thermal shock and temperature cycling usually
results in loss of strength. However, loss of strength
is far lees in concrete using silica—based aggregated
than limestone aggregates (71). Crispino (5) reports
a steady loss in the compressive strength of limestone
aggregate with number of thermal cycles, although the
largest loss is after the first cycle. After six temperaturi
cycles up to 300°C the limestone concrete specimens
had a reduction in strength of 80% of similar specimens
cured under water at 20°C of the same age. He also
reports that the 1088 in strength increases as the value
of the temperature cycle is increased.
The theories put forward to explain the loss
in strength of limestone aggregate conceete are reviewed
in the section on limestone aggregate later in this
chapter.
(c) Modulus of Elasticity.
The elastic deformation of concrete depends
upon the modulus of elasticity. The problem is defining
modules of elasticity, as the stress/strain relationship
for concrete is curved (as shown in Fig.2:9). Three
values of modulus are recognised:
(i) The Dynamic modulus: This is determined from
sonic or ultrasonic velocity measurements in the
specimen. The dynamic modulus is approximately equal
to the slope at the origin of the statically determined
8treSS/straifl relationship.
(ii) The Tangent Modulus: This is the slope of the
tangent to the stress/strain relationship as determined
by static methods.
(iii) The Secant Modulus: This is the slope of a
line from the origin of the stress/strain relationship to
an arbitrary point on the curve.
In general, the elastic modulus of concrete
increases as the strength increases. Neville (16) also
reports that wet concrete specimens have a higher modulus
than dry ones. The properties of the aggregate influence
the modulus of elasticity while they do not really affect
the compressive strength; the higher the modulus of
elasticity of the aggregate, the higher the modulus of
the resulting concrete. Elastic modulus usually
increases with age in the same way that strength does.
The relationship between modulus of elasticity and
temperature is the same as for compressive strength and
temperature, with the modulus decreasing slightly with
tempprature (71). Crispino (5) reports a 75% reduction
in the static elastic modulus of limestone concrete
after a temperature cycle up to 300°C compared with
specimens cured in water at 20°C of the same age.
(d) Poisson's Ratio.
This is the ratio between the lateral strain
accompanying an applied axial strain and the latter
strain. The value ranges from 0.15 to 0.20 when
measured by 8tatic methods for both ordinary and light-
weight concretes (16). However, dynamic measurement
yields a higher value of around 0.25. It is generally
believed that Poisson's Ratio is lower in high strength
concrete.
Crispino (5) investigated the effect of tempera-
ture and temperature cycling on the Poisson modulus. All
kinds of concrete showed a tendency for decreasing values
at 110°C, but at higher temperatures, increasing values
were obtained. He concluded that the increa8e at high
temperatures was due to cracks caused in the structure by
the heat treatment. Thermal cycling also tended to
increase the values. He found that the value for limestone
concrete after six thermal cycles from 23°C to 300°C
increased from 0.2 to 0.5, while similar specimens cycled
from 23°C to 350°C showed an increase in Poisson modulus
from 02 to 0.7.
Information on the effect of age, strength,
sustained load and water content on the Poisson modulus
as yet does not appear to have been reported.
(e) Thermal Expansion of Concrete.
The coefficient of thermal expaneionof concrete
depends upon the mix proportions of the concrete,
although the most influencing parameter is the coefficient
of thermal expansion of the aggregate which generally
occupies between 65% and 80% by volume of the concrete.
The thermal expansion coefficient of the aggregate varies
greatly with geological type, and great variations exist
even within a particular rock group. Griffiths (72)
observed that the coefficient of thermal expansion
generally increased with silica content of the rock.
However, when considering silica aggregates, Harada et al (71)
'o.
observed an abrupt expansion of silica aggregates at
500°C due to the transformation of quartz, and this of
course would be detrimental to concrete required to
withstand such a temperature. They reported that
limestone aggregate. concrete tested by them agreed
closely with the value measured for the original rock
and that no sudden changes at elevated temperatures
were observed. Crispino (5) measured the thermal
expension of limestone rock in three orthognal directions
and found considerable differences as 8hown in Figure 2:10.
He concluded that this played a major role in the
behaviour of lime8tone concrete under thermal cycling
conditions.
The coefficient of thermal expansion of cement
paste has a value between 10 x l0/°C and 20 x l0/0C,
(lb) and that this value increases with richness of
cement. Iormal1y, the thermal expansion of concrete
is close to, and slightly above, the value for the
aggregate used.
The expansion of saturated and dessicated
concretes of the same mix is similar and lower than for
intermediate water contents. However, autoclaved
concrete does not exhibit this behaviour, leading one
to conclude that the coefficient of thermal expansion
depends on the amount of gel water present.
In limestone aggregate concrete, which is
generally accepted to have a low coefficient of expansion
- 0(Hannant (7) measured value of 4 x 10 per C), differ-
ential expansion can occur between the cement paste and
aggregate, which could cause high internal stressing.
This was observed by Qispino ( 5 ) who found that the
differential expansion was enhanced by temperature.
The coefficient of thermal expansion of concrete
is often found to be greater on first heating than for
cooling and any subsequent thermal cycling. This is
due to internal dislocations caused on the first heating
and existing compressive stresses, that both act as a
restraint on temperature cycling.
(f) Heat flow Properties of Concrete.
The heat flow properties of concrete are
important to the prestressed concrete pressure vessel
designer in order that he can assess such parameters as
the thermal stress development and temperature rise
caused by radiation absorption in the concrete. They
are also used in design calculations for the cooling
system. The three heat flow properties are Thermal
Conductivity, Thermal diffusivity and Specific heat.
(i) THERMAL CONDUCTIVITY: This measures the ability
of the concrete to conduct heat, and the general magnitude
is fixed by the thermal conductivity of the aggregate.
The conductivity of rocks varies widely, with Quartzitic
rocks having the highest values, and Bisalt and Gabbros
the lowest. Limestones and Granites fall approximately
in the middle (4). The thermal conductivity is very
sensitive to water content, the value decreasing linearly
as the concrete dries out.
An increase in temperature tends to decrease
the thermal conductivity of the aggregate but increases
the conductivity of the hardened paste. The overall
effect in the temperature ranges at present experienced
in concrete pressure vessels is a small variation with
temperature. However, Harada et al (71) report that
thermal conductivity decreases with temperature when
the temperatures rise above 100°C, and that in the
range 700 - 800°C, the value is down by 50%. In
practice, the thermal conductivity in a concrete pressure
vessel should be kept as high as possible so that heat
escapes quickly and the rate of drying is kept low.
(ii) THERMAL DIFFUSIVITY: This represent8 the rate
at which temperature changes within the concrete mass
can take place, and is given by the expression:—
K
Ce
where D is the Diffusivity, K the Thermal Conductivity
e the concrete density and C the specific heat. The
main influence on the thermal diffusivity is the
aggregate type, quartzitic rocks having the highest
value. The thermal diffusivity is not significantly
affected by moisture loss or age or Ly decreases with
temperature quite markedly due to the increase in
specific heat value with temperature. Harada et al (71)
report a reduction of 40% at temperatures of 700 - 800°C.
(iii) SPECIFIC HEAT: The specific heat is important
when calculating the temperature rise caused by radiation
absorption and represents the heat capacity of the concrete.
It is less affected by the type of aggregate than thermal
conductivity and thermal diffusivity, as the specific
heat of hardened cement paste is similar to that of
*2.
aggregates. The specific heat, like the thermal
conductivity increases with moisture content. The
specific heat also increases significantly in a linear
way with temperature.
Limestone, although not possessing the most
ideal heat flow properties for use in a concrete pressure
vessel, when considered in conjunction with its thermal
expansion properties, can be seen to be a reasonable
choice aggregate for use in the concrete mix used.
(g) Carbonation.
Carbon Dioxide in the presence of water can
attack concrete by a chemical reaction with the calcium
hydroxide. This represented by the equation:-
Ca(O}J)2 1120 * CO2 CaCO3 + 2H20
The carbon dioxide can attack other hydration products,
causing decomposition, and significantly altering the
microstructure of the cement paste. The two main
properties of concrete known to be affected by carbona-
tion at present are strength and shrinkage. Piblajavaara
(73) reports increases of strength over 100% due to
carbonation. However, he also reports carbonation
shrinkage recorded of the same order of magnitude as
drying shrinkage. Carbonation leads to a change of
water content and is accompanied by an increase in the
weight of the concrete.
In general, carbonation is a surface phenomena,
penetrating the surface of the concrete extremely slowly
and as such only has minor effects on massive sections.
However, as yet no assessment of the effect of temperature
.3.
on carbonation appears to have been done, although
Neville (16) reports that the rate of carbonation
depends more on the moisture content of the concrete
and the relative humidity of the ambient medium.
Pihlajavaara (73) verifies this, finding that the amoLint
of carbon dioxide intake is dependent on the bound water
content, and increases with relative humidity.
(h) Shrinkage.
Shrinkage is a time dependent strain shortening
of concrete and is one of the most important properties
of concrete as it is responsible, directly or indirectly,
for cracking in a great many situations.
The tendency to shrink depends on loss of
water. Two stages of shrinkage have been distinguished:-.
(i) An initial or plastic shrinkage. This occurs
before the setting of the cement paste. It depends
upon the curing arrangements, and also is aggravated
by loss of water by evapouration from the surface of
the concrete or by suction by dry concrete below.
The neat cement and water occupy a greater volume than
the products of reaction and so the plastic shrinkage
increases with cement content.
(ii) Drying Shrinkage. This occurs after the setting
and is due to the shrinkage of cement gel.
Two other forms of shrinkage have also been
identified, carbonation 8hrinkage and autogenous
shrinkage. Carbonation shrinkage i8 a direct consequence
of carbonation, reviewed in the previous section.
Autogenous shrinkage is a small amount of shrinkage
that occurs in practice in the interior of a large
concrete mass. Generally, both these types of
shrinkage are small in practice compared with drying
shrinkage.
The mechanism of shrinage is not fully
understood, but it is thought that it is a function of
the loss of the water adsorbed onto the gel or within
the gel. While capillary water is being lost no
shrinkage takes place, but when drying causes gel water
to be lost, drying shrinkage occurs. Theories concer-
ning the mechani8m of shrinkage have varied from
capillary tension effects in the gel pores to gel swelling
and shrinkage analagous to the behaviour of true gels
such as silica gel. None have proved satisfactory,
mainly because the physical structure of cement paste
is more complicated than those put forward in the theories.
The shrinkage mechanism is best described as a balance
of external restraint8 on water and stresses in water.
These stresses induce strain associated with drying,
but shrinkage is not simply a mechanical effect of
drying. If concrete is re—wetted, swelling takes
place. However, the shrinkage and swelling is not
fully reversible as there is an irrecoverable shrinkage
which i8 about a third of the total drying shrinkage.
Shrinkage rate and ultimate shrinkage strain
both depend on the mix in concrete. Aggregates which
du not themselves shrink tend to rtstrain shrinkage,
and thus the elastic modules and proportion of aggregate
in the mix are significant. The permeability of the
aggregate is also important. If the aggregate is
not permeable, the water being expelled from the cement
pa8te has to travel around the aggregate and as such has
a loflger path to leave. Therefore, the shrinkage is
lower. Shrinkage increases with cement content for a
constant water/cement ratio, though for constant water
content per unit volume, cement content has little
influence. These relations imply that an economical
mix will have a low shrinkage.
The effect of curing conditions on shrinkage
is variable. Prolonged curing gives concrete with a
higher strength and a higher elastic modulus and so it
is able to resist the tendency to shrink better than a
concrete which has not been cured to the same extent.
However, well cured concrete tends to shrink more rapidly.
Thus, prolonged curing has a variable effect on drying
shrinkage, although in general it probably has a small
beneficial effect in that it reduces cracking.
Another faclor that influences drying shrinkage
is the surface to volume ratio and size of section of
the concrete structure. A high surface to volume
ratio specimen loses water more rapidly and hence tends
to shrink more rapidly. The centre of large sections
cannot lose water to the atmosphere and thus ha no ten-
dency to shrink other than that associated with the
initial setting action. The shrinkage tendency of the
surface will be resisted by the interior and differential
stresses will be set up; small shrinkage strains will
be observed on the surface although the surface appears
cracked. This is often called Differential shrinkage.
The effect of elevated temperature on
shrinkage is shown in Figure 2:11. It appears that
both the initial rate of shrinkage and its final value
increase with a rise in temperature. An increase in
temperature increases the drying rate and Parkinson (9)
showed that drying shrinkage was proportional to the
loss of water from the concrete. Thus one would
expect that the rate and final value of shrinkage in
a specimen under a thermal gradient will be proportional
to value of the temperature gradient.
(i) Creep.
Creep can be defined as the increase in
strain under a sustained stress. It is the total
strain minus the elastic and shrinkage strains as shown
in Figure 2:12. The creep properties of concrete are
closely related to the shrinkage properties. Theories
explaining the mechanism causing creep in concrete
are fully reviewed by Neville (74).
The factors causing high creep strains are
similar to those producing large shrinkages. In
general, economical design and good practice give a
concrete with a low creep. The strength of concrete
has a considerable influence on creep, which is
inversely proportional to the strength of concrete at
the time of' application of the load. Creep and creep
rate are found to be directly proportional to the
applied stress when the Stre8S does not exceed about
one half of the ultimate strength. The effect of the
aggregate in concrete is to restrain creep as it
restrains shrinkage, and so it is the modulus of
of elasticity of the aggregate that is important as
well as the proportion in the mix. Age of loading
greatly affects the magnitude of creep. This
influence is probably due to the increase strength with
age, and hence an ability to restrain creep.
Elevated temperatures have the effect of
increasing the creep rate and its ultimate value.
England (23) when considering concrete between 20°C and
100°C defined specific thermal creep as the creep strain
per . unit stress per degree centigrade, and found that
it was a property of the sample tested and varied with
time only. A specific thermal creep curve for a
sealed concrete loaded and heated at an age of ten days
is shown in Figure 2:13. Zielinski and Sadowski (75)
report on creep tests carried out on cylinders at a
temperature of 60°C at various moi8ture contents. They
found that the relationship between the creep of concrete
and the water content was approximately linear. They
also found that the ratio of creep at elevated tempera-
tures to the creep at normal temperatures is approximately
constant and irrespective of time and moisture content
in concrete. A Lull account of the influence of
temperature on creep can be found in a paper by Neville
(7t).
The creep recovery is normally far less than
the initial creep since the 8trength of the concrete is
greater at the time of unloading than at the time of
loading. Illston and Sanders (77) suggest that the
irrecoverable creep in concrete is a function of the
temperature and maturity of the concrete, and not of age
at loading. Figure 2:14 shows the elastic and time-
dependent deformations of stressed concrete, illustrating
creep, creep recovery and irrecoverable creep.
2-VI LIMESTONE AGGREGATE CONCRETE.
(a) Introduction.
Limestone has been chosen for use in prestres8ed
concrete pressure vessels both in Britain and abroad on
several occasions. Apart from being readily available
near to the construction site, the main reason for
choosing limestone is its low coefficient of thermal
expansion. However, several investigations have
revealed that limestone aggregate concrete exhibits a
reduction in strength at elevated temperatures (9, 58,
69, 70, 20). Several theories have been put forward to
explain this phenomena.
In this section, the physical and chemical
structure of dolomitic limestone, thetype of limestone
used in the experimental work described in this thesis,
is reviewed in detail. The hydration products under
various curing temperatures detected previously and
subsequent scanning electron micrographs obtained are
looked at in detail. Lastly, the theory of the cement
paste/aggregate reaction, and its affect if any on
strength at elevated temperatures are reviewed.
(b) The Chemical and Physical Structure of Dolomitic
Limes tone.
Limestone cQnsists essentially of calcium
carbona te, although )flagfle8ium carbonate and siliceous
matter such as quartz are often present in small quantities.
The proportion of magnesium carbonate is small except in
dolomite and dblomitic limeatones. The calcium carbonate
is present in the form of crystals of' calcite (is a
trigonal crystal) and Aragonite (orthorhombic crystal1,
less common and less stable than calcite and often
associated with gypsum), and as amorphous calcium
carbonate. Limestone is a bedded rock often containing
fossils and except in the form of dolomite, effervesces
on the addition of dilute hydrochloric acid.
Dolomitic Limestone, as opposed to other types
of limestone such as Oolitic limestone, Shelly limestone
or Chalk, contains the double carbonate MgCO3.CaCO3.
The mineral Dolomite also contains the double carbonate
and is a rhomb-shaped crystal. The difference between
dolomitic limestone and dolomite is that dolomitic
limestone contains both dolomite and calcite, the full
ratio of magnesium to calcium needed for the double
carbonate not being present. Dolomite thes not effervesce
readily on the application of cold dilute hydrochloric
acid, and this is how it is distinguished from calcite.
The mineral dolomite often replaces original calcite in
a rock, the magnesium being derived from sea-water and
introduced into the limestone by solutions passing
through it. This replacement process is called
"Dolomitization". The change from calcite to dolomite
involves a volume contaetion and this often results in
a porous rock. Beds of dolomite are found in Carboni-
ferous limestone, in such places as the Mendips and the
Forest of Dean.
Gillott (78) reports on the study of the micro-
structure of several dolomitic limestones. They all
reveal a fairly fine grained texture dispersed by
micro-channels of magnesium carbonate. The proportions
of magnesium carbonate varies from limestone to
limestone. Also present were traces of soluble alkali
salts Na 20 and K20.
The limestone used in the experimental work
derived from the Mendips and was identified as a
Dolomitic Limestone.
(c) Effect of Curing Temperature on the Hydration
Products of Limestone Aggregate Concrete.
As ha been stated in previous sections,
Imlach (20) studied the effect of prolonged curing at
elevated temperatures with reference to the chemical
changes takingplace under these conditions on 8everal
mixes of cement paste and different aggregates. Many
of the results obtained are reviewed in previous
sections, but of specialinterest in this section are
the results of two mixes, ordinary Portland cement and
Olbury limestone (a dolomitic limestone) with a water/
cement ratio of 0.3 and an aggregate/cement ratio of
2.7:2. The other mix of interest was one of ordinary
Portland cement, pulverised fuel a8h and Olbury
limestone. The water/cement ratio and aggregate/cement
ratio were the same as the previous mix, but 25% by
weight of cement was replaced by pulverised fuel ash.
" cubes were made in order to investigate the reduction
in compressivestrength of limestone concrete and see
'I.
whether this could be related to the chemical changes
taking place under the curing conditions. The cubes were
cured for 6 months at 25°C in water and then a further
6 months at temperatures of 25°C, 90°C and 140°C.
These cubes were then crushed, fractured and examined
under a scanning electron microscope, or examined using
X-ray diffraction apparatus. As a dolomitic limestone
was used in the experimental work described later, the
results and micrographs produced in this investigation
are a useful reference and comparison for the Author's
micrographs obtain from the specimens used in the
practical work.
The strength results obtained with the
ordinary Portland cement and Olbury limestone confirmed
the results of previous investigations, a steady decrease
in strength with increasing temperature. The average
strength of' the cubes cured at OC was 69% of those
cured at 25°C, while the strength of those cured at
140°C was only 28%. The results of the cubes with
ordinary Portland Cement, pulverised fuel ash and
Olbury limestone had similar strengths when cured at
25°C and 140°C, but showed a reduction of' 20% in those
0cured at 90 C.
Apart from examining the cubes cured under
the various conditions with X-ray diffraction apparatus,
examinations were also made of the starting materials.
The phases identified in the Olbury limestone were
calcite, dolomite and quartz; those in pulverised fuel
ash, mullite (3Al 20 3 .2Si09 - occurs in natural rocks from
the Isle of' Mull), quartz and calcium oxide, while the
'2.
ordinary Portland cement has tricalcium silicate,
dicalcium silicate, tricalcium aluminate, tetracalcium
aluminoferrite and traces of bassanite (2Ca804.I120),
magnesium oxide and calcite. The results of the
phases identified in the cubes are given in Table 2:5.
TABLE 2:5.
CuringMix used temp. Hydrated Phases Identified.
0.P.C. + 0.L. 25°C Calcium Hydroxide Ca(OH)2
Ettringite C3A.3CaSO4.321120
0.P.C. + 0.L. 90°C Calcium Hydroxide Ca(OH)2
Form of Calcium AluminateHydrate
0.P.C. + 0.L. 140°C Calcium Silicate HydrateForm of Calcium Aluminate
Hydrate
O.P.C. + P.P.A 25°C Calcium Hydroxide ca(OH)2+0.L. Ettringite C3A.3CaSO4.32H20
0O.P.C. + P.F.A 90 C Tobermorite, form of calcium
+ 0.L. silicate.Hydrate, calcium aluminate,
hydrate.
0.P.C. + P.F.A 140°C Tobermorite, calcium alumin-+ 0.L. ate hydrate.
The cubes of ordinary portland cement and Olbury limestone
0cured at 90 C showed traces of magnesium hydroxide, as
did the cubes of ordinary Portland cement, pulverised
fuel ash and Olbury limestone cured at 90°C and 140°C.
The cube of this mix cured at 25°C showed traces of
calcium silicate hydrate. In all the cubes examined,
3.
traces of the starting materials were detected in all
the cubes showing that prolonged curing for a year at
all the temperatures did not produce full hydration of
the cement.
The cubes of ordinary portland cement and
olbury limestone were examined at three separate establish-
ments, under slightly different conditions. These
differing conditions were ones of coating, which although
makes no difference to the actual material, affects the
size which they appear on the micrographs, as various
thicknesses of coating are used, depending on the
conducting coating used.
A specimen of ordinary portland cement and
Olbury limestone cured at 25°C for 9 months was examined
by Mr. E. Raask at C.E.R.L. This 8pecimen was uncoated.
The micrographa showed long fibres (over lO r m in
length and about O.Srm wide) and platey material.
The fibres were almost certainly ettringite, while the
plates are probably calcium hydroxide; gel bodies that
have been identified as calcium silicate hydrate were
also seen at an aggregate/cement interface.
A sample of ordinary Portland cement and
Olbury limestone cured at 25°C for one year was examined
at Newcastle University. This specimen was coated with
a mixture of gold and Palladium. The material seemed
equally divided between fibrous material and gel—like
material. Two types of fibre seemed present: long
entangled fibres that were almost cer.tainly ettringite,
m in length, while the other fibres were much
shorter (2rm to 4rm in length) and straighter. These
q4.
were probably calcium cilicate hydrate similar to that
identified by Chatterji and Jeffery (31), although the
X-ray results did not reveal this present. The gel-like
material was almost certainly calcium 8ilicate hydrate.
Specimens of the ordinary Portland cement and
Olbury limestone cured at 25°C and 140°C for one year
were examined by Imlach at C.E.R.L. These specimens
were coated with Aluminium. The 25°C cured cube
showed some large fibrous material (ettringite), platey
material (calcium hydroxide) in abundance, and some
"gel-bodies" as seen in the other specimens. The
l4o 0C specimen consisted mostly of "gel-like"
material, in clusters rather like cumulus cloud. Also
present were a few hexagonal plates, which could have
been calcium hydroxide or calcium aluminate hydrate,
although neither of these were detected by X-ray
aflalysi8.
The micrographs of the ordinary Portland
cement, pulverised fuel ash and Olbury limestone of
the various specimens cured at the different temperatures
all were very similar. They all consisted of the
"gel-like" material seen in the ordinary portland
cement and Olbury limestone specimens, and all most
certainly a form of calcium cilicate hydrate, identified
by Speakman and Majumdar (35), with particles of
pulverised fuel ash also visible in all the micrographe.
All the specimens examined in this thesis
were coated with carbon, so although direct comparisons
of size cannot be drawn, the types of' microstructure
q5.
observed can be compared with those published by Imlach.
(6) Cement/aggregate reation and the loss of strength
at elevated temperatures.
Several theories have been put forward to
explain the loss in strength in limestone concrete
at elevated temperatures. Lea and Desch (15)
suggest that the 1088 of strength is due to some
deleterious reaction between cement paste and limestone.
Seligmann and Greening (79) support this theory and
further suggest that the aluminate constituents of cement
are specifically involved.
Another theory put forward by Thorvaidson (80)
is that there is no deleterious chemical reaction between
cement paste and limestone, but a reduction in the
strength of cement paste occurs at elevated temperature.
This is more noticeable with limestone aggregate than
siliceous aggregate, which he suggests reacts with
cement paste in a strength conferring reaction.
Another theoryput forward by Raask (81)
is that there is a chemical reaction between limestone
and the calcium hydroxide liberated on the hydration of
Portland Cement. This is because limestone usually
contains some magnesium carbonate that would be the
first to be attacked in the alkaline media of hydrated
cement. This reaction, which also occurs between
dolomite and the alkalis sodium and potassium in cement
is known as Dedolomitization.
MgCO3 + Ca(OH) 2 + Mg(O!I) 2 + CaCO3
Raask tested two samples of limestone with
various contents of magnesium carbonate and showed that
the magnesium carbonate fraction decomposes in limestone
before the calcium carbonate, and that the reaction
between magnesium carbonate in limestone and the calcium
hydroxide is a diffusion controlled process where ionic
species migrate inside the micro—channels of the limestone.
This reaction could well have something to do with the
loss of strength, because the reaction between
Dolomitic limestone and calcium hydroxide formed on
hydration of cement is prevented by the addition of
a pozzlanic material (20) (The definition of a
pozzlanic material is one that is capable of combining
with calcium hydroxide liberated on the hydration of
Portland cement).
Another theory is that the strength reduction
is purely due to physical changes such as differences
in thermal expansions of aggregate and cement paste.
Fure 2:10, obtained by Crispino (5) shows the anistropic
behaviour of limestone with respect to thermal expansion.
This is due to the calcite in limestone, and could well
be a contributing factor.
The results of Imlach's (29) investigation
seems to suggest that the loss in strength and general
behaviour of limestone aggregate concrete, and especially
dolomitic limestone, is a combination of factorB put
forward in the various theories outlined above. As
limestone was used in the experiments described later,
and specimens kept at various elevated temperatures and
moisture contents were viewed under the scanning electron
microscope, special attention was paid to see whether
any evidence could be found from the micrographs
obtained to support the theories outlined previously
for the behaviour of limestone concrete at elevated
temperatures.
CHAPTER THRbE - INSTRUMEtTATJON.
4BSTRCT.
The selection, development and manufacture
of instrumentation is described to monitor pore pressures,
temperatures, shrinkage strains and evaporable water
contents in concrete specimens at both elevated and
non-uniform temperatures. Methods are given of
measuring tile weight loss from the specimens during
heating. The measurement of evaporable and non-
evaporable water contents is described in samples
removed from specimens at the end of heating.
The measurement of pore pressure by a null
method was developed using a pressure transducer and
special attachment over a range from 0-500 psig.
Temperature was recorded using copper-constantan
thermocouples accurate to over a range of 0-200°C.
Shrinkage strain was measured using demountable gauges.
lectrical resistance meters were used to monitor the
evaporable water contents inside the concrete specimens
during heating. The methods of manufacture and
calibration are given in detail in this chapter.
Weighing of the specimens to monitor the
loss of moisture during heating was performed using
a steelyard, mechanical and electrical balances,
depending on the size and weight of the specimen.
,U1 three methods had an accuracy and repeatability
to within 1 grm. The evaporable and non—evaporable
water contents at the end of heating were also
obtained using a Gravimmetric method.
CHAPTER THREE - INSTRUMENTATION.
I. Experimental Programme.
II. Measurement of Pore Pressure in Concrete.
(a) General Principles
(b) Tranaducer Details and Calibrations
(c) Porous Plate Details
(d) Pressure Attachment details and measurement
procedure.
III. Temperature Measurement.
(a) Thermocouple design
(b) Temperature meter
IV. Measurement of Shrinkage Strain.
(a) Introduction.
(b) Measurement of Horizontal Shrinkage Strain
(c) Measurement of' Vertical Shrinkage Strain
V. Measurement of Evaporable Water Content.
(a) Introduction
(b) Moisture Meter Details.
(c) Moisture Meter Calibrations.
vi
The We4ghig of Testecimens.
(a) In troduction
(b) Procedure for weighing Migration and Venting
Test series specimens.
VII. The measurement of various tyes of water in
Migration Specimens at end of' testig.
(a) Introduction.
(b) Evaporable water content
(c) Non—Evaporable water content
(ii) Assessment of the Loss of Water in Specimen.
CHAPTER THREE - flSTRUMENTATION.
3 - I. EXPERIMENTAL PROGRAMME.
The development of the instrumentation used
in the experimental work described in this thesis was
done very much with the experimental programme in mind.
Most of the experiments were intended to run for a time
scale of several hundred days, with relatively high
pore pressures and temperatures involved. The size
of the specimens, the sealing techniques employed and
the methods available for introducing the instrumen-
tation into the specimens were factors that were also
considered in the development of the instrumentation.
In most specimens it was planned to monitor the varia-
tions of the Pore Pressure, Temperature, Evaporable water
content and weight loss with respect to time and, in
one experiment, the extra parameter of shrinkage strain
was measured. Therefore, before deciding on the use
of any measuring technique, the following requirements
had to be met:-
(1) The methods of measuring Evaporable water content,
temperature and shrinkage strain had to be non-
destructive to enable variations with time to
be observed.
(ii) The techniques employed must give reproducible
results with no zero drift in the case of
electrical gauges.
(iii) 1ith the large number of specimens planned, the
instrumentation either had to be easily manu-
factured at a low cost, or easily moved from
2..
observation point to observation point, with
the added factors of the size of some of the
migration series specimens taken into account
here.
(iv) The instrumentation had to be durable to with-
stand the temperatures and pressures expected
over a fairly long time span.
3-Il. MESUREMErT OF POflE PRESSURE IN CONCRETE.
(a) Gexual Principles.
The technique of the measurement of pore
pressures in undrained triaxial test samples of soils
has been developed for over twenty years and is described
fully by Bishop and Henkel (82). The main difference
between the pore pressures in soil samples and heated
concrete samples is that the pore pressures in soils
are hydraulic, while in heated concrete they are
mainly due to gas and vapour.
If a porous component is cast in concrete,
then provided the volume of the inclusion is small com-
pared with the surrounding concrete, the pressure
in the pores of the component and the surrounding
concrete will be equal, and it is, therefore, possible
by measuring the pressure in the porous component,
to obtain the pore pressures in the concrete. Bremer
(60), in work reviewed in Chapter Two , connected a
"Bourdon" type pressure gauge directly to a porous
inclusion and recorded pore pressures. This type of
device was considered for the work described in this
O3
thesis, but was rejected since the internal volume of
this type of gauge varies with the applied pressure,
which could cause fluid to flow from the porous inclusion
and hence produce an error in the recorded pressure.
Sharp (10), whose previous work at King's
College can be regarded as the forerunner to the work
described in this thesis, developed a technique of
measuring pore pressures in heated concrete samples by
casting a sintered bronze porous plate in concrete as
the inclusion, and measuring the pressure in its pores
by connecting it to one limb of a mercury-filled "U"-
tube. The other limb was connected to a "Bourdon"
Pressure Gauge and screw operated ram, and as the pressure
in the porous plate increased, the mercury level in
the limb closest to the specimen dropped. To restore the
level to its original position, the ram was screwed in so
that the pressure on that side of the "U"-tube was
increased. The fluid in the porous plate was at its
original volume and provided the "Bourdon" gauge was at
the same level as the porous plate the pressure recorded
on the "Bourdon" gauge was the true pressure of the fluid.
A diagram of this system used by Sharp is shown in
Figure 3:1. The arm nearest to the porous plate was
made as narrow as possible. This was so as to control
the volume of fluid in the porous plate as accurately as
possible, since a small change in the volume produced a
large movement in the mercury level. This was found to
work well when the concrete was under fairly saturated
conditions, but extreme care had to be taken during
heating and when the concrete was not saturated, since
the pressures were changing much faster under these
conditions and the mercury could all be forced into one
limb, allowing fluid and vapour to escape from the
porous plate.
This technique of measuring pore pressure was
considered for this work, but wae.rejected on three
counts. Firstly, the number of individual perspex
"U"-tubes that would have to have been manufactured was
very large. Some of the detail in these tubes used by
Sharp were of an intricate nature, taking many man hours
to produce. Secondly, these perspex "U"-tubes would
have had to have been permanent features of the specimens
and in the case of the lOft. migration specimens, the
fixing of them would have been difficult. Thirdly, the
number of joints that would, by necessity, have been
sealed "in situ" was high. This would have made the
chance of leakage that much higher. Instead, it was
decided to use transducers to record the various pore
pressures by modifiying a system developed by STTHAM
INSTRUMENTS INC. of LOS ANGELES.
(b) Transducer Details and Calibrations.
The type of transducer used in the measurement
of pore pressure was THE UNIVERSAL TRANSDUCING CELL,
MODEL no UC3 produced by Statham Instruments InC. of
Los Angeles and marketed in the United Kingdom by
P.K. Morgan, Ltd. of London. The sensing element in
the transducer is an unbonded, hi-active-arm 8train gauge
bridge with a rated excitation of 6 volts (.C. or D.C.)
and a full scale output of 16 mv/volt. It is an all—
purpose transducer, that can be used to measure - with
the appropriate accessories - force, pressure, stress or
displacement. A photograph of one of the transducers
along with the pressure attachments can be seen in Plate
111:1 By changing the Diaphragm it is possible to
select ranges of pressure of 0 - 1 psig (1 psig being
Pull scale output) to 0 - 5000 psig. With all the
pressure accessories there is an overload protection
device in the form of a slip which is placed over the
sensing tip. This gives a load limit on the transducer
of 150% (i.e., on 0-10 psig diaphragm load limit is
15 psig).
One of the problems that has been previously
experienced with most fl'ansducers is the problem of zero
drift, especially when used in long—term experiments,
so before they could be used in the experimental
programme subsidiary tests were carried out on the
transducers to prove their reliability and to determine
whether there was any zero drift.
A specially designed Invar rod with arms on
each side was placed between the Diaphragm and the sensing
tip on the transducers. This was surrounded by another
specially machined piece made of Invar that screwed into
the base of the pressure accessory and into which the
transducer was screwed. The inserts were made of Invar
to utilise the thermal properties of the metal and so
eliminate any errors introduced by temperature fluctua-
tions. By successively hanging known weights from the
Invar connecting rod a graph of Transducer putput against
weight added was obtained. This was repeated after
first applying a known pressure on the diaphragm and
hence already having some transducer output before
adding the weights. This test was repeated at intervals
of one month for a period of 1 year with the transducer
under a constant excitation of b volts. It was found
that the results did not vary and a typical graph for
a given transducer can be seen in Figure 3:2. Two
other subsidiary tests that were performed on the
transducers were, firstly, a transducer was left under
a given pressure and With the input voltage kept constant
the variation in output was observed with time and,
secondly, the output was plotted against pressure
applied for different input voltages to the transducer.
In the first case, it was found with all the transducers
that the output remained constant with time. In the
second case it was observed that the output voltage was
tothe input voltage. The results
for a typical transducer are shown in Figure 3:3.
The importance of these subsidiary tests which
were done on all the transducers were threefold. Firstly,
they showed that there was no zero drift with these
transducers during the period of the tests (1 year) and
it was fairly safe to conclude that this would hold
true for a longer time interval. Secondly, they showed
output was -e.io1kQd. to input, and so it
was important during the main experiments that all
measurements of pressure should be made at a specific
input voltage for the transducers, and this was set at
6 volts. Thirdly, the Invar rod and addition of
known weights provided a quick method of checking the
calibration of any transducer at any time during a
test.
The input voltage for the transducers was
taken from the mains and transformed down using a
Parnell Laboratory Transformer to give 6 v d.c. The
output from the transducer was read on a Universal
Transducer Readout Model No. U.R. 4 purchased from
Statham Instruments. This had its own inbuilt
Balance, excitiation and sensitivity controls that could
be used if the other controls failed.
(c) Porous Plates.
The porous plates used in this work were
sintered bronze porous plates with a porority of 39%
and " thickness. This plate was obtained in large
sheets and cut to the size required. This size was
made the same for all specimens in same series of tests,
but was not necessarily the same throughout the whole
experimental programme.
To prevent the porous plates being filled
with grout during the casting they were first covered
with filter paper, and this remained on them throughout
the testing.
A small hole was drilled in the side of each
porous plate large enough to take a small bore copper
tube, which was then soft-soldered on to the edge of
tie porous plate to prevent movement during ca8ting of
the concrete. This copper tube then passed through
the 8eal of the specimen, which varied from series to
series. Outside the seal this copper tube was bent to
form a U. At the end of the tube a " B.S.P. Brass
Nipple and Union Nut was soldered on aid this was tightened
onto one side of a klinger valve. Since the porous
plate and concrete were raised to a high temperature and
the Copper Tube and klinger valve were at room temperature,
the copper tube was filled with non-volatile oil to prevent
condensation. This was done using some very thin
polythene tubing that was fed inside the copper tubing
to the required distance. The other end of the polytbene
tube was attached to a hyperdermic filled with oil. The
oil was then forced in under pressure and when it started
to flow back out through the end of the copper tube, the
polythene tube was gradually withdrawn. This procedure
ensured that both sides of the "U" formed by the copper
tube were filled with oil.
(d) Pressure Attachment and Measurement Procedure.
The cost of a transducer precluded the possibi-
lity of having separate transducers at every pressure
tapping point. Instead, ten transducers were used for
all the pressure tappings in the Migration, Venting and
Pore Pressure/Shrinkage Tests. To measure all the
various pressures expected to be encountered, these
transducers were fitted with different range diaphragms,
with values of 0-20 psig, 0-50 psig, 0-100 psig and
0-200 psig.
These transducers were moved from pressure
tapping point to point. To ensure that no volume
change occurred when measurements were being made, a
special attachment was made up. A diagram of this is
shown in Figure 3:4. A stainless steel hexagon plug
was silver-soldered into a j" B.S.P. Brass Nipple with
a Union Nut surrounding it. Two holes were drilled in
opposite sides of the hexagon plug which was hollow in
the centre and small bore copper tubing inserted in the
holes and silver-soldered into place. One side of
the copper tubing was bent into a U shape and a -i" B.S.P.
Brass plug was silver-soldered to the end. This was
screwed into the top of the pressure accessory and the
pressure seal completed with a " Dowty seal. To the
other copper tube a " B.S.P. Brass Nipple and Union Nut
were soft-soldered. Into the top of hexagon stainless steel
piece, a " B.S.P. plug with a Dowty seal was screwed in
so making the pressure seal.
To fill the attachment with oil, the j" nipple
was loosely screwed into a klinger valve which had been
topped up with oil. The plug in the top of the hexagon
was tight, the " Brass nipple was attached to a screw
operated ram and the pipe passing through the j" Brass
pipe was blocked manually. Oil was then pumped through
the system until it flowed freely out of the joint
between the " B.S.P. Nipple and the klinger value with
no air in attendance. This joint was then lightened.
The Copper tube passing through the j" Brass Plug was
then unblocked and oil pàmped through until it flowed
freely out of the tube with no sign of any air. This
plug (+ Dowty Seal) was then screwed into the top of a
too.
pressure accessory which had previously had a diaphragm
fitted and been topped up with oil.
This plug and pressure accessory were then
tightened together to form a pressure seal. Finally, the
j" Plug in the top of the attachment was loosened and the
air bled off. This was then tightened down again, with
the attachment full of oil in all the limbs. Lastly,
the transducer was screwed into the base of the pressure
accessory until the sensing tip (with overload safety
device on) just touched the diaphragm. This was
observed simply by activating the transducer and screwing
the transducer into the pressure accessory until an
output was just attained. The lock nuts keeping the
transducer in place were then tightened using spanners
provided with the transducer. check wasthen made on
the transducer calibration by applying a pressure using
the screw operated ram and comparing the output reading
with one on a "Bourdon" Pressure Gauge in the same
circuit as the Ram.
The attachment was then ready for use and,
provided it was stored carefully when not in use, the
limb with the "U" bend remained full of oil. A
photograph of this attachment and transducer, ready
for use, is shown in Plate 111:2.
The procedure to measure the pore pressure in
a specimen was taken as follows. The oil in the klinger
valve attached to a pressure tapping point was topped
up and the " Nipple and Union loosely attached. Oil
was then pumped through until it freely flowed from the
0I
joint and the union nut was then tightened. The plug
in the top of the attachment was then undone and any
air bled off. The plug was then tightened up to form a
pressure seal. The copper tubing with " B.S.P. Iipple
and union nut were joined to a screw-operated ram which
also had a "Bourdon" Pressure Gauge in the circuit.
The whole of the system had been de-aired up to the
klinger valve and was, therefore, full of incompressible
oil. The copper tube on the specimen side had been
partly filled with oil as described previously. With
the temperature of the porous plate now raised, the oil,
if allowed to, would flow out. Therefore a back pressure
was applied on the transducer side of the klinger valve.
This back pressure was usually made equal to the previous
reading at that tapping point, although a comparison of
temperature and resistance meter readings at that point
with the corresponding previous readings were also
indicative of any changes in the concrete at that point
and if large changes were suspected from this comparison,
then a relevant adjustment in the back pressure applied
was made. The klinger valve was then quickly opened, and
the pressure on the transducer readout recorded. Also
recorded was any difference between the back pressure
applied and the pressure on the transducer readout.
Any difference between these two meant that the measure-
ment was not a true "no volume change" measurement.
Consequently, if the pressure on the transducer readout
was lower than the back pressure applied, the back pressure
was adjusted to a valve halfway between the initial back
pressure and that recorded. The klinger valve was then
opened quickly and the new readout pressure recorded.
Using this method, the back pressure was adjusted until
it was equal to the pressure in the porous plates and
hence there was no difference between the back pressure
applied and the pressure recorded on the transducer
readout. A subsidiary test using a calibrated capillary
tube and pressurised pot showed that this method became
self-adjusting as far as flow in and out of the pressur-
ised pot was concerned,provided the difference between
the back pressure applied and the actual pressure in
the pot was not greater than 15 psig. Since, in
practice, the worst difference encountered was 8 psi,
this pressure measurement method did not lead to changes
in weight due to flow of fluid into or from the porous
plates. In fact, as the tests progressed, the differ-
ences between back pressure and that recorded on the
readout when the valve was opened, even when the
concrete was unsaturated, became so small that usually
only two cycles were needed to give a "no-volume change"
measurement.
111. TEMPERATURE MEASUREMENT.
(a) Thermocouple desn.
All the temperature measurements made in the
cx perimental work described in this thesis were made
with copper/constantan thermocouples. These thermo-
couples consisted of copper tubing of 3/32 in. outside
diameter and l/l in. bore, through which was passed a
piece of 2ti S.W.G. constantan wire which was enamelled
and covered with a double layer of cotton. At one end
of the copper tube the constantan and copper were sol-
'o1
dered together with silver solder to form the hot junction
of the thermocouple. This joint was tested using
compressed air to ensure it would prevent leakage along
the inside of the copper tube.
These thermocouples were designed with more
emphasis on robustness than accuracy, as they had to
withstand the mechanical ill-trea tment associa ted with
Concrete casting. Sample thermocouples were subjected
to test in hot water and the indicated temperature was
Pound to be within 1°C of a mercury-in-glass thermometer
throughout the measured range which was regarded as
quite satisfactory. A photograph of a typical thermo-
couple used in this work is shown in Plate III;3.
(b) Temperature Meter.
The temperatures were read by connecting the
thermocouples to a Cambridge Temperature Indicator.
3 photograph of one of these meters can be seen in Plate
111:4. The cold junctions of the thermocouples are
incorporated in these indicators, and the indicators
give the true temperature of the hot junction as these
meters are auto-compensated; that is, the temperature of
the cold junction is added to the thermocouple effect
automatically.
All the copper tubes were linked by a common
copper wire in all specimens, and this along with the
constantan wires were oonnected to a multipole socket.
A multipole wander plug was then used, through a channel
selector switch to connect each thermocouple with the
temperature indicator. The reason for the plug and
socket arrangement was so that a specimen could be
'ow,
detached from the temperature indicator in order that
it could be weighed, and also that several specimens
could use the same temperature indicator.
All the temperature indicators have a specified
external resistance between the hot and cold junctions
that gives the correct temperature to °C on the
indicator. This value for all the meters used in
this work was 20 ohms. Since the resistance across
the plugs and sockets used and the channel selector
switch were liable to vary as they became worn or dirty
during the tests, the resistance of the wiring to each
thermocouple was made much less than 20 ohms. A
20 ohms variable resistance was put in the circuit
and provision made for the indicator to be switched out
and a Wheatetone Bridge to be brought in. This enabled
the resistance of the thermocouple leads to be adjusted
to give exactly 20 ohms. A circuit diagram of the
thermocouple leads and Wheatstone Bridge circuit are
shown in Figure 3:5.
The procedure to measure the temperature at
the hot junction of a given thermocouple was as follows.
The thermocouple is connected to the indicator using the
channel selector switch, then using the two pole, two-
way switch shown in Figure 3:5, the thermocouple and
leads are switched in as one arm on the Wheatatone
Bridge. The variable resistance is then adjusted till
the bridge is balanced. The temperature indicator is
then switched into the circuit and the temperature
obtained.
3-IV. MEASURE?ET OF SHRINKAGE STRAIN.
(a) Introduction.
In the experimental programme only one test
was planned that involved the measurement of shrinkage
strain, andtbese measurements were to be made in con-
junction with pore pressure measurements. The design
details of this experiment are discussed later in
Chapter Four, but it is worth recording at this point
that the methods of measuring shrinkage strain available
had a considerable influence upon the final design of
the experiment.
There is, of course, now a large number of
methods available for the measurement of strain.
However, many of these were found to be quite unsuitable
forlong term measurements on concrete. Electrical
gauges of all types suffer from zero drift unless
specially treated, and the large number of gauge lengths
planned in the test ruled out vibrating strain gauges
on the grounds of cost. In fact, it was decided to
use a demountable gauge.
The larger the gauge length of a demountable
gauge the less the chance of minor inhomogenities in
the concrete causing errors in the readings. For the
horizontal shrinkage strain measurement a concrete
gauge length of 10" was selected, while for the vertical
strain readings a gauge length of 9" was used spanning over
four slices of the specimen (see Chapter Four for
details). The same gauge holes were used to measure
both the horizontal and vertical strains. The gauge
tob•
hole was 0.030" diameter punched into the centre of
a brass hexagonal section rod which in turn screwed
into a 2 B.\. "County Cap" which had been cast into
the specimen. The stud was 2" overall length, of
which 5/10" was threaded 2 B.A. and was g" across
corners. A photograph of one of these studs and
a County Cap is shown in Plate 111:5.
(b) Measurement of Horizontal Shrinkage Strain.
't diagram oftbe system to measure horizontal
shrinkage strain is shown in Figure 3:6. The principle
behind the system is a gauge used to measure the
relative movement of the two gauge holes in the brass
hexagonal slices. To achieve this, a special
"Whittemore" type gauge was designed and a photograph
of this gauge is shown in PlateIII:b. It was constructed
out of Monel Alloy, a special alloy with very low
thermal properties similar to those found with Invar.
The advantage of this material over Invar was the ease
with which this could be machined and brazed compared
with Invar. This was essential as Monel Alloy was
only available in Rod Form of " diameter. Stiffness
is provided by the cross—bracing and all the joints
are brazed. in adjustable foot that could be set and
locked in position is on the L.H.S., and this fits
into one of the gauge holes in the brass hexagonal
gauge studs. The other foot is provided by the arm
of the dial gauge with a special piece locked on the
end to fit the gauge hole in the Brass Stud. As the
two "county caps" diametrically opposite in the specimen
'c.
moved, this movement was detected by the change in dial
gauge reading on the Strain Gauge when the two feet
were inserted in the gauge holes. This movement was
read on the Dial gauge which read to liT4in. per dial
division and the maximum relative movement that could be
detected was
The procedure to measure the horizontal
movement across the diameter of the specimen used in the
Pore pressure/Shrinkage specimen was as follows. The
fixed and moveable feet of the Strain Gauge were placed
in the two holes made in the Invar setting bar and the
Dial Gauge Reading adjusted to a predetermined set
value. This was done at various temperatures from
0 012 C to 30 C and the Dial Gauge reading did not vary
by more than one half of one dial gauge division
(5 x lO 3 ins.) throughout the whole temperature range.
The strain gauge was then placed across the various gauge
lengths and the dial gauge readings recorded. By adding
or subtracting these readings from the initial set of
readings taken across the gauge lengths before the
experiment commenced, the total relative movement of the
two gauge slices could be calculated. This reading
included thermal movement as well as shrinkage movement,
so to obtain the shrinkage strain the thermal effect
had to be assessed. This was done in a subsidiary
test, described in Chapter 9.
(c) Measurement of Vertical Shrinkage Strain.
Vertical movement was measured using a canna-
balised 10" Whitt .emore shown in Plate 111:7. The standard
10g,
Whittemore Strain Gauge measures the relative movement
of two feet over a gauge length of 10". Unfortunately,
the slices of the Pore Pressure/Shrinkage test specimen
were 3", and since the "County Caps" abd brass gauge
slices were cast at mid-height in the slice, this gave
potential gauge lengths of multiples of 3". It was,
therefore, decided to amend a standard Whittemore gauge
by putting an extension arm over one of the feet and
locking it into place to give a nominal 9" gauge length.
&s with the horizontal movement readings, the
Dial Gauge reading on the Whittemore Gauge was adjusted
to a predetermined value using the setting bar specially
mantfactured for it and also shown in PlateIII7. Readings
were then taken on the various gauge lengths. These
gauge lengths spanned four slices, and by assuming that
the movement over a gauge length was made up of the
sum of all the vertical movements of the four slices,
a series of simultaneous equations were obtained which
were solved to give the individual vertical movements
of each slice. By dividing this vertical movement by
the thickness the vertical strain was obtained.
;-V. MEASUREMENT OP EVAPORABLE WATER CONTENT.
(a) introduction.
In order to monitor the variation of the evapor-
able water content with time while the specimens were
under test, it was essential that a non-destructive
technique had to be employed, since all destructive
methods involved the removal of a sample of the concrete.
Several different non-destructive techniques have been
8uCCe8sfUlly used by workers, and a review of these
methods is given by Waters (83). The use of microwaves
to measure the water content by passing them through
the concrete and measuring the attenuation in a similar
fashicmto the way described ty Ptr:[nsori () w.ts co;iderd
for the Pore Pressure/Shrinkage test, but was ruled out
as SOOfl as the steel sealing jacket was found
essential, as this would have prevented the microwaves
entering the concrete. Other methods that have been
successfully employed in moisture migration tests in
concrete are the use of a surface back scatter nuclear
gauge by McDonald (12) and a Neutron probe by Hundt and
Schimmelwitz (13). However, as with all non-destructive
methods, the measurement does not necessarily give the
total water content, but generally selects some particular
property of the water in concrete and indicates this
quantity present. In fact, it was decided to utilise
the variation of electrical resistance of concrete with
water content to monitor the variation of water content
throughout the tests.
The use of electrical resistance as a method
of measuring moisture content in porous materials has
been known since 1897, but a large amount of the recent
development was done at the Building Research Establish-
ment by Lee and Bryden-Smith (tb) and at the Central
Electricity Research Laboratories by Hornby (84).
Although the design details of' the meters were
different, both Hornby and Lee and Bryden-Smith used the
same principle of casting two copper electrodes into
autoclaved mortar. Calibration tests were made to
obtain the variation of resistance with water content
and temperature and attempts were also made to assess
the ageing effects of the meters. Sharp (10) in his
experimental work used meters similar in size and
pattern to those of Lee and Bryden.Smith with minor
differences. All his meters were cast in one batch
and extensive calibration tests were performed to
obtain the variation of resistance with temperature,
water content and age. It was decided, on the
strength of the success of Sharp's meters to use
exactly the same design for the Resistance meters in
this work.
(b) Moisture Meter Details.
The details of the resistance meters used in
the experimental work are illustrated in Figure 3:7 and
a photograph can be seen in Plate UJ:8. The electrodes
were formed of 18 s.w.g. copper wire. The surrounding
mortar was mixed by hand in the following proportions
by weight:-
2 parts of sand evenly graded between B.S.
sieves numbers 7 and 200.
1 part of normal Portland Cement.
0.52 parts of water.
As the number of meters required was almost
three hundred, the meters were cast in nine separate
batches of forty. The mould used for casting is shown
in Plate ]II:9.. Each batch was demoulded twenty-four
'I''
hours after the mortar was Ca8t and stored in limewater
for six days. They were then autoclaved in saturated
0steam at a temperature of 190 C and pressure of 182 lb/in
The autoclave used is shown in Plate 111: 10 , and .i pressure
transducer was connected to the lid to check that the
correct temperature and pressure were attained. The
purpose of the autoclaving was to stabilise the mortar,
and the nominal time of autoclavirig was twenty-four
hours, but this varied slightly from batch to batch and
showed itself in the differing calibrations obtained.
After autoclaving, all the meters from oi batch were
weighed and their saturated resistance measured. Prom
the results the meters were either selected for use in
the experimental programme or for use in the temperature
and calibrations that were performed separately. Those
meters whose weight and resistance varied greatly from
the average for that particular batch were rejected
outright.
The meters selected for use in the experimental
programme were stored in limewater and used for the -
variation of resistance with water content before casting
in the various test specimens. The temperature
calibration moisture meters were cast in sealed pots
described later in this chapter, as were several of the
Age calibration meters, while the rest of the Age
calibration meters were stored in limewater in a room
kept at a constant temperature of 17°C and 88%
Relative Humidity.
(c) Moisture Meter Calibrations.
Pour calibration tests were required for each
batch of meters, as the electrical resistance between
the electrodes of a moisture meter embedded in concrete
depends upon the evaporable water content of the concrete,
the temperature of the meter, the water content of the
material between the electrodes, and the age of the
me tere.
The first calibration that was made was to
establish the relationship between the resistance of a
meter and its temperature. Pour meters from each batch
were used to determine the temperature characteristics.
Two meters from different batches and two thermocouples
were Cast into a cylindrical, concrete specimen 4 in.
long and 4 in. in diameter. This was cast inside a
steel jacket " thick with machined ends, that had been
coated on the inside diameter with autoplax resin and
fin* aggregate sprinkled on the resin while still wet
to improve the bond between the steel jacket and concrete.
The electrical leads from the thermocouples and moisture
meters were led out of the steel jacket via a glass to
metal seal that had been high-temperature soft-soldered
into the side of the steel jacket. This seal consisted
of seven metal pins set in a glass matrix and surrounded
by a metal ring, and could withstand temperatures up
to 200°C before it failed. The electrodes were
connected to th glass to metal seal using a 22 s.w.g.
copper wire in "Thermofit" sleeving. This sleeving
can be contracted on to the wire by heating it to 450°C
and the bare ends were coated with "Araldite" resin to
prevent electrical leakage. The end plates were made
of j" thick mild steel with " inserts into which the
tube fitted. The sealing at the joints between the
end plates and the tube were two washers of 1/32"
Jointite. Inserted in the centre of the top end plate
was " B.S.P. Union into which a " B.S.P.. Blocked
Nipple and Union Nut were tightened to form a seal.
This permitted controlled leakage to occur from the
specimens so that the meters could be tested with
the concrete at various water contents. A photograph
of a typically temperature specimen ready for casting
can be seen inPlateIII:11, whilePlate 111:12 shows a specimen
cast and ready for testing.
The resistance of the meters was measured using
a Wheatetone Bridge that was designed and built for
previous work at King's College by Parkinson (9) and
he gives the accuracy of the bridge as 2%. A circuit
diagram is shown in Figure 3:8. Several specimens were
placed in an oven with the thermocouples and moisture
meters wired up and the temperature gradually increased
to 160°C and then decreased back to room temperature,
readings of resistance and temperature being taken at
various times during the cycle. Special care was
taken with the first heating cycle, as this should repre-
sent the meter behaviour that occurred in all the Migration
and Venting tests after heating commenced. This cycling
process was repeated 8everal times and a graph of resis-
tance against temperature was plotted. It was
observed that provided the autoclaving of the meters
%114.
had been far greater than twenty hours, the cycling
process did not exhibit any noticeable hysterisis, and
the cirve of resistance against temperature was effectively
one line. Some 4ater was then allowed to escape from
the specimen and the weight loss of the specimen
recorded. The temperature cycling process was then
repeated to obtain a second graph of resistance against
temperature. In this way, a family of resistance and
temperature curves at variousweight loss values of the
specimen was obtained for each meter. A typical family
of curves for a moisture meter is shown in Figure 3:9.
The temperature is plotted to a linear 8cale and the
resi8tance to a logarithmic scale.
Using values taken from the plot off resistance
against temperature, it is possible to plot a graph of
resistance (Rb) against the corresponding resistance
at a temperature of 20°C (R 20 ) for various temperatures.
If these two values are plotted to a logarithmic scale,
a series of straight lines are obtained as can be seen
in Figure 3:10. This indicates that there is a
linear relationship between the logarithm of resistance
at a constant temperature and the logarithm of the
corresponding resistance at 2o°C, with the ratio of
log R20 to Log Rt increasing as temperature increases.
This can be written mathematically in the term:—
log R20 - (a (t - 20) + 1) log + b (t - 20)
where t is the temperature in degrees centigrade, a and
b are constants and the logarithms are to the base 10.
The results of all four moisture meters from
onebatch used in the temperature calibration tests
were taken and using a "least squares" fit computer
programme developed by Dr. S. A. Jeffries of King's
College, the values of a and b were different for each
batch of moisture meters cast. s stated earlier,
the casting conditions and autoclaving times for each
cast were never exactly the same and it appears this
has an effect upon the temperature characteristics of
the meter. However, in closer examination, it is not
the variation of resistance with temperature that varies
greatly from cast to cast, but the "inherent resistance"
developed in each batch that explains for the variation
in values of a and b. Table 3:1 gives the values of
a and b determined by the "least squares fit" method
for each Cast of moisture meters cast.
TABLE 3:1.
Values of a and b for various Batches of Meters
Cast ?umber a b
1 - 0.00306 0.0131
2 0.000122 0.0055
3 0.00210 0.0029
4 - 0.00107 0.0083
5 0.00219 0.0013
b 0.000995 0.0027
7 0.00212 0.0012
8 0.00145 0.0008
9 0.00078 0.0030
The relationship between the resistance of
the meters at 20°C and the water content of the mortar
was obtained using the meteré that were to be used in
the experimental test work. These meters were first
dried to constant weight at 105°C and then weighed. As
the weight of electrodes in each meter had been recorded
before casting, the weight of mortar could be calculated.
The meters were then soaked in Limewater until they
ceased to take up any more water. They were then
surface dried with filter paper, weighed and their
resistance measured. The meters were then left to stand
in the laboratory to dry out gradually. Their weights
and resistances were measured at intervals that varied
from twelve hours at first to two days at the end of
the test, which ended when they stopped losing weight.
In this condition the weights were very clo8e to the
oven dried weights and their resistances in some cases
were too high to measure on the Wheatstone Bridge.
The laboratory temperature was recorded each time so
that the resistances could be corrected to 20°C.
The weight per unit weight of dry mortar (i.e., the
constant weight at 105°C) was calculated, and thi8 was
plotted against the resistance corrected to 20°C. A
typical curve for a moisture meter is shown in Figure
3:11. It was found that the readings of water content
against logarithm of resistance at 20°C for the meters
of one batch were in fairly good agreement. Therefore,
using the "least squares" method, the relationship
between water content and resitance at 20°C was obtained
for all the meters from one batch in the form:-
4 3 2W= a log R + b log R + clog R= d logR + e
where W is the ratio of water to dry mortar by weight,
R is the 20°C resistance and the logarithms are to the
base 10. The values of the constants, a, b, c, d and
e for each batch are given in Table 3:2.
T%BLE 3:2.
Castumt)er a b a 6 e
1 0.000848 -0.019b 0.169 -0.656 0.9799
2 0.000934 -0.0214 0.185 -0.712 1.055
3 0.001023 -0.0237 0.207 -0.815 1.245
4 0.000622 -0.0150 0.136 -0.547 0.843
5 0.000821 -0.0187 0.160 -0.613 0.910
6 0.000977 -0.0195 0.154 -0.568 -.839
7 0.000784 -0.0174 0.148 -0.567 0.849
8 0.00167 -0.0311 0.224 -0.748 1.008
9 0.000145 -0.00609 0.0729 -0.347 0.619
The relationship between the water content of
the mortar and the evaporable water content of the
concrete was determined by removing meters from age and
temperature calibration specimens and also from migration
test specimens and taking samples of concrete surrounding
those meters in the specimens after the tests had been
completed. The meters and concrete samples were
dried to constant weight at 105°C to determine their
evaporable water contents. The ratio of the water
content in the concrete to the water content in the
mortar was then obtained for each meter location.
The mean value of this ratio was:-
W in concrete0.78
W in mortar
where W is the ratio of water to dry concrete or mortar
by weight. The standard deviation of this ratio was
0.07.
The first three calibration tests of the moisture
meters that have been described are exactly the same
as performed L)y Sharp (10) in his work at King's College.
However, the fourth calibration test to investigate the
ageing effect of the moisture meters has been performed
in this work in much more depth than ever before.
Lee and Bryden-Smith (b6) in their development work
with moisture gauges, monitored the change in resistance
in gauges cast in control cubes that were stored in
water at a constant temperature of 17.8°C. They
reported an additive ageing correction that varied
linearly with time of + 0.04 ohms per year.
Two tests were performed for the age calibration
of the moisture meters from each cast. Firstly,
meters were Cast into concrete specimens exactly the
same as were used in the temperature calibration tests.
These specimens were sealed in the same way and stored
for a period of two years. At intervals of three months
these specimens were weighed to check there was no
weight loss, and the resistance of the meters cast in
the concrete measured. The temperature of the concrete
specimens was also recorded so that the resistance of
the meters was corrected to the resistance at 20°C.
0The Variation of the resistance of a moisture meter at 20 C
with time in a sealed specimen can be seen in Figure
3:12. The resistance is plotted on a logarithmic
scale while time is plotted on a linear scale. As can
be seen, this gave a linear relation8hip, which was
found to hold true for meters of all the casts. The
additive ageing correction found by this te8t for all
meters was found to be + 0.0001051 log ohms/day.
In the second test to find the ageing effect
of' the moisture meters, sample meters were stored in
Limewater at a temperature of 17°C, with the surrounding
atmosphere kept at a Relative Humidity of 88%. Again,
at intervals of' three months, these meters were surface
dried, weighed and their resistance measured. They
were then replaced into the Limewater. By recording
the temperature of the laboratory when this resistance
measurement was taken, the resistance of the meter
could be corrected to 20°C. At the end of the two
years, these meters were taken out of Limewater and
0dried to constant weight at 105 C. This then enabled
the water contents of the meters when under test to be
calculated. Then using the relationship between the
water content of the mortar and its resistance at 20°C,
the average value for the resistance at 20°C for that
particular water content was calculated.
The difference between the theoretical and actual values
of resistance was obtained and this pbtted to a logarithmic
value against a linear scale of time. A typical plot
can be seen in Figure 3:13. This gave a linear
relationship, which again was found to hold true for
the meters of all the casts. The additive ageing
correction found by this method was +0.0001131 log ohms/day.
The values found by the two methods for the
ageing correction were so close that for experimental
purposes, the mean of these two values was taken as
the ageing correction for the moisture meters of all
the nine casts.
In order to calculate the evaporable water
content throughout the various tests, the age correction
was expressed in the form
Log R - Log R + C1D1
where R is measured resistance, R the corrected
resistance, C 1 the correction in log ohms/day and
the age in days when R is measured. This was
slightly amended when the total duration of the tests
were known and the results were being calculated, to:-
Log R Log R +
log cD2
where C was the correction in ohms when the test on a
meter was completed and D2 was the duration of the
test in days.
It appears from the four calibration tests
performed on the nine different casts of moisture meters
that several conclusions can be drawn for the future.
Clearly, it is the temperature and water content
variations with resistance that vary from cast to cast,
and appear to be a function of the time of autoclaving
and the casting conditions employed, while the ageing
characteristics and relationship between the water Content
of' the mortar and the water content of' the concrete
seem more to be a function of the meter design than
its manufacture.
The equations used at the various stages of
converting a measured meter resistance into an evapor-
able water content per unit weight of concrete, dried
to constant weight at 105°C, were written into a
computer programme so that the experimental moisture
meter readings could be reduced on a digital computer.
3-VI. THE WEIGHING OF TEST SPECIMENS.
(a) Introduction.
The main reason in wanting to weigh the test
specimens at frequent intervals was to determine the
decrease in weight due to water losses. The weighing
also acted as a check upon the evaporable water distribu-
tions obtained from the resistance meters embedded in
the various specimens. The vast difference in weight
between the various test specimens precluded the use of
the same weighing apparatus throughout the experimental
programme. The total weight of a 10 ft. migration
specimen, including instrumentation, heaters, insulation
and the steel jacket was in the region of 227 kg.
(500 lbs), while the 5 ft. specimens complete for
testing were in the region of 125 kg (270 lbs). To
attempt to weigh specimens of these weights at regular
intervals during the tests to determine the decrease
in weight due to water losses required careful considera-
tion. The total weight of water in these specimens
(evaporable + non-evaporable) was in the region of 1%
of the total weight, and during the tests even at the
highest base temperature, nowhere near all this water
was expected to be lost. Therefore, one can see that
the accuracy of the weighing method was of paramount
importance and it was decided to adopt the counterbalance
system developed by Sharp (10) during previous work
at King's College, which had an accuracy of detecting
weight loss to the nearest gram (0.0022 lbs.)
The principle of this system is shown in
Figure 3:14. Basically, most of the weight of' the
specimen is balanced by the counter weights and the
"out of balance weights" is recorded on a set of
scales. This "out of balance weight" is the reading
on the scale when the whole system is just on the point
of equilibrium.
Using the notation used in Figure 3:14,
(W1 - W 3 )a W2b for the system to balance. In the
counterbalance system used, the ditance "b" was made
twice the distance "a". This reduces the sensitivity
of the system, ut this is negligible if the three
knife edges are in a straight line. The weight W2
was kept Constant so that any change in W1 produced an
equal change in W. The selection of the value of
was important, so that - 2W2 never exceeded the
capacity of the scales. A description and details of
va3,
the apparatus used is given in the next 8eCtiOn.
The weighing of the steel jackets and instru-
mentation prior to the casting of the specimens was
done using a steelyard. This steelyard had a capacity
of 600 lbs.and could be read to an accuracy of 1/10 lbs.
The measurement of the weight of the Release
Test specimens was also required to an accuracy of the
nearest gram, but this proved no problem as the total
weight of each specimen was in the region of 9 kg. and
was, therefore, easily accommodated on a 10kg. capacity
balance. The balance used was a "Berkel" flat pan
balance with a scale range of 200 grams in 1 gram
divisions, and can be seen in use in Plate IV:b.
electrical balance manufactured by the
"Mettler" company of Switzerland was used for weighing
small concrete samples for the water content determinations
and various calibration work. This balance had a
capacity of 1.2 kg. (2.6 lbs.) and was calibrated in 0.0].
gram (o.000022 lbs.).
(b) Procedure for weighing 1 migration and venting
Test Series Specimens.
As stated previously, the counterbalance
system for monitoring the weight losses of the Migration
and Venting Test series specimens was designed and
developed by Sharp (10). photograph of this system
can be seen in Plate 111:13. The apparatus was supported
on a gantry which consisted of a 3 in. x 4 in. deep I
section rolled steel joist. The beam in the counter-
balance system was made of mild steel of square section
t2 i
1 in. x 2 in. deep. The knife edges and knife edge
seatings were made of' hardened steel, and the edges
themselves fitted into holes in the beam. The outside
knife edges were 12 in. and 24 ins, respectively from
the central one; this central was held in a bogie that
ran along the gantry. At the end of the counterweight
a hanger was attached to support these, while at the
other end the top pulley block of a lifting hoist was
fitted to the hanger; this hoist was used to raise and
lower the specimens.
1 Governor was bolted to the counterweight end
of the beam. This consisted of two steel rollers
located between the flanges of the I-section beam and
controlled the rotation of the weighing beam. A pointer
was fixed to the beam beneath a central knife edge; when
this was located in the centre of a scale that was
attached, the system was in balance.
The whole counterbalance system had remained
intact from the work of Sharp. Before testing
commenced, the whole system was dismantled, checked
and overhauled. The knife edges and their seatings
were removed and reground. They were then replaced
and lubricated with anti-scuffing paste that reduced
the friction between the moving parts at high pressure.
A new raising hoist was fitted and the whole counter-
balance system tested with known weights to see that
the knife-edge settings were correct.
Since the maximum weight of the 10 ft.
migration specimens was far in excess of the total
weight of specimens previously weighed on this system,
calculations were performed to check that the sections
used for the gantry, weighing beam and knife—edges
were adequate for the loading conditions that would be
experienced. It was found that the maximum direct,
bending and shear stresses that could be expected to be
encountered by the above sectione were well inside those
laid down in the Codes of Practice for steelwork (85).
The counterweights used varied depending
upon the size of specimen being weighed. They were
hung from the hook by means of a chain and metal hanger.
The weights were hung on one of' the four metal arms on
the metal hanger. These weights were standard
laboratory weights of varying value. For the 10 ft.
specimens, the counterweights in total (including chain
and metal hanger) were in the region of 252 lbs.,
while for the 5 ft. Migration series and Venting
Test series, the counterweight value was in the region
of 126 lbs.
In order to be able to raise and lower the
specimens easily with the hoist, especially the 10 ft.
Migration series specimens, a mobile scaffold of the
type used by painters was used.
To weigh a 5 ft. specimen the appropriate
counterweights were placed on the hanger, while the
specimen was raised with the lifting hoist until the
10 kg. capacity flat pan Berkel scales could be placed
underneath. The specimen was then lowered slowly onto
the scales until the beam was horizontal, as indicated
by the pointer. The hoist was then secured and
the scale reading recorded. The difference in scale read-
ing from the previous weighing indicated the loss in
weight of the specimen. Plate]fl:14 shows a 5 ft.
specimen being weighed.
The method used to weigh the 10 ft. specimens
was slightly different. The main problem was that the
headroom available was not enough to allow the specimen
to be raised enough to be able to lower it onto the flat
pan of the Berkel scales. Consequently a frame was
made up so that the "out of balance weight" W 3 could be
measured.
This frame is shown in Plate 1I]15. It was
constructed out of 1" angle iron bolted together.
The vertical pieces at the extremeties were supported
on the scale pans of the two balances used to support
it. The frame was firstly balanced using a 10 kg.
Berkel flat pan balance and a 24kg. capacity portable
steelyard balance that had a scale reading to 1 gram.
The readings on the two balances W . and W . wereLi Ri.
recorded. The 10 ft. specimen, which had been raised
by the lifting hoist about 2" off the ground, was
ten lowered onto the frame, and the two balances were
then readjusted using weights until the weighing beam
was horizontal and the whole system in balance.
The new readings W and were recorded. The
out of balance weight 143 was then calculated.
143 - WLj) + 14RB -
It was found that great care had to be taken
during the weighing of these 10 ft. specimens, as an
increase in the weights on the two balances by 2 gram.
could put the whole system Out of balance.
Before any weighing was commenced, a check
was made in case the knife edges had moved. This
check consisted of placing a check weight of known
weight on the specimen side of the system. A reading
was then obtained on the scales in the same way as
for the specimens. If this reading varied from the
initial reading, then a correction was applied to the
specimen's weight. This correction was of the opposite
sign to the apparent change in weight of the check
weight, and depended upon the ratio of the counter-.
weights used for the specimen to the counterweight used
for the check weight.
(Apparent change ) (Counter Ut)- (in scale reading) x (used for )
(Correction to) ( of Check Wt ) (specimen )(specimen's ) -(scale reading)
(Counterweight used for Check Ut)
Throughout the series of tests, the Check Ut.
altered by 18 grams, and corrections were applied
accordingly.
3-Vu. THE MEASUREMENT OP VARIOUS TYPES OP WATER IN MIGRATION
SPECIMENS AT END OP TESTING.
(a) Introduction.
When the heating had ceased of tIe Migration
series specimens, the final distribution of evaporable
and non-evaporable water was determined. Samples of
the concrete were removed from various positions along
%11
the length of each specimen (as de8cribed in Chapter
Four).
These samples were of the same diameter as
the original specimen from which they were removed and
were about 1 inch thick. They weighed approximately
0.75 kg.
(b) Evaporable water content.
Immediately after being removed from the
specimen the samples were weighed (Wet) and then
,0dried to constant weight at 1OD C and were then
reweighed (Wd). The weight of evaporable water
per unit weight of dry concrete is given by:-
W = W
e wet dry
w W
dry dry
In order to assess the amount of water lost
during the test, we must express the evaporable water
in terms of the total mixing water (We). We can
consider the dry concrete as being made up of aggregate,
cement and non-evaporable water. This is not strictly
correct as part of the cement is combined with the
non-evaporable water in a chemical bond to form various
compounds, or products of hydration. Therefore:-
W =W +W +Wdry c a fl
(l+W/W W )=W a C + fl
•( W/W Wt)
= Weight of cement
= Weight of aggregate
= Weight of non-evaporable water.
WC
Wa
Wn
The ratio of W/W is the aggregate to cement ratio by
weight at casting and its value throughout the whole
of this work was 6.b7.
Therefore: -
We .(Y -W )- wet dry
W Wt dry
7.L7 +
wt/Wc ç
In this work, the ratio W/W was the water to cement
ratio by weight, and was 0.6.
W (W - W ) (12.7833 + W )e wet dry
_______________ nW wt dry ( Wt)
The ratio W/Wt is the weight of non-evaporable
water per unit weight of niixin water and was determined
as described below.
(c) Non-Evaporable Water Content.
The same sample was used to determine the
non-evaporable water content as for the evaporable water
content. After the evaporable water content was
determined the concrete samples were placed under water
in a desiccator and a vacuum in the region of 1 atmosphere
was applied. When the concrete specimen was de-aired,
which was generally after a week, the vacuum was released
and the specimen left submerged for another few days.
The samples were then weighed while they were submerged
(Wsb), the temperature of the water at the time of the
submerged weighing being recorded also. The samples
were then dried to constant weight at 105°C in the same
manner as for the evaporable water content determination
and the samples reweighed (Wd).
By Archimedes Principle the apparent loss in
weight of a body immersed in liquid divided by the density
of the liquid equals thc volume of liquid displaced.
Thus:-
-wdry sub V + V + Va C n
wherea
volume oP aggregate in sample
= volume of cement in sample
V1apparent volume of non-evaporable
water such that (V + V ) - volumeC fl
of hydration products plus volume
of unhydrated cement.
edensity of liquid at temperature
submerged weighing.
Therefore :=
- = W (W 1W . 1 + 1 + Wn. Wa C e e
w...
.. (1)
ew
*where ea = density of aggregate, density cement,
eapparent density of non-evaporable water.
now,
W -W +W +Wdry a c n
w w
W w (W/W +1+t
dry = c a C. . . . . . (2)
Dividing equation (2) by equation (1):-w U Wa +1+ fl. t
U -- -ry e w Wc Ut WC
Udry sub (W 1 - U U 1 ) .. .. .. (3)a + 1 + n. t.
(C C t C
now U 1W - t.(7 as before.a c
U /W - O. as before.t C
e a ande were measured by the relative density bottle
method as laid down in JLS.1377 "Testing of Soils",
and were found to be 0.0982 lb/in 3 and 0.1163 lb/in3
respectively. The value of e is taken as 0.0481 lb/in3
which is an average of results reported by Powers (40)
and Neville (16).
In the experimental work, the weighings were
all performed using metric balances.
Re-arranging expression (3) and taking into
account weighings performed in metric units:-
7.67. W - Wdry sub - 2.764Wde
w . w -11.t 0 4 0 6 dry subwdry e
(d) Assessment of the Loss of' water in Specimen.
The loss of water can be found by subtracting
the total evaporable and non-evaporable water weights in
a specimen from the total missing water. By plotting a
graph of evaporable and non-evaporab].e water content
per unit weight of mixing weight against the distance
from the bottom of the specimen, the weight loss can be
calculated by measuring the area representing the weight
loss, as shown in Figure 3:15, by using a planimeter
or "counting squares". This weight loss is then
compared with the actual values measured during the
experiments.
CJ1PTER FOUR - EXPERiMENTS AND EXPERIMENTAL TECHNIQUES.
\BSTR -\CT.
Details of the concrete mix used throughout
the main experimental work are listed. % description
is given of the design, manufacture, casting, curing,
sealing, heating and test procedures used in:-
(1) The Releas• Test Series
(ii) The Migration and Venting Test Series
(iii) The Pore PressureJShrinkage Test and
its subsidiary tests.
The removal of samples for further testing and viewing
under a Scanning Electron Microscope is outlined.
Limestone Aggregate concrete with a w/c ratio
of O.0 and an a/c ratio of t.7 was used. All the
main test specimens were cylindrical prisms of various
diameters and lengths. All specimens were sealed
against moisture loss to some degree in order to
control or dictate the nature of the movement of
moisture. Sealing was 100% in the Release Tests
only. The Release Test specimens were heated uniformly
in an oven; in the other main experiments, which
necessitated non-uniform temperature states, localised
base heaters were applied.
Measurements of pore pressure and temperature
were made and readings of weight loss were monitored in
(i) The Release Test Series
(ii) The Migration and Venting Test Series.
Measurements of evaporable water content was made in
(i) the Migration and Venting Test Series
(ii) Pore Pressure/Shrinkage test.
Longitudinal and Transverse Shrinkage measurements were
recorded in the Pore Pressure/Shrinkage Test.
Evaporable and non-evaporable water content
determinations were made on samples removed from the
cylindrical specimens at the end of heating.
A description is given of the preparation
and coating of samples from the Release Test and
Migration Test series for viewing under a Scanning
Electron Microscope.
CHAPTER FOUR - EXPERIME?TS ND EXPERIMENTAL TEChNIQUES.
I. Introduction.
11. The concrete mix.
III. The Release Test Series.
(a) Introduction.
(b) Specimen details.
(c) Casting, Sealing and Curing of Specimens.
(d) Assembly of specimens in oven
(e) Testing Procedure
I \T
The Migration and Venting Test Series.
(a) Manufacture, Preparation and General Details
of the Specimens.
(b) Casting of the Specimens and Introduction of
Inst r u ni en tat ion.
(c) Curing of Specimens and completion of Assembly
(d) Heating and Insulation
(e) Testing Procedure
(f) Determination of Final Water Distributions.
V. The Pore Pressure/Shrinkage Test.
(a) Introduction
(b) Design of Mould
(c) Casting and Curing Procedure
(d) The Casting and Construction of the Sealing
Jacket.
(e) Heating of Specimen
(f) Testing Procedure
(g) Casting of Subsidiary Tests
(h) Testing Procedure for subsidiary tests.
Vi. The Preparation and Examination of the Electron
Microscope Specimens.
(a) Introduction.
(b) Extraction of sample from concrete specimens
(c) Coating of sample for the stereoscan Microscope
Cd) Procedure employed in examining specimens in
the Stereoscan Microscope.
CHPTLR FOUR - EXPERIMENTS .ND EXPERIMENTfL TECIIMQUES.
4-I. INTRODUCTION.
The experimental work described in this thesis
may be divided into three main parts: the Release tests,
the Migration and Venting tests and the Pore Pressure/
Shrinkage test.
The object of the Release tests was to
investigate the relationship between the pore pressure
developed inside a sealed concrete specimen under a
constant temperature and the loss in weight of that
concrete specimen when fluid was permitted to escape.
This relationship was investigated at varioustemperaturee
and ages.
In the migration series the aim was to monitor
the variation of pore pressure and water content with
time when concrete is subjected to a thermal gradient.
The concrete specimens were sealed along their sides
and lower ends, heat was applied from the bottom and
steady state temperature conditions were maintained
throughout the tests. Two different lengths of specimen
and various base temperatures were used in this
investigation. The venting series of specimens were
designed and constructed in exactly the same manner as
the Migration series. The object of the investigation
in this series was to see the effect of breaking the
pressure seal at various points along the specimens at
various times and observing the dissipation of the
pore pressures and subsequent migration characteristics
of the water in the concrete. The Pore Pressure!
Shrinkage Test, as it8 name states, was an attempt
to monitor the pore pressure and shrinkage in a concrete
specimen that was sealed along the sides and bottom
and heated from the sealed end.
common link between all the series of tests was
that samples of the concrete were taken after testing
and their evaporable and non-evaporable water distribu-
tions plotted. Samples of some specimens were taken
and viewed under a scanning electron microscope and
comparisons made between the microstructures of the
samples tested and samples of concrete of the same
age that were not sbected to any of the series of
tests.
4-Il. THE CONCRETE MIX.
The concrete mix used throughout the experi-
mental work was:-
l parts by weight Portland cement
Fine aggregate
))Coarse aggregate
4 parts by weight sand
3 parts by weight g" aggregate
3 parts by weight " aggregate
Water/cement ratio 0.6 by weight.
The cement was obtained from .\ssociated
Portland Cement Manufacturers Limited in sealed bins.
All the cement was of the normal Portland type and taken
from one batch (manufacturers' code number 81) and was
the only cement used throughout the whole experimental
programme. The properties of this batch are shown
in Table 4:1, which was supplied by the manufacturers.
T'BLE 4:1
PROPERTIES OP PORTL thD CEi1irT.
CFIE11ICL rLYSIS
Component Percentageliy weight.
SO2 20.70
Insoluble Residue 0.70
il2034.70
203 3.00
CaO 64.41
MgO 1.20
so3 2.40
Mn203
P900.20
Loss on Ignition 1.50
K 00 0.35
Na 2 0 0.30
T1.02 0.26
Total 99.78
Fineness
Specific Surface cm2fgm J
3405
Both coarse and Pine aggregate was carboniferous limestone
t'rom Cheddar in Somerset. The aggregate was obtained
by blasting limestone rock from a quarry and crushing
the rock to the appropriate size. The fine aggregate
was angular and evenly graded between 3/lb in. and B.S.
sieve No. 200. The combined aggregate grading curve is
shown in Figure 4:1, together with standard curves for
a " aggregate found in the British Standard for
aggregateS (8b).
The batches of concrete for the Migration and
Venting series and for the Pore Pressure/Shrinkage test
were mixed in a Liner "Cumflow" Paddle Mixer. Thi
took a maximum of 250 lbs. of material and for the 10 ft.
Migration specimens and Pore Pressure/Shrinkage experi-
ment two batches were mixed to complete the specimen.
For the release test specimens a 56 lb. capacity
"Multiflow" mixer was used. For all casts, the moisture
content of the aggregate was checked before use by taking
samples and drying them in an oven at 105°C. It was
found when preparing for all casts except one, the
aggregate was at a moisture content of less than 0.5%
by weight, this being due to its storage in sealed bins
In the case where a greater moisture content was detected
the aggregate was first dried in an oven before mixing
with the cement and water.
Six 4" cubes were cast with concrete from each
batch. These cubes were demoulded twenty—four hours
after casting and stored under water at 17°C. They
l4O
were removed from the water immediately prior to testing
and were surface dried. Three were te8ted in compression
at an age of seven days and the other three at twenty-
eight days. The mean seven-day compressive strength
was 4810 lb/in. 2 with a standard deviation of 5t0 lb/in2
and the mean twenty-eight day compressive strength was
bilO lb/in 2 with a standard deviation of bOO lb/in2.
-III, TIlE RELE\SE TEST SPECfl1EIS.
(a) Introduction.
is described in Chapter Sharp (10), used
a specimen 4 ins, long and 4 in. diameter in order to
investigate the relationship between pore pressure and
loss in weight at a constant temperature. It was
decided to use specimens of the same dimensions for
comparative purposes but to modify and try and improve
upon several features of his experiment. Five test
specimens were Cast in a preliminary investigation to
determine the best methods of introducing the instru-
mentation through the sealing jacket and also to test
the behaviour of several materials at high temperature
and pressure when used for sealing purposes. It was
from this preliminary investigation that the specimen
details, casting, sealing and curing procedures
described in this chapter were developed.
(b) SpLecimen Details.
The concrete specimens used in the Release Test
series were 4 ins, long and had a diameter of 4 in. as
shown in Figure 4:2. The specimens were sealed in a
mild steel cylinder of length 4 in. and thickness in.
The ends of this cylinder were machined flat and 4
No. -i" B.S.F. holes were drilled and tapped diametrically
opposite each other at the heights shown in Figure 4:2.
The area around these holes was milled flat on the
outside of the steel cylinder so that a pressure seal
could be made with a "Dowty Seal", and a " B.S.F. Bolt.
The two end plates that completely sealed the concrete
specimen were t. inches square mild steel and thickness ".
n area 4 in. diameter and in. depth was machined
out on each plate so that the machined ends of the mild
steel cylinder fitted into the end plates as shown in
Figure 4:2. A in. B.S.F. Union nut was inserted in
the top plate, the pressure seal being completed by a
Dowty seal and a in. B.S.P. Brass union nut and nipple
that had been filled with silver solder. tt the joint
between the end plates and the cylinder gaskets of
"Jointite", a material used for sealing joints in high
pressure steam pipework were seated to prevent leakage.
The endplates were held onto the ends oe the cylinder by
6 No. " diameter rods evenly spaced around the diameter
of the specimen.
Each specimen contained two thermocouples and
one porous plate. The thermocouples were inserted
through the metal cylinder via the " tapped holes.
Each thermocouple was passed through the 2mm. diameter hole
drilled down the longitudinal axis of a " B.S.F. bolt.
This thermocouple was silver—soldered to the bolt so
that no moisture could escape through the 2mm. hole.
These silver—soldered joints did not interfere with the
working of the thmocouples. The bolt plus thermocouple
was screwed into the tapped hole and the pressure seal
made with a -i" Dowty seal. The porous plate was 2 in.
square and in. thick, and covered with filter paper to
prevent grout flowing into them during casting. 2mm.O.D.
copper tube passed from the porous plate through a hole
down the ltngitudinal axis of a ," B.S.F. bolt that had
been inserted in the metal cylinder in the same fashion
as with the thermocouples (the copper tube was silver—
soldered to the bolt in the same way as with the thermo-
couples and the pressure seal completed with a Dowty
seal), and after a U bend, the end was nipped to prevent
any bleeding of mix water during casting and curing.
The inside of each cylinder was coated with
autoplax resin after the instrumentation had been inserted,
and limestone sand was spread over the surface so as to
give a rough surface and provide a good bond between the
metal and the concrete specimen.
view of the sealing jacket is shown in
Plate IV:l. The jacket was assembled prior to casting
and hydraulically pressurised to 200 lbf/in 2 to check
for leaks.
(c) Casting, Sealing and Curing of Specimens.
Each specimen was Cast with its axis vertical.
Plate iV:2 shows several moulds ready for casting. The
moulds were clamped down onto the vibrating table so
they could not move. fter they had beencast and
I -
vibrated thoroughly, the top plate and gasket were
bolted into position. Each bolt was then tightened
in a pre—determined order using a torque spanner so as
to provide an even pressure all around the specimen.
Each bolt was tightened to a value on the torque spanner
of 10 ft/lb.
The complete jackets were weighed before and
after casting 80 that the weight of concrete in each
specimen was known. Six 4 in. cubes were cast from
the same mix. A check was made to ensure that the
density of the concrete in the specimens and the cubes
were similar.
The 8pecimens were kept in a curing room at
a temperature of 17°C and relative humidity of 75%.
While the specimens were kept in the curing room, the
top and bottom joints between the metal cylinders and
the end plates were given a coat of' autoplax resin.
This was an extra seal, and also provided a method of
checking whether a leak had occurred on the joint between
the cylinder ends and end plates. The thermocouples
were also wired up, the " B.S.F. Union nut and nipple
soldered to the copper tube and attached to a klinger
valve as shown in Plate ]V:3. The specimens were then
ready for testing and left in the curing room until
required.
(d) Assembly of' 8pecimens in Oven.
fter each specimen was removed from the curing
room prior to testing, the first task to be performed
was to fill the copper tube connected to the porous
plate with non-volatile oil to the top of the "U"
bend. This was done using a hyperdermic as described
in Chapter Three. The special pressure attachment and
transducer were then attached to the specimen with the
open end of the special attachment being attadd to a
screw-pump and pressure gauge via small bore copper
tubing. All the copper pipe was filled with fluid.
The transducer was fixed rigidly to a piece of handy
angle that was bolted to the specimen. The purpose
of this was to reduce any strain put on the connecting
copper tubing.
The specimen wa8 then mounted in an oven which
had its door removed and replaced ty a sheet of
asbestos. . special hole was cut in the asbestos
so that the transducer and klinger valve were on the
outside of the oven and to protect them from the high
temperatures. Plate IV:4 shows a specimen assembled
in the oven, with the transducer linked to the Readout
and the special pressure attachment connected to the
Screw Pump and Pressure Gauge, the iahole system being
ready for testing.
(e) Testing Procedure.
At the beginning of the test, the temperature
and pressure in the specimen was measured and recorded.
The temperature was taken as the mean reading of the
two thermocouples. The total weight of the specimen
including the transducer and pressure attachment was
also measured and recorded. Plate IV:3 shows a typical
specimen being weighed before and during the test.
Heating of the specimen was then commenced,
and as the temperature increased, readings of temperature
and pressure were taken at intervals of approximately
5°C. In order to ensure that no weight was lost or
gained in measuring the pressure, a back pressure was
applied using the screw pump, as in the procedure des-
cribed for measuring the pressure in Chapter Three.
When the temperature at which the specimen was to be
tested was attained, the specimen was quickly removed
from the oven and reweighed to check that there had been
no leakage during the heat-up period. The specimen
was then quickly replaced in the oven and left for
several hours to allow steady state conditions to be
reached. Readings of temperature and pressure were
then taken, and the pressure recorded was taken as the
pressure attained inside the specimen at zero weight
loss.
The release valve was then opened using a
spanner and a small amount of fluid, air plus water
vapour , was allowed to escape. The specimen was then
re8ealed, removed from the oven and reweighed before
being quickly replaced in the oven. It was then left
for several hours to allow conditions to become steady,
when readings of pressure and temperature were again
taken. Further fluid wasthen allowed to escape, and
the whole process of weighing and taking readings was
repeated.
In the early part of the test, the release
valve was only opened for a matter of seconds to allow
fluid to escape. flowever, as more fluid was
progressively lost, the time for which the relea8e
valve was left open to obtain a measurable weight loss
from the specimen grew longer, until near the end of
the tests, when the pressure in the specimens was only
just above atmospheric, it was found necessary to leave
the valve open for nearly two hours to get a loss of
1 gram.
It was also found that as the tests progressed
the time between the releasing of fluid and the recording
of pressure at steady conditions increased from approxi-
mately three hours at the beginning of the test to two
days near the end. In fact, it was found that when
the concrete had lost so much water that the partial
pressure of the water vapour was nowhere near equal
to the saturated vapour pressure, the temperature of
the specimen was cycled by about lO°C around the
test temperature after each weight loss and the pressure
value was obtained by interpolation from a plot of
temperature against pressure.
tt the end of the test, the specimen was
stripped down and samples taken to see how much more
water could be driven off at the temperature of the
test. lso, samples were taken for viewing under the
scanning Electron Microscope.
-lV. THE 1IGRATION thD VENTING TEST SERIES.
(a) 'anufacturePreparation an_Gneral Details of
the Specimens.
The concrete specimens used in both the
igration and Ventinj test series were cylindrical in
shape and had a diameter of (a ins. Two lengths of
I'.'.
specimen were used in the migration series, 5 ft. and
10 ft., while only 5 ft. specimens, exactly the same
as those used in the migration series were used in the
venting tests. One end of each specimen in both sets
of tests was left open to atmosphere during the test,
while all the other faces were sealed.
The seaiingjackets used were -i," thick mild
steel tubing. The tubing was delivered in lengths
of 5 ft. and 10 ft. straight from the factory with a
layer of protective grease and oil on thenu. This
was cleaned off from both tie outside and inside
surfaces using "Gunk", an industrial cleaning agent
used in the motor industry for the removal of oil from
concrete floors. It was applied "neat" using a broom
or brush to the metal surface, and washed away using
water from a hose. The oil and grease are soluble in
"Gunk", which in turn can be removed by a jet of water,
leaving a clean steel surface.
In order to introduce the instrumentation
into the specimens, l in. diameter holes were drilled
through the steel jacket. Figure 4:3 shows the centre
line of these holes in relation to the base for both
the 5 ft. and 10 ft. specimens. ?tround these holes
was welded a lft" B.S.P. socket of the type used in
the Gas Industry. The base of this socket that was
to be in contact with the outside of the sealing
jacket was shaped on a milling machine to take into
account the curvature of the surface. Figure 4:4
I
shows a section through a typical socket after welding
to the jacket with a gas plug inserted to maintain
the pressure seal.
The end not open to atmosphere was sealed by
welding onto it a piece of mild steel plate 8 ins.
square and j- ins, thick that had already been drilled
and tapped to take a heater after the welding and
casting procedures had been completed.
Ui the welds wore tested for leaks by applying
a hydrostatic pressure of 200 lb/in 2 to the inside of
the tube and checking that the outside of the weld was
dry. Plates IV:b and IV:7 show 5 ft. and 10 ft. jackets
under test for leaks. For the pressure testing the
sockets were sealed using gas plugs similar to those
used after the concrete was cast. The outside of the
plug was covered in Plumber's "Boss White", and the
threads were filled with hemp to help improve the
pressure seal.
•fter the welds had been tested and the gas
plugs removed, the inside of the steel jacket, which
also acted as the mould for the concrete during casting,
was coated with "Autoplax" polyester resin, and limestone
sand sprinkled on the resin just before it set. This
was to improve the bond between the concrete and the
inside of the jacket. .This was a precautionary measure.
Calculations showed that differential expansion between
tne steel jacket and concrete assuming no bond would give
rise to a gap of just under one thousandth of an inch
S
at the highest temperature experienced in the test
series. This, of course, would be intolerable and was
the reason for improving the bond between the concrete
and steel jacket.
(b) Casting of Specimens and Introduction of Instrumentation
Casting of each specimen was done separately and
all the specimens for the Migration and Venting Test
series were cast over several weeks. The reason for
this was that the mixer had a capacity that could only
handle enough concrete for one 5 ft. specimen, and for
the 10 ft. specimens, two batches were required to complete
the casting. Six 4" cubes were cast from each batch
used for strength measurements and density checks.
ll the specimens were cast with their axes
vertical. Before casting, the steel jacket plus gas
plugs, instrumentation and steel hook were weighed on a
steelyard. This weht was recorded so that a density
check and an exact weight of eoncrete in each specimen
could be calculated. The 5 ft. specimens were stood
on the floor, while the 10 ft. specimens sat on four
large rubber pads. Compaction of the concrete in the
5 ft. specimens was attained using a Poker vibrator, while
a 440v/3 phase motored shutter vibrator was clamped to
the 10 ft. specimens to provide vibration for compaction.
PlateIV:8 shows the poker vibrator being used during
the casting of a 5 ft. specimen. Plate IV:9 shows a
series oP 10 ft. specimens either just having been cast
or being prepared for casting. This plate especially
'SO.
illustrates the 10 ft. specimen resting off the floor
on the rubber pads. It also shows the iutter vibrator
clamped to the steel jacket.
The procedure for casting and sealing the
specimens was identical for both the 5 ft. and 10 ft.
specimens. The gas sockets were all left unplugged
and the threads inside were checked and cleared of any
obstructions. The fresh concrete was then poured down
a 2 in. diameter plastic pipe into the steel jacket.
The reason for this was to try and prevent segregation
of the concrete mix. This proved very successful as
did the compaction in all the specimens. Evidence of
this can be seen in Plates VI:1 to Plates VI:6,
Chapter Six, which shows a series of sections removed
from a specimen after testing was completed and the
specimen had been Cut up.
Concrete was poured to give a layer of approxi-
mately 3 ins, in the specimen and then vibration using
the poker vibrator or shutter vibrator was applied to
give compaction. When the level of tlse compacted
concrete reached the base of the lowest socket, the
instrumentation was introduced into the specimen.
This consisted of a porous plate (covered with paper
to prevent grout penetrating the pores and into which
was soldered about a 12 in. length of small bore
copper tube, nilped at the end to prevent any free water
in the concrete from being lost), a thermocouple, and
a moisture meter with electrical lead attached.
Plate IVl0 shows the instrumentation about to be
inserted into a specimen. Concrete was then poured
from above to cover the instrumentation, but no vibration
was applied. The l-j" B.S.P. (ha plug, with its threads
covered with Plumber's Boss White and hemp was then
inserted into the socket and tightened, using a spanner,
This plug had previously been drilled to give a " hole
through its axis. It was through this bole that the
thermocouple leads, moisture meter leads and the small
bore copper tube from the porous plate passed. This
hole was blocked with plasticine to prevent bleeding
of the concrete. It also helped at as an anchor and
prevent the instrumentation moving about during the
vibration of the concrete. In fact, this was found to
be successful. Checks made on the instrumenta&n
after the specimens had been cut up at the end of the
tests showed that it had moved about in most cases by
less than inch from where it was placed during casting.
iost of the porous plates had also remained horizontal
and not rotated. After the concrete had hardened, this
plasticine was removed and a plug of utoplax resin
injected to complete the seal. Just as a precaution,
the whole plug and socket was given a coat of Autoplax
resin, so that should a leak develop, this would quickly
be seen by a break in this coat of resin. PlateIV:ll
shows this coat of resin well.
s the compacted concrete level reached each
gas socket, the relevant instrumentaon was introduced,
the plugs inserted and the seal completed. When the
concrete level reached within l-j" of the top of the
specimen, a " steel bar and hook shown in Plate IV:12
was inserted. (This hook was used for raising and
lowering the specimen during the test for weighing.)
% hole had been specially drilled in the steel jacket
and the horizontal bar was fed through and held in place
by a bolt and washer that fitted into a thread tapped
along the horizontal axis of the bar. The eyelets
were held vertical while the concrete was poured and
vibrated, and the concrete held it in place after that.
The top of the specimen was smoothed off with
a float and covered with a piece of damp sacking until
it had set. A coat of Autoplax resin was then applied
to seal the specimen until required for testing.
(c) Curing of Specimens and Completion of Assembly.
The specimens were left to cure in the position
they were cast in. The reason for this was quite simply
that the specimens when cast were several hundred pounds
in weight and could not be easily moved around. The
specimens were weighed on the steelyard to check their
densities and amount of concrete in each. Work was then
started on them to prepare them for testing.
rirstly, a " B.S.P. Brass ipple and Union
I\ut were soldered to the small bore copper tubing, and
a "U" bend made in this tube so that thepressure
measuring apparatus would be within 2 ins, in height
of the porous plate in the specimen. This is well
illustrated by PlateIV:ll.
Next, all the thermocouples and moisture
meters were wired up so that readings could be taken
from the various positions in the specimens by just
plugging into a standard plug lead to the measuring
apparatus. n extra thermocouple was taped to each
specimen at the base to give the base temperature.
ll the copper leads of the thermocouples were joined
together to niake one common lead. The variou8
constantin loads from each instrumentation position
were gathered together and attached to a 3-way plug in
the case of he 5 ft. specimens and a 15-way plug in
the case of th 10 ft. specimens, along with the common
copper lead. similar thing was done with the moisture
meter leads. The common lead was taken as the outside
electrode. These again were wired up to a 9-way
and 15-way plug in the case of the 5 ft. and 30 l't.
specimens respectively.
Thirdly, a handy-angle frame was built onto
the steel jacket. This was fixed at the bottom to
the end plate by " B.S.P. bolts used for the base
plate heater system (see next section), while at the
top the frame was attached to the lifting book, that
could be screwed in and Out of its connecting bar to the
horizontal bar. This handy angle frame was used to
secure the klinger valves that were attached to the
specimen for the pore pressure measurement. These
klinger valves, which had " B.S.P. male threads on
one side and j" B.S.P. male threads on the other were
placed and fixed after the small bore copper tube had
been filled with non-volatile oil up to the top of
the "U" on the specimen side using a Jiyperdermic syringe
and a very thin piece of plastic tubing. These
klinger valves were then attached rigidly to the
handy-angle frame using steel wire and wood packing
cut to size. This gave them enough rigidity so that
they could be opened and shut easily without putting
any strain upon the small bore copper tubing. The
whole outside of the socket was then covered with
"Cosy-wrap", a household pipe insulator, and the
outside covered with aluminium foil to cut down heat
losses at this point. This is all illustrated in
Plate 1V:l3.
Except for heaters and insulation, the
specimens were then ready for testing.
(d) Heating and Insulation.
' thermal gradient was to be imposed on the
specimens, with the bases to be at the highest tempera-
ture, so that moisture would be driven upwards. In
order to obtain the desired gradients an electrical
heater was clamped to the base plate of the sealing
jacket. . photograph of this heater is shown in
Plate IV:14. This heater consisted of two 450 watt.
eloctrical iron element heaters connected in series,
insulated by sheets of mica. Also between the two
iron elements and on the bottom of the whole heater
was clamped a metal sheet. The whole heater element
was clamped tightly to the base of the sealingjacket,
which had 4 B... holes drilled and tapped in them before
welding to the steel jacket. The metal sheets between
the elements therefore acted as heat sinks. These were
necessary as the elements were live continuously, and
would burn out quickly if they were allowed to hang
155.
loosely. The base heater was surrounded by a -i" thick
metal ring, ins, in diameter. This sat upon a bottom
plate, 8 ins, square and -j" thick of mild steel, which
was held tightly to the plate welded to the steel jacket
by 4 No. " R.S.F. bolts. Surrounding the whole heater
was a punched metal cage, inside which sat glass fibre
insulation. The leads of the base heater were covered
with "Thermofit" sleeving, and this in turn was covered
with bead insulation. These leads passed through holes
drilled in the side of the metal ring, and into a high
temperature ceramic terminal block. Figures 4:5 and
4:6 show the base heater design and how it is attached
to the specimen.
The base heater was fixed to all the specimens
both 5 ft. and 10 ft. after the welding had been
tested and before casting. The reason for this was
simply the steel jackets could be laid on their sides
and more easily handled before the concrete had been
cast. Checks were made after casting the concrete
with an vometer on all the base heaters to ensure
no damage had been done. They all proved robust
enough to have survived the casting procedure and other
movements without damage.
In order to attain the desired thermal gradients,
supplementary heating was found to be necessary in the
form of "Isotape" tape heaters wrapped round the outside
of the sealing jacket. These heaters were 12 ft. long
and 1 inch wide and flexible, so that they could be
wrapped tightly around the specimens. Two wattages
were used, 120 watt and 260 watt, depending upon the
base temperature and gradient required. On the 5 ft.
specimens, one tape was used and wrapped around the
bottom third of the specimen as shown in PlateIV:15.
On the 10 ft. specimens, one tape was wrapped around
the bottom half, while the other was wrapped around
the top half, as shown in PlateIV:1b. These tape heaters
were either connected in parallel with the base heaters
or connected independently to another supply, depending
upon the base temperature and thermal gradient required.
The temperature at the base of the specimen
and the thermal gradient was controlled by varying the
heater voltages. This was done using the control
panel shown in Plate IV:le.,. Twenty-three supply
circuits were mounted on two boards to provide the
panel. Each circuit contained a variable resistor
for altering the supply voltage, a fuse and an indicator
bulb. The power was provided by two "Variac" trans-
formers, one single bank and one triple bank.
The specimens were thermally insulated with
prefabricated pipes made of corrugated asbestos paper
interleaved with aluminium sheets, with the outside
covered with aluminium sheet. This pipe was b in.
bore, 1 inch thick and came in 3 ft. lengths and half
segments. These were held tight around the steel
jacket using aluminium straps. In order to accommodate
the gas sockets welded to the jacket, holes were cut in
'S.;.
the insulation on the edges, so that the half-segments
joined in the line of the sockets. Where there was
a joint of this insulation, layers of pipe insulation
(cosy-wrap) were packed to prevent convection currents
developing.
Finally, the specimens were placed on small
wooden trolleys so that they could be moved more easily
for weighing. PlateIV:lb shows a 10 ft. specimen on
the right-hand side ready for testing, complete with
thermal insulation. Plate 1V:17 shoWs a complete set
of 5 ft. specimens similarily ready for testing.
Ce) Testing procedure.
Testing of the 5 ft. Migration series specimens
started approximately six months after casting. Five
specimens were tested in this series and they were
all started individually, with five ay intervals
between them. Three of the five 10 ft. Migration
series specimens were also tested at around six months
after casting. However, the other two 10 ft. specimens
in this series were not tested until an age oP 10 months.
The reason for this was simply a shortage of klinger
valves for the pore pressure measurements, and the
starting of the testing was held up while more valves
were ordered and delivered. The Venting test series
specimcns, which were all 5 ft. in length, were all
started at an age of 1- years. The starting procedure
and readings taken was the same for both the Migration
series and the Venting Series tests, the only difference
in the tests being that some klinger valves were either
left open to atmosphere or opened after sometime in
the Venting series specimens.
Immediately before heating commenced, the
Autoplax resin seal on the top of the specimen was
removed and the specimen was carefully weighed. This
weighing was done using the counterbalance system
described in Chapter Three, and the weighing was
generally checked twice for no errot. The initial
temperatures and resistance meter readings were taken
and the voltage to both the base heaters and tape
heaters applied. This voltage was controlled very
carefully for the first 24 hours in the case of the
5 ft. specimens and 48 hours in the case of the 10 ft.
specimens. Readings of temperature and pressure were
taken at very frequent intervals during the heating-up
period. When the desired base temperature and thermal
gradient were attained, a complete set of readings was
taken as listed below. This set of readings consisted
of :
Thermocouple Readings.
Resistance Meter Readings.
Pore Pressure Values
Heater Voltage
Weight of Specimen.
The number of different resistance meter and pressure
readings for a 10 ft. specimen was 9, while there were
S for the 5 ft. specimens (both the Venting Test Series
and the Migration Series). An extra temperature
reading was recorded for both 5 ft. and 10 ft. specimens.
Before any specimen was weighed, the weight of the
check weight was measured, and any variations were
adjusted for in the results.
Sets of readings were taken daily for the
first two weeks of all the tests, this frequency
dropping to once a week after three months, and once
every two weeks after nine months. however, during a
venting test, when a klinger valve was opened and a
porous plate vented to atmosphere, the frequency of
readings was reduced again to daily readings. This
dropping back again to weekly readings when the pore
pressure and resistance meter readings had stopped
varying greatly.
The 5 ft. migration series specimens were
run for 1- years, while the 10 ft. migration series
specimens were run for periods between 1 year and
15 months. The venting test series were run for
varying periods. However, as soon as the voltage was
switched off, the tops were sealed to prevent moisture
being sucked into the concrete from the atmosphere
during cooling.
(f) Determination of Final Water Distributions.
s soon as the specimens were cold, samples
were removed from various places along the length of
the specimens for either evftporabl€ and non-evaporable
water contents or for examination under a Scanning
Electron ?icroscope.
T?irstly, the outside of the steel jacket was
marked into inch-thick segments along the length of the
specimen. These were then numbered 1 to t0 in the
5 ft. specimens and 1 to 120 in the 10 ft. specimens.
The specimen was then cut into manageable lengths for
handling using the pipe cutter shown in PlateIV;19.
This is just a larger version of the type used by
plumbers and consists of three cutting wheels on a
half segment. A small groove was cut in the steel
jacket around the circumference using a hacksaw. The
pipe cutter was then attached so that all three wheels
sat in the groove. The pipe cutter was then rotated
around the steel jacket, constant pressure being applied
as the wheels cut in to the steel by tightening thread
on the adjustable handle. In this way the pipe cutter
gradually cut through the steel until the cutting wheels
reached the concrete. The specimen was then laid flat,
and by tapping the steel jacket with a sledge hammer,
the piece fell away. A typical piece of manageable
specimen cut this way, using the pipe cutter, can be
seen in Plate IV:20. This process was too long and
laborious to remove all the samples required, and
the slices required were obtained by cutting th
specimen dry with a "clipper" masonry saw.
Firstly, the steel jacket had tobe Cut through.
As the cutting of the concrete was to be done dry (for
obvious reasons), care also had to be taken to prevent
the concrete temperature risiiig, hence causing drying
out before the sample could be tested. Consequently,
most of the steel was removed on a lathe, and the final
amount removed using the "Clipper" masonry saw, the
cut just penetrating into the concrete. The concrete
was then broken with a chisel, to give a sample 1 inch
thick. Plates VId to VI:( show a series of samples
extracted from a specimen in the manner described
above. Pinally, the steel ring surrounding the concrete
was removed by cutting it with a Hand power saw.
The evaporable and non-evaporable water content
ofeach sample of concrete was then determined as described
in Chapter Three.
On average, 15 one-inch samples were taken
for evaporable and non-evaporable water content
determinations fromthe 5 ft. migration series specimens,
their spacing being determined from the resistance meter
readings (which measured the evaporable water contents).
The number of samples taken from the 10 ft. tigration
series varied from 20 to 30. The pieces of specimen
not used for the water distribution were then carefully
logged and marked, sealed with wax and stored to
provide scanning electron microscope specimens.
The time between the current being switched
off and the samples being removed was kept to a
minimum to forestall any possibility of the water
re-distributing itself due to cooling.
4-V. THE PORE PRESSURE- SHR1NKGE TEST.
(a) Introduction.
The purpose of' this test was to monitor how
the pore pressure and shrinkage of a sealed concrete
specimen under a thermal gradient behaved. The specimen
%
was to be sealed in such a manner that the thermal
gradient caused moisture to migrate from the hot to
cooler parts of the specimen. Such an experiment
would relate to migration tests performed by England
and Ross (14), Parkinson (9) and Sharp (10) at King's
Co1lee when the parameters of shrinkage and pore
pressure were monitored individually in concrete
specimens, within which moisture migration was taking
place under a thermal gradient.
Clearly, the sealing of the specimen in this
test was of the paramount importance. The work of
Sharp, as described in Chapter Two, showed that high
pore pressures would be developed within a concrete
specimen when its temperature was raised, and that
unless the specimen was sealed correctly, these would
soon dissipate in theform of leakage and moisture loss.
Parkinson, whose work is also described in Chapter Two
showed that shrinkage accompanied moisture migration,
and so it was very desirable to cause moisture migration
to occur in any test to monitor both pore pressure and
shrinkage.
It was decided to use a circular specimen made
up of a series of discs. These discs were sealed
in such a manner that moisture could neither escape out
of their edges, nor pass from one to another around the
edges, but through their centres. This series of discs
were c.ist as a homogeneous unit, vertically and
heated at its base with the top of the top disc being
open to atmosphere. In eachdisc,a sintered bronze plate
to measure pore pressure (covered with filter paper to
prevent grout penetrating during casting), a thermocouple
and resistance moisture meter were cast, as were
two county caps diametrically opposite each other so
that transverse and longitudinal shrinkage could be
measured.
In all previous work where pore pressure
was measured, the specimens were sealed and bonded to
a steel sealing jacket. This type of inflexible seal
makes the measurement of shrinkage using a county cap
cast in the surface impossible. It is also worth
noting here that a steel jacket also precludes the
use of vibrating wire gauges to monitor shrinkage,
as the steel jackets interfere electrically with these
type of gauges and make any readings unreliable. The
seal that was to be used was such that it bonded to the
concrete surfacU and did not "blow off" when the expected
pore pressures developed, but at the same time be
flexible enough to allow the concrete to shrink. The
seal selected was one of silicone rubber. This was cast
around the specimen, and the whole specimen was then
encased in a steel jacket. The gap between the steel
jacket and the silicone rubber was filled with grout.
The instrumentation passed through the jackets of
rubber and steel. Brass studs were screwed into the
county caps and these protruded through the sealing
jackets, and were allowed to move freely in both
longitudinal and transverse directiois.
6%.4
(b) Design and Construction of mould for concrete
Specimen.
In order to relate to previous work done at
King's College and also to the migration series tests,
described and reported in this thesis, it was decided
to u8e a specimen 5 ft. in height. however, in order
that each disc could be treated as an element at constant
temperature, the height of each disc was made 3".
Similarily, in order to obtain some significant movement
across the specimen, it was desirable to make the
diameter of a reasonable dimension, and 10 inches was sel-
ected. To stop a moisture path forming up the outside
of the specimen, water stops were to be inserted between
each disc. These water stops were made of P.T.F.E.
rings that protruded an inch outside and inside the
specimen surt'ac. P.T.F.E. does not normally adhere
to any materials; in this case, however, it had to
adhere to the silicone rubber jacket. This was done
by "etching" the P.T.F.E. in a sodium/ammonium bath
at -3°C. This process is an extremely dangerous one,
the fumes being given off from the bath being highly
poisonous, and the P.T.F.E. rings used in this specimen
were treated in this manner by "Flurocabon & Co. Ltd.,"
a specialist company in this process.
In order to design a mould to cast this
specimen several requirements had to be met. Firstly,
the size and weight of the proposed specimen meant the
mould had to be strong enough to withstand a fairly
large weight of concrete. Provision also had to be
made for introducing the instrumentation (plus leads)
and water stops during casting, arid yet these must
remain after demoulding in an undamaged form. The
eventual mould design settled upon is shown in
Figure 4:7. The mould was made a high quality cast
iron by J. II. May, Ltd. of Hackney, and cast in half
segments. These were machined at King's College and
also drilled and tapped with a 2 B.A. thread around
the surface as shown in Figure 4:7 so that the instru-
mentation could be introduced. The two half-segments
were held together to form a disc using a half-inch
whitworth bolt. The weight of a complete disc was in
the region of 19 lbs. The holes drilled in the
bottom flange were spaced at 15° in order that " long
bolts could be passed through to the mould below and
tightened so that "bleeding" could not occur at the joint
of each disc. This also had the effect of putting
ti' e P.T.F.E. rings in compression.
The bottom mould was bolted onto a " thick
steel plate, 14" square and covered with Latex rubber,
with holes tapped to take the " bolts. This steel
plate sat on four rubber bungs which were in turn
supported on the top flanges of two rolled steel joists.
The instrumentation was inserted into the mould at this
time. Two mm. holes were drilled down the axis of a
2 B. %. brass bolt that was screwed into the holes
tapped in the side of the mould. % thermocouple and
the small bore copper tubing from a porous plate were
fed through these holes and out of the specimen. Leads
from a resistance meter passed through another 2 13.t.
66
brass bolt and the gap filled with plasticine to stop
"bleeding" from the specimen during casting. County
caps were held in place against the sides of the mould
using 2 B.A. screws that also kept the threads clean
during casting. These county caps were located
diametrically opposite each other. typical mould
with instrumentation c3n be seen in Plate IV:21.
fter the bottom mould was bolted to the steel
base plate, a P.T.P.E. ring was placed on the top flange
of' the mould and the next individual mould with
instrumentation previously inserted placed on top and
bolted down to the base plate. - P.T.P.E. ring was
then placed on the top flange of' the mould and the
next individual mould placed on top. This was fixed
to the mould below it using -i" long bolts, with the
tightening nuts on the bottom face of the flange of the
mould below. The bolts fixing the individual mould
to the one below it were staggered around the circum-
ference, leaving every second hole in the bottom
fLinge clear. In this way, the six fixing bolts did
not interfere with those clamping the mould above to
itself. The instrumentation was positioned so that
the leads were in the same vertical plane. The
complete specimen mould ready for casting is shown
in Plate P1:22.
(c) Casting and Curing Procedure.
The total weight of the mould alone was over
400 lbs, and with the weight of the concrete specimen
being in the region of 500 ibs, some considerable
thought was given to the method of vibration to be used
to give good compaction in the concrete. In the end
it was decided to try a shutter vibrator which developed
a centrifugal force of 600 lbf. at 3000 cycles per
minute. . full scale test specimen was cast in order
to try this method of vibration and also test the
casting procedure to see what snags arose. This
specimen can be seen in Plate IV:23, ar.d proved to be
an unqualified success. This specimen was used in a
study to examine various water seals on concrete.
The shutter vibrator was clamped to the outside of the
specimen mould and can be seen in PlatiIV:22.
The specimen was cast in two batches. Six
cubes were cast from each batch and used to determine
strcnths and check densities. The concrete was
poured down a 2 " diameter plastic pipe in the same
manner as for the migration series specimens. When
enough concrete had been poured into the mould to
complete the bottom disc, the vibrator was turned on
until all the air had been expelled. This meant
watching very carefully to ensure that no air was
trapped under the P.T.P.E. ring and for the early
discs at the bottom of the specimen, extra care and time
was taken. In this manner the specimen was cast.
The total time to cast the specimen was over 2 hours,
great care being taken throughout to expel all air
from the fresh concrete.
The top of the specimen was smoothed with a
float and covered with a damp sack for 24 hoars. This
was then removed and a layer of autoplax resin cast on
the top surface. The specimen was then left in the
laboratory where it was cast to cure for seven days.
(d) The Casting and Construction of the Sealing Jacket.
Fter seven days the top four moulds were
removed and the specimen surface was coated with autoplax
resin. This was applied with a paint brush after all
four discs had been removed and the brass studs had been
screwed into the county caps.
Special care was taken as each individual mould
was undone and split in two. Firstly, the long bolts
clamping it to the mould beneath it were slackened and
removed. The 2 B. '. screws and bolts that were in
the sides of the mould were then removed carefully.
Lastly, the two -i" bolts holding the half-circles
together were undone and the moulds pulled apart. The
small bore copper tube from the porous place was
positioned such that it was at 900 to the join of the
two segments, and so the mould was pulled out so that
this copper tube was not damaged.
This procedure was repeated the next day on
the next four individual moulds and the surface of the
concrete just demoulded coated with autoplax resin.
The first four discs demoulded were coated with the
layer of silicone rubber applied with a spatula. This
rubber came in 5 kg. drums as a liquid and was mixed
with a hardener in order for it to set in solid form.
The hexagonal brass studs that were screwed into the
county caps had their surface covered with etched
P.T.P.E. The rubber jacket bonded to this P.T.F.E.
on the outside, while the brass could move freely
inside. This etched P.T.F.E. was loosely wrapped so
that the brass studs had enough freedom to move in a
vertical direction in order to trace longitudinal
shrinkage.
.s soon as the specimen had been demoulded
and the layers of autoplax resin and silicone rubber
had been applied, the main rubber jacket was cast around
the specimen. This gave an overall rubber jacket of
4" thickness. The mould used for casting this jacket
was made by . Tofield, Esq., King's College carpenter,
of wood that had been varnished and is shown in Plate
IV:24. Holes were drilled through the circular surface
to enable the brass studs, small bore copper tubing and
electrical leads for the resistance meters and thernio-
couples to pass through the rubber jacket. The rubber
jacket was Cast in 6" liFts, with an interval of two
days between each lift to allow the silicone rubber to
set. Plate IS: 2 shows the silicone rubber jacket
after four lifts.
After the whole silicone rubber jacket had been
cast, the specimen moved off the base plate upon which
it had been cast onto a lu" square plate that had been
prepared for the specimen to sit during testing. The
surface had been machined to give a l2j" diameter circle
set " into the surface of the plate. In this circle
sat a gasket manufactured from 1/32" thick Jointite, as
used in the Release Test specimens. This steel plate
was drilled and tapped to take the bolts that would
anchor the outer steel jacket, shown in PlateIV:2ti to it.
This outer steel jacket was a 12" internal diameter
tube, 5 ft. high with machined ends that had been slit
in half and drilled so thit the instrumentation from the
concrete specimen could pass out. Six 2" x 2" pieces
of angle had been welded near the base so that the
machined bottom coild be anchored with bolts to the
plate upon which the specimen sat, giving a pressure
tight seal. The vertical joints had been milled flat
so that when the tube was fitted together there were no
gaps in the joint. To hold the two halves of the tube
together, lugs made of 2" x 2" angle were welded at the
two ends and in the middle on each side as shown in
Plate IM2b. These lugs were drilled and " Whitworth
nuts and bolts used to hold the tube together. Details
of the base plate and steel jacket can be seen in
Figures 4:8, 4:9 and 4:10.
The concrete specimen was moved onto the steel
plate by rolling it from the base plate upon which it
was cast with its ixis vertical, special care being
taken not to cause any damage to the instrumentation.
The position of the specimen on the base plate was
carefully adjusted using location points. This was a
long process, th? outer steel jacket being placed
around the specimen and removed several times before
the specimen was correctly located.
The steel jacket was then placed around the
specimen and anchored to the base plate. The gap
between the steel jacket and the silicone rubber
v•; I
varied around the circumference of the specimen from
to ,". This gap was filled with grout that was
injected in from the base with a hand pump. This
grout was a mixture of eenes cement and water and
was deliberately made fairly wet in order to flow and
fill the gap. This it did very successfully. Care
was taken not to allow the grout to surround the brass
studs, and this was done by covering them with plasticine
before the grout was applied.
The thermocouples and resistance meters were
then wired up in the same manner as in the migration
and venting series specimens. These were done in
four groups of' five, there being twenty discs in the
whole specimen. The small bore copper tubing from
the pore pressure plates were bent to form a "U" and
a i," B.S.P. nipple and union nut attached. This was
attached to one side of a klinger valve as shown in
PlateIV:27. This klinger valve was held between two
pieces of handy angle to take the strain off the small
bore copper tubing when the valves were opened and
closed. The small bore copper tubing was filled with
non-volatile oil to the top of the "U" bend as in the
migration arid venting series specimens.
(e) Heating of Specimen.
The specimen was to be heated from the base
only, and this was done by clamping four electrical
heaters exactly the same as those used in the migration
series specimens. These four heaters were clamped to
the bottom of the special base plate upon which the
specimen sat during testing before the specimen was
placed on it. This base plate sat upon two rolled
steel joists, and was insulated on the bottom face to
stop heat losses.
. control panel was. constructed to vary the
voltage to each electrical heater. Each control
circuit consisted of a variable resistor, fuse and
indicator bulb. The panel was powered from the mains
via a "Variac". The specimen, complete and ready
for testing, including the heater control panel, is
shown in Plate IV:28.
(f) Testjn Procedure.
Heating of the specimen commenced 202 days after
casting. Before heating commenced, the temperature
and resistance meter reading was taken for each
individual disc. The initial reading across the
diameter of each disc was taken using the special
gauge constructed and described in Chapter Three. The
initial distance between the brass stud and the one
three above it in the specimen was taken using the
Whittemore Gauge that had been specially adapted, as
described in Chapter Three.
Readings of Pore pressure, temperature,
resistance meter reading, horizontal movement and
vertical movement were taken during the initial heat up
period, which lasted for two days, by which time the
thermal gradient did not alter. Readings were taken
daily for the first seven days of heating. 'tfter
this the time interval between taking readings was
increased until after two months of heating tlie
interval between readings was one week.
complete set of readings consisted of:—
Temperature reading.
!esiotance meter reading
Pore Pressure reading
Horizontal, movement
Vertical movement
for each individual disc, there being twenty in the
specimen. The complete set of readings for the
whole specimen took nearly two hours to record, special
care being taken on the horizontal and vertical.
movements. Plate IV:29 shows vertical movements being
taken with the adapted Whittamore Gauge, while Plate
1M30 shows horizontal movements being recorded using
the special gauge constructed.
(g) Casting of Subsidiary Tests.
Two subsidiary tests were run in conjunction with
the pore pressure/shrinkage test. The purpose of these
were:-
(i) To assess the behaviourof thesealing jacket.
(ii) To determine the variation of
coefficient of expension of limestone
concrete with moisture content at
elevated temperatures.
The specimen used in the first subsidiary
test was exactly the same as the 5 ft. migration series
specimens and cast in exactly the same manner. The
only difference between this specimen and the migration
series specimens was that no thermal insulation or tape
heater was wrapped around the specimen. The reason
for this was that simply the same thermal gradient as
the pore pressure/shrinkage specimen was required,
and it was found necessary to remove any thermal
insulation or tape heater in order to attain the
required thermal gradient. The specimen ready for
testing is shown in Plate IV :31.
The specimen used to determine the variation
of the coefficient of expansion with moisture content
consisted of a series Of 11 slices of concrete, 18"
high by 4" wide by " thick. These slices were cast
separately in mould, one at a time, each slice
separated from the other by a sheet of thin card 18"
high and 4" wide. These sheets of thin card permitted
moisture to travel from one slice to another, while
acting as a shear release. The mould in which the
slices were cast consisted of' two sides of metal faced
plywood that hid been drilled and tapped in such a way
as to allow a metal plate to be screwed in vertically
to give an area the shape of the concrete slice. The
plywood that formed the surface of the concrete slice
was drilled and tapped so that county caps could be
cast into the surface of the concrete in order that
shrinkage stresses could be measured. 1so introduced
through similar holes were theremocoiples and resistance
meter leads, so that each slice contained two thermo-
couples, two resistance meters and four county caps to
give two 10" gauge lengths, one each side of tie slice.
The mould sides were clamped to a base plate and the
whole mould clamped on the vibrating table. The mix
used for the slices was the same as used in all the
work described in this thesis, and three cubes were
taken from each batch used to cast a slice.
Immediately after the specimen had been cast and the
top smoothed over with a float and covered with a damp
cloth, the whole mould was moved into the curing room
and left overnight. The moveatile steel plate was
removed the next day and replaced by a sheet of thin
card. The moveable steel plate was moved along the
mould, so that the area between thin card and steel
plate represented the volume of a slice. This next
slice was cast in the same manner as the previous one.
The only slice that was cast in a slightly
different manner from the others was the fifth.
Instead of inserting instrumentation in the form of
resistance meters and county caps, two glass tubes were
inserted vertically through the slice, and two thermo-
couples were inserted in the normal manner. These
two glass tubes were to carry two spiral element
heaters that were used to heat the specimen.
fter the eleven slices had been cast, the
two sides of the mould were removed and the sides and
top of the specimen were coated with autoplax resin.
While this resin was still "tacky", aluminium foil was
applied. This can be seen clearly in PlateIV:32, which
hows the specimen ready for testing. ftur the
aluminium had been applied, the leads of the thermocouples
and resistance meters were gathered together and fed
into a single plug for ease of taking readings.
2 Brass bolts were inserted into the county caps.
These bolts had a hexagonal head and a small hole was
punched in them to accommodate the feet of the
Wliitte.more Gauge. The specimen was mounted on a sheet
of asbestos, which in turn was supported off the bench
by legs. The spiral 450 watt element heaters were
fed through the glass tube and wired in parallel.
These were then linked to the heater control panel used
for the migration and venting series specimens as
described earlier. The specimen was then ready for
testing.
(h) Testing Procedure for Subsidiary tests.
Heating of the specimen used to assess the
behaviour of the sealing jacket commenced at the same
time as for the pore pressure/shrinkage specimen.
Readings were taken on the specimen at the same time as
on the main specimen, so a direct comparison could be
made. The comparison was made by comparing the
resistance meter and pore pressure readings at the
various positions on the subsidiary specimen with
similar positions from the base in the main specimen.
The thermal gradients of the two specimens were approxi-
mately equal, arid although their ages were not exactly
the same, the degree of hydration of the concrete in
each specimen ws not so dissimilar as to be expected
to affect the readings by a large amount.
complete set of readings from this specimen consisted
of:-.
u Thermocouple readings.
5 Resistance meter readings.
S pore pressure readings.
Supply voltage.
Weight of specimen.
heating of the second subsidiary specimen commenced
40 days after the casting of the last slice. Readings
of temperature, water content (through the resistance
meters) and shrinkage movement fusing Whittemore
Gauge) duc to the moisture migration were taken at
daily intervals for the first three weeks of heating.
This was gradually reduced to weekly readings after
(0 days, and the supply voltage was switched off after
100 days of heating, by which time changes were very
small and a good assessment of the coefficient of thermal
expansion with changing moisture content could be made.
4-VI. TIlE PREPAR.TIOt \ND FXAMINATIOI\ OF THE ELECTRON
MICROSCOPE SPEC1MES.
(a) Introduction.
The relatively recent commercial introduction
of the scanning electron microscope (Stereoscan) had
made po8sible the direct examination of rough surfaces
of solid bulk specimens. Much of the work done using
a stereoscan on the examination of the microstructure
of cement paste and concrete is reviewed in Chapter Two..
The author was offered the opportunity of examining
samples taken from his various concrete specimens on
the scanning electron microscope at the Department of
Crystallography, Birkbeck College, the UrAiversity of
London.
The extraction of the samples for viewing
under the stereoscan was done in the Department of
Civil Engineering, King's College. These samples
were then transported to Birkbeck College, were dried
under vacuum, coated with a layer of carbon and viewed
under the microscope.
(b)
Extraction of sample from concrete specimens.
Samples for viewing under the stereoscan were
taken from the Release Test specimens, the 5 ft.
migration series specimens and from cubes and cylinders
that had been cast and subjected to various curing
conditions. The samples from the Release test and
migration series specimens were to be viewed with
reference to their moisture and temperature histories.
The samples for examination were taken in
the form of " cores. This was done using the
diamond—tipped coring drill shown in Plate IV:33. This
was attached to a workshop drill at King's College
and ran at a speed of 1300 r.p.m. The concrete from
which the sample was to be taken was clamped to the
table of the drill. In the case of the migration
series specimens, this was in the form of slices cut from
the specimen after heating ceased in the manner described
previously. In the case of the Release Test specimens
and the cubes and cylinders, this was usually the
actual specimen. . lubricating fluid was passed down
the centre of the coring drill. This was necessary
to enhance the coring drill's action and also to cool
it down. The choice of' this lubricating fluid was very
important in the case of the samples from the Release
Test and migration series specimens. Water would
obviously affect the evaporable and non—evaporable
water contents of the samples, and hence may be the
microstructure. Consequently, Petroleum Fraction that
0 0had a boiling point between 190 C and 205 C was used
as the lubricating fluid. This had no effect on
either the microstructure or water contents of the
sample and was evaporated away when the sample core
was dried under vacuum. In the case of the cubes
and cylinders that had been cured under water, water
was used as the lubricating fluid. Figure 4:11
shows a schemmatic of the lubricating system for
the coring drill.
(c) Coating of Sample for the stezscan microscopy.
Concrete is a non—conductor to electrons and
so before a fractured surface can be viewed under a
stereoscan microscope, a thin film of conducting
material must be applied. This conducting material
can be either a metal or carbon, which is the usual
medium used at l3irkbeck College.
PlatelV:34 shows a typical core taken from
one of the concrete specimens in the tests described
previously. This core was placed in a vacuum
desiccator under a vacuum of t w o atmospheres and
left for 48 hours. The core was then removed and
fractured to give a specimen about " in length.
This specimen was then 8tuck on the brass stub shown in
'we
PlateIV:34 with the freshly fractured surface upwards.
When the adhesive ("Durofix") that held the concrete
specimen to the stub had set, the vertical sides of the
specimen were painted with "Silver Dag", a conducting
paint.
The stub was then placed in a high vacuum
chamber over two freshly prepared carbon rods. This
chamber could hold six stubs arid so six specimens
could be coated at once. The chamber was sealed off
and evacuated using a Roughing valve until a vacuum
of 200rm of mercury was attained inside the chamber.
This valve was then closed and a high vacuum valve
opened. When the pressure inside the chamber had
reached 5 x l0 torr, an arc was passed between the
carbon rods which caused carbon particles to disperse
around the chamber and hence coat the freshly fractured
concrete surfaces. The arc was passed between the
carbon rods until an increase of 7O had been
recorded on a small mica strip that was linked to a
very accurate balance. This was the equivalent of a
coating of carbon of approximately 600 on the
concrete specimens. The high pressure vacuum was then
released and the specimens removed from the chamber.
They were then ready for viewing under the steroscan
microscope.
(d) Procedure employed in examining specimens in the
Stereoscan Microscope.
The scanning electron microscope used at
3irkbeck College was manufactured by the Cambridge
Instrument Co. Ltd. The stereoscan ha8 a better
resolution and a greater depth of' focus that optical
microscopes, whereas its resolution is lower than a
transmission electron microscope but rough surfaces of
solid bulk specimens can be examined directly without
making replicas. The stereoscan suffers from a
considerable disadvantage in that it is not possible
to take electron diffraction patterns of the material
under observation as is possible with a transmission
electron microscope. However, the micrographs obtained
give a good three-dimensional view of the fracturea
surface. The maximum resolution of the stereoscan
is of 200 and needs a fairly good conducting surface
to avoid Ouild-up of charge and image distortion.
In the stereoscan, a beam of electrons is
focussed to a fine spot and is made to scan the surface
of the specimen. Electrons which are reflec ted or
emitted from the specimen as a result of the primary
bear are collected. These are used to modulate the
brightness of a cathode ray tube beam which scans a
screen in synchronism with the primary beam of electrons
scanning the specimen. This produces animage of the
specimen having a marked three-dimensional appearance,
since the contrast of the image is produced by variations
in the number of electrons emitted or reflected from
different parts of the specimen. The stereoscan may
be applied to both conducting and non-conducting
specimens. When a non-conducting specimen (such as a
fractured surface of concrete) is examined, a
conducting coating is applied and a lower accelerating
voltage is used for the primary beam of electrons to
reduce "charging up" of the specimen.
Details of the operation of the stereoscan
will not be listed here. Purther information can be
obtained from the "operator's Manual" published by
the Cambridge Instrument Co. Ltd. Useful background
reading on the scanning electron microscope can be
found in a book on this subject by Thornton (87) and
in a paper by Oatley, Nixon and Pease (88).
Generally, two cores were taken from any one
region in a concrete specimen. These samples were
viewed in great detail at various magnifications.
Micrographs were taken of regions that the author felt
were fairly representative as well as of any detail
that appeared of special interest. The process of
examining these samples was very long and laborious,
with nearly two hours spent examining every sample.
The micrographs were developed by Mr. IL Hunt of
King's College, and examples can be seen later in this
thesis.
CHAPTER FIVE - RESULTS OF TIlE RELEASE TEST SPECIMENS.
ABSTRACT.
The "Release Test" programme is presented
and a brief description is given of the testing procedure.
Typical results, showing the variation of pore pressure
with temperature and weight loss, are presented. The
effect of age at testing is shown in some comparative
results for two ages and four temperatures.
An idealised model of the experiment involving
a sealed body of inert porous material is discussed.
Comparisons between this idealised model and actual
concrete are made, and the main differences highlighted.
From these comparisons, new definitions of water in
concrete, called Active, Passive and Bound water, are
proposed. The sum of these three types of water per
unit weight of mix water is unity. The experimental
results are then discussed in the light of these new
definitions and attempts are made to explain some
anomalies observed.
Scanning Electron micrographs of samples taken
from specimens tested at various elevated temperatures
are presented and compared with the microstructure
of unheated samples.
It is concluded that the quantity of Active
water in concrete increases with both age and temperature;
the quantity of Passive water is temperature and pressure
dependant; and the Bound water depends solely upon
temperature. The micrographs illustrate the influences
of raised temperature and loss of moisture, which cause
disintegration of the microstructure into smaller masses
of hydrate.
CJIPTER FIVE - RESULTS OF TIlE RELESE TEST SPECIE1\S
I.. Introduction.
xi
Test Procedure and Control Specimens.
ill. Presentation of Results.
(a) Control Specimens
(b) Results of Test Specimens
IV. Discussion of Results.
(a) Control Specimens
(b) Idealised model of a sealed porous
material subjected to a "Release" test.
(c) Comparison of the Idealised model and
the actual concrete specimens.
(d) Examination of Experimeiital results.
(e) The 105°C Specimen nomalies.
V. Scanning Electron Microscope Photographs.
(a) introduction
(b) Dummy Specimen Results
(c) l0°C Release Test Results.
(d) 150°C Release Test Results.
(e) l7°C Release Test Results.
(F) 200°C Heated Specimen Results.
(b') Comparisons and conclusions from
Scanning Electron Microscope
I'ho toraphs.
vi. Conclusions.
CH\PTER FIVE - RESULTS OF THE RELEkSE TEST SPECIMENS.
5..I. INTRODUCTION.
This series of tests consisted of monitoring
the pore pressures developed inside sealed cylindrical
limestone aggregate concrete specimens, whose seal
was periodically broken to allow fluid to escape.
The casting and construction of the specimens is
described in detail in Chapter four. PlatelV:4 shows
one of the specimens inside an oven, under test,
while Figure 4:2 shows a cross—section. Table 5:1
shows the esting conditions used in this series.
The specimens numbered I - 5 were cast
individually and were used as pilot studies in order
to develop and test the techniques used later throughout
the main test programme. The only previous similar
study was performed by Sharp (10), who tested a
cylindrical concrete specimen made with flint aggregate
at an age of 41 days and heated to a temperature of
150°C. The results of this test have been reviewed
elsewhere iiid can be seen in Figures 1:7 and 2:7.
TBLE 5:1.
Temperature at NotesSpecimen 'tge at start release ofNumber of test. moisture
i—s Used to testappara tusdesign andact ascontrols.
b 403 days 175°C
7 361 days 150°C
_or,8 30o days 10
9 385 days Used as acontrol
-o10 370 days 12 C
11 39 days 150°C
12 7 days 105°C
13 52 days Used as acontrol
014 31 days l7 C
15 31 days 105°C
lb 74 days 125°C
Ui the specimens numbered from 6 to lb
were cast and sealed at the same time. This procedure
is described in detail in Chapter Four. Four
temperatures were selected for testing: these were
105°C, 125°C, 150°C and 175°C, and were chosen so as
to correspond with Pour of the base temperatures of
the migration and venting series specimens described
later in this thesis. Tests were performed at
two ages. The first group of specimens were tested
at an average age oP 49 days, this corresponding to
a fairly young concrete in terms of the life of a
prestressed concrete pressure vessel. The second
group was tested at an average age of 365 days,
corresponding to a fairly mature concrete.
5-11. TEST PROCEDURh ND CONTROL SPECIErS.
detailed description of the actual test
procedure used and the way in which the measurements
were made is given in Chapter Four. After casting
and sealing the specimens were weigh e d periodically
while they cured to ensure that no loss of moisture
occurred due to faulty sealing. The transducer and
pressure attachments were generally assembled on the
specimen the day before heating commenced. Specimens
were run in pairs as only two ovens were available
for heating and it was this lack of heating facilities
that was the reason why specimens were not all tested
at exactly the sante age in the two test series.
During heating to the test temperature,
readings of pore pressure were taken at about 5°C
intervals. This gave an indication that the specimen
seal was intact and that th specimen was behaving
normally. s soon as the test temperature was reached,
tile specimen w ts weighed to check thit there had been
no weight loss during tire heating cycle, ..nd then left
for day at tire test temperature to ensure that steady-
stite coiidi tions cxi s ted inside the specimen. Readings
of pore pressure and temperature were then taken and
the Releae valve opened for a few sccOnds, and fluid
in tire form of steam and air allowed to escape. This
was usually accompanied by a characteristic hissing
noise in tire early stages of the tests, indicating
just how significant the pore pressures can be when
they are not allowed to dissipate. Readings of pore
pressure, temperature and weight were taken immediately
after tie release valve was shut. This was done for
in terest, arid in general it was noticed that tire pore
pressure value showed quite a significant drop, which
gradually rose to a constant value when steady-state
conditions wereaain achieved inside the specimen.
Tire time to achieve steady-state conditions
inside the specimen varied throughout the test.
During the early stages this time was about three
hours, but as tire fluid loss ir.creased, so did the
time interval, until near the conclusion of' the
test several days were needed to restore steady-state
condi tjon.
When steady-state conditions had again been
attained inside tie specimen (indicated by constant
readings of pore pressure with tiive) the specimen
values of pore pressure, temperature and weight were
recorded. The release v,.ilve was then opened and
more fluid allowed to escape. As mentioned earlier,
in the early stages the fluid escaping was accompanied
by a characteristic hissing sound, arid the valve was
only left open for a few seconds for a significant
moisture loss to be obtained. however, during the
latter stages of the test the release valve was left
open for periods up to one hour before a measurable loss
in weight was recorded.
Towards the end of the tests, when it became
difficult to obtain moisture loss even when the
release valve was left open for a considerable period,
it was obvious that considerable drying had taken
place in the pores of the specimen. Consequently, to
obtain the pore pressure reading, the temperature of
+ _othe specimen was cycled by - C around the test
temperatare, and readings of pressure and temperature
taken (see Figure 5:5). The pore pressure value
was tbEn obtained by interpolation.
The tests were regarded as complete when no
more moisture loss could be obtained when the release
valve was left open for one hour. The total time
taken for th test to be completed varied from specimen
to specimen, and depended uçon the amount of fluid
that could be driven off. tt the end of the test
the specimen was dismantled and the steel sealing
I'I -
)acket removed, leaving the thermocouples and their
leads intact. The specimen was tb-n weighed and
dried to constant weight at the test temperature.
Samples from some of the specimens were then removed
for study under a scanning electron microscope.
Run in conjunction with the test specimens at
Loth ages were control specimens. These were used to
determine the evaporable and non-evaporable water
contents of the test specimens. The control specimens
were exactly the same as the test specimens and cast at
the same time. These specimens were generally run at
an age that corresponded to the average age of the
particular test series. The sealing jackets were rem-
oved, leaving the thermocouples and leads intact, and
the specimens were then wehed and dried to constant
weight at lO 0C. The temperature was then raised to test
value. From these results, and those obtained from the con-
trol test specimens, the evaporable and non-evaporable
water contents of the specimens were calculated.
For the comparative studies of tie micro-
structure using the scanning electron microscope,
pecimen 9, the control specimen for the test series
of an average age at test of 3t5 days was dried to
constant weight at 200°C, and samples taken for viewing.
Similarly, specimen S from the pilot studies, which was
of comparative age to the older test series, was used
to provide unheated samples for viewing under the
scanning electron microscope.
5-111. PRESFNT\T1OT\ OP RUSULTS.
(a) Control SiLecimens.
The purpose of the control specimens was to
calculate the proportions of evaporable and non—evaporable
water in the test specimens. One accepted definition
of non—evaporable water is that water remaining in
the concrete when it is dried to constant weight at
105°C. The test specimens, except for the 105°C
specimens, were all subjected to a temperature higher
than 105°C, and one would expect a certain amount of
non—evaporable water to be driven off.
In order to determine this quantity, the
control specimens were first dried to constant weight
at a temperature of 105°C. They were then re—dried
to constant weight, first at 125°C, then 150°C and
lastly 175°C. One would expect the ratio of the
water remaining to the weight of cement left in the
control specimen to be the same as the value obtained
when the test specimen was dismantled and dried to
constant weight at the test temperature. Knowing
the weights of mixing water and cement in both the
control specimens and test specimens (from the mix
proportions), the amount of non—evaporable water given
up in the release test was calculated. By adding
this value to the amount of water left in each of the
test specimens after drying to constant weight at the
test temperature, the total amount of non—evaporable
water in each test specimen was obtained.
The evaporable water was calculated by
subtracting the non-evaporable water calculated above
From the total mix water in each specimen. From the
mix proportions, the theoretical amount of non-
evaporable water for 100% hydration was also calculated,
and using this figure, the degree of hydration in each
specimen determined. From the degree of hydration,
the quantities of gel and capillary water were also
found.
The amounts of non-evaporable water driven
off from each control specimen at the various tempera-
tures are listed below in Tables 5:2 and 5:3. Also
included are the ratios of the amount of water left
to the weight of cement in both 'the control and test
specimens after drying to constant weight at the test
temperature.
T.BLE 5:2.
Specimen o. 9 - ge at Test 385 days.Total Weight of mixing water l3t grams.
Weight of cement in specimen 22b gramsWeight of water driven off when dried at 105 C, 87 gr.
mount given Ratio of water Ratio ofoff when re- left to weight water leftdried at t°C of cement in to wt. of
Temperature
t°Cgrams. control sped- cement in
men test speci-men.
0105 C 0 ' 0.218 0.219
125°C 5.0 0.194 0.190
150°C 10.0 0.172 0.177
175°C 12.0 0.lt,3 0.158
THLE 5:3.
Specimen t'o. 13 - at test 52 days.Total Weight of Mixing Water 137 gramsWeight of cement in specimen 228 gramsWeight of water driven off when dried at 105 C, 91 gr
.niount given Ratio of water Ratio ofof? when re- left to wiight water left
Temperature dried at t0C of cement in to weightt°C grams control speci- of cement
men in testSpecimen.
105°C 0 0.201 0.200
125°C 2.5 0.190 0.l8
150°C 7.5 0.11)8 0.11)4
175°C 8.5 0.1b0 0.15o
(b) Results of Test specimens.
Graphs of Pore pressure against temperature
during the heating cycle of the test specimens were
plotted for each specimen. Figures 5:1 and 5:3 show
the relationships of gauge pressure against temperature
for specimens ?os. 1) and 14. The curves for all the
other test specimens can be found in ppendix I.
Typical graphs of gauge pressure against
weight of water remaining in the sjeimen are shown in
Figures 5:2 and 5:4 for specimens Nos. t.i and 14.
On these graphs the amounts of evaporable and non-
evaporaole water in each specimen are plotted, as
are the quantities of gel and capillary water in each
specimen, and the weight remaining after drying to
constant weight at the test temperature. .tlso
marked are the proportions of .ctive, Passive and
Bound water, which are defined later in this chapter.
The horizontal scale at the top of each graph gives
the ratio of the water content per unit weight of
cement. lso plotted on these curves are the time.
after heating, in days, to illustrate the time scale
for the testing of each specimen. The curves for
all the other test specimens are also Pound in ppendix
I.
Figure 5:3 illustrates, for Specimen o. 10,
a typical set of graphs of pore pressure against
temperature obtained when the temperature of specimens
was cycled avound the test temperature at various
weight losses.
Figures 5: and 5:7 compare the behaviour
of Gauge Pressure against total water content per
unit weight of cement at the various test temperatures
For specimens of approximately the same age. These
were plotted front the individual curves obtained
during the tests.
Pigures 5:8 to 5:11 compare the behaviour
of t auc Pressure against total water content per unit
weight of cement at various ages for test specimens
at the same test temperature.
Pinally, Table 5:4 gives a summary of the
ratios of evaporable and non—evaporable water to the
weights of cement and the degree of hydration in the
various test specimens.
TBLE 5:4.
e at Temperature 14start at release c /W Degree of
Specimen Cof of iioxsture Hydration
Nuni ler test
__________ (days) _____________ _______
403 175°C 0.388 0.212 0.923
7 3b1 150°C 0.380 0.220 0.95t
8 30u 105°C 0.38u 0.214 0.930
10 370 125°C 0.383 0.217 0.945
11 39 150°C 0.40à 0.194 0.84
12 f.7 105°C 0.401 0.199 0.85
14 31 175°C 0.4l 0.184 0.803
15 31 105°C 0.408 0.192 0.838
it., 74 125°C 0.504 0.195 0.848
These were calculated from the measurements
made at the end of each test, and so correspond to
the state of the concrete at this stage of the test.
3-TV. DISCUSSION OP IES1ILTS.
(a) Control Specimens.
The most important point to be noted from
the results of the control specimens are the ratios
of the weight of water remaining in the specimens to
the weight of cement when the control specimens were
dried to constant weight at the various test temperaturt.
,s tables 5:2 and 5:3 show, these values are in good
agreement with the values obtained in the test specimens
when they were dried to constant weight at the test
temperature after the test.
Prom this one can conclude that the drying
techniques employed do not affect the proportions of
the various types of water in the specimens. It
also indicates that all the concrete tested around
the same ago is very similar. This shows that the
behaviour of all the test specimens during the period
of curing was similar, and that the casting and sealing
techniques used dttring curing were satisfactory. It
also shows that the degree of hydration of the test
specimens were dependent upon time of curing only.
third point to emerge from the control
specimen tests is that the amount of non—evaporable
water (chemically bound water) thtt can be driven
off from a specimen when heated above lO°C is
dependent not only upon the temperature (the higher
the temperature, the more water that can be driven
off), hut that the technique used for driving off
the water also is an important factor.
It is interesting to note that no more
moisture loss could be obtained by breaking the seal
of the specimen in the test specimens, but that when
this seal was totally removed, more moisture could be
driven off by drying in the oven, even though time
amount driven off was often small, especially at the
high test temperatures. This again leads one to
suspect that different mechanisms are responsible
for the loss of moisture from concrete at elevated
temperatures when different drying techniques are
employed.
(b) Idealised model of a sealed porous material
sut)jected to i "elease" Test.
Before examining in detail the actual results
from the 'elease Test specimens and their significance
it is worth considering an idealised model of the
experiment consisting of a sealed inert porous material
containing in its pores liquid water, saturated water
vapour and air, subjected to a "elease" test at a
constant and elevated temperature. The pores of the
body are assumed to have a finite volume and be inter-
connected. The pressure inside the pores is taken
to be equal to the sum of the partial pressures
exerted by the entrained air and water vapour.
If the temperature of the sealed porous
material is raised, then the pore pressures inside
the body will rise. This is due to several effects.
rirstly, the saturated vapour pressure of water
increases with temperature, and this will therefore
increase the pore pressure. Secondly, raising the
temperature oP the mass of air, and also decreasing
the volume occupied by it due to the expansion of the
liquid inside the pores, will result in an increase
in the partial pressure exerted by the air. This
increase in pressure of the air depends upon the
ratio of the volumes of liquid water to air inside
the pores at the start of heating, and also the increase
in temperature to which the water and air are
subjected. This can be examined in a theoretical.
manner assuming that the pore volume remains constant.
Pigitre 5:1.2 shows the gauge pressure of air and how it
varies inside a pore as the ratio of the initial volumes
of liquid water (V1 ) to air (V) is varied (V1/V)i.
The points on the curve are obtained by first fixing
the ratio of the volume of liquid to air in a pore.
The temperature of the pore is then raised (in
plotting Pigure 5:12 four temperatures were chosen,
_o o 010 c, 12 C, 150 C and 173 C to correspond to the
temperatures used in the release test specimens),
and from steam Tables (89) it is possible to calculate
the volume of liquid that turns into steam. This,
at all Four temperatures, was a very small quantity
compared with the original volume of the liquid.
Then, using the coefficient of cubical expansion of
water values obtained from Kempe's Engineers Year
Book for 1975 (90), the expansion of the rest of
the liquid water was calculated. This then gave
the volume which the original mass of air in the pore
now occupied, and by applying the Gas Equation, the
gauge pressure of the air was calculated. The
value of gauge pressure when the value of
0, is the value one would expect if there
was no volume change of the liquid water. Clearly
one can see from Figure 5:12 that as the ratio oP
(V/Vg ) increases (i.e., the pores become more
full of liquid water), the partial pressure exerted
by the air increases considerably from the case when
no volume change of the liquid water is taken, and
that also this increase is temperature dependent.
This increase in air pressure also has the
effect of "compressing" the liquid. That means
that at equilibrium, the pressure exerted by the
liquid is greater than its saturation pressure, at
which under atmospheric conditions the liquid would
Vapourise.
We now consider the sealed porous material
to have reached steady—state conditions at an elevated
temperature with conditions in its pores as described
*bove. The seal of the specimen is then broken for
a few seconds and a certain amount of fluid, in the
form of steam and air, is allowed to escape. This
"J? elease" of fl ui d can be considered in thermodynamic
ternis to be similar to an isothermal expansion (i.e.,
as the body is at a constant temperature, the mass
left will occupy a larger volume that it previously
did and has effectively expanded). The fluid
given off should in theory be air, but in practice
is air and steam, as the act of "releasing" is similar
to that of "throttling". The mass of air inside the
body will be reduced, and therefore so will its
partial pressure. The saturated water vapour pressure
remains constant. However, the reduction in air
pressure reduces the pressure inside the liquid
water, and hence its degree of compression. Some of
the liquid will vapourise to saturate the vapour,
replacing that steam that is given off in the "release".
The overall effect is a drop in the total pore
pressure by a magnitude equal to the reduction in air
pressure.
20'.
Further "releases" of fluid will cause more
drops in the value of the pore pressure. However,
a point should be reached where nearly all of the
air will have been released, and the pores are mainly
full of saturated water vapour and liquid water.
The pore pressure will then be ual to the saturated
vapour pressure at the test temperature. luid then
released will be in the Form of steam. The action
of' allowing some of the saturated water vapour (steam)
to escape permits more liquid water to vapourise until
the vapour becomes saturated. The overall pore
pressure remains constant and equal to the saturated
vapour pressure at the test temperature while liquid
water remains in the pores.
However, as more fluid is allowed to escepe,
the situation arises where all the liquid water has
vapourised to steam. The pores are then full of
superheated steam. A further release of' fluid has
the effect that the density of the superheated steam
is reduced, and the total pore pressure begins to
drop. One would expect this drop to follow the
relationship between the density of superheated
steam and the pressure exerted by it at the test
temperature. Figure :l3 is a plot of absolute
pressure against saturated density of superheated
steam from Steam Tables (89) at various constant
temperatures. This shows that this relationship
is temperature dependent, the slope becoming steeper
the higher the temperature.
Figure :l4 shows in diagrammatic form how the pore
pressure varies inside the idealised model as fluid
is allowed to escape as described above at various
constant temperatures. The points marked "k"
are the maximum value of pore pressure measured.
This value is dependent upon the test temperature,
and the ratio of the initial volumes of air and
liquid water in the pores at the start of the test.
The slope and shape of the curve from the points
marked "V' to the points marked "B", the point at
which all the air has been expelled and the pores
are just full of liquid water and saturated water
vapour, depends upon several factors. The rate of
fluid release and the time for which the seal is
broken influence the position of point "B".
The slope of the line arid the position of "B" are
also dependent upon the permeability of the material
to the escaping fluids. In a material with large
interconnected pores, the point "R" would be a
definite point on the curve, with an abrupt change
of slope. flowever, as the pores get smaller, and
become only partially connected, the shape of the
curve would be more as indicated by the dotted lines
shown in Figure 5:14. The test temperature will,
of course, affect the positionof point "B" in
relation to the vertical axis, and should correspond
to the saturated vapour pressure at the test tempera—
tire. If ore considers that the liquid in the pores
is water and that the porous ma terial is inert, the
effect of the test temperature on the posi tion of
the point "B" in relation to the horizontal axis
is difficult to Forecast. Xs the porous material
is inert, the test temperature should have no
effect Upon the pernieability of the material to the
escaping fluid, and so the only factor the test
temperature could influence is the rate of fluid
release. One would expect that for any given time
for which the seal is open, the higher the tc-:npecature
tic faster the air will be expelled, and so the steeper
the line -R will be. However, one would also
expect the higher temperature specimen to lose
stcam a well as air in this part of the test, and
this of course will have the effect of lessening the
slope of . -B, since the position of "B" will irtove
to the left.
The points marked "C" on the diagram are
thc situations when the density of the superheated
steam are such that the saturated vapour p ressure
can just be sustained at the test temperature.
tgairi, this will be a definite point in a specimen
with large interconnected pores. The dotted lines
correspond to specimens with small pores that are
only partialiy interconnected, and in these
specin'cna point "C" is harder to define. s
tLe test temperature increases, the point "C" should
move to the right (i.e., the saturated density of
superlieated steam at any given temperature to sustain
stui't vapour prcsure increases with temperature
as seen in Figure 5:13). Similarly, the slope of
the line C-D (the point "D" beir.g defined as the
point when the pore pressure is just above atmospheric
pressure and no more fluid can be driven off using
this "relcae" technique) will increase as the test
temperatire increases. For the case of idealised
model of an inert porous material, the point "D"
should ie the same, regardless of the test tempera-
ture. The dotted lines shown in Figure 5:14
illustrate what appears to happen in practice in
concrete Specimens.
In the next section, a closer examination
of the differences between the idealised model
described here and concrete specimens in practice
is made. However, by first looking at such an
idealised model, a general insight into the type
of behaviour one should expect to occur in a sealed
concrete specimer1 is gained.
(c) Comparison of the Idealised Model and the
.\ctual Concrete Specimens.
When building the idealised model of a sealed
porous material subjected to a "Release" test,
several assumptions were made to simplify the situation.
However, in actual practice when considering a sealed
concrete specimen subjected to a "Release" test, several
of the assumptions made for the idealised model are
not valid, and hence must be taken into account
when studying the experimental results.
-aes
The main assumption made when considering
the idealised model was that the porous material
was inert. This meant that the porous material
was unaffected by the increase in temperature,
did not react with, the liquid water contained in
its pores and was physically unaffected by the
loss of liquid and vapour from its pores.
flowever, concrete cannot by any means be consid-
ered as inert. Firstly, a sealed specimen of
concrete will expand when heated. There is also
often a differential expansion between the cement
gel and the aggregate, and this can lead to niicrocracks
being formed between the cenient gel and aggregate,
so effectively increasing the pore volume.
Secondly, the cement gel itself will expand, and
this increases the size of the pores inside the
gel (i.e., the gel pores) but will slightly deaea.se
the volume of the capillary pores. The overall
effect upon the concrete is to increase the pore
volume with heating.
second important difference in concrete
as opposed to the idealised model is in the pore
structure. .lthough the pore volume in concrete
can be considered as finite, its value
varies with the degree of hydration. also, there
are two types of pore in concrete, capillary and
gel pores. The capillary pores are on average
between 10 and 1000 times larger than the gel pores,
whose average diamater has been estimated as 18
Depending upon the degree of hydration, the
water/cement ratio of the niix used and the amount of
compaction during casting, the capillary pores can
be interconnected. Hydration increases the solid
content of the paste, and in mature and dense paste
the capillaries may become blocked by gel and seg-
merited, so that the capillary pores become inter-
connected solely by gel pores. The degree of
interconnection between the capillary pores will
affect the concretds degree of permeability to
air and water vapour, and so influence the ability
for the fluid to escape when the seal is broken.
This in turn will influence the shape of the
curve at the two ends of the plateau (points "13"
and "C" on Pigure 5:14). s was seen in the
idealised model, if the pores are interconnected
and fairly large, the points "B" and "C" are
definite points. Uith a material such as
concrete, the slope of the curve changes slowly
and the points "B" and "C" are less well defined.
Both the capillary and gel pores can
be partially or fully filled with liquid water
(called capillary and gel water respectively).
Entrained air is also present due to incomplete
compaction (there will always be some entrained
air in a fully vibrated concrete) and due to the
reaction between the non—evaporable water with the
'2.04.
unhydrated cement causing a decrease in the volume
of solids. -s has been seen in the idealised model,
the initial ratio of the volumes of liquid to gas
in the pores has an important influence upon the
pore preseure attained when the specimen is heated
before any fluid is released.
When considering this ratio, one must
decide how the expansion of the gel and capillary
water, when the concrete is heated, affects the
volume occupied by the entrained air. Sharp (10),
using sealed specimens similar to those used in this
test series, studied the variation of pore pressure
with temperature. From this study he concluded that
the pore pressure was not affected by the volumes of
the non-evaporable or gel water, but only affected
by the relative volumes of capillary water and
entrained air. This means that one can discount
the expansion of the gel water inside the gel pores
and only consider the expansion of the capillary
water as influencing the volume occupied by the gas
on heating. Consequently, knowing the volume of
the porous plate, and calculating the volumes of
cipillary water and gas, it is possible to make an
assessment of the initial ratios of the volumes of
liquid to gas in the specimens.
V Y V= :cap + pw
(V) Vg pg
where V volume of capillary watercap
V = volume of water in porous platepw
= volume of gas in porous platepg
V = volume of gas Pilled capillaries.
VW /pw (O.t - O.42h)cap C
V = O.058h Wg-
pw
where W = Weight of cement in specimenC
PW Density of water
h = Degree of hydration.
Tatle 5: lists the ratios of(V../V).for the test
specimens used in the Release Test series. The two
values given for each specimen are assuming that the
porous plates are either completely saturated with
water or completely dry, arid are the upper and
lower values for each specimen.
•ao
T'BLI3 5:5.
(V/V).when (V/V).1 gi 1 giwhenSpecimen
No. V = 0, porous V 0, porouspw pg
plate completely plate completelydry saturated
() 2.t)0 4.50
7 2.38 4.08
8 2.55 4.40
10 2.45 4.21
11 3.18
14 3.53 t).23
15 3.21 5.t.9
it) 3.15 5.52
In practice, the porous plates are probably only
partially saturated, and the ratio lies somewhere
between the two extreme values given. nother
possible way of assessing the ratio of (V1/V,).
is from the miJration series test specimen final
water distributions. These are given fully in
Chapter six, but Pigure 5:15 gives in diagrammatic
form the shape of the distributions outained. The
ratio of the amount of evaporable water at the end
of the test to that value obtained by subtracting
the non—evaporable water from unity in th areas
where there appears to be a gain in water content
gives an idea of how much space in the pores was
occupied by gas, and hence the initial ratio of
V 1 / Vg•
\nottier major difference to note between
tLe idealised model and concrete is that the liquid
inside the pores is not pure water, but contains
dissolved salts. Powers and Brownyard (43), in
studying Adsorption Isotherms of hardened cement
paste specimens found that the dissolved salts in
the water had the effect of reducing tue vapoui'
pressure. The salts found in cement paste are
Calcium Hydroxide, Potassium Hydroxide and Sodium
Hydroxide. In practice, the solubility of Calcium
Hydroxide is so low that it can be neglected.
However, the soluhilities of Potassium Hydroxide and
Sodium Ilydrox ide must be considered, and these
solubilities increase with temperature, as, shown
in Tigure 5:lb (91). An assessment of the amount
of dissolved salts can be made from a knowledge
of the chemical composition of the cement used.
The cement used throughout tile practical work
described in this thesis contained 0.3% ?a. ) 0 and
0.35% K0 by weight. These correspond to 0.0019 grm/
grm of certent of Sodium Hydroxide (NaOH) and
0.0021 grifi/grin of cement of Potassium IJydroxide (KOui).
These figures mean that if all the cement is
hydrated, there is 0.004 grm of soluble salt available
per gramme of cement. However, the proDability of
small quantities of Na 20 and K 90 being present in
the limestone aggrc.gate is quite high, and this
could give rise to a nuch higher figure of
dissolved salt. however, if as a first assessment
we neglect any soluble alkali present in the
aggregate and assume that the proportion of
dissolved alkali salt is related directly to the
degree of hydration, the molar concentration of
the soluble salts in the evaporable water is
approximately 4 gram mols/litre of water.
Table S:t (92) gives the depression of the value
of the siturated vapour pressure of pure water at
van otis molar concentrations of dissolved salts
thiit can occur in the water inside concrete.
T[LL
Concentration Depressioii of s.v.p. in lb/in 2 forgram inol/ given salt.ii tre ofwater Calcium Calcium Potas- Sodiumsi.umbuiphate Chloride hlydrox-
liy d r ox - ide.ide__________
0.5 0.19 0.32 0.29 0.22
1.0 0.44 0.7b 0.57 0.44
2.0 1.08 1.84 1.23 0.93
3.0 2.05 3.22 1.91 1.49
4.0 - 4.66 2.70 2.07
5.0 - h.17 3.50 2.t9
- - 4.4t 3.45
8.0 - - ô.l9 4.87
10.0 - - 7.7o 6.28
With a mixture of Potassium and Sodium
Hydroxide the depression will be somewhere between
the two figures given for the individual salts (92).
Clearly, the presence of soluble salts in the
aggregate will increase the molar concentrations
and one would expect that these would rise with
temperature. The overall effect of the dissolved
salts will be to depress the value of the pore
pressure for the plateau of' the curve between the
positions "3" and "C" on the Release Test graphs
(see Figure 5:14).
When building the idealised model, all
the water in the porous material was considered to
be free liquid contained in the pores. However,
as doscrioed in Chapter Two, the water in concrete
is held in a complex state, ad it is worth nOtinJ
the differences between water in concrete and that
in the idealised model at this stage. Basically,
water has been arbitrarily divided into two states
(43):
(i) Evaporable water
(ii) Non—evaporable water.
The Evaporable water is considered to be
made up of the capillary water and gel water, while
the non—evporable water is approximately equal to
the chemically boind water. .s stated, these
states are arbitrary, and one really must consider
water to be held in a continuous varying degree
of firmness from tight chemically bound water in
the hydrate products to free liquid water in the
capill4rY )Ores, as shown in Figure 5:17.
Powers aid Brownyard (43), when studying
the fixation of water in concrete, looked in depth
into the zone between the chemically bound and
adsorbed or cl water. They identified three types
of water in this zone held by different forces.
(i) ZEULITTC tJ%TER: This water is
rearded as being packed between layers
of crystal or in the intorst.ices of the
structure. It may be removed without
giving rise to a new solid phase and gives
a smooth Isobar on dehydration.
(ii) LTTICE WATER: This is water of
crystallisation in a hydrate that cannot
be supposed to be associated chemically
with the principal constituents of the
crystal lattice. It represents the
borderline case between the chemically
bound and zeolitic water and the Isobars
and Isotherms for this type of water are
stepped.
(iii) ADSORBED WATER: Physical forces
known as Va der Waal forces hold this
water to the outer surface of the hydrate.
This can be a large proportion of the
water in concrete if the specific surface
of the hydrate is large. These forces
get weaker as one moves away from the
-
surface of the hydrate. This water
represents the ease between the zeolitic
and capillary water.
The amounts of Zeolitic and A.dsor bed water
were found to be dependent upon the temperature and
pressure of the water Vapour surrounding the solid
and these amounts vary continuously with changes
in either pressure or temperature.
On tue graphs of gauge pressure against
weight of water remaining in the specimen for the
individual Release test specimens (Figures 5:2
and 5:4 and in ppendiz I), the proportions of gel
and capillary water were calculated knowing the
degree of hydration and using equations given in
Chapter Two and were marked on the horizontal scale.
Clearly, these do not appear to relate closely to the
position of C, the point when the density of the
superheated steam cannot sustain the saturated
vapour pressure of the vapour in the pores any
loner. This leads one to suggest new arbitrary
definitions for water held in concrete in the
Release Test specimens:-
(i) \CTIVE WTER: This is the amount
of water in the specimen which, when lost
from a Rvlase Test specimen at a given
temperutire, results in the pore pressure
being less than the saturated vapour
pressure at that temperature. It is thn
quantity of water from "t" to "C" on the
horizontal scale in Pi.gure 5:14.
(ii) PSSIVE WTER: This is the
quantity of water that can be driven off
from a specimen at a constant temperature
when the pressure inside its pores is
below the saturated vapour pressure of
the liquid for that temperature, but
above atmospheric pressure. It is the
quantity of water from "C" to "D" on
the horizontal scale in Figure 5:14.
(iii) BOUND WATER: This is the
quantity of water that remains in a
Release Test specimen when the pressure
inside its pores is Just above atmosphere
and no more fluid can be driven off using
the Release test technique.
Depending upon the temperature and age of
the specimen, these types of water consist of varying
amounts of the types of water defined as Capillary,
Adsorbed, Zeolitic and Lattice, and this will be
examined more closely in the next section.
The last major difference to be considered
between the idealised model and the actual concrete
specimens is the portion of the curve from the points
"C" to "D" in !igure 5:14. When comparing the
practical results with the relationship between the
density of superheated steam and the pressure
exerted by th steam, the theoretical line is almost
vertical. The practical significance of this
point is that more fluid is given off for any given
pressure drop in the actual release test specimens
than in the idealised model, where the only source of
fluid was inside the pores. Clearly water that was
previously not influencing the pore pressure is
being released and filling the pores. The source
of this water must be from inside thehydrate
structure, and the amount driven off will be time,
temperature and pressure dependent. ts the Passive
water is removed, the effect on the hydrate products
will be a shrinkage, so causing an increase in'the
pore volume. This in turn will decrease the density
of the superheated steam and so the pore pressure
will fall. The situation is probably never fully
in equilibrium and during the actual expethnents,
when this part of the test was reached, the tempera-
ture of the specimen was cycled by about S degrees
Centigrade around the test temperature and the pore
pressure monitored. Thepore pressure taken was
found by interpolation of the curve obtained. A
typical curve obtained for specimen 10 is shown in
Figure 5:5 and this should be compared with Figure
5:18, which shows the relationship between pressure
and temperature at various densities of superheated
steam plotted Prom Stm Tables (89). It can be
seen that as the density decreases, the variation
in pressure with temperature decreases. This trend
also occurred in the experimental results; however,
the variation in pressure was much greater for any
given temperature change in the test specimens, and
that this variation increased with test temperature.
The curve obtained was not reproducable on cycling
more than once, but showed a hysteresis effect
that increased with the number of cycles. This
indicates that the behaviour of the Passive water
varies to some extent upon time, and also very much
upon temperature. Tue mechanism by which the
Passive water is driven from the hydrate is uncertain.
It probably has a connection with the free energy of
the passive water held by the hydrate and that of the
superheated steam causing the pore pressure. Powers
and Brownyard (43), when studying the Thezinynamics
of the \dsorption of water on flardened paste,
showed that the dsorption of water occurred when
the free energy of the free water or free vapour
greater than the free energy of the adsorbed or
capillary condensed water. Taking the converse
of this, the free energy of the Passive water must
be greater than the free energy of the superheated
steam, for it to be lost by the hydrate. The free
energy must be provided by the input of heat to
maintain the constant temperature, and so as the
test temperature increases, more passive water
can be driven off to contribute to the pore pressures.
The hysteresis obtained when the temperature is
cycled indicates that the release of the Passive
water from the hydrate structure is not a fully
reversible process. The fact that more water can
be driven off when the sealing jacket is removed
indicates the dependence of the Passive water upon
the pore pressure as well as the test temperature.
t the lower test temperatures, when the gauge pore
pressure was never high, as soon as the pore pressure
was just above atmospheric, no more fluid could be
lost in the "release" test technique, and hence no
more Passive water was lost by the hydrate to the
pores. However, when the jacket was removed and
drying performed at atmospheric pressure, more water
could be driven oN'. Obviously, a different
mechanism Causing moisture loss was at work.
the test temperature increased, the amount driven
oN' after the end of the Releis Test decreased.
Clearly, the higher temperatures and pore pressures
affected the amount of Passive water that could be
"released", and this along with the trends and
amounts of ctive, Passive and Bound water, are
examined closely in the next section.
(d) hxamination of Experimental Results.
In all the specimens, the maximum recorded
press'ire was higher than the value calculated
assuming that the volume of the gas inside the pores
rentined constant after hati ng. Table 5:5 in the
ort-vious section lists the initial ratios of the
volumes oP liquid to gas inside the pores of the
specimens assuming the porous plate was completely
dry ( V= 0) and also completely saturated
(V = 0). Ejsing the relationships plotted inp
Figure 5:12, it is possible to assess the increac.
in gauge tcure above the expected theoretical
gauge pressure assuming no decrease in the VolUme
occupied by tie gas. T1iee are. listed in Table 5:7
1oiig the actual experirneiital I ncruases in gauge
pressure above the theoretical gauge pressure
assuming no volume change.
T.PLL 5:7.
Specimen Test Theroretical increases ctua1 Ex-nur:ier Temper- from Figure 4:12 ierimental
ature. increases
V =0 V =0
0175°C 8.83 lb/i r 22.13 lb/in 2t . t lh/jn
7 150°c 5.18 " 11.58 8.5
8 105°C 2.34 " 4.34 5.8
10 125°C 3.34 7.04 " 7.0
-11 150°C 7.78 " 19.48 ti.0
14 17°C 14 23 " 4u.23 " 21 5
105°C 2 74 " 5.34 " 0.1
125°C 4.74 " 10.44 1.0
Clearly, the increase in the maximum recorded pressure
can be explained by an expansion of the liquid water
in the pores so causing a decrease in the volume
2:2o.
occupied by the entrained gas, and hence an increase
in the pre3sure exerted by it. J'owever, the actual
nature of the liquid water causi rig this expansion is
not clear. In calculating the ratios (V1/V).
for the test SpeCifliens, only the volume of
capillary water was considered as expanding on
Jie. .t ti ng. Por the older test series (specimens €,
7, 8 and 10) all the experimental increases are much
higher than for the case when the porous plate is
considered dry, arid for specimens ti and 8 th€ increases
are even greater than the theoretical values when
considering the porous plate completely saturated.
Clearly, tkercfz'e, it is not correct to assume
what is generally krtown as the capillary water alone
expands on heating to decrease the volume of the
entrained gas in the pores, and that the liquid
water corresponds more likely to the Active water.
This is as defined in the previous section, and
appears to be temperature and age dependent for any
specimen. The age dependence is illustrated by the
fact that th younger test series specimens, whose
proportions of Active water are much lower than the
older ones (see Table 5:8), have much lower
experimental increases in the gauge pressure for
any given test temperature. If one considers that
it is the capillary water that expands, then the
opposite case shodid occur, since it is normally
hi.ld true that for any given water/cement ratio,
the proportion oL' capillary water coes down with
age. .ctive water on the other hand appears to
increase with ae and test temperature.
Table 5:8 lists the ratios of Active water
Passive Water (Wp/WC) arid Bound water
(WB/W) to the unit weight o cement for each
speCi.nteri, where
+ W += WT Total mix water
and
W /W + W /W + W /W W JW = 0.(, the.' c P c B c T c
original water/cei'ent ratio and the Active, Passive,
and found waters are defined as in the previous
sec tion.
TBLE 5:8.
Specimen Test tempera- W/Wnumber ture
8 105°C 0.298 0.02o 0.27b
10 125°C 0.337 0.059 0.20.4
7 150°C 0.341 0.077 0.182
175°C 0.327 0.092 0.181
15 105°C 0.05( 0.024 0.520
125°C 0.290 0.053 0.257
11 350°C 0.300 0.120 0.180
14 175°C O.29t 0.128 0.17
Pirst of all, comparing specimens of the
same average age but different test temperature,
the proportion of Active water to unit weight of
cement increases with test temperature when
comparing the 105°C, 125°C and 150°C specimens at
both ages. The 175°C specimens at both ages
show a slight decrease in the ratio of ctive water
to the unit weight of cement when compared with the
0150 C specimens of the same age. However, when
comparing just the 125°C, 150°C and 175°C specimens
of the same age the ratios 14 /14 c appear to be
fairly constant. In a comparison of specimens
tested at the same test temperature but of different
ages, the older specimens exhibit a greater
proportion of Active water. The fact that for any
0given age, the proportion of active water for 12 C,
150°C and 175°C specimens appears constant, and
that this proportion increases with age, perhaps
gives some insight into the nature of Active water.
s hydration increases, the specific surface
of the hydration products increases quite consider-
ably. This in turn leads to a larger surface area
to which the water described in the previous sectiion
as "Adsorbed water" (i.e. , the water held to the
surface of t1e hydrate by physical forces of attrac-
tion known as Van der I4aal forces )can exist.
1so present between the interstices of the hydrate
structure is what was described in the previous
section as Zeolitic water. Up to 105°C, the Active
water probably consists of the capillary or free
water inside the pores plus the water held to the
surface of the hydrate by physical forces. However,
at some temperature between 105°C and 125°C, the
zeolitic water is released from between the hydrate
structure and contributes to the Active water. The
reason why the older specimens have more Active water
is probably due to some form of free energy considera-
tion. The younger specimens wilihave more free or
capillary water and less Adsorbed jnd Zeoli tic water.
As stated in the previous section, the free energy of
the Adsorbed arid Zeolitic water must be greater
than that of the ctive water for it to be released.
In the case of the younger specimens, the increase
in temperature couplcd with the larger volume of'
free water initially increases the free energy of
this water to a greater extent than the Adsorbed and
Zeolitic water, so less of these types contribute
to the ctive water.
The ratio of the Bound water to the unit
weight of cement decreases as the test temperature
increases for both series of test specimens. This
is as one would expect, since the Bound water is that
amount of water left that cannot be driven off using
the release tost technique. Since the amount of
fluid lost by this technique will depend upon the
test temperature, the pressure gradient between the
pores of the specimen and the atmosphere when the seal
is broken, and the coefficient of permeability of
the specimen to the movement of steam and air at
various pressure and temperature gradients, compari.-.
cone between differing ages at the same test
temperature are a little harder to make. The 105°C
specimen from the younger test series appears to
have a high amount of Bound water compared with the
same test temperature specimen of the older test
series. The 105°C specimens of both ages, and
especially the younger age appear to show inconsistencies
and are dealt with in the next section. The amount of
(._OBound water in the younger aged 12 C specimen is
more than in the older specimen. This illustrates
the influence of the pore pressure gradient on the
quantity of Bound water. The depressionof the
pressures compared with those expected at this test
temperature from Steam Tables for pure water, was
on a percentage basis greater than for any other
specimen of the same age. Therefore, when comparing
the younger and older 125°C specimens, the pressure
gradient in the younger was much less, and this
probably explains why lees fluid was lost using the
Release Test technique.
The amounts of Bound water in the 175°C and
150°C specimens of both ages are approximately the
same. Por these specimens the pressure and temperature
gradients when the seals are broken are high, and
any effect of age is probably removed by these, giving
consequently roughly the same amounts of Bound water.
By definition, the amount of Passive water
is the difference between the sum of the ctive and
Bound water subtracted from the total mix water.
For all the specimens the amount of Passive
water increases in each test series with an increase
in test temperature. The amount of Passive water
appears to decrease with age for all the specimens.
This is consistent with the increase in .ctive water
in the older specimens, and means that the slope of
the line C-I) (i.e., the.drop from the point where
the saturated vapour pressure cannot any longer be sus-
stained to the point where no more fluid can be lost
using the Release Test technique) increases with age.
However, it should be noted that the upper
limit for the amount of Passive water in any specimen
is the total mix water minus the sums of the active
water and the amount left in the specimen when
unsealed and dried at the test temperature under
atmospheric conditions.
In comparing the experimental results with
the idealised model, one of the significant differ-
ences is in the movement of the posi tion "C" (i.e.
the point where the saturated vapour pressure can no
longer be sustained) in specimens of the same age at
differing test temperatures. In the idealised
model, the position of "C" was determined by the
density of the superheated steam inside the pores.
').26
When the density feli below the saturated density of
superheated steam at that temperature, then the
saturated vapour pressure could not be sustained,
and as more fluid was lost, then the pore pressure
fell. This saturated density increases with tempera-
ture, and so as the test temperature increased, the
amount of ctive water decreased inside the idealised
model. however, this does not occur in the test
specimens of either average age. The position of
"C" remains fairly static with the various test
temperatures For the 123°C, 130°C and 175°C specimens
of the same age. However, when comparing the various
test temperatures at different ages the posi tion
of "C" moves to the right (i.e. , the pore pressure
sttrts to drop at a smaller weht loss) in th younger
speCimens whtn compared with the idealised curve.
This is contrary to what one would expect if it was
the capillary water alone that was generating the
pore pressures, since more capillary water occurs in
theyounger specimens. The position of "C" is
determined, as by definition, by the quantity of
\ctive water in the specimen. C. is the point
when all the ctive water has been driven from the
specimen using the elease Test technique.
Perhaps the most major difference between
the test specimens at various ages are the "pressure
plateaux" (the values of pore pressure whi c .i'.
constant while fluid is being lost, the line B—C on
riure 5:14) that in the idealised model corresponded
to the saturated vapour pressure of pure water at
the test temperature. Table 5:9 lists the saturated
vapour pressures.(gauge pressures) of pure water
for the test temperatures of the various specimens,
and th difference between these values and the
value or the pore pressure at the pressure plateau.
TA3LE 5:9.
Specimen Test tern- Satu1'ated VapOUr Difference ofnumber perature pressure (psig) pressure
p1attaii fromS • '. p.
6 175°C 115.0 psig + .0 psi
7 150°C 54.4 " + 3.0
8 105°C 2.8 " + 3.0
10 125°C 19.0 " + 3.0
11 150°C 54.4 " - 2.0
14 175°C 115.0 " - 9 0
15 105°C 2.8 + 1 0
lt 125°C 19.0 " - 4.0
There are several factors that may explain
the differences between the values of the pore
pressure at the pressure plateaux and the saturated
vapour pressure of pure water at the test temperature.
Two explanations that can be discounted however, are
errors in both the pressure and temperature
measuring devices. Firstly, it was thought that
the Prossur recording system was at fault in some
way. This was shown to be not to be so, as the -
pressure transducers were continuously checked
t)iroaghout the tests against a Bourdon Gauge, and
they were never found to be in error by more that
o.l.
The younger test series specimens were tested
during the late autumn in quite cold weather, while
the dder test series specimens were tested in the
late summer and early autumn, when the weather was
significantly warmer. The teiperature recording
meter had an inbuilt compensating cold junction.
This was checked over a range from 0°C to 200°C
against known temperatures and found to read to
+. 0tit1iin - k C. Clearly, therefore, any error in
the temperature meter could not explain the pressure
plateau differences.
The first factor that may explain the
depression of the pressure plateau below the satura-
ted vapour 'ressure of pure water in the case of
specimens 11, 14 and lb are the presence of dissolved
Potassi'n ydroxide arid Sodium flydroxide in the active
water. .s outlined in the previous section,
dissolved salts in liquid water have the effect of
depressing the saturated vapour pressure, and Table
S:b gives values for this depression in lb/in2.
uso shown in this table are the effects of Calcium
Sulphate and Calcium Chloride, which can also be
present in concrete in small quantities. t room
temperatures, the concentration of salts in the
active water was calculated, from the amount of
avLible soluble salt (from cement composition) and
the degree of hydration, as approximately 4 gram
mol/litre of water. However, at elevated tempera-
tures the concentraon could be higher due to the
increased solubilities of the salts. .tlso, the
aggregate Contains traces of alkali that will
dissolve if the temperature is raised (78). Clearly,
just assuming a concentration of 4 gram mol/litre,
r)from Table 5:6, a depression of 3 lb/in is quite
possible, and this may well explain the depressions
.2 .2of' 2 lb/in and 4 lb/in respectively for the
pressure plateaux of specimens 11 and 16. The
depression of 9 psi for specimen 14 seems a lot
harder to explain just by the action of dissolved
salts, although as Powers and Brownyard (43) noted,
the amount of dissolved salt varies greatly from
specimen to specimen, and specimen 14 could contain
an abnormally large amount, especially when one
remembers that its test temperature was 175°C.
However, when one considers the movement of the
pressure plateaux upwards of the other test specimens,
then perhaps the behaviour of all the specimens
becomes clearer.
The elevation of vapour pressure is explained
in thermodynamic terms by studying the dependence
of vapour pressure on the total pressure and applying
the Gibbs Function G, a measure of the free energy
of the system.
G = LI - TS
where H = Total Enthalpy of the system
T = bso1ute Temperature of the system
S Entropy of tIie system
By considering a system at constant temperature,
Sears (93) showed that if a small amount of gas
is introduced into a system containing liquid and
saturated vapour, then the total pressure and
vapour pressure are both increased.
\s reviewed in Chapter Two, several workers
have tried to explain the loss in strength of sealed
specimens of limestone concrete by a series of
chemical reactions. One of these that has been
suggested is a reversal of the Carbonation reaction
CaCQ3 + 21120= Ca(OH) 2 + 11 20 + CO2
This reaction could well be occurring inside the
release test specimens, especially when any equilibrium
was upset (i.e., after fluid was allowed to escape).
The Carbon Dioxide given off would increase the
total pressure, and hence the vapour pressure in the
pores. The amounts of' gas given off would vary
each time the equilibrium was upset, and hence the
vapour pressure would alter slightly. This,
along with the slight temperature fluctuations (due
to meter and thermocouple accuracy) could explain
the slight variations in pressure along the pressure
plateaux.
There is no experimental evidence to show
that either gas is present and being liberated
along the pressure plateau and hence elevating
its value, or that dissolved salts are present in
the liquid water and depressing the value.
However, there is a high probability that both
phenomena are present in all tle specimens, and
that whether the pressure plateau is elevated or
depressed depends upon the amount of dissolved
salts and the test temperature, which will effect
the liberation of gas.
(e) The lO°C Specimen Anomalies.
0The results of specimen 8, the 105 C
specimen of the older test series, exhibited the
same. shape of curve as the other specimens in its
age group. However, the amount of Active water
in this specimen was considerably less than the
other specimens in the test series, the opposite
to what one would expect from the idealised model.
It was this experimental evidence that confirmed
that the .ctive water was, to a certain extent,
temperature dependent. One oI' the reasons why no
more fluid co.ild be lost using the Release Test
technique was that the pressure gradient between
-a2.
the pores and the atmosphere, coupled with the
relatively low value of permeability of concrete
were not a sufficient driving force to permit
fluid to escape under the test condithns. However,
when the sealing jacket was removed, the amount of
extra water driven off revealed that this specimen
was compatible with the others of the test series,
and the difference in behaviour was solely due to
the test temperature. nother significant point
that can be concluded from the behaviour of this
specimen is that in a sealed specimen, moisture
migration caused by the pore pressures generated
inside the concrete due to an increase in temperature
is comparatively small, at 105°C, compared with
specimens heated to much higher temperatures. This
is due to several factors. The first is that tile
actual pressure gradient does not increase in a
linear fashion, but increases more mali xponeaitil
manner. The second ractor is that some of the
water contributing to the pore pressures in the
higher temperature specimens originates from the
hydrate structure, and loss of this water causes a
shrinkage of the gel, so increasing the permeability
of the concrete to liquid and water vapour, and
hence making migration easier. A third factor
is that the temperature of 105°C is close to the
boiling point of pure water. However, the water
inside the pores may well have contained some
dissolved salts which can elevate the boiling point
of the liquid inside the pores. This in turn will
23
ctsange the nature of the fluid trying to escape
when the seal is broken. Instead of being a
vapour, much of this will be liquid which could
not escape as easily.
The behaviour of specimens 12 and 15, the
105°C specimens for the younger ae test series, is
more difficult to try and explain. Specimen 15
was the first of these two to be tested. As can
he seen from the curve obtained (see ppendix I)
the general shape followed the expected pattern,
but th amounts of c tive and Passive water were
very srntll, leaving a large mass of Bound water in
the specimen. When the seal was removed and the
specimen heated in an oven at atmospheric pressure
at 105°C, the amount of water driven off left a
similar proportion of water to the original weight
of cement in the specimen, as the control specimen
of tI' e same age dried under the same conditions.
Consequently, a second specimen ( specimen 12) was
tested at 105°C. This specimen, however,
behaved in a comç'letely different manner to all the
other specimens. Firstly, the maximum recorded
pressure was 11 psi below the calculated value,
assuming that the vol ume of entrained gas remained
cons tint. Secondly, when the e] ease valve was
npenod, tie pore pressure agai nst weight loss drop
ehi hi ted no pressure plateau, and the quantity of
fluid 4iven off when the pore pressure reistcred
ZErO :tUC pressure ws, like specimen 15, small
compared with all the other test specimens. lfoweVer,
when the sc.tl was removed, again the amount of
moi sture rcm:iinin in the pccinin when dcicd dt
105°C was corn j atihlc wi. th the control specimen.
One explanation for the much smaller losses
of fluid usiri tie Pelease test technique with
S ? eCifl.eflS 12 and 15 is that the concentration of
ai ts in the ctive water was high eioagh to
elevate the Boiling 'oir.t of th liquid above 105°C.
When the seal was broken, the preosure and tempera-
turo gradicnt were not ufCicient to drive off the
quanti ties lost in the other specimens. This,
Powever, does not fully explai.t the behaviour of
specimens 1 and They remain an anonialy
ntOnt the other test specimens, and perhaps this
test temperature should be examined more closely in
any future similar tests.
5_V. SC ITLECT'OT\ 1' 1CTOCOII? FJ1OTOGT SPflS.
(a) Introduction.
Saniples of tile older test series specimens
test.d at 105°C, 150°C and 175°C were removed at the
end of the release test before the rest of th
&peCiii.efl was dried to constant weight at the test
temperature. These samples contained iouiid water
only jrid were prepared for vi ewing under the Scanriir.g
Electron 'icroscope as described in Chapter Four.
.Uio Vi e4d were samples taken front a elease test
specin.cn of the same ae that had rer'iained scaled and
unheated, arid a sanip] e from a specimen that was dried
to constant wiht at atmospheric pressure in an OVefl
at 200°C.
The photorahs presented show typical
feature, seen in these specimens. In general,
photographs were not taken of any special features
unless viewed at least three times in any given
specimen. Two ainplcs from each test specimen were
viewed to ct a more representative picture of the
mic ros truc ture.
(b) DumiuySpecinien esults.
The photoraphs for ti-is specimen are
Plat V - 1 to Plate V - t. Two types of needles
arc evident, and these almost certainly correspond
to Ettri r'jtc and Calcium Silicate Hydrate. The
small needles, rowir1g mainly in bunches up to about
in lcndth are almost certainly Calcium ilicatC
Hydrate as identified by Chatterji and Jeffery (31).
These are clearly seen in Plates V -1, 2, 3, 5 and
, the small needles appear to be growing from tie
el structure in rather random directions.
The larger needles on view in Plates V-2
and 3 are more than likely Ettringite. In Plate V-2
the are the thicker individual needles, about r m
in length and °3r ni thick. The main difference
between ttringite and Calcium Silicate Hydrate is
that the aspect ratio (i.e., thickness to length
ratio) is much larger for Ettrinito.
morphous "el-bodies" similar to those
observed by both Imlach (20) and Diamond (34) can
be seen in Plates V -1 and 2. These are almost
certainly just another form of Calcium Silicate
Hydra te.
The white amnorphoi's material seen in Plate
V.- 4 is similar to Calcium Silicate Hydrate Produced
artificially by Fajumdar and Speakmari (35). This
is lOrm across and is more titan likely a conjomorate
of hydrate.
Calcium Hydroxide can almost certainly be
seen in Plates V - 4 and 5. The "platey" material
to the right of the amorphous mass in Plate V - 4
is similar to that identified by previous workers as
Calcium Hydroxide. In Plate V - 5, the large, thick
"platcy" material seen front the side could be a crystal
of Calcium Hydroxide. This is about tm in diameter
and irm thick.
(c) 105°C Release Test Results.
The photographs for this specimen are
Plate V - 7 to Plate V - 10. The first noticeable
feature is a lack of clear defined needles.
atcrial in Plate V - 7 appears as a
honey-combed structure in small balls. These small
balls are between 2rm and 4rm in diamater, and
appear to be the central cores of structures similar
to those published by Chatterji, Moore and Jeffery (33).
-
These structures were like sea-anemones, and the
ones in this photograph seem similar with the ends
of the needles dropped off.. This behaviour is
also suspected of being illustrated in Plate V - 9.
The little stumps seen in the centre and right of
the photograph, between o.lrm and O.25rm in
diameter are thought by the author to be the bases
of broken needles. .\ll this material is thought
to be Calcium Silicate Hydrate.
The large body seen in Plate V - 8 is over
30r m in length and could be Calcium Hydroxide.
This is much larger than seen in any Dummy Specimen.
The large "platey" crystals in Plate V - 10, which
are between rm and lorm in diameter are also
thought to be calcium Hydroxide.
Large amorphous bodies, from about Srm to
m in maximum dimension, and "glassy" in appearance
can be seen in Plates V - 9 and. 10. These are
almost certainly some form of ( alcium Silicate
Hydrate.
(ci) 150°C Release Test Specimen.
The photographs for this specimen are
Plate V - ii to Plate V - it,. I'late V - 11 and
Plate V - 13 exhibit small spheres of amorphous
material from irm to 2rm in diameter. These give
the impression of a honey-combed ot-ructure, and the
g.zps measure from 600 to 2m.
Plate V - 12 shows the surface of the
aggregate which appears to have been chemically
attached. This phenomena appears throughout all
the limestone aggregate specimens, but was quite
prevalent in this specimen, suggestin, some chemical
reaction between either the cement gel or liquid
water and the aggregate.
Plates V -14, 15 and 15 should be viewed
together. Plate V - 14 shows a crater or void
in the aggregate surface, 4Orm across. Plates
V - 15 and 1€ are magnified photographs of the
material growing inside this crater. These are
clearly hexagonal and rhombic plate crystals, which
measure up to 2r m in diameter and in thick.
These areprobably Calcium Aluminate or Tetracalcium
Terroaluminate, although one or two of the larger
crystals could be Calcium Hydroxide. It is
interesting to note the haphazard way they appear
to be growing in the crater.
_o(e) 173 C Release Test Specimens.
The photographs for this specimen are
Plate V - 17 to Plate V - 20.
Plate V - 17 is very typical of this
specimen with a lot of debris lying on the aggregate
surface. The large amorphous material in the
centre of the photograph that measures 8rm by
4im is probably a form of Calcium Hydroxide.
Plates V - 18 and 19 should be viewed in
conjunction with each other. The large body in
Plate V - 18 is about m in diameter and could
well be a small speck of aggregate with hydration
products growing from its surface. Plate V - 19
is a close-up of' the previous photograph and the
hydrate products clearly consist of a number of
spheres or rhombic crystals up to about O.5 r m in
diameter bunched together into conglomorates.
The gaps between the bunches are up to °2r m
(2000 ) and therefore must be regarded as capil-
lanes. It is worth noting the sponginess and
porosity of' the hydrate and the absence of needles.
Plate V - 20 is a very high magnification
shot of' hydrate material. This also exhibits
small spheres or rhombic crystals less than °•2r m
in diameter clustered together to form "cotton-ooU'
balls 2rm to 4m across.
(f') 200°C heated Specimen Results.
The photographs for this specimen are
Plate V - 21 to Plate V - 24.
Plate V - 21 is a fairly low magnification
photograph showing amorphous material from about
lrm in diameter to over lorm in diameter with a
background of an aggregate particle. This structure
is also exhibited in Plate V - 22 at a slightly higher
magnification, although the material is in the same
range of size from m to m across. However,
the top left of this photograph indicates that
at onetime all the debris was in fact one continuous
amorphous mass as seen in the Dummy specimens,
and that the debris is the result of the heat
treatment.
Plates V - 23 and 24 show similar material
to that seen in the previous photographs. The
larger conglomorates are about t>r
m in diameter, but
on closer examination appears to be made up of a
mass of smaller spheres about O.lrm in diameter. In
plate V - 24, small stubs approximately O.5rm across
appear in the right hand side of the photograph.
These may well have been some form of needle structures
and from their size might have been ettringite. Ui
the amorphous spheres and crystals seen in the
specimen are almost certainly a form of Calcium
Silicate Hydrate.
(g) Comparisons and Conclusions from Scanning Electron
Microscope Photographs.
lthough no chemical and X—ray analysis was
possible on the samples viewed under the microscope,
the photographs themselves provide a useful comparison
of the microstructure of the different specimens.
These photographs sould also be viewed in conjunction
with those from the Hydration Test specimens and
Miration test series specimens seen later in this
thesis, as these would then provide .i vuch wider
comparative study.
The most obvious point to note from all the
photographs 18 that, except for small stumps visible
in Plate V - 24 (200°C heated specimen), no clear
defined needles of either Ettringite or Calcium
Silicate Hydrate appear in the specimens that have
been subjected to high temperature treatient.
Plates V - 7 and 9 both exhibit traces of small
stumps as though needles have broken off, and these
two photographs both come from the 105 0C Felease
Test Specimen.
second interesting feature to emerge is
that examples of Calcium Hydroxide seen appear to be
largest in th 105°C eleast Test specimen. The
solubility of Calcium Hydroxide, which although is
small compared with potassium ad Sodium Hydroxide
at room temperatures, decreases as temperature increases,
0(91) and is almost zero at 100 C, while the solubili-
ties of the other two salts increases. This means
that as the temperature of the liquid water is
raised, calcium Hydroxide crystallises out. Above
105°C, however, the temperature starts to break down
the physical structure of the crystals of Calcium
Hydroxide. This almost certainly explains why the
examples of Calcium Hydroxide are largest in the 105°C
Release Test specimen.
Comparisons between the loS°C elease Test
specimen photographs and the higher temperature test
specimen photographs shows a trend o a break-up of
amorphous gel with increasing temperature, giving a
honey-combed structure. This structure is made up
of conglornorates of si.iall spheres or rhmbic crystals
less than m in diameter that cluster together to
form larger bodies about 2 r m in diameter. This
appears to be consistent with the idea of ACtiVe and
Passive water coming from the Zeolitic and Lattice
water defined by Powers and Brownyard (43) and that
the loss of these types of water from the hydrate
structure is temperature and pressure dependent.
Plate V - 12 shows an example of some
chemical reaction between the cement gel or liquid
water arid the limestone aggregate. This is consistent
with the concept of chemical reactions inside the
specimens giving rise to dissolved salts and gases.
The photographs of the 200°C heated
specimen are a useful comparison. They show similar
microstructures to the 175°C Release Test specimens,
and it must be concluded that at this temperature,
the T e1ease test technique produces similar results
to drying in atmospheric conditions. This is also
borne out when one considers the amount of Bound
water that was driven of? when the 175°C Release
Test specimen was dried to constant weight at 175°C
in the oven was very small.
Overall, the photographs show a general
trend of a "break—up" 0 the white amorphous bodies
seen in the unheated and low temperature specimens,
and that the higher the test temperature, the
greater the break—up of the niicrostructure appears
0to be up to 175 C. There appears to be little
difference in the microstructure of the 1750C
Release test specimen and the 200°C heated specimen.
Teedles and lath-like structures do not appear in any
of the specimens heated to 105°C or higher tempera-
tures.
5-VI. COrCIUSIOf'S.
The pore pressures inside sealed cylinders
of limestone concrete heated to various test tempera-
tures at two average ages were measured by means of
an embedded porous plate and a pressure transducer in
a no-volume change method. The concrete specimens
were then kept at an elevated and uniform temperature
and fluid periodically allowed to escape. The pore
pressure inside the pores was plotted against loss
of fluid and the experiments were continued until
no more fluid could be lost by breaking the pressure
seal.
n idealised model of the experiment has
been constructed and its behaviour predicted from
a knowledge of the behaviour of a mixture of air,
liquid water and water vapour undergoing an iso-.
thermal expansion at an elevated and uniform tempera-
ture. The results of the actual Release test
specimens and the idealised model have been compared
and the differences in behaviour noted, and attempts
made to explain them.
From the results of the Release test
specimens, the water in these specimens has been
divided into new arbitrary states, known as 4ctive,
Passive and Bound water. The quantity of Active
water in a specimen is both temperature and age
dependent, and this state of water is made up of
water previously defined as Capillary water, plus
varying amounts of Adsorbed, Zeolitic and Lattice
water, depending upon the age of the specimen and
the test temperature. The quantity of Passive
water is mainly temperature and pressure dependent,
although time also appears to have some influence.
This water consists of varying amounts of Adsorbed,
Zeolitic and Lattice water with some chemically
hound water, depending upon the test temperature
and time of heating. The quantity of Bound water
is temperature and pressure dependent, and consists
of most of the chemically bound water with some
amounts of Lattice and Zeolitic water, depending
upon the test temperature.
The sum of the amounts of Active, Passive
and Bound water equals the total mix water in any
specimen.
The maximum recorded pore pressures in a
sealed concrete specimen depend upon the initial riio
of the volumes of liquid to gas inside the pores,
and the temperature to which the sealed specimen is
raised. The tctivc water expands and decreases the
volume occupied by the entrained gas, so increasing
the expected pore pressure.
The pressure plateau of the idealised model
corresponds to the saturated vapour pressure of'
pure water at the test temperature. This value of
pore pressure at the same test temperature can be
depressed or elevated in the Release Test specimens
by dissolved 8alt$ and gas.
Photographs of the microstructure of both
unheated and heated Release test specimens have
been studied. The microstructure appears to
disintergrato into smaller masses in the heated
specimens due to the action of the temperature and
the loss of water.