high strenght concrete
TRANSCRIPT
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High Strength Concrete - Durability Investiga-
tions by Using the CDF-Test - First Results
Robert Krumbach1, Katrin Seyfarth2, Wolfgang Erfurt2, Karen Friedemann1
SUMMERY
Some unusual observations with respect to the durability of some high-strength
concretes (HSC) caused investigations on the influence of typical properties of
HSCs like hydration behaviour and strength development, especially with regard
to the frost de-icing salt resistance (FDSR). The research project is a co-
operation with the F. A. Finger-Institute of the Weimar-University. In this pa-
per first results are presented.
1 INTRODUCTION
Resulting from their high density, high-strength concretes differ from normal
strength concretes in a higher durability, e.g. a high frost de-icing resistance and a
high resistance to chemical substances. Some unusual observations on HSCs
were made since the development and practical utilization of HSC which caused
objections regarding the durability. Tests on building members produced of HSCshowed, that a decrease of the compressive strength and a formation of mi-
crocracks can occur gradually. This happened primarily when concretes were
exposed to high temperatures during the hydration such as inside building mem-
bers. It was also observed that their resistance to freeze de-icing cycles can be
considerably lower than expected [1-4].
The guideline of the DAfStb (German Committee for Reinforced Concrete) [5]
reflects these problems: The utilization of concretes with a strength class higher
1Dipl.-Ing., Institut fr Massivbau und Baustofftechnologie, University of Leipzig2 Dipl.-Ing., F. A.-Finger Institut fr Baustoffkunde, Bauhaus-University of Weimar
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than B 95 for exterior building members and the use of superplasticisers is lim-
ited.
Causes of the peculiarities of HSCs and the risk of a possible microcrack forma-tion mechanism are conceivable e.g. the entering of moisture into building
components owing to capillary suction - which induce or intensify building dam-
age:
Temporary temperatures up to 70 C inside the structural component owing to
fast heat development during the hydration [6, 7]
Stronger formation of monosulphate/AFm,
At the same time entering of more water e.g. through cracks, makes the
chemical reaction Monosulphate/AFm
Ettringite/AFt possible (thevolume growing 2,3 times)
Incomplete hydration connected with inside drying-out owing to extreme
low w-c ratios
At the same time entering of more water, e.g. through microcracks makes
the swelling of the cement gel and the formation of new phases possible
being the reason for a considerable volume growing (e.g. C3A, C4AF,
C3AH6, Monosulphate/AFm Ettringite/AFt).
No capillary pores content, dense mortar matrix
At the same time entering of more water e.g. through microcracks spacefor expansion there is no for especially:
De-icing water at freezing under hydrostatic pressure (volume growing
connected with blast effects)
Development of new phases under imposed deformation (microcracksinto the concrete structure or flakings can occur)
A reduced durability of HSCs cannot be excluded as mentioned above. Further-
more, unfavourable additional effects caused by high superplasticizer content (up
to 70 g/ml per kg cement) can occur depending on the moment of adding.
Our investigations shall clarify, whether and in which extent a reduced durability
must be expected, what is causing this and by which practical steps the risks can
be counteracted.
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2 GENERAL ASPECTS
In our research project the influence of the typical properties the production of
HSC, like:
extremely low w-c-ratio
high content of superplasticizer at different times
suffering high temperatures during the hydration
on the hydration process and on the strength development, especially on the
FDSR (CDF-test method), shall be investigated.Additionally, the arrangement of the concrete structure as well as possible modi-
fications of the structure will be observed before and after different kinds of cur-
ing and after the CDF-test (ultrasonic). To get a wide spectrum of various
parameters influencing the concrete properties the concrete compositions indi-
cated in table 2 were selected. For comparison, the mixtures will be produced
with two cements with different sulphate resistance: CEM I 42,5 R and CEM I
42,5 HS (sulphate resistance, DIN 1164, tab. 1). The HS-cement showed after 28
days of hydration that is before the frost de-icing salt test slightly more AFm
phases than the ordinary Portland cement containing more C3A.
The concretes have w-c-ratios of 0,40 (no superplasticizer) and of 0,25 (5 M-% of
superplasticizer). The used superplasticizer (FM) is a mixture of melamine and
naphthalinsulphonic resin.
Depending on the concrete mixture microsilica (MS) was used (content: 0 and
10 M-%, tab. 2) in the form of a slurry with 50 M-% solid content.
As aggregates we use sand (0/2) from the river Main and gravel (2/8; 8/16) from
the river Rhine.
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Table 1: Analysis of cements used
CEM I 42,5 R CEM 42,5 HS
Blaine fineness, cm/g 3581 4449
Initial setting, start, min. 2:42 3:32
Required amount of water, M.-% 28,3 29,7
Compressive strength, N/mm
(2d; 28d) 31,9; 59,0 31,4; 57,7
SiO2 20,5 19,5
Al2O3 5,3 4,0
Fe2O3 2,6 6,7
CaO 65,3 64,2
MgO 1,9 1,6
K2O 1,1 0,7
SO3 3,2 3,1
3 FIRST TEST RESULTS
3.1 Consistence and development of durability
The determination of the consistence of any concrete mixture is performed ac-
cording to DIN 1048.
Clear differences were found between the concrete series I (w/c = 0.40; no FM)and series II (w/c = 0,25; 5 M-% FM added during mixing).
In spite of the extremely low w-c-ratio of the concretes of series II slumps be-
tween 43 cm and 55 cm are achieved by adding 5 M-% FM (II/III: 4,5 M-%;
II/IV: 4 M-%).
In contrast the concretes of series I were very sticky. The working quality was
unsatisfactory and the concrete had to be compacted on average for 2-3 min.. The
slumps achieved sizes below 35 cm.
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The consistencies of the concretes of series III (2 min. compacted, too) were also
unsatisfactory. The aim of these series was to test concretes with a w-c-ratio of
0,25 concerning their utilization as ready mixed concrete. We had planned toinvestigate the hydration by delayed FM addition. It was the intention to add
3 M-% at the beginning of the mixing process and the second dose after 45 min.
(time of transport!). Between the beginning and 45 min the mixer should work
periodically (5 min.) 1 min. each.
However, the tests did not show the expected results because the consistencies of
the mixtures with only 3 M-% were so sticky, except for mixture III/I, that we
feared an early development of solidification. Therefore 4 M-% or 5 M-% FM
had to be added at the beginning of mixing, already. The slumps of the compara-
ble mixtures of series II, however, (see tab. 3) were not achieved also after 45min. On the other hand the slumps of mixture III and IV from series III (10 M-%
silica) showed improved results.
The compressive strength tests according to DIN 1048 were carried out after 7 d,
28 d, 56 d and 182 d to estimate the strength development over a longer period.
The specimens were moist-cured in accordance with ENV206.
Table 2: Slumps and development of compressive strength
Compressive strength W.200 inN/mm
Series FM-additionin %
Slumpsin cm
Bulk densityin kg/m
7d 28 d 56d 182d
I/I - 29 2,37 51 62 73
I/II - 29 2,39 54 69 73
I/III - 32 2,31 48 71 76
I/IV - 29 2,34 60 76 80 82
II/I 5 43 2,37 60 76 78 80
II/II 5 55 2,40 66 80 89 88
II/III 4,5 45 2,38 74 96 105 107
II/IV 4 51 2,36 70 95 97 98
III/I 3+2* 35 2,39 59 75 78 78
III/II 5+1* 29 2,39 71 84 83 87
III/III 4+1* 40 2,38 70 88 93
III/IV 4+1* 38 2,40 85 107 110
* after 45 min. added FM
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All concretes (tab. 2) met the 28 d-compressive strength-criterion of HSC of the
guideline of the DAfStb (w.200 > 55 N/mm, see tab. 2). Its noticeable that the
tested compressive strengths of the concretes with HS-cement (concretes II andIV) are higher than those of concretes with R-cement (concretes I and III; comp.
tab. 2, fig. 1).
As expected the concrete mixtures with 10 M-% silica (concretes III and IV of
series I III) yielded higher compressive strengths than the concretes without
silica (concretes I and II of series I III, tab. 2).
Mixtures with FM (w/c = 0,25) produce more favourable properties of green
concrete (slumps > 40 cm). The concretes can be compacted better and higher
compressive strengths are reached compared to concretes with w/c = 0,4 (withoutFM, comp. tab. 2).
The bulk densities amount to 2,31 - 2,40 kg/cm (tab. 2).
Fig. 1: Compressive strengths w.200 of series I w/c = 0,4; without FM
Compressive strength w200 of series I, w/z = 0,4 without FM
40
45
50
55
60
65
70
75
80
85
90
0 182
Concrete age [d]
Compressivestrengthw
200[N/mm]
CEM I 42.5 RCEMI 42.5 HS
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R, 10 M-% silica
7 28
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Table 3: Survey of concretes and the different kinds of curing and test methods
Series of concrete/
concrete
Cement Water-
cement
ratio
Content of
silicaslurry
in %
Content of
FM
in %
Curing
DIN 1048I CEM I 42,5 R 0,4 0 0
48 h 60C,.till 7th d water
immersion
DIN 1048II
CEM I 42,5 HS
0,4 0 0
48 h 60C, till 7th d water
immersion
DIN 1048III CEM I 42,5 R 0,4 10 0
48 h 60C, till 7th d water
immersion
DIN 1048
I
IV CEM I 42,5 HS 0,4 10 0
48 h 60C, till 7th d water
immersion
DIN 1048I CEM I 42,5 R 0,25 0 5, immedi-
ately
60C und 80C, no curing*
DIN 1048
II
II CEM I 42,5 HS 0,25 0 5, immedi-
ately
60C und 80C, no curing*
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Series of concrete/
concrete mix
Cement Water-
cement-
ratio
Content of
silicaslurry
in %
Content of
FM
in %
Curing
CEM I 42,5 R 0,25 10 5, immediately DIN 1048III
60C und 80C, no cur-
ing*
CEM I 42,5 HS 0,25 10 5, immediately DIN 1048
II
IV
60C und 80C, no cur-
ing*
DIN 1048I CEM I 42,5 R O,25 0 5, dosed (3+2)
60C, no curing*
DIN 1048II CEM I 42,5 HS O,25 0 6, dosed (5+1)
60C, no curing*
DIN 1048III CEM I 42,5 R O,25 10 5, dosed (4+1)
60C, no curing*
CEM I 42,5 HS O,25 10 5, dosed (4+1) DIN 1048
III
IV
60C, no curing*
* planned curing and test methods
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Compressive strength w200; w/c=0.25; 5 M-% FM (series II)
40
50
60
70
80
90
100
110
120
0 182
Concrete age [d]
Compressivestregthw200[N/mm]
CEM I 42.5 R, 10 M-% silica
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R
CEMI 42.5 HS
287 56
Fig. 2: Compressive strengths w.200 of series II, w/c = 0,25%; 5 M-% FM
40
50
60
70
80
90
100
110
120
0 182
Concrete age [d]
Compressive strength w200 of series III, w/c=0.25;
5 M-% FM, splite
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R, 10 M-% silicaCEM I 42.5 HS
CEM I 42.5 R
7 28 56
Fig. 3: Compressive strengths w.200 of series III, w/z = 0,25; 5 M-% FM delayed
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3.2 The frost de-icing salt resistance of the investigated high strength
concrete
Our investigations base on different questions. On the one hand the influence of
the concrete composition (w/c = 0,4 or 0,25; FM contents = 0 or 5 M-%; silica
contents = 0 or 10 M-%, comp. tab. 2) on the FDSR shall be analysed. On the
other hand the influence of the cement composition itself shall be investigated.
The CDF-Test according to SETZER [8] was carried out on specimens
150x150x75 mm3. All specimens of series I III were exposed to 28 freeze de-
icing cycles, one cycle takes in any case 12 h.
The first FDSR investigations (tab. 2) produced the expected results. HSC with
w/c 0,4 shows a high or very high FDSR. The very dense structure of these
concretes prevents the increase of mass by capillary suction of the 3 %-NaCl-
solution. The amount of the increasing mass was always below 0,35 M-% (tab. 4;
compared with normal concrete: 0,8 1,5 M-%). Especially the concretes with
w/c = 0,25 have a very small mass increase: 0,07 m 0,12 M-% (tab. 4).
The results correspond to the CDFtests: the rates of scaling of concretes with
w/c = 0,4 (28 cycles: 188 603 g/m) are higher than the rates of scaling withw/c = 0,25 (28 cycles: 51 104 g/m).
As expected silica containing specimens (concretes III and IV) have lower rates
of scaling than specimens without silica (concrete I and II, tab. 4, fig. 4-6). This
points to a denser paste matrix by the filling effect caused by silica [9].
An influence of the cement quality on the results can be recognised on concretes
of series I (w/c = 0,4, tab. 4, fig. 4). Concretes produced with CEM I 42,5 HS
yield significant higher rates of scaling (compare 28 cycles, tab. 4) than those
with CEM I 42,5 R. This tendency was also observed in earlier investigations
[10]. Furthermore, the two cement qualities lead to a different formation of the
concrete structure. This connection has to be considered, too.
Compared with that, the concretes of series II and III (w/c = 0,25) have smaller
differences between the rates of scaling with respect to R- and HS-cements. Here,
these small differences correspond with the results of pore radii distribution
(PRV) of the mortar matrix, because the concretes produced with R- and HS-
cements show a nearly identical PRV.
It is known, that CEM I 42,5 R , with w/c 0,4, shows a higher content of capil-
lary pores and a lower content of gel pores as well as microairpores than the HS-
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cement [10]. Concerning this an improved FDSR of HS-cement should be ex-
pected. However, the opposite tendency could be observed. Consequently, a valid
physical correlation between CDF-Test result and PRV cannot be derived [11].The causes for that are based on occurring chemical reactions, probably.
Table 4: Frost de-icing salt resistance
Rates of scaling (CDF-test) in g/m
Number of cycles:
Concrete Mass growth by
capillary suction
in %
4 8 14 28
I/I 0,32 31,6 93,2 260,4 603,1
I/II 0,33 35,3 133,5 348,1 921,1
I/III 0,30 - 75,3 105,3 188,1
I/IV 0,21 - 106,4 198,3 347,3
II/I 0,07 20,2 38,1 66,2 96,9
II/II 0,12 16,8 30,2 53,3 104,4
II/III 0,07 8,9 17 31,1 56,8
II/IV 0,07 6,2 9,3 42,2 84,0
III/I 0,09 8,2 25,6 65,8 98,2III/II 0,08 6,5 7,8 37,3 79,5
III/III 0,08 10,8 12,1 26,7 51,1
III/IV 0,08 - 16,1 26,7 56,0
The measured rates of scaling (after 28 cycles) of all series meet the acceptance
criterion of 1500 g/m.
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Frost de-icind salt resistence (CDF-test)
w/c = 0.4; without FM (Series I )
0
100
200
300
400
500
600
700
800
900
1000
0 28
Number of freezing and thawing cycles
8 14
CEM I 42.5 R, 10 M-% silica
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R
CEMI 42.5 HS
Fig. 4: Frost de-icing salt resistance of series I
Frost de-icing salt resistance (CDF-test) w/c = 0.25; 5 M-% FM
(Series II)
0
20
40
60
80
100
120
0 28
Number of freezing and thawing cycles
Scaling[g/m]
4 8 14
CEM I 42.5 R, 10 M-% silica
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R
CEMI 42.5 HS
Fig. 5: Frost de-icing salt resistance of series II
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Frost de-icing salt resistance (CDF-test) w/c = 0 ,25; FM split (series III)
0
20
40
60
80
100
0 28
Number of freezing and thawing cy cles
CEM I 42.5 R, 10 M -% silica
CEM I 42.5 HS, 10 M-% silica
CEM I 42.5 R
CEMI 42.5 HS
4 8 14
Fig. 6: Frost de-icing resistance of series III
3.3 Results of ultrasonic investigations
All specimens (150x150x75 mm3
) of the series I III (tab. 2) were tested byultrasonic before and after the CDF-Test. The aim of these investigations was to
get an information about possible damage inside the concretes structure or modi-
fications underneath the concrete surface.
To guarantee the required sensitivity high frequency testing heads (eigenfre-
quency in any case 250 kHz) were used. By a broad band receiver the sound
signal is detected. Afterwards, the detected vibration signal is digitalized and
evaluated by a transient recorder. The sound signal is generated by an ultrasonic
generator of the company GEOTRON-ELEKTRONIC. The specimens are cou-
pled to the sound converters in a special measuring device. Thus, an adjustment
of a reproducible coupling pressure is possible. Clay was used for coupling. The
position of the sound converter during the measuring is shown in figure 7.
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Ultrasonic generator
Scaledsurface
Transient recorder
Specimen
Transmitter
Receiver
A A
B
B
Fig. 7: Position of the sound converter during the measuring
The sound converters are coupled to the specimens as shown in figure 7 (both
positions A A and B B). So, exact localisation of occurring damage near thesurface or inside the concrete structure is possible.
Figure 8 shows the differences between the measured sound velocities before and
after the CDF-test. At the series I (w/c = 0,4) the sound velocities are lower than
the sound velocities of specimens of the series II and III (w/c = 0,25).
The measured sound velocities after the 28 freeze de-icing-cycles are slightly
higher than the sound velocities before the CDF-test, except concretes I/II and
II/I. This fact may be caused by the crystallization of NaCl during the drying of
the specimens after the CDF-test.
The obtained differences of sound velocities of 50 m/s can be neglected due to
low water penetration.
Consistent damage of the material structure could not be observed.
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4400
4500
4600
4700
4800
4900
5000
5100
I/II I/III I/IV II/I II/II II/III II/IV III/I III/II III/III III/IV
Soundvelocity[m/s]
bevore CDF-test after CDF-test
Fig. 8: Sound velocities before and after CDF-test
4 FURTHER INVESTIGATIONS
Up to now, we have also made observations indicating different behaviour of the
investigated HSCs under extreme conditions. Especially if the specimens were
exposed to higher temperatures in the first two days during the curing higher rates
of scaling could be obtained. The further investigations shall simulate the devel-
opment of the hydration heat of 60 C and 80 C. It is also planned, to increase
the number of the freeze de-icing cycles up to 56 (tab. 2).
For the evaluation of the hydration process extensive analytical investigations by
XRD, REM/ESMA or ESEM, DTA/DTG/TG shall be carried out.
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