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STRENGTH AND DEFORMATION PROPERTIES OF MASONRY TO BE USED IN COHPUTERCALCULATIONS.

A. Th. Vermeltfoort ir University of Technology Eindhoven Department of structural engineering Postbox 513 Postvak 7 5600 MB EINDHOVEN The Netherlands

R. van der Pluym ir TNO Building and Construction Research Department of stuctural engineering Postbox 49 2600 AA DELFT The Netherlands

ABSTRACT

- For the use of Finite or Discrete Element Method computer programs to calculate masonry strength and deformation, accura­te figures for the parameters of bricks, mortar, and the interfaces between these two are needed. A series of tests on different size-levels have been started. The preliminary results of the tests carried out uptill february 1991 are discussed in this paper. - The properties of the surface of the bricks have a large influence on the deformation of specimens under compression. - The properties of the mortar are influenced by the bricks. The Youngs module of the mortar used with a soft mud brick was 420 MPa and 2100 MPa when used with a wire cut brick. - Tensile test results obtained from small specimens indicate that the scatter in the tensile bond strength within one batch is caused by the variation of the net bond surface. The fractu­re energy is approximately 5 N/m for specimens that reach a gross tensile bond strength between 0.3 and 0.4 MPa

Introduction

with the development of numerical techniques it is no longer sufficient to think of masonry as a homogenious material. A complete material model requires the definition of constitutive relations before and after failure and suitable failure cri te­ria. For the use of Finite or Discrete Element Method computer programs like UDEC [1] or DIANA [2] to calculate masonry strength and deformation, accurate figures for the parameters of bricks, mortar, and the interfaces between these two are needed. In order to obtain more of this numerical information a series of tests will be carried out. The information which then becomes available will be used in the computerprograms Udec and DIANA. The commission B50 of CUR, a dutch institute stimulating structural research, is participating and coordinating these tests on different size-levels. The smallest test is a tensile test on a brick, the largest is a full scale wall testo with the information obtained from small sized experiments combined with the computer programs mentioned above the results of larger sized elements will be predicted.

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These calculated results will be compared with experimental data obtained from the laboratory-tests of these elements. The first part of this paper deals with the micro-leveI com­pression tests, the second part with the tensile tests.

1 compression tests

The masonry used in the complete series of investigations is constructed from solid clay bricks (two types), calcium silica­te bricks (one type) and 3 different types of morta r in the following cement, lime, sand ratios: 1:2:9 & 1:1:6 & 1:\:4\. AlI masonry has been constructed in the laboratory under close supervision to minimize workmanship effects. At this moment two brick qualities have been tested on micro leveI. A brick fabri­cated by VIJF EIKEN which is a soft mud brick, an other type fabricated by JOOSTEN which is a rather strong wire cut brick. The specimens are made as small as possible, but the bricks are kept as natural as possible, because when a brick is divided into parts residual pressures are released which will disturbe the actual measurements. The smallest height-depth ratio of a specimen in which an area with uniaxial stress will occur is about 1.5 to 2. That means for bricks which are 52x100x210 mm3

a heigth of about 300 mm is requested. The heigth of the speci­mens with mortar and the ones with the grinded bricks is almost equal so the results can be compared. Specimens with grinded bricks have been used to establish the deformation properties of the brick itself. The specimens with mortar consists of 5 bricks with 4 joints. The contact areas of the upper and lower brick with the loadingplatens are grinded leveI. The other specimens consist of 7 grinded bricks. AlI grinded surfaces have a difference in heigth of 0.02 mm average (0.05 mm max), measured along the diagonal of the brick. The maximum diffe­rence in thickness of the grinded bricks is 0.10 mm.

r unnded surf

1

2 1-

3 ,~ ~ ,

I,

5 \ . Gr.nded

oce $' = LVDT

o o (Y)

I

1

2

3

I, ~ 5

6

7

~ ~

7 gnndod br;cks

fig 1. Oimensions Testspecimens

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The following deformations were measured (see figures 2 & 3): a : the deformation of one brick with two joints. The measuring­length was 78 mm (mortar) and 52 mm (grinded). b: the deformation of the brick on four points divided around the specimen. The measuringlength was 45 mm (mor tar) and 40 mm . c: the deformation in lateral direction of the brick on four places. Measuringlength about 100 mm on the front and back sites and 50 mm on the other sites. d: the deformation in lateral direction of the joint on four places, with the same measuring lengths as for the lateral deformation for the bricks. ~: the deformation of the complete specimen on the four corners of the loadingplatens.

400-+-__ -f-_________ ~..--_

300~ __ +-~~~~~~~~

t 250 ---fo~"i! Z ..:<:

200~~-4 ______ ~~~ __ _

140

100

00

00

I I I I I I

I *

E ",,2700 N/mm2

, IjIJF EIKEN 1:'i:1.{

1000 1500 _ ;; m

During tests the displacement velocity of the loadingplatens was 0.28 mm/min. Complete testing took about 30 minutes. In this paper only the prelimina­ry results of the deformations ~, a and b will be discussed.

'd--u d_ N--f

Above. figo 2 Measurements on a specimen Left. figo 3 Deformations of the complete specimens.

1.1 Deformation of the complete specimen.

The four "Vijf Eiken" specimens with morta r showed an almost equal deformation. The differences found for the grinded specimens were higher. In the begin-phase of the tests the stifness of the grinded VE-bricks is about the same as the ones with mortar. 80 the influence of the seams on the deformation is considerable. six seams have the same deformation as four morta r joints. The Joosten bricks react a lot stiffer then the VE-bricks. The JO-specimens with morta r compared with each other, show an almost equal deformation. Uptill now two JO­specimens with grinded bricks have been tested, the stiffnes and the strength of these specimens is much higher .

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1.2 Deformation of the bricks.

It was expected that during testing a certa in measure inaquara­cy due to the inclination of the connecting pins would develope and therefore special tests have been carried out, which showed that this error was to big to give reliable results for the Vijf Eiken bricks used with mortar. As a global view it can be mentioned that the deformations cover a large area and that a number of sensors only started recording after a force of 150 kN or more. The scatter in the measured brickdeformations is large. The average brickdeformation as measured on the grinded VE-specimens is showed in figo 4. It is remarkable that for one of the specimens the deformations only started when the applied force was 200 kN and that this recorded deformation-line is parallel to the lines of the other tests. The measure inacqua­racy is much smaller here, but comparing the brickdeformations with those from the complete specimen no correspondence could be found. The deformation of the complete specimen (with seams) gave a Youngs-module of about 3.500 MPa, other tests gave a value of 6000 MPa.

150"""1""1"--1

100-++1--1---.

Deformation

G ri nded br icks

Vijl Eiken

3specimen

50-+f----+------t-

O 50 }Jm

figo 4 & Brickdeformations.

kN

200

10-,,-0+--_

00 r-00 50

Vijf Eiken and

BRICK DEFORMATION

JOOSTEN 1 : i't: 4Y..

T t 100 }Jm 150

Joosten

The results from the Joosten tests with mortar show the same tendency as the Vijf Eiken tests. All LVDT's reacted only after a force of 120 kN minimal and the lines in the graph are parallel. An explaination can be that there was a visible schrincage-crack at the joint which had to be closed before the brick started to deform o For the deformation of the brick measured on the grinded specimens also remarkable results are

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found. Two LVDT's gave deformations in opposite direction. The average of the four deformation measurements is almost zero. A special test showed that the measureinaqcuaracy due to the inclination of the LVDT-connectionpins was acceptable for the Joosten bricks which are much harder. From the deformation of the complete specimen a Youngs-module of 17.500 MPa can be establisedi other tests gave the same value. Heasurinq the deformation of bricks is very difficult, because: - only a short lenqth (thicknes of the brick) is available. - a larqe scatter in deformations occurs. - the connectionpins for the LVDTs rotate

1.3 The deformation of the mortarjoints

The deformations of the mortarjoints show a large scatter also. The averages of the deformations of the joints between JO- and VE-bricks are presented in figo 5. For the JO-bricks the deformation of the joints established from the complete speci­men is about the same as when measured on two joints seperate­ly. (40 mm x 0.006 = 0.240 mm, see figo 3). For the VE-bricks the measure inacquracies are of the same magnitude as the VE­brick deformations and when the brickdeformations are neglected also a good resemblance is found. Calculating the joint-defor­mation from the complete specimen using the astimated Youngs module of 6000 MPa gives a E-module of about 850 MPa for the morta r , about 2 times as large as measured and about 40 percent of the Youngs module when the mortar is used with JO-bricks. The basicmaterial of the mortar has been the same however.

Z .::L

r 25º-.;.

I

I I

140 1 ---r

I ! i I

I

I 00 1

00

JOOSTEN E ~2100 N/mm2

Vijf Eiken

I 6%0 179'00

These large differences in measured stiffnes properties of the morta r and the inter­faces can be accounted for by: a) The hardening of the mor­tar which depends on the su­ction- (and other) proper­ties of the brick. b) The confinement of the mortar which is better when the brick is harder. c) The structure of the morter near the contact surfaces which depends on the verry different rough­ness of both bricks.

The stiffnes of the mortar if heavily influenced by the brick with which it is used.

figo 6. Deformation of the mortar in the joints

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2 Tensile tests

It was expected that the behavior of mortar prisms would differ from the behavior of mortar in a joint as a result of the influence of the bricks on the curing condition of the mortar in a joint. For this reason, the deformation of the bricks and the mortar joints have been measured on small masonry speci­mens. In this way, the properties of the mortar in the joints can be obtained from the masonry specimens and be compared with those from tests on mortar prisms. Brick prisms have been used to obtain material properties of the bricks. AlI the specimens were ma de with the same materiaIs and at the same time and in exactly the same way as the specimens used for the compression test described in paragraph 1 of this paper. The masonry specimens (lOOxl00xl17 mm3 and 100xl00xll0 mm3

)

consist of three half bricks and two joints or two half bricks with one joint (see figo 7). The middle cross-section of the morta r prisms (40x40x160 mm3 ) was reduced by two pieces of plastic, placed in the mould. The middIe cross-section of the brick specimens (60x52x150 mm3 ) was reduced by two saw cuts.

B"·l~ 174 i 160 {] 150

100

(a)

figo 7.

40 60 100 +--t +---+

(b)

Test Specimens: (a) masonry specimensj (b) morta r specimenj (c) brick specimen.

(c)

The deformation controlled tensile tests were performed in a test rig of the stevin Laboratory of the Delft University of Technology [3].

On masonry specimen A the following deformations were measured. The deformation of one brick and two joints on four points around the specimen. These deformations were used to control the actuator. The average measuring length was 85 mm. The measuring length over one brick and two joints covers also a littIe slices of the Iower and upper brick. The deformation of the upper joint with two diametrical placed sensors and the deformation of the lower joint with two sensors. The average measuring length was 35 mm. Both measuring Iengths over the two joints cover also littIe slices of the bricks that borde r on the joints.

On masonry specimen B the following deformations were measured. The deformation of the joint with four LVDTs around the

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specimen. The average measuring length was 35 mm. The measur1ng length over the joint covers also a little slice of the lower and upper brick.

2.1 Preliminary test results.

In figo 8 the deformation controlled stress-deformation relati­on of three masonry specimens (1VC, 2vc and 3VC) which consist of Vijf Eiken bricks and 1:~:4~ morta r are presented. The strain in the joints is calculated with the assumption that the total deformation takes place in the joints and that the deformation in the little sI ices of bricks as mentioned in the previous paragraph can be neglected. This neglect will be corrected if necessary, when the total test program is comple­ted. Fig. 8 also shows that the tensile bond strength has a great scatter. The tensile bond strength is calculated by dividing the measured force by the gross cross-sectional area of the specimen. From the cracked specimens it can be observed that the net bond surface is smaller.

Ô Il.

2-b

In In

~

0.3

0.2 -

0.1 -

~ specimen 1 vc ............... specimen 2vc ............... specimen 3vc

o. o -h-rT1-rT"TTTTrT1rrT"TT~T"r'N"ocr-r-r-rrrTTTTTT-rT, I 1 1 1 1 1 1 1 1 '1 defortna\ion d (1 o - 'tn) o W M 1W 1~ 200

figo 8. stress-deformation relations of the joint of specimens 1vc, 2vc and 3vc. Vijf Eiken brick with mortar 1:~:4~.

In figo 9 the net bond surfaces of the specimens are presented. The scatter becomes much smaller when the tensile bond strength is based on the net area of the surface (see table 1).

I gross area I net area

1vc I 0.073 2vc I 0.290 3vc I 0.043

I 0.60 I 0.54 I 0.54

table L Tensile bond strength [MPa] of 1vc, 2vc and 3vc.

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figo 9. Net bond surfaces of specimen 1vc, 2vc and 3vc.

Calculating the tensile bond strength in this way, indicates that a great part of the scatter within one batch is caused by the variation of the net bond surface. This way of interpreta­tion also gives the possibility to compare masonry from the laboratory with masonry produced in the building practice in the future in an more objective way. The fracture energy Grof a crack is defined as the amount of work that is needed to create a stress free crack. The area under the diagrams presented in figo 8 is equal to the fracture energy. preliminary analysis of the tests shows that the fracture energy is approximately 5 N/m for specimens that reach a gross tensile bond strength between 0 . 3 and 0.4 MPa.

Aknow1edgement.

The support of KNB and CUR is gratefully aknowledged.

References

[1J UDEC-manual. Universal Distinct Element Code Version ICG1.5 Itasca consulting group inc. suite 210 1313 5 th Street SE Minneapolis, Minnesota 55414.

[2J DIANA-manual. DIANA Finite Element Analysis. Institute TNO for Building Materiais and structures. p.o.box 49 2600AA Delft, The Netherlands.

[3J Hordijk, D.A. and Reinhardt, H.W.; "Testing and modelling of plain concrete under mode I loading"; in Micromechanics of failure of quasi brittle materiais; Elsevier Applied Science, pp 559-568