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THE INFLUENCE OF CAPPING MATERIAL AND PLATEN RESTRAINT ON THE FAILURE OF HOLLOW MASONRY UNITS AND PRISMS

A.W. PAGE Associate Professor

by

P.W. KLEEMAN Senior Lecturer

Department of Civil Engineering and Surveying The University of Newcastle, NSW, 2308, Australia

ABSTRACT

Hollow masonry unit and prism tests are used as a means of predicting wall strength from small specimens. These specimens should be face-shell bedded where relevant, and tested using face-shell capping to reproduce as closely as possible the compressive failure mechanism of the wall. This paper describes an investigation aimed at clarifying the influence of testing machine platen restraint and capping material on the observed compressive strength of units and prisms. It is shown that differences in capping stiffness can influence the results significantly, and that the effects of platen restraint are appreciable, although the mechanism of this restraint is different to that observed in solid test specimens.

INTRODUCTION

In Australia as in many countries, hollow masonry is a common form of construction. Units are of concrete or clay, and walls are normally built with mortar in contact with the face- shells only. When a compressive load is applied to the wall, significant transverse stresses are developed in the web connecting the face-shells . It is these transverse tensile stresses in the web of the units that precipitate compressive failure, by vertical splitting of the web in a plane parallel to the face of the wall. Standard tests to evaluate the strength of hollow masonry should therefore reproduce these effects as closely as possible. Australian Codes require hollow units and prisms to be tested using face-shell capping, with the stack bonded prisms being built with face- shell bedding.

Many factors can influence the observed strength in a standard prism or unit test. These include specimen size and shape, specimen end conditions, the properties of the capping material, the moisture content and age of the specimen, the method of load application, and accidental loading eccentricities. The influence of many of these factors has been previously investigated, and these have been reviewed recently (1).

This paper describes an investigation aimed at clarifying the influence of platen restraint and capping material on the observed compressive strength of hollow units and face- shell bedded prisms tested in uniaxial compression. The influence of these factors on the strength of solid specimens is well known. However, hollow units and prisms loaded only through their face­shells exhibit a completely different mechanism of failure to solid specimens. The influence of capping and platen restraint for solid specimens cannot therefore be assumed to be the same as for their hollow counterparts.

ST ANDARD UNIT AND PRISM TESTS

The provisions of the Australian Masonry Code (2) contain two methods for estimating masonry compressive strength. The first is from a lower bound empirical relationship between the hollow unit compressive strength and masonry strength for a particular mortar type. The

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compression test on the unit is performed with either plywood or hardboard face-shelI capping. Alternatively, the compressive strength can be obtained from tests on stack bonded prisms of at least two units, built with face-sheIl bedding and tested with face-sheIl capping of plywood or hardboard. Capping material and platen restraint wilI affect the results of both types of tests, and influence the relationship between the observed smaIl specimen strength and waIl strength. It is therefore important to obtain a basic understanding of the influence of these two variables.

THE INFLUENCE OF CAPPING MATERIAL AND PLATEN RESTRAINT

Capping is used between the test specimen and the testing machine platen to absorb the effects of irregularities on the ends of the specimen and provide a more uniform distribution of stress when the load is applied by the testing machine in the form of a prescribed displacement of the loading plate. The distribution of stress within the specimen is influenced by the properties of this capping material and the restraint of the platen as the specimen attempts to expand laterally. Artificial strengthening can result from this restraint. The stress state is influenced by the deformation properties of the packing, and the observed strength in the specimen depends upon the relative stiffness of the packing material and the sample. The type of packing used in standard compression tests varies considerably from country to country. In Australia, either plywood or hardboard with a thickness between 4 rum and 6 mm is used.

EXPERIMENT AL STUDY OF THE INFLUENCE OF CAPPING MATERIAL

Properties of Capping Materiais

The results of a study of the in-situ properties of various capping materiais carried out by the authors have been published recently (3). They are reviewed briefly here as they are relevant to the present investigation. Compression and shear tests were carried out on four capping materiais: 4 mm plywood (3 plies); 6.7 mm hardboard (resin bonded compressed fibreboard); 12.5 mm fibreboard (low density board with cane fibres); and 19 mm particIe board (bonded wood chips). To simulate the effects of platen and specimen restraint, compression and shear tests were performed with the capping material being sandwiched between steel plates with the deformations of the capping material being observed. For the shear tests, a range of precompressions were applied to the specimens and held constant while applying the shear load. Poisson's ratio was also determined by testing prisms made up of a number of layers of each material.

AlI capping materiais exhibited non-linear load deformation characteristics with both the normal and shear stiffnesses increasing with the levei of compression. This is not unexpected as considerable compaction of the material occurs. This reduction in thickness was considered when evaluating tangent moduli from the compression test results. The load deformation curves for the shear tests were more linear, and secant moduli were used as a measure of stiffness in this case, again aIlowing for the change in specimen thickness with precompression. Some selected results are given in Table 1. Detailed results are contained in reference (3).

Implications for the Compression Test

The two aspects relevant to the loading of the specimen and influenced by the properties of the capping material are the uniformity of loading over the loaded area, and the lateral restraint imposed by the loading platens through the capping material.

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TABLE 1. IN-SITU CAPPINO PROPERTIES

SHEAR SECANT MODULUS AT

25 MPa NORMAL STRESS LEVEL

(MPa)

MATERIAL TANOENT NEW USED THICKNESS STIFFNESS AT AT25 MPa

25MPa STRESSLEVEL

(MPa/mm) (mm)

PlywoodIP 22 150 140 2.5

Plywood/N 250 240 2.5

Hardboard 68 240 430 5.7

Fibreboard 26 160 170 2.9

Particle board 7.2 300 200 10.9

NOTE:

The value of Poisson's ratio was small for alI four materiaIs and generally in the order of 0.04.

Tangent stiffness = Tangent modulus/actual thickness Shear secant modulus at a shear stress = 10% of normal stress New = Fresh specimen at each compressive stress leveI Used = Same specimen at increasing stress leveIs P/N = Shear direction parallel/normal to middle ply grain direction

Uniformity of loading is dependent on the extent to which the loaded surfaces are not plane. The variation in stress across the top of a non-planar specimen will vary with the stiffness of the packing, with greater variations occurring with increased packing stiffness. It can be seen from Table 1 that hardboard had a much higher tangent stiffness than the other materiaIs, and should therefore only be used when the broad variation from plane of the loaded surface is smal!. Oreater variability of test results using hardboard packing might be expected in cases where the bearing surfaces of the specimens are uneven.

For solid specimens the lateral restraint imposed upon specimens by the loading platens through the packing is a function of both the Poisson's ratio and the shear stiffness of the packing material. Ideally the platen restraint is minimised when the tensile force in the specimen due to the Poisson effect in the packing, and the compressive force due to differential transverse displacements of platen, packing and specimen are equal. A detailed discussion of this mechanism is given in reference (3). In face-shell capped specimens, the contact area is confined to two relatively narrow strips, and the influence of the restraint on Poisson ratio expansion of the packing material may be less important. However, the two loaded strips are connected by the p1aten itself, resulting in overalllateral restraint of the specimen (this aspect is investigated later in the paper).

N01E:

l. 2.

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TABLE 2. SUMMARY OF UNIT AND PRISM TESTS

UNITS PRISMS

ULT. LDAD C. DF V. ULT. ULT. LOAD C. OF V. ULT.

(kN) ('rol LOAD (kN) (%l LOAD

RAllO RAllO

UNIT NOM

TYPE DIM P H P H P/H P H P H P/H

Ixwxh

Clay 400>< 899 722 5.8 16.3 1.25 714 609 5.9 17.9 1.17

200><

200

Clay 400>< 905 781 5.9 10.1 1.16 623 548 6 .7 19 .3 1.14

200x

100

CIay 400>< 911 762 5.8 13.1 1.20 637 549 15 .7 16.9 1.16

150><

200

Clay 400>< 800 781 7.6 8.6 1.02 600 508 4.5 12.4 1.18

100><

200

Cone 400>< 630 562 3.2 6.8 1.12 453 486 8.2 8.6 0.93

20.01 200x

200

Cone 400x S44 427 14.9 9.1 1.27 412 355 9.0 8.7 1.16

15.01 150><

200

Cone 400x 579 556 4 .8 18.9 1.04 460 503 9.8 9.9 0.91

10.01 100x

200

NOMENO-A TURE:

All values are me mean af ten tests. I = length; w = width; h = height Face shel! and web thicknesses in the range of25 mm-35 mm F = plywood capping; H = hardboard capping

C. of v. = cocfficient of variation

Compara tive Study of the Influence of Capping for Hollow Masonry

To assess the differences in the use of plywood and hardboard capping for tests on hollow masonry, a series of uniaxial compression tests were performed on a range of concrete and day hollow units and prisms using plywood and hardboard capping 4 mm-6 mm thick. Ali procedures were in accordance with the relevant Australian Code. Prisms were two high and stack bonded, and built with a medi um strength mortar (l : 1:6 cement:lime:sand for the day, 1:0:5 + water thickener for the concrete). Ali materiais were sampled from the same respective batches. A parallel set of tests were performed on each unit type and a set of prisms constructed from those units, with ali specimens being face-shell capped. Ten replicates were used for each test. The results are summarised in Table 2. This study formed part of a much larger joint investigation with CSIRO Division of Building Construction and Engineering on test methods for hollow masonry, the results of which are to be published shortly.

It can be seen from Table 2 that identical specimens tested with plywood and hardboard capping showed consistent differences both for unit and prism tests, with the observed strength with hardboard capping being consistentIy lower than their plywood counterparts. The variability of the hardboard results was also higher. particularly for the hollow day units . This is consistent with the results of the capping investigation which found the hardboard stiffness in compression to be much greater than the plywood and the hardboard shear stiffness (G/t) to be lower than that of the plywood (see Table I). The day units in particular have surface variations on their loaded surfaces produced by firing. They are therefore more sensitive to a higher capping stiffness. resulting in earlier failure. Because of the differences in the results for

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the two capping types, and the higher variability of the hardboard results, there is case for the deletion of the use of hardboard from the Australian standard.

EXPERIMENT AL STUDY DF PLATEN RESTRAINT

The influence of platen restraint on the observed compressive strength of a solid test specimen is well recognised. Corrections can be made for this apparent strengthening effect by applying a factor which is usually a function of the height/thickness ratio of the specimen. The Australian Masonry Code has such factors for solid masonry units and prisms. These are the result of Australian and other research (4). However, the effect of platen restraint on the failure mechanism in compression of face-shell capped hollow units and prisms is not as well understood. In this case the specimen is subjected to a much more complex stress distribution. In particular the web stresses are strongly influenced by the restraint against lateral expansion by the platen connecting the two loaded strips.

This investigation was aimed at studying this phenomenon by testing a series of units and prisms with differing degrees of platen restraint: by the use of standard test methods and brush platens to minimise platen effects. The brush platens had been developed for previous masonry compression tests and found to be effective in minimising the effects of lateral restraint (4). Each brush platen consists of a series of slender steel filaments which are capable individually of transmitting their share of verticalload, but which deflect laterally as the specimen expands because of their low shear stiffness. This investigation forms part of a larger study of the fundamental behaviour of hollow masonry loaded in compression (5).

Unit Tests

To assess the degree of platen restraint in standard hollow unit tests, compression tests were carried out on 20.01 hollow concrete blocks (nominal dimensions 400 mm long x 200 mm high x 200 mm wide) using the standard Code procedures with plywood face-shell capping, and with brush platens. In the brush platen tests, the platens were located on layers of plaster placed only on the face-shell width, to ensure that contact occurred only between the platen and the face-shell. To prevent the plaster flowing across the unit, a strip of flexible polystyrene foam was placed along the centre of the block before applying the plaster. To monitor the progressive cracking of the units, linear potentiometric displacement transducers (LPDTs) were mounted at the top and the bottom of the central and end webs on a gauge length of either 70 mm or 160 mm depending on their location. Four replicates of each test were performed.

Failure Mode

For the standard unit tests, ali specimens cracked initially at the bottom of the end web at a load of approximately 33% of the ultimate load (see Table 3). This was followed by progressive cracking of other webs and finally spalling of the face-shells in the region between the central and outer web. For the brush platen tests, failure frrst occurred by cracking at the bottom of the central web at a load of approximately 35% of the ultimate load (see Table 3). With increased load the crack continued to open and new cracks formed in the other webs. At the ultimate load the web cracks had progressed through to the top of the unit. There was also local spalling at the bottom of the face-shells in each case. Typical failure modes for the two cases are shown in Figure 1.

Although the failure modes were similar, the results show that there was significant platen restraint present in the standard face-shell bedded test. This is indicated by the lower values for both the cracking load and ultimate load for the brush platen test (the ratio of cracking and ultimate loads for the brush platen and standard tests were 0.73 and 0.71 respectively).

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J

Standard Brush Platen

FIGURE 1. FAILURE MODES FOR UNIT TESTS

TABLE 3. SUMMARY OF UNIT TESTS

TESTTYPE MEAN CRACKING MEAN ULTIMA TE CRACKING LOAD

LOAD LOAD ULTIMA TE LOAD

(kN) (kN)

Standard 165 490 0.33

Brush Platen 120 348 0.35

Mechanism of Platen Restraint

The postulated mechanism is shown in Figure 2. Although contact between platen and specimen only occurs at the face-shells, sufficient frictional resistance can be created on the contact surfaces to restrain the lateral movement of the specimen, with the platen acting as a "tie" between the two contact areas. This results in a higher web cracking load, since the lateral restraint results in reduced lateral displacements and transverse tensile stresses, a reduced change in slope at the capping, more uniform compressive stresses on the face-shell and hence a higher ultimate load. These effects have been confirmed qualitatively by three-dimensional elastic finite element analysis of this test (5).

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PLATEN

S-WEB i CRACK

WEB

I ~

..

FACE SHELL

:1

\OEFORMEO : SHAPE

i CAPPING

~ r-~~====~====~P;LA:'~7EN;=====~q~~~";----1 FIGURE 2. MECHANISM OF PLA TEN RESTRAINT FOR

FACE-SHELL CAPPED HOLLOW MASONRY

The increased degree of restraint also influences the final failure mode. For the standard test the web cracking did not propagate as freely due to the restraint against lateral expansion. Failure was delayed until sufficient compressive stresses had developed to initiate crushing in the face­shells. For the brush platen specimens, the web cracking extended for the full height of the unit for all webs, and only minor crushing of the face-shells occurred. There was a greater degree of variability in the location of the first web cracks for the standard specimens, probably also a result of the increased restraint against lateral expansion.

Prism Tests

To allow comparison of the behaviour of an individual block tested in isolation to that of a block forming part of a prism, a series of tests were also carried out on a set of four, three high stack bonded prisms manufactured from the same batch of concrete units as used for the tests on the individual blocks. The prisms were constructed in face-shell bedding using the same mortar as used in the earlier investigations. To allow the monitoring of progressive web splitting, the webs of the central block were instrumented in a similar fashion to that previously described for the individual block tests. The standard manufacturing and curing procedure was followed. The specimens were tested at an age of seven days. The load was applied monotonically to failure through plywood face-shell capping. The testing arrangement and the prism after failure is shown in Figure 3.

Except for a few minor differences, all four prisms behaved in a similar fashion. Failure occurred by progressive cracking of the webs of the units as shown in Figure 3. In two cases the web cracking commenced at the bottom of the top unit and then propagated into the top of the central block. In the other two cases web cracking first occurred at the bottom of the central

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Test Arrangement Failure Mode (Note LPDT mounted on internaI web)

FIGURE 3. PRISM TEST

unit. The mean cracking load was 105 kN. With increasing load the cracking propagated vertically through the webs of the central unit. At failure the two face-shells of this unit separated, together with outward rotation of the face-shells of the upper unit. This occurred in conjunction with the compression failure of the mortar on the inside of the joint, and spalling of lhe face-shell on the outside of the unit above the joint. The mean ultimate load for the four specimens was 409 kN, with a ratio of mean cracking load to ultimate load of 0.26.

The location of the first crack would be expected to be at the bottom of the web of one of the unils, as the tensile stresses are a maximum in this location due to the web taper. However cracking would not be expected to occur at the bottom of the lowest unit due to the restraining effect of the bottom platen. This was consistent with the observed behaviour. The mean ultimate loads for the two types of block tests and the prism tests are shown in Table 4. Ir can be seen that the prism value falls between the value for the standard unit test (with platen restraint of the face-shells) and the unit tests with brush platens (with negligible restraint of the face- shells). Since the failure in ali three test types involved splitting of the webs of the units, ir is apparent that there is some lateral restraint present in the prism test. The face-shells of the top and bottom blocks are restrained by contact with the upper and lower platens, and the face­shells of the central block are restrained to some extent by contact with the units above and below. This latter restraint must also be present in a storey height wall. The effects of the platen will obviously depend on the number of units in the prism, but in a different way to solid specimens, since the mechanism of restraint in this case is different. To simulate as closely as possible the mechanism of wall failure, at least three high prisms should be used to ensure that at least one unit is not clirectly restrained by the platens.

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TABLE 4. ULTIMA TE LOADS OF UNITS AND PRISMS

TESTTYPE MEAN ULTIMA TE LOAD

(kN)

Standard Block Test 490

Brush Platen Block Test 348

Three High Prism Test 409

CONCLUSION

This investigation has shown that both the propenies of the capping material and platen restraint can significantly influence the observed strength of hollow units and prisms tested in compression. The mechanism of platen restraint in face-shell bedded hollow specimens is different to that for solid specimens, with restraint against lateral expansion being provided by the platen acting as a tie between the two loaded areas on the bearing surface. For a prism test therefore, the degree of platen restraint is more a function of the number of units in the prism rather than the overall dimensions of the specimen. Prisms should consist of at least three units to produce a failure mechanism eloser to that in a wall.

In-situ tests on various capping materiaIs has revealed non-linear behaviour of the materiaIs, and large variations in stiffness particularly for plywood and hardboard. lt is recommended that the use of hardboard be discontinued as an alternative to plywood in the Australian Code.

ACKNOWLEDGEMENTS

This work forms part of a larger research project on the compressive behaviour of hollow masonry. The support of the Australian Research Council, CSIRO Division of Building Construction and Engineering, the Clay Brick and Paver lnstitute, and the Concrete Masonry Association of Australia is gratefully acknowledged. The assistance of Mr Lai Fook Ming and Mr Mike Lewis in perforrning the tests is also appreciated.

REFERENCES

1. PAGE, A.W. and SHRIVE, N.G., "A CriticaI Assessment of Compression Tests for Hollow Block Masonry", Masonry International, Vol. 2, No. 2, 1988, pp.64-70.

2. AUSTRALIAN STANDARD 3700-1988, SAA Masonry Code, Standards Association of Australia.

3. KLEEMAN, P.W. and PAGE, A.W., "The In-Situ Properties of Packing MateriaIs Used in Compression Tests", Masonry lnternational, Vol. 4, No. 2, 1990, pp. 68-74.

4. PAGE, A.W. and MARSHALL, R., "The lnfluence of Brick and Brickwork Prism Aspect Ratio on the Evaluation of Compressive Strength", Proc. 7th IBMAC, Melbourne, 1985, pp. 653-664.

5. PAGE, A.W. and KLEEMAN, P.W., "A Study of the Compressive Behaviour of Hollow Masonry", Research Report No. 060.02.1991, Department of Civil Engineering and Surveying, The University of Newcastle, April, 1991.