Some experimental comparisons of mechanical resistance between cellular day blocks and Iittle walls built with the same blocks

Norberto Tubi, Franeesco Cantoni, Adriano Fantueci CNR-ICITE

1) ABS1RACT This report coneerns some aequisitions following experimentations which were carried out aecording to a research program eoncerning the mechanica1 behaviour of sample-linle waIIs realized with ceIlular clay blocks. The tests implied monotone loads for the blocks (three types), monotone and eyelic loads for the linle walls (realized with these three types of blocks). Important differenees in the trends were reported between the behaviours of isolated blocks and the behaviours of the walls. It is possible to get some information in order to start some studies for the improvement of the products.

2) IN1RODUCTlON Research goal The research aims at obtaining compared data about the mechanical behaviour in masonries realized with different types of eellularclay blocks - alI of them measuring 25x30x19 cm - with hole rates between 45 and 55% (these are the highest limits aceording to the Italian standard for hollow blocks to be employed in loadbearing masonries). The only differenee between these blocks is to be found in the posiúon of the internai clay webs). The little waIIs were realized with a 30x50x(lOO+ 12+ 12 ) cm size; the 12 em size is taken by concrete lumps on the top and on the foot The final aim of the research is to determine the variation of the degradation in the ductility of the masonries aeeording to the variation of hole rate percentage of clay blocks under stresses due to a centered verticalload. The cyelic tests were used to get some infoxmation about the "life cycle" behaviours of walls of buildings where important stresses also show cyclic features (even if with periods that are 10nger than those that are applied in the laboratory).

Keyword: Masonry; Cellular Clay Bricks; Monotone and cyclic tests; Influence of internal webs.

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3) TYPES OF TESTS The compressive strength tests of blocks (Ioads parallel to the vertical webs) were carried out according to the conventional method. Monotone and cycIic tests on Iittie walIs folIowed two methods: the former method implied the interposition of a rubber distance spacer and a steeI sheet between the press plates and the concrete lumps - upper and lower -, while the latter used plasterrenderings at the foot and on the top of the little walls. The only variables are to be found in the blocks and they concern the hole rate percentage and the position of the internal webs The internal webs of blocks 45 and 55 are lined up: block 50 has offset webs in the orthogonal direction as to the wall's plane.(fig. 1).

Fig. I) The figure c1early shows the differencesof the three types of blocks, in which, type 45 and 55 have lined up webs and type 50 (in the middle) has offset webs.

To obtain even blocks, these were produced in the sarne plant using the same clay and applying the same baking cycles, to avoid differences due to the application of different technologies: all the Iittle walls were produced by using the same mortar (premixed mortar). The aim was to obtain perfectly even mortar beds by fixing the value of the joint thickness at 1 em. The constancy of the thickness was obtained by using templates during the manufacturing stage. Thus, the thicknesses evenness alIows for more uniform elastic and plastic behaviours in the connexion of a single little wall and better references between the little walls. AlI the little walls were manufactured during 20 days and Iet to cure for about four months in order to obtain a suitably homogeneous curing, obtaining this way a behaviour that is sufficiently undifferentiated as to the performance of the long-lasting immediate and cyc1ic tests. The choice of a kind of block with offset webs having a hole rate percentage which lies between the hole rates of the other two kinds of blocks is to be reIated to an assumption (which carne out during a previous research) of a lower horizontal tensile strength in the orthogonal direction as to the wall plane in the case of blocks with offset webs stressed in the wall by the tensile action (horizontal) due to compression (vertical) produced by the beds mortar.

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Fig. 2) Bending scheme in the cJay webs produced by the horizontal traction caused by the compression of the horizontal joints mortar.

Since the webs are placed with the offsets in the onhogonal direction as to the wall plane (assuming a reduction of the thermal transmittance), the traction acting in the onhogonal direction as to the wall plane would meet the direction with the less horizontal tensile strength of the block since the offset of the webs would produce bending strains in the webs which are parallel to the wall plane. It must be also considered that the main plan si de of the three kinds of blocks employed is orthogonal to the wall plane; thus, it can be assumed that the most part of the tracúon energy is developed on the block, in the onhogonal direction as to the wall plane.

Fig. 3) This figure shows how the horizontal traction produced by the mortar, has no 'confinements' in the orthogonal direction as to the wall plane.

This way, if during the experimental stage, the behaviours of the mechanical strengths of the walls with offset webs had not been 'linearly interpolated with reference to the hole rate percentages' related to the other two kinds, it would have been easy to find out and understand whether the trend was towards a higher or rather towards a lower resistance. It should also be pointed out that the three sets of blocks with offset webs (during the performance of qualificarion tests) showed the best compression mechanical strength (the

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action is orthogonal to their lying); this strength is even better than that of blocks with a lower hole rate percentage. This can be due to the lower horizontal distance existing between the connections of the block 's offset webs, which turned out to be a better 'bracing' of the webs.

Fig. 4) This figure shows how the offset of the webs in one direction thickens the connection between webs, thus alIowing for a bener useful solidation in case of pure compression of the block.

4) QUALIFICATION OF BLOCKS AND MORTAR This qualification was carried out only under compression centered orthogonally as to the lying plane.

-Blocks

Kind of clay Mean strength Characteristic strength Coefficient of

fb (N/mm2 ) fbk (N/mm2) variation

45 15 13 0,08 50 17 15 0,08 55 11,5 9,1 0,013

It is evident that the block with a hole rate of 50% and offset webs has a comparatively better performance for what concems the mechanical resistance.

- Mortar Mean strength = 3,0 N/mm2

5) SETTING OF THE TESTS WITH MONMTONE LOAD ON LlTTLE WALLS AND ABTAlNED MEASURES. The monotone tests were executed by fixing load 'steps': the press load was increased each time of 2000daN, followed by a stop during which the values were read and 8 transducers automatica11y recorded by means of a data processor; each pause lasted some seconds; so the test on a little walllasted about 12-16 minutes.

A) Monotone load tests with hard rubber soles The contact with the press plates is realized by interposing a 1,5 cm-thick hard rubber sole, which is used to obtain an effect of nearly-concentred load ('dome' pressures) and also to reach the regularization of the tlatness defects on the top and at the foot. A 1 mm­thick sheet is placed between the panel and the rubber, both at the foot and on the top, in order to avoid the effects due to the horiwntal tracion, caused by the vertical compression

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of the rubber on the little wall.

The maximum strengths of the little walis tested are hereafter reported:

45/1 = 56000 daN 45!2 = 56000daN

50/1 = 34000 daN 50/2 = 38000 daN

55/1 = 34000 daN 55/2 = 30000 daN

average 56000 daN

36000 daN

32000 daN

B) Tests with monotone loads with plaster renderings at the foot and on the top of the little walls

Plaster renderings are used to reach the best distribution of pressures at the contact points and to obtain the regularization of the flatness defects on the top and on bottom

The maximum strengths of the little walls tested are hereafter reported:

45/1 = 70000 daN 45!2 = 72000 daN

50/1 = 52000 daN 50!2 = 60000 daN

55/1 = 54000 daN

average = 71000 daN

average = 56000 daN

average = 54000 daN

C) Cyclic tests with hard rubber soles

As already pointed out, the effect of rubber soles, is to distribute in a 'dome-lik:e shape' ali the pressures with analogies under localload on a poor1y effective load dispatcher. Tests were carried out by steps of maximum load with decreases down to the minimum value of 2000 daN each cycle. About 100 cycles were executed each step except for the last one, during which faihrre occurred. The maximum load of the first step was 1/5 of the mean strength measured in the homologous test with monotone loads, with increases towards the following steps which corresponded to 1/5 of the mean strength measured as previously pointed out.

The maximum strengths of the little walis tested are hereafter reported:

45/1 = 55000 daN 45!2 = 55000 daN average = 55000 daN

50/1 = 35000 daN 50/1= 28000 daN average = 31500 daN

55/1 = 42000 daN 55/2 = 30000 daN 55/3 = 30000 daN average = 34000 daN

D) Cyclic loads with plaster renderings

These conventional tests produce practically uniform distributions of pressures in the diagramo

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The maximum strengths of the little walls tested are hereafter reported:

45/1 = 56000 daN 45/2 = 56000 daN average = 56000 daN

50/1 = 44000 daN 50/2 = 55000 daN 50/3 = 55000 daN 50/4 = 33000 daN average = 55250 daN

55/1 = 54000 daN 55/4 = 48000 daN average = 51000 daN

6) COMMENTS OF THE TESTS To be concise, we will hereafter point out the most evident behaviours and make some remarks.

- First remark It has already been said that the blocks tested under centered vertical compression (perpendicular to the laying plane) produced results which are not proportional to their hole rate percentage; as a matter of facto the values were:

<I> 45 = 15 N/ mm2 (average strenght)

<I> 50 = 17

<I> 55 = 11,5

While the mean of the mean strengths of blocks 45 and 55 'should' have corresponded to the 'expected' strength of blocks 50 - (15 + 11,5)/2 = 13,25 N/mm2, blocks 50 being tested with 17N/mm2, we have a 28,3% increase as to the expected value. This 'anomaly' can be explained by the fact that the less the free horizontallength of the offset webs of the block, the better is the webs 'bracing' (whenever the load acts perpendicularly to the laying plane of the blocks in the wall); that is to say, a lower vertical 'slenderness' of the webs is obtained. According to the results of the tests on little walIs, it seems to be inadequate to make statements on the walls strength just on the basis of the tests performed on the blocks, at least on those with hole rate percentages between 45% and 55%.

- Second remark - Monotone tests on little walls As concerns little walls 50 (offset webs) it must be remarked that the performance worsens when related to a 'reliable' value corresponding to the mean of mean strengths of little walls 45 and 55. - Cyclic tests with rubber soles on the little walls As concerns little walls 50 (offset webs) the worsening is very strong as to a reliable value corresponding to the mean of mean strengths of little walls 45 and 55. - Cyclic tests with plaster renderings The performance of little walls 50 is far better than the reliable value.

As above stated, the effect of horizontal traction (due to vertical compression) caused by the mortar beds would be much more dangerous in blocks realized with offset webs, since these - as previously remarked - would be subjected to bendings in the webs parallel to the wall plane. It could, however, be expected that plastic deformations of the mortar wiU reduce, in the long run, these effects, as cyclic tests with plaster renderings seem to suggest.

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- Tlúrd remark During the tests on little walls, crackings under centered verticalload occurred at the beginning and sometimes only along the shortest sides of the little walls (corresponding to the longest sides of the blocks). At the moment, the interpretation of this phenomenon is absolutely not clear. Anyway, by resorting to some superimpositions it could be possible to give a number of reasons. Hereafter are listed, not in order of importance, some of these reasons:

a) it could 'simply' be a lower attitude of the blocks in question to resist the traction perpendicular to the wall plane; this explanation could be supported by the following assumptions: 1) crackings begin to appear on the longest side of the block, that is to say in the direction along which, due to its larger surface, the greatest horizontal action of the vertically compressed mortar is developed; 2) crackings appear along the two directrices of the setting holes, --- the traction acting along the direction of the longest side of the block meets the less resisting section of clay since the presence of the setting holes is responsible for the elirnination of at 1east one clay web per each hole in the direction of the longest side, --- it must be supposed that in the area of the setting holes the bending actions in the webs are higher, --- it can be supposed that in the area of the setting holes the self-induced stresses produced during the manufacturing stage of straining, drying and baking, are to be found in a larger number (the comers of the little walls showed frequent damages); some exploratory tests - which are not described here, and which were carried out without any equipment - in which the block were stressed also by tractions parallel to the lying plane of two hard rubber soles - lower and upper -, showed damages which were reasonably due to lacks 1) in the proportions among the local stresses in the block of the wall

2) in the balances of the "webs-rods" converging in the different connections 3) in the balanced distribution of the resisting masses of clay in the horizontal section of the blocks (vertical compressive strength and vertical shear strength).

b) There is an assumption which has not yet been examined closely, which could, nevertheless, help to explain the factors causing the cracking of the little walls: the values of tensions perpendicular to the lying plane distributed on the two horizontal surfaces of a generical block belonging to the little wall are not uniform since (in alI the three examined blocks) the central third of the block (taken along the base longest side) has less surface of clay-mortar contact with reference to the external thirds, assuming a load which is evenly distributed by the press on the little wall, there are thus more mortar's tensions in the middle, rather than on the sides, producing a different deforrnation in the middle, with reference to the side thirds. In other words, ir produces a shear stress in the two 'boundary ' areas between the described block 's thirds, meeting, in the vertical direction, two less-resistant sections at the setting holes.

Some anticipations about possible dernands for an improvement of blocks design

The following remarks can be applied to the production in general; it must be also pointed out that alI of these comments are not based on the recording of specific and clear defects, but aim at proposing morphological variations in order to obtain functional improvements. What follows , simply means to show areas of study and support new concepts, needing deeper investigation of the geometrical-dimensional design of the blocks and their verification for what concems their static performances:

- each manufacturer should not carry out the analysis of just the compressive strength of the blocks in one or more directions; in fact, he should execute a static verification of the block's performances either during the manufacturing stage, or during the planning stage,

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in order to balance at least the tensile strength behaviours in the two horizontal directions;

- in the case that horizontal tractions of the mortarproduce bendings along the clay webs, so, a reduction of the span (this case has to be verified for ali the different characteristics of each type of block), it would be advisable to eliminate the configurations of those webs producing the bendings and to realize connections andlor dimensions in the webs, according to a diffused balance along the whole section of the block (the setting holes are necessary, but they are an element of discontinuity),

- anyway, the thickness of the clay webs should not be constant, but it could be useful to introduce more distance spacers in the areas in which the stresses are stronger (especially bending stresses in each web) mainly in the perimetrical webs located in the areas of convergence at the corners of the blocks and in the webs surrounding these corners,

- the best response of the blocks with offset webs during the compressive test on the block subjected to the action of the press, could suggest the use of these kind of blocks in those applications where the rnortar produces a smaller horizontal traction,

- allof the pressures caused by the block on specific areas of the mortar should be made hornogeneous in order to avoid vertical shear effects in the blocks and recognizable areas in the blocks should not have reciprocal effects of reduction of vertical shear strength (such an assumption can be applied to elernents with vertical holes),

- the manufacturer should point out the series of characteristics of the mortars to be used with specific blocks in order to produce optimal resluts, by providing for more precise instructions for use as concerns the materials used in the masonries.

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