Ŕ periodica polytechnica Effects influencing the compressive Civil Engineering strength of a solid, fired clay brick 55/2 (2011) 117–128 doi: 10.3311/pp.ci.2011-2.04 Anita Fódi web: http://www.pp.bme.hu/ci c Periodica Polytechnica 2011 RESEARCH ARTICLE Received 2011-01-03, accepted 2011-03-01 Abstract Notation The aim of the paper was to analyze the effect of the differ- fb,65 the compressive strength of the brick specimen of 65 mm ent influencing factors on the compressive strength of a solid, height [N/mm2] clay brick. The effect of the surface polishing, the height, width 2 Aloaded the loaded area [mm ] and length of the specimens, the type of the sampling (drilled or cut) and the effect of the production technology were studied. h the height of the specimen [mm] Though being the moisture content of the specimen an important f ∗ the compressive strength of the specimen of 100 mm x100 factor, it was not examined herein. More than 110 brick spec- b mm loaded area [N/mm2] imens were tested using two different types of measuring pro- cedures: standard (destructive) compressive tests and Schmidt fb the normalized mean compressive strength of the brick hammer (non-destructive) tests were carried out. Formulae [N/mm2] were proposed for determining the normalized mean compres- δ the shape/conversion factor that is suggested to calculate the sive strength from every possible type of specimens that were normalized mean compressive strength of a masonry unit ac- adequate for testing in case of this type of unit. It was found cording to [1] that the behaviour of the brick is orthotropic. It was proved that the effect of the height of the specimen has the most significant Ri the rebound value of the Schmidt hammer tests effect on the compressive strength. A linear relationship was v the smaller size of the loaded area of the brick (the width of found between the normalized mean compressive strength and the brick) [mm] the Young’s modulus of the brick in each direction. 1 Introduction Keywords The determination of the compressive strength of the brick is solid clay brick compressive strength experimental investi- · · not only important because it is necessary for the calculation of gation size effect Young’s modulus · · the compressive strength of the masonry but also it can be ap- plied for determining the Young’s modulus of the brick. The Acknowledgement elastic modulus of the masonry is expressed in the function of The author would like to express her grateful thanks to the the compressive strength of the masonry. For defining the com- colleagues in the Laboratory of the Budapest University of Tech- pressive strength of the masonry Eurocode 6 [2] suggests using nology and Economics and Wienerberger Ltd for offering mate- the normalized mean compressive strength that is the compres- rials and subsidy. This work is connected to the scientific pro- sive strength of a unit with the dimensions of 100 mm 100 mm × gram of the “Development of quality-oriented and harmonized 100 mm in case of air-dried condition. × R+D+I strategy and functional model at BME” project. This The normalized mean compressive strength is hard to be mea- project is supported by the New Hungary Development Plan sured. For most of the bricks, the size of the unit makes it impos- (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). sible to determine the strength of the brick directly. The standard MSZ EN 772-1:2000 [1] does not allow to investigate compres- sive strength on a sample that has a height/smaller length ratio smaller than 0.4 or if the height of the specimen is smaller than Anita Fódi Budapest University of Technology and Economics, Department of Structural 40 mm. Therefore, in case of units with small height, samples Engineering, H-1111 Budapest, Bertalan L. 2, Hungary have to be cut or the standard suggests applying multiple sam- e-mail: [email protected] ples. Large units are appropriate for cutting cubic samples of Effects influencing the compressive strength of a solid, fired clay brick 2011 55 2 117 10 cm height. The standard [1] allows putting more bricks on built in the wall normally lengthwise, the height of the unit is each other. This raises the issue of contact, thus in this paper it 65 mm and as usual it is loaded on the 120 mm 250 mm area. × is intended to be avoided. The thickness of the wall is 120 mm. In the case of buildings, The surface of the brick should be plane; the number of the the cores obtained by horizontal drilling from building always tested specimens should be at least 6. A further problem of the show the compressive strength of the other two directions. investigation of the strength is that brick size tolerance can be 6 mm in 65 mm. Most of the bricks are too flat to investigate 3 The production technology the strength directly. The standard [1] suggests determining the Throughout the world, there are two possible ways of brick mean value of the compressive strength of six bricks in real sizes producing: the brick can be formed either by extrusion or by and then calculating the effect of the dimensions of the brick. moulding. The former is applied in case of clays with high level For the normalized mean compressive strength the mean com- of plasticity. When the plasticity of the clay is lower, the latter pressive strength has to be multiplied by a shape factor. This procedure is used characteristically [5]. In this case the clay factor takes into account only the height and the smaller loaded cake is filled into brick moulds, and then pressed. side of the brick. The compressive strength of the brick can In Hungary only the first technology is typical. First, the clay depend strongly on the production technology. The amount of exploited from the clay field is transferred into the factory where additional sawdust, the splitting process and the firing tempera- it is smashed with additional water and sawdust. After mix- ture, the restraint effect of the platens, the height and shape of ing it is pressed into a machine, and upon extrusion a column the specimen can have an effect on the compressive strength. arises that is split into brick pieces. The raw bricks are con- The size and shape of the loaded area can have an influence on veyed into the drier where the bricks lose the majority of their the results. In addition, the moisture content of the specimen moisture content under 40-100 ˚C. Then units are transferred can also have an effect on the strength. In this paper the effect of into the brick burning oven, where they are fired and afterwards the surface polishing, the height, width and length of the speci- burnished and bundled. That is the reason why visible shrinkage mens, the type of the sampling and the effect of the production cracks can be seen on the surface of the most of the bricks. technology and the effect of the burn in were investigated on the Table 1 shows the results of a survey of the Brick Institute compressive strength. of America for the bricks in the United States. It shows that The novelty of this paper is the investigation of the influ- the compressive strength of the brick depends strongly on the ence of the effect of the production technology (effect of the production technology (extruded or moulded). direction of the loading), the effect of the surface polishing, the Tab. 1. Effect of the production method [6] height, width and length of the specimens and the type of sam- pling (drilled or cut) on the compressive strength. The aim of Mean value of Standard the deviation of the the paper was to find a correlation between the compressive Coefficient of Type of production compressive compressive strength and the mentioned features. Instead of the time con- variations (%) strength strength suming method of the standard [1] a simple, quick determina- [N/mm2] [N/mm2] tion (Schmidt hammer tests) could be appropriate for determin- Extruded 77.9 30.8 39.5 ing the normalized mean compressive strength. Moulded 36.5 12.6 34.5 2 Motivations A comprehensive experimental investigation of the compres- The δ conversion factor of [1] can be applied in every loading sive strength of solid bricks was missing from the literature be- direction. The factor contains the effect of the height and the fore. If the compressive strength of the brick is dependent on smallest width of the unit. The factor orders higher strength to the type of the production technology then brick anisotropy can the bigger blocks, which is not always true. The height of the be assumed. The Eurocode 6 [2] does not contain the effect of brick is defined as the dimension of the sample in the direction the anisotropy. of the load applied. Therefore it is applicable by calculation of The investigation of the effect of the loading direction on the normalized mean compressive strength in every direction of compressive strength is important in the following cases: 1. the units. The δ factor does neither contain the effect of the other when the unit is placed not only in the lengthwise direction of dimension of the brick, nor the difference because of the produc- the brick into the wall (see for example [3] and [4]) or 2. it is tion direction. Most bricks contain holes that cause a smaller built in lengthwise but loaded from the other direction (in case compressive strength.