Compressive Strength of Compressed Earth Block Masonry

Compressive strength of compressed earth block masonry

G. Bei & I. Papayianni Department of Civil Engineering, Laboratory of Building Materials,

Aristotle University of Thessaloniki, Greece

Abstract

Masonry structures constitute a great part of the building stock in the world. Earth block masonry as architectural heritage attracts the interest of engineers for maintenance and modern construction since it is a material of high ecological profile. This issue gains more and more field in the engineering community. Compressive strength is one of the essential mechcal properties to characterise the stress bearing capacity of the materials. The objective of this research is the evaluation of the cornpressive strength of the earth block masonry by using specimens of different geometry aspect ratios. Firstly in the experimental programme, was optimised the synthesis of the mixture for both, earth brocks and earth mortars. For this purpose an adequate number of earth blocks were produced in the laboratory using an apparatus especially designed for that. Uniaxial compression tests are typically undertaken on samples of earth mortars, on single earth blocks, on doublets, and triplets of blocks and lastly on masonry specimens. Therefore a complete image of the behaviour of the earth on the construction is given. In addition, stress-strain diagrams and elasticity modulus are made and discussed. The results of this work could be useful for the design of earth block structures.

1 Introduction

The universality of earth as a building material is fascinating; it is estimated that more than 30% of the world's buildings are made of earth today [l]. These structures constitute an architectural heritage deposit in almost every country worldwide. The extensive use of earth is due to the low cost, availability and feasibility of it. Furthermore under the prism of modem directive of sustainability in construction, earth as building materials is of great interest.

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Wherever earth blocks construction is required to meet minimum regulatory performance requirements it is necessary to complete quality control testing. Compressive strength is one of the essential mechanical properties to characterize the stress bearing capacity of the materials. The objective of this research is the investigation of the range of compressive strength of earth masonry and its components (earth mortars and blocks) and to produce strain - stress diagrams for the evaluation of the structural behaviour of earth walls. This article is a part of a Ph.D. research program, which is still in process at the Department of Civil

Engineering, Laboratory of Buildings Materials.

2 Experimental program

Following tradition in building with earth [2] it was decided to use soil and sand. All specimens were fabricated by using soil from the area of Kilkis (soil used for industrial fixed bricks production) and river sand (0-2mm) from the river Axios. According to Casagrande characterisation the soil used is characterised as CL (CIay Loam) soil. Mineralogical type, Attemberg characteristics, and chemical analyses of this soil as well as the gradation of the soil - sand mixture are outlined in Table 1.

Table 1: Physicochemical and Technical characteristics of earth mixture used for earth mortars and bIocks

Chemical composition (%) Attemberg limits Particle size analysis 1.37

LL : 32,5 Sand fraction: 41,86% cao PL : 19,5 Silt fraction: 35,54% M@ PI : 13,O Clay fraction: 22% Fe203 Classification : CL A1203 (Casagrande) SiOl Mineralogical Loss of ignites comvosition : Quartz, smectite, illite

Six different soil - sand mixture compositions (Kl to K6) were used for the estimation of the earth mortars range of strength. All proportions were mixed in dry conditions to an homogeneous paddle and water was gradually added to attain the mixture optimum moisture content according to the desired workability level of 13,5f0,5 cm after 15 blows [2]. The curing conditions were the same for all specimens prisms (4Ox40x160mm) which were kept in room with 20+2 "C and RH=75% until the day of testing. In the table 2 the mixture proportions of the components are given as well as some other characteristics as the water percentage of the mixture, the specific gravity of the tested specimens, the compressive and flexural strength at the 60 day of mature (mean value of three specimens), the total porosity and shrinkage and the moisture content of specimens when tested.

For the earth block production three different soil-sand mixture proportions (A, B and C) were used. The moisture content of these blocks was determined through modified Proctor tests [3] for the three mixing proportions (see table 3). The production of blocks was made in an apparatus especially designed for this purpose at the laboratory (see figure 1). Earth blocks were plain solid blocks with dimensions of 25Ox 12Ox80mm and weight approximately of 5 kg each one. For the manufacture of every block it was compacted by static compression under 5 ~lmrn' of pressure, which is considered as medium pressure [4]. They were cured in a room of 20f 2 "C and RH=90% for one week and then at room of temperature of 20f 2 "C and RH=65% after, until the testing day.

Figure 1: Earth block production apparatus

Table 2: Earth mortars characteristics

370 Strrcctrrral Srudir~,Rrpam atrd Marnrorancr of Hrriragr Aldzirrcruw V111

Compositions A, B and C were used for the manufacture of single specimens, doublets and triplets (see table 3). Individual blocks were formed in cubes (8Ox80x80mm) by sawing, before tested. All the specimens were sandwiched between two silicon thin sheets and plywood on the top of them to minimize platen restrain effects. Compressive stress was applied continuously at the rate of 3,5 ~/drninup to failure. Doublets (125x120x170mm) of A, B and C earth blocks were produced following RILEM recommendations, with two half-block stack bonded prism mangement with an earth mortar stabilized with ordinary Portland cement [5]. The mortar compressive strength attained the 4,4 ~lmrn~.Triplets with dimensions of 25Ox120x260mm, were produced with earth blocks A, B, and C and mortar composition K3 of thickness of 1 cm. All specimens were tested under uniaxial compressive strength at an age of 60 days after at specified curing conditions (temperature of 20+2 "C and RH=65%). Three specimens of each composition were used of the determination of the compressive strength for both doublets and triplets. In figure 2 is showed the different types of specimens while in the table 4 the mean value of three tested specimens for each composition is presented.

Figure 2: earth block (a) unit specimen, (b) doublet (c) triplet

Table 3: Earth block production characteristics

Mixture Soil (%w/w of solids) Sand (%w/w of solids) W,,, yd compositions ("h) (gdcm3) 12 A 90% 10% 1,975 B 80% 20% 11,9 2,38 C 70% 30% 11,75 2,l

Table 4: Mechanical characteristics of earth blocks and earth walls

Afterwards three wallets were constructed in scale one to one with the following geometry: length 770rnq height 800mm and thickness 120mm. The thickness of the joint earth mortar was 10mm. The mixture composition of the wallets was the following: B earth blocks and K3 earth mortar. These were tested under uniaxial compressive strength [6]. For the stress-strain tests, the specimens were instrumented with a number of displacement transducers in order to record the displacement response during loading of the specimens as well as the deformations at critical points of maximum compression. In the figure 3 the scheme of instrumentation used is depicted. In figure 4, the compressive strength of the wallet (f,,) in ~lrnrn~,the longitudinal deformation (E,,,$ in the direction of the applied load and the transversal deformation (E,,,) in the horizontal direction, is presented.

Figure 3 : Earth block wallet. Figure 4: Stress-strain diagram on three earth

Instrumentation scheme. wallets under uniaxial compressive strength

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3 Discussion of experimental work

3.1 Earth mortars

Using a specific type of soil and sand the tested earth mortars were produced. These mixtures have exhibited compressive strength of 1,94 to 3,21 NI~of range, while their flexural strength was between the 1,13 to 1,83 ~/rnm~.According to their strength they seem to be between the categories 2 (fm>l,5~/mm~)and 3(fm>2,5~/mn?) of those of the recommendation of ARS 672: 1996 of compressed earth mortars standards in order to get mechanical constraints [7]. Category 2 was made for structural elements capable of withstanding important external (live) loads and category 3 was made for structural elements capable of withstanding high external (live) loads). The relatively high deformation plays an important role for the volume stability of earth mortars and it is obvious (table 2) that the addition of sand to the soils reduces the shrinkage tendency whle does not contribute to the strength development. Therefore for the selection of the best mixture for earth mortar a compromise between shrinkage effect and mechanical characteristics should be made.

3.2 Earth blocks

The tests on blocks were principally aimed to the determination of the compressive strength. The experimental results are summarized in table 4. Doublets and triplets of earth blocks can be considered as small-scale masonry prisms (combination of blocks and joints by mortars) although their behaviour is nearer to this of singular earth blocks. Masonry prisms contain vertical joints and different constructions texture type, that doublets or triplets could not simulate them. It seems that the mode of failure in compression is dependent on the test method.

For all the blocks doublets and triplets tested, the onset of failure corresponds to the development of the familiar hourglass shape following spalling of the vertical sides. When the ratio heighdwidth is high (>1,4) the hourglass shape of cracks is less distinguished. The cracks are getting more vertical and are mainly centralized on the specimen surface in comparison with those of cubes of the heightlwidth ratio equal to one. For doublets and triplets however, the failure is more akin to that observed in masonry wall under uniaxial compression. Singular block specimens, doublets, and triplets develop vertical cracks throughout the specimen as the maximum load was approached. The compressive strength values of singular specimens are higher from those in doublets and triplets (from 65% to 80%) (see table 4). Platen restrain effects on unit specimen strength were evident by the apparent increase of strength with the reduction of heighvwidth. An effort was made to take into account the geometrical effects of the experimental confined conditions by using the factors of the table 5 [8]. In the table 6 are summarised the "unconfined" compressive strength for earth blocks B, with the heighttwidth correction factors given in table 5.

Table 5: Aspect ratio correction

Table 6: Unconfined compressive strength

Unconfined

Doublet Trinlet 2.16 0.78 2.05

Comparing the strength-composition relation of earth blocks, the compressive strength of A composition seams to be higher than the corresponding of B and C blocks between all the tested specimens (unit specimens, doublets and triplets). Ths may be due to the higher soil percentage content as well as to the clay content of A composition. It can be explain the higher cohesion between the grains of the earth brick resulting in higher strength development. Compressive strength of blocks ranges between the limits of those of Eurocode 6 that demands 2,5 N/rnm2 for masonry blocks capable to withstand external loads. Minimum requirements for unit compressive strength have been outlined elsewhere [9]. For unconfined dry compressive strength a minimum requirement of 1,4 ~lrntn'is commonplace. This

strength level is indicative of sufficient safety for transportation and wall fixing requirements.

3.3 Earth block masonry

Masonry wallets were tested using B composition on earth blocks and K3 earth mortar. The compressive strength of these wallets is indicative for earth walls made by soil and sand without any stabilization. This type of earth wall can be considered as more representative for traditional earth construction because it uses the same natural raw "traditional" materials. Masonry specimens started to develop cracks much earlier before the ultimate load of the wall. This characteristic behaviour of masonry is attributed to composite interaction between the blocks and mortars joints [10]. The failure mode was typical for wallets of fb,>fm. Detachments of the vertical joints started at the 70% of the ultimate load. This was more obvious where the bonding of the "confined" specimen

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is abated because of hiction at the loading surfaces. Failure is of brittle type and it is developed within the blocks mass. It is emphasizing that earth masonry is not included in Eurocode 6 [ll]. However, an estimation of fwcis attempted to see the relation between experimental values and analytical ones. Eurocode 6: fw, = KX~~'~'X f,0,25 Wallet: fwc=0,6x 3, X 2,50g2'= l,% ~/mm'(22% deviation from experimental mean value of the three tested specimens that is 1,92 ~/mm') The stress-strain diagrams outline a non-linear behaviour. This could be noticed by the low elastic part of the ultimate deformation is ~,,=f~,/~,,=2/1333=1,5xl0~~ which is 6,6 times less from the ultimate E. The ultimate longitudinal deformation

rises the 0,0074. Eurocode 6 adopts 0,2 to 0,35% maximum deformation of masonry. The E,,,,,,,,I of the wall measured at the middle of the specimen reaches the 0,0019 which is very high compared it with the E value of fired brick masonry (see figure 4). The reduction of the stiffness due to the detachments of the vertical joints has influenced to the transversal deformations of masonry during loading.

4 Conclusions

Wherever earth blocks construction is decided it is necessary to design it carehlly and perform quality control in order to meet minimum regulatory performance requirements of building codes.

Earth mortars of soil and sand can be considered as material of strength range 1,94 to 3,21 N/mm2 of compressive strength, while their flexural strength is between the 1,13 to 1,83 ~/mrn~.For the selection of the soil mixture a compromise between shrinkage effect and mechanical characteristics should be made in each case of raw material. High soil content on earth block mixture increases the cohesion between the grains of the earth block resulting to higher strength of blocks of the same compaction rate. The mode of failure in compression of the earth blocks is dependent on the test method. The heightlwidth ratio plays a role in the final crushing value. For doublets and triplets specimens, the failure seems to be more akin to that observed in masonry wall under uniaxial compression. Compressive strength of earth blocks is higher (3,12 NI&) than the lower limits (2,5 ~/mm~)of that of Eurocode 6 for blocks for masonry capable to withstand external loads. The construction with earth masonry is not included in Eurocode 6. The stress- strain diagrams of the tested wallets outline a non-linear behaviour and very small elastic part. The wall deformations are very high (0,007) compared with those of concrete or those of fired brick masonry. The results of this work concern the basic mechanical characteristics of earth block masonry contribute to the establishment of relevant Building Code.

References

[l] Houben H. and Guilaud H., Earth construction: a comprehensive guide, IT

Publications, London, 1994. [2] Bei G., Raw Earth: an ancient and modem building material, Master Thesis, Katholieke Universiteit Leuven, 1996 [3] American Society for Testing and Materials, Standard Test Method for Moisture

- Density relations of soils and soil-aggregate mixtures using 10-lb ramrner and 18- in. drop, ASTM D1557-78 [4] Compressed earth blocks Standards, ARS 670-1996, CO- publishers CDI- CRATerre - EAG, 1998. [5] Olivier M., Mesbah A., El Gharbi, Z., More1 J.C., Test method for strength test on blocks of compressed earth, Materials and Structures, 30, November, 515-517, 1997. [6] CEN, prENI1052-1: Methods of test for masonry - Part 1: Determination of compressive strength 1991.

[7] Compressed earth mortars Standards ARS 670-1996, CO- publishers CDI- CRATerre - EAG, 1998. [8] Middleton G.F. (revised by Schneider L.M.0 Earth wall construction, Construction and Engineering, Bulletin 5, CSIRO Division of Building, 4' Edition, Sydney, 1992.

[9] Walker P,, Specifications for stabilized pressed earth blocks, Masonry International, 10, (1) 1996. [l01 Hendry, A.W ., Structural Brickwork, Macmillan, London, l98 1. [l l] Eurocode 6: Design of masonry structures Part 1-1: General rules for building.

Rules for reinforced and unreinforced masonry, ENV 1996-1-1: Brusselles, 1995.