Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 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. Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 368 Strrcctrrral Srudir~,Rrpam atrd Marnrorancr of Hrriragr Aldzirrcruw VIII 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. Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 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 Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 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 Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 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 Transactions on the Built Environment vol 66, © 2003 WIT Press, www.witpress.com, ISSN 1743-3509 372 Strrcctrrral Srudir~,Rrpam atrd Marnrorancr of Hrriragr Aldzirrcruw V111 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.