Walls of Interlocking Stabilised Soil Blocks (ISSBs) have been considered in low-cost houses around the world especially in developing countries. These were reported to be very weak in resisting the lateral load (e.g. wind or earthquake) without special considerations. In this paper, mechanical properties (compressive strength, elastic modulus, pre/post crack energy absorbed and toughness index) of ISSBs with three configurations and seven combinations of plain and fibrous mortar cubes are experimentally evaluated. Sisal fibre and rice straw (2% and 5%, by cement mass) were considered for fibrous mortar. Empirical equations were developed to predict elastic modulus. It was found that ISSBs had reasonable strength to be considered for masonry. The failure load and toughness index of 2% sisal fibre samples was improved by 10% and 16%, respectively, whereas 2.21 times enhancement was found in elastic modulus. Thus, 2% sisal fibre in plaster (i.e. reinforced coating) would likely improve the lateral resistance of interlocked masonry walling.
Due to increase of population in developing countries the need forlow-cost residential housing has increased considerably around the world (
Mechanical properties of ISSBs by other researchers
Type of block | Compressive Strength (MPa) | Modulus of Elasticity (GPa) | Shear Strength (MPa) | Compressive Toughness (MPa) | Shear Toughness (MPa) | References |
---|---|---|---|---|---|---|
16.48 | 2.34 | 2.65 | 0.56 | 7.59 | Ali et al. 2012 | |
5.42 | - | 0.48 | - | - | Anand et al. 2000 | |
15.2 | - | - | - | - | Jaafar et al. 2006 | |
14.28 | - | - | - | - | Lee et al. 2017 | |
2.5 | - | - | - | - | Fundi et al. 2018 |
As reported in the literature, interlocking block wall showed reduced lateral resistance to wind and earthquake. Therefore, to enhance lateral resistance, different techniques, like plain or reinforced grouting, surface bonding and plastering, were used (
Mechanical properties of fibre reinforced mortar by other researchers
Type of Fibre | Fibre Volume Fraction (%age) | Compressive Strength (MPa) | Modulus of Rupture /Flexural Strength* (MPa) | Toughness Index | Modulus of Elasticity (GPa) | References |
---|---|---|---|---|---|---|
0 | 28 | 5* | 0.45 | - | Pereira et al. (2015) | |
3% | 22 | 8* | 3.28 | - | Pereira et al. (2015) | |
8% | - | 16.8 | 1.41 | 6.07 | Savastano et al. (2009) | |
8% | - | 21.8 | 0.59 | 6.7 | Savastano et al. (2009) | |
4% | 29 | 8.9* | - | - | Teresa et al. (2012) | |
5% | 41 | 10* | - | - | Lertwattanaruk et al. (2015) | |
5% | 36 | 8* | - | - | Lertwattanaruk et al. (2015) | |
0.5% | 21 | 7.50* | - | 7.9 | Islam et al. (2011) |
In otherstudies, the performance of interlocking block masonry wall was tested and it was found that the compressive properties of the wall were directly related to the strength of masonry units (
Schematic diagram of proposed technique for house construction a. Plan, b. Section and c. Elevation.
In the preparation of interlocking soil-stabilised blocks soil and ordinary Portland cement were used. Soil was outsourced from Boko quarries Dar es Salam and it was obtained at a depth of 1 m below the earth surface in order to eliminate the addition of humus materials. The cement was sourced from Twiga cement factory Dar es Salam. Chemical and physical properties as adopted from the manufacturer are shown in
Chemical and physical properties of cement used
Properties | Type | Values |
---|---|---|
Chemical properties | SiO2 | 17.55% |
Al2O3 | 4.70% | |
Fe2O3 | 1.77% | |
CaO | 64.74% | |
MgO | 1.26% | |
Na2O | 0.37% | |
Physical properties | Porosity | 12.21% |
Density (g/cm3) | 2.11 | |
Water Absorption | 5.96% |
Properties of soil used for ISSB
Soil Property | Values (%) |
---|---|
Grain size distribution: | |
Sand | 70 |
Clay | 23.3 |
Silt | 6.7 |
Shrinkage | 7.5-9.17 |
Atterberg limits: | |
Liquid limit | 31.70 |
Plastic limit | 22.90 |
Plasticity index | 8.80 |
The same cement was used for the preparation of plain and fibrous mortar cubes. Three types/conditions of natural fibre were used, including sisal, rice straw and treated rice straw as shown in
Properties of sand used
Properties | Value |
---|---|
Fineness Modulus | 2.14 |
Dry Unit Weight (kN/m3) | 14.89 |
Density (g/cm3) | 2.51 |
Absorption % | 1.77 |
Moisture | 7.08 |
Fibres used, a. Sisal, b. Rice straw, and c. Treated rice straw.
The mix proportions for plain and fibrous mortar cubes are listed in
Mix proportion and labelling of mortar specimens
Combinations | Sample Symbol | Mix Proportion | W/C ratio | Fibre content* |
---|---|---|---|---|
Cement: Sand | ||||
Plain Mortar Cubes | A | 1:3 | 0.50 | - |
2% Rice Straw Mortar Cubes | B | 1:3 | 0.67 | 2% |
2% Sisal Mortar Cubes | C | 1:3 | 0.67 | 2% |
5% Rice Straw Mortar Cubes | D | 1:3 | 0.67 | 5% |
5% Sisal Mortar Cubes | E | 1:3 | 0.67 | 5% |
2% Treated Rice Straw Mortar Cubes | F | 1:3 | 0.67 | 2% |
Note: * by mass of cement
ISSBs were available in the National Housing and Building Research Agency Tanzania (NHBRA) which were prepared using manually pressed machine with dimensions of 300 mm (length) x 150 mm (width) x 100 mm (height). The soil-cement ratio for ISSBs was 12: 1.
The cubes and blocks were tested as per standards (
Typical test set up for mortar cubes.
Test set up of ISSBs, a. Single block, b. 1x2 blocks, and c. 2x2 blocks.
The compressive stress-strain curves are shown in
Stress strain curves for a. Plain mortar, b. 2% Rice straw reinforced mortar, c. 2% Sisal fibre reinforced mortar, d. 5% Rice straw reinforced mortar, e. 5% Sisal fibre reinforced mortar, and f. 2% Treated rice straw reinforced mortar.
Fractured surfaces, a. Plain mortar, b. 2% Rice straw reinforced mortar, c. 2% Sisal fibre reinforced mortar, d. 5% Rice straw reinforced mortar, e. 5% Sisal fibre reinforced mortar, and f. 2% Treated rice straw reinforced mortar.
Compressive strength was taken as the peak value of stress from the stress-strain curve. The area below the stress-strain curve up to the stress of first crack was defined as pre-crack absorbed energy PEp. The area below the stress-strain curve from the first crack stress to maximum load was consideredto be post crack energy absorbed CEp. The total area below the stress-strain curve from initial to maximum load was categorised as total energy absorbed TEp. Compressive toughness index (CTI)p was determined by the ratio of total energy absorbed (TEp) and pre-crack energy absorbed PEp.
Compressive properties of fibre reinforced mortar
Specimen | First crack load (kN) | Max load (kN) | Compressive Strength Cp (MPa) | Failure Mode | Pre-crack energy absorbed PEp (MJ/m3) | Post-crack energy absorbed CEp (MJ/m3) | Total energy absorbed TEp (MJ/m3) | Compressive toughness index CTIp (-) | Modulus of Elasticity Ep (GPa) |
---|---|---|---|---|---|---|---|---|---|
A | 90.5 ± 44 | 160.0 ± 44 | 19.3 ± 5 | Crushing | 0.017 ± 0.02 | 0.0035 ± 0.005 | 0.021 ± 0.02 | 1.21 ± 0.82 | 2.99 ± 0.41 |
B | 52.5 ± 24 | 90.0 ± 24 | 9 ± 1.15 | Buldging | 0.014 ± 0.01 | 0.0043 ± 0.004 | 0.018 ± 0.01 | 1.30 ± 0.76 | 2.05 ± 0.30 |
C | 106.7 ± 52 | 173.3 ± 52 | 19.8 ± 3.8 | Bridging | 0.03 ± 0.02 | 0.012 ± 0.01 | 0.041 ± 0.04 | 1.41 ± 0.70 | 7.17 ± 0.20 |
D | 34.7 ± 12 | 55.0 ± 12 | 6 ± 0 | Tensile | 0.016 ± 0.01 | 0.004 ± 0.003 | 0.02 ± 0.01 | 1.23 ± 0.81 | 1.13 ± 0.17 |
E | 95.0 ± 32 | 153.3 ± 32 | 18 ± 2 | Tensile | 0.03 ± 0.02 | 0.009 ± 0.008 | 0.036 ± 0.003 | 1.31 ± 0.74 | 2.23 ± 1.01 |
F | 39.7 ± 17 | 65.0 ± 17 | 7.3 ± 1.15 | Bridging | 0.005 ± 0.008 | 0.005 ± 0.004 | 0.011 ± 0.012 | 1.57 ± 0.67 | 1.79 ± 0.75 |
Note: Sample size = 3
Comparison of compressive strength of plain and fibrous mortar samples is shown in
Comparison of compressive properties of fibre reinforced mortar.
Modulus of elasticity Ep was calculated as the ratio of stress change to strain change within elastic limits. The averaged Ep of samples A to F is given in
The compressive stress-strain curves are shown in
Stress strain of ISSBs, a. Single block, b. 1x2 blocks, and c. 2x2 blocks.
Fractured surfaces in blocks: a. Single block, b. 1x2 blocks, and c. 2x2 blocks.
The compressive strength was taken as the peak value of stress from stress strain curve of ISSB. The area below the stress-strain curve up to the stress of first crack was defined as the pre-crack absorbed energy PEb. The area below the stress-strain curve from the first crack stress to maximum load was regarded as the post-crack energy absorbed (CE)b. The total area below the stress-strain curve from initial to maximum load was categorised as the total energy absorbed (TE)b. The ratio of total energy absorbed and first-crack energy absorbed was taken as the compressive toughness index (CTI)b.
Compressive properties of ISSBs.
Specimen | First crack load (kN) | Max load (kN) | Top contact area (mm²) | Bottom contact area (mm²) | Top stress (MPa) | Bottom stress (MPa) | Compressive strength Cb (MPa) | Pre-crack energy absorbed PEb (MJ/m3) | Post-crack energy absorbed CEb (MJ/m3) | Total energy absorbed TEb (MJ/m3) | Compressive toughness index CTIb ( - ) | Modulus of Elasticity Eb (GPa) |
---|---|---|---|---|---|---|---|---|---|---|---|---|
1 Block | 11.7±4.8 | 20±4.8 | 15000 | 45000 | 2.50±0.6 | 1.58±0.24 | 1.58±0.24 | 0.006±0.001 | 0.0011±0.0001 | 0.0071±0.0013 | 1.30±0.95 | 0.201±0.04 |
1*2 Block | 34.8±16 | 72.5±16 | 15000 | 45000 | 4.73±0.7 | 0.83±0.16 | 0.83±0.16 | 0.002±0.001 | 0.0005±0.0003 | 0.0025±0.0014 | 1.18±0.78 | 0.209±0.05 |
2*2 Block | 51.1±17.7 | 105±17.7 | 30000 | 90000 | 2.77±0.1 | 0.92±0.04 | 0.92±0.04 | 0.006±0.001 | 0.0009±0.0001 | 0.007±0.001 | 1.14±0.93 | 0.234±0.04 |
Note: Sample size = 3
Comparison of compressive strength of blocks.
The CEb of three specimen configurations (single block, 1*2 blocks and 2*2 blocks) was calculated as 0.006 MJ/m3, 0.002 MJ/m3 and 0.006 MJ/m3 respectively, which showed significant improvement of 66% for 1*2 blocks specimen over the single block and no difference was observed in 2*2 blocks specimen. This increment for 1*2 blocks specimen was due to efficiency of interlocking interface between two blocks. The PEb of three specimen configurations was recorded as 0.0011 MJ/m3, 0.0005 MJ/m3 and 0.0009 MJ/m3, respectively. This had indicated 54% and 18% reduction in post-crack energy absorbed for 1*2 and 2*2 blocks specimens respectively, as compared with single block. This showed that the interlocking mechanism did not perform well after crack propagation. A similar outcome was observed for the values of TEb and CTIb, which showed a reduction of 64% for TEb as compared with single block and a reduction of 13% for CTIb as compared with single block. This proved that the interlocking mechanism did not perform efficiently after the propagation of cracks and required inclusion of another component like plaster to improve post-crack behaviour.
Modulus of elasticity (Eb) was calculated as the ratio of stress change to strain change. The Eb is detailed for each specimen in
Natural fibres have different chemical composition depending on variation in cultivation techniques, soil and environment. Pretreatment of fibres like washing with tap water and retention in boiling water affects their properties. Morphological changes in the fibres were observed using Bruker 3D optical microscope.
Surface contour of fibres, a. Sisal fibre surface roughness, b. Rice straw surface roughness, and c. Treated rice straw surface roughness.
Microscopic images from mortar cubes, a. Plain mortar surface texture, b. Sisal fibre embedment in mortar, c. Rice straw embedment in mortar, and d. Treated rice straw embedment in mortar.
In low-cost masonry housing main-component walling is mostly employed to resist compressive and lateral load (
The following empirical equations [1-2] were developed (
Development of empirical equation for modulus of elasticity, a. Sisal reinforced mortar, b. Rice straw reinforced mortar.
where C is compressive strength in MPa and K = 1, 1.1 and 0.95 for plain, 2% rice straw and 5% rice straw specimen respectively in equation [1]. For equation [2], K (GPa) = 0.293, 0.294 and 0.291 for plain, 2% sisal and 5% sisal fibre reinforced mortar, respectively, Y (1/MPa2) =1 and Z (GPa/MPa) = 1. It may be noted that for each value an average of three readings is taken. In the case of sisal-reinforced mortar a convex quadratic increase is observed, whereas in rice- reinforced mortar a powered linear increase is found.
Experimental and theoretical values of Modulus of Elasticity for plain and fibrous mortar
Specimen | Compressive Strength Cp (MPa) | Modulus of Elasticity Ep (GPa) | ||
---|---|---|---|---|
Empirical Equation | Experimental | Error (%age) | ||
Plain Mortar | 19.3 | 3.10 | 2.99 | 3.7% |
2% Rice Straw Reinforced Mortar | 9 | 2.07 | 2.05 | 1.2% |
5% Rice Straw Reinforced Mortar | 6 | 1.17 | 1.13 | 3.6% |
2% Sisal Fibre Reinforced Mortar | 19.8 | 7.42 | 7.17 | 3.5% |
5% Sisal Fibre Reinforced Mortar | 18 | 2.28 | 2.23 | 2.4% |
Note: 1.
Comparison of modulus of elasticity with experimental and empirical values for plain and fibrous mortar a.
The following empirical equation [3] is developed (
Development of empirical equation for modulus of elasticity of blocks.
where C is compressive strength in MPa and K (1/MPa) = 0.98, 0.65, 0.60 and Z (GPa/MPa) = 1, 0.95, 0.9 for single block, 1*2 and 2*2 blocks respectively. As far as the behaviour of blocks is concerned, empirical modelling reveals that there is concave variation among single blocks, 1*2 blocks and 2*2 blocks.
Experimental and theoretical values of Modulus of Elasticity for ISSBs
Sample Symbol | Compressive Strength Cb (MPa) | Modulus of Elasticity Eb (GPa) | ||
---|---|---|---|---|
Empirical Equation | Experimental | Error (%age) | ||
Single Block | 1.58 | 0.20 | 0.20 | 0.7% |
1*2 Block | 0.83 | 0.22 | 0.21 | 4.0% |
2*2 Block | 0.92 | 0.25 | 0.23 | 6.6% |
Note: 1.
Comparison of modulus of elasticity with experimental and empirical (
The mechanical properties (compressive strength, modulus of elasticity, pre and post crack energy absorbed and toughness index) of interlocking soil-stabilised blocks and fibrous/non-fibrous mortar were experimentally investigated to determine how best to enhance the performance of proposed interlocked fibrous plastered (i.e. reinforced coating) low-cost housing. The following conclusions are drawn:
A knowledge gap identified from the literature review regarding the mechanical properties (compressive strength, modulus of elasticity, pre/post crack energy absorbed and toughness) of ISSB and fibrous mortar which are prime parameters of enhancing resistance to lateral load like wind and earthquake for a masonry structure.
Surface contours of sisal fibres showed a smooth surface, whereas rice-straw fibres exhibited a rough and irregular bumpy surface indication the presence of impurities.
The failure modes of the fibrous cubes were characterised by a bridging, buldging effect and tensile cracks due to the presence of natural fibres, as compared with crushing failure of plain samples. 2% and 5% sisal fibres samples showed very high stiffness at first crack load and ductility at ultimate load, as compared with all other samples.
Mechanical properties with 2% sisal-fibre mortar specimens resulted in an increase of 17%, 8%, 3%, 76%, 221%, 17% and 139% of first crack load, maximum load, PEp, CEp, TEp, CTIp and Ep, respectively, as compared with that of plain mortar specimen.
The compressive strength of 2% rice-straw, 5% rice-straw and 2% treated rice-straw reinforced mortar resulted in a reduction of 210%, 316% and 271% respectively. A 6% increment was found with the addition of 2% sisal fibre from a plain mortar sample and a 5% reduction in compressive strength was observed in a 5% sisal-fibre mortar sample
Microscopic images of sisal-fibre mortar cube showed the embedment of sisal fibre in cement paste without any gap or void, indicating a proper bond of fibre in cement matrix. This resulted in better mechanical properties in sisal- fibrous cubes than in other samples. Untreated and treated rice-straw samples indicated the presence of micro cracks and voids in the cement paste and loose bond with rice straw. This poor bond and void resulted in reduced pre/post crack performance and toughness.
The failure modes of the ISSBs were characterised either by failure perpendicular to bed joint or shear cracks and spalling of block. The compressive stress- strain curves for each sample of single block showed higher stress and strain than did the other two sets of blocks. The reduction in stress-strain of the 1*2 and 2*2 blocks as compared with the single block might have been due to interlocking interaction between the different block units.
The mechanical properties of 1*2 blocks were compared with single block and 1.97 times, 2.62 times and 4% increase was found in first crack, maximum load and modulus of elasticity respectively. Values of compressive strength, PEb, TEb and CTIb were reduced by 47%, 67%, 64% and 9% respectively.
With 2*2 blocks 3.36 times, 4.25 times and 16% increase was found in first crack, maximum load and modulus of elasticity respectively. Values of compressive strength, PEb, TEb and CTIb were reduced by 41%, 1%, 1.5% and 12% respectively, as compared with those of single ISSB.
Empirical relations were developed with the help of experimental data for prediction of modulus of elasticity for fibrous/ plain mortar samples and interlocking soil - stabilised blocks. The experimental and empirical values were found close enough, with a maximum error of 6.6% in ISSBs and 3.7% in mortar cube samples.
In light of the findings and observed behaviour the addition of 2% sisal fibre in plaster (i.e. reinforced coating) of interlocked masonry walling is likely to be effective in improving the performance including the lateral resistance of low-cost masonry house. Further investigation is required to evaluate the performance of ISSB column or wall with fibre- reinforced plaster when subjected to lateral load like wind and earthquake.
The authors would like to thank B Chilla & H Hatibu (NHBRA) and organizations who helped them throughout this research work which was funded by Energy and Low Income Tropical Housing Project (ELITH). The careful review and valuable recommendations by the anonymous reviewers are appreciatively acknowledged.