Table 2 summarizes the density and saturated water content of both the control blocks and fibre-reinforced blocks. The blocks with banana fibre and coconut coir had a lesser dry density and higher wet density than that of the control blocks. It was observed that with the addition of fibre in the mortar, both the saturated water content and porosity were found to increase. The absorbent nature of banana fibre and coconut fibre was compared to the water absorption of the fine aggregates and the volume of pores in the mortar mix. It allowed more water to be absorbed by the mortar. Both the banana fibre and coconut coir reinforced blocks showed a similar dry density, wet density, water absorption rate and porosity.
Table 2
Physical properties of the blocks
| Control | Banana fibre r/f | Coconut coir r/f |
Dry density (kg/m3) | 1792 | 1776 | 1778 |
Wet density (kg/m3) | 2007 | 2050 | 2055 |
Saturated water content (kg/m3) | 215.2 | 274.7 | 277.0 |
Porosity (%) | 21.5 | 27.5 | 27.7 |
3.1 Compression behavior
Figure 3 illustrates the compression test results of the control blocks and fibre-reinforced blocks. For the control mortar, due to shear, cracks occurred and the blocks separated into several parts as shown in Fig. 3(a). In the case of the banana and coconut coir reinforced blocks, they showed more ductile behavior. As shown in Fig. 3(b) and (c), even for the larger deformations, the fibres maintained the integrity of the blocks and did not show any brittle failures. For the visuals, both fibre reinforced blocks showed similar behavior.
Figure 4 illustrates the stress-strain behavior of the control block and fibre-reinforced blocks. The results showed that the compressive strength of the banana fibre and coconut coir reinforced blocks was less than the control mortar. The calcium silicate hydrate gel generated from the cement reacts with water and this is responsible for holding the aggregates together. When the fibres were added to the cement-stabilized soil mortar, it reduces the cement content percentage of the overall mix. This leads to a reduction in the compressive strength of the cement-stabilized soil mortar. In all cases, the stress-strain looked similar until it reached peak stress. The post-peak behavior showed substantial variations. Due to the fibres, there was the establishment the tensile resistance at the crack openings and the stress was distributed throughout the mortar. The existence of fibre reduces unstable crack opening and introduces better-distributed cracking. Hence the cement-stabilized soil blocks with both banana fibre and coconut coir showed better post-peak behavior than the control mortar.
Table 3 summarizes the strain at first crack, compressive strength, compressive stress at the axial strain of 0.06 and the compressive toughness index. The compressive toughness index (CTI) refers to the deformation energy under uniaxial compression for a particular amount of strain [27]. The area inside the stress-strain curve shown in Fig. 4(a) was calculated and the strain was limited to 0.06 in the present study. The higher CTI value for both banana fibre and coconut coir reinforced blocks indicate that the fibre addition improves post-peak behavior and ductility.
Table 3
Compression characteristic of fibre reinforced blocks (standard deviation shown in brackets)
Mix | ∆P (%) | σP (MPa) | σ0.06 (MPa) | σ0.06 /σP | CTI0.06 (10− 3) |
Control | 1.51 (0.09) | 3.44 (0.11) | 0.51 (0.19) | 0.15 | 83 (12) |
Banana fibre r/f | 1.50 (0.11) | 3.16 (0.08) | 1.76 (0.43) | 0.56 | 121 (16) |
Coconut coir r/f | 1.53 (0.07) | 3.11 (0.08) | 1.52 (0.21) | 0.49 | 120 (8) |
∆P - strain at peak load, σP - compressive strength, σ0.06 - compressive stress at the strain of 0.06, CTI0.06 - compressive toughness index |
3.2 Flexural behavior
Figure 5 illustrates the specimen after the flexural tensile test. It is shown that the fibre addition improved the ductility of the beams. The control mortar beam showed a brittle failure while there was a gradual failure for both the banana fibre and coconut coir reinforced mortar beams. The gradual failure of the fibre mortar may be a consequence of the inclusion of fibres distributing the crack before failure. As observed in Fig. 6, the flexural tensile strength is slightly reduced with the addition of fibres. The lower flexural tensile strength observed for the coconut coir reinforced beams was 0.92 MPa, which is 13.2% less than the control beams. The corresponding value for the banana fibre reinforced beams was 1.01 MPa and 4.7% respectively.
Flexural toughness index is an important parameter involved when studying the post-peak characteristics of the blocks. Toughness is determined according to ASTM C1018 [28] by measuring the area under the load versus the deflection curve up to a specific level of deflection. The toughness index is calculated by dividing the toughness at a particular deflection by the first-crack toughness. Three deflections at 3, 5.5 and 10 times the first-crack deflection were considered in this study and the corresponding toughness index was defined as I5, I10 and I20 respectively. Table 4 summarizes the first crack deflection, the modulus of the rupture and the toughness indices. From the results, it can be seen that the value of the toughness indices for the coconut coir reinforced beams is larger than that for the banana fibre reinforced beams. However, both fibre beams had higher toughness indices than the control mortar beam.
Table 4
Test results of mortar beams in flexural (standard deviation shown in brackets)
Mix | Modulus of rupture | First-crack deflection | Toughness indices |
| (MPa) | (mm) | I5 | I10 | I20 |
Control | 1.06 (0.05) | 0.87 (0.05) | 1.21 | 1.23 | 1.24 |
Banana fibre r/f | 1.01 (0.03) | 0.86 (0.02) | 2.09 | 2.45 | 2.59 |
Coconut coir r/f | 0.92 (0.02) | 0.84 (0.02) | 2.16 | 3.04 | 3.76 |
3.3 Sorptivity
Figure 7 illustrates the sorption rate of the cement-stabilized soil blocks. The sorptivity coefficients were calculated from the gradient of the sorption rate versus the square root of the time curve shown in Fig. 7. The sorptivity coefficients in the initial stage were 0.68, 0.98 and 1.00 mm/min½ for the control, banana fibre and coconut coir reinforced blocks respectively. These results reveal that the fibre addition to blocks resulted in the water being absorbed at a faster rate. The main factor that affects the capillary rise is the porous nature of the fibre reinforced blocks.
3.4 Chemical resistance
Figure 8 illustrates the reduction of compressive strength of the cement-stabilized soil blocks which were kept in an outside environment, submerged in water, submerged in saltwater, submerged in a 3% NaOH solution and submerged in a 3% H2SO4 acid solution. The test results showed that:
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The compressive strength reduction of all three types of blocks which were kept outside in the environment, submerged in water and submerged in saltwater varied between 1 and 2%. These conditions therefore do not affect the compressive strength of the blocks.
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Both the control and fibre-reinforced blocks showed that they were the most vulnerable against alkaline and acid attacks. The compressive strength reduction was 6.6%, 6.1% and 4.8% for the control, banana fibre and coconut coir reinforced blocks respectively.
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In the study of their resistance to acid attacks, the compressive strength reduction was 14.7%, 9.4% and 7.9% for the control, banana fibre and coconut coir reinforced blocks respectively.
This demonstrates that the inclusion of fibres, especially coconut coir, within cement-stabilized soil blocks offers an improvement in durability against alkaline and acid attacks.
3.5 Freeze and thaw resistance
Figure 9 illustrates the compressive strength reduction after the particular number of freeze-thaw cycles. It was observed that the compressive strength reduction was increased as the freeze-thaw cycles increases. However, the inclusion of fibres, especially coconut coir, in the blocks resulted in less of a strength reduction. After 12 freeze-thaw cycles, the strength reduction of the control, banana fibre and coconut coir reinforced blocks was 33%, 25% and 19% respectively. This revealed that the mortar reinforced with banana fibre and coconut coir improved the durability of the blocks against freeze-thaw cycles.
3.6 Wet and dry resistance
Figure 10 illustrates the compressive strength reduction after wet and dry cycles. It was observed that the compressive strength reduction was increased as the wet and dry cycles increased. The blocks with the coconut coir addition had a better level of performance than the control and banana reinforced blocks. After 12 wet-dry cycles, the strength reduction for the control, banana fibre and coconut coir reinforced blocks was 25%, 24% and 22% respectively. It has been shown that mortar reinforced with coconut coir improved the durability against wet-dry cycles. However, the improvement was not as much as the freeze-thaw resistance.