The mix proportions of all the CLECA lightweight aggregate concrete (LWAC) mixes with different percentages of fibre volume fractions (0, 0.15, 0.3 and 0.45%) was used in this study are illustrated in Table 3. It is noted that a high-volume fraction (V _f ) tends to “clog” in the mix and create workability problems (Kosmatka et al., 2002). Thus, low V _f (<0.5%) of the barchip polypropylene (BPP) was used in this experiment.

Slump test was carried out in accordance with BS EN: 12350 - Part 2: 2009 to determine the workability of crushed LECA lightweight aggregate fibre-reinforced concrete (CLLWAFRC) with different volume fraction (0, 0.15, 0.3, and 0.45%). Oil was applied on all the surfaces of the moulds before casting. The moulds filled with slumps were vibrated on a shaking table to ensure the uniformity of the mix. The concrete samples were demoulded after 24 +/− 4 h from placing. All the demoulded samples were fully immersed in water at room temperature in a curing tank until they reached the desired testing age.

The 3,000 kN capacity compression test machine was manufactured by Unit Test Scientific Sdn. Bhd. It was set to a constant loading rate of 3.0 kN/s in compliance with BS EN 12390—Part 3 (2009). The same machine was used for the splitting tension test with the loading rate set to 1.5 kN/s in accordance with BS EN 12390—Part 6 (2009). For each mixture sample, cubes with dimensions of 100 mm × 100 mm × 100 mm were cast to test for compressive strength at 7 and 28 days. The splitting tensile strength of mixture samples at 7 and 28 days was examined by cast them into cylinders with a diameter of 100 mm and length of 200 mm. In addition, three prisms (length: 500 mm, width: 100 mm, depth: 100 mm), are used to determine the behaviour of flexural strengths on the 7 and 28 days.

The workability of CLLWAC with different percentages of barchip polypropylene (BPP) fibre is represented by the normal slump value, as indicated in Figure 3.

The addition of PP fibre in CLLWAC has a negative effect on workability. Slump values are reduced remarkably with the increase of BPP fibre %. The slump is declined gradually by 4.6, 13.6 and 27.3% at 0.15, 0.30 and 0.45% inclusion of BPP fibre respectively. Similarly, in order to maintain certain workability, more water is required for lubrication in the case of higher fibre %. Superplasticizer can also be used to compensate for the negative effect of fibre on workability.

Addition of fibre decreases the workability of concrete in a way which bridge and hold the cement matrix, forming a network structure in concrete. This structure hence promotes cohesion and adhesion among the matrices. As the fibre content getting higher, the surface area to cement paste increases, contributing to larger internal friction and work done requirements. Consequently, the viscosity of the mixture increases, and the slump flow is hindered. According to Yew et al., 2015, the inclusion of fibres is well known to affect the workability and flowability of plain concrete intrinsically. However, CLLWAC inclusion of BPP fibre from 0 to 0.45% had achieved high workability with the slump value between 140 and 200 mm.

Demoulded density (DD) and oven-dried density (ODD) were measured for all mixes as shown in Table 4. DD is calculated with the weight of the specimens measured after demoulding; while ODD is calculated with the weight of the specimens measured after being oven-dried for 24 h. All specimens in this study were attributed with DD and ODD in the range of 1965–1995 kg/m³ and 1908–1984 kg/m³, respectively. The outcome fulfilled the objective of obtaining OPSLWC with ODD lesser than 2000 kg/m³. The samples also met the requirement for structural application as structural lightweight concrete (SLWC), defined as concrete with ODD of not greater than 2000 kg/m³ (Newman and Owens, 2003).

underIn general, all densities are observed to have a slight increment as the addition of BPP fibre volume fraction increased. This could be due to the packing density theory, where BPP fibre holds the cement matrix close to one another, causing a packing effect. Therefore, the addition of the fibre material occupied within a unit volume thus increases overall density. In general, density increases as the inclusion of fibre increases. From the previous study, Bagherzadeh et al. (2012) reported a similar outcome.

Figure 5 presents the splitting tensile strength of CLLWAC with different volume fraction percentages of BPP fibre addition at a curing age of 7 and 28 days.

underThe trend of splitting tensile strength enhancement is clear which presents an increase of strength throughout the increase of BPP fibre percentage, as shown in Figure 5. The splitting tensile strength grows exponentially with the fibre percentage until a peak of 2.86 MPa at 7 days curing age and 3.12 MPa at 28 days curing age. The splitting tensile strength develops slower than compressive strength throughout the curing period. The percentage enhancements are 5.69, 5.63, 4.93 and 9.25% with fibre percentage of 0, 0.15, 0.30, and 0.45% respectively.

The addition of BPP fibre significantly influences the mode and mechanism of rupture of the concrete cylinder. This phenomenon might be attributed to arresting of cracks by BPP fibres, therefore, concrete can be subjected to very large deformations before the total uncontrollable collapse. It can be observed that CLLWAC without fibre reinforcement tends to rupture in a way that split directly into half at failure whilst fibre reinforced CLLWAC only cracked along the longitudinal of the concrete cylinder. It can be noticed that CLLWAC-BPP0.45% is prone to fail in a more ductile mode. This is particularly correct where fibre prolongs concrete’s ability to sustain the load and withstand large deformation without fracturing into pieces. Similar behaviour has been reported for PP and PVC fibre OPS lightweight concrete (Yew et al., 2015; Yew et al., 2016; Loh et al., 2021). The failure pattern of CLLWAC-BPP0% and CLLWAC-BPP0.45%, as shown in Figure 6.

According to the study, all specimens are loaded at two points until rupture. Figure 7 records the MOR results of CLLWAC with various volume fractions of BPP fibre at 7 and 28 days curing age.

Based on Figure 7, it can be declared that the MOR increases proportionally to the increment of volume fraction of SPP fibre. The MOR ranged from a minimum of 2.53 MPa to a maximum of 3.53 MPa at 7 days and 2.70 MPa–3.91 MPa at 28 days. The MOR developments as a percentage of CLLWAC-BPP0% are 39.40% at 7 days and 45.01% at 28 days. Therefore, CLLWAC-BPP0.45% had shown the highest MOR similar in the case of compressive strength and splitting tensile strength. Figure 7 distinguish the fracture mode of plain CLLWAC with fibre reinforced CLLWAC. Several views were captured to study their variations in cracking patterns under flexural failure.

Comparing Figure 8, the main difference noticeable was the way the crack propagated through the 100 mm × 100 mm × 500 mm prism under bending. When concrete is subjected to bending, the tensile behaviour is prone to govern its strength as the concrete is brittle and weak in tension. From Figure 8, it can be observed that the presence of fibre inhibits the propagation of crack (bottom). However, crack propagates rapidly parallel to the load applied, splitting the prism into pieces in the case without fibre. Sudden failure usually occurred in the case of lightweight aggregate concrete with lower tensile strength especially when subjected to bending.

The presence of fibres in concrete integrates the cement matrices to minimize the propagation of cracks. As load is applied gradually, the development of cracks initiates, the fibres accommodated the crack faces and control the crack widths or openings. Fibres provide a bridging effect by displacing the minor cracks with the formation of a linking bridge to hold the openings. The stretching of fibres allows stress distribution and promotes an additional energy-absorbing mechanism. These mechanisms delay the failure, at the same time allow greater deformation. Thus, it can be deduced that improvement in the tensile strength of concrete.

Other than the volume fraction, geometry and aspect ratio, the distribution and orientation of fibre in cement matrices also influenced the tensile strength of concrete. The dispersion state of fibre is random due to the influence of aggregates and fibre gravity itself, yet homogenous distribution usually can be ensured at the higher fibre content. Orientation of fibre perpendicular to the load applied resulted in higher tensile strength. Otherwise, paralleled fibres reduce tensile strength because the parallel arrangement increased the weak interfacial transition zone between fibres and cement paste (Jin, 2016).