Splitting Tensile Strength
The split-cylinder test is an indirect test to obtain tensile strength of concrete (AS1012.10 2000). As can be seen in Table 2.3 (Choi and Yuan 2005; Hasan et al. 2011), the macro plastic fibres improve the splitting tensile strength of concrete. In the split-cylinder test, when the stress in concrete reaches tensile strength of concrete, the stress is transferred to the macro plastic fibres. The fibres can arrest the propagation of macro cracks, thus improving the splitting tensile strength (Hsie et al. 2008). The plain concrete cylinders failed abruptly once the concrete cracked, whereas the macro plastic fibre reinforced concrete could retain its shape even after concrete cracked. This shows that the macro plastic fibre reinforced concrete has the ability to absorb energy in the post-cracking state (Hasan et al. 2011).
Flexural test is another indirect tensile test which measures the ability of concrete beam to resist failure in bending (AS1012.11 2000). Three-point loading and four-point loading are normally used in the flexural tests. For the three-point loading flexural test, results are more sensitive to specimens, because the loading stress is concentrated under the centre loading point (Alani and Beckett 2013). However, in the four-point loading flexural test, maximum bending occurs on the moment span (Soutsos et al. 2012). Research has found that the macro plastic fibres have no obvious effects on the flexural strength, which is dominated by the matrix properties (Soroushian et al. 2003). The main benefit of using macro plastic fibres lies in improved ductility in the post-cracking region and flexural toughness of concrete (de Oliveira and Castro-Gomes 2011). Brittle behaviour is always associated with plain concrete (Berndt 2009). When the first crack is produced, the specimen cracks and collapses almost suddenly, with very small deformations and no prior warning. However, in the macro plastic fibre reinforced concrete specimens, the failure progresses with bending, but without any sudden collapse as seen in plain concrete. When flexural tensile cracks occur, the load is transmitted to the plastic fibres. The fibres prevent the spread of cracks as shown in Fig. 2.2 and hence delay the collapse (Foti 2011).
Hsie et al. (2008) tested the flexural strength of macro PP fibre reinforced concrete. The PP fibre had diameter of 1 mm, length of 60 mm, tensile strength of 320 MPa and Young’s modulus of 5.88 GPa. As can be seen in Fig. 2.5, the plain concrete showed a brittle failure. The flexural strength reached the maximum at a deflection of around 0.05 mm without any post-cracking performance. The PP fibre slightly increased the maximum flexural strength to 5.5 MPa at the same deflection point as the plain concrete. However, after the maximum flexural strength, the load is transferred to the PP fibres, thus becoming stable around 1.5 MPa. Similar trends were reported by de Oliveira and Castro-Gomes (2011), Ochi et al. (2007), Meddah and Bencheikh (2009), and Koo et al. (2014).