Soroushian et al. (2003) tested the restrained drying shrinkage of macro plastic fibre reinforced concrete, according to ASTM C157 (ASTM 2008a). They found that the average maximum crack width of plain concrete was 0.3 mm at the 90th day, while 0.19% of PP fibre effectively restrained the crack width to 0.15 mm, and delayed the initiation of cracking. As reported by Najm and Balaguru (2002) and Hsie et al. (2008), the plain concrete can withstand only small drying shrinkage strain, which is usually neglected. However, the addition of plastic fibres significantly increases the strain capacity of concrete, thus contributing to a reduction in crack widths and a delayed crack occurrence time.
Fig. 2.7 Comparison of RDPT results for concrete reinforced with steel mesh, steel fibre and PP fibre (Cengiz and Turanli 2004)
Pull-Out Behaviour of Macro Plastic Fibres
Fibre debonding and pull-out (sliding) at the interface have a substantial impact on total energy absorption during the crack propagation. Therefore, the bond of fibre with matrix significantly affects capacity of the fibres to stabilise the crack propagation in concrete matrix (Singh et al. 2004). Low mechanical bonding strength may not provide sufficient bridging force to control crack development. Moreover, the weak bonding strength can cause internal micro-cracks in the interfacial areas (Ochi et al. 2007).
Oh et al. (2007) explored optimum shape among the various plastic fibres as shown in Fig. 2.8. In their pull-out tests, the crimped-shape fibres exhibited the highest energy absorption capacity. Kim et al. (2008) reported that the embossed fibre had high bonding strength at 5 MPa due to its high surface energy and friction resistance. The crimped fibre also had high bonding strength at 3.9 MPa, but its crimped part was stretched fully during the pull-out tests, thus leading to a rapid increase in displacement and low initial stiffness. The straight fibre had lowest bond strength at 1.7 MPa.