Flexural Strength and Toughness from RDPT
RDPT is another effective test that reflects the behaviour of fibre reinforced concrete. As can be seen from Fig. 4.15, all the fibre reinforced concrete panels reached a peak load at the deflection of 1 mm, before a sudden drop to 5-8 kN. The loads then kept flat until deflection at 10 mm, before a stable downward trend to about 1.5 kN.
The energy absorption is the area under the load curves, which also reflects the performance of fibre reinforcement in dissipating energy. As can be seen from Fig. 4.15a, the 100% recycled PP fibre had slightly lower energy absorption than that of the virgin PP fibre, indicating that the 100% recycled PP fibre produced slightly lower post-cracking reinforcement than that of virgin PP fibre. This result is consistent with CMOD results. The reinforcement of the PP fibres in high-strength concrete depends on both their Young’s modulus and tensile strength. Although the 100% recycled PP fibre had lower tensile strength, its higher Young’s modulus improved its reinforcing effects. Consequently, a comparable reinforcement with virgin PP fibre was produced by the 100% recycled PP fibre in the 40 MPa concrete.
Fig. 4.15 Energy absorption and load curves from Round Determinate Panel Tests: a 6 kg/m3 of PP fibre reinforced 40 MPa concrete, and b 4 kg/m3 of PP fibre reinforced 25 MPa concrete
For the 25 MPa concrete, the 100% recycled PP fibre produced higher post-cracking reinforcement than that of virgin PP fibre (Fig. 4.15b). As we discussed before, the PP fibres have a poor bonding with concrete matrix in the low-strength concrete, hence, the Young’s modulus is more effective on the reinforcement. The 100% recycled PP fibre has higher Young’s modulus, thus producing better reinforcement.