Residual Flexural Tensile Strength with CMOD

Figure 4.11a shows load-CMOD curves of 6 kg/m3 PP fibres reinforced concrete beams of 40 MPa compressive strength. Peaked loads reached for all the recycled and virgin PP fibre reinforced concrete beams were approximately 14 kN, this was then followed by a sharp drop associated with CMOD range of 0.05-0.5 mm. Further, CMOD from 0.5-3 mm was associated with increased loads, which then remained stable at 4-8 kN on further loading. However, the load dropped to zero for the plain concrete beam after the peak load was attained.

Figure 4.11b exhibits load-CMOD curves of 4 kg/m3 PP fibre reinforced concrete beams of target strength 25 MPa. The peak loads for all the beams were approximately around 13 kN, before a sudden drop. Unlike the Fig. 4.11a, the loads then just kept stable at 2-5 kN until failure. It shows inferior post-cracking performance than that of the fibres in 40 MPa concrete, due to lower compressive strength of concrete and lower amount of fibre. As expected, the load dropped to 0 kN soon after the peak load for the plain concrete beam. Figure 4.11 confirms the outstanding post-cracking performance of the 100% recycled and virgin PP fibre reinforced concrete beams, compared to the plain concrete.

Figure 4.12 compares residual flexural tensile strength at the peak load for the 100% recycled and virgin PP fibre reinforced concrete beams compared to the plain concrete beams. As can be seen, for both 40 and 25 MPa concrete beams, the 100% recycled PP fibre reinforced concrete beams have comparable residual flexural tensile strength to that of virgin PP fibre reinforced concrete. Compared to 40 and 25 MPa plain concrete beams, there is no obvious change after adding the PP fibres.

Figure 4.13 compares the residual flexural tensile strength of PP fibres reinforced concrete beams at CMOD1, CMOD2, CMOD3 and CMOD4. As can be seen, for the 40 MPa concrete, the 100% recycled PP fibre reinforced concretes show comparable or only slightly lower residual flexural strength than that reinforced by the virgin PP fibre. Moreover, from Fig. 4.13a-d, with the increase of CMOD, the residual flexural tensile strength of the fibre reinforced 40 MPa concrete beam increases from 1.5 to 2.0 MPa. On the other hand, for the 4 kg/m3 fibre reinforced 25 MPa concrete, the average residual flexural tensile strength of 100% recycled PP reinforced concretes is slightly higher that of virgin PP fibre reinforced concretes. Furthermore, the residual flexural tensile strength just keeps stable around 1.0 MPa from CMOD1 to CMOD4, instead of increasing.

Figure 4.14 shows the fracture faces of the fibre reinforced concrete beams. Figure 4.14a, b represent the fracture faces of 100% recycled and virgin PP fibre reinforced 40 MPa concrete beams, respectively. As can be seen, fibre breakage

Load-CMOD curves for a 6 kg/m of PP fibre reinforced 40 MPa concrete

Fig. 4.11 Load-CMOD curves for a 6 kg/m3 of PP fibre reinforced 40 MPa concrete, and b 4 kg/m3 of PP fibre reinforced 25 MPa concrete was higher than fibre pull out, which indicates good bonding of fibres with a high-strength concrete matrix. Tensile capacity of the broken PP fibres was fully realised, thus producing good reinforcement. As the ultimate tensile capacity was

Residual flexural tensile strengths at the peak load

Fig. 4.12 Residual flexural tensile strengths at the peak load

reached in the broken fibres, the performance of the fibres depended on both their tensile strength and Young’s modulus. From Table 4.4, it can be seen that the 100% recycled PP fibre had higher Young’s modulus but lower tensile strength than the virgin PP fibre. Consequently, the 100% recycled PP fibre produced similar or slightly lower performance than the virgin PP fibre in the 40 MPa concrete beams. Moreover, the failure mechanism of 100% recycled and virgin PP fibre themselves are different. In the case of 100% recycled PP fibre concrete (Fig. 4.14a), the fibres broke with relatively brittle mode of failure, while the broken virgin PP fibre was stretched into massive split micro fibres, showing a more ductile failure (Fig. 4.14b). This is because the 100% recycled PP fibre has very low elongation at break (6.2%), while the virgin PP fibre is more ductile and has much higher elongation at break at 12.6%.

The fracture faces of 25 MPa concrete beams are different with those of 40 MPa concretes as shown in Fig. 4.14c, d. In the low-strength concrete, nearly all the fibres were pulled out without being broken, indicating that the low-strength concrete has a poor bonding with the fibres. Because of this poor bonding, the majority of the fibres remained intact and their full tensile capacity was not realised. Therefore, Young’s modulus of the fibres is more effective on their reinforcement than the tensile strength. Since the 100% recycled PP fibre has higher Young’s modulus than that of virgin PP fibre, thus producing better reinforcement in the CMOD tests.

Residual flexural tensile strength of PP fibres reinforced concrete beams at a CMOD, b CMOD, c CMOD and d CMOD

Fig. 4.13 Residual flexural tensile strength of PP fibres reinforced concrete beams at a CMOD1, b CMOD2, c CMOD3 and d CMOD4

(continued)

Fig. 4.13 (continued)

Fracture surfaces of PP fibres reinforced concrete beams

Fig. 4.14 Fracture surfaces of PP fibres reinforced concrete beams: a 6 kg/m3 of 100% recycled PP fibre, b 6 kg/m3 of virgin PP fibre, c 4 kg/m3 of 100% recycled PP fibre, and d 4 kg/m3 of virgin PP fibre

 
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