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Compressive Strength of Concrete

Figure 5.3 shows compressive strength of the PP fibre reinforced concrete cylinders. As can be seen, the addition of fibres does not have significant effects on the compressive strength. All the recycled fibre reinforced concrete cylinders have comparable compressive strength compared to the virgin fibre reinforced concrete. This is because the compressive strength of concrete was not influenced by the relatively low dosage of fibres (4 kg/m3) according to the research of Hasan et al. (2011).

Residual Flexural Tensile Strength with CMOD

Figure 5.4 shows load-CMOD curves of PP fibre reinforced concrete beams. Three samples for each fibre and concrete type were tested. One plain concrete beam was tested as a control specimen in this research. With increase of the CMOD, the loads of all the PP fibre reinforced concrete beams reached peak (around 20 kN) at the limit of proportionality (LOP), followed by a sudden drop in terms of the CMOD ranging from 0.05 to 0.5 mm. The loads then increased slightly with the increase of CMOD from 0.5 to 2 mm, and remained stable on further loading within the range

Load-CMOD curves of PP fibres reinforced concrete beams

Fig. 5.4 Load-CMOD curves of PP fibres reinforced concrete beams

of 1-5 kN. However, for the plain concrete beam, after the LOP, the load dramatically decreased to 0 kN. This indicated that all PP fibre reinforced concrete beams showed outstanding post-cracking behaviour.

Figure 5.5 shows residual flexural tensile strength at LOP for different fibre reinforced concrete beams compared to the plain concrete. Although all the fibre reinforced concretes have slightly lower residual flexural tensile strength at LOP, it should be noted that the PP fibres have not obvious effects on the residual flexural tensile strength at LOP. This is because the residual flexural tensile strength at LOP mainly depends on concrete properties rather than fibre properties. Moreover, since the CMOD test is for evaluating the post-cracking performance, only one plain

Residual flexural tensile strength of PP fibres reinforced concrete beams at LOP concrete beam was tested as a control specimen, which may have led to some deviation in results

Fig. 5.5 Residual flexural tensile strength of PP fibres reinforced concrete beams at LOP concrete beam was tested as a control specimen, which may have led to some deviation in results.

The Young’s modulus of the fibre significantly affects the performance of the PP fibre in concrete. In the Fig. 5.6 the concrete beams reinforced by recycled PP fibre (Line) and 50:50 virgin-recycled PP fibre (Line) showed higher residual flexural strength than that by the virgin PP fibre (Line), due to the higher Young’s modulus of recycled PP fibre (Line) and 50:50 virgin-recycled PP fibre (Line) than that of the virgin PP fibre (Line) (Table 5.2). From Table 5.2 it can be seen that the virgin PP fibre (Line) has much higher tensile strength than that of the recycled PP fibre (Line), while the recycled PP fibre (Line) reinforced concrete beams showed higher residual flexural strength than that of the virgin PP fibre (Line) reinforced concrete, indicating that for the line-indent PP fibres the Young’s modulus of fibres are more effective on their reinforcement in the concrete than the tensile strength.

As can be seen in Fig. 5.6, the concrete reinforced by the diamond-indent PP fibres, namely recycled PP fibre (Diamond) and 5:95 HDPE-recycled PP fibre (Diamond), produced higher residual flexural strength than that by line-indent PP fibres. This indicates that the diamond-indent PP fibres produced better reinforcement than the line-indent PP fibres. Moreover, with the increase of CMOD, the residual flexural strength of concrete beams reinforced by recycled PP fibre

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

Fig. 5.6 Residual flexural tensile strength of PP fibres reinforced concrete beams at a CMOD1, b CMOD2, c CMOD3, and d CMOD4

(Diamond) and 5:95 HDPE-recycled PP fibre (Diamond) showed an upward trend at CMOD4 (Fig. 5.6d) and CMOD3 (Fig. 5.6c) respectively, while the line-indent PP fibres just kept their flexural tensile strength constant from CMOD1 to CMOD4.

Figure 5.7 shows the fracture faces of the fibre reinforced concrete beams. Figure 5.7a represents the fracture faces of line-indent fibres reinforced concrete beams, including virgin PP fibre (Line), recycled PP fibre (Line), 50:50 virgin-recycled PP fibre (Line). On the fracture faces of these three kinds of fibres

Fracture faces of the PP fibres reinforced concrete beams

Fig. 5.7 Fracture faces of the PP fibres reinforced concrete beams: a line-indent PP fibre, and b diamond-indent PP fibre reinforced concrete, nearly all of fibres were pulled out without being broken, indicating that the line indents have a poor bonding with the concrete. The fibres were pulled out and did not reach their ultimate tensile capacity, hence, the full tensile capacity of the fibres were not exploited. This explained why for the line-indent PP fibres the Young’s modulus of fibres are more effective on their reinforcement than the tensile strength. Since the virgin PP fibre (Line) had the lowest Young’s modulus, hence it offered the concrete the lowest residential flexural tensile strength. Therefore, due to the poor bonding with the concrete, the reinforcement of line-indent PP fibres mainly depends on their Young’s modulus.

From Fig. 5.7b, it can be seen that for the diamond-indent PP fibre reinforced concrete more fibres were broken than pulled out, indicating that the diamond indents had a better bonding with the concrete. The broken PP fibres fully exploited their tensile capacity, thus producing a better reinforcement than the line-indent PP fibres. Since more fibres reached their ultimate tensile capacity and were broken, the reinforcement of diamond-indent PP fibres depends on both their Young’s modulus and tensile strength.

 
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