# Mechanical Performance of Concrete Made with the Addition of Recycled Macro Plastic Fibres
## 4 Results and Discussion
### Toughness  
The experimental curves of the relationship between the applied load (F) and the CMOD as a function of the type and quantity of fibre used in the mixture are presented in Figure 5, where the punctual values of the crack opening are shown on the \(x\)-axis. After reaching the point of first cracking, there was a sudden drop in load, probably caused by both the failure of the anchoring of some fibres and the breakage of others. In correspondence with the broken surface of the specimen, slipped and broken fibres were present. The post-cracking phase followed, during which the plastic fibres began to make their contribution and the deformation energy was dissipated by the fibres with the consequent remaining load, which remained constant up to a CMOD of nearly 4.5 mm, as depicted in Figure 5. This same behaviour was observed by many other researchers using different types of commercial or recycled plastic, and with different amounts of plastic fibre [14, 41, 44, 57]. Figure 6a shows the load-deflection graphs, while Figure 6b shows the initial segments of the graphs obtained by linearly interpolating the forces between the values of 20% and 60% of the peak force, where the first break occurred, as dictated by the UNE 83510: 2004 standard, which allowed us to calculate the toughness and the toughness index. The behaviour of the four mixtures was generally similar, wherein there was a first phase where the force increased until it reached its maximum value (peak value), and where the first  
Figure 4: Flexural strength values for all mixtures, along with the standard deviation bands. Figure 5: Average load–CMOD curves of three specimens of each concrete type. crack occurred in the specimen. With regard to the mixture made with the addition of commercial plastic fibres, the graph displayed a value of peak force greater than that of the other three mixtures. Subsequently, there was a rapid decrease in strength, followed by a slight increase, which may have been due to the contribution of the plastic fibres contained within the mixture. Toward the end, the force gradually decreased until the specimen broke, showing a degrading type of post-cracking behaviour (softening behaviour). The same results were obtained by numerous other researchers who experimented with the addition of recycled plastic fibre to reinforce concrete, demonstrating that it is possible to obtain a concrete that has good mechanical characteristics in terms of toughness, as well as reduces environmental pollution by reusing material resources that would otherwise be disposed of in landfills [12; 27; 58; 59]. In the post-cracking phase, the concrete mix with the highest fibre content (PFRC-6) exhibited a similar behaviour to the concrete mix with commercial fibre (PFRC-REF). Figure 7 shows that the presence of plastic fibres inside the matrix increased both the ability of the concrete to resist the advancement of cracks, and its ability to absorb energy and deform plastically before breaking, giving the compound a certain residual tensile strength after the microcracking of the cement matrix. In Figure 7a, it can be seen how the toughness of the concrete with commercial fibre (PFRC-REF) was higher than that of the concrete with recycled fibre (PFRC-2) when using the same quantity of fibre. This was mainly due to the commercial fibre used in this study having a much rougher surface than the surface of the recycled fibre, which was practically smooth. The increase in toughness index (Figure 7b) resulted in the value of the strength index of the mixture with no fibre addition (C-REF) being equal to 1. An increase in the percentage of recycled fibre confirmed the stitching action of the fibres themselves inside the cement matrix, which was fundamentally dependent on the type of fibre and its quantity. Comparing the RPF concrete specimens with the reference (PFRC-REF), the toughness index increased with RPF dosage, reaching similar levels at 6 kg of RPF per m\({}^{3}\) of concrete.