# Recycled Polyethylene Fibres for Structural Concrete
## 3 Results and Discussion
### Concrete with Recycled Fibres
#### 3.4.3 Mechanical Properties  
Table 6 shows the evolution of the mechanical properties of the three designed concretes.  
In view of the values obtained, it can be concluded that adding fibres to the mixture, in the proportions tested, produces a slight increase in the compressive strength of the concrete. This improvement may be because the addition of fibres reduces the porosity of the concrete, obtaining a more compact and, therefore, more resistant material. The compressive strength of the three concretes after seven days is higher than the 16.5 MPa required for the CEM III used in the mix proportion.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Mix** & **Age (Days)** & **Compressive Strength (MPa)** & **Tensile Splitting Strength (MPa)** \\ \hline \multirow{2}{*}{HFC} & 7 & 26.01 & 2.02 \\ \cline{2-4}  & 28 & 27.16 & 2.07 \\ \hline \multirow{2}{*}{HRFP-2} & 7 & 27.52 & 2.99 \\ \cline{2-4}  & 28 & 28.84 & 2.20 \\ \hline \multirow{2}{*}{HRFP-4} & 7 & 28.17 & 2.78 \\ \cline{2-4}  & 28 & 28.87 & 2.83 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Concrete mechanical properties.  
Figure 14: Comparison of the absorption (**a**) and the porosity (**b**) of the concrete as a function of the concrete/fibre volume ratio.  
To analyse the tensile strength, the values obtained at 28 days will be taken into account. The tensile strength of all the fibre-reinforced concretes was higher than the control value. In the case of the HRFP-2, the tensile strength increased by 6.28%, while for the HRFP-4 the increase was 36.71%. It can be concluded that there is a correlation between a greater volume of fibres and an increase in tensile strength.  
In relation to the tensile strength of the cylinders, a change in the failure mode can be seen when the polyethylene fibres are added. Although the control specimens broke abruptly, typical of a brittle fracture in concrete, the specimens with fibres showed a smooth fracture, maintaining the original shape of the fibre. As can be seen in Figure 15, the fibres are responsible for holding the specimen in its position once the cracking occurs. It can be concluded that the incorporation of fibres decreases the brittle fracture of the concrete and also helps to control the cracks.  
Figure 16a,b shows a cut specimen, where the distribution of the fibres in the concrete can be seen; although the amount of fibres used is not very high, it can be seen that they are distributed randomly over the entire surface. This indicates that the polyethylene fibres mix easily with the concrete, without producing balls or hedgehogs. Furthermore, a good adhesion of the fibres can be seen, due to the roughness of the fibres, and other factors. It must also be taken into account that the direction of the fibres is random, which plays a fundamental role in the tensile strength of the concrete, as the fibres in the longitudinal axis to the specimen will not provide any reinforcement. Several authors have shown that the orientation and distribution of fibres play an important role in mechanical properties [68; 69; 70; 71]. As an example, Figure 16c shows an example of the cylindrical specimen's fibre distribution.  
Figure 15: Breaking surface of a cylindrical specimen after a tensile splitting strength test.