# Recycled Polyethylene Fibres for Structural Concrete
## 3 Results and Discussion
### Comparison among the Properties of the Original Polyethylene, the Recycled One and of the Fibres  
Figure 13 shows a comparison of the mechanical properties of polyethylene before being recycled, after being recycled, and the fibres made from recycled polyethylene.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Mean Diameter** & **Mean Length** & **Density** & **Mean Roughness** \\  & **(mm)** & **(mm)** & **(g/cm\({}^{3}\))** & **(mm)** \\ \hline Recycled fibres & 1.73 & 60 & 0.92 & 0.1173 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Recycled fibre general properties.  
Figure 12: Example of a fibre stress-strain curve.  
First of all, it can be observed from all of the graphs that the fibres are very scattered in relation to the other two products. This phenomenon is because, while the characterisation of the original polyethylene and the recycled one were carried out with standardised test specimens, the fibres are manufactured manually, which leads to irregularities in the geometry.  
Figure 13a shows that the elastic modulus of both the PR and the fibres is increased in comparison to PT. Regarding the yield stress and breaking stress, although the mean values of both the valorised material and the fibres show a slight increase, the three products shown in Figure 13b,c indicate that the recycling process does not significantly modify these properties.  
The main difference is the strain under maximum load, shown in Figure 13d. In the case of the fibres, although the average value is higher than that of the polyethylene before being valorised, the statistical distribution of the results indicates that the ductility of both is similar. However, the strain values of the recycled polyethylene are much lower than the other two. On the one hand, this reduction in ductility can be explained by the increased stiffness shown by the material. On the other hand, it could be due to aspect  
Figure 13: Comparison of the mechanical properties of polyethylene before and after valorisation and of polyethylene fibres. (**a**) Elastic modulus. (**b**) Yield strength. (**c**) Breaking strength. (**d**) Strain at maximum stress.  
ratio differences between the samples used; the higher aspect ratio they show, the lower the ductility they will have.  
The differences in the manufacturing process between the fibres and the valorised polyethylene are in the final steps. In the case of the fibres, the polyethylene in its cast state leaves the cylinder nozzle at a pressure of 150 bars and is then pulled. This produces a stretching of the material so that the molecules are forced to move in one direction, causing a slight induced crystallisation of the polymer. In the case of the PR, the molten material is injected into a mould at a pressure of 1000 bar and a post-pressure is applied for a few seconds and then the mould is opened. Knowing that the pressure with which the PR is formed is almost eight times greater than that of the fibres and considering that the PR cools down more slowly than the fibres, it can be deduced that the PR is much more compact, which can explain why the PR is more rigid and less ductile than the fibres.