# Recycled Polyethylene Fibres for Structural Concrete
## 3 Results and Discussion
### Characterisation of Polyethylene before and after Recycling
#### 3.1.2 Mechanical Properties  
Figure 8a shows the stress-strain curves obtained from the tensile test on the material before it was valorised. The elastic modulus of the material is calculated using the linear elastic section of the curve, for which a linear adjustment of the stress values between 0% and 5% strain was done, as shown in Figure 8c. Figure 8b,d are identical curves but they represent the behaviour of the material once it had been valorised.  
Considering the stress-strain curve, it can be seen that the greatest increase in load was produced in the elastic section. Once the elastic limit had been exceeded, the material showed a yield plateau where, without increasing the load, the specimen reached deformations of around 200% (it was stretched to twice its initial length). Table 2 shows a summary of the main mechanical properties of polyethylene before and after recovery.  
Figure 8: Stress-strain curves of polyethylene before and after being recycled. (**a**) Full stress-strain curves of material before it is valorised. (**b**) Full stress-strain curves of material after it is valorised. (**c**) Elastic section of the stress-strain curves of material before it is valorised. (**d**) Elastic section of the stress-strain curves of material after it is valorised.  
The material parameters were obtained before its valorisation. The mean value of the elastic modulus was 121.3 MPa, so it is a semi-rigid plastic (E\({}_{t}\) between 70 and 700 MPa). This indicates that in the elastic section, the increase in load produces relatively small deformations. At the breaking point there was an average stress value of 13.16 MPa and an average strain of 452.62%, see Figure 9. However, when analysing the results individually, it can be seen that, in the elastic section, the behaviour of the material remained almost constant, while in the plastic zone the results obtained were more heterogeneous, which means that the behaviour at rupture is more difficult to predict. On the other hand, the high percentage of strain indicates that it is a very ductile material. This characteristic is attributed to LDPE being a polymer with a branched chain structure. The ductility value also indicates that it is a polymer with high toughness, understanding this parameter as the amount of energy that a material absorbs before breaking.  
In the case of the material already valorised, the mean values obtained were: an elastic modulus of 194.80 MPa, yield strength of 10.17 MPa, and a breaking stress of 14.28 MPa. As far as ductility is concerned, the strain under maximum load was 136.77%. The results obtained from the different specimens show a low dispersion, so that the behaviour of the recycled polyethylene is almost constant. This is an indication that in the recycling process there were no problems of homogenisation or high porosity that would cause drastic drops in the material's strength.  
If the change in behaviour of the material after its valorisation is analysed, the first thing that can be observed is the values of both breaking stress and yield stress have increased by 6% and 9% respectively, so it can be concluded that the recycling process has not affected the strength of the material. However, the elastic modulus of the recycling was increased by approximately 59%. At the same time, the strain under maximum load of the material decreased by almost 70%; this also indicates that the tenacity of the recycled material is lower.