# Influence of Plastic Waste on the Workability and Mechanical Behaviour of Asphal Concrete
## 4 Discussion
### Volumetric Properties, Marshall Results, and Workability  
Although the air voids measured for the mixture with LDPE(uw) (Figure 6) were under the requirements for 4% and 6% of plastic waste content, the Marshall stability and flow results were satisfactory. Moreover, the relationship between air voids and workability seems to be poor. The results shown in Figure 8 reveal that the type of plastic and, therefore, the viscosity of the resulting binder, seem to have a much higher influence on the workability of the asphalt mixtures than the volumetric properties observed for the different mixtures with different plastic contents.  
The parameter used to assess the ease of laying (Figure 8--void content for one gyration) shows that using 6% of plastic weight by the mass of bitumen is likely to affect the laying down conditions of the resulting asphalt concrete. Nevertheless, no issues were detected throughout the laboratory procedures in handling the asphalt mixtures produced.  
The two parameters used to evaluate the mixtures' ease of compaction lead to quite different conclusions. The compaction curve slope, k, did not depict any noticeable differences between the tested mixtures, probably because the mixtures' air voids were quite similar [58]. On the contrary, CEI revealed much more capacity to distinguish the mixtures' compactability. The stiffer plastic waste, such as HDPE and HDPE500, had a higher contribution to reducing compactability than ABS and LDPE. It must be emphasized that despite the results showing a considerable increase for CEI in the asphalt mixtures with plastic waste, the increase of just 0.5% of neat bitumen content would have a highly noticeable influence [58].  
In order to better evaluate the workability of the studied asphalt mixtures, Figure 15 compares the obtained results (for 6% of plastic content) with SUPERPAVE's recommendations [59].  
SUPERPAVE considers the two following indicators: N\({}_{\text{initial}}\) and N\({}_{\text{design}}\). The first one defines the adequate number of gyrations to achieve the air void content required at the construction site, just after the pavement is finished. If the void volume after N\({}_{\text{initial}}\) is too low, the mixture compacts too easily, and stability problems might be expected after opening to traffic. N\({}_{\text{design}}\) is the number of gyrations that reproduces the probable mixture compaction after the lifespan of the asphalt mixture (4% in SUPERPAVE). The requirements vary as the number of the predicted equivalent standard axles loads of 80 kN (ESALs) for pavements with less than 0.3 million ESALs (very light traffic) to 30 million ESALs or higher volumes (very heavy traffic).  
Figure 15 shows the results and the requirements (in red dots) in terms of the percentage of the maximum density. They reveal that incorporating plastic reduces the values of N\({}_{\text{initial}}\) and N\({}_{\text{design}}\). Consequently, the reference asphalt concrete is not satisfactory for the long term, whereas the asphalt mixtures with plastic waste are suitable for pavements  
Figure 15: Comparison of the workability measured in gyratory compaction tests with the SUPERPAVE limits for (**a**) N\({}_{\text{initial}}\) and (**b**) N\({}_{\text{design}}\).  
with up to 3 million ESALs. Furthermore, even for heavily loaded pavements, the asphalt mixtures with plastic waste show a much better behaviour in compactability than the reference and are close to the acceptable threshold.