# Influence of Plastic Waste on the Workability and Mechanical Behaviour of Asphal Concrete
## 5 Conclusions  
The study described in this paper focused on evaluating the volumetric and Marshall properties, workability, and mechanical performance of asphalt concrete with plastic waste (ABS, HDPE, HDPE500, and LDPE(uw)), added by the dry process, aiming to replace part of the bitumen. The volumetric properties, Marshall stability, and flow were assessed to compare the results with the usual requirements. The study continued to the workability and performance-oriented testing program based on gyratory compactor tests and fundamental properties of the studied blends.  
The analysis of data obtained from the laboratory testing plan allowed for the conclusions presented below:  
* The type of plastic revealed a much higher effect on the workability of the asphalt mixtures than the volumetric properties.
* The plastic types and contents used did not reveal substantial water sensitivity issues for the asphalt concrete.
* Unlike the reference asphalt mixtures (without plastic), the mixes with plastic waste revealed suitability in the long-term (up to 3 million ESALs), based on SUPERPAVE criteria.
* The asphalt mixtures with plastic waste offered values for stiffness suitable for pavement surface layers.
* The use of plastic waste increased the elastic behaviour of the asphalt mixtures.
* The contribution of ABS to the elasticity of the mixture was roughly the same for all the stiffness values.
* The inclusion of 6% of HDPE, ABS, or LDPE(uw) into the reference asphalt mixture did not put the asphalt concrete's fatigue performance at risk.
* Globally, the asphalt concrete with plastic waste revealed better resistance to permanent deformation than the reference. Nevertheless, it must be emphasized that the performance varied with plastic type and content and also with the performance parameter considered.
* Except for LDPE(uw), 6% and 8% of plastic waste tended to be the most effective to increase resistance to permanent deformation.  
Lastly, the activities completed in this study showed that incorporating the types of plastic waste used by the dry method as a bitumen extender can improve some properties of asphalt concrete. Moreover, this way of plastic waste recovery will help to reduce the inadequate disposal of plastic waste in nature. As it happens for other techniques based on recycling of by-products, the authors are aware that the studies on this type of asphalt concrete must continue to obtain a full understanding of its long-term behaviour and environmental impact and the life cycle analysis.  
Conceptualization, S.C.; methodology, M.F., S.C., A.A. and L.P.-S.; formal analysis, M.F., S.C., A.A. and L.P.-S.; investigation, M.F. and S.C.; writing--original draft preparation, S.C.; writing--review and editing, M.F., S.C., A.A. and L.P.-S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Acknowledgments:** The authors would like to thank the technical personnel working at the Laboratory of Highways Pavements (LPR) of Instituto Superior de Engenharia, Instituto Politecnico de Coimbra. The authors would also like to express their special thanks of gratitude to the companies Ambiente--Recuperacao de Materiais Plasticos, S.A. and LifePoly, Lda., for providing the plastic waste used in this study, and Contec--Construcao e Engenharia, S.A., for providing the aggregates and bitumen.  
**Conflicts of Interest:** The authors declare no conflict of interest.