# Influence of Plastic Waste on the Workability and Mechanical Behaviour of Asphal Concrete
## 1 Introduction  
According to Plastics Europe [1], the European member states have been adopting increasing landfill restrictions to plastic waste. They are also supporting efforts to add value to those by-products within the plastic industry and other sectors. Implementing these policies has led to a reduction of quantities of plastic waste disposed in landfills, with a decrease of around 44% from 2006 to 2018 [1]. Nevertheless, the available data show that more than 7.2 Mt of plastic waste was sent to landfills in 2018 in European member states. In Portugal, the recycling rate of plastic from packaging was 36% in 2019, but the recycling into new packages was only 11% in 2020 [2]. The circumstances show that there is still a considerable way to reduce plastic use and the valorization of plastic waste in its industry and others. Therefore, the incorporation of plastic waste in asphalt mixtures for highway pavement has been identified as an opportunity, most intensely studied over the last decade [3; 4; 5; 6; 7; 8; 9]. The opportunity lies in several aspects: the potential to improve the properties of the bituminous mixtures, the cost reduction by using cheaper polymers to improve the rheology of bitumen, the ease of usewhen the plastic waste is added directly to the plant in the production process of asphalt concrete (so-called dry process), and the contribution to value to considerable quantities of plastic waste instead of disposing it in landfills (circular economy). Nevertheless, there are some concerns about the potential for the production of additional emissions when plastic waste is incorporated in asphalt concrete and for the generation of microplastics. There is a possibility of the pavement releasing plastic particles, from a few nanometers to 5 mm in dimension [10], which can be harmful to human and other species' health [11]. These issues associated with microplastic are still under evaluation by the scientific community [12; 13]. Despite that, it must be emphasized that several studies identified the presence of microplastics in road dust from different sources even when asphalt materials do not incorporate plastic waste [14; 15; 16]. The results on the use of plastic waste as a polymer for bitumen modification and as aggregate replacement in asphalt concrete show that there is potential to achieve environmental advantages compared to virgin polymers and natural aggregates [17]. The studies carried out so far show that the performance of asphalt mixtures incorporating plastics waste depends on several aspects, i.e., plastic type [18] and content [19; 20]; temperature and time of mixing [18]; and production process (dry--plastic waste added while mixing all the constituents or wet--plastic waste combined with bitumen before mixing) [21]. The dry process is more straightforward than the wet process because there is no need to use special equipment, such as bitumen stirrs, to blend the bitumen and the plastic waste. Nevertheless, instead of using plastic as a substitute for part of the mineral aggregates as usual in the dry process, plastic waste can be added to work as a bitumen-extender polymer. In this case, it aims to improve the performance of the asphalt concrete, even if it is added in the mixing process when all the constituents are blended [22]. The plastic and the bitumen interact at the manufacturing temperature during the mixing and transport. If the temperature is high enough, the plastic can absorb some of bitumen's lighter components [23]. Moreover, some polymer wastes, such as low-density polyethylene (LDPE), are challenging to diffuse into bitumen in the wet process and tend to separate in storage even after high-shear mixes [12; 24]. Therefore, techniques such as chemical stabilization, chemical functionalization of the polymer, or binder blending at the asphalt concrete plant (dry process) are recommended to reduce those problems [4; 12].