ReviewRepurposing waste plastics into cleaner asphalt pavement materials: A critical literature review
Introduction
One of the most significant environmental issues is the increased waste material produced on the earth every day. Plastic waste is an extreme problem across the entirety of the world. For instance, an Australian Plastics Recycling Survey revealed the total consumption of plastics in 2016–2017 to be 3,513,100 tons while only 415,200 tons were recycled (Chin and Damen, 2019). The extensive production and use of plastics in China led to over 30 million tons of waste plastics per year in recent years (Chen et al., 2019a). It was estimated that 275 million metric tons of plastic waste was generated in 192 coastal countries in 2010, with 4.8–12.7 million metric tons entering the ocean (Jambeck et al., 2015). In the European Union, although plastic waste management is improving, the majority of plastic waste (41%) is sent to incineration and approximately 30% of plastic waste is recycled (Filho et al., 2019). According to the United States Environmental Protection Agency (EPA), 35,370 U.S. tons of plastic were generated in 2017 in the U.S., only 8.4% of which (2960 tons) was recycled and 26,820 tons (75.8%) were landfilled (Environmental Protection Agency (EPA), 2019). Waste plastic threatens the environment and public heath, as it was found that microplastics between 100 and 100 μm existed from leachate samples collected from landfills (He et al., 2019). This indicates that landfill is not the final sink of plastics, but a potential source of microplastics, creating a threat to the quality of drinking water, and other sources of water. A study by Hwang et al. (2019) found that direct contact of polypropylene microplastic particles with cells may have potential to cause health problems by inducing the production of cytokines from immune cells, rather than by direct toxicity to cells.
Waste plastic is one of the interesting materials that have attracted much attention for the past few years (Huang et al., 2007; Ingrassiaa et al., 2019). However, the waste plastic recycling rate in the U.S. is far below other countries that reported recycling rates between 30 and 60%, while Japan has the highest recycling rate of 78% (Khoo, 2019). It is a challenge to recycle waste plastic due to the complex nature of plastic waste mixtures and inefficient mechanical recycling. Instead of shipping waste into developing countries, Australia is taking proactive steps in exploring the alternative to use recycled plastic that is a significant contributor to overall Australian waste generation (Chin and Damen, 2019). The U.S. is also seeking for alternatives to use waste plastics as China has banned importing waste plastics, followed by India (Cockburn, 2019).
Reducing the use of plastic could be the most direct way to reduce waste plastic. For example, a perspective is proposed to call for moving toward zero plastic waste by banning single-use plastics (Walker and Xanthos, 2018). However, this ban may be difficult to put into place and enforce, therefore, other options need to be sought out in order to cut down on the plastic waste problem. Researchers and engineers are exploring alternative methods to repurpose waste plastics that can be utilized in civil infrastructures, such as wood-plastic composites (Keskisaari and Kärki, 2018), concrete blocks (Meng et al., 2018), mortar (Ramli and Tabassi, 2012; Makria et al., 2019). In mortar, Ramli and Tabassi (2012) found that polymer-modified mortars exhibited better engineering properties than the conventional mortar mixes. Arulrajah et al. (2017) explored the possibility of using plastic granules together with crushed brick and reclaimed asphalt pavement (RAP) waste used as base materials.
Taking advantage of waste materials in asphalt pavement has become an interesting yet challenging task. Asphalt is the most employed binder in pavement and derived from petroleum, which is a non-renewable resource (Ingrassiaa et al., 2019). On the one hand, environmentalist favors using various types of waste materials such as waste engine oil, cooking oil, swine manure, and coffee grounds, which can undoubtedly reduce environmental impacts and save virgin materials consumption; on the other hand, engineers are hesitant to advocate the use of recycled materials in a great amount unless the performances of such pavement infrastructure that contains recycled materials are proven to be as good as that contains no recycled materials. One of the challenges in the use of waste plastic is that the deterioration in properties of old plastic (Tam and Tam, 2006). By far the largest share of all PC plastic waste is packaging waste (Ragaert et al., 2017). There are many studies available on the use of various waste polymers in roads (Poulikakos et al., 2017; Sabina et al., 2011), however, there exists a gap in fully understanding the performance of asphalt pavement that contains recycled plastics at various dosages and types.
Section snippets
Research objective and method
The objective of this literature review is to provide a systematic assembling and evaluating the feasibility of various recycled plastics for asphalt pavement with respect to engineering performance, cost benefits, and environmental impact reduction, and to identify the challenge and propose recommendation for future study. The flow chart for conducting this literature review is shown in Fig. 1.
Recycled plastic types
Plastics are present everywhere and are abundant in a variety of types in terms of their applications, which, in turn, creates various waste plastics that can be recycled. Table 1 is a summary of common plastic products that can be recycled according to ASTM D7611 (ASTM International, 2019). The plastic recycling symbol is a solid equilateral triangle surrounding a numeral from 1 to 7, which defines the resin type of polyethylene terephthalate (PETE or PET), high-density polyethylene (HDPE),
Repurpose of waste plastic for asphalt pavement
Asphalt is a hydrocarbon material that has similar chemical function as plastic. Polymer such as EVA has been introduced into asphalt as a modifier to improve the base asphalt’s performance. Polymer modifier is not chemical reacted with the base asphalt, but uniformly dispersed in the base asphalt and absorbed the asphalt’s component (Yu et al., 2017). Compatibility is a property of the modifier fine particles uniformly distributed in the medium of asphalt without stratification, cohesion or
Polyethylene terephthalate (PET)
PET products has been increased dramatically consumed in the past decades (Jamdar et al., 2017). PET is a thermoplastic polymer resin of the polyester family, and its formula is (C10H8O4)n, which has been widely used for drinking bottles. Jamdar The recycled PET bottles can be repurposed into granulates and fibers as shown in Fig. 7. The size of shredded PET bottles is 1.18–2.36 mm (Rahman and Wahab, 2013), which is used to replace aggregate in the asphalt mixture. The specific gravity of PET
Summary of effect of each plastic on asphalt and asphalt mixtures’ performance
As waste plastic is type and composition specific, some studies have been conducted to compare each plastic type to other polymer wastes. For example, Lastra-González et al. (2016) compared PE, PP, PS and rubber from end-of-life tires that were used to replace 1% of the filler fraction by dry method, and they found the asphalt mixtures modified with PE, PP and rubber have a similar performance in increasing the rutting resistance, not affecting the stiffness for fatigue resistance. However, the
Economic benefits
From an economical point of view, using waste material in road construction and pavement is beneficial in different ways. It can be beneficial through improved performance of pavement as well as from reduced landfills. Many of the waste polymers could be hazardous and pose as environmental burden if these are not effectively recycled or reused. By incorporating waste polymers in pavement, not only does it improve the pavement performance and reduce environmental pollution, the need to utilize
Challenges in dealing with waste plastics into asphalt pavement
There is an increasing awareness and action in exploring more feasible methods to incorporate waste plastics into asphalt pavements. Starting from recycling plastic at the recycling center or plant, it is a challenge in sorting out the waste and categorizing into different types of plastic. Some other challenges are summarized as follows, from the engineering performances’ point of view.
One of the biggest technical concerns is the compatibilization between waste plastic and asphalt, let alone
Conclusions and future study
This paper presents a comprehensive literature review on the effect of incorporating plastic wastes into asphalt pavements. Based on more than 200 collected technical literatures, the state-of-art findings are summarized as follows:
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The current recycling practice of waste plastics according to ASTM D7611 include 7 major types: PETE, HDPE, PVC, LDPE, PP, PS, and others (ABS, EVA, PC, PU).
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Plastics are complex materials and are often composed of many different grades of plastic. Precautions must be
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The authors would like to thank SURF program at the University of South Alabama for sponsoring this study. Thanks also go to Ms. Sara Boccardo and Mobile County Recycling Center for providing waste plastic materials needed to further this area of study.
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