By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 25 2024The global road construction industry is confronting the dual challenges of depleting natural resources, which are crucial for producing and maintaining asphalt pavements, and the low recovery rates and environmental impacts of waste plastic disposal.1 Incorporating plastic waste into asphalt offers a promising solution to address both of these pressing issues.
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Plastic waste can be incorporated into asphalt pavement through dry and wet processing methods.1 This approach provides both economic and environmental benefits, and recycled plastic can enhance the high-temperature performance of asphalt mixtures. The characteristics of plastic-enhanced asphalt depend on factors such as the source of the waste, dosage, blending conditions, and plastic pretreatment.2 This article examines how plastic affects asphalt, highlighting its advantages and addressing associated challenges.
Using Recycled Plastic in Asphalt
Asphalt is a thermoplastic material that exhibits viscoelastic behavior in pavements. To enhance the durability and performance of asphalt under varying climatic and traffic conditions, modified asphalt is preferred over the raw form. This modification typically involves adding virgin polymers, which are expensive and challenging to obtain. Consequently, economic factors and growing environmental concerns have led to the use of waste plastics for asphalt modification.2
Various types of waste plastics, including polyethylene (PE), polypropylene (PP), and polystyrene (PS), can be incorporated into asphalt. PE is the most commonly used plastic due to its significant share in the total plastics market, which helps reduce overall solid waste. Additionally, PE enhances the rheological properties of asphalt binders and field mixtures.1
Plastic waste can be added to asphalt through either wet or dry processing methods. The wet process involves mixing recycled plastics directly into the asphalt binder at high temperatures to achieve a homogeneous blend. The type of plastic and asphalt binder determines the appropriate mixing temperature and duration. This method provides better control over the properties of the modified asphalt but requires specialized equipment and storage facilities. It is particularly suited for plastics with melting points below 200 °C, such as PS and PP.1
In contrast, the dry process involves using recycled plastics as reinforcement materials in asphalt mixtures. This method is more cost-effective and can be implemented in existing asphalt plants with minimal modifications. However, it results in asphalt mixtures with relatively poorer water stability compared to those produced by the wet process.2
Advantages
The use of plastics in everyday life, commerce, and trade is projected to grow due to their cost-effectiveness, versatility, lightweight nature, durability, and ease of processing. However, the fact that approximately 50% of plastics are single-use raises significant environmental concerns due to their non-biodegradability and the issues associated with their combustion after disposal. Recycling plastic waste into asphalt presents several environmental, technical, and economic benefits.1
Incorporating recycled plastic into asphalt pavements helps reduce municipal solid waste and supports effective plastic waste management. It also eliminates the need for costly polymers to modify asphalt mixtures used in road paving. Additionally, recycled plastics can enhance asphalt's thermal stability, degradation resistance, and resistance to deformation, thereby extending the material's service life.1
The advantages of using plastic in asphalt depend on the processing technology employed. Both dry and wet processing methods affect the thermorheological and mechanical properties of asphalt pavements. Therefore, the choice between these methods should consider the available production equipment, performance indicators, the source of plastic waste, and the desired properties of the asphalt mixture.1
Challenges and Limitations
Recycling plastic waste into asphalt presents several challenges and limitations. Economically, the production of plastic-modified asphalt may require additional equipment, potentially increasing costs. Furthermore, the pyrolysis of plastic waste can produce impurities such as sulfur, chlorine, solid residues, moisture, and acids, which can hinder its effectiveness in asphalt applications.1
The performance enhancement of asphalt through recycled plastic depends on the type of plastic used and the specific application environment. While plastic waste can improve certain properties, it may also reduce resistance to moisture damage and increase the risk of cracking due to higher creep stiffness at low temperatures.2 Additionally, the fatigue life of plastic-modified asphalt is often lower compared to virgin polymer-modified asphalt.1
Challenges also arise in terms of compatibility and storage stability. Improper storage can significantly impact the mechanical properties of the final product. Although various plastics can be used as modifiers, only those compatible with asphalt's rheological properties are most effective.2 High-density plastics, such as certain types of PE or PP, can cause segregation, leading to uneven surfaces in the asphalt.1
The variable quality of waste plastics and the complex mechanisms of asphalt modification complicate the control of plastic-modified asphalt quality.2 Additionally, the potential for plastic to break down into microplastics and contaminate nearby water sources poses an environmental risk.1
Conclusion and Future Prospects
Significant efforts are being made to enhance the usage of recycled plastic in asphalt by overcoming the current limitations. For instance, researchers from the University of Nevada recently demonstrated the performance of a highway section in California made from asphalt formulated with recycled plastic. The tested pavement performed well with no cracking or rutting despite bearing heavy truck traffic under harsh environmental conditions, including flooding and freezing weather.3
In addition, a recent study in Construction and Building Materials examined the future recyclability of plastic-modified asphalt. The study assessed asphalt modified with plastic using dry, wet, and mixed methods, evaluating its recyclability at the end of its service life. Comprehensive mechanical testing analyzed the compactibility, moisture resistance, cracking tolerance, fatigue resistance, and rutting performance of the recycled plastic-modified asphalt. The findings indicated that this asphalt is fully recyclable and can perform comparably to standard plastic-modified asphalt mixes.4
Overall, using recycled plastic waste in asphalt for eco-friendly road construction offers numerous advantages. However, further studies, including life cycle assessments, are required to ascertain these environmental and economic benefits.
Moreover, the long-term performance of roads fabricated using plastic-modified asphalt should be evaluated. A better comprehension of the influence of different plastic wastes on the properties of asphalt and related microcosmic mechanisms will allow better control over asphalt performance.2 This will promote the application of recycled plastics in asphalt.
References and Further Reading
1. You, L. et al. (2022). Review of recycling waste plastics in asphalt paving materials. Journal of Traffic and Transportation Engineering (English Edition), 9(5), 742–764. DOI: 10.1016/j.jtte.2022.07.002. https://www.sciencedirect.com/science/article/pii/S2095756422000812
2. Xu, F., Zhao, Y., & Li, K. (2021). Using Waste Plastics as Asphalt Modifier: A Review. Materials, 15(1), 110. DOI: 10.3390/ma15010110. https://www.mdpi.com/1996-1944/15/1/110
3. Wolterbeek, M. (2023). Recycled-plastic pavement withstands heavy trucks and extreme weather. University of Nevada, Reno. https://www.unr.edu/nevada-today/news/2023/plastic-pavement
4. Lu, D. X., Enfrin, M., Boom, Y. J., & Giustozzi, F. (2023). Future recyclability of hot mix asphalt containing recycled plastics. Construction and Building Materials, 368, 130396–130396. DOI: 10.1016/j.conbuildmat.2023.130396. https://trid.trb.org/View/2103185
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