Abstract
Many recent headlines proclaim that putting plastics in asphalt is one solution to the problem that only 9% of plastics produced in the USA are reused instead of being disposed of in landfills, burned for fuel, or lost into the oceans. Plastics can provide performance benefits to asphalt mixtures that include increased strength, moisture resistance, cracking resistance, and better bonding within the asphalt mixture. Based on a mixture of good and bad past experiences, the asphalt mixture industry is cautiously exploring using plastics in asphalt. This article provides a history, current status, guide to selecting a technology, and examples of successful uses of plastics in asphalt to demonstrate that plastics in asphalt is a technology for the present and future.
Background
The term “plastics” covers a wide variety of polymer materials but generally refers to thermoplastic materials that can be heated and formed into different shapes. The types of commonly recycled plastics, identified by resin codes commonly found in the recycle symbol, are (1) PET, (2) HDPE, (3) PVC, (4) LDPE, (5) PP, (6) PS, and (7) other types. Details of each resin type can found be here. One of the most common types of resins is polyethylene (PE), which is used to make a variety of products, including plastic bags, plastic wrap, and milk jugs. The melting point of polyethylene plastics is in the temperature range of the production of asphalt mixture, which makes PE plastics suitable for blending with asphalt binder. Some types of plastics, such as PET, have higher melting points, which make them good for fillers and aggregate replacement in asphalt mixtures. Some plastics, such as PVC, should be avoided in asphalt mixtures because heating the plastic can release hazardous gases.
History
The idea of using plastics in asphalt can be traced to Europe in the 1970s. Patents described the usage of waste polyethylene (PE) and polypropylene (PP), also known as polyolefins, as a method to improve asphalt mixtures. The addition of these types of plastics improved the high-temperature properties of asphalt, including the resistance to deformation or rutting and increased strength. Polyethylenes were used in asphalt in France starting in the mid-1980s for high modulus (high stiffness) mixtures designed for heavy traffic. The implementation of plastics in the USA began in the early 1990s. Commercial products, such as Novophalt®, were introduced to the asphalt paving market. Several states approved trial projects of plastics in asphalt to investigate the performance and benefits. Novophalt was produced by modifying an asphalt binder at the asphalt plant with polyethylene using proprietary blending equipment. Some trial sections performed well, such as a runway at Houston Hobby Airport. Other areas did not perform well, such as trial sections in Oklahoma and Pennsylvania. Because of the results and lower oil prices, the USA's interest in plastics in asphalt waned.
Current Status
Interest in the asphalt industry has recently increased in using polymer-modified asphalt as supplies of quality virgin asphalt binder and aggregates have decreased, and the usage of reclaimed asphalt pavement (RAP) has increased. The most common polymer used in asphalt binders is virgin SBS polymers due to their elastic properties at low temperatures and their ability to stiffen an asphalt binder at high temperatures. In the USA, the desire of governments to use recycled polymers increased after China stopped accepting most post-consumer plastic waste plastics in 2018, which has resulted in only 9% of plastics being recycled instead of being burned or dumped in landfills. The asphalt industry has conducted several studies and is cautiously optimistic about using plastics in asphalt because of the history (NAPA Report (NAPA-IS-142)).
Several companies are working to bridge the demands of both governments and the asphalt industry. These companies are marketing plastic-derived additives that improve performance for asphalt mixtures. NewRoad™, produced by NVI Advanced Materials Group, uses several proprietary processes to convert recycled plastic polymers and other materials into a hybrid polymer blend that creates bonds within the asphalt mixture, thereby improving mixture performance. Another manufacturer with a similar goal of recycling plastic into asphalt mixtures is MacRebur, which credits the inspiration of using plastics in asphalt to an experience of watching people in India setting shredded plastic bags on fire to patch potholes. MacRebur promotes the ability to use post-consumer plastics to replace asphalt binders and improve performance with their additives. A third manufacturer is GreenMantra, who recycles plastics and converts them into a wax that is introduced into the asphalt binder. These companies appear to have learned from prior experiences of plastics in asphalt to bring new products to the asphalt market over the past few years.
Keys to Success for Plastics in Asphalt
Successful implementation of plastics in asphalt mixes involves several factors:
Engineering a hybrid polymer blend for performance
The first step for successful implementation is engineering a blend of materials, including plastics, that improves the performance of the asphalt mixture. Experience has shown that using certain types of plastics, such as polyethylene and polypropylene, along with other proprietary materials, improves the performance of asphalt mixtures. Also, not all types of plastics should be used in asphalt mixes. Like a cake recipe, blending plastic polymers and other materials in specific percentages, referred to as a hybrid polymer blend, is required to achieve mixture performance. Furthermore, it appears that the different manufacturers have developed proprietary methods to blend these materials together to achieve performance.
Dosage
An effective dosage should be decided by specifying the desired performance. Some experiments did not go smoothly, because their goal was to use the most plastic possible, rather than focusing on asphalt mixture performance. The asphalt industry also wants to avoid its experience with shingles, in which high dosages of shingles were used for short-term economic gains without sufficient evaluation of the long-term effects. The effect of dosage can be demonstrated using laboratory and field testing.
Consistency of plastics
When using recycled plastics in asphalt mixtures, consistency is very important when producing asphalt mixtures to achieve consistent field performance. Some types of plastics enhance performance in asphalt mixtures, while others merely take up space as fillers or bond poorly with aggregates. Furthermore, the plastics need to be cleaned and sorted before blending. Because asphalt mixtures are naturally variable due to moisture and material components, the hybrid polymer blend must be manufactured under a high level of quality control to produce a consistent hybrid polymer additive in order to prevent compounding the variability that naturally exists in producing an asphalt mixture.
Explaining the benefits
The usage of plastics in asphalt is a promising outlet for using a lot of plastics. Society needs to understand the benefits and challenges of using plastics in asphalt. A benefit is the potential ability to use a lot of plastic waste. In the USA, approximately 500 million tons of asphalt are placed each year. If one pound of plastic was used in each ton, that would equal 50 billion plastic bags, which is approximately all the plastics bags made in the USA each year. Another benefit is using a hybrid polymer blend at the recommended dosage improves the performance and longevity of asphalt pavements, which is important for state highway agencies. A societal concern is whether using plastics in asphalt cause more problems, such as releasing microplastics into the environment. Lab studies on NewRoad™ have shown that the generation of microplastics is nonexistent when using this additive because the recycled polymers improve the bonding within the mixture.
Case Studies
The following case studies show examples of asphalt pavements with different successful hybrid polymer technologies. The results show that asphalt mixtures that incorporate hybrid polymer blends successfully improve the performance of asphalt mixtures.
NewRoad™
Nov. 2017 — Des Moines, IA
Project Description
The project consisted of full-depth construction of a 3-lane industrial road for 1,000 ft as part of a bridge relocation project. The NewRoad™ additive was included in the wear (surface) layer on one lane. The NewRoad™ section used the equivalent of 300,000 plastic bags.
Observations
The control lane started showing cracking (dashed line) along the inside wheel path after 3.5 years of traffic, while the NewRoad section has no cracking after four years of traffic. The surface of the control section has lost some of the fine material in the mixture, resulting in the pavement retaining moisture on the surface longer, which is detrimental to bonds holding the asphalt pavement together. The NewRoad section has not lost the fines and dries much faster after a rain event. Therefore, it is expected that the NewRoad section should last longer based on these field observations.
Source: NewRoad™ — Des Moines, IA — November 2017
Pavement sections after four years.
MacRebur MR Additive
Summer, 2018 — San Diego, California
Project Description
Residential street near the University of California - San Diego student housing. The pavement was a rehabilitation with 4 inches of asphalt mixture over an aggregate base. The MacRebur section used the equivalent of 233,000 plastic bags.
Observations
Based on observations from the project engineer, the performance for rutting due to heavy construction traffic is better than a similar pavement placed adjacent to the experimental section after three years of traffic.
Source: MacRebuber, UCSD Student Housing — San Diego, CA — Summer 2018
(a) Pavement during construction (b) Pavement after three years
Read the full case study.
GreenMantra Ceranovus® Wax
Fall, 2012 — Vancouver, British Columbia
Project Description
Overlay on a thoroughfare in Vancouver with the goal of using the wax derived from recycled plastics as a warm-mix additive.
Observations
Based on observations from the city materials engineer, the performance is similar to other pavements after nine years of traffic. Current GreenMantra products have a higher melt point than the wax used in this project, which reduces the warm-mix benefits of this wax for the City of Vancouver.
Source: GreenMantra Ceranovus Wax — Vancouver, BC — Fall 2012
Read the full case study.
Next Steps for Adopting Plastics in Asphalt
History, research studies, and field case studies have shown that plastics can be successfully implemented in asphalt mixtures. When beginning the journey of implementing plastics into asphalt mixtures, consider the following information when evaluating different technologies:
Benefits
The different plastic-derived additives claim to provide various benefits for the asphalt mixture. Most additives claim to increase the strength of the mixture. In addition to strength, NewRoad™ claims to improve the consistency of the mixture and facilitate increased quantities of RAP. The MacRebur MR8 additive is promoted as a binder replacement. Claims about Ceranovus® wax state that it lowers the mixing temperature and reduces the carbon footprint. Laboratory and field results should back up the claims made by manufacturers.
Laboratory Testing Results
Testing asphalt mixtures with performance or index tests provides an indication of potential long-term field performance. Generally, testing should demonstrate the rutting, cracking, and moisture sensitivity performance of the engineered material blend. When comparing laboratory testing between different manufacturers, especially cracking results, verify the aging protocol used, because aging can significantly affect the results.Implementation at Asphalt Plant
Hybrid polymer blends in asphalt must be easily implemented at the asphalt plant, or they will not be produced. Most asphalt mixture producers prefer the dry method, which is introduced at the asphalt plant, because it allows the producer to control the dosage and reduce the different types of asphalt binders they have to store.
Field Results
The ultimate evaluation of an additive for an asphalt mixture is field performance. Previous experience with plastics in asphalt in the 1990s showed experimental sections had more cracking after three to five years than the control sections. To reduce the risk of implementation, the technology should have a pilot project or test section with the hybrid polymer technology that has been in service for at least five years in a similar climate.
By following in the footsteps of those who have blazed the trail for plastics in asphalt, the asphalt industry can produce a product that has better performance, greater sustainability, and a greener image than ever before.
About the Author
Andrew LaCroix, Ph.D., P.E.
Chief Technologist
Andrew LaCroix considers himself a Road Scholar based on his 15+ years of experience in testing and evaluating asphalt mixtures and pavements. His expertise includes advanced performance testing of asphalt mixtures, developing test methods and equipment for asphalt labs, and investigating technologies to improve the performance of asphalt pavements. Currently, he works for NVI Advanced Materials Group as the Chief Technologist. The best part of his job is teaching others about the benefits of asphalt technologies, such as plastics in asphalt.
References
Willis, R., F. Yin, R. Moraes. Recycled Plastics in Asphalt: State of Knowledge. NAPA-IS-142. National Asphalt Pavement Association. 2020.
Haberl, P. K. Process for Preparing a Mixture Containing a Binder Material and Polyolefin, US Patent 3,853,800. 1974.
Topics: Case Studies, Whitepapers, construction, Resource, testing, Asphalt
