Recycled Asphalt Shingles in Pavement Mixes: Lessons from the US-258 Highway Project

Asphalt producers and paving contractors are constantly seeking ways to reduce material costs while maintaining pavement quality. One promising approach that has gained traction involves incorporating recycled asphalt shingles (RAS) into hot mix asphalt designs. The successful use of RAS on a North Carolina highway project demonstrates how contractors can leverage this recycled material to achieve both economic and performance benefits. Understanding the fundamentals of Concrete Wall Construction Process Including Materials provides useful context for evaluating alternative construction materials like RAS, as both fields require careful attention to material properties and mix design specifications. The US-258 project, executed by Greenville Paving and Contracting, stands as a landmark case study in adopting RAS technology for surface course applications.

Understanding Recycled Asphalt Shingles and Their Role in Mix Design

What Are Recycled Asphalt Shingles?

Recycled asphalt shingles are processed roofing shingles that are repurposed as a component in asphalt pavement mixes. There are two primary sources of RAS:

• Manufacturer scrap shingles: Off-cuts and defective shingles collected directly from shingle manufacturing plants. These are relatively clean and consistent in composition but are often already contracted to specific recyclers.
• Tear-off shingles: Used shingles removed from existing roofs during reroofing projects. These contain aged asphalt binder and are more variable in composition, but they are widely available and represent a significant waste stream.

For the US-258 project, Greenville Paving opted to use tear-off shingles due to limited availability of manufacturer scraps in North Carolina. Tear-offs typically contain higher residual asphalt binder content, which can help offset the cost of liquid asphalt cement in the mix design.

Why Use RAS in Asphalt Mixes?

The motivation for incorporating RAS into asphalt mixes is primarily economic and environmental. At the time the US-258 project was being planned, liquid AC binder prices had reached approximately $700 per ton. By replacing a portion of the virgin binder with the asphalt content already present in the shingles, contractors can achieve significant material cost savings.

Additional drivers include:

• Reduction of landfill waste: Approximately 11 million tons of asphalt shingle waste are generated annually in the United States.
• Conservation of virgin aggregate and binder resources.
• Improved rutting resistance in the finished pavement due to the stiff nature of aged shingle binder.
• Lower carbon footprint for pavement construction projects.

Mix Design Parameters on the US-258 Project

The US-258 project involved multiple mix types across nine miles of roadway in Greene and Lenoir Counties. The project scope included widening, resurfacing, and shoulder reconstruction. The following table summarizes the mix types and quantities used:

Only the S9.5B surface mix incorporated the tear-off shingles. The base and binder mixes were placed in May and June 2010 respectively, while the RAS-containing surface mix was placed between August 13 and 18, 2010, after receiving provisional approval from the North Carolina Department of Transportation on July 27. The surface mat was placed at a rate of 168 pounds per square yard at a thickness of 1.5 inches.

Navigating the Challenges of RAS Implementation

The Asbestos Concern

Asbestos is the single greatest concern for contractors considering RAS. Older roofing shingles may contain asbestos fibers, and proper identification and handling are critical before any RAS material can be processed or used. For the US-258 project, Greenville Paving took the following approach:

• Initial asbestos testing was performed by a hired asbestos inspector on the first batch of shingles to be ground.
• James Ross, the asphalt quality control manager, pursued and obtained certification as an accredited asbestos inspector using polarized light microscopy in both North Carolina and South Carolina.
• His son, Josh Ross, also obtained certification, providing the company with in-house asbestos identification capability.
• Dedicated laboratory equipment was invested in to support ongoing testing.

The cost of certification and lab equipment was significant, but the company viewed it as a long-term investment that would pay for itself across multiple projects. Understanding how materials are tested and certified in other construction contexts, such as Understanding Measurement of Masonry Brick Works in Construction, helps put these quality assurance requirements into perspective.

Shingle Sourcing and Logistics

Securing a reliable supply of tear-off shingles required collaboration with local government. Greenville Paving worked with the Pitt County recycling coordinator to revise the landfill permitting structure, allowing the company to extract shingles from the waste stream. The logistical steps involved:

1. Coordinating shingle collection from the landfill site.
2. Transporting the shingles to Greenville Paving plant.
3. Grinding the shingles to the appropriate size for incorporation into the mix.
4. Managing the processed RAS stockpile to prevent moisture-related clumping.

Grinding and Processing Costs

Grinding shingles into a usable form requires specialized equipment that can be cost-prohibitive for many contractors. Greenville Paving chose to subcontract the grinding operation rather than invest in its own grinding machinery. This decision kept capital costs manageable while still allowing the company to proceed with RAS incorporation. The grinding machines themselves are expensive to purchase and maintain, and the company determined that subcontracting offered better economics given the volume of RAS they anticipated processing.

Stockpile Management

RAS stockpiles require careful management to maintain material quality. Tear-off shingles are prone to clumping when exposed to moisture, particularly in hot weather. Greenville Paving developed the following stockpile management practices:

• Keeping piles relatively small to reduce compaction from material weight.
• Avoiding driving equipment over the RAS stockpile.
• Regularly fluffing the pile to prevent shingles from fusing together.
• Ideally storing the material under cover (though Greenville Paving did not have covered storage, fluffing was used as a mitigation strategy).

Performance Benefits and Field Observations from US-258

Improved Compaction Characteristics

One of the most notable performance observations from the US-258 project was the reduced compactive effort required to achieve target densities. NCDOT specifications require 92% density compaction for RS9.5B mix, but field tests showed that the RAS-containing mix averaged 93.2% density with less rolling effort than the same mix type without shingles. This means:

• Fewer roller passes needed to meet specification requirements.
• Potential fuel savings from reduced equipment operation time.
• Faster paving operations with improved production rates.
• Consistent density results across the project length.

Rutting Resistance and Volumetric Properties

The aged binder present in tear-off shingles contributes to a stiffer mix that exhibits improved resistance to permanent deformation. Ross reported that the RAS-containing mixes showed better rutting performance compared to conventional mixes. Volumetric properties also remained within specification, meeting all design criteria required by NCDOT. The mix was developed through extensive laboratory testing during the winter of 2010, with multiple design iterations evaluated before the final blend was approved.

Cost Savings on Liquid Asphalt Cement

Liquid AC binder is the most expensive component in any asphalt mix. By replacing a portion of virgin binder with the asphalt content inherent in the shingles, Greenville Paving achieved meaningful cost reductions. The 3% shingles content in the S9.5B surface mix, combined with 12% RAP, meant that a substantial portion of the binder required for the mix was sourced from recycled materials rather than purchased at market rates. This cost advantage helps contractors remain competitive in bidding environments where margins are tight.

Equipment Used on the Project

The paving train on US-258 consisted of standard equipment configurations, demonstrating that RAS mixes can be placed using conventional machinery without modifications:

• Caterpillar Paver AD1000 for material placement.
• Two Caterpillar 634D rollers for compaction.
• Roadtec material transfer vehicle for maintaining continuous paving operations.

This equipment setup mirrors what many contractors already use for conventional paving, lowering the barrier to entry for RAS adoption. For related knowledge on material processing in construction, the principles of Dry Mix Process and Wet Mix Process for Shotcrete offer an interesting parallel in terms of how material handling and mixing methods affect final quality.

The Future of RAS in Asphalt Pavement Applications

Regulatory and Quality Control Evolution

Todd Whittington, P.E., asphalt design engineer with NCDOT, characterized the US-258 project as a success that met agency expectations. He indicated that continued successful projects could lead to increased use of post-consumer shingles in future NCDOT projects. However, he emphasized the importance of maintaining proper controls, particularly in stockpile management to meet air quality standards.

Unlike warm mix asphalt technologies that benefit from significant manufacturer support and troubleshooting resources, RAS technology is still developing its support infrastructure. Contractors adopting RAS must be prepared to invest in their own research and quality control processes until the industry matures.

Increasing RAS Content in Mix Designs

The US-258 surface mix contained 3% RAS and 12% RAP, but Ross expects these percentages to increase over time as contractors and agencies gain confidence with the material. Future developments that could support higher RAS content include:

• Development of testing protocols to determine the quality of liquid binders in RAS.
• Greater availability of manufacturer scrap shingles as recycling networks expand.
• Refined processing techniques that produce more consistent RAS material.
• Broader industry experience leading to better troubleshooting resources.

Industry-Wide Implications

The US-258 project demonstrated that RAS can be successfully incorporated into surface course mixes on highway projects while meeting all specification requirements. The environmental benefit of diverting shingle waste from landfills is substantial, with 11 million tons of asphalt shingle waste generated each year in the United States. As more states follow North Carolina lead and approve RAS for highway applications, the technology has the potential to become standard practice rather than a pilot project novelty.

Contractors considering RAS adoption should be prepared for the upfront investment in asbestos testing capability and the learning curve associated with stockpile management and mix design optimization. However, as the Greenville Paving experience shows, the combination of cost savings, performance benefits, and environmental advantages makes RAS a compelling option for forward-thinking asphalt producers. Those looking to deepen their understanding of construction material systems can explore topics like Sand Cement Screed Mix for Flooring to see how material proportioning principles apply across different construction applications.