Reclaimed asphalt pavement (RAP) has long been a material of interest in road construction, but its application in pavement preservation surface treatments has gained significant momentum. As agencies and contractors seek cost-effective and sustainable alternatives to virgin aggregates, RAP has emerged as a viable option for chip seals, slurry seals, and microsurfacing. Greg Duncan, senior engineer at Applied Pavement Technology, presented a Federal Highway Administration (FHWA) webinar detailing how practitioners have adapted typical practices to achieve performance comparable to virgin materials when using RAP in preservation treatments. Understanding the fundamentals of Pavement Construction and Asphalt Equipment a Complete Guide provides essential background context for these advanced surface treatment techniques.
Most contractors currently use between 10 and 30 percent RAP in their asphalt mixtures for new construction, which has created substantial RAP stockpiles across the country. Since these materials improve sustainability and reduce project costs, using them for pavement preservation treatments makes both economic and environmental sense. Agencies have turned to chip sealing, slurry sealing, and microsurfacing as practical methods to put reclaimed materials to work while reducing growing RAP stockpiles.
Understanding RAP in Chip Seal Applications
A chip seal consists of an asphalt binder, typically an emulsion, sprayed directly onto a pavement surface followed by an application of aggregate chips that are immediately rolled to ensure proper embedment. Aggregate retention remains a primary construction and performance concern for chip seals, and understanding how RAP behaves in this context is essential for successful application.
Processing Requirements for RAP in Chip Seals
The chip seal design process for RAP follows largely the same principles as for virgin aggregates, as the material reacts similarly to applications where pre-coated aggregate was used. However, RAP requires different processing before it can be used in preservation treatments:
- RAP must be crushed and screened into single-sized coarse aggregate and fines
- RAP stockpiles must be free of metal, fiber, or soil contamination
- The material should be processed to achieve consistent gradation similar to virgin aggregates
- Moisture content must be controlled to ensure proper emulsion adhesion
Performance Findings from RAP Chip Seal Projects
Contractors who have applied chip seals with recycled materials report that very few application changes are needed to achieve performance comparable to virgin resources. Studies have found that chip seal emulsion and application rates for RAP treatments require minimal alteration from those used for virgin treatments. Notably, RAP-based chip seals have demonstrated several performance advantages:
- Better aggregate bond compared to virgin aggregate chip seals
- Longer retention of darker surface texture, which improves aesthetics
- Comparable chip loss rates when properly processed and applied
- Cost savings through reduced material procurement expenses
For contractors already familiar with Asphalt Plants and Pavement Construction Equipment a Complete guide to production and placement, transitioning to RAP-based chip seals requires relatively minor adjustments to established workflows.
Applying RAP in Slurry Seals and Microsurfacing
Slurry seals and microsurfacing represent another category of pavement preservation treatments where RAP has shown promising results. A slurry seal is a mixture of well-graded aggregate, such as fine sand and mineral filler, combined with an asphalt emulsion and spread over the full width of a pavement surface using either a squeegee or a spreader box mounted on a truck. Microsurfacing uses crushed, well-graded aggregate combined with mineral filler, such as portland cement, and a latex-modified asphalt emulsion applied through similar equipment.
Design Requirements and Material Specifications
While RAP use in slurry seals has been less extensive than in chip seals, research findings have been equally positive. RAP slurry seals and microsurfaces share similar design requirements with RAP chip seals but come with more restrictive material specifications and additional equipment requirements during application.
Benefits of RAP as Aggregate in Slurry Seals
Using RAP as the aggregate source in slurry seals offers several distinct advantages over virgin materials:
- Higher total asphalt content compared to virgin mixes, even though less emulsion is required to reach optimum design properties
- Better resistance to cracking and raveling due to the increased asphalt content
- Smoother surface finishes and better-bonded longitudinal seams
- Reduced material costs from lower emulsion requirements
Application Considerations for Slurry and Microsurfacing
Contractors working with RAP in slurry and microsurfacing applications discovered that additional rolling with a pneumatic roller was necessary to properly set the materials. This extra compaction step helps achieve the desired surface density and ensures adequate embedment of the RAP aggregate particles.
One important finding from the studies is that incorporating too much asphalt in chip seals, slurry seals, and microsurfaces can increase bleeding under traffic loads. However, higher asphalt contents also improve resistance to aging, oxidation, and raveling, creating a balance that must be carefully managed during mix design. Understanding the principles of Asphalt Pavement Engineering Mix Design Construction Methods Rehabilitation is essential for achieving this balance in RAP-based preservation treatments.
California Case Studies: Real-World RAP Preservation Success
Several agencies in California have put RAP-based preservation treatments to the test with consistently positive outcomes. These real-world applications provide valuable data for agencies and contractors considering similar approaches in their regions.
Los Angeles County: A Leader in Sustainable Pavement Practices
Los Angeles County has been at the forefront of using recycled materials in pavement preservation. According to the FHWA report, LAC’s transition from conventional to sustainable practices saved over $52 million and substantially reduced greenhouse gas emissions. Since 2009, the county’s RAP use has grown to the point where, in 2018, approximately 35 percent of LAC’s annual pavement program budget was spent on RAP pavement preservation treatments.
San Bernardino County: Overcoming Aggregate Scarcity
In San Bernardino County, aggregate supply is scarce in less populated areas, making conventional chip seals expensive due to the need to haul materials over 100 miles to project destinations. The county began using reclaimed asphalt pavement aggregate (RAGG) in 2014 as a cost-effective alternative. Key findings from their experience include:
- Lower chip loss using RAGG compared to virgin aggregate, likely due to the asphalt film on RAP chips enhancing bonding
- Fine particles on the RAP chips acting as choke stone, increasing texture density and reducing chip loss
- Cost savings exceeding 25 percent on some projects where contractors could bid RAGG as a substitute for virgin materials
- Performance characteristics for chip seals remained similar whether RAGG or virgin aggregate was used
Lake Los Angeles RAGG Chip Seal Project
A chip seal composed of RAGG and PG 76-22 tire-rubber-modified asphalt binder was placed in the Lake Los Angeles area in 2013, funded by the California Department of Resources Recycling and Recovery. The objective was to determine whether nonpreheated 0.375-inch and 0.3125-inch RAGG could perform as well as virgin aggregate. Despite construction challenges including variable moisture content, RAGG cleanliness issues on the first day, and adverse weather conditions, the chip seal continues to perform positively years after placement.
Comparative Performance and Economic Benefits of RAP Treatments
The accumulated evidence from research studies and field applications demonstrates that RAP can be a cost-effective alternative for pavement preservation, depending on local market conditions. Agencies and contractors both stockpile RAP as a byproduct of milling asphalt surfaces and producing excess material at asphalt plants. Most RAP owners are eager to reuse the material cost-effectively in as many projects as possible, provided it does not compromise performance or service life.
| Treatment Type | RAP Processing Required | Application Adjustment | Performance vs. Virgin | Cost Savings Potential |
|---|---|---|---|---|
| Chip Seal | Crush and screen to single-sized aggregate | Minimal emulsion rate changes | Comparable or better bond, darker texture | 10-25% |
| Slurry Seal | Crush, screen, control fines | Extra pneumatic rolling needed | Better crack resistance, smoother surface | 15-25% |
| Microsurfacing | Crush, screen, control fines and moisture | Additional rolling, stricter specs | Comparable with higher asphalt content benefits | 10-20% |
Finding suitable RAP sources requires identifying owners who have more material in their stockpiles than they can use in standard mix production. The economic viability of RAP-based preservation treatments depends on local factors including aggregate haul distances, virgin material costs, and RAP stockpile availability. Contractors should evaluate the specific conditions in their service areas to determine whether RAP preservation treatments make financial sense.
The use of additives and modifiers can further enhance RAP performance in preservation applications. For practitioners interested in maximizing RAP content without sacrificing pavement quality, exploring Reviving Recycled Asphalt How Additives Enable Higher Rap content provides valuable insights into advanced material optimization strategies.
Considerations for Adopting RAP Preservation Treatments
Agencies and contractors considering RAP for pavement preservation should evaluate several factors before proceeding:
- Assess the quality and consistency of available RAP stockpiles for contamination and gradation
- Verify that processing equipment can produce single-sized aggregate suitable for chip seals or the finer gradations needed for slurry seals
- Conduct trial sections to calibrate emulsion application rates and rolling patterns for local materials
- Monitor performance through regular inspections and compare results with virgin aggregate control sections
- Document cost savings and performance data to build the business case for broader adoption
These technologies are expanding rapidly across the country as more states and agencies deploy RAP in surface treatments. The consistent finding that these materials perform well while reducing costs encourages broader adoption. As the body of successful projects grows, contractors and agencies positioned at the forefront of these applications will have a competitive advantage in their local markets.
The path forward for RAP in pavement preservation is clear. With proper processing, careful application, and attention to the specific requirements of each treatment type, recycled asphalt can deliver performance equal to or better than virgin aggregates while generating significant cost savings and environmental benefits. As RAP stockpiles continue to grow and the pressure to adopt sustainable practices increases, the use of recycled materials in chip seals, slurry seals, and microsurfacing will become an increasingly important tool in the pavement preservation toolkit.