SWUTC Research Project Description
Use of Recycled Asphalt Shingles in HMA
University: Texas A&M University
Principal Investigator:
Fujie Zhou
Texas Transportation Institute
(979) 458-3965
Funding Source: SPR Program
Total Project Cost: $111,369
Project Number: 0-6614
Date Started: 9/1/12
Estimated Completion Date: 8/31/13
Project Summary
Project Abstract:
Use of recycled asphalt shingles (RAS) in hot-mix asphalt (HMA) or warm-mix asphalt (WMA) mixtures has the potential to significantly reduce the cost of asphalt paving mixes while conserving energy and preserving the environment. Research on this subject is relatively limited in the literature.
Project Objectives:
The main objectives of this research project are defined as below:
- Define best practices relative to the use of RAS in HMA and WMA mixes,
- Develop a balanced mixture design method(s) for RAS mixes including characterizing RAS binder and developing new RAS binder blending charts,
- Construct and monitor field test sections containing RAS,
- Define the environmental benefits associated with the use of RAS, and
- Recommend changes in Texas Department of Transportation (TxDOT) specifications to allow optimal use of RAS.
Task Descriptions:
Task 1 – define current best practices by visiting shingle manufacturers and shingle processors in Texas and testing RAS materials as well as conducting literature reviews.
Task 2 – characterize RAS binder properties and develop RAS binder blending charts through extensive laboratory binder testing. Mixture design method(s) for RAS mixes will be proposed based on laboratory evaluation of engineering properties of RAS mixes in Task 3, and will be validated through field test sections in Task 4.
Task 3 – RAS Mixture Design Methods
Task 4 – Field Test Sections
Task 5 – define the environmental benefits and cost savings.
Task 6 – recommend changes to current specifications and mix design methods based on research results from Task 1 through 5.
Task 7 – Conduct two workshops to effectively implement findings.
Task 8 – Document all research activities in a research report.
Implementation of Research Outcomes:
In the last several years, both reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) have been widely used in paving Texas highways. Compared to RAP, RAS has two features: 1) much higher asphalt binder content, and 2) super stiff asphalt binder. Thus, the use of RAS can not only reduce the cost of asphalt mix paving significantly, but it also conserves energy and protects the environment. However, the super high stiffness of RAS binder causes strong concern on durability (or cracking performance) of asphalt mixes containing RAS. Another concern was the variability of processed RAS due to different RAS sources varying from manufacture waste shingles to a wide variety of tear-off shingles. To address these two major concerns (variability and durability), this research was initiated with objectives to:
• Define variability of processed RAS.
• Develop best practices for RAS processing and stockpile management.
• Evaluate methods of improving cracking performance of RAS mixes in the laboratory.
• Construct field test sections to validate laboratory test results.
Impacts/Benefits of Implementation:
From this study, researchers found the following:
• Processed RAS, in terms of aggregate gradation and asphalt content, is consistent and has low variability when the six-step RAS processing and stockpile management are used.
• RAS binders are very stiff. Tear-off RAS binders with an average of high temperature grade of 175°C are much stiffer than manufacture waste RAS binders, which have an average high temperature grade of 131°C.
• Different from virgin/RAP binders blending, the virgin/RAS binders blending is nonlinear. However, for practical application, the linear blending chart can still be used for estimating continuous grade (high and low temperatures) of both virgin/RAS blended binders and virgin/RAP/RAS blended binders, if the RAS binder is limited within 30 percent of the total binder.
• RAS mixes, when properly designed and constructed, can have similar or better performance than virgin mixes.
• Both laboratory and field test sections indicated that use of softer virgin binder (i.e., PG xx-28 or PG xx-34) or design of mixes with higher density can improve the cracking resistance of RAS mixes.
• Laboratory test results showed that use of rejuvenators can also improve cracking performance of RAS mixes.
• The balanced RAS mix design system for project-specific service conditions, as shown in Figure 1, is validated through field test sections.
Based on the findings from this study, which impacts the USDOT strategic goals of state of good repair and environmental sustainability, researchers recommend the following:
• Implement statewide the best practices developed for RAS processing and stockpile management, RAS mix production, and field construction.
• Use soft binders (i.e., PG xx-28 or PG xx-34) or design the mix with 97.5 percent design density when the total binder replacement including both RAS and RAP is 20 percent and above.
• Implement the balanced RAS mix design system for project-specific service conditions for designing mixes containing RAS.
• Construct field test sections in selected districts (e.g., Dallas/Fort Worth, Austin, Paris, Houston) to validate RAS rejuvenators that have shown significant improvement in engineering properties of RAS mixes.
Web Links:
Final Technical Report