Study Examines Mixes Combining RAS and WMA

Annual surveys by the National Asphalt Pavement Association have shown that the use of recycled asphalt shingles (RAS) in asphalt mixtures peaked in 2013 and has since declined sharply. The surveys also show that the national average of reclaimed asphalt pavement (RAP) content has essentially plateaued at about 20% in recent years. The motivations to use RAP and RAS have largely been driven by interests in the conservation of natural resources and potential economic savings by reducing the quantity of new binder needed in mixtures.

The introduction and growth of warm mix asphalt (WMA) technologies over the past decade have provided additional opportunities to reduce energy consumption and emissions in asphalt pavement construction. However, the convergence of RAS and WMA has raised some concerns. Since RAS binders have very high softening points, some experts in the industry have questioned if lower mix temperatures with WMA are sufficient to activate hard RAS binders.

Earlier this year, NCAT completed a study under the National Cooperative Highway Research Program (NCHRP) to quantify the effects of RAS on asphalt mixture properties and evaluate the short-term performance of asphalt mixtures that used RAS in conjunction with WMA. The final report for this project is soon to be released as NCHRP Research Report 890.

The study included an extensive evaluation of mixtures from eight field projects that contained RAS produced at HMA and WMA temperatures. Laboratory performance tests were conducted to evaluate the recovered binders, mixture stiffness over a wide temperature range, moisture susceptibility, fatigue cracking, thermal cracking, and permanent deformation. Statistical analysis of the test results found no detrimental effects associated with using WMA technologies with mixtures containing RAS. As might be expected, laboratory tests showed that using WMA technologies improves the cracking resistance of some RAS mixtures.

Based on the literature review and research conducted at NCAT, no clear method has been identified to determine the degree to which RAS binder activates and becomes an integral part of the composite binder in an asphalt mixture. Some evidence indicates that at least partial activation of the RAS binder increases with higher mixing temperatures and longer mixture storage time. However, increasing the mixing temperature to better activate RAS binder results in increased mix stiffness and negatively affects a mixture’s cracking resistance. Other factors that are likely to affect the degree of activation include the stiffness of the RAS binder, sizes of the RAS particles, chemical properties of the virgin binder and/or rejuvenator or other additives, mixing time, and storage time. It is unwise to assume that all RAS mixtures would have the same amount of activation under the range of conditions that exist in the real world.

A practical outcome of the project is commentary on recent revisions to AASHTO PP 78 that expands its application to WMA/RAS mixtures. The latest Provisional Standard Practice for Design Considerations When Using Reclaimed Asphalt Shingles (RAS) in Asphalt Mixtures (AASHTO PP 78-17) provided new guidance on determining the shingle aggregate gradation and specific gravity, adjusting VMA requirements, and testing of the composite binder for embrittlement using the critical low-temperature difference parameter ΔTc, as well as notes for the mix designer to consider. One new note states that “[a] mixture performance test for cracking implemented by the agency is acceptable in lieu of the binder testing for ΔTc.”

The study also examined the economics of using RAS. The report states that the use of RAS is most economically favorable when contractors are able to maximize tipping fees and use higher percentages of RAS when RAP contents are not impacted and when virgin asphalt prices are relatively high. Highway agencies are more likely to realize an economic benefit when competition among contractors is good and the performance of mixtures containing RAS is equal to or better than mixtures without RAS.

The report also includes recommended best practices for the processing of RAS. The study found that lower mix production temperatures associated with WMA did not cause any plant issues or construction problems for any of the projects evaluated in this study. There were no problems with burners, baghouses, motor amperages, or mix storage. Excellent mixture coating was achieved with all WMA technologies at the lower mixture production temperatures. Roadway performance of HMA and WMA mixtures containing RAS after two to three years was practically the same. It was recommended that the field projects be reexamined in a few more years so that the performance can be compared to the lab test results to aid in setting criteria for the mixture performance tests.