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62 The use of RAM, including RAP and RAS, has increased in highway construction due to potential engineering, economic, and environmental benefits. Although the use of RAM in new asphalt mixtures can reduce the amount of virgin and nonrenewable materials required and increase the rutting resistance of pavements, it may also compromise cracking resistance. To mitigate this issue, asphalt mixtures containing RAM may require the use of a softer binder or RA. Not all state DOTs have experience with mixtures using RAs. The primary objective of this synthesis is to document current state DOT practices and procedures related to the use of RAs in asphalt mixtures containing RAM. This chapter presents a summary of the major findings from this synthesis project along with identified knowledge gaps and research needs. 5.1 Summary of Findings The responses provided by 47 state DOTs and eight CPTAs provided valuable insight into current practices with respect to use of RAs in asphalt mixtures. Detailed interviews were con- ducted with five state DOTs. The information obtained in the interview sessions was used to acquire a more precise idea of the concerns and effective practices for evaluation of RA-treated binders and mixtures. Key findings are: ⢠The use of RAM is widespread. All responding survey agencies allowed the use of RAP in asphalt mixtures. ⢠Most (60%) agencies did not allow the use of RAs in asphalt mixtures on a routine basis. For the majority of the agencies that were using RAs, they were primarily used for demonstration or research purposes. Use of RAs was neither specified nor required in asphalt mixtures. ⢠As identified from the literature review, three important factors can significantly affect the performance or effectiveness of RAs in asphalt binder blends and mixtures: the type of RA, appropriate selection of RA dosage, and the dispersion and diffusion of the RA into RAM. ⢠RA types can be classified based on either the ASTM D4552-20 specification or RA source/ chemical composition. A large number of agencies were unsure of the exact classification type of RA products used in their mixtures. ⢠The literature review indicated that the most prevalent tool for RA dosage determination was use of blending charts to meet desired properties or specifications (e.g., viscosity or penetra- tion values, specific PG grades). This method has also been employed by agencies and contrac- tors for determining RA dosage. ⢠For the agencies that used RAs in asphalt mixtures, no or minimal changes were made to agency QA processes. ⢠As reported by agencies, the most common methods used to evaluate performance of mix- tures with RAs were standard Superpave binder tests, mixture testing on STA/plant-produced mixtures, and monitoring field performance. Most surveyed agencies indicated that they had C H A P T E R  5 Summary of Major Findings
Summary of Major Findings 63  not extensively evaluated the long-term performance of asphalt mixtures with RAs. The litera- ture review shows that a loss of RA effectiveness over time is a major concern for RA-treated binder blends and asphalt mixtures. ⢠There is a wide range of testing methods documented in the literature to evaluate the prop- erties and performance of RA-treated asphalt binders and mixtures (e.g., binder analytical tests and microscopy techniques, various mixture performance tests, and advanced pavement performance simulation and prediction tools). The appropriate RA type and dosage can be preliminarily determined using blending charts and then can be further evaluated and vali- dated using these various testing methods. The five case examples illustrated that for evalu- ation of the RA-treated material, some state DOTs primarily use binder tests, while others focus on mixture performance evaluations in the context of BMD methods, and some states also include both binder and mixture testing for evaluation purposes. ⢠The literature review showed that the addition of RA may increase the rutting and moisture susceptibility of asphalt mixtures containing RAM. Some agencies reported that they have observed premature distresses of bleeding, raveling, and increased moisture damage for RA- treated asphalt mixtures. ⢠The five case example states indicated that they have not undertaken use of LCA and LCCA or sustainability measures for evaluation of the RA-treated asphalt mixtures. The literature review also showed that only a few studies have used these methods to evaluate asphalt materials containing RA products. Little attention has been devoted to directly comparing the various RAs from the sustainability point of view by conducting LCA and LCCA. ⢠The survey indicated that the major challenges for agencies to start or continue using RAs are (1) lack of agency experience in evaluating of asphalt mixtures with RAs, (2) lack of tests and criteria to approve RA, and (3) lack of tests and criteria to determine dosage rate and performance. ⢠Overall, the use of RAs is still relatively new for a majority of the agencies, and most have limited experience with use of RAs in asphalt mixtures. 5.2 Identified Gaps in Knowledge and Future Research Needs Knowledge gaps that were identified from the literature review, surveys, and case examples, as well as suggested future research to address these gaps, are discussed in the following: ⢠The appropriate aging conditioning protocols to use for RA-treated binders and mixtures for the purpose of investigating the long-term performance of RA-treated binders and mix- tures were identified as a knowledge gap through the surveys and literature review. There are several binder and mixture aging protocols that can be followed, but their advantages and disadvantages, as well as applicability to RA-treated asphalt materials, need to be evaluated. ⢠While the ASTM D4552 procedure classifies RA types primarily based on the viscosity of RA products, there is a knowledge gap with respect to the standard method to classify RA prod- ucts based on chemical composition. ⢠A knowledge gap exists on investigation of the role of binder activation and availability of recycled binder from RAM during the mix design and evaluation stages, which is also of great importance for determination of the appropriate RA type and dosage as well as diffusion of RA into RAM. ⢠Surveys and the literature review revealed that binder testing is used by many agencies for determination of RA type and dosage as well as for evaluation of the RA-treated asphalt binders and mixtures. However, testing of binder blends where the RA, RAP, and virgin binder are fully blended during the recovery process does not represent the blending status of these components in the mixture. There are also many existing challenges associated with binder
64 Use of Recycling Agents in Asphalt Concrete Mixtures extraction and recovery (e.g., remaining solvent in the sample and environmental effect due to the use of solvent). Future tools and methods for RA evaluation and type and dosage deter- mination should focus on testing of mixtures. ⢠The evaluation of the RA-treated binder and mixtures at present is mostly limited to the labo- ratory scale. The effectiveness of the various binder and mixture testing methods documented in the literature and used by agencies needs to be validated by field performance over time. ⢠Many studies from the literature review have shown that there is a loss of effectiveness of RA over time; however, little attention has been devoted to comprehensively evaluating the rate of loss in effectiveness for the various RA types or the dosage. ⢠Guidance on RAM stockpile management to ensure consistent properties and good quality of the RAM materials, which is critical in design and production of mixtures including mixtures with RAs, is of interest to agencies due to widespread RAM usage. ⢠Sustainability measures of RA-treated asphalt materials by conducting LCA and LCCA were identified as a knowledge gap through both the survey and literature review.
