Implementing Revisions to the Allowable Maximum Recycled Binder Ratio (RBR) Specification
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2020-09-01
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Edition:July 2017–July 2019
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Abstract:An increase in the use of reclaimed asphalt pavement (RAP) in the design and construction of new roads must not come at the expense of reduced durability or life cycle cost of flexible pavements. In Texas, current mixture design specifications allow the use of a binder with lower high temperature grade in lieu of the specified binder when incorporating RAP into the mixture. Referred to as binder substitution, it is accompanied by other constraints that limit the maximum recycled binder ratio for different mixture types and applications. The current specifications, although straightforward, have a few gaps with regards to the allowed binder substitution and recycled binder ratio. This study evaluated those gaps to determine: (i) impact of binder substitution on performance, (ii) impact of recycled binder ratio on performance, (iii) the influence of different types of RAP on the rheological and performance related properties of asphalt binder, and (iv) efficacy of agents of additives that can potentially be used to improve the properties and performance of asphalt mixtures incorporating RAP. The performance of binders and mixtures were evaluated using systematically controlled materials prepared in the laboratory as well as materials sampled from the field. The results show that addition of RAP to a substitute binder results in an increase in stiffness, non-recoverable compliance, and rutting resistance—but also show a substantial decrease in the fatigue cracking resistance and loss of low temperature grade with an increase in recycled binder content. This is especially true when using a substitute binder grade that is two high temperature grades below the specified binder or using RAP with RAS. The low temperature properties of the recycled binder from RAP can vary significantly from one source to another and have a significant effect on the properties of the binder and mixture. These results suggest that the substitute binder should have no more than one high-temperature grade lower than the specified binder and preferably also one low-temperature grade lower than the specified binder. The quality of the RAP from the specific source and interactions with the rejuvenator must also be evaluated on a case-by-case basis. This project developed fast and effective methods for such evaluation.
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