Enhancing the mechanical performance of mixed recycled aggregates for unbound pavement layers

Abstract

The use of recycled aggregates in road pavements can markedly contribute to more sustainable infrastructures, minimising the extraction of virgin aggregates and using secondary materials that would otherwise be discharged into the environment with negative impacts. In Portugal, as in most Southern European countries, most of the recycled aggregates coming from construction and demolition waste are mixed or non-selected recycled aggregates, containing concrete and mortars, natural aggregates, masonries, among others. These materials are considered as low-quality recycled aggregates, due to poor mechanical properties, namely, their resistance to fragmentation and to wear, and as a result their use in roadway pavement layers is frequently unfeasible. This paper presents the results of a laboratory study on mixtures of a non-selected recycled aggregate with natural and tyre rubber aggregates, with a primary focus on resistance to fragmentation. The study also discusses the fragmentation resistance across different aggregate sizes. The results show that to achieve a meaningful improvement in the fragmentation resistance of the recycled aggregate, it should be blended with very high-quality all-in-aggregates. The addition of tyre-derived aggregate (TDA) to recycled and natural aggregates markedly improves fragmentation resistance; however, the concentration of zinc in the TDA leachate exceeds the regulatory limit.