Despite the several benefits of modifying bitumen with polymer; it is increasingly becoming difficult to ignore concerns about its high energy consumption and fume emissions during asphalt production. Previous studies found on natural rubber (NR) modified binders applied it as a hot mix asphalt (HMA), however in recent years there has been increasing interest in warm mix asphalt. This research explores the possibility of applying NR in warm mix asphalt (WMA) by comparing the physical properties of cup lump rubber modified bitumen with and without warm mix additive. Four percentages (2.5%, 5.0%, 7.5% and 10%) of Cup lump rubber (CLR) was used to modify 60/70 penetration grade bitumen, while three percentages (0.3%, 0.4%, and 0.5%) of Evotherm chemical was added to produce warm cup lump modified bitumen (WCMB). Storage stability, penetration, softening point, viscosity, ductility, and elastic recovery tests were conducted to determine the effect of CLR and Evotherm. The results show that storage stability and viscosity of the cup lump modified bitumen heavily depends on the CLR content. While a positive correlation exists between the increase in binder stiffness with increasing mixing and compaction temperatures. The addition of 0.5% Evotherm reduces the viscosity by 14%. Contrary to expectations, CLR was found to reduce the ductility of the bitumen while the presence of Evotherm in the blend increases the softening point. The use of warm NR modified bitumen in road construction is expected to lower the asphalt production temperature, carbon emission and cost without compromising the performance of the mix.

This paper provides an overview of crumb rubber modified asphalt mixtures with particular reference to the dry process. The dry process involves the blending of crumb rubber with hot aggregates prior to mixing with bitumen. In comparison to the wet process (process of incorporating crumb rubber into bitumen prior to mixing with aggregates), this mixture type has a number technical issues. The lack of standards and inconsistent performance have resulted in scepticism among practitioners and researchers in accepting the dry process for rubber modification even though it has the potential to recycle more crumb rubber compared to the wet process. This has resulted in the overwhelming majority of asphalt paving projects involving crumb rubber using the wet process. Therefore, to better understand the mixture, details pertaining to dry mixed rubberised asphalt are discussed in this paper with sufficient information from previous research. The discussions highlight several critical issues regarding its modification concept (the function and behaviour of rubber particles within the mixture), mixture design criteria and mechanical performance of this mixture type. This is necessary in order to identify the factors that play a significant role in improving the mixtures properties for future studies. Additionally, the review will be a positive step in the direction of achieving an appropriate design standard for dry mixed rubberized asphalt mixtures.

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