Characteristics of aggregate gradation, drain down and stabilizing agents in stone matrix asphalt mixtures: A state of art review

Over the years stone matrix asphalt (SMA) mixtures have evolved significantly and research works have presented some promising results. This made the paving industries to increase the usage of SMA mixtures in the pavement construction. However, SMA mixtures have certain challenges or setbacks, which require comprehensive understanding. The aim of the review paper is to present some of the challenges in SMA mixtures and their mitigations measures. It is observed that several challenges (i.e., aggregate gradation, stone-on-stone contact, drain down and stabilizing agents) related to SMA mixtures are addressed by the researchers. Many agencies have suggested their own aggregate gradation based on their experience, location, climatic conditions and available manufacturing facilities, and they are working exceptionally. Stone-on-stone contact is achieved with proper aggregate gradation and failure to meet the same will reduce the performance. Drain down is major concern in the SMA mixtures since it controls overall mixture performance. Therefore, researchers shown out the most importance to curtail the same. Several researchers followed ASTM D6390 and National Cooperative Highway Research Program (NCHRP No. 424) procedure to evaluate the drain down and concluded that drain down should be less than 0.3% (by weight of mixture). Apart from that, researchers are trying different stabilizing agents to reduce the drain down. Especially, stabilizing agents such as fibers and polymers have a positive impact. Recently, suitable waste and recycle products are used as stabilizing agent to reduce drain down and environmental pollution. Out of which coconut fiber has shown some promising results, however, comprehensive research is required in this regard. On the other hand, 0.3% of cellulose fiber is working as a good stabilizing agent and can be used without conducting drain down test. The review paper provides a complete knowledge about different stabilizing agents used in SMA mixtures to minimize the drain down.