Laboratory performance of stone matrix asphalt mixtures with two aggregate gradations

Stone matrix asphalt （SMA） is a gap-graded bituminous mixture which can be used in surface layer of high volume pavements. The mixture has higher concen- trations of coarse aggregates, providing strength and rut resistance to the mixture, and higher asphalt content giving durability. There must be a proper stone-to-stone contact between the coarse aggregates of SMA, and hence aggre- gate gradation is an important factor in this mixture. In the current study, two aggregate gradations, with nominal maximum aggregate sizes （NMAS） 16 and 13 mm were adopted to prepare SMA mixtures and their laboratory performances were compared. Polymer-modified bitumen （PMB） was used as the binder material and no stabilising additive was used, since drain down was within permissible limits for both mixtures with PMB. Conventional cylin- drical specimens were prepared in superpave gyratory compactor with bitumen contents 5.0 %, 5.5 %, 6.0 %, 6.5 % and 7.0 % by weight of aggregates, and volumetric and Marshall properties were determined. Tensile strength, behaviour to repeated loading etc. were checked for cylindrical specimens prepared at optimum bitumen con- tent, whereas specially prepared slab specimens were used to check the rutting resistance of SMA mixtures. From the laboratory study, it was observed that, out of the two SMA mixtures, the one with NMAS 16 mm performed better compared to the other. These improved properties may be attributed towards the larger coarse aggregate sizes in the mixture.

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-onstone 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 than0.3%(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.