A Study of Effects of Binder Quality and Natural Fiber on Stone Matrix Asphalt Mixtures

Stone Matrix Asphalt (SMA) is a gap graded mix, characterized by high coarse aggregates, high asphalt contents and polymer or fiber additives as stabilizers. High concentration of coarse aggregate maximizes stone-to-contact and interlocking in the mix which provides strength, and the rich mortar binder provides durability. The stabilizing additives composed of cellulose fibers, mineral fibers or polymers are added to SMA mixtures to prevent draindown from the mix. In comparison to dense graded mixtures SMA has higher proportion of coarse aggregate, lower proportion of middle size aggregate and higher proportion of mineral filler. It resists permanent deformation and has the potential for long term performance and durability. In the present study, an attempt has been made to study the engineering properties of mixtures of stone matrix asphalt made with three types of binders namely conventional bitumen 80/100 and 60/70 and modified binder CRMB 60, with a non-conventional natural fiber, namely coconut fiber. The binders and fibers in different proportions are used for preparation of mixes with a selected aggregate grading. The role of a particular binder and fiber with respect to their concentrations in the mix is studied for various engineering properties. For this, various Marshall samples of SMA mixtures with and without fibers with varying binder type and its concentration are prepared. The optimum binder content is determined keeping the suggested air voids content in the mix. Marshall properties such as stability, flow value, unit weight, air voids are used to determine optimum binder content and optimum fiber content for each type of binder for further studies on SMA mixes. Thereafter, the draindown characteristics, both static and repeated indirect tensile strength parameters and moisture susceptibility characteristics in terms of tensile strength ratio and retained stability of different SMA mixtures values have been studied for such mixes. It is observed that only 0.3% addition of coconut fiber significantly improves the Marshall properties of SMA mixes. Addition of nominal 0.3% fiber considerably improves the draindown, indirect tensile strength and fatigue characteristics of the SMA mixes with conventional bitumen, which would otherwise have not been able to meet the prescribed criteria