Analysis of Mineral Fillers and Mastics Used in Stone Matrix Asphalt

Basil M. Harris and Kevin D. Stuart

Abstract

Stone Matrix Asphalts (SMA) have been proven to deliver more durable and stable flexible pavements than conventional dense graded Hot Mix Asphalts (HMA). Recognized for their resistance to rutting, gap graded SMA mixes are used mainly for surface courses subjected to high volume, heavy traffic; used in surface mixtures because of the high stresses near the surface and because SMA is a premium mix that costs significantly more than conventional dense graded HMA. SMA’s inherent stability is due to the framework formed by the large proportion of durable, coarse aggregate particles present in the mix. Unlike open graded mixes, SMA’s have a relatively high binder content and low air voids (3 to 4 percent). The mastic is a combination of the binder, added mineral filler, filler fraction of the aggregate, and stabilizing additive. The effect of the properties of the added mineral filler on the properties of the mastic and ultimately on the performance of the pavement is not well understood. The objectives of this study were: to develop reliable procedures for characterizing mineral fillers; and to explore the relationships between the reported field performances of mineral fillers in SMA mixes and laboratory measured mineral filler properties. Reliable methodologies for obtaining the particle size distributions of minus 150-:m aggregates by laser diffraction are proposed. The proposed procedures include: representative sampling and sample size reduction of mineral fillers and minus 150-:m aggregate fractions, selection of an optimum medium/dispersant solution for testing mineral fillers in a laser diffraction device, and method for obtaining a repeatable analysis of particle size distributions of mineral fillers by laser diffraction. Samples of 18 field tested mineral fillers were obtained from three European countries. The performance of each of the mineral fillers was reported as either being “good” (recommended) or “bad” (norecommended) for use in SMA and HMA concretes. Performance was based on the effects of the fillers on stiffness. A high stiffness may lead to cracking and low workability. A low stiffness could lead to bleeding or shoving. Among the mineral filler properties measured, the Anderson modified Rigden Voids (percent voids in a compacted sample of dry mineral filler) was the only independent variable that could singly distinguish most of the “good” fillers from the “bad”. Several models are proposed that predict the probability that a given filler is “good”.