Influence of Aggregate Morphology and Grading on the Performance of 9.5-mm Stone Matrix Asphalt Mixtures

About the report:

Stone matrix asphalt (SMA) is a gap-graded hot mix asphalt with a high percentage of coarse aggregate and a high asphalt content. SMA is the typical gap-graded mixture used in Virginia that is intended to maximize rutting resistance and durability with a stable stone-on-stone skeleton held together by a rich mixture of asphalt binder.  The field performance of SMA in Virginia has been generally excellent.  However, several SMA pavements have undergone premature failure, and factors related to both pavement structure and materials have been identified as causes for the poor performance.  A detailed forensic study recommended a specification change for gradation of SMA-9.5 mixtures to provide improved stability.  The Virginia Department of Transportation adopted a new aggregate gradation in 2012 for these mixtures.  Aggregate morphological characteristics, including sphericity, flatness ratio, elongation ratio, angularity, and texture, have been recognized as major factors influencing the performance of aggregate and asphalt mixtures. The current study demonstrated via laboratory methods the stability of mixtures conforming to the Virginia Department of Transportation’s new grading specification and the effects of aggregate morphology and asphalt binder properties on stability. 

SMA mixtures designed and produced by different contractors using aggregates from different quarries in Virginia were included in the study. SMA mixtures and samples of the aggregates used in production were obtained for laboratory evaluation of the structural stability and aggregate characteristics of the mixtures.  All mixtures met the criterion of VCAMIX < VCADRC, indicating good stone-on-stone contact and a denser coarse aggregate fraction.  Flow number and asphalt pavement analyzer results showed better rutting resistance of all mixtures with revised gradation.  In both confined and unconfined flow number tests, polymer-modified binders (PG 64E-22 or PG 76-22) showed a lower flow number slope compared to that of PG 70-22 (PG 64H-22) binders, indicating better rutting resistance. SMA mixtures with polymer-modified binder also showed excellent fatigue performance.  All mixtures showed a maximum number of cycles of 1,200 in the Texas overlay test, showing the excellent reflection crack resistance of these mixtures.  Regression analyses were conducted between weighted mean morphological characteristics and performance parameters.  Aggregate morphological characteristics were found to play an important role in the mechanical performance of SMA mixtures and the uncompacted void content of the coarse aggregates.  Results showed that using more spherical (equant), angular, or better-crushed rough coarse aggregate particles in SMA mixtures can improve the resistance of the SMA to rutting.  In addition, aggregates with fewer flat and elongated aggregate particles can contribute to better rutting performance of SMA mixtures.  Results also showed that better rutting resistance can be obtained using polymer-modified binders even if the aggregate morphological characteristics are slightly less favorable.

Authors