Effect of nominal maximum aggregate size and aggregate gradation on the surface frictional properties of hot mix asphalt mixtures

Abstract

Roadway friction is one of the most important features for road safety. Many procedures, tests, and tools have been used widely to measure surface frictional properties of flexible pavements. The main objective of this study is to evaluate the surface characteristics of Superpave laboratory-prepared Hot Mix Asphalt (HMA) specimens made of local resources and to study the role of the Superpave Gyratory Compactor (SGC) in fabricating the top and bottom surfaces of HMA specimens. The evaluation was carried out using two methods; the British Pendulum Test (BPT) and the Sand Patch Test (SPT). Asphalt mixtures were prepared using PG 70–10 asphalt binder and crushed limestone aggregate with three Nominal Maximum Aggregate Sizes (NMAS); 19 mm, 12.5 mm, 9.5 mm and involving two types of gradations for each NMAS; Fine Gradation (FG) and Coarse Gradation (CG). Analysis was performed using descriptive charts and analytical techniques including the three-way Analysis of Variance (ANOVA) and regression analysis. The study has shown that the surface frictional properties of mixtures’ top surfaces were significantly affected by the NMAS and aggregate gradation type. Accordingly, by increasing the NMAS in the mix, the top surface exhibits improved macrotextures and lessened microtextures for both types of aggregate gradations. On the other hand, the frictional properties of the bottom surfaces were only affected by the aggregate gradation type due to the migration of fine materials toward the bottom surface during compaction, and therefore the bottom surfaces are mostly similar to one another. Additionally, it has also been observed that fine-graded mixtures provide higher microtexture values and lower macrotexture values when compared to coarse-graded mixtures. The study helped conclude that the difference between friction and texture values measured on top and bottom surfaces of asphalt concrete specimens is significant.