As part of a wider review of the Series 900 of its Specification for Road Works (SRW), the National Roads Authority (NRA) in Ireland requested TRL to undertake a laboratory based investigation of the friction performance of Stone Mastic Asphalt (SMA) specimens with different combinations of fine aggregate in the mix design. Cl. 942 of the SRW currently permits the use of crushed rock fines (CRF), sand, or a combination of CRF and sand as the fine aggregate (0/2mm or 0/4mm aggregate fraction) in the asphalt mix. Concerns have been raised that this fine aggregate can adhere to the surface of the coarse aggregate in the initial period after laying and that this may be influencing the skid resistance performance of some materials during the initial period of trafficking.
Two methodologies were adopted for the laboratory testing:
1. A “staged” polishing approach to examine the initial friction performance of the specimens, and
2. Grit-blasting and further polishing to represent the specimens’ longer term friction performance.
The results of the study indicated that the nature of the fines in an SMA mix does have an influence on the friction in the initial stages of polishing but this influence may be reduced or eliminated in the longer term. However where low PSV fines are retained at the end of an extended polishing process, they can have an adverse effect on the friction.

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