HA/QPA/RBA Collaborative Programme 2004/07: Surface requirements for asphalt roads

This report describes research for Topic 1, “Surface requirements for asphalt roads”, of the 2004–2007 phase of the Collaborative Programme sponsored by the Highways Agency, the Quarry Products Association and the Refined Bitumen Association. The primary objective was to assess whether current aggregate specifications related to skid resistance are appropriate for modern asphalt surface courses. A database study attempted to compare the in-service performance of existing thin surfacings with current specification requirements for Polished Stone Value (PSV) and traffic. However, the main emphasis of the work was on a series of road trial sites where thin surfacings using the same coarse aggregate in different sizes were laid for direct comparison. Monitoring of the development of skid resistance over a range of speeds commenced at nine sites representing different PSV/traffic combinations. A laboratory study was made of the polishing of aggregates of different sizes using the Wehner-Schulze test, a technique new to the UK. Samples of aggregates used in all the trial sites, plus cores taken from one site after a period of trafficking, were tested. A short study was also made to assess making comparisons of the spray generated by traffic on different thin surfacings. A mobile photographic method was investigated that provided a feasible way of measuring spray in traffic under moderate rainfall but not when it was heavier and the surface texture appeared flooded. Currently, the trials are at different stages of polishing towards their equilibrium skid resistance level and this is necessarily an interim report. Monitoring of the trial sites is continuing in the next phase of the Programme.