Proprietary thin surface course systems have been successfully used on UK trunk roads for over 10 years, providing quiet surfaces while maintaining good friction when the road is wet. However, the process of approving them focussed on compliance with existing specifications that had been derived from the properties of traditional materials such as hot rolled asphalt (HRA) and surface dressings, prevalent at the time. Collaborative research sponsored by Highways Agency (HA), the Mineral Products Association (MPA), the Refined Bitumen Association (RBA) sought to optimise requirements for thin surface course systems to maintain safety standards, further improve durability and make more efficient use of resources. Skid resistance and texture depth were monitored on a range of trial sites, constructed specifically for the project, and in-depth laboratory studies were developed to help give a better understanding of the effects observed in the field. The results from the project have made it possible to make recommendations for changes to the requirements for thin surface course systems regarding the polish resistance of aggregates and texture depth with reference to the size of coarse aggregate in use.

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