Predicting the early life skid resistance of asphalt surfacings

Safety is the most important responsibility of anyone involved in highway transportation. This applies at all stages from initial design, selection of materials to use of the highway by the user. However, it has been found that some aggregates perform better than expected whilst others give a lower in-service skid resistance. This is not unexpected as surfacing aggregate is now used in ways that were not considered in the development of skid resistance standards and specification recommendations that have been used as the basis for many countries throughout the world. Criteria such as noise, negative texture, spray generation, layer thickness, availability and cost of limited sources of high PSV aggregate have resulted in a shift towards thinner, smoother and quieter surfacings. To achieve higher performance these materials typically require the use of modified bitumen or have thicker coatings of bitumen to hold the aggregate particles together. Given that bitumen has poor wet skid resistance, the early life safety of such materials is an issue that needs consideration. The research outlined in this paper considers whether it is possible to develop predictive models that address this issue. A laboratory evaluation using a range of aggregate and bitumen types is detailed. It was decided to use the standard accelerated polishing apparatus used to determine PSV values. The research has shown the effect of rock type, bitumen type and test conditions such as the presence of water on the development of early life skid resistance of test samples in the laboratory. Road-trial data has been obtained from a range of asphalt surfacing types. These have been periodically assessed using a GripTester to assess how their skid resistance has developed from initial construction. The research has shown how the safety of new road surfacings may be better predicted so as to minimize risk to the user whilst helping to improve the performance of asphalt road surfacings.