THE EARLY LIFE SAFETY OF HIGH STONE SURFACINGS

In the 1990s there was a change from the traditional use of hot rolled asphalt (HRA) in the United Kingdom (UK) to high stone content, thinner layer surfacings with porous and semi-porous textures. UK experience with skid resistance has traditionally been with positive textured surfacings such as the use of 20mm chips applied to HRA. The bitumen coatings on the aggregate surface are quickly removed by trafficking to expose the high skid resistant aggregate specified to ensure a safe surfacing for the highway user. In contrast, observation of the high stone types of mix, which typically use modified bitumen or thicker binder films, showed the bitumen to adhere to the aggregate longer under similar trafficking. This posed the question of whether the traditional explanations for the skid resistance and mix requirements still applied. This was considered during a project known as SKIDGRIP run in conjunction with two industrial partners. This paper details a Case Study where a GripTester was used to monitor the development of early life skid resistance over a three year period for a 14mm SMA mix constructed with high PSV aggregate (PSV 65) and 100pen polymer modified binder. This has shown that the development of skid resistance is more complex than traditional explanation. There are distinct stages in the life of a bituminous surfacing controlled by a complex interaction of factors including mix constituents, mix properties, type and amount of traffic, applied stressing and environmental conditions. The creation of secondary textures on the coated aggregate surface and then on the trafficked mastic of the mix influence very early life grip characteristics. Heavy trafficking and/or applied stress then becomes important in controlling exposure of the aggregate surface during early life with a combination of aggregate properties and texture controlling development of grip in mature and older life stages. For the covering abstract see ITRD E121480.