Implications of Different Foamed Bitumen Stabilization Production and Curing Processes on Airport Pavement Thickness and Life

Foamed bitumen stabilization is a useful and well established method for improving crushed rock and natural gravel materials for pavement construction. Like most pavements, those with a foamed bitumen base course (FBB) are usually designed using layered elastic softwares in which the FBB layer is characterized by an elastic modulus and a Poisson’s ratio, with the modulus having a significant influence on pavement thickness. In Australia, FBB characterization is based on the saturated indirect tensile modulus after three days of accelerated curing of samples produced in a laboratory mixer. It is well established that this approach to FBB characterization is not representative of field production and in-pavement curing conditions. To determine the effect of FBB production and curing on FBB modulus, pavement thickness and predicted pavement life, the same FBB was produced using a laboratory mixer, an exsitu pugmil and an insitu stabilizer. Material was sampled and the uncured, cured and saturated modulus was measured after various periods of accelerated laboratory curing. The exsitu produced FBB was also cured using simulated in-pavement conditions. The various FBB modulus values were then used to determine the required thickness and predicted life of a typical aircraft pavement including a 300 mm thick FBB layer. It was found that field produced FBB modulus increased significantly during the first 90 days after production and that laboratory production and curing protocols were not representative of field production and in-pavement curing conditions. Layered elastic pavement modelling showed that more than 80% of the predicted pavement damage occurred in the first 20 days after FBB production. It is therefore recommended that FBB remains untrafficked for 7–12 days after production, wherever possible, and thinner pavements are likely to perform adequately in situations where the FBB is protected from traffic loading for more than 14 days following production.