A large part of Australia's pavement maintenance budget is expended on the prevention or repair of defects associated with cracking of the bituminous seal coat surfacing of unbound granular pavements. This paper is concerned with the mechanical response of this type of pavement to tensile forces. In a series of laboratory tests, a 100 mm thick fine crushed rock pavement has been stretched horizontally by the opening of a crack in the underlying subgrade. The main series of tests were carried out on pavements which had been dried back after compaction. Other tests examined the effects of applying a bituminous seal coat and of testing at the compaction water content. For pavements tested at the compaction water content, an apparent tensile strength of 2 to 3 kPa was developed at crack openings of about 0.6 to 1.0 mm. If the pavement is dried back to about 50 percent of modified optimum moisture content, the apparent tensile strength is increased to 10 to 15 kPa at about 2 to 3 mm crack opening. Even when the dried-back pavement is sealed with C170 residual bitumen which has been hardened fourfold, and the sealed pavement is then loaded at about 100 mm/min, at 200 degrees C the seal coat does not affect the peak strength and has only a marginal effect on post-peak behaviour. Elasto-plastic continuum modelling provides a good description of the observed behaviour of the asymmetrically loaded test pavements. The numerical model shows that the actual tensile strength of the dried-back, crushed rock pavement material was about 60 kPa. Such high tensile strength will have a big influence on the behaviour of the pavement surface under traffic loading. (a).
Abstract