SURFACE CRACKING OF ASPHALT MIXTURES IN SOUTHERN AFRICA (WITH DISCUSSION)

Since the latter half of the sixties, gap-graded and semigap-graded asphalt has been used extensively throughout South Africa with the exception of the Cape Province where continuously graded asphalt is still frequently used. It had been found that gap-graded asphalt was superior to continuously graded mixes with respect to fatigue. The occurrence of surface cracks in the asphalt was therefore unexpected and the behavioral pattern of the asphalt of those projects where cracks were manifested was different from that predicted during the design stages. This paper gives details of field, laboratory and analytical studies that were carried out over a number of years. Both load- and non-load-associated factors were found to contribute towards the problem. Field studies included analysis of a full scale load test, surveys of pavement condition and determination of material characteristics, particularly the viscosity behavior. It was concluded that distress occurs when the structural response in terms of deflection and curvature as well as the viscosity of the aged binder exceeds specific limits. Laboratory studies were done to determine the aging characteristics of asphalt. By using accelerated aging tests, it was found that the widely used gap- and semigap-graded type of asphalt may require special protective measures to prevent cracking due to the aggressive environment in parts of Southern Africa. The problem is further complicated by secondary stresses and strains. Residual stresses resulting from elasto-plastic behavior of the pavement materials may cause fatigue cracks to develop in the surface of the asphalt depending upon the pavement material characteristics and structure. Furthermore, thermal effects, which have until now been ignored, were also found to have the potential to cause surface cracking. This stems from the interaction between thermal and load stresses upon aged asphalt. Interim guidelines are presented for improved design and pavement management procedures.