Greg White

Two runways at the same Australian airport were resurfaced with 50-60 mm of asphalt between September 2010 and July 2011. The Marshall-designed 14 mm nominal sized asphalt was typical of airport-quality asphalt in Australia. A premium acid modified bitumen, locally known as multigrade or M1000, was used. Approximately six months after construction, horizontal shear creep deformations were observed. The failures were only observed in the braking zone associated with one landing direction. Only the second of the two resurfaced runways was affected. Around 60 isolated failures presented over two to three years, after which, new failures ceased to appear. During construction, the fine aggregate (dust) source changed from one quarry to another. It followed that one runway received asphalt made with dust from one quarry and the other runway received asphalt made with dust from the other quarry. Both dust sources were olivine basalt with a track record of adequate performance in road asphalt production. Subsequent investigation identified that the second dust contained predominantly Hisingerite clay minerals. Hisingerite is a rarely encountered member of the Smectite-group of clays and possesses physical properties indicative of potentially adverse impacts on asphalt stability and shear stress resistance. The original dust contained Nontronite clay minerals, a more common and less concerning member of the Smectite-group.