Developing a framework for diagnosis of shear distress in asphalt surfaces

Braking and turning by heavy vehicles can trigger shear-related distress in asphalt surfaces. The evolutionary increase in vehicle tyre pressure and wheel load will result in an increase in such surface failures in the future. Shear-related distress may result from delamination at the surface layer interface, or excessive shear creep within the asphalt surface layer itself. Both deformation and delamination distress appear visually identical at the surface. When subsequently cored, secondary (or symptomatic) debonding of the interface may result in asphalt deformation distress being misdiagnosed as a delamination failure. Regardless the mechanism, a broad range of properties may be responsible and a systematic diagnosis is required to determine the root cause. A framework for the diagnosis of shear distress in asphalt surfaces is presented. The framework contains three levels of investigation, including the location of the distress (at the surface layer interface or within the surface layer), the element within the interface/asphalt responsible and finally, the components or characteristics that resulted in a reduced shear resistance. The framework relies on the assessment of surface performance relative to comparable surfaces of adequate performance. Surface age, thermal gradient and traffic-induced shear stress states must all be comparable for a meaningful diagnosis to be made. The developed framework has been used to successfully determine the root cause of shear creep in the heavy braking zone of a runway at an Australian airport. Its application is recommended for other surface distress investigations, where the environmental and traffic factors are comparable. The framework may also be adapted to forensic investigation of other distress modes.