Shear stresses in an asphalt surface under various aircraft braking conditions

Aircraft braking forces have increased in the past and will continue to increase in the future. This has resulted in instances of shear related distress in asphalt runway surfaces. The shear stresses transferred from braking tyres to the pavement surface can initiate failures of poorly bonded interfaces. Shear creep deformation within the mix can also occur. Octahedral shear stress (OSS) has been recommended as an indicator of asphalt distress. OSS is unaffected by the geometry of loading and reference coordinate system. In this research, the peak surface forces in three dimensions were calculated. Stresses through the surface layer were then calculated using mePADS/GAMES software. The calculated stresses in the surface layer were compared under various braking conditions. The OSS was used as the primary basis for these comparisons.The maximum calculated OSS induced by a heavy braking truck was only 53% of that for the aircraft during only moderate braking. Contrary to expectation, the peak shear stress under the leading edge of the tyre did not increase significantly with increased aircraft braking effort. Analysis of the shear stress distributions, however, identified a significant change with increased braking effort. For non-braking aircraft, a zone of near-zero shear stress was found under the central portion of the tyre. As the horizontal surface force increased, this became a zone of near-constant shear stress. Observed differences in field performance of nominally identical asphalt in landing/braking zones of an airport were not explained by the peak OSS values calculated. It was, however, concluded that the near-steady shear state under a passing tyre during aircraft braking could explain different asphalt responses in the field. This conclusion was consistent with the observed nature of such failures, which are creep related, rather than the result of conventional slip circle shear. It was suggested that the presence of shear related failures only in the braking area would be indicative of shear creep deformation within the mix rather than delamination at the surface interface.