Quantitative Field Evaluation and Effectiveness of Fine Mix under Hot-Mix Asphalt Base in Flexible Pavements

Testing was conducted with the main objective of predicting the effect of incorporating a fine hot-mix asphalt (HMA) layer under an HMA base on the long-term fatigue performance of flexible pavements. Testing at the Virginia Smart Road allowed the determination of the vertical compressive stress and horizontal transverse strain induced by a steering-axle tire of 25.8 kN (5,800 lb) under the HMA layer of two pavement designs, one of which included a fine surface mix below a base mix. Stresses and strains were measured for four different speeds [8, 24, 40, and 72 km/h (5, 15, 25, and 45 mph)], for three tire inflation pressures [552, 655, and 724 kPa (80, 95, and 105 psi)], and at different temperatures. Stresses were measured with pressure cells, while strains were measured with H-type strain gauges embedded in the HMA layers during construction. As expected, temperature was found to significantly affect the vertical compressive stresses and horizontal transverse strains measured under the HMA layer. Speed, on the other hand, did not affect the magnitude of the vertical compressive stress measured in any of the layers but did affect the loading time. However, speed was found to significantly affect the horizontal transverse strain measured under the HMA layer. The compressive stress and horizontal transverse strain measured at the bottom of the HMA layer at depths greater than 150 mm (6 in.) were found to be independent of tire inflation pressure ranges from 552 to 724 kPa (80 to 105 psi). It appears that incorporation of a fine HMA at the bottom of a HMA base layer would increase the fatigue lives of flexible pavements.