Measurement of Vertical Compressive Stress Pulse in Flexible Pavements: Representation for Dynamic Loading Tests

Testing at Virginia Smart Road allowed determination of the vertical compressive stress pulse induced by a moving truck and by falling weight deflectometer (FWD) loading at different locations beneath the pavement surface. Testing was performed on 12 different flexible pavement sections. Stress and temperature were measured using pressure cells and thermocouples, respectively, that had been installed during construction of the road. Target testing speeds were 8 km/h, 24 km/h, 40 km/h, and 72 km/h. The considered depths below the pavement surface were 40 mm, 190 mm, 267 mm, 419 mm, and 597 mm. A haversine or normalized bellshape equation was found to be a good representation of the measured normalized vertical compressive stress pulse for a moving vehicle. Haversine duration times varied from 0.02 s for a vehicle speed of 70 km/h at a depth of 40 mm to 1 s for a vehicle speed of 10 km/h at a depth of 597 mm. For the FWD loading, a haversine with a duration of 0.03 s was found to approximate the induced stress pulse at any depth below the pavement surface. Currently, laboratory dynamic testing on hot-mix asphalt (HMA) specimens is performed using a haversine wave at loading duration of 0.1 s. Because HMA is a viscoelastic material, the loading time affects its properties and, therefore, it is recommended that the loading time of HMA dynamic tests be reduced to 0.03 s to better match loading times obtained from moving trucks at average speed and from FWD testing.