Asphalt Concrete Strain Responses at High Loads and Low Speeds At the National Airport Pavement Test Facility (NAPTF)

Results are described from asphalt strain gage measurements made during pavement response tests on the flexible pavement test items at the National Airport Pavement Test Facility (NAPTF). Tests were run at speeds of 0.08, 0.15, 0.23, 0.3, 0.6, 1.5, and 2.2 m/sec (0.25, 0.5, 0.75, 1.0, 2.0, 5.0, and 7.33 feet/sec) with dual-wheel configurations at wheel loads of 106.8, 133.5, and 160.2 kN (24,000, 30,000, and 36,000 pounds) and tire pressures of 1378 kPa (200 psi). The strain gages were located at the bottom of the 125-mm- (5-inch)-thick surface asphalt layer of the conventional and stabilized-base test items and additionally at the bottom of the 125-mm- (5-inch)-thick stabilized-base asphalt layer of the stabilized-base test items. Gages were oriented along the travel direction and transverse to the travel direction. Measurements were made at asphalt temperatures of 11.1°C (52°F) and 22.2°C (72°F). Significant permanent deformations were found in the measurements, particularly in the transverse direction. The measured strains were found to vary strongly with temperature and test speed, spanning the range of 300 to 2,000 microstrains. The upper range of these values is much larger than was anticipated, and the strains are 2 to 3 times higher than layered elastic computer program predictions of asphalt strain responses for the structural and load conditions existing during the tests. Longer duration of loading results in reduced asphalt stiffness. However, the longer duration of loading at slow speeds increases the amount of viscous flow and leads to significant increases in the total strain within the asphalt mix as speed decreases.

Authors: Navneet Garg and Gordon F. Hayhoe