Viscoelastic Behavior of Asphalt Concrete at Various Constant Strain Levels

The asphalt concrete pavement (flexible pavement) practices vehicular traffic loading which consists of dynamic flexural stresses throughout its service life. The resistance of asphalt concrete mixture to such dynamic stresses is essentially related to the viscoelastic properties of asphalt concrete mixture such as phase angle, dissipated energy, and resistance to flexural stresses. In the present investigation, slab samples of asphalt concrete mixture were compacted in the laboratory at optimum binder content of 4.9 % with the aid of roller compactor. Beam specimens of asphalt concrete were extracted from slab samples and tested under dynamic flexural stresses using a four-point bending beam test with three constant strain levels (750, 400, and 250) microstrains. The resistance of asphalt concrete beam specimens to dynamic flexural stresses, as well as changes in viscoelastic properties such as phase angle, flexural strength, and dissipated energy, were measured using the elapsed time required for the initiation of micro cracks. After three seconds of dynamic loading, the flexural strength of the asphalt concrete mixture was (80, 150, and 450) kPa when tested at constant strain levels of (250, 400, and 750) micro strain, respectively. The cumulative energy dissipation of asphalt concrete mixture declines. However, the cumulative energy dissipation rises as the loading time proceeds. The phase angle increases as the repetitions of flexural stresses proceed. Significant decline in the phase angle could be noticed when the constant strain level increases.  It was concluded that from the phase angle point of view, failure of asphalt concrete mixtures decreases by (97.1, and 97.5) % when the constant strain level increases from (250 to 400 and 750) micro strain respectively.