Significance of confined dynamic modulus laboratory testing for asphalt concrete: Conventional, gap-graded, and open-graded mixtures

The objective of this study was to evaluate the effect of confinement on the dynamic modulus E* values for different asphalt mixtures. Traditionally, only unconfined tests have been conducted, mainly driven by the simplicity of the test procedure and laboratory equipment needs. In addition, the moduli of the most widely used, dense-graded (conventional) asphalt mixtures are thought to be less influenced by confined testing. However, the increased use of gap- and open-graded asphalt mixtures necessitates the consideration of confined laboratory testing to obtain their true material properties. After five confinement levels were evaluated, a stress of 138 kPa (20 psi) was selected as a rational confinement level for confined E* laboratory testing, because an increase of confining stress beyond 138 kPa did not significantly increase the moduli of the mixtures. The scope of work included testing and analyses of 26 mixtures: four conventional, 12 gap-graded, and 10 open-graded mixtures. The AASHTO TP62-07 procedure was used to measure E* values of the asphalt mixtures in both unconfined and confined states of stresses. Results for the conventional mixtures showed that there was no significant effect of confinement at lower temperatures, but the confined moduli were higher at the highest test temperature. For the gap- and open-graded mixtures, confined test values were higher than unconfined test values at all test temperatures. Statistical hypothesis testing of the data confirmed these findings.