Effect of Test Condition Parameters on ${\mathrm{IRM}}_r$

The index of retained resilient modulus (${\mathrm{IRM}}_r$) is used to assess moisture‐induced distress in asphalt concrete mixtures. The ${\mathrm{IRM}}_r$ represents the ratio of two moduli and is sensitive to slight deviations in each measured value of modulus. A laboratory and analytical research program was conducted to examine the effect of variations in test condition parameters on the resilient modulus, $M_r\text{,}$ and the ${\mathrm{IRM}}_r\text{,}$ In the laboratory test program, the strain and temperature dependencies of the $M_r$ were determined for a dense‐graded asphalt concrete mixture. The results indicate that constant stress testing may result in a misinterpretation of the ${\mathrm{IRM}}_r$ and, further, tests conducted within the currently accepted temperature range may result in a $\pm 20\%$ deviation in the ${\mathrm{IRM}}_r\text{.}$ In the analytical program, the effect of diametral test boundary conditions on the measured value of $M_r$ was evaluated using two‐ and three‐dimensional finite element models. The results indicate that the resilient‐modulus diametral test is adequately represented by elastic theory based on an assumed plane stress condition.