Influence of loading frequency on mechanical properties of unbound granular materials via repeated load tests

Unbound granular material (UGM) is widely used in flexible pavement to carry vehicular dynamic loading with different loading frequencies. In this study, the influence of loading frequency on the mechanical properties of UGM was investigated via repeated load tests using the precision unbound material analyzer (PUMA). The confining stress, plastic strain, plastic strain rate, resilient strain, resilient modulus, and Poisson's ratio of two granular materials were analyzed at four loading frequencies and two stress levels. The results showed that confining stress, plastic strain, plastic strain rate and Poisson's ratio decreased as the increase of loading frequency. Resilient strain and resilient modulus did not show obvious trend with loading frequency. Compared with 1 Hz, confining stress of 19 mm gravel at 240 kPa decreased 28.1% at 5 Hz and 33.7% at 8 Hz. The maximum difference of confining stress at 5 Hz, 8 Hz and 10 Hz was only 0.78 kPa. Other indicators showed similar tendency at different loading frequencies. The results reveal that loading frequency has significant influence on mechanical properties of UGM, while the influence becomes small when the frequency exceeds certain value. Moreover, the loading stress and gradation of material had interaction effect with loading frequency. The coefficients of variation of measurement indicators decreased as the increased loading frequency, which suggested that appropriate frequency can help reduce the error of the testing results. (c) 2021 Elsevier Ltd. All rights reserved.