Performance of Unbound Aggregate Materials under Single-Stage and Multistage Loading Modes Based on Precision Unbounded Material Analyzer

This study explored the performance of unbound aggregate materials (UAM). Repeated load tests of single-size, two-size mixed, and three-size mixed gravels were conducted using the precision unbounded material analyzer (PUMA) in single-stage loading mode (SSLM) and multi-stage loading mode (MSLM). The permanent deformation and elastic modulus of the first 400 loading cycles were analyzed under the two loading modes. The modulus of PUMA repeated load tests was compared with modulus models of the repeated CBR test and with constant confining pressure (CCP) and variable confining pressure (VCP) models of the repeated load triaxial test. The results of MSLM showed that the permanent deformation of single-size gravel increased rapidly when the loading strength was lower than 340 kPa and then decreased. The maximum permanent deformation of the mixed gravels appeared as the loading strength was lower than 200 kPa. The increment of permanent deformation tended to single-peak with the large-size aggregate. The loading strength only slightly affected the permanent deformation once it reached a certain extent. The initial-stage permanent deformation of SSLM was the main part of the total, and the proportion increased with the increase in loading strength. A slight difference was observed between the single-size and mixed gravel when the loading strength was under 240 kPa. Then, the permanent deformation of single-size gravel increased rapidly. The permanent deformation of MSLM was smaller than that of SSLM. MSLM can improve the carrying capacity of the unbound materials and reveal the principle of three-stage compaction for UAM. Although the calculated equivalent modulus of the full-friction model was lower than those of the existing studies, it presented the same tendency and was appropriate for the calculation of modulus. The elastic modulus of CCP model was the largest among the four models, and the growth rate of elastic modulus was the highest for VCP model. The effect of confining pressure on the modulus increased with increasing loading strength.