Accumulated deformation characteristics and shakedown analysis of recycled filler from construction and demolition waste

To promote the application of recycled filler from construction and demolition waste (CDW) in railway subgrades and to evaluate its long-term dynamic performance under train loading, laboratory dynamic triaxial tests were conducted on recycled filler with different brick-concrete ratios under various combinations of compaction degree, confining pressure, and dynamic stress ratio. The effects of material parameters and stress state on accumulated plastic strain characteristics were investigated, and the applicability of existing shakedown criteria to CDW recycled filler was assessed. A new shakedown index that accounts for both accumulated plastic strain and its development rate was then proposed, and the critical dynamic stresses associated with different shakedown limits were identified. The results show that recycled brick specimens produced the largest final accumulated plastic strain and the highest strain growth rate, particularly at high dynamic stress ratios, where strain development became unstable. By contrast, recycled concrete aggregate specimens developed only small plastic strain during the initial loading stage at low stress levels and then stabilized rapidly. Based on the evolution of accumulated plastic strain and strain rate, the dynamic shakedown response of recycled filler was classified into three ranges: plastic stability, with final accumulated plastic strain below 2% and the proposed index below 0.003; incremental collapse, with final accumulated plastic strain above 4% and the proposed index above 0.02; and plastic creep, for intermediate cases. Compared with existing criteria, the proposed index better captured the accumulated deformation response of recycled filler under cyclic loading and showed improved discriminatory capacity. The critical dynamic stress of recycled brick filler was markedly lower than that of natural subgrade filler, whereas recycled concrete aggregate and mixed brick-concrete fillers showed higher critical dynamic stresses than natural filler. These findings provide theoretical bases for using CDW recycled filler in railway transportation infrastructure.