Abstract: In recent years, inert construction and demolition (C & D) waste has increased rapidly. Large-scale projects, such as airport expansion involving land reclamation and runway subgrade backfilling, provide stable and scalable opportunities for the resourceful and beneficial reuse of such materials, while also placing stringent demands on backfill performance in terms of bearing stability and deformation control. However, the dominant factors governing the shear strength of waste-derived backfill and the mechanisms underlying strength variations across different gradations remain unclear. In this study, four gradations of crushed concrete are prepared and tested using consolidated drained triaxial compression tests. Peak shear strength is interpreted within the frameworks of critical state soil mechanics and the state parameter. The results show that crushed concrete exhibits pronounced strain softening and dilatancy characteristics. As confining pressure increases, the peak deviator stress increases whereas dilatancy is suppressed. Differences in peak strength induced by gradation are mainly attributed to variations in dilatancy potential rather than changes in critical state strength. These findings provide a quantitative basis for gradation optimization and engineering suitability assessment of recycled crushed-concrete backfill materials.