Study on the Constitutive Model of Desert Sand–Fly Ash Cenosphere Lightweight Aggregate Concrete Under Steel Tube Confinement

The compressive behavior of desert sand–fly ash cenosphere lightweight aggregate concrete (DFCC) under steel tube confinement differs from that of conventional lightweight aggregate concrete, and existing constitutive models may not adequately characterize its stress–strain response. This study aims to develop a compressive constitutive model for DFCC under steel tube confinement and evaluate its applicability to the axial response analysis of DFCC-filled steel tube stub columns. Uniaxial compression tests were first conducted to characterize the complete stress–strain response of unconfined DFCC, followed by finite element analyses of DFCC under different levels of steel tube confinement. Axial compression tests and numerical simulations of five DFCC-filled steel tube stub columns were further used for component-level evaluation. The results showed that unconfined DFCC exhibited pronounced post-peak softening, and the Hu model reproduced the measured complete stress–strain curves with coefficients of determination ranging from 0.9646 to 0.9794. Steel tube confinement enhanced the compressive resistance, deformation capacity, and post-peak residual strength of DFCC. Based on the confined stress–strain responses, a modified constitutive model was established by recalibrating the peak parameters and post-peak branch. When implemented in the finite element analysis of the stub columns, the ratios of experimental to numerical ultimate loads ranged from 0.97 to 1.06, with a mean value of 1.02 and a standard deviation of 0.035. The proposed model can therefore provide a practical constitutive description for the axial analysis of DFCC under steel tube confinement within the investigated parameter range.