A two-scale constitutive model for steel-confined ultra-high-performance concrete (UHPC) considering fibre–confinement interactions

Defining a comprehensive constitutive law for designing steel fibre-reinforced ultra-high-performance concrete (UHPC) structural members necessitates a complete understanding of steel fibres’ contributions to both confined and unconfined behaviours under compression and tension regimes. This study introduces a complete set of stress–strain (constitutive) models tailored for the simulation of reinforced UHPC columns subjected to varying levels of confinement and incorporating commonly used types of steel fibres within practical volumetric ranges. The proposed models integrate critical modifications to conventional confinement models for reinforced concrete columns, calibrated to better reflect the actual fibre orientation, improved ductility (through actual stress levels in transverse reinforcement at peak strength), and the elevated peak and residual strengths characteristic of steel-confined UHPC. Validation against independently tested specimens through both analytical and numerical approaches confirms the reliability and generalisability of the models, establishing them as effective tools for predicting the structural performance of UHPC members across a diverse range of fibre parameters and confinement conditions.