| Abstract: |
Ultra high-performance concrete (UHPC) has emerged as a transformative structural material, offering compressive strengths exceeding 150 MPa, superior tensile ductility, and enhanced durability. Despite these advantages, its behavior under combined loading conditions such as simultaneous flexure and axial compression, or concurrent shear and torsion remains inadequately characterized. This study presents an empirical investigation into the structural response of five UHPC mix designs subjected to combined loading scenarios. A total of forty beam-column specimens with systematically varied steel fiber volumetric fractions (1.0% to 3.0%) and water-to-binder ratios (0.16 to 0.20) were tested under controlled combined loading protocols in the laboratory. The experimental matrix included uniaxial compression, pure flexure, flexure-axial interaction, and shear-torsion interaction tests. Results indicate that increasing fiber content from 1.0% to 3.0% enhanced the compressive strength from 148.6 MPa to 214.7 MPa and the ductility index from 3.18 to 5.53. Under combined flexure-axial loading, the interaction ratio increased non-linearly from 0.72 to 0.89, revealing a progressive shift in failure mode from purely flexural to coupled shear-flexural mechanisms. Under shear-torsion interaction, the shear capacity and torsional capacity improved by 86% and 103%, respectively, over the range of mixes studied. Critically, the current ACI 318 and AASHTO interaction equations, calibrated for normal-strength concrete, underestimate UHPC combined capacity by 12–18%. These findings provide empirically grounded data for revising design interaction diagrams and extend the knowledge base for UHPC structural applications in bridges, high-rise buildings, and seismic-resistant construction. |
