Experimental and parametric investigation on flexural performance of post-tensioned reinforced concrete beams with large-diameter strands

Post-tensioned (PT) concrete beams have emerged as a prominent solution for the long-span concrete structures due to their superior prestressing efficiency, crack resistance, bearing capacity and convenient construction. However, the flexural behavior of PT beams with large-diameter strands and their compatibility with current design provisions remain insufficiently understood. This study experimentally investigated the flexural performance of full-scale bonded PT reinforced concrete (RC) beams with large-diameter steel strands (17.8 mm and 21.6 mm) through four-point bending tests. Ten specimens were designed and fabricated with varying diameters of PT strands, non-prestressed reinforcement ratios ( ρ s ), and sectional depths to elucidate the coupled effects of prestressing and non-prestressed reinforcement. Key performance metrics, including failure modes, load-deflection response, crack propagation, stiffness degradation, and ductility, were systematically analyzed. Experimental results demonstrated that increasing the strand diameter significantly enhanced cracking resistance (up to 77%) and initial stiffness. Conversely, while increasing ρ s improves yield and ultimate capacities, it compromises ductility by inducing premature concrete crushing. Theoretically, analytical models for the cracking moment and ultimate capacity were established based on sectional analysis. An improved crack-width prediction method and a modified stiffness-reduction coefficient are proposed, yielding higher accuracy than existing provisions. A three-dimensional (3-D) finite element (FE) model, incorporating concrete damaged plasticity and bond-slip interfaces, was developed and validated against the experimental stiffness, strength, and cracking patterns. Parametric analyses further confirmed the superior cracking resistance of large-diameter strands. This study provides a comprehensive experimental and theoretical basis for the safe application of large-diameter strands in PT structures and informs future code development.