Effect of Sheath Modeling on Unbonded Post-Tensioned Concrete under Blast Loads

Unbonded post-tensioned (PT) concrete systems are widely used in safety-critical structures, yet modeling practices for prestress implementation and tendon-concrete interaction remain inconsistent. This study investigates the effects of sheath (duct) implementation and confinement assumptions through nonlinear finite element analysis. Four modeling cases were defined, consisting of an explicit sheath without tendon-concrete confinement (S) and three no-sheath variants with different confinement levels (X, N, A). One-way beams and two-way panels were analyzed, and panel blast responses were validated against experimental results. In both beams and panels, average initial stress levels were similar across models, through local stress concentrations appeared when the sheath was modeled. Under blast loading, these local effects became critical, and the sheath-implemented model reproduced experimental behavior most accurately, whereas non-implemented models deviated. Reduced blast intensity diminished the differences among models, thereby reaffirming that sheath-induced localization and damage propagation are critical factors. These findings highlight the importance of explicit sheath implementation for realistic numerical assessment of unbonded PT structures under extreme loads.