Stress Distribution in Concrete with Nonuniform Passive FRP Confinement

Understanding the stress distribution within concrete structures under complex loading conditions is crucial for investigating their failure mechanisms and ultimate strength design. Nevertheless, internal-stress distribution measurements in concrete are very challenging. This study addresses this issue by employing an innovative pressure mapping sensor to directly measure the axial stress distribution in concrete subjected to both uniform and nonuniform confining stresses. These different confinement conditions were achieved by applying fiber-reinforced polymer jackets to concrete specimens with various cross-sectional shapes. The obtained stress patterns reveal that the stress distribution within concrete depends not only on the instantaneous confining stress field but also on the stress path applied to concrete. In situations of inadequate confinement, stress concentration within an effectively confined area results in premature and severe damage, which subsequently leads to that concrete within the ineffective confinement region playing a major role in stress resistance, exhibiting a higher axial stress distribution. This finding contradicts the widely accepted notion that high stress always occurs in the effective confinement area. A series of local stress–strain curves corresponding to effective and ineffective confinement areas were also obtained, providing insights into the variation in load resistance contribution during the loading process. Furthermore, a comparison between test data and finite-element analysis results exposes disparities in the predicted stress distribution, underscoring limitations in the employed capacity of the concrete plastic-damage model to simulate concrete heterogeneity and its damage process under weak confinement. The stress distribution data presented in this study hold critical significance for comprehending concrete material failure mechanisms, advancing constitutive theories, and establishing corresponding mechanical models.