End-Friction Effect on Concrete Cubes with Passive Confinement

True-triaxial test data are fundamental in obtaining the constitutive laws of fiber-reinforced polymer (FRP)-confined concrete, especially concrete under nonuniform (passive) confinement. A new true-triaxial test system, which exerts passive confinement on concrete cubes, has been developed by the authors to fill the passive confinement gap in existing tests. To obtain true constitutive behavior from cube testing, it is essential to resolve the end-friction effect. In this study, the design of a friction reduction pad with polytetrafluoroethylene (PTFE) sheets was investigated, and a quantitative analysis was carried out to determine the pad's friction reduction performance. Moreover, an equivalent confinement method was proposed to quantify pad performance under different pad configurations. The results demonstrate that a two-ply greased PTFE pad works efficiently for concrete under uniaxial compression. However, it does not prevent end friction from regenerating in the strain-hardening stage in passively confined concrete. The equivalent confinement method was further developed to quantify the regenerated end-friction effect and to obtain the inherent constitutive behavior of concrete under uniform passive confinement in a true-triaxial test system. This study is the first quantitative investigation of friction reduction performance specifically for concrete cubes with strain-hardening behavior. (C) 2018 American Society of Civil Engineers.