Retrofitting heat-damaged non-ductile RC beam-to-column joints subjected to cyclic and axial loading with FRCM composites

Abstract

An experimental program is conducted in this research to investigate the cyclic behavior of exterior reinforced concrete (RC) beam-to-column joints retrofitted with a fabric reinforced cementitious matrix (FRCM). FRCM is a structural strengthening system made with a bidirectional polyparaphenylene benzobisoxazole (PBO) mesh and a stabilized inorganic matrix for reinforced concrete constructions. Six RC joints were designed to represent non-ductile exterior joints in old RC structures; three joint specimens were subjected to a temperature of 720 °C for 2 h, while the other specimens remained unheated. Two heat-damaged and two unheated joint specimens were then retrofitted with FRCM composites adopting a U-shaped and an X-shaped strengthening configuration. The other two specimens were not strengthened; one of them was subjected to heat in order to study the effect of heat on the cyclic behavior of the non-ductile joints. The specimens were subjected to lateral cyclic loading on the beam and axial loading on the column. Test results showed that repairing the non-ductile beam-to-column joints using FRCM composites changed the mode of failure from brittle joint shear failure to a desirable ductile beam hinge. FRCM composites also significantly increased the ductility, drift ratio, and initial stiffness of the joint specimens up to 166%, 66%, and 11%, respectively. The U-shaped configuration was better than the X-shaped configuration in enhancing the ductility of the joint. The heat had an adverse effect on the strength, ductility, and initial stiffness of the specimens where it decreased them by 25%, 38%, and 51%, respectively, while it increased the deflection of the joint by 26%. FRCM composites restored the ductility of the heat-damaged joints and increased their initial stiffness, deflection, but they did not restore the strength capacity of the joint specimens. ACI-440 and ACI-352 guidelines were used to predict the analytical joint shear strength of the repaired joints, and the results showed that ACI guidelines are applicable but conservative in predicting the shear capacity of the RC joints strengthened with FRCM composites.