Monotonic/cyclic behavior of postfire RC beam-column joints without/with CFRP retrofitting: Experiments

Abstract

In this study, one control, four 45-min, and four 75-min fire-exposed reinforced concrete (RC) joints without/with CFRP retrofitting were tested under monotonic/cyclic loadings. In each of the four postfire joints, two joints were not retrofitted, and two joints were retrofitted with CFRP. The results showed that, under monotonic loading, the control joint formed a plastic hinge lp = h/2 from the column surface. Fires elongated the plastic hinge length to lp = h. Cyclic loading formed a thorough crack running from one side to the other side at the beam end. CFRP retrofitting further concentrated a main crack on the beam-end section and developed toward the joint center. The combination of fire, CFRP retrofitting, and cyclic loading further shifted the damage and cracks to the joint center and prevented the damage in the retrofitted regions of beams and columns. The performance and mechanical properties of postfire RC joints were affected detrimentally by fire and/or cyclic loading, and beneficially by CFRP retrofitting. Compared with the control joint, fire decreased the stiffness of postfire RC joints by 14.2%−25.5%, ultimate load by about 11%, while it marginally reduced the yield load. The combination of fire and cyclic loading decreased the stiffness of postfire RC joints by 12.8%−27.0%, ultimate load by 21%, ductility by 82.2% (to low ductile behavior), and yield load by 4.6%−9.1%. CFRP retrofitting almost recovered the stiffness and improved the ultimate load by 15% and yield load by 0.1%−9.7% of postfire RC joints compared with the control joint. It improved the ductility to ‘highly ductile’. Cyclic loading caused a significant degradation in secant stiffness at an average value of 4.0%/cycle. It also caused minor strength degradation for small-amplitude cycles, significant strength degradation for cycles larger than critical cycles. Un-retrofitted and retrofitted joints under cyclic loading had an average pinching index of 0.75, showing a significantly high level of pinching.