Static and seismic strengthening of RC frame joints by using CFRP sheets with various anchoring methods

Abstract

This study proposes an indirect strengthening method using carbon fiber-reinforced polymer (CFRP) and hybrid bolt-plate locking anchored CFRP hoops, which is more suitable than direct externally bonding strengthening on the reinforced concrete frame joints due to the orthogonal beam and slabs and debonding problem, especially for improving seismic performance. To understand the structural behavior of the joint using the proposed strengthening method, static and quasi-static cyclic tests were carried out on 8 joint specimens in a scale of 1:2. Three different anchoring systems were compared including through-slab CFRP closed hoops, border-beam-slab U-shaped CFRP anchored hoops, and the newly proposed hybrid bolt-plate locking anchored CFRP hoops. Results show that the hybrid bolt-plate locking anchored CFRP hoops has the least damage to the original structure and can be the most effective method in delaying the slip of the longitudinal rebars and the debonding of the CFRP on the beam, which can significantly enhance the seismic performance of the joint. Compared with the unstrengthened case, the yield load, ultimate load and ductility coefficient improved 25%, 19.7% and 55.7%, respectively. Under the cyclic loading, the utilization ratio of longitudinal CFRP sheet can be further improved with respect to the other two methods, up to 1.5 and 1.75 times respectively. Additionally, theoretical calculations were performed to predict the bearing capacity of the joints, which shows good agreement with the experimental values. The findings are significant in the engineering application to improve the structural behavior and performance of the joint using the proposed strengthening method.