Compressive behavior of post-heated circular CFST short columns externally strengthened with CFRP sheets

Due to the increasing usage of concrete-filled steel tube (CFST) members in structural engineering, there will be more chances of fire hazards on these structures in the near future. Externally bonding carbon fiber reinforced polymer (CFRP) composites has emerged as a popular method for repairing damaged steel-reinforced concrete members. However, limited research is available to evaluate the behavior of CFRP strengthened CFST members. This paper presents the results of an experimental investigation on the compressive behavior of the post-heated circular CFST short columns externally strengthened with CFRP sheets. A total of twenty-one specimens are tested to investigate the influence of temperature and the number of CFRP layers on the mechanical behavior of repaired specimens. The ultimate strength is obtained from monotone static tests. The extensometer technique based on gauge strain is used to measure strains of electric gauges glued to the external surface of specimens. The results indicate that the increase in the number of CFRP layers leads to a significant change in the mechanical properties of post-heated CFST columns. Furthermore, it is shown that increases in the number of CFRP layers remarkably enhance the ultimate strength and initial stiffness of specimens subjected to the same heat treatments, while deteriorate the ductility. Based on extensive experimental analysis, simplified formulae are proposed to estimate the compressive ultimate strength of all specimens tested, providing reasonably good correlation with the experimental results. Besides, the proposed formulae are compared with some existing empirical models, and validity of the proposed formulae is evaluated.