Mechanical performance of reinforced concrete beams after fire and strengthened with thin-walled U-shaped steel plate

Reinforced concrete (RC) beams after fire can be strengthened by installing external thin-walled U-shaped steel plate and filling non-shrinkage grouting material inside. It has the advantages of effectively avoiding the shedding of the grouting materials to ensure a long-term reinforcement effect. In order to verify the effectiveness of the reinforcement method, the static loading tests of 14 concrete beams were carried out. The effects of U-shaped steel plate thickness and fire exposure time on the bending performance of strengthened RC beams, and the effects of U-shaped steel plate thickness and shear span ratio on the shear performance of strengthened RC beams were investigated. The calculation formulas for the flexural strength and the shear strength of the strengthened beams were developed by referring to the calculation formulas for the capacity of composite beams at ambient temperature. The results show that the flexural strength of concrete beam decreases by 9.4% and 10.4% after experiencing fire for 1.0 h and 1.5 h, respectively. After experiencing fire for 1.5 h, the shear strength of concrete beam with the shear span ratio 1.74 and 2.58 decreases by 19.5% and 10.6%, respectively. The flexural strength and the shear strength of concrete beams firstly exposed to fire and then strengthened with thin-walled U-shaped steel plate is about 127.4%-151.7% and 21.3%-56.7% of that without fire, respectively. The spaced ring formed by welding angle steel in the inner side and above the flange of thin-walled U-shaped steel plate can ensure the U-shaped steel plate and concrete work together. The strengthened specimens can be seen as full shear connection composite beams. The longitudinal shear force of the spaced ring is larger when the flexural failure occurs in the strengthened beams, and is smaller when the shear failure occurs in the strengthened beams. © 2017, Editorial Office of Journal of Building Structures. All right reserved.