Mechanical Performance Of Reinforced Concrete Beams After Fire And Strengthened With Thin-walled U-shaped Steel

A new strengthening technology for reinforced concrete beams after fire by installing external thin-walled U-shaped steel and irrigation non-shrinkage grouting material is presented. It has the advantages of convenient construction, reducing the amount of template and effectively avoiding the shedding of the grouting layer to ensure a long-term reinforcement effect.In order to verify the effectiveness of the reinforcement method and establish the corresponding design method, four point loading tests of 14 concrete beams were carried out. The effects of U-shaped steel thickness and fire exposure time on the bending performance of strengthened specimens, and the effects of U-shaped steel thickness and shear span ratio on the shear performance of strengthened specimens were investigated. The calculation formulas for the flexural strength and for the shear strength of the strengthened beams were developed by referring to the calculation formulas for the capacity of U-section steel-encased concrete composite beams at ambient temperature. The results show that the flexural strength of concrete beam decreases 9.4% and 10.4% after experiencing fire for 1h and 1.5h, respectively. After experiencing fire for 1.5h, the shear strength of concrete beam with the shear span ratio as 1.8 and 2.5 decreases 20.2% and 14.6%, respectively. The flexural strength and the shear strength of concrete beams firstly exposed to fire and latterly strengthened with thin-walled U-shaped steel is about 127.4%~151.7% and21.3%~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 can ensure the U-shaped steel and concrete work together. The strengthened specimens can be seen as full shear connection composite beams. The longitudinal shear force bore by 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.Finite element model of reinforced concrete beams after fire strengthened with thin-walled U-shaped steel have been established based on the finite element analysissoftware ABAQUS. The test of 3 bending reinforcement beams and 4 of shear reinforcement beams were simulated respectively.Simulation results of bending reinforcement beams control error within 5%.Simulation results of shear reinforcement beams control error within 13%.Using the numerical analysis model systematically analyzes the influence law of full scale beam in the engineering bending bearing capacity when changed in the fire exposure time, the thickness of reinforced and the number of the spaced ring. By reference to the calculation formula of the bearing capacity of thin-walled U steel concrete composite beam at normal temperature, the calculation formula of the bearing capacity and the shear capacity of the reinforced concrete is proposed.