Research On Shear Behavior Of Damaged Reinforced Concrete Beam Strengthened By Ultra-high Performance Concrete

The shear cracking problem of reinforced concrete（RC）beams is widespread.The shear failure is harmful and the shear mechanism is complex.It is urgent to improve the shear behavior and prolong the service life of RC beams.Ultra-high performance concrete（UHPC）has excellent mechanical properties and durability.When UHPC is applied to strengthen the shear performance of RC beams,the interface bonding is reliable,the reinforcement is efficient and long durability.In order to investigate the shear strengthening method,shear mechanism and shear behavior of damaged RC beams strengthened by UHPC,failure tests,finite element simulation and theoretical analysis of calculation method were carried out.The main work and conclusions are as follows:Eight I-shaped RC beams with high longitudinal reinforcement ratio were designed and fabricated.Six of them were strengthened by UHPC on both sides of the web after cracking under pre-damaged loading,and two of them without strengthening were used as contrast beams.The effects of shear span ratio,pre-damaged degrees of RC beams and high-strength steel mesh in UHPC layer on shear behabvior of strengthened beams were studied through three-point bending static loading testThe test results showed that the shear strengthening method of UHPC layer can effectively improve the crack resistance and shear capacity of damaged RC beams.All tested beams were shear failure with some ductile,and the strengthened beams showed good cooperative working performance without UHPC-RC interface peeling failureThe smaller the shear span ratio,the higher the shear capacity of strengthened beams.The performance of RC beams degraded after pre-damaged loading,and the greater pre-damaged degree of RC beams reduced the improvement of crack resistance.The addition of high strength steel wire mesh can further improve the crack resistance,stiffness,deformation capacity and shear capacity of the strengthened beams.ABAQUS was used to establish nonlinear finite element models.For different shear span ratio models,two kinds of concrete tensile constitutive models were used to simulate the softening behavior of concrete.The reliability of the simulation models was verified by comparing and analyzing the numerical simulation results and test data.The simulation of ultimate shear capacity and concrete cracking was accurate,which proved the accuracy and reliability of the finite element models.The finite element models further verified that the strengthened beams were shear failure mode,and the main factor leading to the failure of the strengthened beam was the excessive damage and failure of the internal normal concrete.Combined with parameter analysis,it was found that the ultimate shear capacity of beams can not be improved by increasing the thickness of UHPC.Excessively damage degree was unfavorable to the shear behavior of strengthened beams.The shear span ratio had a significant effect on the ultimate capacity.The critical longitudinal reinforcement ratio of strengthened beams from shear failure to bending failure was 2%～2.5%.This paper introduced the shear calculation method of increasing section reinforcement method in the current strengthening code of China.The calculation method of code was too conservative according the test data of this paper.Based on the test data and advanced codes at home and abroad,a recommended formula to calculate the shear capacity of RC beams strengthened by UHPC was proposed.The average value and the coefficient of variation of the ratio between the proposed and experimental results were 0.89 and 0.16,which indicated that the proposed calculated formula was applicable to predict the shear capacity of strengthening beams.