Study On Shear Mechanism And Calculation Method Of RC Beams Strengthened By CFRP Grids And PCM

CFRP(Carbon Fiber Reinforced Polymer)grid,a new type of concrete structure reinforcement material,has some favorable properties,such as high strength-to-weight ratio,corrosion resistance,excellent durability,convenient construction and bidirectional loading on horizontal and vertical grids compared to the conventional CFRP composites.CFRP grid and polymer cement mortar(PCM)strengthening technique is affixing the CFRP grid on the surface of concrete structures by sprayed PCM so as to have the CFRP grid-PCM reinforcing layer work together with concrete structures.This method for shear strengthening of reinforced concrete(RC)beams has been proven to be a more effective method than conventional shear strengthening methods,and has wide application prospect.It is clear that maximizing the performance advantages of the CFRP grid is essential to obtaining best shear strengthening effect for RC beams.In order to achieve the purpose,the shear mechanism and capacity calculation method of the shear strengthening RC beam with CFRP grids and PCM should be investigated in details.In this paper,the four-point bending RC beams strengthened by CFRP grids and PCM were taken as the research object to carries out the further investigation by the finite element analysis method based on the completed experimental study of the shear strengthening RC beams with CFRP grids and PCM.This study was conducted from six aspects,including the experimental investigation on shear behavior of RC beams strengthened by CFRP grid and PCM,the parameter influence analysis on stress performance of shear strengthening RC beams based on the finite element analysis,the study on cooperative working performance and interaction between CFRP grids and steel rebars,the investigation on shear mechanical model,the investigation on effective strains of CFRP grids and shear capacity calculation method of RC beams strengthened by CFRP grids and PCM.The main research works and conclusions can be drawn as follows:(1)The shear behavior of RC beams strengthened by CFRP grids and PCM was investigated based on the four-point bending tests.Four-point loading was carried out on three test RC beams with the different reinforced arrangements of CFRP grids,stirrups and longitudinal steel bars.The stress performances of longitudinal steel bars,stirrups and CFRP grids were analyzed.The stress behavior of CFRP grids in the shear strengthening RC beams and the cooperation work performance between CFRP grids and steel bars were discussed preliminarily.The research results indicated that shear contributions of the vertical and horizontal CFRP grids were similar to those of the stirrup and longitudinal steel rebar,respectively.The stress distribution of vertical CFRP grid was not uniform due to the resistant action of grid points.Moreover,the vertical CFRP grids bore the lateral extruding action with opposite directions in tension and compression zones due to the compressive and tensile behavior of horizontal CFRP grids,resulting in the complex stress state of the vertical grid which differs from the stirrup.There exists the cooperation work between CFRP grids and steel rebars in shear strengthening RC beams.(2)The stress performance of the shear strengthening RC beams with CFRP grids and PCM was simulated analysis by the finite element software ABAQUS/Standard.At first,the finite element models were established based on the four-point loading tests of RC beams strengthened by CFRP grids and PCM,and the accuracy of the models were verified through comparing the experimental results with the finite element analysis results.Then,the effect of CFRP grid interval,cross-sectional area and mechanical properties of grid,shear-span ratio,concrete strength,strength and thickness of PCM,as well as stirrup reinforcement ratio on the stress performance of shear strengthening RC beams with CFRP grids and PCM were investigated through the results analysis of the finite element simulation of 23 RC beams strengthened by CFRP grids and PCM.Finally,the cooperative working performance and interaction between CFRP grids and steel rebars were systematically analyzed.The research results showed that the finite element analysis models have satisfactory accuracy and can be used to predict the shear behavior of RC beams strengthened by CFRP grids and PCM.The CFRP grids and steel rebars have good cooperative working performance and can share stresses with each other.(3)The shear mechanism model and capacity calculation formula of RC beams strengthened by CFRP grids and PCM were proposed.Firstly,the influences of horizontal CFRP grids and interaction between CFRP grids and steel rebars on shear mechanism of RC beams strengthened by CFRP grids and PCM were comprehensively investigated based on the finite element analysis of 12 shear strengthening RC beams and 3 unstrengthened RC beams.Then,the effective strains of vertical and horizontal CFRP grids were obtained through the regression analysis of several significant influence parameters(reinforcement ratio and elastic modulus of CFRP grids,concrete strength,shear-span ratio)on the shear strengthening RC beams and real strains of CFRP grids.Finally,on the basis of the above research,the shear capacity calculation method of RC beams strengthened by CFRP grids and PCM was proposed.Moreover,the accuracy of the proposed shear capacity calculation method was validated by the comparative analysis of numerical results obtained from existing method and the method proposed in this study.The results indicated that the effect of horizontal CFRP grids on stress behavior of shear strengthening RC with CFRP grids and PCM should not be neglected.The vertical CFRP grids share about 16% of the stirrup strain.The proposed shear capacity calculation method based on the improved truss-arch model and effective strain theory of CFRP grids has better accuracy and applicability compared to the existing approach.