Analysis Of Direct Shear Properties Of ECC-NC Interface Under Salt Freeze-Thaw Cycle

Engineered Cementitious Composites(ECC),whose ultimate tensile strain can reach more than 3% under tensile load,has the significant advantages of strain hardening and multiple fine cracks.Research shows that ECC not only has good tensile and crack resistance,It also has good bending resistance,self-healing,corrosion resistance and impermeability,so it is widely used in the repair and reinforcement of concrete Bridges.In cold areas,the bond interface between ECC and concrete may also suffer from salt freeze-thaw erosion,resulting in loss of peripheral concrete and decreased strength,further weakening the bond performance of the interface between ECC and concrete,and reducing the durability of ECC repair or enhance the concrete structure.In order to study the durability of the bond surface between ECC and existing concrete under the salt freeze-thaw cycle,this paper carried out a detailed study on it by combining the extrusion-type direct shear test and finite element simulation.The research contents are as follows:(1)Through direct shear tests,the influences of ECC strength,roughness,interface treatment mode and other factors on the interface shear performance between ECC and existing concrete under normal environment were studied.The optimal working mode of ECC and existing concrete under shear load was explored from the aspects of test phenomenon,failure mode and load-slip curve.(2)The mass loss rate,freeze-thaw failure mode and mechanical behavior of composite specimens under different freeze-thaw cycles were studied through freeze-resistance test and interfacial shear performance test in salt-frozen environment,which revealed the failure mechanism and bond degradation law of ECC and concrete interfacial bond under shear load in salt-frozen environment.(3)Finite element analysis was carried out on a test beam,and the bridge in cold area was simulated to be strengthened by epoxy resin bonded prefabricated ECC plate.The influence of structural bond degradation on the overall structure performance was explored and the comprehensive repair analysis based on the prefabricated ECC reinforced beam was carried out.The main conclusions drawn are as follows:(1)Here are two types of interface failure modes in common environment: Interface failure,interface failure developed to matrix material failure,in which the interface treatment mode and interface agent type had an effect on the failure mode of the specimen,ECC strength had no effect on the failure mode of the specimen,only had an effect on the ultimate load,and the interface treatment mode and interface agent material had a significant effect on the interface bond strength.(2)Through the analysis of the interface bonding-slip curve of each specimen under ordinary environment,it is found that the descending section cannot be measured in the failure process of the specimen with smooth treatment,interface chisel and UDC bonding,and the failure mode is mainly brittle.For the bolted and epoxy bonded specimens,the failure process can be divided into three stages: elastic stage,elastoplastic stage and failure stage,and the ductility is obvious in the failure process.(3)Failure modes of specimens after freeze-thaw can be divided into three types:Interfacial failure,interfacial failure transition to bonding material failure,interfacial failure transition to matrix material failure,freeze-thaw environment,interfacial treatment and interfacial agent type have effects on specimen failure mode,ECC strength has no effect on specimen failure mode,only has effects on ultimate load Pu,and interfacial treatment and agent type have significant effects on interfacial shear strength degradation.The smoother the interface is,the faster the degradation of shear strength is.The use of high performance interfacial agent can effectively inhibit the degradation of shear strength.(4)The frost heave force caused by icing in freeze-thaw environment is actually a uniaxial tensile cyclic stress,and the damage caused by such force is actually a axial low-period fatigue damage.Moreover,based on the research of other scholars on freeze-thaw damage and combined with the test results,a prediction model for the residual direct shear strength of epoxy resin bonded specimens under the action of salt-freeze cycle was established.The predicted value of the verified model is in good agreement with the experimental value.(5)Through finite element analysis,it is found that the interfacial bond slip degradation has a certain effect on the overall working performance of the structure,but the influence degree is not large,and the influence degree of freeze-thaw cycle on the interfacial bond slip performance is greater than that of the whole working performance of the reinforced structure.