Research On Factors Affecting Bonding Behavior Between Existing Concrete And ECC

Engineering cementitious composite(ECC)is a new type of short fiber reinforced cementitious composite,which has the characteristics of stress-strain hardening,higher tensile strength and excellent ability of crack control.When ECC reaches the ultimate tensile strain,the width of cracks is still less than 0.1mm.Therefore,as the repair materials,ECC can be used to replace the damaged part of the structure,and it has broad prospects in application of improving the bearing capacity and durability of existing structures.However,there are few domestic or overseas studies on the bonding factors and the mechanism of interface bonding.Besides,the constitutive relation between concrete and ECC has not been established.Based on the research of the predecessors and the research group,a series of tensile and shear experiments were carried out to study the influence of roughness,interfacial agent,strength of existing concrete and strength of ECC on the bonding strength between ECC and existing concrete.The slip curves of the loading end and the free end of ECC were obtained through the shear experiments.Besides,the bonding mechanism between ECC and existing concrete was also explored.Finally,this paper established the interface bond model and bond slip constitutive model.The main conclusions are as follows:(1)Roughness and interfacial agent have great influence on the bond behavior between the ECC and existing concrete.Within a certain range,the relation between bond strength and roughness is hypo-two function and the best roughness is 3-4mm.Silica fume and expansive agent can effectively solve the problem of shrinkage of cement paste,and make the interface more compact.Under the same condition,interface agents of type E can increase the tensile and shear strength by 40% and 50%.The bond strength increases with the increase of compressive strength of concrete and ECC,but the increase is small in general.(2)A series of multi factor formulas were established for calculating the bond strength between the ECC and existing concrete under the condition of axial tension,oblique shear and straight shear.The theoretical calculation results are in good agreement with the experimental results,which can provide reference for the practical application.Among them,the maximum tensile strength is 1.98 MPa,the maximum shear strength is 2.73 MPa,and the maximum oblique shear strength is up to 12.74 MPa.The results revealed that the ECC is more suitable for strengthening the members with larger shear strength or improving the shear capacity of the existing structures.(3)Based on the experimental results,this paper established a Three Zone-Three Layer bonding model of ECC/existing concrete.This model helps the bonding analysis from the macroscopic bond strength data into the weak transition layer—the transition layer L1 of the existing concrete-interface agent.Influence mechanism of all factors were investigated based on this model.The observation results of high power optical microscope indicated that the model can be consistent with the experimental results.(4)Based on the bond slip curve measured in the direct shear experiments,this paper established a constitutive relation model which can describe the whole process of ECC/ concrete bond slip.The bond slip can be divided into four stages: elastic stage,elastoplastic stage,plastic strengthening stage and failure stage.The test curve is fitted and calculated,and the results proved the model is in good agreement with the experimental curves.