Experimental Study On Mechanical Properties And Durability Of Ultra High Toughness Cementitious Composite-to-Concrete Interface

The engineered cementitious composites(ECC)has many advantages,such as ultra high toughness,multiple micro-cracking behaviors,self-healing characterization,better fatigue resistance,better permeability resistance,better freeze-thaw resistance,better corrosion resistance.Nowadays,it has been widely applied to repaire the concrete structures.However,the interface properties between ECC and concrete are the most critical but the weakest part of these ECC repaired concrete structures.By far,the study of ECC-to-concrete interface is very limited.Therefore,the experimental study on mechanical properties and salt freeze-thaw durability of ECC-to-concrete interface were performed.The main research conclusion is as follows:1)According to the experimental study on the influence of ECC strength grades,polyvinyl alcohol(PVA)fiber types,interface roughness,ECC construction methods(i.e.,normal casting and spraying ECC)on the axial tensile properties of ECC-to-concrete interface,the influence laws of various factors on the interface axial tensile failure mode,axial tension-displacement curve and nominal tensile strength were obtained.The micro bonding mechanism of ECC-to-concrete interface under axial tensile load was analyzed,and the interface axial tensile micromechanics model based on the bonding mechanism was deduced.In addition,a calculation method of interface roughness critical value based on the axial tensile micromechanics model was also proposed.2)According to the experimental study on the influence of ECC strength grades,PVA fiber types,interface roughness,ECC construction methods(i.e.,normal casting and spraying ECC)on the shear properties of ECC-to-concrete interface,the influence laws of various factors on the interface shear failure mode,shear-slip curve and shear strength were obtained.The micro bonding mechanism of ECC-to-concrete interface under shear load was analyzed.Combining with experimental results that the ECC strength grades and interface roughness had a significant influence on the interfacial shear strength,an interface shear strength model with ECC strength and interface roughness as variables was deduced based on the bonding mechanism.3)According to the experimental study of ECC-to-concrete interface under salt freeze-thaw cycles effect,and the damage of interface shear performance under salt freeze-thaw cycles effect,the influence laws of ECC strength grades and interface roughness on the interface salt freeze-thaw cycle life,and the influence laws of salt freeze-thaw cycles on the interface shear strength and shear stiffness were obtained.The damage mechanism of ECC-to-concrete interface under salt freeze-thaw cycles effect was analyzed.Subsequently,a damage prediction model of interface shear strength was established based on the interface damage mechanics by the damage mechanics method.4)According to the experimental study of fracture characteristics of ECC-to-concrete interface under salt freeze-thaw cycles effect,the influence laws of salt freeze-thaw cycles on the load-crack opening displacement curve,ultimate splitting load and ultimate crack opening displacement were obtained.The experimental results show that the fracture characteristics of ECC-to-concrete interface showed obvious nonlinearity no matter whether salt freeze-thaw cycle effect was applied.Therefore,the double parameter fracture criterion could be used to describe the nonlinear fracture characteristics of the interface.Based on the related theory of linear elastic fracture mechanics,the analytical formulas for crack fracture toughness and unstable fracture toughness of interface were deduced.The degradation laws of interface fracture characteristics under salt freeze-thaw cycles effect could be reflected by variations of crack fracture toughness and unstable fracture toughness,and then degradation laws of interface fracture characteristics was analyzed.5)According to the experimental study of the interface bond properties and crack resistance of ECC repaired reinforced concrete beams,the influence laws of ECC strength grades,interface roughness,PVA fiber types,ECC construction methods(i.e.,normal casting and spraying ECC)and ECC repaired methods(i.e.,precast ECC plate and cast-in-place ECC)on the interface bond properties and bending crack control performance of ECC repaired reinforced concrete beams were obtained.The finite element model of ECC repaired reinforced concrete beams were established,and the validity of finite element analysis method was also verified.