Study On Mechanical Behaviors At Tendon-Concrete And FRP-Concrete Interfaces Based On Fracture Mechanics

As the techniques are widely developed and applied in fields of anchorage and rehabilitation for concrete, researchers have gradually paid more attention to bond properties between repairing materials and concrete. The present study aims at the mechanical behaviors at both the tendon-concrete and FRP (fiber-reinforced polymer) -concrete interfaces based on fracture mechanics. Two models are mainly analyzed, namely, tendon-mortar-concrete anchorage and CFRP (carbon fiber-reinforced polymer) sheet strengthened three-point bending notched beam of concrete. Details of the present study are introduced as follows.(1) First, an analytical model is presented to calculate the effective fracture toughness of concrete. Only if the elastic modulus and flexural tensile strength of the concrete are given, the critical effective crack length and maximum applied load can be predicted by using the proposed model. Then the size effects of the effective fracture toughness are studied.(2) An analytical method is proposed to predict the load carrying capacity of three-point bending notched beams of lightly reinforced concrete based on the fictitious crack model and deformation compatibility conditions at the steel bar-concrete interface. Two cases are considered. Case 1 allows interfacial debonding at the steel bar-concrete interface without yielding of the steel bar and Case 2 means the steel bar can yield without interfacial debonding. Results show that there are two peak values of the applied load during the loading stage. And the second peak value is corresponding to the initiation of steel bar yielding in Case 2. Then the calculated two peak values in Case 2 are verified with experimental results. Therefore, when the elastic modulus and flexural tensile strength of the concrete, and the yielding strength of the steel bar, are given, the loading carrying capacity can be predicted for lightly reinforced concrete beams with different ratios of initial notch length to beam height in Case 2.(3) When interfacial debonding failure occurs in the tendon-mortar-concrete anchorage, the variations of shear stresses along the thickness of the mortar layer are obtained based on the deformation compatibility conditions at the two interfaces and shear deformation compatibility conditions in the mortar layer. Then the expressions of tensile stress in the tendon and interfacial shear stress are yielded. The possibilities of interfacial debonding at the two interfaces are judged according to the two interfacial shear strengths, diameter of the tendon and thickness of the mortar. Only the interfacial debonding at the tendon-mortar interface and the interfacial debonding at both of the two interfaces are studied, respectively, in the present study. Moreover, the interfacial debonding is modeled as the interfacial shear crack propagation. Then the appearance possibilities and orders of the interfacial shear cracks from the loading and free ends of the tendon are judged according to different boundary conditions of the anchorage. The expressions of the pullout load with respect to the interfacial crack lengths are established and the maximum pullout load and critical crack lengths are obtained using theories of extremum. Besides, the effects of the embedment length, concrete rigidity, mortar thickness and interfacial parameters on the calculated results are discussed.(4) When the failure mode of the anchorage is shear failure of the mortar, the expressions of tensile stress in the tendon and shear stress in the mortar are obtained based on the deformation compatibility conditions at the interfaces and shear deformation compatibility conditions in the mortar layer. The appearance possibilities and orders of the shear cracks in the mortar from the loading and free ends of the tendon are judged according to different boundary conditions of the anchorage. Moreover, the expressions of the pullout load with respect to the shear crack lengths are established and the maximum pullout load and critical shear crack lengths are obtained using theories of extremum. The effects of the embedment length, concrete rigidity, mortar shear modulus and shear fracture energy on the calculated results are discussed.(5) An analytical method is presented for the anchorage by considering both the concrete cone failure and interfacial debonding. The effects of the two failure modes on each other are studied. Then the appearance possibilities and orders of the two failure modes are judged. Moreover, the effects of the embedment length, concrete rigidity, mortar thickness and concrete tensile strength on the failure modes are analyzed.(6) The analytical methods for the anchorage are applied to study pullout of an anchor from mortar filled steel tube. Then an analytical method is proposed to predict the maximum pullout load based on the deformation compatibility condition at the anchor-mortar interface and shear deformation compatibility condition in the mortar layer. The calculated tensile capacity is verified with experimental results. Therefore, when four interfacial parameters determining the interfacial behaviors and a few material parameters are given, the maximum pullout load can be predicted for the anchor-mortar-tube anchorage with different sizes.(7) An analytical method is presented to predict the loading carrying capacity of CFRP sheet strengthened three-point bending notched beams of concrete based on the fictitious crack model and deformation compatibility condition at the CFRP-concrete interface. Both the vertical crack propagation in the concrete and horizontal interfacial shear crack propagation are considered in the proposed model. Results show that there are two peak values of the applied load during the loading stage. Then the calculated two peak values are verified with experimental results. Therefore, when three interfacial parameters determining the interfacial behaviors and a few material parameters are given, the loading carrying capacity can be predicted for CFRP sheet strengthened three-point bending notched beams with different ratios of initial notch length to beam height.