Study On Bond-Slip Behaviors Between Steel Shape And HSHP Concrete In Steel Reinforced Concrete Structures

Steel reinforced concrete structure, SRC structure for short, is a main kind of steel and concrete composite structures. SRC structural members have a relatively small sectional dimension and a high bearing capacity. SRC structures obtain a better integral stiffness, and have excellent seismic behaviours. So SRC structures are put into application extensively. The fine bond behavior between steel shape and concrete is the base of their cooperation. The cooperation is the reflection of the advantages of SRC structures. The bond behaviors directly affect the bearing capacity, failure modalities, crack, deformation and caculating methods etc.Although the calculating methods in SRC codes are different in many countries, they commonly simplify or neglect the effect of bond-slip between steel shape and concrete. There are many differences and conflicts among the present researches, and many problems about the bond-slip behavior are not solved at present. So the bond-slip behaviors between steel shape and concrete, especially high strength and high performance (HSHP) concrete, is necessary to be studied and perfected further.Through twelve simply supported SRHSHPC (steel reinforced high strength and high performance concrete) composite beams of various concrete strength, shear span ratio, and stirrup ratio, the distribution mode of bond stress and relative slip is obtained. Steel strain and relative slip increase exponentially from the free end to the loading end before bond stress reaches the local bond strength; after that, the strain difference between the gauges around the vicinity of the loading end becomes smaller and smaller. This is the interfacial bond softening, leading to progressive reduction of shear transfer capacity between steel and concrete. By studying the influence of the main parameters on interfacial bond-slip behavior, the occurrence and evolution of bond softening is made clear mechanically and the relationship between local bond strength and its influential parameters is analyzed. Based on the test results, the regressive formulas of local bond strength are established.The bond softening behavior, governed by the relation of bond stress and relative slip, leads to progressive reduction of shear transfer capacity between shaped steel and concrete. A simplified constitutive relationship of bond stress versus slip is proposed and employed to analyze the interfacial bond-slip process. The basic governing equations are established and solved for the steel strain and bond stress distribution. Thereby the expression for initial softening load and anchorage strength is derived. The application of the model is verified through comparison with experimental results.Steel-concrete composite structural member is considered as a generalized elastic body with both the applied load and the interfacial shear stress acting as boundary stresses, and the debonding process is modeled as crack propagation along the interface. According to energy principle in fracture mechanics, the energy relationship is discussed in the process of debonding and an energy-based criterion for steel-concrete composite structure is proposed. Following, the debonding process is analyzed through energy-based criterion. The analysis is first performed for special case with constant shear stress along debonded interface, and then for the general case with shear stress softening. From the results, a direct correspondence between energy-based and strength-based analyses can be established for arbitrary softening behavior along the interface. Through the proper definition of effective interfacial shear strength, the strength-based approach can be employed to give the same results as the energy-based analysis.The nonlinear fracture mechanics (NLFM) method is employed to investigate the interfacial behavior between embedded steel and concrete for SRC composite structural members. Considering the stress equilibrium, the interfacial governing differential equation is derived based on some basic assumptions. By introducing 5 different bond stress-slip constitutive models, the closed-form solutions for the distribution of bond stress, relative slip and steel stress are obtained and the anchorange force between embedded steel and concrete can be calculated accordingly. The distributions of the interfacial shear stress are different, but the anchorange forces converge to the same expression. At last, the effective anchorge length for SRC composite structure, which is defined as the length needed to attain the maximum anchorage force, is proposed on the basis of the analytical results.All these research results may contribute to the improvement of calculating theory of strength, stiffness, deformation and crack width, and can be applied in the finite-element method analysis of steel reinforced concrete structures.