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Bond between reinforcing bars and concrete under impact loading Yan, Cheng

Abstract

The bond between reinforcing bars and concrete under impact loading was studied for plain, polypropylene fibre reinforced, and steel fibre reinforced concretes. The experimental program included setting up an impact test system, in which an impact load with considerable energy could be generated, and in which the applied loads, accelerations and the strains along the reinforcing bar could be recorded at a rate of 200 microseconds per data point. The experiments consisted of both pull-out tests and push-in tests. For both types of tests the experimental work was carried out for three different types of loading: static, dynamic and impact loading, which covered a stress (bond stress) rate ranging from 0.5 • 10' to 0.5 • 10-2 MPa/s. The other important variables considered in the experimental study were: two different types of reinforcing bars (smooth and de-formed), two different concrete compressive strengths (normal and high), two different fibres (polypropylene and steel), different fibre contents (0.1 %, 0.5% and 1.0% by volume) and surface conditions (epoxy coated and uncoated). All of the test data were processed by computer, and the output included the stress distributions in both the steel and the concrete, the bond stresses and slips, the bond stress-slip relationships, and the fracture energy in bond failure. The energy balance at different stages in the bond process was examined. The internal crack development was also investigated. It was found that for smooth rebars, there existed a linear bond stress-slip relationship under both static and high rate loading. Different loading rates, compressive strengths, types of fibres, and fibre contents were found to have no great influence on this relationship and the stresses in both the steel bar and the concrete. For deformed rebars, the shear mechanism due to the ribs bearing on the concrete was found to play a major role in the bond resistance. The bond stress-slip relationship under a dynamic (high rate) loading changes with time and is different at different points along the reinforcing bar. In terms of the average bond stress-slip relationship over the time period and the embedded length, different loading rates, compressive strengths, types of fibres, and fibre contents were found to have a great influence on this relationship. Higher loading rates, higher compressive strengths of concrete, and steel fibres at a sufficient content all significantly increased the bond resistance capacity and the fracture energy in bond failure. All of these factors had a great influence on the stress distributions in the concrete, the slips at the interface between the rebar and the concrete, and the crack development. It was also found that there is always higher bond resistance for push-in loading than for pull-out loading. The bond resistance and the fracture energy in bond failure decreased when the rebar was epoxy coated. This influence of epoxy coating on the bond strength for push-in loading was much more significant than for pull-out loading. However, high rate loading, high concrete strength, and the steel fibre additions effectively reduced the above negative effects. The addition of polypropylene fibres was found to have very little effect on the bond behaviour, in terms of the bond strength, the stress distributions both in the rebar and the concrete, the crack development, the slips, the bond stress-slip relationship, and the fracture energy in the bond failure. In the analytical study, finite element analysis with fracture mechanics was carried out to investigate the bond phenomenon under high rate loading. The analytical method took into account all of the important variables in the bond-slip process. In this approach the chemical adhesion and frictional resistance between rebar and concrete were considered only during early loading in the elastic stage. After that only the rib bearing mechanism was taken into account. The fiber concrete composite and the high strength concrete were appropriately modelled. In the finite element analysis quadratic solid isoparametric elements with 20 nodes and 60 degrees of freedom were employed for the rebar and the concrete before cracking. After cracking the concrete elements were replaced by quadratic singularity elements, which were quarter-point elements able to model curved crack fronts. A special interface element, the 'bond-link element', was adopted to model the connection between the reinforcing bar and concrete. It connected two nodes and had no physical thickness at all, and so could be thought of conceptually as consisting of two orthogonal springs, which simulated the mechanical properties in the connection, i.e. they transmitted the shear and normal forces between two nodes. A new approach was proposed in this study for the establishment of the stiffness matrix of the 'bond-link element'. Then a bond stress-slip relationship at the interface between the rebar and the concrete would be one of the output results of the finite element analysis, rather than an input parameter required before the analysis could proceed. The dynamic constitutive laws of both steel and concrete, the criteria for crack formation and propagation in concrete based on the energy release rate theorem for mixed mode fracture, and the criterion for concrete crushing were used in the finite element process. It was an iterative program with rapid convergence. Not only could the bond stress and crack distribution be found through the analysis, but also a bond stress-slip relationship under high rate loading could be established analytically. The results obtained from the finite element analysis were compared with those from the experimental method, and reasonably good correspondence was found.

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