Bond of Ribbed Bars Modeled through Concrete Confinement


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Title: Bond of Ribbed Bars Modeled through Concrete Confinement

Author(s): A. J. Bigaj, J. A. den Uijl and J. C. Walraven

Publication: Special Publication

Volume: 180


Appears on pages(s): 195-214

Keywords: Bonding; bond splitting; bond stress; concrete durability; confined concrete

Date: 10/1/1998

Bond properties are usually described by empirical relations that are based on pull-out tests of bars with short embedment lengths cast in low to medium strength concrete. The limited validity of these formulations is recognized and their applicability in structural analyses is reconsidered. Results of two test series with various confining conditions and concrete strengths provide the basis for the derivation of a new bond model for ribbed ibars. Pull-out bond failure in confined concrete and splitting bond failure in unconfined concrete have been studied. Steel yielding is found to have a iconsiderable influence on bond strength. Significant differences in bond of NSC and HSC are confirmed. An analytical bond model for ribbed bars is developed. It is based on the confining capacity of the concrete surrounding the bar. This is evaluated with a /thick-walled-cylinder model, from which the relation between the radial displacement and the radial compressive stress at the steel-to-concrete interface i s derived. The radial displacement at the interface is linked to the slip of the ribbed bar, distinguishing between the two modes of bond failure: pull-out and cover splitting. The model takes into account concrete toughness and bar contraction, also after yielding. Verification of the model against selected experimental results reflects the potential of the model to be used in a broad range of applications. In its present form the model is used for the analysis of isolated bond problems using a one-dimensional finite difference approach, but its application is also considered to count for three-dimensional deformations in FE codes that treat the bar as a one-dimesional element.