International Concrete Abstracts Portal

The increasing significance of performance-based criteria in modem structural design has motivated new considerations in bond design of conventional reinforcing steels, relating to more reliable assessment of both the demand and the supply sides of the anchorage/development design problem. Accurate identification of the required anchorage lengths needed to ensure strain compatibility, by proper consideration of the conditions affecting bond, is necessary to limit slippage of the steel relative to the concrete. While minimum development lengths calculated by designers imply that the bar is fully anchored, it is well established by experimental observation that in practice there is always some bar slip. Recent research results from around the world provide the basis for improved understanding of the effects on bond performance of critical parameters such as confinement, spacing, and material properties. Much of this work has been empirical in nature and the applicability of empirical design expressions in calculations is limited. Nonlinear finite element calculations and other sophisticated analysis re-quires more information as to how the bond failure proceeds than simply an upper limit. This paper will summarize the available information that exists both within North America through AC1 and within the CEB as to the viable approaches and philosophies that can be applied to the bond problem. The range of application of the various techniques will be identified as will limitations and needs for more re-search.

Splitting does always occur in some way prior to bond failure, in the form of either partial splitting (quite often undetected) or full splitting, the latter being the subject of several recent papers, owing to the importance of cover splitting in R/C elements. Starting from the test results on fully-split specimens (like those by the authors on special specimens having a fabricated crack) it is possible to formulate suitable bond stress-confinement stress relationships. These models can be introduced into the limit-analysis models developed lately for the description of partial splitting up to the onset of full splitting and bar pull-out in short anchorages. In this way, a linkage between the bar-concrete pressure (studied here through a limit-analysis elastic-cohesive model) and the bond stress is established, in order to evaluate the ultimate bond capacity and to investigate the transition from a splitting-type failure to a pull-out failure. At the same time, such important topics as concrete tensile strength and fracture energy, crack cohesion and localization, concrete cover and bar diameter, fiber content and external pressure can be incorporated into the model. A set of diagrams showing the bond capacity and crack number/opening/penetration versus concrete cover is presented, and the design implications of both the theoretical and experimental results are discussed.

The bond between reinforcement and concrete should ensure high structural stiffness and small cracks in the serviceability limit state, generate small splitting forces and allow full utilization of the reinforcement ductility in the ultimate limit state. While bond behavior at service load and splitting behavior has been investigated intensively, bond behavior at large inelastic steel strains is not known very well. Therefore, in this paper the contribution of concrete between cracks at inelastic steel strains is investigated numerically based on a rational mechanical model and using realistic constitutive material laws. The model predictions agree rather well with a large number of test results. According to the results of the parametric study, after steel yielding the ratio of mean steel strain to the steel strain at the crack is mainly influenced by the reinforcement percentage and the shape of the steel stress-strain curve. It is much lower than at service load. Due to this lower ratio of mean steel strain to steel strain at the crack, the rotation capacity of plastic hinges and thus the structural ductility is reduced significantly and may be very low if reinforcement with low ductility is used. Therefore an optimization of bond seems to be necessary. Corresponding extensive numerical and experimental studies are under way in Germany.

A bond model is presented that uses an interface idealization of the bond phenomena and incorporates dilation to characterize the wedging effect of the ribs. Since this type of model can potentially predict both pull-out and splitting failures, it may provide an approach for characterizing the observed experimental response of bond specimens in a form that can be used to better understand the progressive failure of complicated structural components. Using the mathematical framework of plasticity theory, the model is defined to characterize the effects of damage in the region near the bar. The form of the model is based upon experimental results that include a variation of the confinement stress; thus, the model fully couples the tangent and normal response so that it: (1) exhibits a sensitivity to the confinement stress and (2) produces longitudinal cracking in models of bond specimens. Validation problems based on experiments from several research groups are considered to highlight the strengths and weaknesses of the model. The model reproduces the experimental data with acceptable accuracy using a single calibration, but the results also suggest that the limitations of the interface idealization merit further investigation.

This paper describes a theoretical and experimental analysis designed to characterize the initial branch of the bond-stress/slipping curves (5-s) for normal-strength and high-strength concretes. The theoretical analysis is used to interpret the results of experimental trials on reinforced concrete ties prepared with class 50 and 100 lMPa concrete mixtures and submitted to tensile forces without inducing any yield in the bar. The purpose of the investigation was to study any changes in bond behavior (over the limited range of slipping values considered) due to the better mechanical features of the 100 MPa concrete, and thus contribute to a better understanding of how high-strength reinforced concrete elements behave in a ser-viceability state.