1 DISCRETE BOND ELEMENT FOR 3D FINITE ELEMENT ANALYSIS OF REINFORCED CONCRETE STRUCTURES J. Ožbolt*, S. Lettow* and I. Kožar** *University of Stuttgart, Germany Institute of Construction Materials, Pfaffenwaldring 4, 70569 Stuttgart **University of Rijeka, Croatia Faculty of Civil Engineering, V. Cara Emina 5, 51000 Rijeka SUMMARY Reinforced concrete structures perform well only if the transfer of tensile forces from reinforcement bars into concrete is possible. This transfer relies on the bond resistance. Therefore, for modeling of reinforced concrete structures one needs a simple, robust and realistic bond model. In the present paper a discrete bond model that can be used in the three- dimensional finite element analysis of concrete structures is described. The model response is controlled by two groups of parameters: (i) basic model parameters: the bond strength, the shape of the stress-slip curves and the loading-unloading-reloading rules; and (ii) the parameters which depend on the geometry of the concrete specimen as well as on the stress- strain state of the reinforcement bar and concrete in the vicinity of the bar, respectively. The basic performance of the model for monotonic and cyclic pull-out load is demonstrated by means of a few numerical examples. The results and possibilities for further improvements of the proposed model are discussed. 1. INTRODUCTION In the past a number of experimental investigations have been carried out in order to clarify and understand the behavior of deformed bars pulled out from a concrete block under monotonic and cyclic loading conditions. These experimental results are well documented in the literature (CEB Bulletin 230, 1996). Based only on the experimental results it is difficult to filter out the influences of material and geometrical parameters on the bond resistance. Therefore, to better understand bond behavior one needs a reliable bond model that can be employed in three-dimensional finite element analyses. Numerical modeling of bond is principally possible at two different levels: (i) phenomenological modeling based on the bar- concrete inter-face type of the formulations of smeared or discrete type and (ii) detailed analysis in which the geometry of the bar-concrete connection is modeled by a three- dimensional model that is formulated in the framework of continuum or in the framework of discrete type of the modeling (e.g. lattice model, random particle model, etc.). Due to the complexity of the detailed analysis in the practice phenomenological models are normally employed (see fib Bulletin No. 10 (2000)). In the phenomenological models concrete and reinforced concrete members as well as steel bars are modeled by two- or three-dimensional finite elements. The link between the steel bar and concrete can be realized either by a continuous or discontinuous connection. In the continuous connection the macroscopic stress-strain constitutive relationship has to be employed whereas in the discontinuous approach bond is defined by discrete, zero thickness elements (springs) whose behavior is controlled by the stress-slip relationship. Both approaches are able to realistically predict bond resistance for different geometries and for