A material model for plain concrete formulated within the framework of multisurface elastoplasticity-damage theory is proposed in this paper. Anisotropic sti ness degradation as well as inelastic deformations are taken into account. The applicability of the model encompasses cracking as well as the non-linear response of concrete in compression. The e ect of di erent softening laws on the stress-strain relationship and on the dissipation is investigated in the context of a 1D model problem. The integration of the evolution laws is based on the standard return map scheme. Further computational issues include the stability of the local iteration procedure and the treatment of the apex region of the damage surface. The model is employed for re-analyses of a cylinder splitting test and of a notched concrete beam. Results from the composite elastoplastic-damage model are compared with test results and results from other material models for concrete, respectively. ? 1998 John Wiley & Sons, Ltd.KEY WORDS: damage; plasticity; concrete; cracking; Rankine criterion; ÿnite element analysis INTRODUCTORY REMARKSBrittle materials such as geomaterials and concrete exhibit distributed as well as localized degradation of the mechanical properties with increasing loading. The phenomenological response of plain concrete subjected to predominantly tensile stresses is characterized by a more or less linear ascending branch of the stress-displacement curve followed by a progressively decreasing residual strength resulting in the formation of macrodefects in the form of discrete cracks. When unloaded in the post-peak regime, non-recoverable deformations as well as a degradation of the sti ness of the unloading branch is observed. From a microstructural point of view, the progressive degradation of the elastic moduli, commonly referred to as damage, is the result of growth and coalescence of existing microcracks and microvoids along the interfaces of the cement paste and the aggregates. This deterioration process prevents a complete closure of microcracks in unloading processes. As a consequence, permanent strains develop. On the phenomenological level, this e ect is often modelled by means of classical plasticity theory. Depending on the level of hydrostatic

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