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A new damage model, based on continuum damage mechanics and simulating the opening, closing and reopening of cracks in concrete using only one surface of discontinuity, is proposed in this article. The model complies with the thermodynamics principles of non-reversible, isothermal and adiabatic processes. INTRODUCTIONMicro-cracking and sliding between granular particles cause the highly non-linear behavior of concrete (Oller 1988), and it is difficult to represent this behavior through constitutive models. These phenomena can be characterized by the cohesion and the inner friction angle, the softening stress strain curve and the volumetric dilatation (Tao and Phillips 2005). They cause the decrease of the material's elastic modulus and strength. If the material is exposed to alternating tensilecompressive stress states, the opening, closing and reopening of cracks can occur. These phenomena must be adequately represented by constitutive models ensuring continuity of the response to complex loads. Traditionally, constitutive models for concrete use the fracture mechanics theory (CFM) and the continuum damage mechanics theory (CDM). Classical fracture mechanics gives a basis for simulating the opening and closing of cracks. Hillerborg et al. (1976) describe the mechanical behavior of concrete by developing a non-linear fracture model and using the finite element

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