MICRO AND MACROMECHANICAL ASPECTS OF THE BEHAVIOR OF CONCRETE MATERIALS WITH SPECIAL EMPHASIS ON ENERGY DISSIPATION AND ON CYCLIC CREEP V. P. Panoskaltsis and S. Bahuguna Department of Civil Engineering Case Western Reserve University, Cleveland, Ohio 44106-7201, U.S.A. ABSTRACT In this work energy dissipation of concrete materials in the process of loading- unloading is studied. Loading-unloading is talcing place in the viscoelastic, hardening and softening region of the material. A unified model, which has been recently developed is used. Cyclic creep is also studied analytically and a comparison to experimental results is discussed. 1. INTRODUCTION Concrete and geomaterials, such as rock, mortar, sands, clays, cemented soils etc., are characterized by shear strengths that increase with increasing normal stresses; i.e. they exhibit frictional strength, hence they are often termed "frictional" materials. Our interest lies in a class of frictional materials with cohesion, namely concrete materials. The increased use of concrete in traditional structures, dams and advanced technological structures, such as nuclear reactors has resulted in a great need for better understanding of these materials. Due to recent rapid advances in computer technology, their constitutive behavior is no longer intractable, no matter how complex it may be. In this article two important aspects of the behavior of concrete materials are studied in particular. Firstly, their energy dissipation during loading-unloading is considered. The principles of thermodynamics are used and an effort is made to identify the different mechanisms involved in the dissipative phenomena. In addition, some aspects of irreversible thermodynamics are discussed. Second, cyclic creep is studied analytically and comparisons to experimental results are discussed. A unified model developed by the first of the authors is briefly reviewed and is used in this work. This model is based on the theory of internal variables. Internal variables provide a connection to the material's microstructure (Aifantis [1984], Lubliner [1989]) and will be chosen so as to effectively represent the irreversible mechanisms. The local state of the material depends on the instantaneous values of the stress tensor (or strain, whenever their interchange is justifiable), the temperature and the vector of internal variables. This is sometimes called the "article of faith" of material scientists (see Kocks in Miller [1987]). The history dependence of the material is provided through the rate (evolution) equations of the internal variables. The possibilities and some guidelines of employing viscoelasticity and plasticity theories in order to describe permanent deformations are also presented in Section 2. A discussion about the concept of "plasticity" of geomaterials is also provided. 119