DAMAGE EVOLUTION IN COMPRESSED CONCRETE P. Stroeven and J. Hu Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands ABSTRACT This paper deals with different aspects of damage evolution in compressed concrete. Contributions are paid to mechanisms of damage evolution in compression, and supporting evidence from analytical approaches and experiments. This approach is basically at the mesoscopic level, although also microscopic aspects are briefly introduced. Fracture roughness and fractality are touched upon. Quantitative image analysis and stereological tools are presented for the study of damage evolution on engineering level. The stochastic concept of heterogeneity is introduced as sampling concept. A distinction is made between structure-insensitivity and structure-sensitivity of material properties. Sub-sampling (image analysis areas, or specimens in material testing) is demonstrated leading to biases when material properties are sensitive to structural details. This is the case for crack initiation strength, because governed by local tensile strength capacity. Experimental approaches to ultimate strength or post-ultimate properties are less liable to such size effects. 1 INTRODUCTION Concrete is a particulate composite material on different levels of the microstructure. It is also referred to as a macroscopically heterogeneous quasi-brittle material. Already in the early 1960s research efforts demonstrated virgin concrete to contain myriads of tiny cracks resulting from stresses due to shrinkage and differential settlements [1]. They are not visible by naked eye, though. The underlying concept of a continuous range of micro-structural dimensions as well as the three discrete levels of aggregation, denoted by macro-, meso-, and micro-level in concrete technology, have been recognized for a long period of time in the physics and mechanics of deformable bodies [2]. They form reference for the stages of damage evolution described in this paper. Composite material behaviour under forces reflects the properties of the composing parts of the material body and the material structure on the various aggregation levels. This behaviour can be defined in terms of macro or engineering properties, such as the mechanical ones. Properties are denoted as structure-insensitive when solely governed by material composition, e.g. mass, and to a lesser extent UCS. Contrary, structure-sensitive properties, such as the crack initiation strength, are affected by the so-called group pattern or configuration of particles. As a consequence, particle size, shape and spacing are involved. An engineering property can only be attributed to a material element of at least representative dimensions, the representative volume element (RVE). Similarly, the quantitative image analysis approach to damage evolution stages should be based on a representative area element (RAE) [2]. Sub-RVE/RAE elements are frequently employed in experimental studies. They reveal an increasing degree of stochastic heterogeneity at reducing volume dimensions. This is reflected by the increasing width of the probability density curve of the investigated material property or structural characteristic (like cracking). This is a complicated field, because each geometrical parameter or material property has its independent scale of heterogeneity, so that required sample sizes vary significantly. Implications are size effects. 2 GLOBAL MECHANICAL BEHAVIOUR IN COMPRESSION On engineering level the material behaviour is generally expressed in terms of the stress-strain diagram. Strength, stiffness and toughness are derived from such diagrams. Secants value of