Abstract
First of all it is shown that the structure of a composite material such as concrete can be subdivided into different levels. Characteristic properties of hardened cement paste will be described in terms of the micro-level. Pores, cracks and inclusions are introduced on the meso-level. On the macrolevel the material is looked upon to be quasi homogeneous. On this level material laws are developed. Mechanics and mechanisms of deformation and crack formation can be discussed on the corresponding levels. It is pointed out in particular that mechanisms of the micro-level cannot be linked directly with the material behaviour, this implies that mechanisms derived from direct observation of macroscopical behaviour are generally meaningless. It is important to note that different mechanisms must be related to the observed materials response according to the hierarchic structural system. On the micro-level the Munich Model is introduced. The influence of moisture content on creep and strength of hardened cement paste can be described satisfactorily by means of this model. The influence of porosity on strength is also discussed on the micro-level. As a typical example for the mechanisms involved on the meso-level, crack propagation from large pores and cracks and interfaces is treated. This is essentially a micromechanics approach of a composite porous material. It can be concluded that numerical methods have become a powerful tool to complement experimental research. It is possible to base material laws for application in structural engineering on results gained on the micro-level and on the meso-level if the hierarchic structural system is taken into consideration.
