The concrete works, during their exploitation are exposed to operation or environmental conditions which can inflict certain degradations to them. Establishing a good diagnosis needs a particular knowledge of the behavior of concrete when exposed to aggressive agents, especially the mechanical behavior. The determination of the causes of degradation is a complex matter and the interaction between different pathologies makes it more difficult. The choice of the materials and techniques used to repair the damages is also very important to succeed in the intervention. Our objective is the development of a knowledge base which will be used as a base to formulate the rules used to organize, rationalize and optimize the diagnosis process of these pathologies. The knowledge base gathers the majority of known phenomena related to the degradation of the concrete works. It is organized according to a reasoning which enables to describe or to identify most damages and degradations of chemical or mechanical origins or of implementation. Used with an expert system it will enable us to evaluate the importance of the damage, the need to intervene and finally to propose recommendations about the appropriate procedures and materials needed to repair the damage.