The structural elements of buildings are subject to degradations affecting their service life, due to the use to which they are subjected and to the context where they are located. It is possible to determine the degradations to which a structure is subjected through the use of fault trees and then calculate its actual service lifetime. SAC$\hat$3 is a computer system that integrates an expert system which implements fault trees by means of inference rules, as well as durability models to diagnose the potential degradations of a structure and its service lifetime. This paper presents the relevant explanations of concepts in the field of civil engineering, the development corresponding to the expert system as well as its application to the particular case of reinforced concrete structures.