The water content in concrete (nuclear structures and nuclear waste repositories) is a major topic to understand and predict the behaviour at the end of the operating period. That is the reason why ANDRA and EDF are involved in research programs dedicated to concrete Thermo-Hydro-Mechanical (THM) modeling and to in situ water content assessment technologies. Another example concerns the bridges which include “external” post-tensioned cables to reinforce these structures. These cables are not into the concrete material, hence potentially accessible for measurement. They are generally placed in High Density Poly-Ethylene (HDPE) ducts, where the residual space is filled under high pressure with a cement grout intended to prevent corrosion. Nevertheless, in some cases, the cables breaking occur in non-protected zones because of the presence of a “white paste” or grout voids. To remote diagnosis anywhere and in real time of post tensioned ducts or to measure the water content in concrete, we propose a structural health monitoring method based on Frequency Domain Reflectometry (FDR). Today’s, advanced reflectometry methods provide an efficient solution for the fault-detection and for their diagnosis in electric transmission lines. This paper presents a direct model of the FDR method based on telegrapher’s equations. An analysis of these signals, based on scattering theory, enables one to retrieve the impedance distribution of the electric line. The impedance distribution depends on damage into the duct or the water content in concrete. An inversion algorithm is realized with software 'ISTL' provided by INRIA. FDR method has been applied to two real cases: measurement of the water content in concrete and the diagnostic of the external post tensioned duct.