The behaviour of civil engineering works (structures, buildings, dams, etc.) in time is a current problem which is the subject of deep consideration and numerous research projects. These studies - which are aimed at adopting a better approach to repair, maintenance and reinforcement operations -have revealed a significant need for the development of means to diagnose and monitor structures. Many nondestructive testing techniques already exist but a major difficulty in applying them arises from the fact that they are not universal. It is therefore necessary to analyse their limits and define fields of application. Choosing a suitable technique is always a delicate process. In addition, the results obtained are generally affected by a considerable degree of uncertainty; cross tests using different techniques make it possible to improve the quality of the diagnosis. Thermal approaches are currently emerging and being developed quickly. They are typically based on infrared thermography measurements. These techniques involve a contact-free analysis and provide overall information on the structure. They are adapted to a qualitative type of research in which the prime objective is to highlight anomalies. However, it is generally complicated and difficult to make a quantitative interpretation of the results [1]. This article presents a new thermal method based on the concept of thermal impedance, which can be measured at the surface of a structure. It is adapted to a local quantitative analysis and should be used as a complement to the overall measurements taken by infrared thermography to quantify and refine the analysis.