The monitoring of civil engineering structures is a major challenge and still the main topic of numerous researches. In particular, issues of cost, sensor durability and measurement reliability remain fundamental elements to be considered for any monitoring operation. Distributed optical fiber sensors have become widely used for strain and temperature monitoring in reinforced concrete structures given the benefits they offer in terms of accuracy, measurement range over long distances and low intrusivity compared to “conventional” sensors. Optical fiber sensors are usually bonded to the concrete surface or embedded within the concrete medium. Under the operating conditions of the structure, bonded sensors are exposed to the external climatic environment, while those inserted into the structure are subjected to the alkaline cement matrix. Therefore, this thesis aims to investigate the durability of optical fiber sensors used for distributed strain measurements in reinforced concrete structures.
The study involves an experimental program performed on instrumented concrete specimens exposed to accelerated ageing tests that are representative of the considered applications. Two commercially available optical fiber cables were selected for this study. Mechanical, physico-chemical and geometrical analysis are carried out on the specimens at different ageing times to assess the durability of these two optical fiber cables for the two possible configurations (embedded and bonded). The evolution of the strain transfer process from the concrete to the optical fiber core is evaluated numerically using a simple finite element model.
In parallel, several concrete blocks of the ODOBA project, under accelerated aging cycles to initiate concrete pathologies, were instrumented using embedded and bonded optical fiber cables. The purpose is to prove the aptitude of this methodology for the detection and long-term monitoring of the pathologies usually met on nuclear power plant (delayed ettringite formation: DEF, alkali-silica reaction: ASR and DEF/ASR coupling). The findings of this study will then allow to propose objective criteria when selecting suitable monitoring systems for power plant structures as well as methods for interpreting measurements over time.​