Preliminary Development and Evaluation of Fiber-Optic Chemical Sensors

Conventional methods of measuring carbonation and chloride ingress exist, but they are either inaccurate, expensive, or involve destructive testing. In seeking new solutions to this problem, the potential to use novel fiber-optic chemical sensors developed specifically for this application and designed to monitor chemical changes in cementitious materials, in situ and nondestructively, was explored in this study. Three types of fiber-optic sensors were thus constructed, tested, and evaluated, viz a temperature sensor, a pH sensor, and chloride sensors. The temperature sensor was based on the fluorescence decay of temperature-dependent materials, whereas the pH and chloride sensors were based on sol-gel technology, with pH and chloride sensitive indicators impregnated in the sol-gels. All the sensors were tested in situ, and subsequently, the temperature and the pH sensors were embedded in mortar and tested. It was found that both the temperature sensor and the pH sensor could function correctly for over 18 months after placement, but there was an issue with alignment of the sensor each time it was reconnected to the hardware. However, the laboratory tests showed that the chloride sensor was not reversible, and therefore further refinement was considered to be necessary before it could be used in situ in environments where the chloride content was known, from other measurements, to decrease. Research is ongoing to refine the sensor performance and expands the in situ testing program. The sensors themselves are inexpensive to fabricate, but the sensing hardware used in this work is costly because of its high versatility.