Development of a fibre optic crack sensor for concrete structures

The condition of a concrete structure can be effectively assessed through the monitoring of cracks. Large cracks may be warning signs of severe degradation, while small cracks with openings from 0.2 to 0.4 mm may lead to durability problems associated with the penetration of water and other chemicals. Since the location of cracks in a concrete structure is not known in a-priori, conventional 'point' sensors (e.g., strain gauges) are not effective in the sensing of cracks. In this paper, we will describe recent findings on a fiber optic crack sensor that allows the detection and monitoring of multiple cracks without requiring prior knowledge of crack locations. The sensing principle will be discussed first, following by the theoretical derivation of signal loss vs. crack opening relation. The fabrication of a sensor suitable for both external bonding and internal installation in a concrete structure will be described. Representative experimental results will be shown to demonstrate the applicability of the sensor under both static and cyclic loading. Measured losses are found to be in good agreement with theoretical results. Based on the experimental and theoretical findings, the potential of the sensor for practical applications is demonstrated.