In one of the earliest works involving concrete damage detection

In one of the earliest works involving concrete damage detection using the intensity modulation technique, Rossi and Le Maou [27] conducted experiments with a bare fiber for crack detection in concrete structures. The fiber, with its protective coatings removed, was embedded directly in the concrete, and the transmitted signal was monitored. As the crack reached to the fiber, the fiber broke, causing abrupt cessation of the transmitting signal. Although the simplest, the major limitation of this method is that once the fiber breaks no further detection can be performed. Ansari and Navalurkar [28] designed their sensors for crack detection based on the same intensity modulation method yet with a different configuration. To increase the sensitivity, the fiber was made in a loop shape such that the fiber circumferences the generated crack. The sensor based on this design is limited to small size cracks only. Leung et al. [29] developed a sensor to monitor flexural cracks in the concrete structures. The loss in the back scattered light intensity is related to a mechanical deformation. The arrangement of the fiber which is laid in a zig-zag course inside the concrete is the key feature of this design. This design increases the sensitivity of the system. The sensor is efficient in monitoring flexural cracks under various types of loads. This technique is simple and sensitive, but only responsive to certain orientations of cracks with regard to the fiber’s orientation. Habel et al. [30] demonstrated that an intensity-based FOS can be used in a quasi-distributed configuration to measure crack opening widths. Similarly, Lee et al. [31] showed that even a low resolution and less sensitive intensity based optical fiber sensor constructed with inexpensive instruments can be useful in the cases where precise measurements of strain or cracks are not required, for example, measurements of stiffness.In general, for health monitoring of concrete structures, including damage detection, an ideal technique should have the common desirables: a simple sensing mechanism, a long sensing range, low instrumentation cost, high sensitivity, fast response, insensitive to temperature and light fluctuations, and capability of distributed sensing [32]. In the present work, we describe a new fiber loop ringdown (FLRD) sensor, which potentially meets the aforementioned requirements for crack detection in concrete structures.The FLRD technique originates from cavity ringdown spectroscopy (CRDS). In CRDS, a light pulse is injected into a cavity constructed using two highly reflective mirrors. The trapped light pulse bounces back and forth many times before it dies out completely. In each round trip a small part of the light energy of the trapped light pulse leaks out of the cavity.

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