Crack detection and vertical deflection monitoring in concrete beams, using plastic optical fibre sensors

Plastic optical fibres have been employed to detect initial cracks, monitor post-crack vertical deflection and detect failure cracks in concrete beams subjected to flexural loading conditions. The intensity-based sensor system relies on monitoring the modulation of light intensity within the optical fibre as the sensor is loaded. The sensor design offers good signal stability and sensitivity to the monitored parameter and represents a cost-effective alternative to other more sophisticated health-monitoring systems currently used in civil engineering structures. Here, a series of three- and four-point bend tests was conducted on a range of structures. Initially, the optical fibres were attached to scale-model concrete samples (without reinforcement) to evaluate their ability to monitor beam deflection and detect cracks. Similar tests were subsequently conducted on life-size concrete beams containing reinforcing steel bars. The location and subsequent trajectory of cracks during the loading regime were marked and then compared to the sensor signal to assess the sensor's ability to monitor crack development. The results demonstrate the possibility of using optical fibres to detect hairline cracks and ultimate failure crack in civil engineering structures and highlight their ability to monitor crack propagation up to ultimate failure. In addition to detecting the initiation of a crack, good agreement between the sensor output and crack progression during loading was also obtained in these concrete beams.