New fiber optic sensor for monitoring temperatures in concrete structures during fires

•A new fiber optic sensor for monitoring high temperatures is proposed.•The new sensor is based on the use of Regenerated Fiber Bragg Gratings.•Sensors were embedded in a loaded concrete beam and subjected to a fire test.•The maximum temperatures measured by the sensors were over 950°C.•Sensor measurements were checked by numerical simulations and thermocouples. Monitoring temperatures in structures during fires provides valuable information to 1) the firemen engaged in extinguishing it, 2) those who assess its security, and 3) the organizations who have to decide on its possible repair, renovation or demolition. Developing sensors able to measure extremely high temperatures in actual blaze conditions is therefore a fundamental requirement. This paper proposes a new fiber optic sensor based on Regenerated Fiber Bragg Gratings specially designed to be embedded in concrete structures to monitor temperatures during fire events. A practical test was carried out on a 5.8m long beam subjected to the ISO-834 fire curve for 77min under the typical loads borne by beams in conventional structures. Nine optical sensors were installed at the mid-span section of the beam and were submitted directly to flames and high temperature gradients (of the order of 200°C/min) that make them measure maximum temperatures of 953°C. The temperatures recorded by the new sensors were compared with those obtained from electrical sensors (thermocouples) and a numerical model, with which they showed a good fit, except in those places in which concrete spalling caused distortions in the results and/or failure of the sensors. The paper thus demonstrates the viability of optical technologies in monitoring reinforced concrete during fires and analyzes sensor behavior to point out areas in which additional research is required.