In-Situ Relative Humidity Sensing for Ultra-High-Performance Concrete Using Polymer Fiber Bragg Gratings

We report on measuring, for the first time, the relative humidity in ultra-high-performance concrete mixtures using a Bragg grating in few-mode CYTOP polymer fiber (3 principal modes), inscribed in the fiber using a femtosecond laser technique. The sensor was characterized for temperature, strain and relative humidity noting improved sensitivity coefficients compared to other similar sensors. The polymer optical sensing elements successfully monitored the relative humidity for a period of eight days. We show that fiber optic sensing methods in concrete structures offer the ability to overcome deficiencies with existing electronic solutions such as scalability, reduction of single point monitoring sensing sites, and electromagnetic interference, while enabling ready installation within critical sections of the buildings.