Towards Embedded Fiber Optics for the Ultrasonic Monitoring of Concrete

Abstract

The most widespread sensors embedded in concrete aim to monitor temperature and strain. They are mainly providing local information at the position of the sensors themselves. In recent years new embedded sensors in concrete have been considered by various research groups in view of providing quantitative information in volume so that damage location does not have to be known precisely prior to sensors installation. For instance some piezoelectric ultrasonic sensors are currently being tested in real bridges to monitor damage and/or cracks. Embedded sensors based on electromagnetic waves or electric fields are also under development to monitor water and/or chloride ingress. All those promising ongoing works show that, thanks to embedded sensors, it will soon be possible to monitor in 2D or 3D on site concrete structures. In this paper we show preliminary experimental results using embedded Fiber Bragg Grating (FBG) in concrete to record ultrasonic signals at several locations along a single optical fiber. The advantage of FBG for concrete monitoring is obvious: the footprint of the optical fiber itself is very limited compared to other sensors cables, its durability in harsh conditions such as salty environment or radioactive one is a real advantage, in addition it is a broad band transducer which allows multiple scale analysis. After a description of the FBG sensing principle and the optical fiber installation into decimeter and metric concrete samples, ultrasonic signals recorded with a spacing of 5 cm will be shown and analyzed. The sensitivity of the FBG will be commented with regards to a selection of piezoelectric ultrasonic sources covering body waves as well as coda waves frequency range (typically between 50kHz to 500kHz).