Fiber optic stress wave sensor for detection of internal flaws in concrete structures

Internal cracks are frequently initiated in the form of delaminations at the interface between the reinforcement and the concrete in bridges. The most probable cause for the generation of internal cracks and delaminations is the pressure generated by the accumulation of corrosion byproducts at the interface between the reinforcement and the concrete. Failure to detect these defects at the early stages of development results in time consuming and costly repair and replacement operations. This article describes development of a fiber optic ultrasonic sensor for distributed detection of flaws and defects in concrete bridge elements. Distributed sensing provides the opportunity for the optical fiber sensor to detect anomalies throughout the entire structure. An ultrasonic transmitter source or an impact device is used for point-by-point generation of stress waves within the bridge deck. An optical fiber is adhered to the surface of the structure, for sensing of the echoed ultrasonic signals. The optical fiber senses the stress waves, and a one-ended measurement of the optical signal by an interferometer provides information about the integrity of the concrete. This configuration eliminates the need for using a conventional piezoelectric transducer (PZT) that can only make localized measurements. The delamination detection capability of the sensor is demonstrated through experimentation with concrete beams that contain intentionally induced delaminations.