Large Aperture Ultrasonic System for Testing Thick Concrete Structures

Abstract

A large aperture ultrasonic system (LAUS) has been designed and built for testing thick concrete structures. The scalable system consists of 12 ultrasonic units, each hosting 32 individual shear wave transducers with a mechanical dry point contact (DPC) to the concrete surface. The 12 units are attached to the concrete surface using a vacuum case, which holds them in place during operation. Each LAUS unit can be placed individually on the surface to achieve the optimal ultrasonic condition, e.g., to avoid rebars. For the generation of the vacuum, air pressure is supplied to each unit through an air hose. The 12 units define an aperture, which is necessary for fast reconstruction of the subsurface structure. The well-known synthetic aperture focusing technique (SAFT) algorithm has been adopted for this situation, where the individual LAUS units form a linear aperture with not necessarily equidistant spacing between the units. The exact geometrical position of the units, which are individually marked with retroreflective labels, is determined using photographs and image processing. All transducers are synchronized and work either as a transmitter or receiver. A full scan consists of 12×11 recordings, where each unit acts as transmitter once and all others as receivers. An electronic subunit on the back of the ultrasonic device holds the battery and handles data acquisition, synchronization, and data communication. A computer is used as base unit that communicates with each LAUS unit for control and data acquisition, the synchronization is performed by a radio modem that uses a special algorithm similar to a digital phase locked loop (PLL). No wire connections are necessary between the units and the base system. The LAUS system is designed to investigate concrete structures with thicknesses in excess of 2 m, depending on the acoustic condition of the object under investigation. Highly reinforced concrete may have less penetration depth. The LAUS provides quasi-real-time imaging, once the transducers are put in place and the data have been acquired. The first measurements on a foundation slab confirm that the system can register ultrasonic echoes from the back wall at 4 m distance.