Multilayer piezoelectric PVDF transducers for nondestructive testing of concrete structures

Abstract

Infrastructures such as bridges, railways and roads are vital to every country's economic growth and development. These are usually reinforced concrete structures which need regular monitoring for operation and safety. The large concrete structures pose many challenges for inspection due to surface roughness, inhomogeneities and size of aggregates. Nondestructive testing is one of the most effective ways to ensure the safety and operability of such structures. ELOP has developed a handheld ultrasonic scanner for concrete inspection. Our patented scanner covers large surface areas in short inspection time by using a rolling transducer design. The scanner uses a separate transmitter and receiver transducer array inside a cylinder and a segmented dry coupling to the concrete surface. The internal acoustic path allows the scanner to send and receive signals at any chosen time as coupling between the transducer and the concrete is always maintained. An innovative transducer is developed for low frequency applications (50-500 kHz) based on the piezoelectric polymer PVDF. The transducer sensitivity is improved by stacking multiple layers of PVDF which are connected electrically in parallel and acoustically in series. A rotational encoder keeps track of positional information with reference to a starting point which can be recorded along with 3D data of any located defects. Synthetic aperture focusing technique is used for image reconstruction improving SNR of the received signal. This system setup yields high resolution and easy interpretation of real-time 3D images showing the concrete's internal state. The presented paper will describe the progress on development of 3D ultrasonic scanner along with images obtained from concrete structures using the scanner.