Noncontact Ultrasonic Nondestructive Techniques: State of the Art and Their Use in Civil Engineering

Abstract

Ultrasonic noncontact methods of testing civil engineering materials in air, where both transmission of waves into materials and reception of signals are contactless, are reviewed. A short history and technical details of noncontact methods related to generation, reception, and transmission of waves are presented. Particular attention is devoted to enhancement of the signal-to-noise ratio by using coded signals. Next, examples of currently available solutions dedicated to (1) estimation of bulk properties of materials, (2) evaluation of state of concrete cover, (3) flaw detection, and (4) identification of structural parameters of porous materials are discussed. Applications of laser techniques, classical echo, direct through transmission, and different types of semicontact methods are topics that are not reviewed in this paper. Finally, the benefits and limitations as well as expected directions of further development of the noncontact techniques are indicated.