Noncontact ultrasonic diagnostics in concrete: A preliminary investigation

The use of ultrasonics is well-established in concrete diagnostics. Most current systems use narrow-bandwidth piezoelectric transducers, which must be in contact with the concrete surface. New ultrasonic nondestructive testing (NDT) systems combine electrostatic broadband transducers and signal processing techniques such as pulse compression to improve signal-to-noise ratios (SNRs). This permits air-coupled, noncontact (NC) operation which has many advantages. Preliminary experiments with low-power NC equipment, designed for foodstuffs and packaging, indicate that diagnostics can be performed in this manner through at least 75 mm of concrete. The speed-of-sound versus strength relations thus determined differ from those measured previously since the NC system is more sensitive to paste properties than aggregate properties while the converse tends to apply for traditional equipment. There is significant scope for optimising the NC system to achieve greater penetration and diagnostic capability. (C) 2004 Elsevier Ltd. All rights reserved