In this paper we demonstrate the potential of contactless ultrasonic sensing for rapid and automated characterization of concrete structures. Practical generation and detection of ultrasonic surface waves are made possible using air-coupled sensors, offering the potential to overcome limitations associated with infrastructure assessment measurements. The objective is to demonstrate enhanced performance when miniature, micro-machined sensors (MEMS) and high voltage solid-state capacitance transmitters are used in a scanning configuration, suitable for automation. The employed sensors, system and testing configuration, with respect to sensor height and incident angle, are described. An optimal test configuration is defined and incorporated into a controlled scanning system. Tests with the optimized configuration were carried out on reinforced concrete elements: a pre-stressed concrete rail tie that contains rail seat damage and concrete blocks with varying levels of simulated micro-cracking damage. In both cases, obtained surface wave velocity and attenuation signal characteristics show sensitivity to concrete material damage.

We describe an approach that utilizes ultrasonic surface wave backscatter measurements to characterize the volume content of relatively small distributed defects (microcrack networks) in concrete. A simplified weak scattering model is used to demonstrate that the scattered wave field projected in the direction of the surface wave propagation is relatively insensitive to scatterers that are smaller than the propagating wavelength, while the scattered field projected in the opposite direction is more sensitive to sub-wavelength scatterers. Distributed microcracks in the concrete serve as the small scatterers that interact with a propagating surface wave. Data from a finite element simulation were used to demonstrate the viability of the proposed approach, and also to optimize a testing configuration to collect data. Simulations were validated through experimental measurements of ultrasonic backscattered surface waves from test samples of concrete constructed with different concentrations of fiber filler (0.0, 0.3 and 0.6%) to mimic increasing microcrack volume density and then samples with actual cracking induced by controlled thermal cycles. A surface wave was induced in the concrete samples by a 50kHz ultrasonic source operating 10mm above the surface at an angle of incidence of 9°. Silicon-based miniature MEMS acoustic sensors located a few millimeters above the concrete surface both behind and in front of the sender were used to detect leaky ultrasonic surface waves emanating from concrete. A normalized backscattered energy parameter was calculated from the signals. Statistically significant differences in the normalized backscattered energy were observed between concrete samples with varying levels of simulated and actual cracking damage volume.

The utility of micro-electro-mechanical sensors (MEMS) for application in air-coupled (contactless or noncontact) sensing to concrete nondestructive testing (NDT) is studied in this paper. The fundamental operation and characteristics of MEMS are first described. Then application of MEMS sensors toward established concrete test methods, including vibration resonance, impact-echo, ultrasonic surface wave, and multi-channel analysis of surface waves (MASW), is demonstrated. In each test application, the performance of MEMS is compared with conventional contactless and contact sensing technology. Favorable performance of the MEMS sensors demonstrates the potential of the technology for applied contactless NDT efforts. Objective: To illustrate the utility of air-coupled MEMS sensors for concrete NDT, as compared with conventional sensor technology.

This letter presents an automated acoustic sensing device for rapid detection of delamination in concrete. The device consists of ball-chains for continual impact excitation and multi-channel microphones for acoustic sensing. A ball-chain is formed by multiple metal balls connected by flexible ropes and is dragged on concrete surface to excite vibration of delaminations. Compared to the conventional chain drag test, the ball-chain generates acoustic signals with higher signal-to-noise ratio (S/N) because the balls give isolated but continual impacts on concrete surface during dragging. The proposed method was validated on a concrete specimen with artificial delaminations.