Advancements in evaluation of air-coupled impact-echo test method
This report presents a study and accompanying laboratory work to investigate a recently-developed air-coupled impact-echo (IE) nondestructive testing (NDT) method, in which microphones replace the traditional physically-coupled IE sensors. To develop an optimum testing system and verify the new method, two concrete plates were tested in the laboratory, one of which was a solid concrete slab, and the other was a model of a reinforced concrete bridge deck with artificial defects. An IE testing system was developed using an Omega OMB-DAQ-3000 data acquisition module and a custom program written in LabVIEW. A measurement microphone was utilized as a sensor for the air-coupled test method, and two piezoelectric accelerometers were utilized for the traditional physically-coupled IE sensors. Prior to performing the IE tests, P-wave speeds were measured using the accelerometers according to ASTM specifications. The accuracy and feasibility of the air-coupled test method to determine the concrete structure's solid thickness and to detect defects or flaws, such as delaminations or voids, were verified by comparing test results obtained via the air-coupled and physically-coupled sensors.
The air-coupled IE method thus has the potential to increase the efficiency of IE testing of bridge decks and other concrete structures, by eliminating the need to physically couple and uncouple sensors for each test. However, when using the air-coupled IE method in practice, ambient noise generated by wind, traffic, and machinery will be sensed by the microphones and therefore reduce the signal to noise ratio of the data. Additionally, a portion of the acoustic energy generated by the impacts during testing will be lost due to the mismatch in acoustic impedance between concrete and air. To address these problems, a parabolic reflector and a sound isolation enclosure were studied and found to improve the quality of recorded signals compared to using a microphone alone. Additionally, filtering techniques including band-pass, high-pass, and adaptive filters were implemented in MATLAB for post-processing the test data. Finite element method (FEM) based numerical simulations were conducted using COMSOL Multi-physics software to understand the mechanics of the air-coupled IE test, study the optimum geometry for the parabolic reflector, and investigate the effects of the microphone height. Finally, two-dimensional (2D) IE scanning tests were conducted on the bridge deck with artificial defects to locate the defect positions by the air-coupled and physically coupled test methods. Results obtained by these two methods are in good agreement, demonstrating the accuracy and feasibility of the air-coupled IE test method.