Study of Acoustic Surface Waveguides on Reinforced Concrete Slabs

This paper describes the development of a two-dimensional acoustic surface waveguide system to enhance the transmission of Acoustic Emission (AE) signals in high attenuation concrete materials. The design of the surface waveguide system and the AE source location results are described. In this study, steel wires were selected as a waveguide material and were attached on the surface area of reinforced concrete structures. AE sensors were mounted at the end of the waveguides. The waveguides were connected to a concrete slab at joints with small contact areas using epoxy. This minimizes the amount of AE energy that could dissipate back to concrete. Thus, AE signals can be transmitted a longer distance. Experiments using standard pencil-lead breaks were conducted at 49 locations on a surface of a reinforced concrete floor slab to provide artificial AE signals. High transmission efficiencies were experimentally determined for the epoxy joints developed to attach the waveguides on the concrete surface. Results confirm that the use of the two-dimensional surface waveguides can significantly increase the AE monitoring range. A multi-layer Neural Network (NN) system was employed to predict locations of the AE sources. Four data sets of AE parameters and their corresponding 49 source locations in each data set were used to train the NN system. A testing data set was then used to demonstrate the ability of the NN in identifying the locations of the AE sources. Satisfactory prediction results from the NN were obtained.     

Application of Wigner-Ville Transform to Evaluate Tensile Forces in Seven-Wire Prestressing Strands

Seven-wire steel strands are widely used in various types of prestressed concrete structures. When a strand is subjected to a tensile force, the traveling time of the stress wave will be affected due to the elongation of the strand together with the changes in wave velocities. In this paper, the Wigner-Ville transform technique was used to analyze the measured stress waves in order to identify the arrival time of each frequency component. A numerical calculation considering the elastic waveguide theory and the acoustoelastic effect was conducted. A commonly used 12.7-mm (1/2 in.) diameter seven-wire prestressing strand (Grade 270) was tested. Tensile force up to 142 kN (32 kips) was applied to the strand. The results indicate that evaluation of the tensile force in the strand can be accomplished by measuring the traveling time of a single-frequency component of the propagating stress wave. The Wigner-Ville transform technique can efficiently identify the arrival time of each frequency component of the stress-wave signals with reasonable accuracy. The numerical and experimental results correlate well with each other. Results of this study present a technique that can provide an efficient nondestructive measurement of stress retension levels in long post-tensioned steel strands.