Influence of Steel Bars in Detection of Voids in Concrete Using Ultrasonic Pulse-Echo Method

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Title: Influence of Steel Bars in Detection of Voids in Concrete Using Ultrasonic Pulse-Echo Method

Author(s): Leonel Lipa, Laura M. Navarro, and Enrique N. Pasquel

Publication: Materials Journal

Volume: 116

Issue: 4

Appears on pages(s): 223-234

Keywords: flaw detection; honeycombs; pulse echo; reinforced concrete; ultrasonic; voids

Date: 7/1/2019

Abstract:
In this research, the influence of steel bars in the detection of voids in concrete elements using the ultrasonic pulse-echo method was analyzed. For this purpose, reinforced concrete walls were made that contain No. 3, No. 4, or No. 5 steel bars, and expanded polystyrene spheres 50 mm (1.97 in.) in diameter, which represent internal voids. Spheres of smaller diameter were not used because their detection is very low. An ultrasonic pulse-echo device of 50 kHz frequency transverse waves that uses a synthetic aperture focusing technique (SAFT) was used. The steel bars were placed with different separations to observe the effect they cause in a scan. In addition, the spheres were laid between these different configurations with the purpose of verifying if their detection is possible. Additionally, image processing was carried out to eliminate or totally reduce the effect of the steel bars in the scans. This research concluded that when scans are performed on strips with transverse No. 3, No. 4, and No. 5 bars, these would appear in the ultrasound image with a probability of 35.4%, 41.7%, and 62.5%, respectively. In addition, if the scan is performed on a strip with No. 4 or No. 5 bars, which have the same direction of the scan, they appear in large magnitude in the ultrasound images, so it is very difficult to distinguish if there is another material in these areas. However, in this last case, if the bars are No. 3, they do not appear in the scan. On the other hand, the spheres were located with a probability of 81.3% when they were located between steel bars separated by a distance greater than or equal to 100 mm (1.97 in.).