Evaluation of retroreflective corner echo for detection of surface breaking cracks in concrete by ultrasound

Abstract

The retroreflective corner echo is used, for example, in ultrasonic non-destructive testing of metals to find fatigue cracks in tubes or shafts. If the much weaker crack tip signal is additionally detected, the crack length can also be determined. A corner reflection occurs in cases of surface breaking cracks with predominantly perpendicular orientation to the surface. The intensity of the corner reflection depends on the angle of incidence and on the ultrasonic wave mode used. For the reliable detection of vertical surface breaking cracks in metals, transversal waves are commonly used, which propagate at an angle of 37° to 53° to the inspection surface. As shown in this contribution, the wide spread low frequency ultrasonic arrays with dry point contact sources available for ultrasonic testing of concrete also allow to receive corner echoes. These devices generate transversal waves in concrete structures with a large divergence of the sound field. A series of experiments was carried out with such dry point contact arrays on concrete specimens with artificial test defects and controlled induced cracks of different depths. The ultrasonic time-of-flight signals were recorded, exported and reconstructed utilising the SAFT (Synthetic Aperture Focusing Technique) algorithm. The SAFT reconstruction parameters were adjusted to visualize the corner echo indication. As will be shown, with this targeted processing, the reproducible detection of surface breaking cracks in concrete is possible. The retroreflective corner echo can thus be exploited in civil engineering for non-destructive inspection of concrete.

Imaging of vertical surface-breaking cracks in concrete members using ultrasonic shear wave tomography

“…In concrete, the scattered pulses are analyzed 23 for travel-time-based positioning of the crack tip. However, the latest studies attempt to image the crack tip 24 – 26 through which the depth can be estimated. One such study 24 , 26 proposes the corner echo formed by the crack tip and flat back wall as an indicator to analyze the crack depth.…”

“…However, the latest studies attempt to image the crack tip 24 – 26 through which the depth can be estimated. One such study 24 , 26 proposes the corner echo formed by the crack tip and flat back wall as an indicator to analyze the crack depth. Some other applications with DPC SH-wave arrays include imaging underlying delaminations 27 – 29 , debonded rebars 27 , and other inclined planar defects 30 in reinforced concrete.…”

“…The introduction of “Half-Skip'' travel modes integrates a reflection at a back wall into the measurement and evaluation. It has recently been shown that one such method, the corner echo formed by the vertical crack plane and a plane back wall, can be used to decide whether a crack in concrete is a full-depth crack 24 , 26 . In another study, three techniques, SAFT/TFM, Plane-Wave Imaging, and Half-Skip TFM (HSTFM) 30 were evaluated to find the optimum methodology with respect to defect inclination.…”