Title:
Influences of Reinforcing Bars on Crack Depth Measurement by Stress Waves
Author(s):
Yiching Lin, Tzonghow Liou, and Chiamen Hsiao
Publication:
Materials Journal
Volume:
95
Issue:
4
Appears on pages(s):
407-418
Keywords:
concrete; crack depth; nondestructive testing; steel reinforcing bars;
stress waves;
DOI:
10.14359/383
Date:
7/1/1998
Abstract:
A research program was undertaken to evaluate the influences of steel reinforcing bars on signals obtained from depth measurements of surface-opening cracks in reinforced concrete structures based on transient stress waves generated by elastic impact. Numerical studies were performed to gain an understanding of the interaction of transient stress waves with steel bars in concrete structures containing surface-opening cracks. A reinforced concrete beam was constructed as an experimental specimen. The specimen was simply supported and loaded at the center point until cracks occurred. Experimental studies were carried out on the cracked beam. Two displacement receivers were placed on the opposite sides of a surface-opening crack to monitor disturbances caused by the arrivals of stress waves generated by impact. The first receiver and impact were located on the same side of the crack. The first receiver was used to trace the start time of impact. The second one placed on the opposite side of the crack was used to find the arrival time of P-wave propagating through steel bars and the arrival time of the diffracted P-wave from the bottom edge of the crack. Numerical and experimental results show that the presence of steel bars results in an initial disturbance in the waveform recorded by the second receiver because P-wave can propagate across the crack along the steel bars. It is also shown that the shortest travel path for a P-wave from the impact point through the steel bars to the second receiver is obtained if the P-wave is incident upon the concrete/steel interface with a critical angle of incidence. Equations for determining the depth of steel bars are presented. Because the amplitude of the initial disturbance caused by the steel bars is small, it is still easy to identify the following arrival of the P-wave diffracted from the bottom edge of the crack. Thus, the presence of steel reinforcing bars does not have significant influences on measuring the crack depth.