This paper summarizes recent developments in the impact-echo technique for determining the depth of surface-opening cracks in reinforced concrete beams and detecting voids in grouted post- tensioning tendon ducts. The impact-echo technique is a nondestructive testing method for evaluating concrete integrity based on stress wave propagation. A brief introduction to the impact-echo principle is given. Analysis of impact-echo signals is explained. Laboratory specimens containing a reinforced concrete (RC) beam and a slab were constructed. The RC beam was loaded at the center point until cracks occurred. The cracked beam was used to investigate the influence of the presence of reinforcing bars on measuring the depth of surface-opening cracks. The slab specimen was used to study the difference in impact-echo responses between the fully and partially grouted ducts embedded in the slab. To measure the depth of a surface- opening crack, two receivers were used and located on the opposite sides of the crack to monitor signals caused by the arrivals of stress waves generated by impact. Signal analysis was performed in the time domain. To locate voids in ducts, only one receiver close to the impact point was used and frequency analysis of impact-echo signals was performed. For the crack case, experimental results show that the depth of surface-opening cracks can be easily determined without reinforcing bars. It is also shown that the presence of reinforcing bars does not have significant effects on measuring the crack depth. For the duct case, it is shown that the presence of voids in ducts disrupts the impact-echo spectral pattern associated with the fully grouted ducts. Such a change in the spectral pattern is the key to detecting voids in grouted ducts and the peak frequency in the spectrum caused by multiple wave reflections from the void can be used to locate the void.
Publication Source: Trends in NDE Science & Technology; Proceedings of the 14th World Conference on Non-Destructive Testing, New Delhi, 8-13 December 1996.Vol. 2, pages 831 - 834 Publisher:Ashgate Publishing Company
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