Measurement of Concrete Thickness and Detection of Defects Using Ultrasound Methods

The use of Non-destructive Evaluation (NDE) techniques has proven to be effective in determining the thickness of plate like concrete elements and locating defects such as cracks, delaminations, voids, honeycombing, and debonding. Some of the problems that have plagued the concrete paving industry are testing for quality assurance and verification of thickness of existing concrete slabs without compromising their structural integrity by traditional coring methods. There are several Non-Destructive Evaluation (NDE) techniques that have been proven to be an effective way to combat these problems. This paper outlines one of these techniques, Ultrasound-Echo (UE). UE utilizes two ultrasonic probes, one transmits a stress wave field into the concrete specimen, and the other receives the natural response of the concrete specimen in the time domain. A Fast Fourier Transform (FFT) is then applied to the received response so it can be examined in the Frequency Domain. Using the wave speed of the concrete, the peak or resonant frequency can be used to calculate the thickness of the concrete element where the concrete element is free of defects. In areas of detectable defects, there will be pronounced shift in the resonant frequency. Requirements for self calibration of the wave speed and testing procedures for the ultrasound test equipment are discussed. To verify the accuracy of UE to identify defects, a test slab with purposebuilt delaminations and air voids at known locations was constructed. A comprehensive measurement of this special test slab was performed using an UE device, and analyzed results are presented. Defective sections of the test slab are clearly identified by the UE method with recommendation given for detection of the defects.