Nondestructive Assessment of FRP-Concrete Bond

Acoustic testing and thermal imaging are widely used methods for evaluating FRP-concrete bond. This study assessed the performance of these methods by comparing predictions of good or bad bond in newly wrapped specimens against measured values subsequently obtained at the same locations from destructive pull-off tests. To ensure consistency, acoustic tests were conducted using an instrumented swing hammer with a fixed drop height. This meant that the same force was applied at each test location. Though many commercial thermal imaging systems are available, for the precision required in detecting bond problems they are costly. For this reason, USF developed a Depth Encoder-Infrared Thermocouple (DEIT) system that can achieve similar results but at a fraction of the cost. The DEIT system was custom-built to obtain quantitative information on the size and location of the suspected areas of poor bond in the prestressed concrete pile specimens evaluated. It operates like a scanner where a strip of high intensity light is passed over a page directly adjacent to an array of light receptors. DEIT uses a 500W strip-type heat lamp in combination with a line of ten infrared detectors to measure surface temperature. Destructive pull-off tests were conducted on full-size pile specimens where the FRP had been applied to dry and wet surfaces using pressure vacuum bagging. The intent was to establish the relative merits of these systems compared to applications were no pressure was applied. Results showed that by and large acoustic testing and thermal imaging gave comparable results when evaluating bond in newly applied FRP.

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