Effectiveness of Impact Echo testing in detecting flaws in prestressed concrete slabs

This paper presents application of Impact Echo (IE) technique for evaluation of unsound concrete in the anchorage zones of a post-tensioned (PT) slab in a high rise building. The L-shaped area where the problem occurred was located at the interface of prestressed concrete beams and a depressed floor slab. Single point IE evaluation was used as the Non-Destructive Testing (NDT) evaluation method due to its portability and recent success in similar applications. Repair scheme for the affected portions was devised based on the results of IE investigations. Majority (84%) of the tested locations were classified as Good or Fair and required no further investigation/repair. The remaining 16% of the locations that were identified as Questionable or Poor by IE testing were manually investigated by field chipping. These destructive field investigations revealed that the accuracy of fault prediction by single point IE method in the current application was around 60% for the points that were classified as Questionable/Poor. Adjustment to the commonly used geometric correction factor (K) in IE testing by ±5% improved this accuracy to 81%. This finding points to the need of adopting a revised geometric correction factor in IE testing of actual structural elements under field conditions.