Ultrasonic non-destructive assessment of bonding defects in composite structural strengthenings

A non-destructive investigation technique for bonding evaluation in external FRP strengthenings is presented. The technique is similar to the usual pulsed echo ultrasonic analyses, but bonding defects are located through the amplitude of the reflection echo rather than its delay time. This new technique has been experimentally demonstrated to be rather unaffected by variations in glue thickness, very common in FRP strengthenings of concrete and masonry structures. The effectiveness of the proposed technique has been then successfully tested with different composite materials (CFRP and GFRP), applied to different substrates (concrete, masonry, polyurethane). A theoretical model of the principle of operations has also been developed. Such a technique appears useful for both quality control of the application and in situ surveying.