Detection capability of NDT methods in steel-concrete-steel composite elements

Steel concrete steel (SC) is a structural typology and construction method. Continuous steel plates are used on the surfaces of concrete walls/slabs, having both the roles of formwork and tensile reinforcement. The steel plates completely replace the longitudinal reinforcement, but shear reinforcement may still be present. SC gained popularity in industrial applications, with the notable use in modules of the AP-1000 nuclear power plant (NPP). The presence of thick continuous steel plates on both sides of the construction elements hinders the use of many traditional non-destructive evaluation (NDE) techniques to assess concrete quality or damage. In fact, reliable and cost effective assessment of structures is an outstanding challenge to the use of SC construction in nuclear applications. To understand the limitations and possibilities of NDE techniques, several tests were performed on full scale SC elements. The elements were approximately 0.8x0.8x9.0m beams, tested earlier to static loads and with understood damage of the concrete. The NDE methodologies applied were Ultrasound 3D tomographer, Impact-Echo, Impulse-Response, Ground Penetrating Radar, Betatron and Ultrasonic testing. The objective was to detect cracks of different sizes and configurations in the beams. Width ranges of 0.1…1 mm were studied, on cracks perpendicular or oblique to the surface, generated by pure bending and combined bending and shear. Both crack types may be present in NPP walls/floors damaged by accidental overload. The results show that Ultrasound 3D tomographer (as well as the other ultrasonic testing devices) and Ground Penetrating Radar are not suitable for testing trough the steel surface, due to loss of the transmitted energy in the steel plates. Impact-Echo testing revealed promising results whereas Impulse-Response is suitable for detecting de-bonding of the steel plate from the concrete. Radiographic testing proved to be applicable in cases when the cracks had sufficient opening and length close enough to the film side.