Integrating Complementary NDT Methods for Subsurface Void Detection in Reinforced Concrete: A Field Case Study

Accurate detection of internal air voids within reinforced concrete is a persistent challenge in non-destructive testing (NDT), particularly in heavily reinforced or cast-in-place elements. This paper presents a combined NDT approach implemented on a major industrial infrastructure project in North Africa, designed to evaluate potential entrapped air voids within turbine pedestal foundations. The testing methodology employed synchronized line scans using Pulse Echo Ultrasonics and Magnetometric Rebar Detection to overcome the interpretive ambiguity often associated with ultrasonic reflectors. Since both rebar and voids can create echo signatures in pulse-based systems, the rebar mapping data was used to isolate echo responses not attributable to reinforcement. These suspect areas were further verified through borescope inspection via targeted drilling, confirming the presence of entrapped voids. The aim of the investigation was to determine whether inadequate vibration during casting had resulted in defects requiring structural remediation or if superficial treatment would suffice. The case demonstrates the importance of using multi-method NDT protocols to achieve high diagnostic certainty in structural integrity assessments. The paper outlines the workflow, limitations of each technique, signal interpretation strategy, and practical recommendations for similar applications in infrastructure settings. Acknowledgment: The author acknowledges the contribution of Mr. Georgios Karydis, who acted as supervising engineer for the overall inspection project.