Ultrasonic C-scan imaging preliminary evaluation for corrosion and void detection in posttensioned tendons

Corrosion of the nation's transportation infrastructure is a widespread and costly problem. The most prevalent durability issue in reinforced concrete structures is chloride-induced corrosion of the reinforcing steel. A reliable method of determining grout voids and corrosion levels in posttensioned bridge structures is needed. Traditional techniques of corrosion monitoring (e.g., half-cell potential and corrosion rate measurement) are problematic when used in this type of structure, as are standard nondestructive evaluation (NDE) methods, such as impact echo. C-scan imaging, an ultrasonic technique used primarily in the composites industry for detecting delamination, is examined as a method of evaluating grouted posttensioned tendons. This method exhibits many promising qualities: it can be used for internal or external tendons and on metal or plastic ducts; access to only one side of a specimen is required; strong imaging allows easy interpretation of results; the technique poses no risk to users or the environment; and the method has strong potential for development as a handheld field tool. The C-scan technique may be valuable for the investigation of not only posttensioning applications but other types of reinforced concrete structures as well. Results of preliminary investigations on lab specimens indicate that the C-scan technique holds promise. The ultimate goal of the research is to provide a user-friendly, robust system for the NDE of posttensioned tendons for voids, corrosion, and wire breaks. Recommendations for optimal acquisition and processing techniques as well as for the future development of the equipment as a field tool are proposed.