Applications and Limitations of Impact Echo Scanning for Void Detection in Posttensioned Bridge Ducts

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Findings are summarized from an internal research study to determine applications and limitations of the impact echo scanning (IES) system for void detection in posttensioned bridge ducts. The nominal thickness of the test panels ranged from 0.2 to 0.7 m with plastic and steel ducts inside. The diameter of the ducts ranged from 50 to 120 mm. One tapered section (with wall thicknesses between 0.2 to 0.7 m) had draped ducts, with the rest being straight ducts. Impact echo (IE) testing was performed using the IE scanner system. This system, as rolled, automatically performs an IE test at 25-mm (1-in.) intervals. All IE scans in this investigation were performed on lines perpendicular to the general posttensioned duct direction. The maximum frequency of excitation of the impactor in the scanner used in research is 25 kHz. This internal research is an expanded study from a previous research project funded by the NCHRP-IDEA program titled Nondestructive Evaluation Method for Determination of Internal Grout Conditions Inside Bridge Posttensioning Ducts Using Rolling Stress Waves for Continuous Scanning. The purpose of the study was to better understand the nominal thickness limitation of the structures for IE tests, using a scanner with a maximum excitation of 25 kHz to correctly identified voids in posttensioned bridge ducts.