Sensitivity Studies of Grout Defects in Posttensioned Bridge Ducts Using Impact Echo Scanning Method

Findings are presented from an NCHRP Innovations Deserving Exploratory Analysis Program project, Nondestructive Evaluation Method for Determination of Internal Grout Conditions Inside Bridge Posttensioning Ducts Using Rolling Stress Waves for Continuous Scanning. The study involved a defect sensitivity study of an impact echo scanner to detect and image discontinuities in posttensioned ducts of a mock-up U-shaped bridge girder and a mock-up slab. Various sizes of ducts were included in this study as well as various sizes of void defects. A detailed sensitivity study of nondestructive grout defect detection with impact echo scanning of eight ducts 4 in. in diameter with constructed defects was the main focus in the study. Comparisons of the impact echo defect interpretation and the actual design conditions of the ducts inside the bridge girder or slab are presented. The impact echo results are presented three-dimensionally by using thickness surface plots to provide improved visualization and interpretation of the internal grout to void defect conditions inside the ducts of the girder. The impact echo tests were performed with a scanner, which greatly facilitates the impact echo test process by allowing for rapid, near continuous testing and true scanning capabilities to test concrete structures. The general background of the impact echo technique and the impact echo scanner is summarized. Descriptions of two mock-up specimens used in the experiment and the discussion of the results from the impact echo scanner are presented.