Detection of Grout Voids in Post Tensioning Ducts with Sonic / ultrasonic Impact Echo Testing

Post tensioning used in the construction of bridges and buildings requires the tendon ducts be filled with grout after tensioning. The grout protects the steel strands from corrosion and provides a structural bond for the tendons. Studies conducted by Florida DOT determined that voids occurred in tendon duct grout more frequently than expected with an increased potential for tendon corrosion and failure.

NDT Corporation has successfully used sonic/ultrasonic impact echo testing to identify and map post tensioned duct voids in segmental, balanced cantilever, cast in place bridges and box beams. This technique illustrated below measures the thickness of concrete in which the tendon duct is located. The pulse-echo ray path time from wall to wall is a function of the compressional wave velocity and the element thickness. If there are no anomalous concrete conditions present such as voids, poor concrete etc. the thickness and the compressional velocity are normal. Where the duct (between the parallel concrete faces) is voided, the sonic wave is delayed due to the presence of the low velocity air void which bends the wave effectively increasing the sonic path length time and the “apparent thickness” of the element.

The size of the grout void and corrosion of the tendon, if any, is verified by drilling a small diameter (<1") hole to expose the duct and inserting a small diameter bore scope to video document internal duct conditions. A differential air pressure device is used to determine the approximate volume of the void. Pictured below are voided vertical tendon ducts with corroding tendons and voided draped tendons ducts with grout covered tendons.

The use of sonic/ultrasonic impact echo testing and bore scope video documentation provides bridge owners with a means to efficiently assess the condition of post tensioning and the information necessary to quantify quantities for repairs.

For additional information call NDT Corporation at 508-754-0417 or check our web site www.ndtcorporation.com.