NDE of Grouted Post-Tensioned Tendons

Paul S. Fisk, President NDT Corporation

Void Detection in Post-Tensioned Ducts

Sound waves pass through full PT ducts but deflect and slow as they pass around voided PT ducts.

Post-tensioned bridge tendons are normally filled by cementitious grouting after tendon stressing. In addition to bonding the strands in place, the grout provides an important role in the structure durability by protecting the post-tensioned tendon from corrosion. Unfortunately, many structures are suffering from grout voids, segregated grout and soft grout with elevated levels of sulfates that can cause corrosion.

NDT Corporation's sonic/ultrasonic test method has been proven effective in locating voids and soft grout inside post-tensioned tension ducts. A stress wave is created on the surface of the structure by a metal projectile and the resonant frequency measured. Sonic/ultrasonic testing can detect a range of anomalies including voids, soft grout, soft grout layer surrounding hard grout, duct corrosion and honeycombed concrete. Sonic/ultrasonic testing, in conjunction with video borescope evaluation and chemical testing of grout samples, provides key data to enable engineers to select and quantify repair strategies such as vacuum grouting and corrosion mitigation by the post-tension impregnation process.

The technique has been successfully employed for bridges for many Departments of Transportation such as Florida, Kansas, Massachusetts, Rhode Island, and Virginia. for more information, contact NDT Corporation.

Post-Tension Bridge Evaluation Case Study: Jamestown Verrazzano Bridge.

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