Grout bleeding and/or incomplete filling of tendon ducts can result in the creation of voids within the sleeves of PT tendons, increasing the chance for corrosion and the likelihood of failure.

Ensuring the quality & long-lasting performance of a concrete structure is of the utmost importance. Over the lifespan of a large structure, you want to avoid any major surprises or unexpected expenses and ensure its longevity.

In this instance, our client raised some concerns about the quality of a relatively new structure where post-tension (PT) cables were used in most load bearing walls. They wanted to non-destructively investigate the PT tendons to ensure that the installation had been completed to the specifications by their subcontractor.

Xradar was specifically hired owing to the challenging nature of the PT tendons and associated voids due to either bleeding grout or incomplete filling of the ducts.

-Tensioned tendons in the beams were first located by Xradar enhanced GPR, and then imaged by ultrasound echo tomography across the tendon every 100 mm.

Knowing the location of the reinforcement and the tendon ducts was essential in order to carry out an effective ultrasound investigation. Ultrasound pulse echo tomography (UPE) is based on stress-wave propagation and reflection. Since stress waves propagate through metal (and concrete), the technique is widely used to detect voids in heavily reinforced concrete. However, possible interferences and additional reflections occur from various interfaces within the concrete. To avoid confusion, the precise location of the tendon ducts was identified using Xradar enhanced GPR.

Ultrasound Pulse Echo Tomography was used in several predetermined areas to inspect the tendon ducts for potential associated voids. Data was collected at 10cm intervals along each duct to create a complete profile. In many locations, probable voids were detected and marked on site. Some of these imperfections were verified by carefully drilling into the ducts. The findings indicated sporadic issues throughout the survey area, each of which had to be filled and sealed.

Composite B-scan made from consecutive UPE images aligned on a tendon. Echoes (red) seen at ~50 mm depth come from surface irregularities. Those identified at ~350 mm show the back side of the beam.

The black arrow indicates the drill hole confirming the void in Tendon 1. The tendon sleeve was completely hollow (no grout). The orange line denotes the location of the void.