Innovation In Structural Investigation

GBG Structural Services

Materials Testing and Structural Investigations




The Inspection of Post-tensioned Ducts


Post tensioned ducts are usually located using information from existing drawings although radar and even covermeters can be used in instances where drawings are not available.

Access to the ducts is made using a 25mm drill bit on an electrically powered percussion drill. Caution is used so as not to damage the post tensioning cables. A skilled technician can usually feel when the drill bit makes contact with the duct sheath and can stop the drill before making contact with the cables.

On contact with the duct a pressure test can be carried out to determine the volume of any voids in the grout, if present (see other Appendix for details). Grout is then sampled using a hammer and chisel and the exposed cables inspected using a rigid endoscope.

The access hole is overcored or broken out if difficulty is experienced retrieving sufficient grout or a closer inspection is required (especially in the case of a voided duct which requires inspection with a video endoscope).

Inspection of Post Tensioned Anchorages

The encasement concrete to post tensioned anchorages can be removed using either high-pressure water jetting or percussive breaking tools.

High pressure water jetting can be used to expose anchorages which have a greater amount of concrete cover or in locations were reinforcement congestion is high. Water jetting can avoid the cutting of reinforcement to gain access but does tend to remove any evidence of light corrosion on the anchorages.

When exposed, anchorages are inspected for condition and defects and dimensioned to confirm the compliance with original drawings. Holes used to grout the ducts from the anchorage plate are also inspected for any voids.


Holes drilled to post tensioned ducts and anchorage encasement are usually reinstated using a suitable proprietary repair mortar.

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