The load capacity of post-tensioned elements is governed by the presstress applied from the tensioned steel tendons. The durability of the embedded tendons can be affected where tendon ducts are not adequately filled with grout or grease and a pathway exists for moisture to enter the ducts and cause the tendons to corrode. In extreme cases, failure of tendons due to corrosion can lead to sudden collapse without warning. It is important therefore to periodically check the condition of the tendons and anchorages.
Duct Location – NDT
Post tensioned ducts are usually located using ground penetrating radar, utilising information from record drawings where available drawings.
Tendon Inspection
Access to the ducts is made using a masonry drill bit on a percussion drill. Our skilled technicians can can feel when the drill bit makes contact with the duct sheath and can stop the drill before penetrating the duct. The duct can then be carefully opened up with hand tools to allow inspection of the tendons .
Anchorage Inspection
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.
Void Volume Measurement
Pressure test can be carried out to estimate the volume of any voids encountered in the duct at the inspection location.