Post tensioning certifier UK CARES has highlighted the perils of ungrouted ducts in post tensioned slabs and estimated that approximately 1% of ducts are either partially or completely ungrouted.
"Poor workmanship is generally the cause," said a UK CARES spokesman.
"If people do the job properly, they should check the duct is clear of blockages and undamaged and they shouldn't be grouting in cold weather. Unskilled people may not know what to do when a blockage occurs"
The consequences of ungrouted or poorly grouted tendons may include strand corrosion, reduced shear and flexure capacity due to unbonded behaviour and unpredictable behaviour during demolition.
"The percentage of projects found with no grout is very low and it doesn't occur in most companies," said Ben Ume, a director for post tensioning design specialist Matthew Consultants.
"Also, most cases of ungrouted tendons are found before the building is handed over,” added Ume.
“My advice to clients with bonded tendons would be to use a Post Tensioning Association (PTA) registered company."
The PTA represents post tensioning companies and designers and promotes best practice.
Tests can be carried out on existing buildings by ultrasound, x-rays or by drilling holes, according to UK CARES and should be considered for existing post tensioned buildings where there is low confidence in the installation or if the building is in an aggressive environment.