Determinations of Void Volume
Where voids are discovered or suspected in post tensioned ducts, a simple pressure test using compressed air can be used to determine the approximate void volume.
The equipment consists of a pressure cylinder of known volume (approximately 8 litres) and an expanding plug to seal in the hole used to access the duct. The pressure cylinder is pressurised to approximately 1000 mb and released into the duct via the expanding plug in the access hole. The approximate void volume can be calculated from Boyle's Law, where the factor of pressure and volume (cylinder volume x 1000 mb) before releasing the air will equal the factor of pressure and volume (cylinder volume + void volume x resulting pressure) after release into the duct. The depth of the access hole and the volume of the flexible pipe used are also taken into account.
Sometimes a stable pressure is not achieved after release into the duct, usually caused from the leaking of air from the duct via the expanding plug or elsewhere within the duct. In this situation a data logger is used to record the pressure as the air is released into the duct and as it leaks away afterwards. A simple graph of pressure versus time can then be plotted to determine the pressure after release into the duct but before any leakage occurs. This pressure would then be used to calculate void volume and the leak would not be of concern.
When interpreting the significance of the voids it may be useful to adopt a crude classification which is generally accepted:
|< 0.5 litres||negligible|
|up to 5 litres||small|
|up to 15 litres||moderate|
In any circumstance it is the presence of corrosion, which may occur in the void, which is a cause for concern. A small sealed void where the tendons are covered by a film of grout would not normally be considered a problem.