Steel Corrugated Duct for Bonded PT
Duct is an important element for bonded post tensioning work. It is essential as it creates a void for the tendon or tensile element. By and large, steel corrugated duct provides the most economical solution to bonded post tensioning work.
Steel corrugated duct are manufactured from a long galvanized strip of steel, normally in the range of 0.25 – 0.40mm in thickness and can be 30 – 80mm in width. It depends on the type of duct forming machine used. These strips are then pressed into a corrugated form before they are fed into a turning wheel to make the edges interlock, and forming into a pipe. And in post tensioning terms, we call this pipe a ‘duct’. And duct can come in 2 types of shapes, round and flat. A steel corrugated flat duct is pressed into shape from a steel corrugated round duct and it is generally used for slab anchorage (not limited).
Duct plays a critical role in providing a corrosion protection environment to the tensile element or tendon (eg. PC Strand). And steel corrugated duct offers excellent bonding behaviour to tendon grout and also the concrete around it. And it is precisely because the duct is a protection to the tendon, its quality is paramount.
Due to the way the steel corrugated duct is produced, bad quality duct can cause huge problems like ‘bleeding’ during the construction process. Most of the time, it is the bad interlocking of the edges between adjacent strips that causes the duct to collapse under the weight of the concrete. This results the concrete to leak into the duct (term ‘bleeding’) and bond with the tendon even before stressing commence.
Operators are well aware of the repercussion bleeding can cause. During stressing, it causes the tendon elongation to fall under the required length even though the stressing jack and pump had worked to its required load. To sum it up, we call this prestressing lost (due to frictional effect). This is a situation everybody wants to avoid.
Duct is also available in plastic form, generally made of polyethylene / polypropylene material. Plastic duct has lesser bleeding issue and it is more commonly found in corrosive environment like waste water treatment tanks, acid tanks, silos etc. However steel corrugated duct is still the preferred choice for providing a homogenous environment to prestressed concrete structure, primarily due to its excellent bonding characteristic. And in some countries, plastic duct are barred because of fire safety regulation. This is because the burning of plastic during a fire hazard can emit poisonous gases and it is harmful to humans.
TMG visited a few project sites and with the blessing of the main contractors, we managed to film the process of steel corrugated duct production. The source of the duct forming machine remains unknown.