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Ducts for post-tensioning system

Ducts are embedded inside a concrete structure to form a void for installing and prestressing post-tensioning strands after the concrete attains desired strength. ducts are manufactured from different materials such as steel and plastic. ducts type and material can change according to the location and purpose of use.






Duct types:


  • corrugated steel: corrugated steel ducts are made from steel strips that spirally wound to the required diameter. the thickness of ducts should not be less than 0.45 mm(26 gauges) for ducts with diameter less than 66 mm. and it should be 0.60 mm ( 24 gauges)for ducts with a bigger diameter. ducts should be sufficiently flexible to form the required profile without flattering or crimping. also, it should be mortar tight and galvanized. ducts shall be coupled and sealed properly to avoid blockage.
Figure 1

  • smooth, rigid steel pipe: this type of duct used for the external portion of post-tensioning ducts. for external tendons deviating. in a location such as segmental bridge and deviator locations as shown in figure 2. steel pipe is prefabricated to the required radius and curvature. steel pipe should conform to ASTM A53/A53M.



                            
Figure 2

  • corrugated plastic ducts: these ducts are used for internal tendons. it is fabricated from polyethylene or polypropylene. figure 3 showing plastic ducts.
Figure 3

  • corrugated plastic, tight radius duct: this type of ducts can be used as an alternative for steel pipe. it can be used for external tendons at deviation location, looped vertical tendons for precast and anchorage blisters. ducts used for internal post-tensioning can be coupled with straight ducts from the same materials to provide continuity from anchorage to anchorage. 

Duct size:
AASHTO 5.4.6.2 stating the duct cross-section area shall be twice the tendons area. with an exception for ducts placed by the pull-through method. for this case, the cross-section area should be 2.5 the stand tendons cross-section area.

for oval(flat) ducts. this ducts used for transverse post-tensioning. these ducts are made for 4 strands of 15.24 mm diameter.  but it can accommodate up to 5 strands. the minimum internal clear dimension shall be 25 mm vertically and 75 mm horizontally.

for single bar tendons. ducts diameter shall be 12 mm greater than the largest external bar or bar coupler dimension. 






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