In post-tensioning system tendon forms a continuous bond along its length with surrounding concrete.
Steel (corrugated) ducts are placed in formworks before grouting. Then tendons are threaded into the duct and fixed at its ends by anchors. After the concrete has been poured and has reached required strength, tendons can be stressed. Stressing can be controlled through elongation measurement. When tendons are stressed, they are fixed in their position by wedges, keeping the specified force. Then the grout is injected into the voids of tendon ducts. The bond of tendon and concrete is realized due to this operation. In fully hardened structure the force of stressed tendon is transferred to the concrete.
This system requires the use of high strength bare steel monostrands coated with anticorrosion grease. They consist of seven cold-drawn wires, where 6 external wires spirally wrap one central (king) wire.
• to maintain a voided path for the strands in the concrete member during construction • to transfer the bond between the grout within the duct and the concrete surrounding • оto act as additional protection against penetration of moisture and chemicals into the interior of the duct
The flat ducts housing 4-5 tendons are for the use in thin members, such as slabs. The strands generally share a common anchor, but are stressed and locked off individually.
The larger round ducts are for application in deep members. In this case all strands of one duct are stressed and locked off simultaneously using a specially designed multistrand jack.
• to provide a continuous bond between the strand and the duct; • to increase protection against corrosion by acting as a barrier to moisture penetration;
• through its alkalinity, provide an environment non-conducive for corrosion.
Bonded post-tensioning system is widely used in bridges and transport structures, as well as in construction of heavy loaded buildings.
Application of this prestressing system in large structure members such as, for example, beams, allow increasing the length of span and the capacity, as well as reducing the construction schedule and working labor.