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Unbonded Post Tensioning
Unbonded Flat Slab Anchors

Unbonded Flat Slab Anchors

Unbonded Flat Slab Anchors


Unbonded Flat Slab pc.strandsUnbonded Flat Slab pc.strands
Unbonded Flat Slab pc.strands


Unbonded Post-Tensioning


Unbonded post-tensioned concrete

Unbonded post-tensioned concrete differs from bonded post-tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this, each individual tendon is coated with a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). The advantages of this system over bonded post-tensioning are:
  • The ability to individually adjust cables based on poor field conditions (For example: shifting a group of 4 cables around an opening by placing 2 to either side).
  • The procedure of post-stress grouting is eliminated.
  • The ability to de-stress the tendons before attempting repair work.

Picture number one shows rolls of post-tensioning (PT) cables with the holding end anchors displayed. The holding end anchors are fastened to rebar placed above and below the cable and buried in the concrete locking that end. Pictures numbered two, three and four shows a series of black pulling end anchors from the rear along the floor edge form. Rebar is placed above and below the cable both in front and behind the face of the pulling end anchor. The above and below placement of the rebar can be seen in picture number three and the placement of the rebar in front and behind can be seen in picture number four. The blue cable seen in picture number four is electrical conduit. Picture number five shows the plastic sheathing stripped from the ends of the post-tensioning cables before placement through the pulling end anchors. Picture number six shows the post-tensioning cables in place for concrete pouring. The plastic sheathing has been removed from the end of the cable and the cable has been pushed through the black pulling end anchor attached to the inside of the concrete floor side form. The greased cable can be seen protruding from the concrete floor side form. Pictures seven and eight show the post-tensioning cables protruding from the poured concrete floor. After the concrete floor has been poured and has set for about a week, the cable ends will be pulled with a hydraulic jack, shown in picture number nine, until it is stretched to achieve the specified tension.


Rolls of post-tensioning cablesPulling anchors for post-tensioning cablesPulling anchors for post-tensioning cables
Pulling anchors for post-tensioning cablesPost-tensioning cables stripped for placement in pulling anchorsPositioned post-tensioning cables
Post-tensioning cable ends extending from freshly poured concretePost-tensioning cable ends extending from concrete slabHydraulic jack for tensioning cables


NOOR Post-Tension is the largest supplier of fabricated unbonded post-tensioning materials and services in the United States. Unbonded post-tensioning is used in a variety of residential and commercial cast-in-place concrete building structures. Any floor system, whether in an elevated building or a ground supported foundation, can utilize a post-tensioned reinforcing system; from single family residences to industrial flooring, apartment buildings to sports courts, high rises to parking garages, and everything else in between



Post-Tensioning Systems


Unbonded vs. Bonded


Unbonded


  • Economical
  • Greater layout flexibility
  • Force transmitted solely by the anchors
  • Total force limited by anchor spacing
  • Retrofit openings require more care
  • Replaceable
  • Simple stressing equipment


Bonded


  • Can be more costly due to duct placement & grouting
  • Force transmitted by anchors and bond to concrete
  • Greater total force can be applied
  • Strain compatibility with concrete
  • Openings less difficult
  • Minimizes need for non-prestressed reinforcement
  • More complex stressing equipment required


Myths about P/T


  • P/T Concrete is Crack Free
  • P/T Concrete is Water Proof
  • You Cannot Drill/Make Openings in P/T Slab
  • If You Drill Into a Tendon, it will Fly Out of the Building
  • It is Impossible to Upgrade/Repair a P/T Structure
  • P/T Structures are Not Durable


Comparison with Structural Steel Buildings


  • Reduced structural depth for lower story heights, smaller column and foundation sizes
  • Savings in labor and materials
  • Mechanical
  • Electrical
  • Elevator
  • Cladding