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External Prestessing



 


External Prestressing

Technical Features of External Prestressing

External prestressing is characterized by the following features :

- The tension elements resp. prestressing tendons are placed on the outside of the physical cross-section ( mostly in concrete ) of the structure
- The forces exerted by the prestressing tendons are only transferred to the structure at the anchorages and at deviators
- No bond is present between the cable and the structure, unless at anchorages and deviators bond is intentionally created
- The application of external prestressing is indeed not bound to the use with concrete, but it can be combined with any construction material as composite materials, steel, timber, steel and concrete combined and other modern plastic materials. This can considerably widen the scope of the post-tensioning applications.
- Due to the absence of bond, it is also possible to restress, destress and exchange any external prestressing cable, provided that the structural detailing allows for these actions.


Advantages :

- Facilitating of concrete placing due to the absence of tendons in the webs, and ease for placing tendons.
- Improvement of conditions for tendon installation which can take place independently from the concrete works.
- Reduction of friction losses, because the unintentional angular changes, known as wobble, are practically eliminated. Furthermore, in case of the use of a polyethylene sheathing the friction coefficient is drastically reduced compared to standard internal and bonded prestressing using corrugated sheat metal ducts.
- Generally the webs can be made thinner, resulting in an overall lighter structure
- As an overall result, better concrete quality can be obtained leading to a more durable structure and facilitating execution.

Typical Applications for External Prestressing

Typical applications where external tendons are feasible, practical and economical, are:
- Repair work and strengthening of all kinds of structures
- Underslung structures - Precast segmental construction simple and continuous spans - Incremental launching procedure, in particular centric prestressing

The PBL multi strand anchorage systems can be used for external prestressing tendon

- The tendon is formed from plain 0.5 or 0.6 dia. strands according to ASTM 416 or BS standard
- The duct is from high density polyethylene and continuous from one anchorage to the other. The Tendon sheathing passes freely through intermediate diaphragms, respectively deviators, whereby a metal or HDPE sleeve provides the required reservation opening.
- A standard PBL multi strand anchorage assembly consisting of anchor head, wedges, anchor plate casting safely transfers the prestressing force to the end cross-beam
- The tendon is filled with cement grout after it has been tensioned. As no double pipe is provided at the anchor points through the end-diaphragms a certain bond between tendon and structure is created which hinders the replaceability. Depending on requirements, the anchor head may be protected by a cap, or alternatively the anchorage recess is filled with non-shrinkage concrete.


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PBL GROUP LTD. PBL Group Building 8/11 Soi Vipavadi 44 Vipavadi Rangsit Road, Jatujak, Bangkok 10900
Tel. 66(0)2562-0765-9, 66(0)2941-1692-3 Fax. 66(0)2562-0764 PBLGROUP@PBL-Thailand.com