Post Tensioning Systems
Post Tensioning Systems
Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast.
The tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete. Once the concrete has been cast and set, the tendons are tensioned (“stressed”) by pulling the tendon ends through the anchorages while pressing against the concrete. The large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is “locked-off” at the anchorage. The method of locking the tendon-ends to the anchorage is dependent upon the tendon composition, with the most common systems being “button-head” anchoring (for wire tendons), split-wedge anchoring (for strand tendons), and threaded anchoring (for bar tendons).
Tendon encapsulation systems are constructed from plastic or galvanised steel materials, and are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing (bonded post-tensioning); and those where the tendon element is permanently debonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands (unbondedpost-tensioning).
Casting the tendon ducts/sleeves into the concrete before any tensioning occurs allows them to be readily “profiled” to any desired shape including incorporating vertical and/or horizontal curvature. When the tendons are tensioned, this profiling results in reaction forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure
Types of Post Tensioned Systems
BONDED POST TENSIONED SYSTEM
Bonded post-tensioning has prestressing tendons permanently bonded to the surrounding concrete by the in situ grouting of their encapsulating ducting following tendon tensioning. This grouting is undertaken for three main purposes: to protect the tendons against corrosion; to permanently “lock-in” the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain structural behaviors of the final concrete structure.
Bonded post-tensioning characteristically uses tendons each comprising bundles of elements (e.g. strands or wires) placed inside a single tendon duct, with the exception of bars which are mostly used unbundled. This bundling make for more efficient tendon installation and grouting processes, since each complete tendon requires only one set of end-anchorages and one grouting operation. Ducting is fabricated from a durable and corrosion-resistant material such as plastic (e.g. polyethylene) or galvanised steel, and can be either round or rectangular/oval in cross-section. The tendon sizes used are highly dependent upon the application, ranging from building works typically using between 2-strands and 6-strands per tendon, to specialised dam works using up to 91-strands per tendon.
Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end-anchorages to formwork, placing the tendon ducting to the required curvature profiles, and reeving (or threading) the strands or wires through the ducting. Following concreting and tensioning, the ducts are pressure-grouted and the tendon stressing-ends sealed against corrosion.
COMPONENTS OF BONDED POST TENSIONED SYSTEM
SITE IMAGE OF BONDED POST TENSIONED SYSTEM
Among the advantages of this system over unbonded post-tensioning are:
Reduced reliance on end-anchorage integrity
Following tensioning and grouting, bonded tendons are connected to the surrounding concrete along their full length by high-strength grout. Once cured, this grout can transfer the full tendon tension force to the concrete within a very short distance (approximately 1 metre). As a result, any inadvertent severing of the tendon or failure of an end anchorage has only a very localised impact on tendon performance, and almost never results in tendon ejection from the anchorage.
Increased ultimate strengthin flexure
With bonded post-tensioning, any flexure of the structure gets directly resisted by tendon strains at that same location (i.e. no strain re-distribution occurs). This results in significantly higher tensile strains in the tendons than if they were unbonded, allowing their full yield strength to be realised, and producing a higher ultimate load capacity.
In the presence of concrete cracking, bonded tendons respond similarly to conventional reinforcement (rebar). With the tendons fixed to the concrete at each side of the crack, greater crack-expansion resistance is offered than with unbonded tendons, allowing many design codes to specify reduced reinforcement requirements for bonded post-tensioning.
Improved fire performance
The absence of strain redistribution in bonded tendons may limit the impact that any localised overheating has on the overall structure. As a result, bonded structures may display a higher capacity to resist fire conditions than unbonded ones
UN BONDED POST TENSIONED SYSTEM
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 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 de-stress itself and burst out of the slab if damaged (such as during repair on the slab).
SITE IMAGE OF UNBONDED POST TENSIONED SYSTEM
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 on each side).
The procedure of post-stress grouting is eliminated.
The ability to de-stress the tendons before attempting repair work.