Why Post-tensioning and Pre-tensioning?

Why Post-Tensioning?

The use of post-tensioning allows engineers to use thinner concrete sections, longer spans, stiffer to resist lateral loads and resist the effects of shrinkage and swelling.
Concrete has ‘compressive’ strength i.e it can carry its own weight within a structure. And when ‘live’ loads such as vehicles on a bridge are applied the concrete tends to deflect or sag which leads to cracking, thus weakening the structure.

Why Post-Tensioning is done?

We know Concrete lacks ‘tensile’ strength. That’s why steel reinforcing bars – ‘rebar’ – are often embedded in the concrete to limit the width of cracks. However, rebar provides only passive reinforcement – that is, it does not bear any load or force until the concrete has already cracked.
Now, this is the time when Post Tensioning comes into act. Post-Tensioning systems provide active reinforcement. The function of post-tensioning is to place the concrete structure under compression in those regions where load causes tensile stress. Post-tensioning applies a compressive stress to the material, which offsets the tensile stress the concrete might face under loading.

Post-Tensioning

How it's actually done?

Post-tensioned bridge decks are generally composed of in situ concrete in which ducts have been cast in the required positions. When the concrete has acquired sufficient strength, the tendons are threaded through the ducts and tensioned by hydraulic jacks acting against the ends of the member. The ends of the tendons are then anchored.
Tendons are then bonded to the concrete by injecting grout into the ducts after the stressing has been completed.
It is possible to use pre-cast concrete units which are post-tensioned together on site to form the bridge deck.
Generally, it is more economical to use post-tensioned construction for continuous structures rather than in-situ reinforced concrete at spans greater than 20 meters. For simply supported spans it may be economic to use a post-tensioned deck at spans greater than 20 meters.

Advantages.

It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed
Cracks that do form are held tightly together
It allows slabs and other structural members to be thinner
It allows us to build slabs on expansive or soft soils
It lets us design longer spans in elevated members, like floors or beams

Common Applications.

Post-tensioning, or PT, has become increasingly popular over the past 30 years or so as the technology has been perfected. At one time there were problems with corrosion of the cables, especially in deicing-salt-laden parking structures, but better materials and construction methods (plus good training and certification programs) have eliminated most problems.

What is Pre-tensioning?

Pre-tensioning The tension is applied to the tendons before casting of the concrete. The precompression is transmitted from steel to concrete through bond over the transmission length near the ends.

Why Pre-tensioning?

We know that concrete is good for compression and develops cracks when tensile strength is applied. 

The basic concept behind it is rebars are excellent in tension and when we apply tension to rebars and release, it applies compression to the concrete. So we will definitely look forward to compression as much as possible but sometimes concrete is subjected to tension.

For example, any cantilever section hanging out on one end and other end attached to the column will get subjected to tension at the hanging out end as it will be pushed down. Now imagine if we Pre-tension it, the tensile strength of the rebars will apply the compression force to the concrete.

Pre-Tensioning.

How Pre-tensioning is done?

Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete. The concrete is then placed and tendons become bonded to concrete throughout their length. After the concrete has hardened, the tendons are released by cutting them at the anchorages. The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. The tendons are usually stressed by the use of hydraulic jacks. The stress in tendons is maintained during the placing and curing of concrete by anchoring the ends of the tendons to abutments that may be as much as 200m apart. The abutments and other formwork used in this procedure are called prestressing bench or bed.