Prestressed Concrete
According to ACI committee on Prestressed concrete, Prestressed concrete is the one in which there have been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are contracted to a desired degree.
In the Reinforced cement concrete member, the prestress is commonly introduced by tensioning the steel reinforcement.
Thus, the prestressing is the intentional creation of permanent stresses in a structure or assembly for the purpose of improving its behaviour and strength under various service conditions.
Prestressing is the initial application of stresses, controlled in magnitude and direction, to a structural member to counteract the undesirable stresses due to working loads.
Types of prestressing in Concrete
1) External prestressing
2) Internal prestressing
3) Linear prestressing
4) Circular prestressing
5) Pretensioning
6) Post tensioning
7) Full prestressing
8) Partial prestressing
External Prestressing -- This method of prestressing is achieved by external reactions introduced through different support conditions.
Internal Prestressing -- Internal prestressing is obtained by tensioning the steel reinforcement. It is most common because greater degree of accuracy can be achieved in execution.
Linear prestressing -- It is applied in straight members such as beams, slabs, piles, electric poles , etc.
Circular prestressing -- It is applied to circular structure such as tanks, bunkers, silos,etc. where the prestressing tendons are wound in circles.
Pretensioning -- It is that method of prestressing in which tendons are prestressed before the concrete is placed.
Post tensioning -- In this method, the member is concreted and a duct is formed in the member either with a tube or with a metal sheath. when concrete is sufficiently hard, tendon consisting of either a bar, cable, or strand is threaded through the duct; tensioned and anchored to concrete member at its ends.
Full prestressing -- The concrete is said to be fully prestressed if there are no tensile stresses in it under working load.
Partial prestressing -- If some tensile stresses are produced in the concrete under working load conditions, it is said to be partially prestressed.
