What Is Prestressed?
Although steel takes up to the tensile stresses, the concrete at the tensile zone develops minute cracks. The load-carrying capacity of such concrete sections can be increased if steel and concrete both are stressed before the application of external loads.
This is the concept of prestressed concrete. As per the ACI committee, prestressed concrete is that concrete in which internal stresses of suitable magnitude are introduced so that these stresses resulting from the external loadings can be counteracted to a desired degree.
In RCC members, prestress induced is of compressive nature so that it balances the tensile stresses produced due to external load. It makes the whole section effective (the concrete area in the tension zone also !) in resisting loads.
Methods of Prestress
In this system, a prestressing force is applied to the high tensile steel, i.e., the steel reinforcement. It induces internal compressive stresses in concrete. This most commonly used method because of the easy and accurate application.
Internal prestressing can be done by two different methods :
- This method is not commonly used.
- In this method, the prestressing is done by adjusting the external reactions (by introducing different support conditions).
- In as per below fig, simply supported beam is prestressed externally by jacks.
- The externally prestressing system requires very much accuracy in planning and application.
Externally prestressing system for a simply supported beam.
In Pre-tensioning, the steel tendons are tensioned before the concrete is cast. The tendons are temporarily anchored against some abutments and then cut or released after the concrete has been placed and hardened.
The prestressing force is transferred to this concrete by the bond along the length of the tendon. Pre-tensioning is generally done in precasting plants in permanent beds, which are used to produce Pre-tensioned precast concrete elements for the building industry.
In Post-tensioning, the steel tendons are tensioned after the concrete has been cast and hardened.
Post-tensioning is performed by two main operations: tensioning the steel wires or strands by hydraulic jacks that stretch the strands while bearing against the ends of the member and then replacing the jacks by permanent anchorages that bear on the member and maintain the steel strands in tension.
A tendon is generally made of wires, strands, or bars. Wires and strands can be tensioned in groups, whereas bars are tensioned one at a time.
In the post-tensioning process, the steel tendons are placed in the formwork before the concrete is cast, and the tendons are prevented from bonding to the concrete by waterproof paper wrapping or a metal duct (sheath).
Tendons bonded to the concrete are called bonded tendons. Unbonded tendons, left without grout or coated with grease, have no bond throughout the length of the tendon.
Difference Between Pre-Tensioning and Post-Tensioning
|1||Pre-tensioning is done in the factories thus suitable for precast construction works||Post-tensioning can be done in factories as well as on the site|
|2||In this type of concrete, the pre-stressing cables called the strands are tensioned before casting the concrete and then concrete is casted enclosing the tensioned cables.||In this type of concrete, the strands are enclosed within a duct in the form and then concrete is casted. The process of tensioning the strands is carried out after the concrete attains its sufficient strength.|
|3||Small sections are to be constructed||Size of a member is not restricted, long-span bridges are constructed by post-tensioning|
|4||Loss of prestressing isn’t less (about 18 %)||Loss of prestressing isn’t more (about 15 %)|
|5||In this method, similar prestressed members are prepared.||In this method, products are changed according to a structure.|
|6||Pre-tensioning members are produced in a mould.||Cables are used in place of wires and jacks are used for stretching.|
|7||In this method, the concrete is prestressed with tendons before it is placing in position||In this method prestressing is done after the concrete attains sufficient strength.|
|8||This method is developed due to the bonding between the concrete and steel tendons.||This method is developed due to bearing.|
|9||Pre-tensioning is preferred when the structural element is small and easy to transport.||Post-tensioning is preferred when the structural element is heavy.|
|10||It is cheaper because the cost of sheathing is not involved||It is costlier because of use of sheathing|
|11||It is more reliable and durable||The durability depends upon the two anchorage mechanism|