What Is Prestressed?
Concrete in which there have been introduced internal stress of such magnitude and distribution that stresses resulting from given external loading are counteracted to a desired degree in reinforced cement concrete members are called Prestressed Concrete. The prestressing is commonly introduced by tensioning the steel reinforcement.
There is some confusion about what is prestressed concrete and the difference between prestressed concrete and reinforced concrete. In prestressed concrete, high tensile steel is used which will have to be elongated a great deal before its strength is fully utilized. If the high tensile steel simply buried in the concrete as in ordinary reinforced concrete, the surrounding concrete will have to crack very seriously before the full strength of the steel is developed. Hence it necessary to pre-stretch the steel with respect to concrete. By pre-stretching and anchoring the steel against the concrete, desirable stresses and strains are produced in both materials. Compressive stresses and strains in concrete and tensile set ease and strains in steel. This combined action permits the safe and economical utilization of the two materials which cannot be achieved by simply burying steel in concrete as is done for ordinary reinforced concrete. Therefore, prestressed concrete is nothing but an extension of the applications of the reinforced concrete to include steels of high tensile strength.
Methods of Prestress
Prestress may be induced in the structure by various prestressing methods. Some of which are explained below :
1- In this system, a prestressing force is applied to the high tensile steel, i.e., the steel reinforcement.
2- It induces internal compressive stresses in concrete.
3- This most commonly used method because of the easy and accurate application.
4- Internal prestressing can be done by two different methods
1- This method is not commonly used.
2- In this method, the prestressing is done by adjusting the external reactions (by introducing different support conditions).
3- In as per below fig, simply supported beam is prestressed externally by jacks.
4- The externally prestressing system requires very much accuracy in planning and application.
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 cast enclosing the tensioned cables.||In this type of concrete, the strands are enclosed within a duct in the form and then concrete is cast. 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 the use of sheathing|
|11||It is more reliable and durable||The durability depends upon the two anchorage mechanism|