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 are some confusion about what is prestressed concrete and the difference between presressed 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 eases 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.
Types of Prestressing Used in Concrete
Prestressed concrete structure can be classified in number of ways depending upon their features of design and construction. These are follows.
- Externally or internally prestressing: Externally prestressing is a term applied to prestressed structures where stressing is applied externally. Similarly, internally prestressing is used to describe the method of prestressing internally. Most of the prestressed concrete structures are internally prestressed.
- Circular and linear prestressing: Circular prestressing refers to prestressing in circular or round structures like tanks, pipes etc. Where the prestressing reinforcement are wound around in crcle. Prestressing all other members like beams slaps, etc. termed as liner prestressing.
- Pre tensioning and Post tensioning: The term pre tensioning is used to describe any method of prestressing in which the reinforcement are stretched or tensioned before the concrete is placed. The steel rods are temporarily anchored against some posts when tensioned and the prestress transferred to the concrete after it has set. In contrast to pre tensioning, post tensioning is a method of prestressing in which the reinforcement is tensioned after the concrete has hardened.
- Partial or fully Prestressing: when a member is designed so that under the working load there is on tensile stress in it then the concrete is said to be fully prestressed. If some tensile stresses will be produced in the member under working load then it is termed partially prestressed.