What is Prestressed Concrete , Prestressed Concrete

 PRESTRESSED CONCRETE

Introduction

One of the serious limitation of reinforced cement concrete is the cracking which is a natural phenomena for concrete constructions. 

 Once crack occur they do not disappear even after removal of load. If the width of these cracks is to be kept within permissible limit the still stress has to be kept low. Presence of crack lowers the capacity of structure to bear reversal of stresses, impact variation and shocks. Also, the reinforcing bars may get wear away in due course of time due to access of water/moisture and the concrete decay. Besides these disadvantages, the presence of cracks makes theory of reinforced concrete neck irrational.

Efforts were made to eliminate the cracking of concrete by artificially introducing in it either before or cumulatively with the application of external loads, a compressive force of permanent nature. This force is applied that it causes compressive stresses in that zone of the member where tensile stress will be caused by external loads. The tensile stress in concrete will thus be neutralized and it will not crack. 

Prestressed Concrete :

A prestressed concrete is defined as a concrete in which stresses of suitable magnitude and distribution are introduced to retroact, to a desired degree, the stresses resulting from external loads. 

The concept of prestressed concrete was first used by Mandl of France in 1896. In prestressed concrete high strength concrete and steel are desirable. The prior is required because of following: 

• The use of high strength concrete results in smaller cross-section of member and hence smaller self weight; longer spans become technically and economically practicable.
• High bearing stresses are generated in anchorage zones.
• The shrinkage cracks are reduced with higher modulus of elasticity and smaller creep strain resulting in smaller loss of prestess. 

The loss of prestressed at the initial stages is very high and for this reason high strength steel is required. High tensile strength wires with ultimate tensile strength up to 2010 N/mm² are the choice. For prestressed concrete members, the high tensile steel used generally consist of bars

Pre-tensioning and Post-tensioning:

 Prestressing is achived by either pre-tensioning or post-tensioning. In the before the wires or cable are anchored, tensioned and concrete in cast in the modulus. After the concrete has gained strength the wires are released. This sets up compression in concrete which counteract tension in concrete because of bending in the member. In the post-tensioning prestressing  force is applied to the steel bars or cables, after the concrete has hardened sufficiently. After applying the full prestress the cable passages are grouted. The minimum 28-day cube compressive strength for concrete is 40 N/mm² for pretensioned members and 30 N/mm² for post-tensioned members.

Prestressed Concrete Method

Advantages :

• The cracking of concrete is eliminated enabling the entire cross section of the member to take part in resisting moment.
• As dead load moments are neutralized and the shear stresses are reduced, the sections required are much smaller than those for reinforced concrete. This reduces the dead weight of structure.
• In ordinary reinforced concrete (RCC) the economy is not as pronounced as in prestessed concrete (PSC). The prestressing force in most cases in computed strictly from dead load of the structure; consequently, a reduction of the order of 25% may result in the weight of prestressed member.
• A composite member can be formed by joining individual precast concrete units together.
• Shear stressed are reduced.   

Disadvantage : 

• A high degree of workmanship and control is required.
• Special alloy steel are more expensive than traditional steels used in reinforced concrete.
• Expensive equipment is needed and there are complex safety requirements.  

Uses :

It is widely used for construction of precast units such as beams, floors, roofing system, bridges, folded plate roofs, marine structures, towers, and railway sleepers.