Prestressed Concrete

What is Prestressing?

Prestressing is the application of an initial load on a structure, to enable it to counteract the stresses arising from subsequent loads during its service period.

  • Prestressing denotes a method in concrete construction in which the member is subjected to initial compression at the time of manufacturing itself. Thus, prestressing induces compression on the lower side of a beam and when the load is applied, tension so produced neutralize the compression already set up by prestressing.

Advantage of using Prestressed Members: 

1. In the case of RCC, the section below the neutral axis is ineffective (as all the tensile stresses are taken by steel). But in the case of prestressed concrete, the entire section is under compression and thereby increasing the load carrying capacity of the member.

2. Because of the entire section being in compression, the structure is crack free.

3. As the self-load is taken care of by additional eccentricity, the concrete required is about one-third the quantity required for corresponding RCC structure.

4. Thus because of reduced section and reduced weight, the cost of foundation etc. is less.

5. Most of the large span prestressed concrete structures are prefabricated under controlled conditions, the cost of shuttering and centering is considerably reduced.

Limitations in the use of Prestressed Structures:

The main limitation in the use of prestressed concrete is the comparatively expensive equipment and skilled operation.The method is, therefore, economical in large projects.

The grade of concrete used: M35 and above concrete.

Methods of Prestressing:

1. Pre-Tensioning:

In this method, the tendons are stretched to the desired amount. One end of the tendon is secured to an abutment, while the other end is stretched out with the help of prestressing jack. The formwork is then erected around the tensioned tendons and fresh concrete is poured, compacted and properly cured. At the end of the curing period, concrete is hard enough to sustain the compression passed through the bond. The projecting steel wires are then cut. The method is particularly adapted in precast beams, posts and simply supported slabs.

2. Post Tensioning: 

In this method, the structural member is cast, leaving ducts inside for subsequent insertion of prestressing tendons. The ducts are formed by placing CGI sheets inside the formwork or by keeping steel spirals or sheet metal tubes or any other suitable methods. When the concrete has been fully cured and hardened, the tendons are inserted inside the ducts. One end of the duct tendon is fixed while the other end is attached to prestressing jacks. When the required stretch has been given, the jack end of the tendon is also anchored and the duct is grouted with cement.