Prestressed Concrete

Application of prestressed concrete fe415 rightsteel

Concrete and steel are the two most vital components for the construction of a building. Concrete is known for its impressive compressive strength but it lacks the tensile strength. This tensile strength is provided by using reinforced steel bars. So both these components are needed for the construction. However, with the rise of technologies, different variants of steel, concrete etc have been introduced. One such material is Prestressed Concrete.

What is Prestressed Concrete?

Prestressed concrete is a type of concrete in which the material is prestressed i.e compressed in areas that will be subjected to external loads or stresses and to strengthen it against external forces. The main highlight of prestressed concrete is that it combines the high strength compressive properties of concrete with the high tensile strength of steel. In normal reinforced concrete, the compressive stresses are handled by concrete and the tensile strength are handled by the reinforced steel bars.

Since the loads are handled by induced stresses in the prestressed concrete, it makes it more resistant to shocks and vibrations than normal RCC. Like ordinary concrete, prestressed concrete is used for the construction of structures such as bridges, water tanks etc.

The Process

The prestressed concrete is generally prepared using two processes which are pre-tensioning process and post-tensioning process. Let us see what these processes are and how do they differ

Pre-tensioning

This is a variant of prestressed concrete which is prepared by tensioning the tendons before the concrete is cast. Here, the tendons are generally single wires, multi-wire strands or threaded bars that are produced using high-tensile steels, carbon fiber etc. These tendons are stressed by anchoring them at the end of a metal form, which may be up to 120 m in length. Hydraulic jacks may be used as required for stressing the wires. Side moulds are fixed and the concrete is placed around the tensioned wires. Finally, the concrete hardens and shrinks, gripping the steel along its length, and transfers the tension from the jacks to exert the compressive forces in the concrete.

Post-tensioning

In post-tensioning the tendons are stressed after the concrete has been cast and set. Here, the tendons are encapsulated within a protective sleeve which is either cast into the concrete structure or placed adjacent to it and are not placed in direct contact. An anchorage assembly firmly fixed to the surrounding concrete at each end of the tendon. The tendons are stressed by pulling the tendon ends through the anchorages while pressing against the concrete. Helical reinforcements are commonly included in the design due to high local stresses at the anchorage positions. The usage of anchorages in the post-tensioning depend on whether the tendons are stressed individually or as a group.

Pros and Cons

Prestressed concrete has many benefits than normal reinforced concrete. The compressive strength of prestressed concrete is utilized to its maximum. Formation of tension cracks are eliminated and it reduces the corroding of steel components. For any given span and loading condition, weight reduction is achieved by using a component with smaller cross-section. Even though it comes with multiple benefits, there are disadvantages too. Prestressed concrete requires a high degree of workmanship and control. For preparing the equipment required for producing prestressed concrete are quite expensive. Also, prestressed concrete is more economical for spans over 9 m height unlike normal reinforced concrete which is economical for a span of up to 6 m.

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