What is Prestressed Concrete?

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Prestressed concrete is a system into which internal stresses are deliberately induced without any form of external loads to improve its performance. The internal stresses induced in the concrete structure is used to counteract the stresses coming from the external load application. Here the concept and methods in the prestressed concrete system are briefly described.

Concept of Prestressing 

As mentioned before, prestressing is the application of an initial load on the concrete structure, so that the structure is able to counteract or withstand the stresses coming due to the service loads. The concept can be clearly understood by the example of a barrel. A barrel used in olden periods to transports liquids and grains are tightly bound by metal bands as shown in figure-1. These metal bands are fitted so tightly that it creates a hoop compression around the barrel. When this barrel is filled with liquid, it exerts hoop tension. The hoop compression that is created by the metal bands helps to counteract the hoop tension created by the fluid within. This is a prestressing system. Similarly, effective internal stresses are induced into the concrete by means of tensioned steel bars before the concrete structure is subjected to any service loads. This stress counteracts with external stresses.

Need for Prestressing Concrete

The need for prestressing in concrete can be justified by the following issue:

1. Concrete is weak in tension and strong in compression. This is a weak point of concrete that results in early flexural cracks mainly in flexural members like beams and slabs. To prevent this, the concrete is induced with compressive stress deliberately (prestressing) and this stress counteracts with the tensile stress the structure is subjected to during service condition. Hence the chances of flexural cracks are reduced.
2. The pre-compression that is induced as a part of prestressing helps to enhance the bending capacity, the shear capacity and the torsional capacity of the flexural members.
3. A compressive prestressing force can be applied concentrically or eccentrically in the longitudinal direction of the member. This prevents cracks at critical midspan and supports at service load.
4. A prestressed concrete section behaves elastically.
5. The full capacity of the concrete in compression can be used over entire depth under full loading in the case of prestressed concrete.

Methods of Prestressing

The prestressing can be performed by two methods:

1. Pretensionong
2. Post-Tensioning

1. Pretensioning

In the pretensioning method, the stress is induced by initially tensioning the steel tendons. These are wires or strands that are tensioned between the end anchorages. After this tensioning process, the concrete casting is performed. Once the casted concrete has hardened sufficiently, the end anchorages arranged are released. This releasing transfers the prestress force to the concrete. The bond between the concrete and the steel tendons facilitates this stress transfer. As shown in figure-2, the tendons that are protruding at the ends are cut and a finished look is achieved. In order to induce prestress force in the pre-tensioning method, a large number of tendons and wires are used. This arrangement hence demands a large area of surface contact to make the bond and stress transfer possible.

2. Post Tensioning

The procedure in post-tensioning is depicted in the figure-3 below. Here, the steel is prestressed only after the beam is cast, cured and attain strength to take the prestress. Within the sheathing, the concrete is cast. For the passage of steel cables, ducts are formed in the concrete. Once the casted concrete hardens completely, the tendons are tensioned. One end of the tendon is anchored and the other end is tensioned. In some cases, the tensioning can be performed from either side and anchored subsequently. Once the prestressing is complete, there is space between the tendons and the duct. This leads to:

1. Bonded Construction
2. Unbonded Construction

1. Bonded Construction In bonded construction, the space between the duct and the tendon is filled with cement grout. The grouting process helps the steel to resist corrosion to a large extent. The ultimate strength is increased as this method increases the resistance to live loads acting. The grout mixture is cement and water combined with or without admixture. No sand is used in this grout. 2. Unbonded Construction If no grout is used to fill the space between the duct and tendon, it is called as unbonded construction. Here, the steel is galvanized to protect from corrosion. A waterproofing material is used for galvanizing. Read More: Pre-Tensioning and Post-tensioning in Prestressed Concrete Mix Design

Advantages of Prestressed Concrete

The major advantages of Prestressed Concrete are:

1. The prestressing of concrete by using high tensile steel improve the efficiency of the materials
2. The prestressing system works for a span greater than 35m.
3. Prestressing enhance shear strength and fatigue resistance of concrete
4. Dense concrete is provided by prestressing systems thus improving the durability
5. Best choice for the construction of sleek and slender structures.
6. Prestressing helps to reduce the dead load of the concrete structure
7. Prestressed concrete remains uncracked even at service load conditions which proves the structural efficiency
8. Composite construction by using the prestressed concrete unit and cast-in-unit derives the economical structure

Disadvantages of Prestressed Concrete

1. Higher material costs
2. Prestressing is an added cost
3. Formwork is more complex than for RC (flanged sections, thin webs) – thus, precast not as ductile as RC