Prestressing in Prestressed Concrete

Prestressing of Concrete | Pre-stressed Concrete | Types and Advantages

Prestressing of concrete is done by inducing predetermined compressive stresses to concrete by tensioning the steel, before subjecting it to service loads. In prestressed Concrete, the stress developed during the service stage is countered by the already induced compressive stresses. Prestressing is a technique that combines the strong compressive strength of concrete with the high tensile strength of steel. This page discusses prestressed concrete, various prestressing methods, and how prestressing works.

Why Prestressing of Concrete?

Concrete has remarkable properties that make it the most preferred material for structural members, but it also has flaws. Let us Consider two scenarios in which a concrete beam is subjected to loads.

  • CASE 1 – PLAIN CEMENT CONCRETE BEAM ON LOADS
  • CASE 2 – REINFORCEMENT CEMENT CONCRETE ON LOADS

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Plain Cement Concrete Beam on loads (Case 1)

Consider a Plain Cement Concrete (PCC) beam subjected to the loads depicted in Fig 1. The tensile zone of the beam experiences bending and crack development. This demonstrates that concrete is extremely strong in compression but very weak in tension.

PCC Beam on loads
Fig 1 – PCC Beam on loads

Reinforced Cement Concrete beam on loads (Case -2)

Consider a reinforced cement concrete beam that is being loaded with the loads indicated in fig 2. The beam would not bend or crack in this instance. This is because reinforcement steel is available in the tensile zone. Or, to put it another way, we can say that the reinforcement steel handles the tensile loads and prevents the member from cracking.

RCC beam subjected to loads
Fig 2 – RCC beam subjected to loads

The RCC beam with steel behaves as a composite member in this scenario. Notably, the reinforcement steel’s high tensile strength and ductility compensate for concrete’s inferior tensile strength and ductility.

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Prestressing of Concrete – Significance

Despite having the ability to withstand acceptable compression, concrete has the following drawbacks.

  • Tensile strength is weak.
  • Brittle
  • Non-ductile

A competent designer foresees potential failure areas and designs the structure to address them. Furthermore, the design developed by this method is optimised. The design is based on design criteria ( Goal of the design). The design criteria of ultimate strength and serviceability must be met by each design, though.

Let us go through the details of Ultimate strength and Serviceability

Ultimate strength

The structures in the Ultimate strength design Criteria are designed for ultimate strength and will not collapse even under the most adverse conditions. For instance, the proposed structure for a bridge satisfies the design criterion of ultimate strength if it can carry a traffic load without collapsing.

Serviceability

The structure designs must be checked for serviceability conditions such as stability analysis, deflection tests, and so on. If the structure tends to deflect at moments during the service stage, the serviceability condition is not met.

Let us examine the effect of service loads on RCC structures such as bridges.

Deflection of steel structures

Deflection On Service loads
Fig – 3 Deflection On Service loads

Figure 3 depicts the effects of service loads on an RCC structure. Significantly, the structure deflects as a result of the moments. To handle the loads, the ductile reinforcement elongates. However, concrete with low tensile strength breaks when subjected to tensile loads.

The cracks absorb moisture and gradually corrode the reinforcement steel. This, in turn, causes concrete spalling and eventually leads to the structure collapsing. In order to reduce deflection cracks, prestressed concrete is used, which contributes to the members’ increased strength. With the help of prestressing, designers can optimise their work while tackling wide spans.

Prestressing of Concrete – Definition

Prestressed concrete
Prestressed concrete

Prestressing is a method of inducing compressive stress into a structural member by tensioning the steel prior to subjecting it to service loads. Prestressing is frequently used for buildings with enormous spans, pile foundations, silos, reservoirs, prefabricated components, etc.

Photographs – Prestressed Concrete Structures

Prestressing in Prestressed Concrete
Prestressing in Prestressed Concrete

Prestressed Concrete – How it works

The figure below explains how an RCC member subjected to loads deflects and gets cracked

Deflection and cracks on service loads
Deflection and cracks on service loads

Prestressing in Concrete – Methodology

Before pouring concrete in prestressed concrete, the steel/tendons are stretched along the axis, as seen in fig. Once the concrete has reached the desired strength, the tendons are released. When the tendons separate, they cause compressive stresses in the structural part.

Mechanism of Prestressed Concrete
Mechanism of Prestressed Concrete

The compressive stress induced in the structural member when tensioned steel is released counterbalances the compression caused by loads applied during the service stage. Tensioning steel in prestressed concrete causes negative deflections in the member. These deflections balance the compressive stress generated by service loads and prevent cracking in the concrete.

Prestressed bridge on service Loads
Prestressed bridge on service Loads – No deflection

The prestressing process gives designers much-needed freedom when designing large spanned structures. In contrast, it is difficult to derive cost-effectively and optimised designs for RCC.

Prestressing of Concrete – Methods

Prestressing is done in two methods

  • Pre-tensioning Method
  • Post-tensioning Method

Pre-tensioning Method

Before pouring the concrete, the tendons are stretched in the pre-tensioning method. Tendons are released once the concrete has attained the desired strength. The structure is subjected to service loads after the tendon has been released.
Tendons made from high-strength steel are located between two abutments/buttresses as shown in the figure. The tendons are stretched to around 70% of their ultimate strength or as specified by the designer. Tendons are kept stretched when concrete is poured. Once the desired strength of the concrete is achieved, the tendons are released. Because of its strong ductility, steel attempts to regain its original length when released. Tensile stress in steel is converted to compressive stress in concrete during this process, resulting in a negative deflection. These compressive stresses induced in the structural member counter the compressive stress generated in the service stage.

Pre tension bed and process
Pre-tension bed and process

The pre-tensioning method is used for precast girders of bridge spans, metro lines and flyovers, railway sleepers, piles, and prefabricated elements subjected to heavy loads. The structures are prestressed in the prestressing yards, conveyed, and lifted for erection on site. Size restrictions apply to pre-tensioned structures since they must be transported from the fabrication yard to the construction site and then erected there.

Post-Tensioning Method

Once the concrete has attained its design strength, tendons are tensioned in the post-tensioning method. Ducts or profiles are strategically embedded within the concrete during casting for this purpose. Once the concrete has hardened and reached design strength, the tendons are inserted through the previously installed ducts or profiles. Tendons are tensioned using jacks in accordance with the design requirements. Once post-tensioning is finished, the structure is released for service loads..

Post tensioning system
Post-tensioning system

After tensioning, the tendons in bonded type post-tensioning are grouted using special grouts. Tendon grouting is not required in unbounded types. Post-tensioning is performed on-site rather than in a fabrication yard, as is the case with pre-tensioning. The post-tensioning method is used in viaducts, segmental construction of huge bridge spans, massive slabs, reservoirs, large cement plant silos, coal washeries, and other structures.

What are tendons in prestressed concrete?

Tendons are formed up of single wires, multi-wire strands, or threaded bars, which are generally manufactured of high-tensile steels, carbon fibre, or aramid fibre. A tendon is a steel cable or wire that is used in Prestressed Concrete structural elements such as beams and columns. It is a medium through which tensile stresses are induced into the concrete. It could be a single steel wire or a collection of wires twisted together to withstand the appropriate tensile stresses.

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