All posts by Vinod Gopinath

Prestressing methods in Prestressed Concrete – Types and methodology

The prestressing in Prestressed 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 combination of the high-strength compressive properties of concrete with the high tensile strength of steel. This article is about prestressing in prestressed concrete, different methods of prestressing, and how prestressing works.

Prestressing in Prestressed concrete

Concrete got excellent properties, making them the most preferred material for structural members, but has its weakness too. Let us consider two cases where a concrete beam is subjected to loads.


Plain Cement Concrete Beam on Loads

Let us consider a Plain Cement Concrete (PCC) beam subjected to loads as shown in Fig. The beam bends and cracks are developed in the tensile zone. This confirms that the concrete is very weak in tension and strong in compression.

Beam subjected to loads
Beam subjected to loads

Reinforced Cement Concrete beam on loads

Consider a reinforced Cement Concrete beam subjected to loads as shown in fig. In this case, the beam will not bend or cracks. This is due to the presence of reinforcement steel in the tensile zone. The reinforcement steel takes care of the tensile loads and prevents the member from cracking.

RCC beam subjected to loads
RCC beam subjected to loads

In this case, the RCC beam with steel behaves as a composite member. Concrete’s poor tensile strength and ductility are countered by the reinforcement steel having high tensile strength and ductility.

Significance of Prestressed Concrete

Even though concrete owes the property of good compressive strength, it has the following disadvantages.

  • Tensile strength is weak
  • Brittle
  • Non ductile

A good designer anticipates the areas of failure and designs the structure to overcome them. The design developed through this method is optimised.
The Design is based on Design criteria ( Goal of the design). Each design should satisfy the design criteria of ultimate strength and Serviceability.

Let us go through the details of Ultimate strength and Serviceability

Ultimate strength

In this design Criteria, the structures are designed on ultimate strength and will not collapse even in the worst condition. For example, if the proposed structure for a bridge can handle a load of traffic without a collapse then it satisfies the Design criteria of ultimate strength.


The structures are to be checked for serviceability conditions like stability analysis, deflection checks, etc. In the service stage if the structure tends to deflect on moments, then the serviceability criteria is not satisfied.

Let us analysis the impact of service loads on RCC structure like a bridge.

Deflection of steel structures

Deflection On Service loads
Deflection On Service loads

The figure presents what happens when an RCC structure is subjected to service loads. The moments cause the structure to deflect. The ductile reinforcement elongates to negotiate the loads. However concrete with poor tensile strength fails on tensile loads and develops cracks.

Beam subjected to loads
Beam subjected to loads
Cracks developed on deflection
Cracks developed on deflection

The cracks absorb moisture and gradually rust the reinforcement steel. This leads to spalling of concrete and initiates an ultimate collapse of the structure.
Prestressed concrete is introduced to minimise deflection cracks, for increasing the strength of members. Prestressing gives the designers, the flexibility of optimising the design while negotiating large spans.


Prestressed concrete
Prestressed concrete

Prestressing is a method of inducing Compressive stress into a structural member by tensioning the steel, before subjecting to service loads.

Prestressing is adopted for structures like bridges,large spanned auditoriums, silos, reservoirspile foundationsprefabricated elements etc.


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

Principle of Prestressed Concrete

In prestressed concrete, the steel/tendons are stretched along the axis before pouring concrete as shown in fig.
The tendons are released once the concrete reaches the desired strength. On detaching, the tendons induce compressive stresses in the structural member.

Mechanism of Prestressed Concrete
Mechanism of Prestressed Concrete

The compressive stress induced in the structural member on releasing the tensioned steel, counterbalances the compression arose due to loads applied in the service stage. In prestressed concrete, tensioning of steel initiates negative deflections in the member. These defections balance the compressive stress due to service loads and prevent the concrete from cracking.

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

Prestressing method provides the designers with the much-needed flexibility in designing large spanned structures. Whereas deriving economical and optimised designs in RCC seems difficult.

Method of Prestressed Concrete

Prestressing is done in two methods

  • Pre-tensioning Method
  • Post – tensioning Method

Pre tensioning Method

In Pre tensioning method, the tendons are stretched before pouring the concrete. Once the concrete attains the desired strength the tendons are released. After releasing the tendon the structure is subjected to service loads.
The High-strength steel tendons are placed between two abutments/buttress. The tendons are stretched around 70% of their ultimate strength or as per design requirements. Concrete is poured with Tendons kept stretched. The tendons are released once the concrete attains its desired strength. On release, the steel tries to regain its original length due to its high ductility. During this process, the tensile stress in steel is converted to compressive stress in concrete and thereby initiates a negative deflection. These compressive stresses induced in the structural member counters the compressive stress in the service stage.

Pre tension bed and process
Pre tension bed and process

The post-tensioning method is for precast girders of bridge spans, metro lines, flyovers, railway sleepers, piles, and prefabricated elements subjected to heavy loads. The structures are prestressed in the prestressing yards, conveyed, and lifted for erection at the site.

The post-tensioned structures have size limitations as they have to be carried from fabrication yard to site and erected at the site.

Post-Tensioning Method

In the post-tensioning method tendons are tensioned, once the concrete attains design strength. For this purpose, ducts or profiles are strategically placed within the concrete during casting.

Once the concrete hardens and attains design strength, the tendons are inserted through the already placed ducts or profiles. The tendons are tensioned using jacks as per design requirements. On completion of post-tensioning works, the structure is released for service loads.

Post tensioning system
Post tensioning system

In bonded type post-tensioning, the tendons are grouted with special grouts after tensioning. In unbounded type, tendon grouting is not necessary.

Post-tensioning is done at the site and not in the fabrication yard like a pre-tensioning system. The post-tensioning method is used in viaducts, segmental construction of large bridge spans, large slabs, reservoirs, big silos of cement plants, coal washeries, etc

What are tendons?

Tendons consist of single wires, multi-wire strands or threaded bars that are most commonly made from high-tensile steels, carbon fiber or aramid fiber.

Foundation types- shallow and deep foundation

Foundation is the most significant part of any structure/building which transfers the total loads of the structure and its components to a competent surface on the ground. Foundations are broadly classified into two types. ie. Shallow and Deep Foundations.

Foundation is the last part of the structure which touches the ground. The area of contact with the ground is called the foundation bed.

Every structure is divided into:

a) Sub structure

b) Super structure

Components of a structure that are coming below the ground level are called substructure, and above ground level is called superstructure. Foundations are coming in the substructure category. Foundations are responsible for transferring loads of superstructure components to the ground.


The selection of foundations depends on the bearing capacity of the soil and the purpose of the structure. Geotechnical engineering is a field of Civil Engineering, which analyses the physical and chemical properties of soil and furnish designers with the inputs on the soil properties and proposed types of foundations. The Safe bearing capacity of the soil determines the foundation type and dimensions.


Bearing capacity is the capacity of soil to support a structure without settlement or failure. To keep the structure safe, the bearing capacity has to be calculated at different locations. The ultimate bearing capacity has to be divided by a factor to derive the safe bearing capacity of the soil. Safe bearing capacity is defined as the maximum load per unit area soil can withstand without settlement and failure. The safe bearing capacity is determined by conducting field tests or soil investigations.



A well-designed foundation is supposed to possess the following qualities.

a) Have to distribute the total load on the structure to a larger area.

b)Have to counter unequal settlement in case of any displacement.

c) Has to prevent the structure from lateral moments.

d) Foundations are responsible for the total stability of structures.


Foundations are classified into

a) Shallow Foundation

b) Deep Foundation



Shallow foundations transfer the load laterally to the soil. It is also called stripped foundations. The depth of a shallow foundation is less than its width.

Characteristics of shallow foundations

Shallow foundations are adopted when the load acting on a structure is reasonable and has a competent soil layer capable of negotiating the loads available at a shallow depth or shorter depth.

Shallow foundations are placed on the surface of the ground. The depth of a shallow foundation can range from 1 meter to 3.5 meters and sometimes more.

The width of the shallow foundation is greater than the depth. Shallow foundations are very easy to construct and do not require highly skilled manpower and professional supervision. These foundations can even be done with the help of medium-skilled workers. A shallow foundation is very economical when compared with a deep foundation. Shallow foundations are end bearing type foundations that transfer loads to the end of the foundation.

Shallow foundations are considered as the most preferred option when the safe bearing capacity of the soil is reasonable and the structural loads are within the permissible limits.



Characteristics of deep foundation

The width of the deep foundation is less than the depth. The depth can even go up to 60 meters or more depending on the design, loads, and availability of capable strata.

Deep foundations require technical expertise, sophisticated equipment, and highly skilled manpower for interpreting and executing works.

The deep foundations are costly due to their way of execution involving the infusion of quality materials, skilled labor, professional engineering support, and equipment

Deep foundations do not rely only on end bearing for transferring the loads. The skin friction developed between the foundation surface and the soil surrounding it may also be considered in the design stage.

The deep foundations can resist uplift pressure much more than shallow foundations and hence the chances of failure are less compared to shallow foundations.