Prestressed Concrete – Definition, Working, Components, Advantages & Applications

Prestressed concrete is one of the greatest innovations in modern civil engineering. It overcomes the limitations of ordinary reinforced concrete (RCC) and allows engineers to design longer, thinner, and more durable structural members.

In this article, we’ll explore what prestressed concrete is, how it works, its components, advantages, and where it is commonly used.

What is Prestressed Concrete?

Prestressed concrete (PSC) is a type of concrete in which internal compressive stresses are intentionally introduced before applying external loads. The initial load or pre-stress is applied to enable the structure to counteract the stresses arising during its service life. 

Before the invention of modern prestressed concrete, the idea of introducing internal stresses to strengthen a structure was already used in simple, everyday objects. Let's understand this in detail, so that you can easily understand the concept of prestressing and prestressed concrete. 

 

Force Fitting of Metal Bands around Wooden Barrels

When a barrel is made, metal bands (called hoops) are tightly fitted around the wooden staves. The hoops are stretched slightly as they are placed, so they pull inward on the wood when they try to contract. This action compresses the wooden staves together, keeping them tightly sealed even before the barrel is filled.

Now, when the barrel is filled with liquid, the liquid pushes outward, creating tensile stresses in the wood.
But since the wood is already under compressive stress, the outward pressure first reduces that compression instead of pulling the barrel apart.

The result? — The barrel stays strong and leak-proof.

So here, the internal compressive stress (from the hoops) counteracts the external tensile stress (from the liquid pressure).

 

Pre-tensioning the Spokes in a Bicycle Wheel

The spokes of a bicycle wheel are tightened before the wheel is used. This tightening puts the spokes under tension and the rim under compression, even when there’s no load. When the rider sits on the bicycle, the lower spokes tend to lose some tension due to the load, but they never go completely slack because of the initial prestress.

This pre-tensioning helps the wheel carry heavy loads and stay perfectly circular while rotating at high speeds.

Concept of Prestressing Concrete

These two examples show a common principle — a structure is strengthened by introducing internal stresses before applying external loads. That is exactly what happens in prestressed concrete.

In concrete members, we apply a compressive force before loading — usually by tensioning high-strength steel wires or tendons inside the concrete.

So when the external loads (like the weight of a bridge deck or building floor) act later, the internal compressive stress neutralizes part of the tensile stress produced by the load. The result is a concrete member that:

• Doesn’t crack under service loads,
• Can span longer distances, and
• Uses materials more efficiently.

So we can defined prestress as:

Prestressed concrete is concrete in which internal compressive stresses are intentionally introduced to counteract tensile stresses from external loads.

It is considered an advanced version of reinforced concrete, mainly used in bridges, flyovers, long-span roofs, and industrial structures.

Why Prestressed Concrete?

In ordinary RCC beams, cracks develop when tensile stresses exceed the tensile strength of concrete.
However, in prestressed concrete, the prestressing force keeps the concrete in compression even under working loads, thus minimizing or completely preventing cracks.

Working Principle of Prestressed Concrete

Let’s understand the concept through the figure below (RCC vs PSC deflection):

1. RCC Beam (Case 1)

When a reinforced concrete beam carries a load, it bends downward due to the dead load and self-weight of the structure.
This results in a sagging deflection (δ₁).

2. PSC Beam (Case 2)

Before the beam carries any external load, a prestressing force is applied to the steel tendons. This introduces compressive stress in the concrete, causing a small upward (hogging) deflection (δpsc).

When the beam is later subjected to external loads, the final deflection is:

δ = δ₁ – δ'

This total deflection is much smaller than in RCC beams, resulting in a stiffer and crack-free structure.

Hence,

Deflection in R.C.C [δ₁] > Deflection in Prestressed Concrete [δ]

This principle of “preloading the structure” before applying actual loads is what makes prestressed concrete a superior design system.

Components of Prestressed Concrete

Prestressed concrete systems consist of several essential components that work together to apply and maintain the prestressing force:

Components of Prestressed Concrete     

1. Concrete Member

High-strength concrete is used to resist the large compressive stresses developed due to prestressing. The concrete must have low creep, shrinkage, and high modulus of elasticity.

 

2. Prestressing Steel

The steel used to apply the prestress force is called prestressing tendon. It may consist of wires, strands, cables, or bars made of high-tensile steel.

 

3. Anchorage System

Anchorages are devices used to transfer the prestressing force from the tendon to the concrete. They hold the tendons in place once tensioning is complete.

 

4. Ducts or Sheaths

In post-tensioned members, steel tendons are placed inside ducts or sheaths to allow for tensioning after the concrete hardens.

 

5. End Blocks and Bearing Plates

These components ensure the safe transfer of prestress to the concrete member and distribute the stress uniformly.

Types of Prestressing Methods

There are two main methods used to introduce prestress into the concrete. The prestressing steel can be stressed first before concreting. This is called as pre-tensioning. Whereas, we can also induce prestress after concreting my special techniques, this is called as post-tensioning. Let look into each type in detail:

1. Pre-Tensioning

• The steel tendons are first tensioned before the concrete is poured.
• Once the concrete gains sufficient strength, the tendons are cut, and the prestress is transferred to the concrete through bond action.
• Commonly used in precast factories for sleepers, poles, and small bridge girders.

2. Post-Tensioning

• The concrete is first cast with ducts or sheaths.
• After hardening, tendons are inserted and tensioned using hydraulic jacks.
• The force is transferred using anchorages.
• Commonly used in large-span bridges and continuous beams.

Types of Prestressing Steel

Prestressing force is induced using high-tensile steel, available in different forms:

1. Wires – Single steel units used for small-scale elements.
2. Strands – Two, three, or seven wires twisted together to form a stronger unit.
3. Tendons – A group of strands or wires used together for applying larger forces.
4. Cables – A group of tendons bundled together for very heavy prestressing requirements.
5. Bars – Large-diameter steel bars used as individual tendons in short-span members.

Advantages of Prestressed Concrete

• High Load-Carrying Capacity: Can resist greater loads compared to RCC members.
• Crack-Free Under Service Loads: Compressive stress prevents crack formation.
• Efficient Use of Materials: Both concrete and steel are used to their full strength potential.
• Reduced Structural Depth: Allows thinner sections and lighter members.
• Longer Spans Possible: Ideal for bridges, roofs, and auditoriums.
• Better Control Over Deflection: Prestressing counteracts sagging and improves stiffness.
• Improved Durability: Crack-free concrete prevents corrosion and water ingress.
• High Resistance to Shear and Torsion: Prestressing improves structural performance.
• Better Fatigue Resistance: Withstands cyclic loading effectively.
• Economical for Large Structures: Though costlier initially, it ensures long-term savings.
• Versatile Application: Suitable for both precast and in-situ construction.

Read More On: Difference Between R.C.C and Prestressed Concrete

Disadvantages / Limitations

• High initial investment for equipment and skilled labour.
• Requires precise design and execution.
• Not economical for small-scale or short-span structures.
• Complex anchorage and tensioning procedures.

Applications of Prestressed Concrete

Prestressed concrete is widely used in:

• Bridge decks and girders
• Railway sleepers
• Roof slabs and domes
• Poles and piles
• Water tanks and pressure pipes
• Multi-storey and industrial buildings
• Precast structural components

Conclusion

Prestressed Concrete (PSC) is a remarkable engineering solution that combines the compressive strength of concrete with the tensile strength of steel.

By introducing internal stresses before loading, PSC ensures minimal cracking, reduced deflection, and longer service life — making it the preferred choice for modern bridges, decks, and durable structures.

Frequently Asked Questions (FAQs)

1. What is prestressed concrete?
Prestressed concrete is concrete in which internal compressive stresses are intentionally introduced to counteract tensile stresses from external loads.

2. Why is prestressed concrete preferred over RCC?
It offers higher strength, crack resistance, and longer spans than RCC, making it ideal for heavy and long-span structures.

3. What are the components of prestressed concrete?
Concrete, prestressing steel, anchorage system, ducts, and end blocks are the main components.

4. What are the types of prestressing?
Pre-tensioning and Post-tensioning are the two main methods.

5. Where is prestressed concrete used?
PSC is used in bridges, poles, tanks, precast slabs, and large-span roofs.