Short Answer:
Prestressed concrete is a special type of concrete in which internal stresses are introduced before applying external loads to improve its strength and performance. This is done using steel tendons or wires that are stretched and anchored to compress the concrete, making it better at resisting tension forces.
The key difference from RCC (Reinforced Cement Concrete) is that in RCC, steel bars resist tensile forces after the load is applied, while in prestressed concrete, the structure is already under compression due to pre-applied forces, which controls cracking and allows for longer spans with less material.
Detailed Explanation:
Prestressed concrete and how it is different from RCC
In traditional RCC (Reinforced Cement Concrete), steel rebars are embedded in concrete to resist tensile forces because concrete is weak in tension but strong in compression. However, when long spans or heavy loads are involved, RCC may crack or deflect under tension, especially at the bottom fibers of beams and slabs.
To overcome this, engineers developed prestressed concrete, where internal compressive forces are applied to the concrete before any external load is placed on it. This is done by tensioning steel wires or tendons, either before or after the concrete is cast, and anchoring them to generate a compressive force that opposes the expected tensile force. As a result, when actual loading occurs, the member remains mostly in compression and performs better.
How Prestressed Concrete Works
- Pre-Tensioning Method:
- Steel wires are stretched on a frame before the concrete is poured.
- Once the concrete gains strength, the wires are cut, and the force is transferred to the concrete.
- Commonly used in precast factory-made elements like railway sleepers and bridge girders.
- Post-Tensioning Method:
- Concrete is cast first with ducts or holes for tendons.
- After it gains strength, steel tendons are inserted and tensioned, then anchored and grouted.
- Used for cast-in-place large beams, slabs, and bridge decks.
Difference Between Prestressed Concrete and RCC
| Aspect | RCC | Prestressed Concrete |
| Tensile Resistance | Provided after loading using rebars | Provided in advance by tensioned steel |
| Cracking | Cracks under load due to tension | Minimizes or eliminates cracks |
| Span Capability | Suitable for short/medium spans | Ideal for long spans |
| Material Usage | More concrete and steel | Less material, more efficient |
| Cost | Lower initial cost | Higher initial, but long-term benefits |
| Application | Buildings, low-rise slabs/beams | Bridges, large span roofs, high-rise buildings |
Advantages of Prestressed Concrete
- Better crack control and durability
- Longer spans with less deflection
- Efficient material use
- Higher load-bearing capacity
- Lighter and thinner sections
- Good for repetitive factory production
Applications
- Bridges and flyovers
- Precast railway sleepers
- Multistorey building floors
- Large water tanks and silos
- Industrial buildings and parking garages
Conclusion:
Prestressed concrete is an advanced form of concrete where internal compressive stress is applied using stretched steel tendons to counteract future tensile stresses. Unlike RCC, where steel works only after loading, prestressed concrete starts resisting stress from the beginning. This makes it suitable for large spans, heavy loads, and crack-free performance, especially in bridges and long-span structures.