Short Answer:
There are two main types of prestressing used in concrete structures: pre-tensioning and post-tensioning. In pre-tensioning, the steel tendons are stretched before the concrete is poured, and in post-tensioning, tendons are stretched after the concrete has hardened.
Pre-tensioning is commonly used for precast elements like railway sleepers and bridge girders, while post-tensioning is used for cast-in-place structures such as slabs, beams, and bridges. Both methods help improve strength, reduce cracks, and allow longer spans with thinner sections.
Detailed Explanation:
Types of prestressing used in concrete structures
Prestressing is a method used to strengthen concrete by introducing internal compressive stresses that counteract tensile forces due to loads. Since concrete is weak in tension, prestressing helps in improving performance and preventing cracks. There are two major methods used in construction based on when and how the stress is applied to the tendons: pre-tensioning and post-tensioning.
- Pre-tensioning
In pre-tensioning, high-strength steel tendons or wires are first stretched between fixed anchors before the concrete is placed. The concrete is then poured around the stretched tendons. After the concrete gains enough strength, the tendons are released. This action transfers the tensile force to the concrete through bond, compressing the concrete and improving its strength.
Features of Pre-tensioning:
- Performed in factory setups (precast units).
- Common for mass production of items like railway sleepers, poles, girders, planks.
- Very strong bond between steel and concrete.
- Economical for repetitive elements.
Advantages:
- Better control on quality due to factory conditions.
- Faster installation at site.
- High bond strength due to pre-casting.
Limitations:
- Not practical for on-site casting.
- Requires special pre-tensioning beds and equipment.
- Post-tensioning
In post-tensioning, ducts or sleeves are kept inside the concrete member during casting. After the concrete gains strength, steel tendons are inserted into these ducts. Then, the tendons are stretched using jacks and anchored at both ends. The duct is finally filled with grout to protect steel from corrosion and ensure bonding.
Features of Post-tensioning:
- Mostly used on-site.
- Suitable for long-span bridges, slabs, and heavy beams.
- Flexible for large and irregular shapes.
Advantages:
- Can be applied in place without factory setup.
- Better for larger structures or custom designs.
- Allows for thinner sections and less concrete usage.
Limitations:
- Needs skilled labor and tensioning equipment.
- Grouting must be done properly to avoid corrosion.
Conclusion:
The two types of prestressing in concrete—pre-tensioning and post-tensioning—both aim to improve the strength and serviceability of concrete structures. Pre-tensioning is used mostly in factory-made precast components, while post-tensioning is flexible for site work and large structures. Each method has its own advantages and is selected based on project size, location, and design requirements.