A method of prestressing concrete by tensioning the tendons against hardened concrete is known as:
post-tensioning
Prestressing Concrete: An Introduction
Prestressing concrete is an advanced construction technique used to enhance the performance of concrete structures. It involves introducing internal compressive stresses into the concrete, which helps to counteract the tensile stresses caused by external loads. This process significantly improves the concrete's strength, durability, and crack resistance, allowing for longer spans and more efficient use of materials.
Understanding Post-Tensioning
The question specifically describes a method of prestressing concrete where the tendons are tensioned against hardened concrete. This particular technique is known as post-tensioning. It is a widely used method, especially for large-scale construction projects such as bridges, high-rise buildings, and parking structures.
- In post-tensioning, empty ducts or sheaths, usually made of metal or plastic, are precisely placed within the concrete formwork before the concrete is poured. These ducts define the path the tendons will follow.
- After the concrete has been poured and cured to achieve a specified strength, making it hardened concrete, high-strength steel tendons (which can be strands, wires, or bars) are threaded through these pre-installed ducts.
- Hydraulic jacks are then used to pull and tension these tendons from one or both ends of the concrete member. The force exerted by the jacks pulls the tendons against specialized anchorages embedded within the hardened concrete.
- Once the desired amount of tension is reached in the tendons, they are securely anchored, and the stressing equipment is removed. This action transfers the compressive force from the tensioned tendons directly into the hardened concrete.
- Finally, in bonded post-tensioning systems, the ducts are often filled with cement grout. This grouting provides a permanent bond between the tendons and the concrete, enhancing the structural integrity and protecting the tendons from corrosion.
Distinguishing Pre-Tensioning
To fully grasp post-tensioning, it is beneficial to understand its primary alternative, pre-tensioning. While both are methods of prestressing concrete, they differ significantly in their application timing relative to the concrete hardening process.
| Feature | Pre-Tensioning | Post-Tensioning |
|---|---|---|
| Timing of Tensioning | Tendons are tensioned before the concrete is poured and hardens. | Tendons are tensioned after the concrete has been poured and become hardened concrete. |
| Process Overview | High-strength tendons are stretched between external anchor blocks. Concrete is then cast around these tensioned tendons. Once the concrete cures, the external anchorages are released, and the stress is transferred to the concrete through bond. | Ducts are placed within the formwork. Concrete is poured and cured. Tendons are then threaded through the ducts, tensioned with jacks against the hardened concrete, and anchored. Ducts may be grouted. |
| Bonding Mechanism | Relies on the bond formed between the tendons and the fresh concrete as it cures around them. | Can be unbonded (greased and sheathed tendons) or bonded (grouting the ducts after tensioning). |
| Typical Applications | Primarily used for precast concrete elements manufactured in factories, such as hollow-core slabs, railway sleepers, and concrete piles. | Used for both precast and cast-in-place concrete, especially for long-span structures like bridges, large floor slabs, and containment structures. |
Evaluating Other Options
- Anchorage: An anchorage is a component or device that holds the tensioned tendons in place and transfers the prestressing force to the concrete. While essential to post-tensioning, it is a part of the system, not the method of prestressing concrete itself.
- Full Prestressing: This term refers to the degree or level of prestressing concrete. It implies that sufficient prestressing force is applied to ensure that no tensile stresses (or very minimal tensile stresses) develop in the concrete under service load conditions. It describes the desired outcome of the prestressing design, not a specific method of applying the prestress.
Conclusion on Concrete Prestressing
Given the description in the question, where tendons are tensioned against hardened concrete, the method accurately identified is post-tensioning. This technique allows for significant engineering advantages, creating robust and efficient concrete structures.
Important Questions from Analysis of Prestress
In pre-stressed concrete, high-grade concrete is used for -
Which of the following pre-stressing systems employs high tensile bars with thread at ends?
For prestressed concrete, which code is to be used?
Determine the eccentricity of a load balancing cable for a beam of size 350 × 750 mm at centre of it. The beam subjected to a live load of 10 KN/m over a span of 9 m and is simply supported. The prestressing force applied is 1700 KN.
As per IS:1343-2012, the minimum characteristic strength of pre-stressed concrete to be used for post-tensioned and pre-tensioned structural elements are respectively:
