JNTU B.Tech III PRESTRESSED CONCRETE Previous Question Paper (August - 2022 - R18)

Pre-tensioning is a method of prestressing where the tendons are tensioned before the concrete is cast. The tendons are temporarily anchored against external abutments, concrete is poured around them, and once the concrete reaches the required strength, the tendons are released. The prestress is transferred to the concrete through bond along the length of the tendon.

Post-tensioning is a method where the tendons are tensioned after the concrete has hardened. Ducts are placed in the concrete formwork before casting. Once the concrete reaches sufficient strength, the tendons are inserted, stressed against the hardened concrete using hydraulic jacks, and anchored at the ends. The prestress is transferred through end-bearing.

Advantages of Pre-tensioning include suitability for mass production of standardized units, excellent bond characteristics, and elimination of end anchorages. Its limitations include the requirement for heavy casting beds and the inability to easily create curved cable profiles.

Advantages of Post-tensioning include its suitability for large-span cast-in-situ construction, the ability to use curved cable profiles to optimize stress distribution, and the reduction of dead load effects. Its limitations include the requirement for specialized anchorage devices, the need for grouting ducts to prevent corrosion, and higher labor costs due to complex site operations.

Prestressing methods are broadly classified into two categories based on the sequence of tensioning the tendons relative to the casting of concrete:

1. Pre-tensioning: In this method, tendons are tensioned before the concrete is cast. The tendons are anchored against strong abutments, concrete is poured, and once it gains sufficient strength, the tendons are released. The prestress is transferred to the concrete through bond between the steel and concrete.
2. Post-tensioning: In this method, tendons are tensioned after the concrete has hardened. Concrete is cast with ducts for the tendons. Once the concrete reaches the required strength, the tendons are tensioned using hydraulic jacks and anchored against the hardened concrete member itself. The force is transferred through end-bearing plates.

The necessity for high-strength materials arises from the significant losses of prestress that occur over time. High-strength materials are essential for the following reasons:

• High Strength Concrete: High-strength concrete is required to resist the high compressive stresses induced at the anchorage zones during prestressing and to minimize losses due to elastic shortening, creep, and shrinkage. It also provides superior bond strength for pre-tensioned members and better durability.
• High Strength Steel: Prestressing steel must be of high strength to maintain a significant effective prestress force after accounting for losses such as relaxation of steel, creep, and shrinkage of concrete. If low-strength steel were used, the total initial prestress would be largely lost, rendering the prestressing ineffective. High-strength steel allows for the application of large tensioning forces while keeping the cross-sectional area of the tendons relatively small.