BTECH-413-MODULE-4

PRESTRESSED CONCRETE STRUCTURES

Reinforced Concrete

  • Definition: Concrete is inherently strong in compression but weak in tension. To overcome this limitation:

    • Steel is introduced to resist tension forces.

    • Concrete absorbs compression, while steel bars (reinforcements) maintain their position.

  • Tensile Strength of Concrete: Neglected (considered zero).

  • Impact on Structures: Reinforced concrete beams can develop cracks under service loads due to tensile stress.

Pre-stressed Concrete

Definition of Pre-stressed Concrete

  • Concept: Internal stresses are intentionally induced to counteract external loads.

  • Historical Note: In 1904, Freyssinet aimed to introduce permanent forces within concrete to resist elastic forces under loads, coining the term "Pre-stressing."

Load Analysis of Pre-stressed Concrete

  • Typical Load Conditions:

    • Reinforced Concrete: Showcases cracks under dead and full service loads.

    • Pre-stressed Concrete: Shows no cracks; exhibits initial camber under dead load.

Concept of Pre-stressing

  • Historical Background:

    • Originated from the construction of barrels where metal bands were tightened around wooden staves.

    • Compression achieved through tensile stress between the metal bands and the wooden materials.

    • This concept is linked directly to the ability of the materials to resist internal pressure.

Principle of Pre-stressing

  • Methodology:

    • A compression force is applied to the concrete section to mitigate tension stress.

    • Goal: Ensure that tensile stress remains below the cracking threshold, preventing concrete damage.

  • Visualization:

    • Concrete subjected to both internal (pre-stressing) and external forces (like dead and live loads) where they counterbalance each other.

Mechanics of Pre-stressing

Stress Analysis

  • When Pre-stress is Applied Centrally:

    • Results in large compression forces with minimal or no tensile forces present in the cross-section.

  • When Pre-stress is Applied Eccentrically:

    • Leads to smaller compression forces and variations in resultant stress across the concrete section.

Methods of Pre-stressing

  • Basic Techniques:

    1. Pre-tensioning:

      • Tendons are elongated against abutments pre-concrete placement. Upon hardening, the tension is released, inducing compressive force due to bond.

      • Commonly used in precast concrete applications.

    2. Post-tensioning:

      • Tendons are tensioned post-hardening using ducts within the concrete. Stress is applied after achieving adequate strength. Anchorage is provided to maintain tension.

Advantages of Pre-stressed Concrete

  • Leverages high-strength concrete and steel, allowing for:

    • Reduced material usage.

    • Smaller and lighter structures.

    • Absence of cracks in service.

    • Maximized load-bearing capabilities of the entire section.

    • Enhanced corrosion resistance.

    • Benefits for deflection control and improved shear resistance.

Disadvantages of Pre-stressed Concrete Compared to Reinforced Concrete

  • Requires:

    • Higher quality materials.

    • More technical complexity in design and execution.

    • Increased overall costs.

    • Challenges in recycling methods.

Applications of Pre-stressed Concrete

  • Suitable for various structures including:

    • Bridges

    • Slabs in buildings

    • Water Tanks

    • Concrete Piles

    • Thin Shell Structures

    • Offshore Platforms

    • Nuclear Power Plants

    • Repair and Rehabilitation works.