Pre-Stressed Concrete Beams

Overview of Concrete Beams

• Traditionally Reinforced Concrete Beams
  • Strong in compression, weak in tension (1/10 strength in tension).
  • Bending load causes:
    • Top half: Compressed
    • Bottom half: In tension (cracks form and propagate leading to failure).
  • Reinforcement:
    • Steel bars placed in tension zones to take on tensile loads when concrete cracks.
    • Other types of bars may be used for thermal effects, but not covered in this context.
  • Tension Zones:
    • Not always at the bottom (e.g., near columns, cantilevers).

Pre-Stressed Concrete Beams

• Definition: Combines steel and concrete actively rather than passively.
• Manufacturing Process (for pre-cast, pre-tensioned beams):
  1. Cables are placed in a form and tensioned.
  2. Concrete is poured and allowed to cure.
  3. After curing, cables are released, inducing compression in the bottom half.
• Load Bearing:
  • Pre-stressing allows beams to handle more load compared to traditionally reinforced ones.
  • Tension must counterbalance pre-compression before net tension occurs.

Considerations in Pre-Stressing

• Tensile Cracking Management:
  • Must calibrate pre-stressing to avoid excessive tensile cracking.
• Concrete Shrinkage:
  • Curing causes shrinkage for up to 18 months, impacting tension in pre-stressed steel.
  • Requires careful planning to offset losses from shrinkage to maintain required pre-stress levels.
  • Uses high strength reinforcing to prevent steel rupture from excessive pre-stress.

History and Development

• About 100 years ago, designers recognized the need for high strength reinforcing to enhance pre-stressing.
• Development in material science for:
  • High strength reinforcing
  • High strength concrete

Post-Tensioning

• Definition: Similar to pre-stressing but applied on-site after curing before load application.
• Process:
  • Tendons are draped throughout beams/slabs to meet bending/deflection criteria.
  • Can be sheathed and greased or unsheathed, affecting construction and load transfer.

Benefits of Pre-Stressing

• Advantages:
  • Engages steel and concrete actively, leading to:
    • Stiffer sections
    • Options for reduced material usage
    • Ability to span longer distances and carry more loads
• Drawbacks:
  • Increased design and construction complexity.
  • Application depends on specific project needs.

Conclusion

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