Pre-Stressed Concrete Beams

Overview of Traditionally Reinforced Concrete Beams

• Strength Properties:
  • Concrete is strong in compression but weak in tension (about 1/10th as strong).
  • Tension in a pure concrete beam leads to cracks in the tension zone under bending loads.
• Role of Reinforcing Steel:
  • Structural bars are placed in the tension zone to handle tensile loads when concrete cracks.
  • Additional bars may control thermal effects (not covered in detail).
  • Tension zones can vary (e.g., at columns or cantilevers).

Comparison with Pre-Stressed Concrete Beams

• Active Engagement:
  • Pre-stressed concrete actively engages both steel and concrete, unlike traditional methods that allow passive interaction.

Making Pre-Stressed Concrete Beams

1. Pre-Cast, Pre-Tensioned Concrete Beam:
   • Cables are placed in a form and tensioned.
   • Concrete is poured and allowed to cure.
   • After curing, tension is released from cables.
   • Induces compression on the bottom half (cambering effect).
2. Load Handling:
   • When load is applied, pre-stressed cables induce tension that counters the pre-compression, allowing for higher load capacity compared to traditional beams.
   • Must balance pre-stressing to avoid cracking and maintain structural integrity.

Shrinkage Considerations

• Concrete Curing and Shrinkage:
  • Concrete shrinks as it cures (up to 18 months), affecting the tension in pre-stressed steel.
  • Loss of pre-stressing due to shrinkage requires careful calibration.
• High Strength Reinforcement:
  • High strength materials needed to manage pre-stressing without rupturing the steel.
  • Development of material science has improved the use of high strength reinforcing and concrete.

Post-Tensioning

• Definition:
  • Similar to pre-stressing but applied on site after curing.
• Tendon Placement:
  • Tendons are draped throughout slabs and beams to meet bending and deflection criteria.
• Types of Tendons:
  • Can be sheathed and greased or unsheathed, each with specific benefits for load transfer and construction ease.

Benefits of Pre-Stressed Concrete

• Advantages:
  • Results in a stiffer section.
  • Allows for reduced material usage, longer spans, and increased load capacity.
• Considerations:
  • Comes with design and construction complexity.
  • Application depends on specific project requirements.

Conclusion

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