Lecture-2-Type of Prestressing

Key Concepts:

• Prestressing steel forms (wires, strands, tendons, bars, cables)
• Bonded vs. Unbonded tendons
• Stages of Loading (Initial, Intermediate, Final)
• Advantages of Prestressing (reduced cracking, increased durability, higher stiffness, increased shear capacity, high span-to-depth ratios, suitability for precast construction)
• Limitations of Prestressing (skilled technology, costly materials, auxiliary equipment, quality control)
• Types of Prestressing (based on source, location, sequence, shape, amount, direction)
• Pre-tensioning vs. Post-tensioning
• Full, Limited, and Partial Prestressing
• Uniaxial, Biaxial, and Multiaxial Prestressing

1. Forms of Prestressing Steel

• Wires: Single steel units.
• Strands: Two, three, or seven wires wound together (typically a seven-wire strand with six wires around a central wire).
• Tendons: A group of strands or wires wound together.
• Bars: Single, larger diameter steel units.
• Cables: A group of tendons forming a prestressing cable.
• Anchoring Systems: Tendons are attached using anchor blocks, guides, and wedges (jaws) to hold them in place. Individual wires can be anchored using button heads.

2. Nature of Concrete-Steel Interface

• Bonded Tendon: Adequate bond exists between the prestressing tendon and concrete. Pre-tensioned members and grouted post-tensioned members are examples. Concrete is all around the tendons.
• Unbonded Tendon: No bond exists between the prestressing tendon and concrete. Ungrouted post-tensioned tendons are examples. Concrete does not come in direct contact with the tendon.

3. Stages of Loading

• Initial Stage:
  • During tensioning of steel.
  • At transfer of prestress to concrete.
  • Purpose: Understand the transfer of forces between steel and concrete.
• Intermediate Stage:
  • Loads during transportation of precast members.
  • Analysis required for stresses during transportation.
• Final Stage:
  • At service: Member under normal operation.
  • At ultimate: Extreme event, member reaches final strength.

4. Advantages of Prestressing

• Reduced Cracking: Concrete remains uncracked under service conditions.
• Increased Durability: Reduction of steel corrosion due to uncracked concrete.
• Full Section Utilization: Higher moment of inertia, increased stiffness, less deformation, improved serviceability.
• Increased Shear Capacity: Uncracked section leads to higher shear resistance.
• Suitability for Pressure Vessels and Liquid Storage Tanks: Prevents liquid seepage through cracks.
• Improved Performance Under Dynamic and Fatigue Loading: Better resilience under repeated loading cycles.
• High Span-to-Depth Ratios:
  • Non-prestressed slab: Span-to-depth ratio of 28:1.
  • Prestressed slab: Span-to-depth ratio can increase to 45:1.
  • Enables larger spans for bridges and buildings with large column-free spaces.
• Reduced Self-Weight: Less depth compared to RC members, leading to more aesthetic appeal and economical sections.
• Suitability for Precast Construction:
  • Rapid construction.
  • Better quality control.
  • Reduced maintenance.
  • Suitable for repetitive construction.
  • Multiple use of formwork.
  • Availability of standard shapes (T-sections, double T-sections, hollow core sections, piles, L-sections, inverted T-sections, I-girders).

5. Limitations of Prestressing

• Skilled Technology Required: Not as common as reinforced concrete due to complexity.
• Costly High-Strength Materials: High-strength steel and concrete increase material costs.
• Additional Cost of Auxiliary Equipment: Specialized equipment for tensioning and anchoring.
• Need for Better Quality Control and Inspection: Critical to ensure proper prestressing and prevent failures.

6. Types of Prestressing (Classifications)

• Based on Source of Prestressing Force:
  • Mechanical: Weights, levers, geared transmissions, screw jacks, wire-winding machines (suitable for mass production).
  • Hydraulic: Hydraulic jacks with calibrated pressure gauges (most common).
  • Electrical: Electrically heated steel wires anchored before concrete placement (thermo-electric prestressing).
  • Chemical: Expansive cements used, expansion controlled by curing conditions.
• Based on Location of Prestressing Tendon:
  • External Prestressing: Tendons located outside the concrete member (e.g., cables outside a beam, used in retrofitting bridges).
  • Internal Prestressing: Tendons located inside the concrete member (most common).
• Based on Sequence of Casting and Tensioning:
  • Pre-tensioning: Tension applied to tendons before casting concrete. Pre-compression transmitted through bond.
  • Post-tensioning: Tension applied to tendons after hardening of concrete. Pre-compression transmitted by anchorage devices.
• Based on Shape of Member:
  • Linear Prestressing: Straight or flat members (beams, piles, poles, slabs).
  • Circular Prestressing: Curved members (tanks, silos, pipes).
• Based on Amount of Prestressing Force:
  • Full Prestressing (Type 1): No tensile stress allowed in concrete under service loads.
  • Limited Prestressing (Type 2): Tensile stress under service loads is within the cracking stress of concrete.
  • Partial Prestressing (Type 3): Cracking allowed under service loads, crack width limited.
• Based on Directions of Prestressing:
  • Uniaxial Prestressing: Cables parallel to one axis (longitudinal prestressing of beams).
  • Biaxial Prestressing: Cables parallel to two axes (biaxial prestressing of slabs).
  • Multiaxial Prestressing: Cables parallel to more than two axes (prestressing of domes).

7. Manufacturing of Pre-tensioned Railway Sleepers (Step-by-Step)

1. Tensioning the Steel:
   • A traveling pre-tensioning stress bench is used.
   • Tendons are anchored at one end (dead end) using wedges.
   • The other end (stretching end) is pulled by hydraulic jacks.
   • Extension is measured using a scale.
2. Casting the Concrete:
   • Ingredients (coarse aggregate, fine aggregate, cement, water, admixtures) are weighed by an automatic batching machine.
   • Concrete is mixed and dropped onto the stress bench.
   • Vibration is applied to compact the concrete.
   • Surface is finished.
3. Curing:
   • The mould is placed in a steam chamber for 24 hours for initial curing.
4. Transferring Prestress:
   • Wires are cut, transferring prestress to the concrete through bond over the transmission length.
5. Demoulding and Stacking:
   • Sleepers are taken out of the mould.
   • Sleepers are stacked and cured in a water tank for final strength gain.
6. Dispatch:
   • Sleepers are loaded onto wagons for transport to the railway site.

8. Construction of Post-tensioned Bridge Girder (Step-by-Step)

1. Fabrication:
   • Reinforcement cage is fabricated.
   • Ducts for tendons are placed based on analysis.
2. Casting Concrete:
   • Side formworks are placed.
   • Concrete is cast and cured.
3. Tensioning Tendons:
   • Steel tendons are passed through the ducts.
   • Tendons are stretched by hydraulic jacks.
   • Wedges are placed in anchor blocks.
   • Jacks are released, transferring tension to the concrete as compression.

9. Notable Quotes/Statements

• "A fully prestressed concrete member is usually subjected to compression during service life. This rectifies several deficiencies of concrete."
• "For the same span, the depth of a prestressed member is less than the corresponding RC member. It leads to reduction in self weight, more aesthetic appeal due to slender sections and finally it gives a more economical section because the amount of concrete used is much less."
• "Hence, in a post-tensioning operation, one has to be extremely careful during the stretching of the steel because that is the testing time for the prestressed member."

10. Synthesis/Conclusion

Prestressed concrete offers significant advantages over traditional reinforced concrete, including reduced cracking, increased durability, and higher span-to-depth ratios. However, it also requires skilled technology, high-quality materials, and careful quality control. The various types of prestressing, including pre-tensioning and post-tensioning, are suited for different applications. The manufacturing processes for pre-tensioned elements like railway sleepers and post-tensioned structures like bridge girders involve distinct steps that must be carefully executed to ensure the desired performance and longevity of the structure. The choice of prestressing type depends on the specific project requirements and desired performance characteristics.