IRC Code: IRC:SP:67-2005 – Guidelines for Use of External and Unbonded Prestressing Tendons in Bridge Structures

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The Indian Roads Congress (IRC) is the primary authority in India for establishing technical standards for road and bridge construction. One of its essential publications, IRC:SP:67-2005, provides comprehensive Guidelines for the Use of External and Unbonded Prestressing Tendons in Bridge Structures. This guideline serves as a crucial reference for engineers and designers, ensuring structural efficiency and durability in prestressed concrete bridges.

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Prestressing is a widely adopted technique that enhances the load-carrying capacity of bridges by introducing pre-compressive forces. The use of external and unbonded prestressing tendons offers advantages such as ease of maintenance, reduced material usage, and improved stress distribution.

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Scope of IRC:SP:67-2005

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This IRC Code covers the design, detailing, materials, protection, and applications of external and unbonded prestressing tendons. The key aspects include:

• Types of external and unbonded prestressing applications
• Material specifications for prestressing steel and sheathing
• Design guidelines as per IRC:18 and IRC:21
• Protection measures for prestressing steel, anchorages, and deviators
• Structural detailing and construction techniques

The code is applicable to new bridge constructions, retrofitting, and strengthening of existing structures.

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Applications of Unbonded Prestressing

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The use of unbonded prestressing tendons is categorized into three main applications:

1. Primary Prestressing System
   • Used in segmental bridge superstructures where tendons act as the main prestressing force.
2. Supplementary Prestressing
   • External prestressing used for strengthening or rehabilitation of existing bridges.
3. Load Redistribution System
   • Used as an auxiliary mechanism to alter load paths and stress distribution in bridges.

These methods ensure that bridges remain structurally sound, safe, and cost-efficient over their service life.

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Design Considerations

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Structural Behavior of Unbonded Prestressing

Unlike bonded tendons, unbonded tendons do not adhere to concrete, making them more flexible in stress redistribution. However, they require anchorage points and deviators to maintain stability and effectiveness.

Load Combinations and Safety Factors

The design philosophy follows Limit State Method (LSM), ensuring compliance with:

• Ultimate Limit State (ULS) for structural strength
• Serviceability Limit State (SLS) for deformation control

The applicable load factors are derived from IRC:6-2000, with prestressing force variations considered for different conditions.

Shear and Torsion Considerations

The shear capacity of a bridge with external tendons is assessed using:

• Concrete section strength calculations
• Anchorages and deviators to resist torsional effects
• Load redistribution strategies for better performance

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Materials and Construction

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Prestressing Steel and Sheathing

Materials must meet the specifications of IRC:18-2000. Suitable prestressing steels include:

• High-strength galvanized strands
• Epoxy-coated steel tendons
• Sheathed and lubricated strands for corrosion protection

Sheathing Systems

Sheathing must be leak-tight and pressure-resistant, with materials such as:

• HDPE (High-Density Polyethylene) ducts
• Metallic sheaths with protective coatings

Anchorages and Deviators

Anchorages are critical components that transfer forces to the concrete structure. IRC:SP:67-2005 mandates:

• Replaceable and adjustable anchor systems
• Proper detailing to minimize stress concentrations
• Protection using epoxy coatings or sealed enclosures

Protection Measures for Prestressing Tendons

To ensure long-term durability, prestressing tendons must be protected using:

• Cement grout or nuclear-grade grease for corrosion resistance
• Steel protective enclosures for exposed anchorages
• Periodic inspections and maintenance routines

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Advantages of External and Unbonded Prestressing

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1. Ease of Inspection and Maintenance

External tendons allow visual inspection and force monitoring, reducing the need for costly repairs.

2. Improved Structural Performance

Unbonded tendons provide flexibility in load redistribution, enhancing bridge resilience and earthquake resistance.

3. Economical and Efficient Design

With optimized material usage and simplified construction, external prestressing offers cost-effective solutions for bridge engineering.

4. Retrofitting and Strengthening Existing Bridges

Unbonded prestressing is widely used for bridge rehabilitation projects, extending service life without major modifications.

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Conclusion

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The IRC:SP:67-2005 serves as an essential guideline for the design, detailing, and application of external and unbonded prestressing tendons in bridge structures. By implementing these provisions, engineers can enhance bridge longevity, optimize structural efficiency, and ensure long-term durability.

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