External prestressing of concrete is a technique used for strengthening and rehabilitating existing bridge structures that have experienced deterioration, increased load demands, or structural deficiencies over time. Unlike conventional prestressing where tendons are embedded within the concrete cross-section, external prestressing involves placing the prestressing tendons outside the structural member, anchored at the ends and deviated using saddles or deviators. This technique is particularly suited for bridge rehabilitation because it can be applied with minimal disruption to traffic and does not require extensive demolition of existing concrete.
Applications of External Prestressing in Bridge Repair
- Deck Strengthening: Adds external tendons to increase flexural capacity.
- Span Continuity: Links spans to reduce joint stress and improve load distribution.
- Shear/Flexural Upgrade: Supplements internal reinforcement for higher load demands.
- Damage Repair: Restores strength after corrosion, cracking, or impact.
- Bridge Refurbishment: Replaces or supports failed tendons in complex bridge types.
How External Prestressing Works?
External prestressing systems function by inducing a pre-compression force into the bridge component through high-strength steel tendons (usually seven-wire strands or parallel wire cables). These tendons are placed along a predetermined path, usually external to the concrete section and are tensioned using hydraulic jacks. The key components include:
- Anchorages fixed at both ends to transfer the prestressing force.
- Deviators or saddles to provide the necessary tendon profile (drape).
- Bearings and guides to ensure tendon alignment.
- Corrosion protection systems such as sheathing, greased ducts, or HDPE pipes.
Advantages of External Prestressing for Bridge Repair
- Minimal Intrusiveness: Repairs can be conducted with minimal demolition of the existing structure, often allowing partial or full use of the bridge during execution.
- Ease of Inspection and Maintenance: External tendons are visible and accessible, simplifying routine inspection, tension verification, and replacement if needed.
- Flexibility in Design: Tendon profiles can be easily adjusted during construction to optimize stress distribution.
- Cost-Effective Strengthening: Compared to replacement, external prestressing is more economical and avoids the environmental impact of demolishing large concrete structures.
- Adaptability to Multiple Damage Types: Whether the structure has experienced flexural cracking, shear deficiencies, or even support settlement, the system can be designed to address the specific issue.
- Reusability of Infrastructure: External prestressing allows for the continued use of foundational and substructure elements, extending the overall life of the bridge asset.
Heavy Equipments Used in External Prestressing for Bridge Repair
External prestressing for bridge rehabilitation involves complex installation and tensioning procedures that require a coordinated deployment of heavy equipment. These machines facilitate the safe lifting, access, anchorage installation, and tensioning of prestressing tendons, especially in elevated or constrained bridge environments. The following heavy equipment types are commonly used in such operations:
Mobile Cranes and Truck-Mounted Cranes
Cranes are indispensable in the installation phase of external prestressing systems. Mobile cranes and truck-mounted cranes are typically used to lift and place large structural components such as anchorage blocks, steel deviator frames, tendon reels, and tensioning jacks. On multi-span or high-clearance bridges, mobile cranes allow erection of steel supports or platforms beneath girders. Their capacity to handle loads ranging from 10 to over 100 tonnes makes them suitable for transporting heavy prestressing materials to specific locations along the bridge span, even in confined urban corridors or difficult terrain.
Suspended Rope Platforms
Suspended rope platforms are commonly used in external bridge prestressing to provide access beneath bridge decks or girders, especially in locations where ground-based access is not feasible, such as over water or high elevations. These platforms, suspended by wire ropes from the bridge superstructure, allow workers to install anchorages, guide external tendons, and carry out tensioning operations using hydraulic jacks. They are also used for inspection and adjustment of tendons and deviators during or after prestressing. Operated with manual or motorized hoists, the platforms must comply with safety standards, including harnesses and fall protection systems.
Aerial Work Platforms (AWP) and Boom Lifts
To ensure safe access to elevated sections of the bridge, such as girder undersides or vertical sides, articulated and telescopic boom lifts are deployed. These aerial work platforms enable technicians to carry out pivotal tasks including drilling, saddle installation, cable routing, and grouting without relying on extensive scaffolding. Especially in projects where traffic must remain operational on the bridge deck, AWPs allow workers to access the structure from below without disrupting vehicular flow. Their mobility and height-adjustability make them an essential tool for bridge retrofitting tasks.
Self-Propelled Modular Transporters (SPMTs)
In projects involving large-span bridge segments or full-span precast units, SPMTs are occasionally used to reposition structural components during or before external prestressing application. These computer-controlled transporters can maneuver massive loads with millimeter precision, allowing them to carry segments or girders that are being retrofitted with external tendons. While not required in every project, they play a key role in complex segmental bridge rehabilitation schemes or in staged bridge construction that involves external prestressing of prepositioned units.
Hydraulic Jacks and Stressing Equipment
Tensioning in external prestressing involves the use of hydraulic jacks to apply force to high-strength steel tendons. Depending on the tendon configuration, either mono-strand or multi-strand jacks are used, capable of delivering several hundred kilonewtons of force. These are operated with hydraulic power packs and precision gauges that measure elongation and applied load. Stressing equipment is positioned on mobile platforms or anchored temporarily near the tendon anchorage zone and is operated under controlled conditions to achieve the specified prestress force defined in the structural design.
Scaffolding and Gantry Systems
Temporary access structures such as modular scaffolding systems and under-bridge gantries are essential for conducting installation work at height. These platforms are erected below or alongside the bridge deck and provide a stable working surface for tasks such as fixing anchorage plates, installing saddles or deviators, and routing external tendons. Gantries may be suspended or movable and are designed to support the weight of personnel, equipment, and materials. For larger bridges or long spans, specially designed mobile gantries or overhead systems may be used for continuous tendon installation and tensioning operations.
Concrete Core Drills and Pneumatic Breakers
During the anchorage installation phase, precise recesses must often be created in existing concrete elements to house bearing plates or support brackets. Heavy-duty concrete core drilling machines are used to bore through reinforced sections without causing excessive vibration or damage to the surrounding structure. In cases where demolition is required, pneumatic breakers and rotary hammer drills are employed to remove deteriorated or obstructing concrete. These tools allow clean preparation of anchorage zones and ensure proper load transfer from the tendon to the existing structure.
Tendon Uncoilers and Cable Payout Machines
Handling long lengths of prestressing cables or strands requires controlled feeding systems to prevent damage or misalignment. Tendon uncoilers, also known as cable payout machines, are designed to support and smoothly release coils of steel strand or cable during installation. These machines often come with tension control mechanisms and rollers to guide the tendon along the specified path through deviators and anchorages. By minimizing friction and preventing kinks or twists, they help ensure tendon integrity and ease of tensioning.
High-Pressure Grouting Pumps
Once tendons are tensioned, they must be protected against environmental exposure, particularly in external configurations where corrosion risk is higher. High-pressure grouting pumps are used to inject protective grout or grease into ducts or sheaths encasing the tendons. These pumps generate sufficient pressure to fill the entire void around the tendons, ensuring proper coverage and eliminating air pockets. For permanent corrosion protection, the grout mix often includes additives to enhance bond strength and moisture resistance.
Cable Pusher Systems and Winch Pulling Units
In long-span or cable-stayed bridge repairs involving external prestressing, cable pushers and motorized winch units are used to feed tendons over considerable distances. These systems are important when tendons need to pass through multiple saddles or across expansion joints. Cable pushers apply a continuous pushing force to prevent buckling, while winch pulling mechanisms ensure steady tension and directional control during installation. Their coordinated use ensures damage-free routing of unbonded tendons along curved profiles in segmental or box girder bridges.
Post-Tensioning Duct Installers and HDPE Duct Handling Rigs
External prestressing often uses high-density polyethylene (HDPE) ducts or sheathing to encase the tendons and provide corrosion protection. Installing these ducts over long spans requires dedicated handling rigs, reels, and duct threading systems. These systems are used to unroll, cut, align, and fix the ducts along the girder or deck profile before the tendons are inserted. Careful installation of ducts ensures uniform cover, correct alignment through deviators, and adequate space for later grouting operations.
Strand Pushing Jacks and Pre-Stressing Chairs
In addition to standard stressing jacks, projects may require strand pushing jacks to assist in the positioning of tendons through ducts with multiple deviations. These hydraulic devices push individual strands through long or curved tendon profiles. Pre-stressing chairsāadjustable steel framesāsupport stressing jacks at the anchorage zone and help maintain alignment during load application. They provide a rigid setup during simultaneous tensioning of multiple strands in multi-strand external tendon systems.
Segmental Bridge Gantry Cranes and Launching Systems
For externally prestressed segmental bridges, heavy gantry cranes or launching girders may be re-engaged to reposition precast segments during repair and realignment. While these are not used in every repair job, they are indispensable when segmental adjustments or re-leveling is needed before introducing new external tendons. The same equipment may also be retrofitted with tendon feeding tracks or access platforms for placing external cables and supports.
Conclusion
External prestressing has emerged as a strategic solution for bridge rehabilitation, offering both flexibility and structural enhancement without extensive dismantling. Its ability to extend service life, adapt to various bridge types, and accommodate in-situ conditions makes it especially valuable in aging infrastructure scenarios across India. When combined with detailed structural assessment, quality equipment, and robust corrosion protection, external prestressing becomes a reliable technique for modern bridge repairing.
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