Repairing Damaged or Broken Blisters in Box Girder Bridges

USAMA KHAN
13 hours ago
10 min read

Abstract

Blisters—the localized anchorage blocks in box girder bridges—are critical structural elements that transfer concentrated post-tensioning forces into the girder body. When damaged, they represent a significant structural vulnerability requiring immediate attention. This comprehensive article details standardized methodologies, safety protocols, and engineering practices for repairing damaged blisters, drawing from AASHTO, Eurocode, and UK Highways England standards. Proper repair not only restores structural integrity but extends the service life of the entire bridge system.

Pre-Repair Assessment and Investigation

Initial Inspection and Condition Survey

Visual Inspection: Document all visible damage including cracking patterns, spalling extent, reinforcement exposure, and water staining
Damage Classification:

o Minor: Hairline cracks (<0.1mm), surface spalling without reinforcement exposure

o Moderate: Cracks (0.1-0.3mm), localized spalling with partial reinforcement exposure

o Severe: Cracks (>0.3mm), extensive spalling, reinforcement corrosion, deformation

o Critical: Complete blister failure, tendon anchorage compromise, imminent collapse risk

Non-Destructive Testing (NDT) and Investigation:

Concrete Assessment:

o Ultrasonic Pulse Velocity (UPV): Assess concrete quality and detect voids

o Rebar Locators: Map reinforcement layout and cover depth.

o Half-Cell Potential: Measure corrosion risk of embedded steel (NDT)

o Carbonation Depth Testing: Determine concrete passivation loss

Tendon Condition Assessment:

o Void Detection: Ground penetrating radar (GPR) to locate grout deficiencies

o Strand Corrosion: Endoscope inspection of duct interiors where accessible

Standard Repair Methodologies

Method A: Epoxy Injection Repair (Minor to Moderate Cracking)

Epoxy injection represents the premier method for restoring monolithic structural integrity to cracked concrete without adding mass or altering geometry. Its primary significance lies in its ability to penetrate hairline cracks as fine as 0.1mm, creating a molecular bond that restores the tensile capacity of the concrete across the crack plane, effectively rejoining separated sections into a single load-bearing element. This process not only re-establishes load paths but also provides a critical barrier against moisture and chloride ingress—key contributors to reinforcement corrosion. By using low-viscosity, high-strength structural epoxies, this method addresses both structural and durability concerns simultaneously, preventing progressive crack propagation that could otherwise lead to spalling, water penetration into tendon ducts, and eventual blister failure. Furthermore, epoxy injection is minimally invasive, requiring no removal of sound concrete and preserving the original reinforcement layout and cover depth, which is crucial for maintaining the designed fire resistance and long-term durability of the anchorage zone.

When to Resort ?

This method is the definitive choice for treating active structural cracks measuring between 0.1mm and 3.0mm that are subject to movement but have stabilized, and where the underlying concrete substrate remains sound and the reinforcement is uncorroded.

It is particularly appropriate for repairing map cracking or isolated linear cracks radiating from anchorage points—classic signs of bursting or spalling stresses—prior to the onset of spalling. It should be implemented when crack width monitoring indicates the movement is within the elastic deformation capacity of the epoxy (typically 1-2% strain). This technique is not suitable for dormant cracks that may reopen, for cracks wider than 5mm (which require mortar filling first), or in cases where the reinforcement has begun to corrode, as the corrosion products will continue to expand and fracture the repair.

Epoxy injection is also contraindicated in permanently wet conditions unless specifically formulated hydrophilic resins are used, and it should not be applied to cracks that are

merely aesthetic or not affecting structural performance.

Method B: Partial Depth Removal and Patching

Partial-depth removal and patching is the cornerstone repair for localized concrete deterioration, addressing the most common forms of blister damage: spalling, delamination, and surface corrosion. Its fundamental significance lies in its surgical precision—it removes only compromised material, preserving the healthy structural concrete beneath while completely excising contaminated or carbonated concrete that harbors chlorides.

This method allows for the direct treatment of corroded reinforcement through cleaning, passivation, and application of corrosion inhibitors, interrupting the electrochemical corrosion cycle. By utilizing advanced repair mortars—such as polymer-modified, shrinkage-compensated, or microsilica-enhanced formulations—the patch achieves superior bond strength (often exceeding the tensile strength of the parent concrete), durability, and resistance to future environmental attack. The technique restores not only the structural section but also the vital concrete cover protection for the reinforcement, re-establishing the blister's original geometry and load-transfer mechanism.

When to Resort ?

This method is the standard protocol for localized spalls or delaminations where reinforcement is exposed but corrosion is limited to a shallow depth (typically less than 25% of bar diameter loss) and where the damage does not extend through the full section thickness. It is specifically indicated when sounding surveys reveal hollow areas or when visual inspection shows rust staining, pop-outs, or minor concrete disintegration without structural deformation. Resort to this method when the substrate concrete behind the reinforcement remains sound with a compressive strength within 10 MPa of the original design, and when the repair area is manageable (generally less than 0.5m² per discrete patch to minimize thermal and shrinkage stresses). It is not appropriate for widespread corrosion (affecting more than 50% of the reinforcement in the blister), for damage that extends to the tendon ducts, or when the remaining concrete substrate is of questionable quality.

Methodology

1. Demolition Control:

o Saw-cut perimeter (minimum 25mm depth beyond damage)

o Remove unsound concrete to sound substrate

o Maintain minimum 90° edges (no featheredging)

Substrate Preparation:

o Sandblast to achieve CSP 3-4 (Concrete Surface Profile)

o Remove laitance and expose aggregate

o Clean with high-pressure water (minimum 3000 psi)

Bonding and Patching:

o Apply bonding bridge (epoxy or polymer-modified cementitious slurry)

o Place repair mortar in 50mm maximum layers

o Use shrinkage-compensated concrete or polymer-modified mortar

o Consolidate thoroughly, avoid overworking

o Wet cure for minimum 7 days. Use curing membranes compatible with waterproofing systems

Method C: Full-Depth Replacement (Critical)

Full-depth replacement represents the most comprehensive and structurally definitive repair for severely compromised blisters, essentially reconstructing the anchorage zone to its original—or enhanced—design capacity. Its paramount significance is the complete elimination of all deteriorated and contaminated material, including compromised reinforcement, allowing for a fresh installation with modern corrosion protection systems. This method provides the opportunity to correct original design deficiencies, upgrade reinforcement detailing, incorporate improved materials (such as higher strength concrete or stainless steel reinforcement), and install advanced monitoring instrumentation during reconstruction.

By creating a completely new monolithic section, it restores confidence in the blister's ability to transfer concentrated tendon forces without uncertainty about residual damage or hidden defects. Furthermore, it permits thorough inspection of adjacent areas, including tendon anchorages and ducts, that are normally inaccessible.

When to Resort ?

This aggressive intervention is reserved for CRITICAL DAMAGE SCENARIOS where corrosion has compromised more than 25% of the primary reinforcement cross-sectional area, where extensive delamination affects over 50% of the blister's surface area, or where structural deformation indicates a loss of load-bearing capacity. It is mandatory when tendon ducts are exposed or damaged, when the anchorage hardware itself shows signs of distress, or when multiple repair attempts have failed.

This method becomes necessary when non-destructive testing reveals widespread concrete deterioration with compressive strength loss exceeding 30% of specified strength, or when the blister exhibits progressive movement under load. It should only be undertaken with complete temporary support systems in place and with the bridge either fully closed to traffic or with drastically reduced load limits, as the blister will be completely non-functional during reconstruction. The considerable cost, traffic disruption, and engineering complexity justify this method only when no lesser repair can guarantee long-term safety.

Methodology

Temporary Support:
- Install temporary post-tensioning or propping to relieve blister loads
- Install reaction frames for tendon retensioning if required
Demolition:
- Sequential removal using hydraulic splitters or diamond sawing
- Protect adjacent tendons with sacrificial buffer zones
Reinforcement Replacement:
- Replace corroded bars with equivalent or higher grade steel
- Use mechanical couplers for continuity
- Ensure minimum 50mm cover with corrosion inhibitors in concrete mix
Reconstruction:
- Match original geometry within ±3mm
- Use microsilica concrete (minimum 50 MPa or higher as per girder concrete design for abrasion resistance

Method D: Weld Repair of Cracks

Weld repair is the fundamental restoration technique for steel blister stiffeners and protrusions, offering the unique ability to restore metallurgical continuity and load path integrity in a single process. Its critical significance lies in reconstituting the original material properties through fusion, effectively eliminating the stress concentration at crack tips and restoring the full sectional capacity of the steel element. Properly executed weld repair not only addresses visible defects but can enhance the local fatigue resistance through controlled weld profiles and post-weld treatments. The method allows for the preservation of the original geometry and alignment of critical connections without requiring complete replacement of components, maintaining the complex fit-up of diaphragm connections and tendon anchorages. When performed with certified procedures and materials, weld repair creates a joint that often exceeds the base metal properties in strength and can be designed to provide improved toughness in fracture-critical applications.

When to Resort ?

Weld repair is specifically indicated for fatigue cracks in steel blisters, particularly those originating from weld toes, bracket connections, or cut-outs where stress concentrations have initiated progressive cracking. It is appropriate for repairing fabrication defects discovered during inspection, such as lack of fusion or undercut that has developed into service cracks. This method should be employed when crack lengths are manageable (typically less than 300mm) and accessible from both sides for proper groove preparation and welding.

It requires that the base metal remains ductile and weldable without pre-existing embrittlement, and that the surrounding structure can tolerate the heat input without distortion. Crucially, weld repair is only valid when the root cause of cracking has been identified and addressed—otherwise, cracks will inevitably recur.

It is contraindicated for cracks in corrosion-thinned sections (where material loss exceeds 15% of thickness), for cracks in high-strength steels without specialized procedures, or in constrained areas where proper weld profiles cannot be achieved and inspected.

Methodology

Crack Removal:
- Drill stop holes at crack ends (minimum 6mm diameter)
- Gouge or grind out entire crack, creating U-groove
- Magnetic particle test to verify complete removal
Welding:
- Preheat to 100-150°C for mild steel, higher for high-strength steels
- Use low-hydrogen electrodes (E7018 or equivalent)
- Implement multi-pass technique with peening between passes
- Complete full penetration welds with UT verification
Post-Weld Treatment:
- Slow cool under insulation blankets
- Grind flush and apply zinc-rich primer within 4 hours

Method E: Stiffener Replacement

Stiffener replacement is the definitive solution for irreparably damaged steel blister components, offering complete renewal of the load transfer mechanism with contemporary materials and detailing. Its primary significance is the elimination of compromised material that cannot be economically or effectively repaired, particularly sections with advanced corrosion, fatigue damage across multiple elements, or deformation from overload. This method provides an opportunity to upgrade the original design with improved details—such as larger weld access holes, smoother transitions, and better corrosion protection—that directly address the failure mechanisms observed.

By installing entirely new components, it resets the fatigue life of the connection and allows for the incorporation of modern steels with enhanced toughness and weathering characteristics. Furthermore, replacement permits thorough inspection and potential upgrading of the parent girder web or flange to which the blister attaches, addressing secondary issues before they become critical.

When to Resort ?

Complete stiffener replacement becomes necessary when corrosion has reduced plate thickness by more than 15% of the original design, when multiple intersecting cracks indicate widespread fatigue damage, or when deformation has altered the geometry beyond practical straightening tolerances. It is mandatory when connection failures have occurred or when previous repairs have been unsuccessful. This method is particularly appropriate for older bridges - as preventative upgrade before catastrophic failure.

Replacement should be undertaken when access conditions permit the installation of temporary bracing to maintain stability during removal and when the new components can be properly aligned and fitted. It is not advisable when only minor damage exists that could be addressed with local repairs.

Methodology

Temporary Bracing:
- Install reaction frames before cutting
- Maintain geometric stability during replacement
Sequential Replacement:
- Replace no more than 25% of connections simultaneously
- Use match-drilled holes for bolted connections
Fit-Up and Alignment:
- Achieve maximum 2mm gap for welding
- Use strongbacks to maintain alignment during welding

Advanced Strengthening Techniques

Method F: FRP Strengthening

FRP strengthening represents a paradigm shift in blister rehabilitation, providing substantial capacity enhancement without the significant mass addition, geometric alteration, or complex temporary works of traditional methods. Its transformative significance lies in its high strength-to-weight ratio (approximately 10 times that of steel), corrosion immunity, and ability to be custom-formed to complex geometries, making it ideal for the confined spaces and irregular shapes of blister blocks.

The application process is minimally invasive, often requiring only surface preparation rather than concrete removal, which preserves the existing structure's integrity and reduces downtime.

When to Resort ?

FRP strengthening is deployed as a preventative or enhancement measure for blisters showing and is particularly effective for treating distributed cracking patterns where traditional repair would require extensive concrete removal, or for strengthening regions around anchorage zones where adding conventional reinforcement is impractical. This method excels in environments with severe corrosion challenges, as the non-metallic materials are immune to chloride attack.

Resort to FRP when access constraints limit the use of heavy equipment, when minimizing dead load is critical, or when the blister geometry is too complex for conventional formwork.

Methodology

Surface Preparation: Achieve ICRI CSP 4-5 profile
Laminate Design: Typically carbon fiber (CFRP) with 1.5% strain capacity
Application: Wet layup or pre-cured systems with certified adhesives
Anchorage: Additional U-wraps around blister perimeter
Capacity Increase: Typically 30-40% additional bursting resistance

Method G: External Prestressing

External prestressing offers a fundamental re-engineering solution for severely compromised blister regions by introducing a new, independent load path that bypasses the damaged area entirely. Its paramount significance lies in its ability to apply controlled, active forces that directly counteract the service loads causing distress—essentially putting the damaged region into a beneficial state of compression that closes cracks and reduces stress amplitudes. This method provides measurable, verifiable strengthening through jacking force readings, offering quantifiable evidence of restored capacity that is often superior to passive repair methods.

External tendons are inspectable, replaceable, and adjustable throughout the structure's life, representing a maintainable solution rather than a permanent fix. By transferring forces away from damaged zones to stronger sections of the girder, this method can salvage blisters that would otherwise require complete reconstruction, often with significantly less traffic disruption and at lower cost than full replacement.

When to Resort

For Severely Damaged Blisters

External prestressing becomes the strategy of choice when internal tendon systems are compromised or when blister damage is so extensive that local repairs cannot restore sufficient capacity. It is appropriate for addressing systemic issues affecting multiple blisters, for upgrading structures to carry higher loads, or for repairing damage caused by tendon corrosion or anchorage failure.

This method should be considered when traditional repairs would require extensive temporary works or prolonged closures, as external prestressing can often be installed with less invasive procedures. It is especially valuable for historic structures where preserving original materials is important, as it minimizes intervention on the existing fabric. The method requires specialized design to ensure proper load transfer and fatigue performance of the new anchorages.

Primary Standards:

AASHTO LRFD Bridge Design Specifications (2020) - Section 5: Concrete Structures
AASHTO Manual for Bridge Evaluation (2018) - Condition Assessment Protocols
BD 63/07 (Highways England) - Inspection of Highway Structures
BD 79/19 (Highways England) - Strengthening of Concrete Highway Structures
ACI 224.1R - Causes, Evaluation, and Repair of Cracks in Concrete Structures
ACI 546R - Concrete Repair Guide