Assessing Fire Damage: Structural Investigation of Bridge Girders After Fire

Severe fire damage beneath a pedestrian bridge, showing charred concrete, soot deposits, and structural deterioration. The assessment focused on evaluating fire-induced degradation to ensure bridge safety and restoration.

Overview

In response to a fire incident in June 2023, at a bridge in Walnut Creek, CA, Concrete Science, Inc. conducted a comprehensive fire damage assessment on bridge girders. The fire, caused by burning rubbish beneath the structure, lasted approximately 2.5 hours and subjected the girders to an extremely high temperature. The assessment aimed to determine the extent of structural degradation, evaluate concrete integrity, and identify areas requiring repair.

The work involved field & laboratory testing. Our engineers conducted pulse velocity tests, per ASTM C597, on fire exposed as well as non-fire exposed concrete girders. We identified critical areas and obtained core samples through the web of the girders. Our petrographer performed petrographic (microscopic) analysis, per ASTM C856, on core samples to determine the depth of fire damage, changes in hydrated cement, aggregate, and microcracking. The objective was to provide actionable recommendations to restore the bridge's integrity.

Scope of Work

Fire-exposed concrete girder with visible spalling, cracking, and surface degradation. Markings indicate areas of structural assessment and testing for material integrity.
Soot-covered girder with structural investigation markings from fire damage assessment. The surface condition highlights the extent of heat exposure and material degradation.

  • Conducted pulse velocity testing, ASTM C597, on both fire-exposed and non-fire exposed sides of the bridge girders.

  • Compared pulse velocity readings across different sections of the girders to identify variations in concrete quality.

  • Documented visible fire damage, including spalling, cracking, and delamination.

  • Assessed surface conditions to determine the depth of fire damage and material degradation.

  • Obtained concrete core samples from both fire-exposed and non-fire exposed areas of the girders.

  • Conducted petrographic analysis to examine fire-induced effects, including cement paste alteration, aggregate reactions, microcracking.

  • Measured depth of carbonation and fire-induced changes in concrete composition.

  • Verified microstructural changes in fire-exposed samples using petrographic microscopy and chemical analysis.

Concrete core sample labeled from the fire-exposed side, showing surface porosity and heat effects. Measurement scale provides context for material loss and fire damage assessment.
Petrographic microscope image of a fire-damaged concrete sample, highlighting microcracks, aggregate changes, and heat-induced discoloration. Analysis reveals structural alterations from high-temperature exposure.
Fire-damaged concrete surface with exposed rebar imprints and a measuring tape indicating material loss. The assessment focuses on evaluating fire-induced deterioration and structural integrity.

Material Condition Assessment

  • Inspected condition of reinforcing steel and its bond with the concrete.

  • Evaluated concrete integrity using petrographic data and field test results.

  • Identified areas of significant material loss.

  • Provided targeted repair recommendations to address damage and enhance structural durability.

Key Findings

  • Fire-Exposed Side:

    • Web sections of all bridge girders showed minor surface spalling but remained structurally intact.

    • The areas of girder which exhibited severe spalling, cracking, and delamination, had significantly reduced pulse velocity readings. The pulse velocities in these areas were as low as 5,700 ft/sec, far below the remaining areas of the girder, indicating serious fire-induced deterioration.

    • Typical pulse velocity readings in the unaffected sections were higher, averaging 13,600 to 15,300 ft/sec, confirming that structural integrity remained largely intact in these areas.

    • Pulse velocity data showed a clear contrast between fire-damaged and unaffected sections, aiding in the targeted repair strategy.

  • Non-Fire Side:

    • No significant damage observed.

    • Pulse velocity readings ranged between 15,200 to 15,350 ft/sec, confirming that concrete remained in good structural condition with no fire-related deterioration.

Pulse velocity testing being conducted on a fire-damaged concrete girder to assess internal integrity. The test helps determine the extent of material degradation and guides repair decisions.

  • Fire-Affected Concrete Cores:

    • Surface spalling, black soot deposits, and severe microcracking detected.

    • Petrographic analysis revealed cement paste dehydration, carbonation, and fire-induced cracking within the concrete matrix.

    • Aggregate discoloration observed in concrete confirmed high temperature exposure.

    • Pulse velocity and petrographic results were consistent, confirming significant material degradation in fire-exposed areas.

  • Non-Fire Cores:

    • No fire-related deterioration.

    • Minor carbonation was observed.

    • Pulse velocity confirmed no internal voids or damage, ensuring structural stability.

Fire-damaged concrete core with visible spalling and soot deposits on the surface. The image highlights material degradation from heat exposure and structural assessment markings.
Microscopic view of a fire-exposed concrete sample, showing aggregate reactions, heat-induced discoloration, and microcracking. Petrographic examination identifies structural changes from thermal exposure.

Solutions and Recommendations

Structural Repairs & Girder Rehabilitation

  • Identified critical areas for repair.

  • Recommended repairs to the girder web & flanges.

  • Recommended repair to minimize future carbonation.

Soot-covered bridge girder with minor surface spalling, indicating exposure to fire but minimal structural damage. Assessment focused on evaluating material integrity and surface degradation.
Heavily fire-damaged concrete girder with visible spalling, soot accumulation, and exposed reinforcement imprints. Structural assessment focuses on evaluating degradation and recommending fire-resistant protection measures.

Recommendations

  • Recommended that the girder sections be rebuild.

  • Recommended further structural evaluation to determine optimal strengthening solutions for the long-term durability.

  • Recommended anti-carbonation coatings to mitigate potential reinforcement corrosion.

Close-up of a pulse velocity test being performed on a fire-damaged concrete girder. Non-destructive testing helps assess internal structural integrity and guide rehabilitation strategies.
Under-bridge view showing fire damage before repairs, with soot-covered surfaces and structural exposure. Assessment focused on evaluating fire-induced deterioration and planning rehabilitation.
Depth measurement inside a drilled concrete core used for petrographic and structural analysis. Core sampling provides insights into fire-induced damage and material degradation.

Outcome of the Fire Damage Investigation

The client received comprehensive, data-driven recommendations for bridge restoration. Findings from this fire damage assessment provided critical insights for informed decision-making regarding bridge maintenance and long-term structural preservation.

Revisiting the site 2 years later (2025)…

The bridge has been repaired and is now in regular use.

Under-bridge view after fire damage rehabilitation, highlighting clean surfaces and reinforced concrete. The structure has been restored to full functionality with enhanced protection.
Post-repair view of the bridge showing resurfaced girders and restored structural components. Rehabilitation efforts focused on improving durability and fire resistance.
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