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Public Roads - Winter 2024

Winter 2024
Issue No:
Vol. 87 No. 4
Publication Number:
Table of Contents

Intumescent Paint Protection in the Transportation Industry

by Tyler Hebert
Two men in safety vests and hard hats look at the underside of a bridge following a collapse. Image Source: FHWA.
Engineers inspect the collapsed bridge on I–95 in Philadelphia, PA.

With the recent collapse of the bridge on I–95 in Philadelphia, PA, there is a spotlight on fire as a threat to transportation infrastructure. On June 11, 2023, a tanker carrying gasoline caught fire under an overpass and caused the steel girders of the bridge to become weak and fail due to the extreme temperatures. While these events are rare, bridge collapses can lead to large portions of fundamental highways being shut down for extended periods of time and possibly put many lives at risk. In the United States between 1997 and 2015, over 165 bridge fires occurred, and 30 resulted in collapse due to this damage. The majority of these fires are caused by tankers and trucks either leaking or spilling flammable chemicals on or below bridge decks. Of these bridges, steel bridges were the most susceptible to this particular kind of collapse, because they are higher than both wooden and concrete structures. Steel, in the presence of fire, generally remains unchanged until temperatures reach 600 °F; however, when temperatures of 1,100 °F occur, the steel loses half its strength. Gasoline fires quickly reach temperatures exceeding 1,500 °F, which contributed to the fast deterioration of the steel girders located on the I–95 bridge. As a result, fire is a very important concern for any kind of steel structure, especially buildings that are more susceptible to frequent fires.

There are a variety of methods used to eliminate this issue in the construction industry, such as using specialized materials to decrease the spread of fire and increase the structural resistance, but the same cannot be said for the transportation industry. One such method is the use of intumescent paint, a fire-retardant coating used to insulate different materials exposed to fire. While there are many varieties of this paint available to engineers and contractors, the general goal of these coatings is to withstand high temperatures for an extended period to allow for a longer emergency response time. When exposed to heat, the coating sublimates and expands up to 100 times its original thickness in the form of a carbonaceous char. This char creates a barrier between the specific structural component, such as essential girders or abutments, and the intensity of the flame. Intumescent paint can be applied to any structural material, whether steel, wood, concrete, or plaster. Based on the particular product, these coatings can withstand high temperatures for anywhere from 30 to 120 minutes. The emergency responders in Philadelphia arrived at the scene in only 10 minutes after they were alerted to the incident. Intumescent paint could eliminate large disasters by increasing the likelihood that firefighters will arrive on scene and extinguish the fire before any major structural impacts. Certain intumescent paint brands have also been tested and determined compliant in accordance with ASTM E-84, Surface Burning Characteristics of Building Materials, and ASTM E-119, Standard Test Methods for Fire Tests of Building Construction and Materials.

Visualization of a steel beam being burned from below by fire. A callout shows the intumescent coated substrate on top and the expanding char layer below. Image Source: FWHA illustration; images source: © Intel / Andrey / tiero /
Visual depiction how intumescent paint works in a bridge fire.

Intumescent paint is not the first proactive fireproofing technique, which raises the question as to what the current fireproofing methods are for highways and bridges. The National Fire Protection Association (NFPA) has various design criteria and standards to protect bridges from the possibility of collapse by fire. The NFPA standard for road tunnels, bridges, and other limited-access highways includes Standard, which states that “structural elements shall be protected in accordance with this standard in order to achieve the following functional requirements: support firefighter accessibility, minimize economic impact, and mitigate structural damage.” Currently, NFPA does not have the enforcement authority to ensure these standards are being implemented on highway bridges. Other agencies, such as the Occupational Safety and Health Administration, have applied some of these standards, but NFPA criteria are not legally binding on their own. As a result, it would be advantageous to review the current policies and guidelines to ensure all proper precautions are being exercised.

One issue with the implementation of intumescent paint in bridge applications is the cost. Intumescent coatings generally cost $4 to $12 per square foot; however, the avoided emergency replacement cost and economic impact of a bridge collapse would greatly offset this initial cost. To examine this mitigated cost, another bridge can be observed to determine the economic impact of losing a vital portion of an essential transportation system. A bridge collapse occurred in Minnesota on I–35W in 2007, which was traversed by over 140,000 daily commuters. A study conducted by the Minnesota Department of Employment and Economic Development on this catastrophe calculated upward of $17 million in losses in 2007 and $43 million in 2008. While this collapse was not caused by a fire, the lack of this crucial bridge for commuters resulted in a large economic hindrance. As a result, it is essential to provide proactive fire protection to all major traveled bridges, which can possibly mitigate extreme costs in the future.

Since intumescent paint is typically applied inside buildings with relatively controlled environments, additional research is needed to analyze the impact of precipitation, freeze-thaw cycles, and temperature fluctuations on intumescent paint performance and longevity. Furthermore, detailed investigations of the current fire protection practices need to be evaluated by State departments of transportation to prevent future incidents and ensure proper standards are being implemented. Overall, intumescent coatings have the potential to provide critical protection to bridges, which are a major component of transportation systems throughout the United States.

Tyler Hebert is a student at Marshall University in Huntington, WV. Tyler is currently studying civil engineering and will graduate in 2024.

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