Fireproof Coatings

Diagnosing Failure in Fireproofing [Intumescent Coatings]

The project specification for a recent building addition at a pharmaceutical manufacturing facility required that much of the structural steel be fireproofed. Fireproofing in this instance meant the application of a fire-resistive or intumescent coating.

Intumescent coating products are designed to passively protect substrate materials—typically steel—from reaching combustion temperatures. These coatings function by charring and “swelling” upon exposure to high temperatures such as when a fire occurs. The charred coating then acts as an insulating layer to slow heat transfer to the substrate. A coating’s performance is rated in terms of hours, which indicates how long the coating can adequately protect the substrate.

Photos courtesy of KTA Tator Inc.
The article discusses a coating failure with a commercial fireproofing system. The image above shows typical crack formation in the fireproofing material. Cracks in the dark blue topcoat often followed cracks in the underlying fireproofing.

The fireproofing system for this project, as with most architectural applications, required certification in accordance with ANSI/UL 263, “Fire Tests of Building Construction and Materials.” Fire ratings under UL 263 are expressed in hours and are applicable to floor-ceilings, roof-ceilings, beams, columns, walls and partitions. It should be noted that there is a more stringent classification for fire-resistive coatings under ANSI/UL 1709, “Rapid Rise Fire Tests of Protection Materials for Structural Steel.” This classification provides fire-resistance designs for protecting structural members subject to petrochemical exposure fires—such as at a refinery, off-shore oil platform, etc.

Starting point: Specification and planning

The project specification for the building addition required that fireproofing materials be field applied to structural steel framing members (i.e., steel columns and floor beams) that were critical to the support of the building in the event of fire.

The total applied dry film thickness of the fireproofing material was based on the size of the member being protected and the number of hours that the members needed to be protected.

A steel framing plan for the building provided details concerning the fire rating needed for individual members and the number of layers of fireproofing material and/or total dry film thickness needed. The plan indicated that the number of coats required on a given structural member ranged from two to 12, with total dry film thicknesses ranging from approximately 50 to 300 mils. The structural steel was to be shop primed with an epoxy coating.

One component, multiple coats

There are primarily two generic coating types that comprise intumescent coatings—one component acrylic/vinyl/polyvinyl acetate coatings or high-build epoxy coatings. The single component acrylic based coatings are applied in multiple coats, with the number of coats dependent on the thickness necessary for the coating to provide the required fire rating.

Since multiple coats are required, the time frame for a complete application may span several days based on the recoat time between coats. After application is completed, additional time may be needed to allow for sufficient curing of the intumescent coating layers before application of any required exterior finish coat. Finish coats may be required since the intumescent coating alone may not be suitable for prolonged exterior exposure.

The intumescent coating approved for the project was a one-component, solvent-based coating applied at up to 25 mils dry film thickness per coat. The specification required that an epoxy topcoat be applied after the material was sufficiently cured. The coating manufacturer provided criteria to evaluate coating cure based on a Durometer hardness test.

The applicable test method is based on the depth of penetration of a specific type of indenter gauge that is forced into the coating film under specified conditions. The indentation hardness is inversely related to the penetration where less penetration equates to a higher numerical reading which indicates a harder film that is assumed to indicate a greater degree of cure (based on the manufacturer’s product information). The product information also stated that the fire-resistive coating was not intended for exterior exposures or interior environments exposed to freeze/thaw conditions.

Problems call for consultant

The coating application for the project occurred in late fall in a Northeastern coastal location.

At that time, the building addition was only partially complete, and some areas where the steel was coated with the intumescent were not fully enclosed. The space was not conditioned until late in the year when the building addition had been completed.

Structural support steel
Cracking and disbonding of the applied intumescent prompted a comprehensive coatings evaulation.

By spring when the construction was nearing completion, various problems with cracking and disbonding of the applied intumescent were reported. An independent consultant was brought in to evaluate the problem.

At the site, the consultant observed that the structural steel exhibited pervasive, scattered areas where the coating system (fireproofing and topcoat) was cracking, disbonding, or had slightly raised up and lifted away from the steel. The raised and lifted areas of coating were sometimes visually apparent because they bulged out from the surface. Other raised lifted areas were less apparent and were only identifiable by sounding (tapping) the steel with a metal wood chisel or hammer. Sounding on areas where the fireproofing had lifted from the steel produced a “hollow thud” while areas that had not lifted produced a metallic-like ring. When a metal chisel was used to fracture and remove the raised/lifted areas, the entire fireproofing system typically disbonded cleanly from the shop applied primer layer.

Moreover, the backside of the disbonded tan-colored fireproofing was smooth and exhibited cracking that radiated in all directions. The fireproofing could be easily removed for several inches around the periphery of the raised/lifted area. As coating was removed outward from the lifted area, adhesion eventually improved. Cracks typically extended through the fireproofing layer and some also extended through the dark blue or gray epoxy topcoat.

In some more isolated instances, cracking was observed on the backside of disbonded fireproofing but the cracks did not extend through the entire fireproofing thickness to the surface.  In those instances, there were sometimes shallow depressions in the surface of the fireproofing that followed the cracking pattern within the depth of the film. The surface of the epoxy shop primer from which the fireproofing disbonded was also smooth and often displayed a whitish, haze-like stain on the surface. There were also a few areas where cohesive disbonding within the fireproofing layer was observed. In these instances, the epoxy topcoat typically disbonded with a thin layer of fireproofing on the backside of the epoxy chip and a thicker layer of fireproofing remained on the surface.

Structural support steel
The coating system cracked and peeled off of structural support steel in the pharmaceutical facility.

Generally, the consistency of the fireproofing layer was hard and brittle, comparable to that of a thick layer of dried plaster. However, there were isolated areas where the fireproofing layer was found to be soft and pliable. In these areas the fireproofing layer could be indented with only light to moderate thumb pressure.

When cut with a utility knife, the fireproofing layer was soft and pliable, with a “sticky” putty-like consistency and also exhibited a strong solvent odor.  The soft fireproofing did not display cracking or disbonding as did the harder, brittle areas of fireproofing.

Adhesion of the hard, brittle fireproofing around the cracked or raised/lifted areas was typically rated as poor. When probing along the periphery of disbonded areas, a metal wood chisel could be inserted between the fireproofing and the epoxy shop primer. Once inserted, the entire fireproof system could be removed for several inches until adhesion eventually improved.

Adhesion of the soft, pliable areas of fireproofing was rated slightly better (fair to poor) primarily because the fireproof material was somewhat sticky. Representative samples of the coatings were obtained for laboratory analysis.

Review reveals exposed system

Following the site visit, inspection reports from the coating application were obtained and reviewed. The review showed that application of the fireproofing material was substantially complete prior to the installation of the exterior shell of the building. As a result, the fireproofing material was applied to structural steel that was exposed to weathering. In fact, the information indicated that it took several months to complete the building enclosure.

Structural support steel
The fireproofing had been exposed to considerable direct and indirect weathering and freeze/thaw cycling.

Additionally, once the building was complete, large and small openings still remained (i.e., doors of all sizes, windows, equipment openings, etc.) that prevented the building interior from being weather-tight. Also reported was that the building was not weather-tight for a considerable time period and at the time of the consultant’s visit, the building had not been environmentally controlled.

Given this information, it was clear that the fireproofing had been exposed to considerable direct and indirect weathering and freeze/thaw cycling over the winter and ensuing spring. The expansion and contraction caused by alternate freeze/thaw cycling would have produced internal stress within the fireproofing layer (even more so in the thicker layers). In an effort to relieve the stress, the fireproofing cracked. The freeze/thaw cycles were likely further exacerbated by the inclusion of water and/or solvent remaining in the fireproofing system.

Problems replicated in the lab

The laboratory analysis included microscopic examination and application of the specified material to test panels at various thicknesses—e.g. both thick and thin samples. These samples were subsequently exposed to water after curing for two weeks. The water exposure consisted of placing the samples in a beaker containing water for four hours and then removing.

After removal, both thin and thick samples were found to have absorbed water and were swollen as compared to the areas that were not immersed. The thicker film sample, which already exhibited cracking before water exposure, exhibited additional cracking and the thinner sample had lifted from the panel surface. Additional cracking, even beyond the immersed area, was noted on both the thin and thick applications after drying. The failure (cracking, lifting) on these panels was essentially similar to the problems that were observed in the field.

The testing indicated that untopcoated fireproofing applied to structural steel in the building addition that was exposed to moisture would have reacted similarly. Additionally, the expansive stresses on the fireproofing film would have been magnified if moisture was absorbed into the film and subject to freezing.

Cause determined

The results of the investigation concluded that weathering exposure of the fireproofing was the primary cause of the observed failure.

Further review of the application records lead to the conclusion that proper recoat intervals between coats of the fireproofing material likely were not observed. This was supported by the laboratory microscopic analysis of samples. This analysis observed that, while the cross section analysis of some samples displayed visible lines of demarcation between individual layers (coats), it was more common that the total fireproofing thickness appeared as a single layer.

This observation was of interest because in most cases, coating films that are properly dry and cured, form a skin on the surface as the film naturally dries from the surface downward through the body of the film. This surface film typically provides a line of demarcation between coats that is visible when a cross section of the film is viewed microscopically. The lack of a clear line of demarcation between coats is possible evidence that the multiple coats of fireproofing were not sufficiently dry/cured (surface skin did not form) before subsequent coats were applied.

As a result, successive layers of fireproofing applied over previous insufficiently dry/cured layers may essentially have melted together and appeared as a single layer. The insufficiently cured coating layers may also remain soft as was observed in some areas of the building.

System knowledge is key

The problems observed underscore the importance of understanding the coating materials specified and applied for a project—particularly when the material plays an important role such as in fireproofing. Although it was obvious that much of the coating system would need to be removed and replaced, in this case, the coating manufacturer was asked to develop a written repair procedure for evaluation of the fireproofing system to determine whether intact areas could be salvaged and repaired.

About the author

HelselJayson L. Helsel is a senior coatings consultant with KTA-Tator Inc. He has an MS degree in chemical engineering from the University of Michigan, is a registered professional engineer, an SSPC Certified Concrete Coatings Inspector, and a NACE Certified Coatings Inspector. At KTA, Helsel manages coating projects, performs failure investigations and coating surveys, writes coating specifications, and is a regular instructor for KTA coating inspection courses. Helsel previously served as a Lieutenant Commander in the U.S. Coast Guard, with experience in marine-vessel inspection.

6 thoughts on “Diagnosing Failure in Fireproofing [Intumescent Coatings]”

  1. I have a close friend who has been in the sales and application side of high-performance coatings for many years, and shared many successful and not-so-successful coatings stories. My knowledge and experience in coatings is… less than very limited; but I really enjoyed reading this article. Thanks for a clear, comprehensive and intelligent case study.

  2. I didn’t realize that fireproofing materials need to be tested to determine what thickness they operate at. It would make sense that thicker coats of fireproofing material is best because it would offer the greatest distance from a fire source. However, your information that weathering can make materials fail testing is something to keep in mind.

  3. Hi, just gone through this very precise and comprehensive investigation study. so, does that mean the fireproofing intumescent coating is not suitable for exterior use (directly exposed to a open, external or outdoor environment)? even if there is a protective paint coating on top of it? Thanks.

    1. Jason Weslager

      The following is from the author, Jay Helsel.


      For your question whether the intumescent coating is suitable for exterior exposure – it depends on if the appropriate top coat is applied.

      The intumescent coating used in this case was not suitable for exterior exposure without an appropriate top coat. The required topcoat is typically specified in the approval/testing process for the intumescent coating. When the required top coat is applied, the system can be used for exterior exposures.

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