Steel is used as a structural and fabrication material in tall buildings around the world. This hard, strong alloy contains iron, carbon, and other elements that impart strength and durability. Steel offers many advantages but exhibits poor performance when there is a rapid rise in temperature, such as during a fire in an office tower or apartment building. These high temperatures cause elongation, deformation and a dangerous decrease in steel’s mechanical properties.
Intumescent materials such as coatings and sealants have long been used for fire protection by the building and construction industry. These elastomeric materials can be applied directly to steel and used in conjunction with fire barriers such as composite sheets, flexible duct wraps, mats, tapes, and putties. The designers and builders of tall towers can choose from a variety of fire protection systems, so it’s important to specify materials that protect metal and support the safest designs.
Trans-Atlantic Tragedies
The tragic events of September 11, 2001 demonstrate the limitations of steel. When terrorists crashed airliners into New York City’s World Trade Center (WTC), the fire from the burning jet fuel wasn’t hot enough to melt the many metric tons of steel used for structural support. Steel’s melting point is significantly higher than the temperature at which jet fuel burns, but the steel girders in the WTC lost significant strength because of the fire’s high heat. Yet the investigation didn’t end there.
As the National Institute of Standards and Technology (NIST) later determined, a failure in fireproofing materials also played a role. NIST, a physical sciences laboratory and non-regulatory agency of the U.S. Department of Commerce, reported that “the towers withstood the impacts and would have remained standing were it not for the dislodged insulation (fireproofing) and the subsequent multi-floor fires.” (Source: https://bit.ly/2Kozb5Q).
In the United Kingdom, the Grenfell Tower fire of June 14, 2017 also involved material selection. The tower’s cladding, an aluminum composite, was designed to improve heat and energy efficiency. Two types of cladding were used, and both were installed over insulation. The immediate cause of the fire was a faulty refrigerator inside an apartment, but the blaze engulfed the building’s exterior. Later, investigators determined that neither the cladding nor the insulation met fire safety tests.
The failure of these building materials wasn’t the only reason for the Grenfell Tower tragedy, but using the wrong materials played an important role. The manufacturer of the metal composite panels offered both combustible and non-combustible products, but combustible panels were chosen to reduce costs. Moreover, these panels were installed over a polyisocyanurate rigid foam product with an aluminum foil facing. This composite material had a fire performance rating, but only for the surface spread of flame.
Three Fire Protection Categories
As these two incidents demonstrate, the designers and builders of tall structures need to select materials with the required fire-resistance and install them so that they provide optimal fire protection. During this material selection process, there are many solutions for asset protection, fire control, and delaying. National and international fire ratings vary, however, and there are differences in material testing and certification. The range, cost and availability of new products also affects buying decisions.
Asset protection, the first category of fire protection, involves applying a disposable layer to a structural or fabricated material. A form of temperature control, asset protection can reduce the high heat to which steel girders are exposed during a fire. By creating an insulating layer, it can also be used to protect aluminum, copper, or other metal components. Unlike other types of fireproofing, asset protection materials are bonded to the substrate and won’t become dislodged by impact.
Fire control, the second broad category of fire protection, is also used in tall buildings. For example, fire stop doors seal-off rooms and prevent the spread of flame. Containing a fire can keep it from reaching additional sources of fuel, but that’s not the only consideration. Because many building materials release harmful substances when burned, tall buildings need seals that prevent the spread of smoke and gases that can overwhelm the occupants of a burning building before they have a chance to escape.
Delaying, the third and final category of fire protection, is also used to protect people and property. In some conflagrations, it’s only a matter of time before the fire destroys everything in its path. Containing the blaze isn’t possible, but the right materials can slow a fire’s advance from one room to another. Delay tactics can allow firefighters to rescue occupants from other parts of a burning building. The right materials can also reduce the temperature that a structure experiences, which reduces the risk of collapse.
Understanding Intumescence
Intumescent materials are used in fire protection because they react with heat to generate a layer of char. Intumescence, the process of becoming enlarged or swollen, describes how these materials can expand up to 20 times their initial volume. The char is a very good thermal insulator, remains stable at high temperatures, and is difficult to ignite because it consists mainly of burned carbon. The expansion that’s associated with intumescence is directly affected by the chemical composition of the specific material.
Among all of the products for fire protection, intumescent coatings are some of the most popular. Durable, attractive, and lightweight, they are easy to apply and repair. Yet they require special considerations. For example, if an unprotected steel beam is connected to a protected steel beam, the unprotected member may conduct high heat to the protected member during a fire. There may also be uncertainties about the temperature distribution in the steel cross-section.
Intumescent coatings, or intumescent paints, may be divided into two basic types: single-component and multi-component. They can be solvent-based or water-based and are usually applied in several coats over multiple days. The tack free time is relatively short, but the full cure time can be as long as several weeks. Applicable standards include ASTM E 814 and UL 1479. Both tests are for penetration firestops. When the intumescent coating is fully cured, a decorative paint may be applied.
Intumescent materials aren’t limited to paints or coatings, however. In addition to composite sheets, duct warps, mats, tapes, and putties, the designers and builders of tall towers can specify intumescent gaskets for fire stop doors. There are other gasket applications, too. When conduits pass through fire-rated ceilings and walls, each side is terminated with a stainless steel box that’s fitted with an intumescent gasket. Fire-resistant sleeves are also used to protect cables that pass through one side of a wall and out the other.
Intumescent Gasket Considerations
Because they’re made of elastomers, intumescent gaskets form flexible seals and can resist compression set, the permanent deformation of a material when an applied force is removed. That’s important with door seals because a fire door may remain closed for long periods of time. If the door seal fails to “spring back” when the door is opened, compression set is said to have occurred. Yet it’s also a problem if the door seal is so rigid that it won’t compress and allow the fire stop door to close securely.
The designers and builders of tall towers also need to consider that intumescent gaskets lose some of their elastomeric, or rubber-like, properties in order to provide fire resistance. These gaskets are disposable, too. If an intumescent gasket reacts in response to a heat source, char occurs and the gasket must be removed and replaced during building repairs. By definition, sacrificial materials aren’t indestructible; however, they can help protect valuable assets.
Elasto Proxy, a global supplier of heat barrier and fire protection products, is developing an intumescent silicone elastomer with applications that may include gaskets for tall buildings. Specific uses include the seals on fire stop doors and in the BESA boxes that meet requirements from the Battery and Electric Specialists Association (BESA). Elasto Proxy’s intumescent gasket material is is still in development but inquiries are welcome.
For more information, go to www.elastoproxy.com/contact-us/
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