Fireproofing

• 1. Introduction

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  Fireproofing is the art and science of making things resistant to fire. In the Channel Tunnel connecting the United Kingdom and France, an intense blaze once burned so fiercely that it reduced the concrete lining of the undersea tunnel down to about 50 millimetres. That moment raises a question that engineers have wrestled with for centuries: how do you build something that holds up when everything around it is burning? The answer turns out to depend on the material, the temperature, the duration, and the stakes. Structural steel begins to lose strength as heat climbs toward a critical threshold. Paper in a fireproof vault faces a different enemy than magnetic data tape. Traffic tunnels face different threats than office towers. This documentary follows the many faces of fireproofing, from the ancient problem of keeping a steel frame standing in a building fire, to the precise temperature thresholds that separate a saved document from a destroyed one.

• 2. Steel Under Heat

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  Structural steel loses strength as temperatures rise, and the critical threshold engineers work around sits at approximately 540 degrees Celsius. Once steel climbs past that point in a building fire, the frame can fail. Building codes in North America and elsewhere set restrictions on how much steel can be left exposed, and engineers have developed a long list of methods to keep the metal below that limit.

  The oldest approach is encasement. Wrapping steel i-beams in brick masonry or concrete delays the time before the steel is exposed to dangerous heat. The tradeoff has always been weight. Masonry encasement uses large amounts of heavy material, which increases the load on the very frame it is protecting.

  Newer methods chip away at that weight penalty. One technique wraps an i-beam in a thin layer of metal lath and covers it in gypsum plaster. Gypsum plaster contains water crystals that resist heat, making it an effective barrier for its mass. Multiple layers of gypsum board around an i-beam achieve a similar result. Spray-applied fire-resistive materials, known in the industry as SFRM, use an air-pressured spray gun to coat beams with gypsum plaster, mineral fibres mixed with an inorganic binder, or a cementitious formula using magnesium oxychloride cement.

  One of the more counterintuitive methods involves hollow structural columns filled with liquid water or antifreeze. When fire reaches part of the column, the liquid distributes heat throughout by convection, keeping any single point from reaching the critical temperature. This method was patented in the nineteenth century, though the first prominent example arrived 89 years after that patent.

• 3. Spray Plasters and Boards

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  Purpose-designed spray fireproofing plasters have become widely available across global construction markets, falling into three main categories: gypsum plasters, cementitious plasters, and fibrous plasters. Each takes a different approach to the same problem of slowing heat transfer.

  Gypsum plasters incorporate chemical additives that create air bubbles within the material, displacing solid mass and reducing bulk density. Lightweight polystyrene beads can be mixed in at the factory to reduce density further, which generally produces more effective insulation at lower cost. The resulting material has qualified to the A2 combustibility rating under the German standard DIN 4102.

  Fibrous plasters take a different path to low density. Mineral wool or ceramic fibres entrain air by their nature, displacing heavier solid components. The result is a light material that insulates by trapping air rather than by chemical reaction.

  Beyond spray products, proprietary boards and sheets made from gypsum, calcium silicate, vermiculite, perlite, and mechanically-bonded composite boards of punched sheet-metal and cellulose-reinforced concrete have all been used to clad steel and other structural elements. The goal in every case is the same: to delay the moment when the protected material reaches the temperature at which it fails.

• 4. Concrete and Tunnel Fires

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  Traffic tunnels carry a particular fire risk that ordinary buildings do not face. Vehicles transporting petrol, liquefied petroleum gas, and other hydrocarbons can cause a very rapid temperature rise in the event of an accident, reaching far higher ultimate temperatures than a standard building fire. Fire-resistance standards account for this with what engineers call the hydrocarbon curve, a steeper and hotter test profile than the one used for ordinary building elements.

  Concrete, which performs well in typical building fires, cannot withstand severe hydrocarbon fires on its own. The reason is a destructive chain reaction inside the material. Unprotected concrete holds hydrates and unbound humidity. When exposed to intense heat, those materials undergo a sudden endothermic reaction that generates internal pressure high enough to spall the concrete, breaking it into small pieces that fall onto the tunnel floor. Humidity probes are inserted into concrete slabs during fire testing to monitor this, even when testing to the less severe building elements curve under standards including DIN 4102, ASTM E119, BS 476, and ULC-S101.

  The Channel Tunnel fire demonstrated what happens when hydrocarbon heat attacks a concrete lining without adequate protection. The response from European researchers came through the Eureka Fire Tunnel Research Project, which produced the building codes that now govern tunnel fireproofing. Cementitious spray fireproofing in tunnels must be certification-listed and applied in the field according to that listing, using a hydrocarbon fire test curve such as the one specified in UL 1709.

• 5. Fireproof Vaults and Thresholds

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  Fireproof vaults built for paper documents rely on a mechanism that most people would not expect: steam. Concrete and masonry blocks used in vault construction hold chemically-bound water. When fire surrounds the vault, that water is forced into the interior chamber as steam, soaking the documents inside and keeping internal temperatures below the critical threshold of 176.7 degrees Celsius. Above that point, the information on paper is destroyed.

  If a fire is extinguished before interior temperatures reach 176.7 degrees Celsius, water-soaked paper can be treated through freeze drying and recovered. An alternate construction method uses dry insulating material instead of concrete, at lower cost and with less labour, though it does not carry the same self-dampening property.

  The steam that saves paper is lethal to other storage media. Microfilm is destroyed at 65.5 degrees Celsius, a threshold known in the industry as Class 150. Magnetic media such as data tapes lose data above 51.7 degrees Celsius, classified as Class 125. Vaults designed to meet that more stringent standard are called data-rated vaults. Every component of a fireproof vault, including doors, HVAC penetrations, and cable penetrations, must meet the fire protection rating of the vault as a whole.

• 6. Asbestos and What Replaced It

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  Asbestos was once a standard fireproofing material, used on its own, mixed with binders such as cement, sprayed onto surfaces, or pressed into sheets. It appeared as an additive in fabrics for protective clothing and in building materials across many industries. Its heat resistance made it popular; its later history made its removal an industry in its own right.

  When asbestos was proven to cause cancer, a large removal-and-replacement industry emerged to strip it from existing structures. Endothermic materials stepped in to fill the gap. Gypsum, concrete, and other cementitious products had been in use alongside asbestos and outlasted it. More technically demanding versions of these endothermic materials went on to serve in aerodynamics applications, intercontinental ballistic missiles, and re-entry vehicles including the Space Shuttles.

• 7. Fireblocking and Fire Walls

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  Wood-frame construction creates a hidden network of hollow spaces. Gaps formed by joists and studs inside floor and wall partitions act as channels through which fire can travel easily from one area to another. Fireblocking closes those channels by dividing the hollow spaces into smaller intervals. Materials used for this purpose include solid lumber, plywood, OSB, particle board, gypsum board, cement fiberboard, and glass fibre insulation batts.

  Fire walls work at a larger scale, separating a building into distinct units to restrict or delay fire moving from one section to the next. They typically run the full length of a building, from the foundation to the roof. Fire barriers and fire partitions operate similarly but are limited in height to a single floor, running from the slab of one level to the underside of the next floor above. Each approach creates a physical break that forces fire to slow or stop at a boundary rather than spreading freely through a structure.