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Fire-Resistive Tests, Specs Challenge Industry

WEDNESDAY, JUNE 13, 2018

By Allen Zielnik


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On June 14, 2017, a 24-story concrete public-housing high-rise tower, on Grenfell Road, in West London, home to about 300 people, caught fire. It is believed that a refrigerator-freezer in a fourth-floor unit initiated the fire. By the time the fire was extinguished—some 60 hours later—it caused 71 confirmed deaths with at least another 70 others injured. While many factors contributed to the toll, such as blocked stairways and otherwise poor maintenance, the root cause of the rapid flame spread, engulfment of the tower in flames, and ensuing tragedy was the use of a particular construction method and materials.

Tower History

Completed in 1974, the Grenfell Tower sits in a populous area of London, surrounded by low-rise residences, many of them of them of wealth and status, typical of the mingling of socioeconomic strata that is much of London. The tower underwent a major renovation from 2012 to 2016 and, as part of the project, new windows and an exterior rainscreen skin of aluminum composite panels were installed.

© iStock.com / AmandaLewis

While many factors contributed to the toll, such as blocked stairways and otherwise poor maintenance, the root cause of the rapid flame spread, engulfment of the tower in flames, and ensuing tragedy was the use of a particular construction method and materials.

(Rainscreens rely either on a drained/back-ventilated approach to both drain and dry out residual water captured by the rainscreen, or otherwise employ a pressure-equalized design that uses a ventilated and drainable cavity. Rainscreens are good from a weatherproofing perspective but can create an effective “chimney” within the exterior wall system. This can allow materials above to be preheated at a quicker rate than in a more conventional wall system and potentially increase flame spread.)

The material used, Arconic’s Reynobond PE cladding panels, consisted of two coil-coated aluminum sheets fusion-bonded to a polyethylene core. The rainscreen cladding was attached to the facade over Celotex RS5000 PIR polyisocyanurate foam core thermal insulation panels in what is called a “drained and ventilated” facade. These panels represented a savings of 293,000 pounds in comparison with an earlier plan to use a fire-resistant grade cladding. The total cost of the renovation was 8.6 million pounds—about the price of a four-bedroom house in the district.

The Fire, the Cladding & the Testing

In buildings, fire has the potential to spread externally by breaking out through windows, travelling over or through the cladding system and then breaking back into the building at another level. Flames can extend over 2 meters above the window opening from which they break out, leading to potential secondary fires, even with a resistant cladding. Anything less can result in rapid flame spread.

The fire spread up the outside of Grenfell Tower surprisingly quickly and fiercely. Initially, suspicion focused on the Reynobond PE ACM cladding rainscreen panels. This type of panel, with a flammable polymer core, is not allowed in North America or most of Europe following similar, but smaller, fires in France and Dubai. However, in the ensuing investigation, small-scale fire tests had revealed that both the cladding and the Celotex insulation had failed in subsequent testing.

ChiralJon, CC-SA-BY 2.0, via Wikimedia Commons

Flames can extend over 2 meters above the window opening from which they break out, leading to potential secondary fires, even with a resistant cladding. Anything less can result in rapid flame spread.

Detective Superintendent Fiona McCormack said, "The insulation was more flammable than the cladding. Tests show the insulation samples combusted soon after the test started."

United Kingdom building regulations require that buildings more than 18 meters tall must use materials of limited combustibility in accordance with BS 8414 Fire Performance of External Cladding Systems. This is a large-scale assembly test representing a high-rise building, measuring fire spread. It replicates a fire starting inside a room, breaking out through a window and exposing the external facade to fire.

When sold, Celotex stated that the product met BS 8414 requirements for Fire Class 0. Class 0 is not a classification defined by a British Standard; rather, it is defined in other documents as a sample that achieves a certain rating in surface spread of flame tests and achieves an index of performance in a standard fire propagation test. The details aren’t as important as is the fact that products are tested to simulate their end-use application, so potentially products that have different end-use applications can achieve different classifications in each case. This includes the orientation of the product as it is to be used and its position in relation to other adjacent products such as the substrate, insulation materials and any mechanical fixings to the building. So, the fire performance rating of the materials, while useful, does not guarantee specific performance in a given construction.

Another standard, BR 135, Fire Performance of External Thermal Insulation for Walls of Multistory Buildings, defines design principles, methodologies and fire spread requirements for these types of non-loadbearing cladding systems. Fire resistance and flame spread are also the result of design as well as materials. Buildings are systems; an effective facade design requires a wide range of inter-related technical provisions, such as maintaining ventilation to ensure that the durability of the system is not compromised while minimizing the risk of fire spread through the cavities. Small-scale component testing cannot always reflect the overall fire performance of the complete system when installed in a real building.

Other Fire-Resistive Shortcomings

While not involved in the Grenfell incident, the industry faces similar issues with defining tests and specifications for other fire-resistance products such as intumescent coatings. Upon exposure to flame, these coatings, which can otherwise appear mostly like regular decorative product, are designed to greatly swell and form an insulating char layer on the substrate. This thermal insulation can help to limit the flame spread and provide added time for firefighting and egress.

Natalie Oxford, CC-SA-BY 4.0, via Wikimedia Commons

Buildings are systems; an effective facade design requires a wide range of inter-related technical provisions, such as maintaining ventilation to ensure that the durability of the system is not compromised while minimizing the risk of fire spread through the cavities. Small-scale component testing cannot always reflect the overall fire performance of the complete system when installed in a real building.

Flammability is a general word used to describe the reaction to fire behavior of a material and can be subdivided into many individual parameters including ignitability, flame spread and rate of heat release. It is often defined by the method used to measure it, but is most widely stated as the ease of which a material will ignite. The purpose of any test is to meet a regulatory requirement or to demonstrate that the tested material will perform adequately in a fire scenario. However, they cannot fully account for all substrates, application conditions, in-service degradation factors, fire scenarios, etc. Rather, they serve as comparative, and not predictive, tools.

Another factor regarding the PIR insulation is that it can generate hydrogen cyanide gas when combusted. In fact, medical discharge papers related to the Grenfell accident indicated that one 12-year old girl, along with her sister and mother, were treated for cyanide poisoning. Celotex RS5000 PIR has since been voluntarily withdrawn from the market.

The British Research Establishment, which performed the BS 8414 fire test on Celotex, made clear that they did not design, select or install the test system, and were not involved in the sample selection process and, as such “the test results have been withdrawn.”

"The cladding system in that test included Celotex RS5000 and fibre cement board rainscreen (declared reaction to fire classification A2)—this was not an Aluminium Composite Material (ACM), nor was it the cladding system on Grenfell Tower,” according to the BRE’s statement.

So, while the polyethylene core exterior panels were initially blamed for the catastrophe, the actual root cause issue appears to be more complex. Were the insulation panels primarily to blame? Perhaps the combination of the two was responsible? Did the original building design, or other renovation changes, contribute? Were the product fire tests deficient or improperly performed? The real answer to all of these may well at minimum be “probably.” What is likely generally agreed on is that while small-scale fire tests can help lead to the development and selection of better materials (in concert with better building code requirements), they simply cannot be made to be completely predictive when used in combination with other materials or with specific building designs.

Certainly, the concept of integrated fire safety doesn’t rest only with the building material manufacturers, it also rests with the architectural community designing buildings and selecting materials. As in medicine, this probably requires that more formal attention be paid to fire safety in architectural school and the cooperation of fire-safety and fire-testing experts, building code authorities, building product manufacturers and first responders. Man has needed fire since our earliest days, but controlling it still remains a formidable challenge.

ABOUT THE BLOGGER

Allen Zielnik

Allen Zielnik has 42 years of experience in both chemical and physical instrumental methods of analysis of materials. He has been with Atlas Material Testing Technology for the past 22 years, specializing in the effects of solar radiation, weather, and the environment on the durability and performance of materials and products, including coatings. A frequent speaker at various worldwide technical symposia, he is the author of more than 120 publications and conference presentations. Zielnik has degrees in electronics engineering and analytical chemistry.

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Tagged categories: Aluminum; Cladding; Fatalities; Fire; Fire-resistive coatings; Intumescent

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