It’s what’s on the surface that counts

When the Twin Towers of the World Trade Centre in New York collapsed, it was one of the most shocking and memorable sights of that terrible day. In the aftermath, post-event analysis would find clear evidence that the structural integrity of the buildings had failed catastrophically when the inferno that ensued the initial impact caused the steel skeletons of the towers to soften and buckle.

Engineers John Skilling and Les Robertson designed the 415m-plus towers to withstand immense gravitational and windsheer forces through a clever system of steel box columns, which formed a central and an outer tube, with floor trusses extending to the central core and lattice-work steel adding strength to the outer frame. The engineers had even considered the possibility of a Boeing 707 accidentally flying into one of the towers, but a long-burning fire fed by aviation fuel was never imagined when construction began in 1966.

Attempts to understand the performance of the buildings have included an examination of the fire-proofing insulation on the towers’ steel structures. One side has argued that had the insulation been thicker, the towers would have stood for longer or might not have collapsed at all. This has been countered with the argument that the impact of the planes caused much of the fireproofing on the steel trusses to be dislodged anyway. Investigators also concluded that the heat from the many fires ignited in the minutes immediately following the impacts was of the order seen by large commercial power generators.

The US National Institute of Standards and Technology is not surprisingly making a huge effort to put into practice the 30 recommendations that have come out of the Federal Building and Fire Safety investigation into the World Trade Centre disaster. Among those recommendations is the need to better understand the link between temperature and the physical properties of fire resistant materials for structural steel. The WTC investigation and the NIST’s work with manufacturers of fire resistant materials have also highlighted the critical importance of understanding the durability of these materials. Thus, a first of its kind durability standard for fire resistant materials is being drafted to support the industry in its research and development.

International Protective Coatings, a subsidiary of Dutch paint giant Akzo Nobel and one of the biggest names in the field of fire protective coatings, says the repercussions of 9/11 have forced the construction industry to re-evaluate traditional methods of fire protection in commercial buildings and spurred on development of a new generation of materials.

Above 500C (930F), unprotected steel starts to lose its structural integrity; this can happen in as little as 20 minutes. At 600C (1112F) steel loses 75 percent of its strength. However, the same steel coated with an intumescent (which means ‘swollen layer’) fire proofing material lasts for longer than 60 minutes before heating to the critical temperature where it will weaken. Temperatures inside a burning building can exceed 1000C.

Intumescent coatings work by expanding to form an insulating layer around the steel. When subjected to heat, the coatings change from a decorative paint into a swollen layer of carbonaceous char. The layer of char can be up to 50 times the thickness of the initial coat. This is formed as the paint is heated to around 200C and above. At these temperatures, the resin system in the paint melts and allows the release of a mineral acid, which reacts with a carbon rich element in the paint to form a carbon char. At the same time, a foaming agent, which provides a gas, is released. This expands the foam to form the thick layer. While the reaction is taking place, energy is being absorbed within the insulating layer, limiting the amount of heat passing through the coating to the steel. The char layer gradually becomes thicker until the outer surface finally becomes soft and powdery.

The intumescent coatings typically used to protect steel in office blocks or sports stadia and other civil structures are known as thin-film intumescents, because applications are usually up to about 1mm. Their purpose is to protect against cellulose fires, meaning fires involving wood and paper. In these fires, heat rises relatively slowing and peaks at about 950C, according to International. These coatings are traditionally either water or solvent-based.

A greater level of fire protection can be achieved with so-called thick-films using epoxy-based coatings. These epoxy-coating systems were developed over 30 years ago to meet the needs of the NASA Apollo space programme, as well as offshore oil rigs. They can protect steel from volatile hydrocarbon-induced fires, explosions and jet fires that can reach temperatures of more than 1100C. Thickness varies from 3-20mm depending on the desired level of fire resistance, says International. They don’t tend to be used in commercial construction applications because of their aesthetic limitations.

Last autumn, however, International announced a new material, Interchar 212, which it says will give ‘unprecedented protection’ to high-rise structures and public buildings. The new material is based on the technology originally developed for NASA. It can be applied to structural steel in as little as two coats and is capable of offering up to four hours’ fire protection to help prevent steel infrastructures from collapsing prematurely in extreme heat situations. Interchar 212 is primarily designed for cellulose fires but has the big advantage of also working well in hydrocarbon fires, where oil, petrol or gas is fuelling the fire, and has a certified hydrocarbon fire rating. It is an epoxy, rather than acrylic, intumescent, and hence is harder, durable and more resistant to damage. It can be applied offsite and is designed to be mechanically handled, transported and erected at site. But if it is damaged, it can be hand trowelled or even sprayed.

Furthermore, says Bill McPherson, Akzo Nobel’s Marine & Protective Coatings general manager, the new formulation provides the ‘aesthetic versatility’ that architects want for steel designs, and also provides a level of anti-corrosion and extreme-heat protection that has not been available before. It is a significant step forward, he says.

Before epoxy-based coatings were first introduced in the 1970s, various latex-based intumescent materials had been used in the oil and gas industry, but in some cases it was found that they would lose much of their fire-resistant capability over time as weathering leached out the fire-active ingredients. According to John Dunk, International’s worldwide director of fire and insulation, the binder is the key to the longevity of intumescent coatings, and epoxy binders have proved to be the superior approach.

Epoxy-based coatings have proved to be sufficiently durable, and to require so little maintenance, says Dunk, that they have largely replaced the much cheaper, but relatively high-maintenance cement-based coatings. For example, vermiculite cements are soft, and so susceptible to damage; cracking leads to water ingress, and hence both corrosion risk and reduced fire protection. He says good surface preparation and the right primer contribute to the durability of epoxy-based intumescents.

Glenda Thisdell is editor of Coatings COMET, published by the Paint Research Association, UK. The PRA offers technical research and consultancy services for the coatings industry.

Steel solutions

The Association for Specialist Fire Protection (ASFP) outlines various methods used to protect steel:

Fire resistant boards are fitted around steel columns and beams, forming a box that provides a heatproof insulation. The boards can be made from gypsum-based plasters or calcium silicate, or fibres and specialist vermiculite containing material.

Vermiculite cement sprays are probably the least expensive form of fire protection, says the ASFP, and is often used in multi-storey car parks and basement areas of buildings, where a very rough and thick coating has been applied to the profile of the steel. Thickness varies from around 10-12mm up to 50mm. As the vermiculite cement is highly alkali, it requires three epoxy or other alkali-resisting primers to be applied to the steel after blast cleaning.

Fibre sprays contain mineral wool, and insulate by providing a thick layer of heat resistant material.

Dry lining systems rely on creating a void around the steelwork. Cladding with mineral wool slabs or blankets are often used in ceiling voids.

Intumescent coatings offer the most complex method of providing structural fire protection. When subjected to heat, these coatings change from being a decorative paint, which has been applied to the steelwork, into an intumescent or swollen layer of carbonaceous char. The layer of char can be 50 times the thickness of the initial coat, and is formed as the paint is heated to around 200C and above. At these higher temperatures the resin system melts and allows the release of mineral acid, which reacts with a carbon rich element in the paint to form a carbon char. Released at the same time, is spumific, which provides a gas, which expands the foam to form the thicker layer. The char layer gradually becomes thicker until the outer surface finally becomes soft and powdery. While this reaction is taking place, energy is being absorbed within the insulating layer limiting the amount of heat passing through the coating to the steel.