The materials and methods collectively described as passive fire protection for tunnels are of varied types and mixed benefits. The importance of passive fire protection is often not just a matter of reducing recovery costs by protecting the tunnel structure. Most types only come into play at higher temperatures when life in the tunnel is, hopefully, saved.
It is also worth noting that the strict fire controls practiced by the Netherlands (RWS curve fire design basis) have a lot to do with the consequences of tunnel fire. Severe structural damage to tunnels below sea level can cause collapse and widespread flooding damage.
Tunnel users cannot survive a sustained fire due to lack of oxygen, high levels of heat and the possibility of toxic substances. The first priority of the fire and rescue services is to save life by aiding rapid evacuation. In the vital minutes immediately after a fire takes hold, tunnel users may have to save themselves by reaching the nearest refuge or escape route. It is here that passive fire protection can perform a role in holding back the worst heat and so preserving life until an escape or rescue can be made. Passive fire protection can also promote fire life safety by preserving separate ventilation ducts.
The general principles of passive fire protection are outlined in separate articles in this issue’s review and also by some of the references below. Despite their robust nature in normal conditions, both concrete and ferrous materials should be shielded from the highest temperatures in order to survive a major fire.
Primary means of passive protection employed in tunnels are:
– Fire resistant panels shaped and cut to fit the tunnel profile
– Sprayed-on linings with the same function
– Internal protection for structural concrete in the form of thin polymer fibres to deter concrete spalling
– Various forms of fibrous fleece that can sometimes be used to wrap around important ducts and cables to prolong the useful life of services and communications if threatened by fire, but rare in tunnels
– Increasing the thickness of permanent concrete lining can limit structural damage from fire in percentage terms but necessary extra excavation and concrete will likely make this impractical. Spalling could be even more unsafe.
Strength loss
It has been pointed out (Clement and Focaracci, 2011) that the improvements achieved in concrete tunnel lining design life by forming high quality, dense concrete can be detrimental in fires due to a higher tendency to spall, despite low thermal diffusivity. Spalling can be explosive and unpredictable (Bruenese, 2009).
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By GlobalDataSprayed lining
Another approach in creating fireproofing barriers is spraying fire-resistant material onto the surface to be protected. Advantages include a close fit to the structure or service lines to be protected.
Today, sprayed on products are based on materials that resist heat themselves or form heat insulation of the material (concrete and steel) beneath, perhaps, as with expanding vermiculite, initiated by the heat of a fire. According to chemical giant BASF Meyco vermiculite-based products are relatively weak and require mechanical bonding to the tunnel structure such as by stainless steel mesh. Its own cement-based product, Meyco Fireshield 1350, is claimed as six times the compressive strength of a typical vermiculite-based product.
Using advanced robotic applicators such as the Meyco’s Robojet, applying the correct thickness of lining in the correct places can also save costs and achieves tolerances that are impossible with manually applied products. Application rates at 150-250m2 per hour depending on the specified thickness. Recent applications of Fireshield 1350 have included the Bodio section of the Gotthard rail base tunnel and the Soderledtunnel safety upgrade in Stockholm, Sweden.
Morgan Thermal Ceramics’ main product used in tunnels is FireMaster Firebarrier, a cementitious material that can be sprayed or cast onto surfaces to be protected. Firemaster Firebarrier 135 is a special formulation for tunnel protection that has passed a large range of tests including on prestressed large concrete slabs under RWS fire conditions. It provides an easily cleaned, high-quality surface finish.
Another sprayed material is Promat’s Cafco Fendolite MII.
Panels
The principle of protecting a tunnel’s structure from fire and possible physical damage has long been established, although the days of using asbestos in insulating fire protection materials have long gone for good health reasons.
Today, panels employ various methods and material to create sufficient heat insulation, durability and ease of installation, as well performing other functions not associated with fire. These include ceramic refractory fibres, aerated concrete and vermiculite-type minerals.
Two main types of panel produced by the German Xella group have been used for tunnel fire protection. Contractor Multiclad Facade Systems installed 45,000m2 of Xella Fermacell Aestuver lightweight, sandwichpanel, cement board in Brisbane’s dualhighway Clem Jones Tunnel.
Subsequent to Rijkswaterstaat tests, Xella Hebel panels were chosen for the N57 road beneath the Walcheren canal near Middleburg. The Hebel panels are 150mm thick and consist of aerated concrete to line sheet steel walls at the portals.
Promat offers Promatect H boards as used in the Westerscheldetunnel and Velser Tunnel in the Netherlands. These sub-water applications have shown the ability of the board to be unaffected by water leakage but still reveal ‘wet spots’ to facilitate inspections. Promat Vermiculux is designed for structural steel fire protection.
Confidence in modern passive barrier fire protection, whether panels or sprayed, is demonstrated by a non-traditional and more economical means of creating an escape route. The suspended emergency escape route, which is trapezoid or rectangular in section and fixed to bars or bolted plates in the tunnel crown is made of concrete and/or steel and covered with passive fire protection. Tunnel users access the escape route via stairways from laybys. Tests have shown that temperatures in the escape tube can be limited to acceptable levels by insulation.
Fibres against fire
Another fairly recent development is the use of polymer fibres within the concrete mix to be cast with it. Unlike other forms of passive fire protection that form an insulting barrier between the fire and the material to be protected, thin (monofilament or micro-) polymer fibres work internally to prevent greater damage to concrete in a high-energy fire. They are therefore mainly a safety measure, as the concrete affected will still need replacement or repair.
Such fibres are also designed and supplied to reduce surface cracking of the concrete during curing but as far as fire protection they will melt at a temperature lower than that causing concrete damage. The calcium hydroxide in cement dehydrates with heat from the fire, so increasing the vapour pressure in addition to any free moisture within the concrete. The small passages created by the melting of the fibres allow this excess pressure to be dissipated and released without serious deterioration in concrete strength. Increasing internal pressure would cause serious, possibly explosive, spalling.
A number of manufacturers and distributors supply thin polymer fibres for use in fibre protection including Propex (Fibermesh 150 etc) and Adfil. These fibres should be distinguished from macrofibres, the purpose of which is mainly structural.
