Many tunnels are constructed using either cast concrete sections in cut and cover or by lining of bored tunnels with concrete. There is a general perception that concrete will not burn and therefore will be fireproof. In the event of a fire however, concrete can suffer explosive spalling caused by the residual moisture in the concrete boiling and causing high pressure steam which, in trying to escape, will cause the concrete to crack and shatter, in some cases explosively.
This problem has been found particularly in high strength concrete. In 1976 the American National Institute for Standards and Testing (NIST) initiated research into concrete fire performance. Evidence was demonstrated in the Channel Tunnel fire of November 1996, where pictures of the railcars show their roofs sagging under the weight of spalled material.
NIST indicated that explosive spalling can be expected at temperatures of 300 – 450ºC. Furthermore, work on bored tunnels using concrete linings in the Netherlands shows that explosive spalling can occur within 10-15mins at surface temperatures as low as 200ºC. The Dutch experience claims that where the moisture content is over three per cent of the mass, the risk of explosive spalling is 100 per cent.
The Rijkwaterstraat (The Dutch Transport Ministry) takes the view, based on extensive research, that tunnels must be protected from fire in the ‘worst case’ scenario. Fire tests are required on an individual project basis and the tests are done to the special heating regime known as the RWS Curve. This has a maximum temperature of 1,350ºC maintained for up to two hours.
The Dutch authorities require the surface temperature of the test piece to be restricted to 350ºC and the temperature at 25mm cover depth to 250ºC. Imposition of this limit leads to a need for fire protective insulation to the concrete lining, and tests of prospective materials must show no loss of bond, failure of fixing, or explosive spalling during the test.
To protect tunnels from the ravages of high intensity fires holistic solutions combining active measures, such as fire detection systems, and management systems, such as evacuation procedures and passive fire protection systems, should be employed after a detailed risk assessment has been carried out as part of the tunnel’s design process. The use of proprietary systems should give confidence to the client and the insurer.
Passive fire protection systems used to protect tunnel concrete are easy to install. In case of fire such materials can result in the quick reopening of the tunnel as they are quick to replace in comparison to the reinstatement of fire-damaged concrete.
Typical systems include proprietary spray applied cementitious products and autoclaved calcium silicate boards utilising inert fillers. The mechanism for the protection of the concrete by cementitious and calcium silicate products is twofold. Firstly, they contain trapped moisture, which in a fire situation will boil, and keep the concrete temperature around 100°C until all the water has disappeared. Secondly, the product acts as an insulator.
Material benefits
The added insulation that these materials provide will, after a fire, eliminate repairs to the concrete since temperatures will not have reached a high enough level. Temperatures as low as 160°C can mean that repairs are needed, for example concrete containing polypropylene fibres will need reinstatement as these products will have melted.
Once temperatures reach 300°C the bond between the concrete and the rebar will have significantly reduced. At 380°C it is generally accepted that all exposed concrete will need removal and subsequent repair. Even smouldering fires can dehydrate the surface of concrete and, if a serious hydrocarbon fire follows, the level of explosive spalling is likely to be greater.
Secondly, passive fire protection coverings do not impinge upon the long-term durability of concrete as they have no effect on the porosity of the material, nor do they cause the formation of any cracks or channels that can admit agents harmful to rebar. Similarly, as proprietary passive fire protection products are placed between the concrete and the fire they have no effect on the properties of the concrete during placement or in its lifetime.
Another major benefit is that the products are specifically designed, factory manufactured, and assessed for their end use, and can be installed by companies that are members of third party certification schemes. Schemes are found in Approved Document B Fire Safety to the Building Regulations for England and Wales.
The use of third party certified applicators for products has become even more important since the Regulatory Reform (Fire Safety) Order (RRO) became law in England and Wales in October 2006. The RRO places the onus of the fire safety on the shoulders of the Responsible Person who is either the employer, the person in control of the structure, the owner, or any other person who to any extent exercises control over the structure. The Responsible Person is required to ensure that an assessment of the risk of and from fire is undertaken. Identified hazards must be removed, or reduced, so far as is reasonable and special consideration shall be given to the risks posed by the presence of dangerous chemicals or substances and the risks that these pose in case of fire. Special consideration will also be given to any group of persons who may be especially at risk in case of fire.
