Concrete in fire conditions

Concrete is the ideal building material to use in order for the structure to exhibit the highest resistance and the best possible response in case of a fire. As a construction material it does not contribute to the fire load. Without the use of additional protective materials, it provides the necessary fire protection, thermal insulation and barrier function to fire spreading between compartments, thereby ensuring escape routes and structural integrity during the fire. In comparison with other building materials, therefore, the use of concrete provides an easy, reliable and cost effective fire protective construction.

The Windsor Tower in Madrid is a well documented case of a modern high-rise building exposed to fire. The building had a bearing structure made of reinforced concrete (slabs, shear wall cores and interior columns) and steel (perimeter columns). It was destroyed in 2005 by a fire which started on the 21st floor (out of 29 floors in total) and lasted 25 hours. The use of strong deep collector beams of reinforced concrete in the mechanical floors, as well as the remaining vertical load bearing concrete elements (shear wall cores and columns), helped to prevent the complete collapse of the structure and resulted in an extremely long duration of resistance of the structure to the fire.

An independent study, by the Instituto Técnico de Materiales y Construcciones (Intemac) found that the Windsor Tower performed remarkably well in this extreme fire event, even better than would have been expected, should the existing provisions for fire protective design of reinforced concrete buildings had been implemented. Furthermore, based on a comparison of the response of the steel columns between floors (in some of which protective coating had been applied due to upgrading works during the time of the fire), the need to implement fire protective coating on steel columns, in order for them to function as required by design, was established. It was concluded that if these had been protected, the building would not have collapsed in the upper floors.

Comparisons: In comparison with other structural materials, concrete is the ideal structural material against fire due to the following:
  • It does not burn and does not contribute to the structure fire load
  • Due to the increased fire resistance of the material, fire spreading is prevented
  • It is a very effective fire barrier, ensuring the function of escape routes with proper design
  • It does not melt, something that would enhance the transfer of fire
  • It does not produce smoke and toxic gases, thereby reducing the risk to personnel and firefighters
  • It holds the fire in, thereby reducing its spread to the surroundings
  • It acts as a fire protective material by itself; hence, under normal circumstances, no additional measures, materials and expertise is required in construction
  • It has an extremely high resilience in extreme temperatures, which makes it an ideal material for housing of storage areas that contain large fire loads.
  • It maintains during the fire generation and extinction phases the integrity of the structure without the development of large deformations and overall movement of the structure or the individual spans, thereby facilitating the process of fire extinction.
  • Due to the smaller thermal movements during the fire it is ensured that the structure will not destabilize and/or collapse, while ensuring at the same time that the fire will not spread due to loss of the barriers between the compartments.
  • It is not adversely affected by the water thrown in during the fire
  • It is easily repaired after the fire, something that improves both in terms of time and costs the rehabilitation works that follow the fire, thereby providing in a very short time the bearing structural frame ready for use.
  • Its use in tunnels as a paving material, in contrast with asphalt which is a combustible and rheoplastic material during a fire, ensures that the fire will not spread inside the tunnel. Moreover, the fact that concrete retains its rigidity makes it possible to intervene immediately with fire fighting means and to evacuate the personnel, without allowing for time for the pavement to cool. For this reason, in Austrian tunnels it is now mandatory to use concrete for the road pavements.
 
 
  • Its use in buildings in urban environments, prevents the spreading among buildings of a fire, after a strong earthquake, when there is a higher risk of this happening due to the increasing use of natural gas in the city.

Behaviour of reinforced concrete in fire. According to the European Standard EN 13501-1 (2002), in which construction materials and elements are classified for reaction to fire, both concrete as well as the individual materials that are used for making concrete, are rated in the highest category of fire resistance A1: “materials that do not burn at temperatures that usually occur during a fire”.

As far as concrete is concerned, the properties that determine the response of a structural element in a fire depend on its composition, the type of aggregates, the initially contained moisture and its age. In summary, the effects of temperature on the properties of concrete are:
  • The density of the material is only affected to a very small percentage (ignoring the transfer of moisture through the pores during the fire) and so is its thermal conductivity,
  • The specific heat of material is affected, with an increase of up to 20% relative to the value at room temperature, when the temperature of the concrete mass (for limestone aggregates) reaches 1100oC (this percentage may be exceeded because of contained moisture)
  • The compressive strength is reduced, when the temperature of the material exceeds 500oC. 
As for the steel reinforcement, its response depends on whether the steel is hot-rolled or cold-formed and is more sensitive than concrete at high temperatures, due to the fact that both the tensile yield strength and the modulus of elasticity are affected (reduced) already from 500oC and 300oC, respectively. For this reason, fire design regulations (such as the Greek Fire Safety Regulations and EN 1992-1-2) prescribe a minimum cover of the reinforcement in the design of an element for a specific period of fire resistance, in order to delay the increase in temperature of the steel, making use of the insulating property of the concrete cover. The methodology for the analysis of individual structural members using the 500o isotherm follows along the same lines (Simplified calculation method, App. B in 1992-1-2).

Design of a reinforced concrete structural element for fire: In general, depending on the type of aggregates used (i.e. siliceous or limestone) and the conventional exposure time to the fire (based on the design options and regulatory requirements), the design of a reinforced concrete structural element for fire involves the selection by the designer of the following:
  • The member dimensions, in order to:
    • Achieve a desired reduction of temperature from the exposed side of the element to the non-exposed side to about 140oC - in the case of slabs and walls, (Condition I), and
    • Verify the load bearing capability of the element against the loads that are considered to act during the fire, after neglecting the portion of the cross section where the concrete has exceeded a given temperature (of the order of about 500oC (Condition R), in line also with the simplified design method proposed in EN 1992-1-2.
  •  The distance of the centre of the tensile reinforcement to the exposed surface, in order to limit the temperature in the reinforcement during the exposure time to the fire to a level below 500 - 550oC, at which point the yield strength of the heated steel is reduced to service stress levels under the requirements of conventional load Regulations (condition R).
  • The function of the element:
    • If it is a column, the concurrent action of the axial load is taken into account (stability check under the axial load action during fire).
    • For beams and slabs, the relevant prevailing design requirements are used with a relaxation of the relevant provisions towards the less restrictive side, in order to take into account the anticipated favourable action of moment redistribution from the span to the supports during the fire, under the condition that this is possible to take place (continuity of the reinforcement, sufficient anchorage, monolithic construction).

Details. In the case of a thick structural element with a relatively thick cover additional surface (skin) reinforcement is required in the form of individual rebars or wire mesh and the inclusion in the mix of polypropylene fibres; this is essential in order to avoid explosive spalling of the cover, a phenomenon that depends on the service moisture content in equilibrium with the environment before the start of the fire and the compressive strength of the concrete (the requirements being more restrictive for high strength concrete).

Photographs taken in Aliveri after the fire of 2007

Prefabricated concrete building
 
Mezzanine floor of composite construction that collapsed
 
Industrial steel metal building that collapsed. Industrial steel metal building that collapsed.