In most cases, concrete does not require any additional fire-protection because of its built-in resistance to fire. It is a non-combustible material (i.e. it does not burn), and has a slow rate of heat transfer. Concrete ensures that structural integrity remains, fire compartments are not compromised and shielding from heat can be relied upon.
Concrete as a material
Concrete does not burn – it cannot be set on fire, unlike other materials in a building and it does not emit any toxic fumes when affected by fire.
Concrete is proven to have a high degree of fire resistance and, in the majority of applications, can be described as virtually fireproof. This excellent performance is mainly due to concrete’s constituent materials (cement and aggregates) which, when chemically combined within concrete, form a material that is essentially inert and, importantly for fire safety design, has relatively poor thermal conductivity. It is this slow rate of conductivity (heat transfer) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage.
Concrete structures perform well in fire. This is because of the combination of the inherent properties of the concrete itself, along with the appropriate design of the structural elements to give the required fire performance and the design of the overall structure to ensure robustness.
Fire performance is the ability of a particular structural element (as opposed to any particular building material) to fulfill its designed function for a period of time in the event of a fire. These criteria appear in UK and European fire safety codes.
The impact of a major fire at Tytherington County High School, Cheshire (August 2006), was limited due to the fire resistance of the concrete structure. Rather than taking a year to be demolished and replaced, as was the case with an adjacent lightweight structure, the concrete classrooms were repaired ready for the following term.
Figures from the RISC Authority, calculate that 40% of fires that resulted in a loss of over £150 000 involved structures built using new methods of construction – notably timber frame and light gauge steel frame.
Andrew Minson, executive director of The Concrete Centre said: “Every fire causes financial loss and in most cases insurers have to pay for the damage and repair. For this reason, insurance companies keep comprehensive databases on the performance of construction materials. In mainland Europe, this information often results in reduced insurance premiums for concrete buildings due to their proven fire protection and resistance.”
Minson pointed to France where insurance premiums for warehouses built from concrete can be reduced by up to 20%. He said: “The growing emphasis on risk avoidance means that the inherent fire resistance of concrete is being increasingly recognised, and it will be of no surprise if this is more widely recognised in lower insurance premiums in the UK.”
Concrete can offer up to four hours fire resistance, well beyond the periods often stipulated by the Building Regulations for life safety. It offers this high level of protection for buildings whether under construction or completed, with no need for fire-proof boards or finishes that might be compromised due to poor installation, alternations or refurbishment.
“Concrete offers insurers and policy holders the potential for minimal damage, and therefore, smaller claims and lower premiums,” said Minson.
Concrete’s performance in fire
Concrete and its mineral constituents enjoy the highest fire resistance classification (class A1) under EN 13501-1. Concrete has low thermal conductivity and it is this slow rate of heat transfer that enables concrete structures to perform well in a fire. Concrete also acts as an effective fire shield between adjacent spaces. In an intense fire, concrete can spall. Spalling occurs when entrapped air expands, causing pieces of the concrete to break off. Measures such as the introduction of polypropylene fibres to the concrete mix can reduce or eradicate the incidence of spalling. The performance of reinforced or pre-stressed concrete elements (columns, beams, floors etc.) in a fire is largely dependent on the performance of the reinforcing bars or strands. Steel reinforcement counteracts high tensile stress in concrete elements. However, steel loses its high tensile properties at relatively low temperatures (350 °C to 400 °C). The fire performance of reinforced and pre-stressed concrete elements, therefore, is closely related to the ‘depth of cover’ of the concrete (i.e. the distance from the steel reinforcement to the surface of the element).
Fire engineering and whole building performance
For any building or structure, regardless of its complexity, design for fire safety should address the following four principal objectives:
- Ensure stability of the load-bearing construction elements over a specific period of time.
- Limit the generation and spread of fire and smoke.
- Assist the evacuation of occupants and ensure the safety of rescue teams.
- Facilitate the intervention of fire fighters and other rescue parties.
Good practice in design for fire safety incorporates these aspects and more, in what is termed 'fire engineering' for large, complex structures that warrant additional design effort. Although prescribed data (such as dimensions for thickness and cover) may be used, the aim of fire engineered structures is to move away from the traditional methods and create a fire strategy dedicated to the project in hand, based for example on the building's design, how it will be used, fuel load and the probability of a fire occurring. For this reason, computer software is used to perform a probability analysis of the behaviour of both fire and people.
From a whole building standpoint, concrete can satisfy the four principal objectives of fire safety through its inherent fire resistance and the utilisation of its structural continuity in fire engineered design.
A forum for sharing best practice and discussion for concrete and fire performance is The Concrete Centre’s Fire Forum. This will take place on Monday 17 November 2014 at the BRE Centre for Fire Safety Engineering, University of Edinburgh. To attend, submit a paper for presentation at the event or for more information, email firstname.lastname@example.org.
Written by MPA The Concrete Centre.
Read the article online at: https://www.worldcement.com/europe-cis/22082014/concrete-and-fire-material-and-structural-performance-343/
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