Reinforced Concrete and Fire

Fending off the flames, Structural Engineer, January 2004, discusses the performance of structural concrete in building fires, and advocates the virtues of this material's inherent resistance to the damaging effects of fire. Some topics discussed include:

The Benefits of Reinforce Concrete Construction
This article touts the benefits of concrete's inherent fire resistance, including for example:

• Application of additional fireproofing material is not required
• Connections are not vulnerable and do not require special attention for fireproofing
• Concrete walls and slabs are well suited to containing the spread of fire
• The added protection afforded by concrete construction can lead to lower building insurance rates
• Reinforced concrete structures typically survive building fires with only minimal structural repairs required
• Redundancy in concrete structures reduces the risk of structural failure due to fire


• Concrete in fire does not generate smoke or toxic gasses

Rational vs Empirical Design Methods
Two strategies are available for fire safety design. Rational design is the more complex and more expensive to perform. It consists of using fire test data, analysis of materials behavior at high temperature, and consideration of other thermal effects, to evaluate the performance of a particular structure under assumed design conditions. In other words, the structural behavior of a particular structure under building fire conditions is analyzed on a case-by-case basis. More on these techniques can be found in the Concrete Reinforcing Institute's Reinforeced Concrete Fire Resistance manual. Rational design is most appropriate when its results are likely to lead to sufficient cost savings to justify the extra effort required in design.

Empiricle design relies on standardized fire test results dictating design parameters such as critical member thickness, minimum concrete cover for reinforcing, etc. This is the design strategy most commonly applied to typical building projects.

Concrete Firesafe Design
The remainder of the article focuses primarily on empirical design methods. The predicted fire-resistance rating of concrete members is provided in a number of tables, relating factors such as aggregate material, restraint conditions, concrete cover for reinforcing, and type of structural member.

The discussion of lightweight concrete is noteworthy. Most commonly formulated with lightweight aggregates such as expanded clay, shale or fly ash, lightweight concrete has a density of 90 to 120 pounds per cubic foot, compared to approximately 150 lb/cu. ft. for normal weight concrete. Generally speaking, lightweight can expect to achieve a higher fire resistance rating than normal weight concrete due to its lower thermal conductivity.