High-Strength Concrete and Fire
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It is vital that buildings be capable of protecting people and property against the hazards of fire. Concrete, as the most commonly used building material, has a major role to play in resisting fires. When exposed to fire, normal strength concrete performs well in an engineered structure, but how does high-strength concrete behave? This article is intended to provide state-of-the-art information on this crucial subject.

Designers of high-rise buildings ask several critical questions regarding the resistance of high-strength concrete to fire and elevated temperatures.

• What is the endurance of high-strength concrete (HSC) in a fire?


• Do shear walls built of HSC have comparable fire resistance to regular HSC columns?


• How closely do fire tests account for the conditions of “real world” fires?


• Will HSC spall after prolonged fire exposure?

Endurance of Concrete

Concrete can not be set on fire like other materials. As a non-combustible material, it does not emit toxic fumes, produce smoke, or drip molten particles when exposed to fire, unlike plastics or metals. Also, concrete does not add to the fire load in a building. For these reasons, concrete is readily accepted in building codes as having a high degree of fire resistance. In many applications concrete is virtually fireproof. Concrete’s excellent performance in fire is due to its main constituent materials—cement, water, and aggregates—which, when combined, form a material that is non-combustible and has a thermal conductivity equal to 1/21 that of steel. It is this slow rate of heat transfer that enables concrete to act as an effective fire shield, not only between adjacent spaces but also in protecting reinforcing bars and internal concrete from damage.

Concrete’s fire resistance has been proven by hundreds of fire tests, some of which are discussed below. Testing has shown that the rate of temperature increase through the cross section of a concrete column or shear wall is relatively slow, and so internal zones do not reach the same high temperatures as relatively thin sections of steel beams and columns when exposed to fire. The minimum concrete column dimension typically used in very tall high-rise buildings is 24” square, due to structural requirements. Using these typical dimensions, HSC columns with fire exposure on four sides are rated at four hours.

Typical shear wall thickness for high-rise buildings 50 stories and above is 24”, which is required for serviceability (drift control). The structural thicknesses required for shear walls and columns are much greater than what is required for a three fire rating, providing even greater endurance in a fire. In addition, shear walls with fire on one side will have improved performance.

Standard Fire Tests versus Real World Fires

The current ASTM E 119-2000 (Methods for Fire Tests of Building Construction and Materials) is more conservative for building materials and components than so-called “real world” fires. For example, in the World Trade Center fire, combustibles and debris shoved to one end of the building lowered the fire load as compared to a typical office. Areas the combustibles were shoved from had lower fire load because the materials were not there, and areas where the materials piled up also had a lower fire load because the materials mixed with non-combustibles such as concrete and ceiling tiles. Figure 1 below shows time temperature curves from calculated “real world” fires and Figure 2 shows the ASTM E 119 time temperature curve. The E 119 curve rises at the same rate as the calculated fires, and at two hours is about 10 degrees C higher than the “real world” peak temperature. The E 119 curve continues to rise after the “real world” fire temperatures start falling. The figures demonstrate that design requirements based on ASTM test curves are conservative enough to handle far more than what has been encountered in reality.

Figure 1. Typical time temperature curves for given ventilation factor and different fuel loads (MJ/m2 of total internal surface area) (Magnusson and Thelandersson 1970)

Figure 2. Time temperature curves for ASTM E119

Spalling of High Strength Concrete

The endpoint for the ASTM E 119 column fire test is that point at which the column can no longer support load. Minor spalling may occur during the test without affecting the column’s fire resistance rating. At the end of the test, when columns are loaded to failure at elevated temperatures, splitting and spalling also occur.

Numerous tests on lightweight, normal weight, and high-strength concrete conducted by the National Institute of Standards and Technology (NIST), Construction Technology Laboratories (CTL), and the National Research Council (NRC) of Canada have reported various degrees of spalling in fire tests. Ten ASTM E 119 HSC column fire tests performed at NRC of Canada showed that minor spalling occurred within 15 minutes of the start of the test for some of the specimens, but that large spalls and cracks were observed only towards the end of the test. No explosive spalling occurred in any specimens.

“Explosive spalling” is a term used by laboratory technicians to define a spall that occurs with a loud popping sound. This does not mean that the concrete creates projectiles endangering occupants or emergency responders. The spalled concrete typically drops to the floor around the column or may even remain somewhat attached. Since major spalling occurs when temperatures exceed 815 degrees C (1500 degrees F), occupants or personnel would not be in the area of the spalling.

During tests on small (4” dia x 8”) unreinforced concrete specimens at NIST, only six out of a total of 76 specimens failed by explosive spalling. During CTL tests on small (3” dia x 6”) unreinforced HSC concrete specimens, none failed by explosive spalling. It should be noted that tests on small specimens are not comparable to tests on full-size reinforced concrete specimens and therefore are not conducted in accordance with ASTM E 119. Instead, small electric furnaces are used with a radiant heat source, so the size of specimen and rate of temperature rise in the electric furnaces may significantly influence the observed performance of concrete.

It is also worth noting that the longer concrete cures, the less water retained and the less chance for spalling, since fire-induced spalling can occur in concrete with high water content. Because HSC has a low w/c ratio, and since tall buildings using HSC are not occupied until more than a year after concrete is placed, high water content can be avoided.

HSC and Fire Resistance in Building Standards

These tests provide data indicating that HSC columns are easily designed and detailed to provide structural integrity during a fire; even so, the results are most likely very conservative. The limitations of testing equipment to apply loads on HSC columns means that test specimens were much smaller than the smallest HSC columns allowed by ACI 216.1-06.

The American Concrete Institute (ACI) standard 216.1-06 (Standard Method for Determining Fire Resistance of Concrete and Masonry Construction Assemblies) requires certain minimum sizes (8” to 14”) for fire resistance of one to four hours when the specified concrete compressive strength is 12,000 psi or less. For columns made with a compressive strength above 12,000 psi, ACI 216.1-06 requires that the minimum dimension of a column be no less than 24” and that column ties be formed with 135-degree hooks so that the ends of the column ties do not open as the ties expand and contribute to spalling.

There is no differentiation between normal strength concrete and HSC for aspects of fire design which depend on concrete’s low thermal conductivity, such as fire barriers like walls and floors. In any case, the use of HSC is normally reserved for heavy columns and shear walls in the lower floors of very tall buildings. Current testing and design standards should reassure designers that HSC’s fire endurance will continue to make it a superior choice of material.