Mythunderstanding #1:  Spalling of Concrete
Spalling is a physical process of the breakdown of surface layers of masonry (typically concrete) which crumble into small pebble-like pieces in response to high temperatures and/or mechanical pressure.  The physical appearance of spalling may also include striated lines, discoloration of the masonry and pitting or rough texture to the concrete that remains attached to the slab.   Spalling is caused by heating, mechanical pressure, or both.  This heat and/or pressure causes uneven expansion of the materials that make up the concrete (such as sand and gravel), the reinforcing steel and the concrete mix, or the surface and subsurface layers of the concrete.  Alternatively, the heat may release the water in the concrete. Pressure created by rapid changes in temperature, such as application of cold water to the heated concrete surface during firefighting operations, can also cause spalling.  These processes break the bond that holds the solids together and thus cause the concrete to crumble.


(Click to enlarge photo)
Concrete spalling above a doorway.

Whether or not concrete will spall when heat is applied is highly variable and depends on the complex interaction of the composition of the masonry material, temperature and temperature change from burning fuel load, configuration of materials, pressure on the concrete and local conditions. If the concrete is “green,” meaning recently poured, it is more susceptible to spalling because its water content is higher than concrete that has cured. If the concrete is reinforced with rebar, it may also be more susceptible to spalling because applied heat is absorbed at different rates by the metal and by the surrounding masonry; the metal heats faster and expands, putting pressure on the concrete that may cause it to fracture. How well the surface absorbs liquid may also be a factor in whether or not spalling is produced.


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Closeup of spalling above same doorway.

The presence of spalling at a fire scene was traditionally seen as an indicator that the fire had burned very hot, which could only occur if an ignitable liquid was used. However, physics and research have not borne this out to be fact. When an ignitable liquid is poured, it pools on the hard surface, such as a concrete floor. When the pool is ignited, what is burning is the vapors being given off by the flammable liquid, not the liquid itself. This burning can only occur where the vapors mix with oxygen and have access to flame—namely on the top of the pool and at the edges of the pool. The bottom of the pool, the part in contact with the masonry, does not burn. The burning does heat the liquid, but the maximum temperature of the liquid cannot reach higher than the liquid’s boiling point, which in the case of gasoline is 300ºF—typically not hot enough to cause the expansion of water in the concrete that creates spalling. In addition, an ignitable liquid pool on concrete burns quickly, only a few minutes, which is typically not long enough to heat the concrete to temperatures conducive to spalling.

In fact, it has been shown that spalling is more likely to occur from radiant heat released by burning materials over a period of time than from the comparatively short burn period of a pool of ignitable liquid. This excessive radiant heat can be created by a variety of conditions, including burning of modern furnishings and collapse bringing hot materials into contact with the masonry.

Spalling can also result from other causes that are unrelated to fire, including corrosion of supporting rebar that places stress on the concrete, the expansion and contraction of the freeze/thaw cycle, and chemicals like deicers and fertilizers. If spalling is present, the investigator should also consider whether that spalling existed prior to the fire.

However, none of this should be taken to mean that spalling is never related to use of an ignitable liquid. Two examples are illustrative. First, radiant heat from a burning ignitable liquid pool can heat the concrete around the edges of the pool more significantly than the concrete under the pool, which is protected by the liquid’s inability to heat beyond its boiling point. Therefore, spalling may occur around the edges of the pool, and therefore a “protected” area with spalling around its edges MAY indicate that an ignitable liquid was present. This spalling may extend to the areas with cracks or joints where the ignitable liquid might have seeped. The second example requires the presence of a floor covering to increase the radiant heat output. If an ignitable liquid is poured on carpet that covers a concrete floor, the intense radiant heat from the burning carpet in close contact with the concrete surface may cause spalling, something that may not have happened if the ignitiable liquid had not been present.


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Closeup of the spalling of the concrete pad in the four-story apartment
building under construction.

The presence of spalling at a fire scene cannot be taken as a definitive indicator that an accelerant was used in the fire. Rather, the presence of the spalling should be explained, if possible, and then treated as one of many factors that enter into assessing the totality of the circumstances at the scene and their relationship to determining the heat source and first material ignited.

For more information on spalling, consult:

DeHaan, John D. Kirk’s Fire Investigation, 6th Edition. 2006. pp 258-261.

NFPA 921: Guide To Fire and Explosion Investigations. 2004 Edition. Section 6.6.

Lentini, John J. Scientific Protocols for Fire Investigation. 2006. pp 453-459.

Redsicker, David R. and John J. O’Connor. Practical Fire and Arson Investigation, 2nd Edition. 1997. pp 101-103.

Next month's Mythunderstanding: #2: The area of greatest damage and the lowest point of burning is always the area of origin.