SPALLING AND DETERMINATION OF ORIGIN AND CAUSE
TRAINING NOTEBOOK
Concrete is a universal building material. Its use in construction ranges from single-story dwellings to high-rises and everything in between. For many years, fire investigators have studied the effects of fire exposure on concrete to determine the origin and cause of a fire. Many investigations uncovered the presence of spalling— the breaking off of layers or pieces of concrete from the surface of a structural element when it is exposed to the high and rapidly rising temperatures of fires.
There are three types of spalling: surface pitting, explosive spalling, and corner break-off. Spalling may occur during the concrete’s exposure to heat —and be accompanied by violent explosions—or some time after heat exposure of a now-weakened concrete member. Minor spalling—limited surface pitting, for example—may be insignificant. Hut more extensive spalling can seriously jeopardize structural stability.
A RESULT OF ACCIDENTAL FIRES, TOO
The old fireman’s tale explanation for spalling is that it is caused by the use of an accelerant and is an indication of an incendiary fire. Relying tin this erroneous explanation could lead to an innocent person’s being denied insurance proceeds and/or being charged with the crime of arson. A scientific look at the phenomenon of concrete spalling clearly shows this explanation to be highly questionable. Recall that concrete is a mixture of different materials. Each component expands at a different rate, and the differences in expansion can cause spalling under accidental fire conditions. An increase in temperature increases the volume of almost all materials. Portland cement expands when heated even if aggregates are not used, but if it is heated above the boiling point of water, the resultant loss of moisture actually can cause the material to shrink. The result of this expansion and contraction can be spalling. When aggregates, which have additional different rates of expansion, are added to the mixture, the possibility of spalling increases even more.
The critical factors influencing spalling are moisture content, compressive stresses, aggregate types, reinforcement cover, profile of section, and thermal barrier.
Concrete is a mixture of portland cement, water, and inert materials called aggregates. To produce goodquality concrete, the water and cement must be in proper portion; the aggregate must be of proper type, size, and amount; and the mixing must be thorough. In addition, the concrete must be properly poured and finished to prevent air pockets from forming or separating. Finally, it must be properly cured.
Concrete can be cast in place or precast, and it can be plain, reinforced, or tensioned. Plain concrete has little or no reinforcement, while reinforced concrete is a mixture of concrete and steel. Pretensioned or posttensioned concrete has been put under compression. Pretensioned (often called prestressed) concrete contains steel strands that are stretched through the form in which the concrete is poured. The concrete is cured until it reaches a specified strength. The tensioned cables then are released from the ends of the forms, placing the concrete under compression; this increases its tensile strength. Pretensioned concrete is made in a manufacturing facility, and the finished units are installed on the job location.
In posttensioned (often called poststressed) concrete, steel strands or bars that are wrapped to prevent adhesion between the steel and concrete are placed in the form into which the concrete is poured. After the concrete reaches a specified strength, the steel strands or bars are stretched and anchored at the ends of the unit. This posttensioning places the concrete under compression and increases its tensile strength. Posttensioned concrete—unlike pretensioned—is poured at the job site.
All types of concrete, whether cast in place, precast, plain, reinforced, or tensioned, are subject to spalling during an accidental fire. Some investigation texts indicate that a temperature of 2,000°F is required for spalling to occur. As previously stated, however, moisture content is one of the critical factors influencing spalling. Since the spalling sometimes is the result of the moisture’s having been converted to steam, a temperature of only 212°F is necessary for spalling to occur. Other pressures or the shock of a fire stream rapidly cooling the surface also can cause spalling.
CONSIDER EXPANSION RATES
Most common aggregates, such as sand, contain a high level of quartz. Quartz expands steadily when heated up to 573°F, at which time it undergoes a sudden expansion. This expansion, coupled with the shrinkage of the portland cement due to moisture loss, has a pronounced effect on the concrete and can produce severe spalling. Laboratory studies have shown that even the most fire-resistant concrete begins to break down at temperatures above 900°F. Since accidental structure fires can and normally do reach temperatures of 1,500°F and above, the presence of spalling in an accidental structure fire is to be expected.3
Tensioned concrete by its very nature is under more compressive stress than plain or reinforced concrete. While this compression increases the strength of the component, it also increases the possibility that spalling will occur. There is a greater tendency for prestressed concrete to spall in fire.4>
As a fire indicator, while spalling can indicate the presence of an accelerant, remember that a liquid does not burn, its vapors burn. The pool of liquid actually cools the concrete and protects it from higher temperatures. Because of this, spalling may be more pronounced along the edges of the pool of liquid. In addition, the spall pattern may be stained from the tarry residues left from combustion. Such discoloration, however, is not proof of the presence of an accelerant.
Accidental fires with normal fire growth and progression can and do produce spalling of concrete surfaces. When one understands the dynamics of fire and the properties of concrete, it becomes obvious that spalling is a normal result of exposure to fire and that interpreting the presence of spalling as an indicator of incendiarism is not based on sound scientific facts or principles.
Endnotes
“Spalling of Concrete in Fires,” Construction Industry Research and Information Association, London (1984), p. 6.
Ibid., p. 2.
“Defining and Recognizing the Effects of Concrete Spalling,” Fire and Arson Investigation Research Paper, National Fire Academy, Emmitsburg, MD (1982), p. 3-
“Misconceptions About Fire Investigation/Part III,” The National Fire and Arson Reporter, Vol. 7 No. 5 (1989), p. 4.
