The fire is then on the surface of the pool of accelerant and the liquid accelerant acts as an insulator to the concrete. If this is so, then concrete under gas tanks should not be spalled, since the area is so protected by the volume of gasoline. Anyone who has worked a fire scene where petroleum products were stored can see that the petroleum products do cause the concrete to spall. Whether spalling occurs depends on many factors that have not been duplicated in a laboratory setting. These factors would include radiant heat in the specific area of the accelerant, age and porosity of the concrete, usage of the concrete, the type of construction surrounding the concrete, the type of crust or finish of the concrete during construction, the type of aggregate used in the concrete, etc.
Concrete is usually very porous, unless it has been sealed.
It retains accelerants longer than wood or most other products. Imagine pouring this type of fuel on porous concrete and allowing it to soak in for a minute or two while other preparations are being made. Then a trailer is ignited and the liquid that has not been soaked up by the concrete vaporizes and burns. The more of the liquid burned, the hotter the concrete will get. Perhaps this is the reason that while using various laboratories, my success rate for finding accelerant in concrete samples is about 50 percent.