For those of you who have not experienced live fire-training exercises, we will take a moment to summarize operations ongoing in a burn building. Burn buildings are generally concrete or steel structures in which firefighters arrange fuels to be ignited and extinguished for training exercises. These fuels are usually “Class A” (wood, straw and paper products). Certain newer facilities contain controlled propane or natural gas burners and steel live fire training props to simulate uncontrolled fires. During a normal training day, firefighters may ignite and extinguish these fuels many times. One ignition and extinguishment of a fire is called an “evolution”. Three or four evolutions per hour during the course of the training session are normal. The session may last four hours for one department. It is not unusual for several departments to use the same structure in one day. Therefore, a structure could see thirty to fifty evolutions during a busy day. A certain amount of heat soaks into the structure when an evolution is conducted. A little more heat sink is generated with each evolution. By the end of the day the building is saturated with heat and will take a full day to cool off. The building may not have a chance to fully cool if fire stimulation training is ongoing the next day. Many fire departments use live fire training props for their training as well.

The size of the room, the size of the fuel loading and the average duration and number of evolutions will all affect the temperatures generated in the building. A typical “Class A” loading, including a bale of hay and two to four wooden pallets, will develop temperatures of about 200 degrees Fahrenheit near the floor and about 900 to 1100 degrees near the ceiling. It is not uncommon to experience ceiling temperatures between 1300 to 1700 degrees. Once the fire is allowed to fully develop, firefighters extinguish the fire using pressurized water delivered with fire hoses. Fire hose are capable of delivering hundreds of gallons of water per minute at pressures of generally 50 to 100 pounds per inch. Some of the water immediately converts to steam, expanding 1700 hundred times its’ volume. This pressure plays havoc with the firefighters and the building. In addition to steam and the effects thereof, the building is subjected to its’ worst enemy ~ thermal shock. What is thermal shock? Have you ever taken a hot glass from the dishwasher and immediately placed it under a cold stream of water? What happened? The glass will often shatter.

That phenomenon is called thermal shock. When a material is heated or cooled to extreme temperatures, and then suddenly subjected to oppositely extreme temperatures the material tries to react by expanding or contracting. The characteristics of the material may not allow it to adjust so quickly. When this happens, the binding element within the material fails. Such failures often show up as simple cracks; such as in structural concrete ceiling or floor slabs. Occasionally the reaction is more violent, as with concrete explosions. Concrete can actually explode like a hand grenade if water inside the concrete turns to steam and is unable to escape. The pressure developed by the steam is greater than the tensile strength of the concrete. When this happens, chunks of concrete and stone are blasted into the room with extraordinary force. More frequently however, the concrete has cracked over time and steam escapes through those cracks. Eventually, however, reinforcing within the concrete corrodes and large slabs of concrete fall away from the ceiling or walls. These are called spalls. Spalls result in weakened concrete sections.

When a slab begins to spall, it is a clear indication that the slab has been exposed to too much heat and, consequently, is failing. Consider this, a 24″ diameter spall averaging 3/4″ deep weighs about thirty pounds. Clearly, no firefighter wants thirty pounds dropped onto his or her head from 8 feet high when crawling through a smoke filled room! Spalls usually expose reinforcing.

Subsequent fires then work on the reinforcing which eventually loses tensile strength. A structural concrete slab relies on both the compressive strength of the concrete and the tensile strength of the reinforcing steel. Once either property is compromised, the structural integrity of the slab is compromised. In summary, concrete deterioration progresses as follows. As a building absorbs heat, the walls and slabs expand. Rarely is the building designed to compensate for this extreme movement. Frequently, expansion cracks develop in the slabs and walls. Those cracks often do not present structural concerns. In addition, however, the products within the concrete (sand, cement and gravel) are all expanding and contracting at slightly different ratios. Further, the extreme heat begins to affect the cement binder holding everything together.

Eventually, the binder softens and particles of sand gravel and cement separate. When this occurs, fine cracks develop in an irregular pattern called crazing. When a slab shows crazing, it is a sign that the concrete is suffering from the heat. However, a structural problem may still not exist. Eventually, water and steam penetrate the slab through these cracks and micro-cracks. This moisture finds its’ way to the reinforcing steel that is critical to the strength of the slab. The reinforcing begins to corrode, developing pressure from crystallization within the slab. After a period of time, the bond between the concrete and steel fails, resulting in an un-reinforced section of concrete that is now simply hanging onto the rest of the slab. This is called a delamination. More water builds up in the void between the concrete and steel. When that water turns to steam, it blows the loose un-reinforced concrete off (a spall).

Again, reinforced concrete slabs rely on the great compressive strength of concrete and the tensile strength of steel. These elements must be bonded together. When concrete and steel are no longer bonded, structural slabs are severely weakened. We have inspected slabs that have entirely delaminated horizontally into two un- reinforced slabs above and below a mat of reinforcing steel that now serves no purpose except, perhaps, to act as a cage to keep the top layer of concrete from collapsing to the floor.

Many of the same reactions to heat and steam are ongoing inside masonry walls that support many slabs and roofs in burn buildings. Masonry generally deteriorates slower than concrete. This can be attributed to the fact that some concrete masonry products contain pre-fired materials that have already reacted to a considerable level of heat associated with manufacturing. Also, concrete block walls are relatively porous allowing vapors to pass without immediate cracking or explosion. Nevertheless, many, if not most burn buildings will show cracking in masonry bearing walls. These cracks are commonly found near the corners of buildings and generally do not present structural stability problems, though they should be repaired to prevent further deterioration. In many cases, an appropriate recommendation for these cracks is to saw cut a straight joint in the wall along the line of the crack and to simply leave it. This will act as an expansion joint to minimize future cracking in that area. Consult with your structural engineer.

Finally, Architects have included lights, doors, windows, handrails, etc. inside burn buildings that have all quickly failed. Doors and windows are a particular maintenance headache and must be thoroughly considered during the design stage. It is rare to see a seasoned burn building with doors and windows still in operable condition.