Evaluating Fire-Damaged Concrete
Date Posted: March 7, 2024
Posted In: Discussion, Ideas,
From JLC: Can an intense fire damage a poured concrete or slab foundation?
Peter Marxhausen, a forensic structural engineer and civil engineering instructor at the University of Colorado Denver and author of “Engineering Evaluation of Fire Damage to Concrete Foundations” (Structure, August 2014), responds: Yes, an intense wildfire or structural fire can cause significant damage to a poured-concrete, CMU, or slab-on-grade foundation. The damage is generally associated with structural degradation due to high heat combined with internal nonhomogeneous thermal stresses. With the combination of externally applied forces, internal thermal-induced stresses, and the degradation of the concrete strength, the foundation may continue to stand, or it may collapse.
I have evaluated hundreds of structures that have been damaged by fires, including wildfires. Over the past 25 years, I have found that if the structure is wood-framed and burns without being quickly extinguished, the resulting temperature increase can be sufficient to cause irreparable damage to the concrete foundation at the areas of exposure to high heat.
I have also evaluated concrete foundations that have been damaged by ordinary house fires and have always found that the concrete foundation becomes unusable where the structure is burned to ash without the benefit of being extinguished. In those cases where the heat is high for several hours, the exposed concrete becomes soft and powdery and can easily be broken apart with a framing hammer to reveal perfectly intact rebar (which, by the way, doesn’t melt in a fire).
On the other hand, firefighting professionals typically extinguish most structure fires before the concrete is heated to an extent that it becomes weakened. Where a traditional fire response extinguishes a structure fire, the evaluation methods that I discuss below can help determine whether a foundation is safe to be reused in the repair of a structure.