During a fire, the heat released by combustible materials can reach extremely high temperatures—around 1,200 °C—which can affect the mechanical properties of concrete foundations. After a fire, one might wonder whether the foundations can be preserved so that the structure can be rebuilt directly on top of them, thereby saving significant reconstruction costs. It is estimated that a new residential foundation costs between $20,000 and $40,000, not including demolition, landscaping, and other expenses if the home must be rebuilt after a fire.
To determine whether a concrete foundation has been significantly damaged by a fire, there are a few warning signs:

• Surface delamination: A portion of the concrete surface, ranging from a few millimeters to a few centimeters, peels away from the surface. These areas can be detected by tapping the surface with a hammer. This damage generally does not affect the structural properties of the concrete, as these defects are superficial.

• Surface spalling: Spalling, unlike delamination, involves significant loss of concrete (leaving a hole in the surface). This type of defect is often found at the edges of foundations and occurs due to thermal shock caused by firefighting operations or internal pressure generated by fluid migration within the concrete. During a fire, the concrete heats up, and upon contact with the cold water sprayed by firefighters to extinguish the fire, a significant thermal gradient develops within the concrete, which can cause it to spall. This type of defect is more likely to compromise the foundation’s mechanical strength.

• Color change in concrete: Concrete tends to change color depending on the temperature to which it is exposed, which provides a good indication of the fire temperature and, consequently, the rate of loss of compressive strength. At a temperature of approximately 300 °C, the concrete will begin to take on a pinkish hue. This is a sign that chemical changes are occurring within the cementitious matrix, leading to a significant loss of strength. The pinkish hue will turn red up to 600 °C. The color will change to gray between 600 °C and 900 °C and to beige between 900 °C and 1,000 °C. Above 1,300 °C, the concrete may melt. However, it is important to note that a residential fire generally does not reach this temperature.

The graph below shows the reduction in the compressive strength of concrete subjected to different temperatures:

• Cracking: The presence of cracks may be a sign of significant fire-induced damage to the concrete. However, it is necessary to check whether soot is present in the cracks to determine whether they existed before the fire. According to one study, concrete cracking begins at 600 °C ¹ .

How to assess the reusability of foundations?
After an initial examination, the portions of the foundation that appear to have been exposed to the fire must be stripped of interior finishes to allow for visual and acoustic inspection. The type, extent, and location of the damage to the foundation will determine whether it is still capable of supporting the loads of a building of the same dimensions.
If there is any doubt regarding the reusability of the foundation, core drilling, impact tests, and hardness tests can be performed to assess the concrete’s compressive strength. According to the Quebec Building Code, new concrete is considered adequate if its compressive strength exceeds 15 MPa. If this is the case, the foundation can be reused after performing the necessary repair work, such as sealing cracks and stabilizing the foundation, if needed.

The Role of the Expert
Several signs of damage may be present on a foundation following a fire. The expertise of a civil engineer will help identify these damages and determine whether they significantly affect the foundation’s mechanical properties, in order to assess the feasibility of retaining it for reconstruction.
Finally, a cost estimate must be prepared by an estimator if the foundation can be retained, as these costs may be significant. Additionally, the cost of demolishing the building above the foundation may be higher due to the precautions that the machinery operator must take to avoid damaging the foundation. These are factors that the appraiser and the insurer must consider when deciding whether to retain or demolish the foundation.

Sources:

• ¹ Joongwon Lee, Kwangho Choi, and Kappyo Hong, “The Effect of High Temperature on the Color and Residual Compressive Strength of Concrete,” Proceedings of FraMCoS-7, May 23–28, 2010.