What is Fire damage in concrete?
While concrete structures are very resistant to fire and generally perform well when fire occurs within them, they not impervious to fire damage.
Exposure to heat alters concrete both mechanically and chemically and can cause significant structural damage, including spalling of the concrete cover and alterations to the steel reinforcement's microstructure, potentially modifying its strength and stiffness.
The most obvious damages are cracks, spalling and extensive blackening of the concrete surface.
What causes Fire damage in concrete structures?
The primary causes of fire damage in concrete structures involve a series of thermal and chemical reactions leading to structural deterioration.
High temperatures induce extreme thermal gradients, leading to differential expansion between the concrete's components, which results in surface cracking and micro-cracking. These cracks not only weaken the structure but also facilitate further damage through increased exposure to elements and stresses.
When exposed to temperatures above 100°C, the cement hydrates which give concrete its strength begin to dehydrate and decompose. Dehydration results in the decomposition of Portlandite into lime, an inert filler material, reducing the strength, integrity and load bearing capacity of concrete structures. High temperatures cause water loss from both the cement paste and pore structure of concrete which increases porosity, further reduces strength, and often results in pink, red or white-grey discolouration
Oxidation at high temperatures, particularly of the aggregates and steel reinforcement within concrete, further exacerbates fire damage. In aggregates, oxidation can lead to volume expansion, while in steel reinforcement, it can weaken the bond with the surrounding concrete. At temperatures in excess of 704°C (1300°F), rebar can lose 20% of its design strength and structural failure of reinforced concrete often occurs when the effective tensile strength of steel reinforcement is lost through heating.
In high-strength concrete, these phenomena can cause explosive behaviour under fire conditions due to the rapid buildup of steam and pressure within the material, leading to explosive spalling and significant risks to the structure's safety and integrity.
What are the signs of Fire damage in concrete structures?
Surface cracking.
Disintegration of concrete surface.
Degradation of surface coatings.
Possible degradation of reinforcement.
How can I identify Fire damage in concrete structures?
Visual survey | Non-destructive testing | Destructive testing |
SEM Microscopy Cement content |
How can I prevent Fire damage in concrete structures?
Preventing fire damage in concrete structures involves several strategies, including the use of fire-resistant materials and construction techniques, applying protective coatings to concrete surfaces, and ensuring adequate concrete cover over reinforcement to protect against high temperatures.
Regular inspections and maintenance can identify and address potential vulnerabilities, while the incorporation of design features that enhance the structure's resilience to fire, such as firebreaks and compartmentalization, can also be effective.
Additionally, understanding the specific risks associated with the building's use and location, and implementing appropriate fire safety measures, can significantly reduce the likelihood and impact of fire damage