Concrete is clearly a tough material, but when it is exposed to the extremely high temperatures of fire than can occur when a building burns, it will chip and flake in a process that is known as “spalling.” Spalling of concrete during a fire causes the material to weaken and eventually can lead to the collapse of walls, floors, and ceilings of the building.
Now, researchers at Empa, the Swiss Federal Laboratories for Materials Science and Technology, have discovered a new way to add specialized polymers to the concrete mix that vastly improve the concrete’s resistance to spalling during a fire.
An interesting article on the Science Daily website provides the exciting details:
Wood crackles as it burns in a chimney or campfire. When concrete is exposed to fire it chips and flakes — a process known as spalling. Both effects are due to the same phenomenon: water trapped within the piece of wood or concrete element vaporizes due to the high temperature. As more water vapour is produced the pressure within the wood or concrete structure increases. In wood this causes the cells to burst with a crackling sound, creating cracks in the logs. In concrete structures, chips split away from ceilings, walls, and supporting pillars, reducing their loadbearing capacity and increasing the risk of collapse in a burning building.
The resistance of conventional vibrated concrete to the heat of a fire can be optimized by adding a few kilograms of polypropylene (PP) fiber per cubic meter of concrete mixture. When exposed to fire the fibers melt, creating a network of fine canals throughout the concrete structure. These allow the water vapour to escape without increasing the internal pressure, so the concrete structure remains intact.
Self-Compacting High-Performance Concrete (SCHPC) is used in the commercial construction industry, and is highly valued for its ability to self-compact, which saves time during construction. When it comes to making SCHPC more fire resistant using polypropylene, there’s a catch 22. We explore that conundrum and the Empa researchers’ solution on the next page…
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