Fire-Resistant Self-Compacting Concrete Is Developed
Engineering360 News Desk | January 06, 2016Scientists from the Swiss Federal Laboratories for Materials Science and Technology ("Empa") have developed a method of manufacturing fire-resistant self-compacting high-performance concrete (SCHPC) that maintains its mechanical integrity when exposed to fire.
When concrete is exposed to fire, it chips and flakes—a process known as "spalling"—resulting from the vaporization of water trapped within due to the high temperature. In concrete structures, chips split away from ceilings, walls and supporting pillars, reducing their load-bearing capacity and increasing the risk of collapse.
The resistance of conventional vibrated concrete to a fire’s heat 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 that allow the water vapor to escape without increasing the internal pressure. This enables the concrete structure to remain intact.
However, SCHPC—a highly fluid concrete mixture that is able to consolidate under its own weight—behaves differently. Adding more than 2 kg of PP fiber per cubic meter affects its ability to self-compact, so the proportion of PP fiber in SCHPC must be kept relatively low. This, in turn, means that if the concrete is exposed to fire, the network of canals created by the melting fibers is not continuous throughout the structure, allowing spalling to occur.
Empa scientists developed a method to make SCHPC fire resistant while keeping the proportion of polymer fibers low enough that the concrete remains self-compacting. They did so by manufacturing a series of thin-walled concrete slabs that are pre-stressed with cables made of carbon fiber-reinforced polymer. The concrete making up the slabs also contained 2 kg of PP fiber per cubic meter of mixture.
Researchers tested an SAP-containing concrete slab with a radiant heater. Image credit: Luke Bisby/University of Edinburgh.In several slabs, the scientists also added a super-absorbing polymer (SAP), which is a synthetic material that is capable of absorbing many times its own weight in water. They then exposed the concrete slabs to fire, attaining temperatures of up to 1000°C. After 90 minutes, the SAP-containing concrete slabs showed minor cracking. Spalling occurred only in the SAP-free slabs.
The explanation for this behavior is as follows: During the manufacturing process the SAP is saturated with water, swelling to several times its dry volume. As the concrete sets, the water is drawn out of the SAP by capillary action in the porous cement matrix. The SAP shrinks and creates hollow spaces that link the individual, previously unconnected, networks of PP fibers. The result is a “dendritic network” of SAP and PP fibers that permeate the concrete. This allows it to tolerate heat long enough to maintain the building’s structural integrity.
The process, for which a patent has been applied, allows for the use of SCHPC for fire-resistant structures. Previously this was possible only by using self-compacting concrete in combination with a sprinkler system or an external thermal insulation layer. Additionally, SAP-enriched SCHPC can now be used in place of conventional vibrated concrete—the compacting of which generates a lot of noise—without any loss of fire resistance.