Fire resistant self-compacting concrete

When concrete is exposed to fire it chips and flakes – a process known as spalling. This effect is due to water trapped within the concrete vaporising. As more water vapour is produced the pressure within the concrete structure increases. In concrete structures, chips split away from ceilings, walls, and supporting pillars, reducing their load bearing 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 optimised by adding a few kilograms of polypropylene (PP) fibre per cubic meter of concrete mixture. When exposed to fire the fibres 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) behaves differently though. Adding more than 2kg of PP fibre per cubic meter to the SCHPC mixture affects its ability to self-compact, so the proportion of PP fibre in SCHPC must be kept relatively low. This in turn means that if the concrete is exposed to fire, the network of fine canals created by the melting fibres is not continuous throughout the entire structure, allowing spalling to occur. The question is: how to make SCHPC fire resistant, so buildings made of it are safer, whilst keeping the proportion of polymer fibres low enough that the concrete remains self-compacting.

Researchers from Empa’s Concrete/Construction Chemistry and Mechanical Systems Engineering Laboratories have found an answer. They manufactured a series of thin-walled concrete slabs which were pre-stressed with cables made of carbon fibre reinforced polymer. The concrete from which the slabs were made also contained 2kg of PP fibre per cubic meter of mixture. In some slabs the scientists also added a small quantity of super absorbing polymer (SAP), a synthetic material which is capable of absorbing many times its own weight in water. They then exposed the concrete slabs to fire, reaching temperatures of up to 1000°C. After 90 minutes it became clear that whilst the SAP-containing concrete slabs showed some minor cracking, spalling occurred only in the SAP-free slabs.

The explanation for this behaviour is that during the manufacturing process the SAP is saturated with water, swelling to several times its dry volume. As the concrete is setting the water is drawn out of the SAP by capillary action in the porous cement matrix. The SAP shrinks and creates hollow spaces which link the individual, hitherto unconnected networks of PP fibres. The result is a dendritic network of SAP and PP fibres which permeate the entire slab of concrete, allowing it to tolerate the heat of the fire long enough to maintain the structural integrity of the building.

Tom Austin-Morgan

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