Self-compacting high-performance concrete now fire resistant as well

Published:  04 January, 2016

Scientists from the Swiss Federal Laboratories for Materials Science and Technology (Empa) develop a method for manufacturing fire resistant self-compacting high-performance concrete.

When exposed to fire, self-compacting high-performance concrete (SCHPC) flakes and splits which reduces its load-bearing capacity.

These effects are due to water trapped within the concrete vapourising at high temperature, increasing the pressure inside the structure.

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.

However, self-compacting high-performance concrete (SCHPC) behaves differently. 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 challenge faced by the Empa scientists was 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 the 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 special 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 1,000°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.

This is because 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 volume of concrete, allowing it to tolerate the heat of the fire for sufficiently long to maintain the structural integrity of the building.

The new development is expected to increase the opportunities for exploiting the economic and environmental advantages offered by self-compacting concrete (SCC). The new process, for which a patent has been applied, allows for instance the use of SCHPC for fire resistant structures. To date this has only been possible using SCC in combination with a sprinkler extinguishing system or an external thermal insulation layer.

Researchers from Empa tested a SAP-containing concrete slab with a radiant heater. Some discolouring emerged at the surface of the concrete element, but no spalling occurred.

The SAP-containing concrete element resisted temperatures of around 1,000 °C, and its capacitance was preserved. The slab made of concrete without SAP showed some severe spalling which reduced the loading capacity of the slab and raised the risk of a collapse.

  • Operation Florian

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