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TunnelTECH – FIRE SPALLING Fire-spalling of self-compacting concrete May 2012
SP Technical Research Institute of Sweden
Limestone is often used as a filler in the manufacturing process for production of self-compacting concrete, but tests have shown that this leads to greater spalling tendencies when exposed to fire. But what factor does the age of the concrete play in the equation? Samples of various mixes used as part of a research project that concluded in 2009 were stored, affording the opportunity for further fire tests over various periods of time. A notable conclusion by the SP Technical Research Institute of Sweden is that, contrary to usual expectation, there may be certain concrete mixes that actually suffer an increased risk of fire spalling with age.

Fire spalling of concrete: Mont Blanc Tunnel after 2001 fire (left) and the Channel Tunnel after 2008 fire

Fire tests of self compacting concrete that had been stored from six months up to five years showed that, for three out of the four mixes, the spalling tendency when exposed to fire reduced with age. However, the fourth mix, which contained the greatest proportion of limestone filler, exhibited a greater tendency to spall after four years' storage.
Pic 1

Fig 1. Eight test pieces arranged in pairs

A common way of manufacturing self compacting concrete is to use limestone as a filler in the mix. This produces a particularly dense concrete, making it more inclined to spall when exposed to fire.
The 'Self compacting Concrete with Good Fire Spalling Properties' research project, which was concluded in 2009, performed 178 small and large fire tests on 52 self compacting concrete mixes, with the aim of investigating their spalling tendencies and how spalling could be prevented.
The results showed that self compacting concrete has a very high spalling tendency, but that a fire resistant self compacting concrete can be produced by adding a small quantity of polypropylene fibres to the mix. When the work was concluded, those sample pieces that were left over were stored for use in future projects and tests.
In 2010, during SP's annual Construction Workshop, these sample pieces were used in a public demonstration test, providing a unique opportunity to investigate spalling tendency after several years' storage. Theory and experience from ordinary concrete suggests that the spalling tendency should decline as the concrete dries.
Demonstrating the effect of concrete's age on fire spalling
The tests were carried out on compression loaded test pieces of overall size 1.7m x 1.2m x 0.2m, with eight test pieces arranged in pairs in SP's horizontal furnace, as shown in Fig 1.
Pic 1

Fig 2. Test pieces following exposure in the furnace

Following their exposure to fire, the samples were removed from the furnace, and the spalling depth was measured, as shown in Fig 2.
Fig 3 shows the spalling depths of test pieces of various ages. For the oldest pieces, a relationship can be seen between the spalling tendency and the quantity of limestone filler. The spalling tendency declines with age for three of the four mixes that were tested, while the mix containing the highest proportion of limestone filler (140 kg/m³) exhibited a greater spalling tendency after about four years' storage.
At the time of testing, the moisture ratio in this concrete was about 3.5%, which is somewhat higher than the values that are traditionally regarded as safe. The fact that the concrete with the highest quantity of filler has a higher spalling tendency as it ages seems to indicate that we have identified a concrete that becomes more dense more rapidly as the moisture dissipates out of it. T.Z. Harmathy warned of this phenomenon in his publication Fire Safety Design and Concrete (Longman, 1993), but until now nobody had been able to demonstrate this experimentally.
Pic 1

Fig 3. Spalling depths (Medelspjalkningsdjup) over time (0-60 months) for different limestone filled (kalkstensfiller), and limestone-free (ingen kalkstenfiller) concrete mixes

Conclusion
Not much is known about the behaviour of high strength and self compacting concrete containing different quantities of densifying additives such as limestone filler and silica fume, after a longer period of storage. The tests that we have carried out indicate that there may be concrete mixes that do not behave in the same way as normal concrete, i.e. that their risk of fire spalling increases with age.
References
PP fibres to resist fire induced concrete spalling - TunnelTalk, November 2010
Fire damage rebuild of Mont Blanc road link - TunnelTalk, June 2001
Eurotunnel back to full capacity - TunnelTalk, February 2009

           

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