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Concrete slab fire test results questioned

Leading researchers this week hit back at claims that post-tensioned concrete slabs could be much more vulnerable to fire than previously thought.

Independent tests have found a post-tensioned slab had collapsed after just 66 minutes in fire conditions, despite having been designed for a resistance of two hours.

However, experts this week blamed high moisture content for the poor behaviour of the slabs used in this test.

“The [independent] test had an unrealistically high moisture content (4.6%) which does not represent expected moisture values for commissioned buildings,” said Professor Colin Bailey who has been carrying out fire tests on post-tensioned concrete slabs at the University of Manchester.

“In contrast, the 12 tests that were carried out at the University of Manchester had a maximum moisture content of 2.5% and represented reality. It is no surprise that the test had extensive spalling; any expert would have predicted this before the test.”

Peer-reviewed and published by the Institution of Structural Engineers, the now-criticised paper Reinforced concrete structure in fire: a review of concrete rules was written by Peter Brett Associates senior associate Fergal Kelly with John Purkiss, formerly of Aston University.

Kelly rejected criticism of his research questioning the experts’ assumptions about the real moisture content of buildings . “The design amounts of concrete and water determines the initial water content,” said Fergal Kelly. “Over time concrete dries out, but it can take several years to reach a low moisture content.

It has a high moisture content early in its life and you can’t say just ignore it. We did extensive searches for data on moisture content but there was no evidence available to give us a pointer.”

Other concrete experts were more confident about fire performance. “It’s well known in various extreme situations you will have spalling,” said Concrete Centre technical director and head of structural engineering Andrew Minson.

“The significant factor is moisture content. In a tunnel, which is a damper situation, measures are taken to minimise spalling, by adding fibre reinforcement. But modern concrete in normal situations does not spall as many recent tests have demonstrated.”

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