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Concrete row ramps up

More tests are urgently needed to determine a definitive value for moisture content of concrete slabs in operational buildings, researchers said this week.

This follows the collapse after just 66 minutes of a post tensioned slab that had been designed for a two hour fire resistance in an independent test. The results were revealed in the paper Reinforced concrete structures in fire, published by the Institution of Structural Engineers (IStructE).

Professor Colin Bailey of the University of Manchester blamed the premature collapse of the slab’s on "unrealistically" high moisture content of 4.6% (News last week).

The IStructE paper was written by Peter Brett Associates senior associate Fergal Kelly and John Purkiss, formerly of Aston University. Kelly said that buildings have a higher moisture content earlier in their life.

"With regard to moisture content, Bailey states that 2.5% is a realistic moisture content but offers no evidence to prove this," said Kelly. "Bailey’s own paper on the large scale Cardington test pointed out that the moisture content of that building was 3.8% at the time of testing, when the building was already 3.5 years old. Unfortunately, Bailey relies on this assumption rather than evidence," he said.

Bailey admitted that he wasn’t aware of any research that had been carried out to measure moisture content and to support the 2.5% assumption but said that this was a commonly accepted value. Bailey said his own test had been carried out in an unheated hangar slowing the drying process. "No one has done any definitive research measuring moisture contents in buildings after they have been commissioned, although EN1992-1-2 states that it is expected to be between 2.5 and 3%," said Bailey.

But Kelly disputed the assumed figures. "No-one in the fire research community knows where the 3% guidance in the Eurocode came from," said Kelly. "It’s a very crucial gap in our knowledge which is why we are calling for further research in this area."

Bailey and the Concrete Centre last week launched an attack on the Peter Brett Associates paper which was peer reviewed by the IStructE. The paper found that during a fire, concrete spalling in a post tensioned slab began after just 11 minutes and exposed the rebar after just 20 minutes. Within 60 minutes the slab had deflected by 180mm and at 66 minutes the slab collapsed. Such findings led the Peter Brett Associates team to conclude that more testing of post-tensioned slabs is urgently needed to update the fire resistance guidance in British Standards.

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