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Fire test exposes flaws in post-tensioned floor slabs

Test have shown slabs designed for a two hour fire resistance collapsed after 66 minutes.

Post-tensioned concrete slabs could be much more vulnerable to fire than previously thought and more tests are urgently needed, it was claimed this week. In an independent test, a post tensioned slab designed for a two hour fire resistance collapsed after just 66 minutes. The research by a leading consultancy was published last week.

According to the peer reviewed research paper published by the Institution of Structural Engineers, the original concrete cover rules, which give a minimum depth of concrete to reinforcement for a particular fire resistance period, were developed from tests carried out in the 1930s-1940s. But developments in concrete technology mean these rules may no longer be relevant.

"Concrete mixes and the way we use concrete have changed," said Peter Brett Associates senior associate Fergal Kelly, who wrote the paper Reinforced concrete structure in fire: a review of concrete rules' with John Purkiss, formerly of Aston University.

"Permeability has changed. Concrete mixes have changed to accelerate early strength and improve workability."

In their paper Kelly and Purkiss claim: "Extrapolation of historical levels of spalling to provide concrete cover for modern concretes is unconservative and unsafe. A test programme based on modern concrete constituents and structural forms should be aimed at updating these specific areas of code guidance."

Concrete spalls in a fire, reducing the cross section of the structural member and exposes the steel reinforcement. The tendency towards shallower floor construction has changed the way concrete is used, claims the paper. Shallower floor construction is achieved by using computer modelling techniques and higher strength concrete accompanied by higher concrete stresses which increases the significance of spalling.

Post-tensioned concrete is popular as it gives slender floor depths. It is proving additionally vulnerable as the strength of post tensioning strands is reduced to 55% at 350oC and the bond is lost between the strands and the concrete, says the paper. In the post-tensioned test example given in the paper, spalling started at 11 minutes and the post-tensioned ducts could be seen after 20 minutes.

After 28 minutes there was a steady increase in deflection of up to 180mm at 60 minutes, after which runaway deflection started and the deflection of the slab was measured at 246mm just before it collapsed at 66 minutes.

The paper says other recent tests on post-tensioned concrete slabs and columns of various sizes have failed to achieve the fire resistance which they should have done according to British Standards, largely due to spalling. "We need to be sure that when we’re using concrete and new forms of construction that the same rules apply," said Kelly.

"This paper throws up some concerning issues," said Standing Committee on Structural Safety (SCOSS) secretary John Carpenter. "As design advances, any disparity between test samples and full-size behaviour needs to be carefully considered, as does reduction of slab thicknesses, and reduction in 'spare capacities' arising from more sophisticated design methods. Real fire will find the weakest link.

"Post-tensioned floors may be particularly vulnerable and specifically so if not constructed correctly. Previous research has recommended the increase of concrete cover for post tensioned slabs. However Kelly said that these findings would not necessarily lead to thicker slabs. "Post-tensioning is an effective system," said Kelly. "Spalling can be addressed by putting fibres or mesh in to prevent it. A lot of designers put in additional mesh anyway."

Arup head of technology professor Tony Jones played down the implications. "The paper was very one sided," he said. "It looked at the structures that have collapsed, but not at the structures that have withstood fire and been repaired."

■ 11 mins Spalling starts
■ 20 mins Spalling exposes post tensioning ducts
■ 28 mins Steady increase in amount of slab deflection
■ 66 mins Deflection reaches 246mm before slab collapses

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