Post-tensioning in fire

A single fire test published by Kelly and Purkiss[i] in 2008 has raised concerns over the fire resistance of post-tensioning. A key reason for the failure of this slab was high levels of spalling of the concrete, which was induced primarily by the exceptionally high moisture content of 4.6 per cent. Eurocode 2 advises that a moisture content of three per cent is appropriate for fire design and it is generally considered that for an internal environment the long term moisture content is less than one per cent.

The authors of this paper explained their findings at the open Concrete Fire Forum in February 2009. The fire specialists present at that meeting were critical of the test regime used and requested that further details of the test data were made public. The authors agreed to this but to date this has not been forthcoming.

More recently Bailey and Ellobody[ii] have reported a more representative sample of 12 bonded and unbonded fire tests have demonstrated that the majority of post-tensioned buildings the current codes of practice give acceptable fire resistance. The moisture contents ranged from 1.07 to 2.54 per cent. The paper did find that unbonded PT slabs designed using BS 8110-2 may be slightly unconservative. This method of construction represents less than 15 per cent of the UK market and in the light of these tests it is more appropriate to use the guidance in BS EN 1992-1-2 which was found to be conservative.

Post-tensioning is a popular method of construction in the Middle East and recently two post-tensioned structures have been subject to severe fires and have survived. In one building, designed for a three-hour fire resistance the fire raged for five hours without collapse of the floor. Spalling of the concrete did occur leading to large deflections, put the floor remained intact.

[1[KELLY, F. & PURKISS, J. Reinforced concrete in fire: a review of current rules'. The Structural Engineer, Vol 86, Issue 19, 7 Oct 08.

[2]BAILEY, C.G. & ELLOBODY, E.Comparison of unbonded and bonded post-tensioned concrete slabs under fire conditions. The Structural Engineer, Vol 87, Issue 19, 6 Oct 09.