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