Why
Spalling is a problem in HSC & not in Normal Strength Concrete
(NSC)?
The
conventional theory of
explosive spalling is that it is chiefly caused by the
build-up of water vapour
pressure in
concrete during fire. If the
concrete
is not very permeable, water vapour formed within it during heating
will not be
able to dissipate and pressure is formed. When that
pressure exceeds the
tensile strength of the concrete, explosive spalling will result.
HSC
is produced primarily through use of a relatively low
water/cementitious ratio
and incorporates silica fume. This leads to a reduced permeability relative to
normal weight concretes.
Because
of the relatively high permeability of normal
strength concrete, when subject to fire, water vapour is able to
readily
dissipate through it. Vapour pressure within the concrete
therefore
remains within its tensile strength and spalling is generally
avoided.
Whereeas
HSC is more susceptible to explosive spalling under fire conditions due
to the
buildup of pore pressure in the cement paste (due to its reduced
permeability).
The
extremely high water vapour pressure, generated during exposure to
fire, cannot
escape because of the high density (and low permeability) of HSC. This
pressure
often reaches the saturation vapour pressure, which at 300°C is
about 8 MPa.
Such internal pressures are often too high to be resisted by the HSC,
which has
a tensile strength of about 5 MPa.
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