Geoscience Reference
In-Depth Information
227
concrete layers and detachment of a section of concrete
along some plane of weakness, such as a layer of
reinforcement. Forms of cracking include those caused
by differential thermal expansion that often run
perpendicular to the outer surface. Also, differential
incompatibility between aggregates and cement paste
may cause surface crazing. Thermal shock caused by
rapid cooling from fire-fighting water may also cause
cracking. Macrocrack and microcrack patterns are readily
observed microscopically with the aid of fluorescent resin
impregnation (
227
) and fluoresence microscopy (
228
).
The colour of concrete can change as a result of heating
and this may be used to indicate the maximum
temperature attained and the equivalent fire duration. In
many cases, at above 300°C a red discolouration is
important as it coincides approximately with the onset of
significant strength loss. Any pink/red discoloured
concrete should be regarded as being potentially
weakened. Actual concrete colours observed depend on
the types of aggregate present in the concrete. Colour
changes are most pronounced for siliceous aggregates
(especially flint/chert) and less so for limestone, granite,
and sintered PFA (shows very little colour change). The
colour changes are most easily observed in hand specimen
and in thin sections viewed in plane-polarized light. Figure
229
shows red discolouration of concrete aggregate in thin
section. The red colour change is a function of (oxidizable)
iron content and it should be noted that as iron content
varies, not all aggregates undergo colour changes on
heating. Also, due consideration must always be given to
the possibility that the pink/red colour may be a natural
feature of the aggregate rather than heat-induced.
Some widely used aggregate materials contain
naturally red or pink particles. British examples include
Ordovician and Permo-triassic sandstones and quartzites,
which are often various shades of red and any
sand/gravel deposits that include materials derived from
these rocks (e.g. Trent river gravels). In addition, the
Thames river gravels may occassionally include naturally
red-coloured flints. Care must also be taken when white
calcined flints are present as these are commonly
incorporated in decorative white concrete panels and are
also a common ingredient of calcium silicate bricks.
Petrographic examination is invaluable in determining
the heating history of concrete as it can determine
whether features observed visually are actually caused
by heat, rather than some other cause. In addition to
colour changes of aggregate, the heating temperature can
be cross-checked with changes in the cement matrix and
evidence of physical distress, such as cracking and
microcracking. A compilation of the changes undergone
227
Fire-damaged concrete showing a macrocrack
(yellow) running parallel to the outer surface and red
discolouration of a flint fine aggregate particle (left).
Quartz fine aggregate particles appear white and the
cement matrix is dark; PPT, ×35.
228
228
Close view of fire-damaged concrete showing
microcracking (green); UV, ×150.
229
229
Fire-damaged concrete showing red
discolouration of a flint fine aggregate particle near to
the spalled outer concrete surface (left), indicating
heating to 300-600ºC; PPT, ×35.
