Geoscience Reference
In-Depth Information
by concrete as it is heated is presented in
Table 27
.
Careful identification of microscopically observed
features allows thermal contours (isograds) to be plotted
through the depth of individual concrete members. In the
most favourable situations contours can be plotted for
105°C (increased porosity of cement matrix), 300°C (red
Table 27 Mineralogical and strength changes to concrete caused by heating (Concrete Society TR68; 2008)
Changes caused by heating
Heating
temperature
Strength
Mineralogical changes
changes
70-80°C
Dissociation of ettringite, Ca
6
Al
2
(SO
4
)
3
(OH)
12
·
26H
2
O, causing its
depletion in the cement matrix
Minor loss
of strength
105°C
Loss of physically bound water in aggregate and cement matrix
possible
commences causing an increase in the capillary porosity and minor
(<10%)
microcracking
120-163°C
Decomposition of gypsum, CaSO
4
·
2H
2
O causing its depletion in the
cement matrix
250-350°C
Pink/red discolouration of aggregate caused by oxidation of iron
Significant loss
compounds commences at around 300°C
of strength
Loss of bound water in cement matrix and associated degradation
commences at
becomes more prominent
300°C
450-500°C
Dehydroxylation of portlandite, Ca(OH)
2
causing its depletion in the
cement matrix
Red discolouration of aggregate may deepen in colour up to 600ºC
Flint aggregate calcines at 250-450ºC and will eventually (often at
higher temperatures) change colour to white/grey
Normally isotropic cement matrix exhibits patchy yellow/beige colour
in cross-polarized light, often completely birefringent by 500°C
573°C
Transition of
- to
-quartz, accompanied by an instantaneous increase
Concrete not
α
β
in volume of quartz of about 5% in a radial cracking pattern around the
structurally
quartz grains in the aggregate
useful after
heating in
temperatures in
excess of 550-
600°C
600-800°C
Decarbonation of carbonates; depending on the content of carbonates
in the concrete, e.g. if the aggregate used is calcareous, this may cause
a considerable contraction of the concrete due to release of carbon
dioxide, CO
2
; the volume contraction will cause severe microcracking
of the cement matrix
800-1200°C
Complete disintegration of calcareous constituents of the aggregate and
cement matrix due to both dissociation and extreme thermal stress,
causing a whitish grey colouration of the concrete and severe
microcracking. Limestone aggregate particles become white
1200°C
Concrete starts to melt
1300-1400°C
Concrete melted
