Civil Engineering Reference
In-Depth Information
develop. A change from buff or cream to pink tends to occur at about
300°C and from pink to whitish grey at about 600°C. Certain types of
aggregate also show these colour changes which can sometimes be seen
within individual aggregate particles. The change from a normal to light
paste colour to pink is most marked. It occurs in some limestones and
some siliceous rocks - particularly certain flints and chert. It can also
be found in the feldspars of some granites and in various other rock
types. It is likely that the temperature at which the colour changes occur
varies somewhat from concrete to concrete and if accurate temperature
profiles are required, some calibrating experiments need to be carried
out.
3
Dehydration of the cement hydrates: This can take place within the
concrete at temperatures a little above 100°C. It is often possible to
detect a broad zone of slightly porous light buff paste which represents
the dehydrated zone between 100 and 300°C. It can be important, in
reinforced or pre-stressed concrete, to establish the maximum depth of
the 100°C isotherm.
C
HANGES
IN
FIRE
-
DAMAGED
CONCRETE
<300°C Boundary cracking alone
250-300°C Aggregate colour changes from pink to red
300°C Paste develops a brown or pinkish colour
300-500°C Serious cracking in paste
400-450°C Portlandite converts to lime
500°C Change to anisotropic paste
500-600°C Paste changes from red or purple to grey
573°C
Quartz gives a rapid expansion resulting from a phase
change from alpha to beta quartz
600-750°C Limestone particles become chalky white
900°C Carbonates start to shrink
950-1000°C Paste changes from grey to buff
C
HANGES
IN
AGGREGATE
250-300°C
Aggregate colour changes from pink to red
573°C
Quartz gives a rapid expansion resulting from a phase
change from alpha to beta quartz
600-750°C Limestone particles become chalky white
900°C
Carbonates start to shrink
C
HANGES
IN
THE
CEMENT
PASTE
300°C Paste develops a brown or pinkish colour
400-450°C Portlandite converts to lime
500-600°C Paste changes from red or purple to grey
950-1000°C Paste changes from grey to buff
