Geoscience Reference
In-Depth Information
aggregate can often be distinguished from terrestrial
gravels by the presence of unfossilized calcareous shell
fragments and encrusting organisms (see
124
and
125
).
The use of crushed rock aggregate is indicated by
particles with angular shape and often by monomictic
composition. Figure
157
shows concrete with a crushed
gabbro coarse aggregate and Figure
158
shows concrete
containing crushed limestone coarse aggregate.
Some artificial aggregates used in concrete may have
beneficial characteristics, while others are undesirable.
Figure
159
shows lightweight concrete made with
sintered PFA coarse aggregate, which is a low density
aggregate. Figure
160
shows white concrete from a
cladding panel, incorporating flint coarse aggregate that
has been calcined to give it a decorative white colour.
Figure
161
shows concrete where boiler clinker waste has
been used as aggregate. It is important to identify the
presence of boiler clinker as it may contain high levels of
deleterious salts that can corrode metal elements of the
construction, such as steel reinforcement or steel beams.
The aggregate grading (particle size distribution) is an
important characteristic as it affects the workability and
packing density of the concrete mix. The coarse
aggregate grading can be assessed petrographically from
core specimens and finely ground slices, while the fine
aggregate grading is best assessed in thin section. The
157
158
157
Concrete with crushed rock (gabbro) coarse
aggregate and quartz fine aggregate particles. The
uncarbonated cement matrix appears black; XPT, ×35.
158
Concrete with crushed limestone coarse
aggregate (light brown, left and right) and a quartz-
rich (grey/white) natural sand fine aggregate. The
uncarbonated cement matrix appears black; XPT, ×35.
159
160
159
Lightweight concrete containing artificial sintered
PFA coarse aggregate (black/isotropic) and natural
quartz sand fine aggregate (grey/white). The cement
matrix is fully carbonated (light brown); XPT, ×35.
160
Decorative concrete containing crushed calcined
flint aggregate (brown) and traces of natural sand,
including quartz (white) and glauconite (green); PPT,
×35.