Geoscience Reference
In-Depth Information
154
The petrographer should concentrate their aggregate
description on those features that affect concrete
properties. Unless the aggregate is 'all-in', the coarse and
fine aggregate fractions are described separately. Features
described should include aggregate type, nominal
maximum size, shape, grading, distribution, and
orientation. Each rock type present should be identified
and the colour, relative hardness, percentage of total, and
degree of alteration or weathering should be described.
The relative hardness of aggregate constituents (and
cement matrix) can be determined empirically in hand
specimen using the following classification, which is
based on Mohs' Scale of Hardness:
• Very soft - can be penetrated easily by a finger.
• Soft - scores with a fingernail.
• Moderately soft - scores using a copper coin.
• Moderately hard - scores easily with a penknife.
• Hard - not easily scored with a penknife.
• Very hard - cannot be scored with a steel point or
knife.
154 Concrete with natural sand and gravel aggregate
from the Thames Valley river terraces (England),
comprising rounded flint coarse aggregate particles
(brown, lower and left) and quartz (white), ironstone
(black), and limestone (Iight grey) fine aggregate
particles; PPT, ×35.
155
Visual estimation of the relative proportions of different
aggregate constituents using a comparitive chart such as
in Figure 19 , is sufficient for most purposes. If required,
more accurate quantifications may be achieved by point-
counting, either in thin section or of finely ground slices.
The petrographic identity of aggregate indicates its
source and the petrographic description will allow
detection of the source region, or possibly even the
specific source quarry. It is important to remember that
the coarse and fine aggregates may be derived from
different sources.
The aggregate type may be natural gravel, marine
dredged, crushed rock, artificial, or recycled. Natural
gravels are indicated by rounded or subrounded shape,
although some angular/partly angular particles will be
present where oversized gravel particles have been
crushed down during processing. Figures 154 and 155
show concretes containing natural sand and gravel
aggregates from two different river terrace deposits in
England. Figure 156 shows a concrete from the Middle
East that contains a natural wadi gravel coarse aggregate
that has been partially crushed. Marine dredged gravel
155 Concrete with natural sand and gravel aggregate
from the River Trent terraces (England), comprising
rounded quartzite coarse aggregate particles (left and
right) and quartzitic fine aggregate particles
(grey/white); XPT, ×35.
156
156 Concrete (from the United Arab Emirates)
containing partially crushed wadi gravel coarse
aggregate. Rounded serpentinized peridotite (grey/left)
and angular processed peridotite (right) coarse
aggregate particles are seen; XPT, ×35.
 
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