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as it is poured, and a rough surface texture, to improve
the bonding with the cement matrix. A well graded
aggregate needs less cement, because the smaller
particles fill the spaces between the larger ones. Flaky
particles make for a harsher mix of lower workability at
a given water content than nonflaky ones. This leads to
poor compaction and a high void content resulting in
low strength and durability. Rocks that give rise to
excessively elongate or flaky aggregate particles include
gneiss, schist, amphibolite, phyllite, and slate. The
optimum properties for concrete aggregate are found in
natural sand and gravel resources. Crushed rock, being
angular, is less suitable, but is used in areas where gravels
are scarce.
131
132
131 Well graded fine aggregate or 'sharp sand'. The
aggregate is also polymictic consisting chiefly of
quartz with minor flint (brown) and ironstone (black);
PPT, ×35.
132 A uniformly graded fine aggregate or 'soft sand'.
The aggregate is also monomictic consisting wholly of
quartz grains; XPT, ×35.
133
133 Comparison
chart for
classification of
aggregate particle
shape and surface
texture (Ingham,
2005).
Very angular
Angular
Sub-angular
Sub-rounded
Rounded
Well rounded
Equant
Irregular
Flaky
Elongate
Elongate
and flaky
Decreasing sphericity
SHAPE
 
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