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348 Barry’s Advanced Construction of Buildings
Fine and coarse aggregate
‘Fine aggregate’ is the term used to describe natural sand, crushed rock and gravel, most
of which passes through a number 5 British Standard (BS) sieve. ‘Coarse aggregate’ is the
term used to describe natural gravel, crushed gravel or crushed rock, most of which is
retained on a 5 BS sieve. The differentiation of fine and coarse aggregate is made because
in practice the fine and coarse aggregate are ordered separately for mixing to produce a
determined mix for particular uses and strengths of concrete.
Grading of aggregate
The word ‘grading’ is used to describe the percentage of particles of a particular range of
sizes in a given aggregate from fines (sand) to the largest particle size. A sound concrete is
produced from a mix that can be readily placed and compacted in position, i.e. a mix that
has good workability and after compaction is reasonably free of voids. This is affected by
the grading of the aggregate and the water/cement ratio.
The grading of aggregate is usually given by the percentage by weight passing the various
sieves used for grading. Continuously graded aggregate should contain particles graded in
size from the largest to the smallest to produce a dense concrete. Sieve sizes from 75 to
5 mm (from 3 to 3 / 16 inch) are used for coarse aggregate. An aggregate containing a large
proportion of large particles is referred to as being ‘coarsely’ graded and one having a large
proportion of small particles as ‘finely’ graded.
Particle shape and surface texture
The shape and surface texture of the particles of an aggregate affect the workability of a
concrete mix. An aggregate with angular edges and a rough surface, such as crushed stone,
requires more water in the mix to act as a lubricant to facilitate compaction than does one
with rounded smooth faces to produce a concrete of the same workability. It is often necessary
to increase the cement content of a mix made with crushed aggregate or irregularly
shaped gravels to provide the optimum water/cement ratio to produce concrete of the
necessary strength. This additional water, on evaporation, tends to leave small void spaces
in the concrete, which will be less dense than concrete made with rounded particle aggregate.
The addition of extra water, beyond that required in the chemical reaction (hydration),
will weaken the concrete. Water that does not take part in the chemical reaction leaves
voids as it evaporates out of the concrete.
The nature of the surface of the particles of an aggregate will affect workability. Gravel
dredged from a river will have smooth surfaced particles, which will afford little frictional
resistance to the arrangement of particles that takes place during compaction of concrete.
A crushed granite aggregate will have coarse surfaced particles that will offer some
resistance during compaction. The shape of particles of aggregate is measured by an
angularity index, and the surface by a surface coefficient. Engineers use these to determine
the true workability of a concrete mix, which cannot be judged solely from the grading
of particles.
Water
Water for concrete should be reasonably free from such impurities as suspended solids,
organic matter and dissolved salts, which may adversely affect the properties of concrete.
Water that is fit for drinking is accepted as being satisfactory for mixing water for concrete.