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Structural Concrete Frames 347
Workability is affected by the characteristics of the particles of aggregate such as size
and shape so that, for a given mix, workability can be improved by careful selection of
aggregate.
The grading of the size and the shape of the particles of aggregate affects the amount of
cement and water required to produce a mix of concrete that is sufficiently workable to be
compacted to a dense mass. The more cement and water that are needed for the sake of
workability, the greater the drying shrinkage there will be by loss of water as the concrete
dries and hardens.
Natural aggregates
Sand and gravel are the cheapest and most commonly used aggregate in the UK and consist
of particles of broken stone deposited by the action of rivers and streams or from glacial
action. Sand and gravel deposited by rivers and streams are generally more satisfactory
than glacial deposits because the former comprise rounded particles in a wide range of
sizes and weaker materials have been eroded by the washing and abrasive action of moving
water. Glacial deposits tend to have angular particles of a wide variety of sizes, poorly
graded, which adversely affect the workability of a concrete in which they are used.
Crushed rock aggregates are generally more expensive than sand and gravel, owing to
the cost of quarrying and crushing the stone. Provided the stone is hard, inert and well
graded, it serves as an admirable aggregate for concrete. The term ‘granite aggregate’ is used
commercially to describe a wide range of crushed natural stones, some of which are not
true igneous rocks. Natural granite is hard and dense and serves as an excellent aggregate.
Hard sandstone and close-grained crystalline limestone, when crushed and graded, are
commonly used as aggregate in areas where sand and gravel are not readily available.
Because of the depletion of inland deposits of sand and gravel, marine aggregates are
used. They are obtained by dredging deposits of broken stone from the bed of the sea. Most
of these deposits contain shells and salt. Though not normally harmful in reinforced concrete,
limits should be set to the proportion of shells and salt in marine aggregates used for
concrete. One of the disadvantages of marine fine aggregate is that it has a preponderance
of one size of particle that can make design mix difficult. Sand from the beach is often of
mainly single-sized particles and contains an accumulation of salts. Beach sands to be
used as fine aggregate in concrete should be carefully washed to reduce the concentration
of salts.
Artificial aggregates
Blastfurnace slag is the by-product of the conversion of iron ore to pig iron and consists
of the non-ferrous constituents of iron ore. The molten slag is tapped from the blast furnace
and is cooled and crushed. In areas where there is a plentiful supply of blastfurnace slag,
it is an economical and satisfactory aggregate for concrete.
Clean broken brick is used as an aggregate for concrete required to have a good
resistance to damage by fire. The strength of the concrete produced with this aggregate
depends on the strength and density of the bricks from which the aggregate is produced.
Crushed engineering brick aggregate will produce a concrete of medium crushing strength.
Porous brick aggregate should not be used for reinforced concrete work in exposed
positions as the aggregate will absorb moisture and encourage the corrosion of the
reinforcement.