The term 'aggregates' is used to describe the gravels, crushed rocks
and sands that are mixed with cement and water to make concrete.
As aggregates form the bulk of the volume of concrete and can affect its performance, the selection of suitable material is important.
Sand includes
natural sand, crushed
rock or crushed gravel which is
fine
enough to pass through a sieve with 5 mm apertures. Coarse
aggregate comprises larger particles
of gravel, crushed gravel or crushed rock. The size of the sieve used to distinguish between sand
and coarse aggregate
is expected to be changed to 4 mm throughout Europe.
Most concrete is made from natural aggregates that are usually specified to conform to the requirements of BS 882 and BS EN 12620 together with the UK National Annex. Manufactured lightweight aggregates are sometimes used (see page 12).
Sizes of aggregate
The maximum size of coarse aggregate, Dmax is governed by the
type
of work to be done. For reinforced concrete it should be such
that
the concrete can be placed without difficulty, surrounding all
reinforcement thoroughly, particularly in the cover zone, and filling
the
corners of the formwork. It is usual in the UK for coarse aggregate for reinforced concrete to have a maximum size of 20 mm.
Aggregate of Dmax 40 mm can be used for foundations and mass
concrete and similar sections where there are no restrictions to the
flow
of concrete. It should be noted, however,
that concrete with
Dmax 40 mm aggregate is not always available
from
producers of
ready-mixed concrete. The use of a larger aggregate
results in a slightly reduced water demand and hence a slightly
reduced cement content for a given strength and workability.
Smaller aggregate, usually Dmax 10 mm, may be needed for concrete that is to be placed through congested
reinforcement for example. In this case the cement content may have to be increased
by
10 - 20% to achieve
the same strength and workability as with a 20 mm maximum-sized aggregate concrete because the sand
content and water content normally
have to be increased to produce a cohesive
mix.
Quality requirements
Durability
Aggregates should be hard and should not contain materials that are
likely to decompose
or change in volume when exposed to the weather. Examples of undesirable materials are lignite, coal, pyrite and lumps of clay. Coal and lignite may swell and decompose leaving small holes on the surface of the concrete; lumps of clay may
soften and form weak pockets; and pyrite may decompose, causing iron oxide stains to appear on the concrete surface. When exposed to oxygen, pyrite has been known to contribute to sulfate attack. High-strength concretes may call for additional special properties. The mechanical properties of aggregates for heavy-duty concrete floors and for pavement wearing surfaces may have to be specially selected. Most producers of aggregate are able to provide information about these properties, and reference, when necessary, should be made to BS 882 / BS EN 12620.
There are no simple tests for durability or freeze/thaw resistance, and assessment of particular aggregates may be based on experience of the properties of concrete made with the type of aggregate in question with a knowledge of its source. Some flint gravels with a white porous cortex may be frost-susceptible because of the high water absorption of the cortex, which results in pop-outs on the surface of the concrete when subjected to freezing and thawing.
Cleanness
Aggregates should be clean and free from organic impurities;
aggregate containing organic material makes poor concrete. The
particles should be free from coatings of dust or clay, as these
prevent the proper bonding of the material.
An excessive amount of fine dust or stone 'flour' may prevent the particles of stone from being properly coated with cement and thus lower the strength of the concrete.
Gravels and sand are usually
washed by the suppliers to remove excess fines (clay and silt, for example) and other impurities, which, if present in excessive amounts, result in a poor- quality concrete. However, excessive washing can remove all fine
material passing the 300 μm sieve. This may result in a concrete mix lacking in cohesion
and, in particular, being unsuitable for placing by pump. Sands deficient
in fines also tend to increase the
bleeding characteristics of the concrete, which can result in poor vertical finishes due to water scour.
Limits on the amount of fines are given in BS 882 when
determined in accordance with the wet sieving method specified in BS812.
An approximate guide to the fines content of gravel sand can be
obtained from the field settling
test. Results of this test cannot be used as
the basis for accepting
or rejecting material
but they are nevertheless useful by detecting changes
in the cleanness
of sand.
More details are given on page 54 under Testing materials, aggregates.
Where colour of surface finish is important, supplies of aggregate should be obtained from one source throughout the job whenever practicable. This is particularly important for the sand - and for the coarse aggregate if an exposed-aggregate finish is required.