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AIB 005
Quality Aggregates and
Manufactured Sand for Concrete
Aggregate content is a factor which
has direct and far-reaching effects
on both the quality and cost of
concrete. Unlike water and cement,
which do not alter in any particular
characteristic except in the quantity
in which they are used, the aggregate
component is infinitely variable in
terms of shape and grading.
With coarse aggregates graded in
fractions between 5mm and 40mm,
differences in particle shape and
surface texture affect the bulk void
content and frictional properties of
concrete.
Optimum Shape
The optimum shape is cubical.
Similarly, an even gradation of the
total coarse aggregate fraction is
desirable so that the smaller particles
can fit between the larger particles,
thereby minimizing the voids. Well
shaped aggregates also minimize the
incidence and degree of segregation.
It has been proven that more than
20kg of cement can be saved for
every cubic metre of concrete that
is made by replacing a poorly shaped
aggregate with a cubical aggregate.
In addition, both compressive
strength and flexural strength are
improved by using cubical
aggregates, which also increases
workability and reduces bleeding
and shrinkage.
The impact of the physical
characteristics of the sand used in
the concrete mix is even greater than
that of the coarse aggregate fractions,
both in the concrete's plastic and
hardened states.
Void Content
The principles of total internal
friction and void content apply
equally to the fine fraction, but
because of the vastly smaller particle
size and therefore the greatly
increased surface-area-to-volume
ratio, any detrimental or undesirable
shape or texture properties will be
greatly amplified.
Similarly, manufactured sand
presents an opportunity to control
the mineral content in the particles.
Natural sand often contains
undesirable minerals and clays, and
the effect of these materials on both
the fresh and the hardened states of
concrete can be extremely harmful.
For example, the effect of clay
particles in fresh concrete is not
obvious, as the particles absorb
disproportionate volumes of water
and hence swell to many times their
original size. This swelling occupies
a volume in the cement paste in its
fresh state. When it hardens,
however, the clay particles contract
and leave minute voids which in
turn increase the shrinkage and
permeability and hence reduce the
concrete's chemical resistance and
compressive strength.
Other undesirable materials, ranging
from basic chlorides to harmful
chemicals, can exist in this fine
material fraction. The use of
manufactured sand, however,
reduces the risk of impurities.
Shortage
The growing shortage of natural
sand is another question the
construction industry must consider.
While this may not appear to be a
priority topic, pressure from
environmentalists
and
conservationists worldwide will
continue to encourage both
legislators and construction
engineers to look for viable
alternatives to natural sand.
Cubical sand manufactured from
crushed rock is the most desirable
fine material for concrete
production. It is generally accepted
that particle shape depends on the
rock type, breakage energy and the
type of crusher used.
It is also generally accepted that the
crushers most successful at
producing non-flaky aggregates are
autogenous (rock on rock) and
vertical-shaft impactors, such as the
Barmac B-Series VSI crusher.
Flexibility
Accepting feed material up to 50mm
in size, and thus acting as a finishing
crusher, the B-Series VSI offers
considerable flexibility. With the
Hydracascade feature, a single
crusher can, in one pass, produce
the same quality and fraction of
aggregate achieved in up to three
stages by other plants.
The ‘rock-on-rock’ principle also
maximizes sand production without
compromising either the shape or
the texture, thus ensuring that tight
specifications can be met.
The reason for this lies in the crusher
technology which allows the
material to crush itself by the action
of centrifugal force and attrition,
rather than by relying on breaker
bars, hammers or cones. This results
in lower energy consumption and
wear costs than other types of
crushers, providing further
economies in the overall production
cost.
Also, fracture takes place along the
rock's natural grain, producing the
characteristic cubical shape and
surface texture.
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Abrasive
The feed materials predominately
used can be particularly abrasive.
Whether flint gravel or granite, the
effect on conventional crushers can
be unacceptable in terms of the
overall economics.
The B-Series rock-on-rock
principle, however, ensures that
throughput can be maintained
without incurring high wear costs
and prolonged downtime.
Furthermore, as wear does
inevitably occur, the product
grading will remain unaltered,
ensuring continuity of quality.
Barmac B-Series VSI crushers are
in use throughout the world,
producing good-quality aggregates
and 0-5mm concrete sands. These
machines are also well established
in the industrial minerals sector
crushing such diverse materials as
Chinese bauxite, sintered
magnesite, fused aluminas and
recycled glass, where shaped and
fine sizes are desirable
characteristics of the finished
material specification
New Model
Recently Metso Minerals launched
the new B1100 Series VSI which
has been designed to offer higher
capacity for a given motor size,
plus easier servicing and reduced
operating costs.
These benefits have been achieved
through an improved hopper
design and the development of new
rotor design technology.
The new model gives capacity
increases of up to 25%, as well as
an improved capability to handle
coarse feed fractions. Specific
energy consumption can be
reduced by up to 20% and average
wear costs are reduced in many
cases by 15%.
Typical Barmac B-Series VSI
Aggregate and Sand Circuit
New Feed
–40 +20mm
Feed: New 140 TPH
Recirculating: 60 TPH
B7100 VSI
185kW [250 hp]
Tip Speed: 47 m/s
10% Cascade
3-Deck Screen
Cut 1 = 20mm
Cut 2 = 10mm
Cut 3 = 5mm
0–5mm
5–10mm
10–20mm
For more information regarding this circuit see AIB001.
Barmac VSI crushers are also used in the industrial minerals, mining, recycling and general quarrying industries. A wide range
of materials are processed through Barmac VSI crushers worldwide. For more information on this or any other application, please
contact your nearest Barmac representative.
Metso Minerals (Matamata) Limited
Mangawhero Road, Private Bag 4071,
Matamata, New Zealand
Tel. +64 7 881 9060, fax +64 7 888 5244
[email protected]
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