Large quantities of aggregates are obtained
by dredging marine
deposits, and they have been widely and satisfactorily used for making concrete
for many years.
If present in sufficient quantities, hollow
and/or flat shells may
affect the properties of the
fresh and hardened
concrete. Limits
on shell content are
given in BS 882:
1 For 20 mm and larger coarse aggregates (single-sized, graded or all-in) the limit is 8%
2 For 10 mm to 5 mm coarse aggregate
the limit is 20%
3 For sand there is no limit.
In order to reduce the risk of corrosion of embedded metal, limits are specified in BS 5328, BS EN 206-1 and BS 8500 for the chloride content of the concrete. To conform to these limits it is necessary for marine-dredged aggregates to be carefully and efficiently washed with frequently changed fresh water to reduce the salt content.
Chloride contents
should be checked
frequently throughout aggregate
production in accordance with the method
given in BS 812 : Part
117.
Appendix C in BS 882 gives maximum chloride contents for the combined aggregates; these limits have been derived from those given in BS 5328 but do not necessarily ensure conformity to BS 5328 for all concrete mixes, particularly those with a low cement content. For reinforced concrete made with CEM I, in tests for the total maximum chloride content, expressed as percentage chloride ion by mass of combined aggregate, no result should be greater than 0.08% and 95% of the test results should not be more than 0.05%. For concrete made with sulfate-resisting Portland cement the maximum chloride content of the combined aggregate should not be more than 0.03%. For prestressed concrete and steam-cured structural concrete BS 882 recommends that the maximum total chloride content expressed as percentage of chloride ion by mass of combined aggregate should not exceed 0.01 %.
Some
sea-dredged sands tend to have a preponderance of one size of particle and a deficiency in the amount passing the 300 μm sieve. This can lead to mixes prone to bleeding
unless mix proportions are adjusted to overcome
the problem. An increase
in the cement content by 5 - 10%
will
often offset the lack of fine particles in the sand.
Beach sands are generally unsuitable for good-quality concrete, since they are likely to have high concentrations of chloride because of the accumulation of salt crystals above the high-tide mark. They are also often single-sized, which can make the mix design difficult.
Lightweight aggregates
In addition to natural
gravels and crushed rocks, a number of manufactured aggregates are available for
use in concrete. Lightweight aggregates such as sintered pfa are required to
conform to BS 3797/BS EN 13055-1.
Lightweight aggregates have been used in concrete
for many years - the Romans made use of pumice in some of their construction work. Small quantities of pumice are imported and still used in the UK, mainly in lightweight concrete blocks, but most lightweight aggregate concrete is made using manufactured aggregates. All lightweight materials are relatively weak because of their higher porosity, which gives them reduced weight. This imposes a limitation on strength, though this is not often a serious problem because the strength that can be obtained is comfortably in excess of most structural requirements. Lightweight aggregates are used to reduce weight in structural elements or to give improved thermal insulation.
Delivery of aggregates
Quality control of concrete should start with a visual inspection of the aggregates as they are delivered, combined with some quick, simple testing if there is any doubt about their quality or grading.
The cleanness
of sands can be checked quickly by hand. If a
sample of sand is rubbed between the palms of the hands, staining
of the palms may be an indication
that an excessive amount of clay
and silt are present, due to inadequate washing.
Confirmation or denial of this indication
can be determined by the field settling test
described under Cleanness on page 54. Coarse aggregates should be inspected visually for clay lumps and clay
coatings, grading and particle
shape. Clay lumps are not always
obvious and careful
inspection of deliveries is advised. Loads
containing such lumps should be rejected before discharge.
As previously mentioned
on page 11 , layer loading of lorries produces
an
aggregate that is unmixed, and problems
will
occur in obtaining
a uniform concrete. Such loads should be rejected.
A further problem with gravel coarse aggregates may occur when oversized material is crushed. Such material tends to be of an angular particle shape, rather than rounded or irregular, and a load of all crushed material or a load containing a large part of crushed as well as uncrushed, can lead to variations in the water demand, consistence and strength unless adjustments are made to the mix. Coarse aggregate should have a uniform particle shape for production of high quality concrete.
A useful means of detecting changes in grading or shape is by the loose bulk density test in accordance with BS 812 : Part 108.
Storage of aggregates
Aggregates should be stored so that they are kept as uniform as
possible in grading and moisture content, and protected
from intermingling and contamination by other materials.
It is best to put down a layer of concrete over the areas where the aggregates will be stored. The concrete
should be laid to fall away
from
the mixer to allow free drainage of water
from the aggregate, and should extend well out from the mixer set-up so that all deliveries
can
be tipped onto it. If a clean, hard base is not
provided, the bottom 300 mm of each aggregate pile should not be
used, since dirt and water can accumulate there.
It is essential
to provide substantial partitions to separate the
different aggregate sizes and to prevent spillage from one bay to
another. Such partitions can be made using concrete,
brick or block retaining
walls, or by driving H-section steel members into
the
ground and laying heavy timber sections
between them.
Stockpiles should be as large as possible,
as this helps to ensure
uniformity of moisture
content. Variations in the moisture
content of coarse aggregates as delivered, or in the stockpiles, are usually
not
sufficient to have much effect on the control of free water/cement ratio. However,
the variations that commonly occur in the moisture content of sand will require adjustment to be made
in
order to control the free water/cement ratio.
Ideally, stockpiled sand should be allowed to stand
for 12 hours before use so that, apart from the lower part of the stockpile, the
moisture content will be reasonably
uniform at about 5 - 7%.
When sand is very wet (as sometimes happens with fresh deliveries,
or
after it has been raining)
the moisture content can be as high as 12-15% . Unless adjustments are made to the water added at the mixer, excessive variations in workability, strength and durability
will
result.
For large batching plants the aggregate would probably be lifted by a conveyor system to covered overhead storage hoppers discharging directly into weigh-batchers.