The water used for mixing concrete should be free from any impurities that could adversely affect the process of hydration and, consequently, the properties of concrete. For example, some organic matter can cause retardation whilst chlorides may accelerate the stiffening process and also cause embedded steel such as reinforcing bars to corrode. Other chemicals such as sulfate solutions and acids may have harmful long-term effects by weakening the cement paste by dissolving it.
It is important, therefore, to be sure of the quality of water. If it comes from an unknown source such as a pond or borehole it should be checked by making trial mixes. The British Standard 3148 specifies the quality of water and gives the procedures for checking its suitability for use in concrete. BS EN 1008, which supersedes BS 3148, had not been finalized at the time this handbook was published.
Drinking water is suitable, of course, and it is usual simply to obtain a supply from the local water utility. But some recycled
water is being increasingly used in the interests of reducing
the environmental impact of concrete production, and seawater
has been used successfully in mass concrete
with
no embedded steel.
The
use of seawater does not normally
affect the strength of plain Portland cement concrete, but it must not be used for concrete
containing embedded metal because
of the danger of corrosion
of the steel from the chloride
content of the water, nor should it be used where white efflorescence could mar the appearance of the
work.
The important, and most difficult, issue is the correct assessment of
how much water is required. In the sections
dealing with aggregates and batching, the variability of moisture content is
discussed and in the quest to control free water/cement ratio it is
essential to allow
for water contained
in aggregates. Because such a large proportion
of concrete consists of aggregates, a small shift
in the moisture content makes a big change in the quantity of water
to be added.
Devices based on advanced electronics technology exist for measuring the moisture content of a batch of aggregates and for calculating the free water/cement ratio of concrete during mixing,
but
most producers rely on the experience
of the batcher to judge
the point at which the amount of water is correct
from the way in which the concrete moves and the sound it makes in the mixer.
Any
equipment that is capable of indicating the consistence of concrete
while it is being mixed assists the batcher in gauging the
mixing water with greater speed and accuracy.
Where the mixer is
powered by an electric motor an ammeter or kilowatt meter
accurately indicates the power consumed
in mixing the concrete - less power is demanded as concrete becomes more workable. Similarly, truck mixer drums that are turned by hydraulic
drive can have the consistence of concrete indicated by a pressure
gauge (Figure 3).
Recycled water systems are usually
found
at large-scale permanent
mixing plants such as precast concrete factories or ready-mixed concrete depots where water used for cleaning the plant and
washing out mixers after use can be collected, filtered and stored
for
re-use. Some processes are able to reclaim up to a half of the
mixing water in this way.
When reclaiming water for use as recycled
mixing water care needs to be taken to avoid impurities including harmful chemicals, oil or
organic matter, and any traces of powerful admixtures such as air-
entraining agents, retarders or pigments
must be diluted to such an extent that they will have no effect. Any polypropylene or steel fibers need to be filtered out and a careful check kept on the amount of suspended fines carried in the water: after all the effort
and
cost of obtaining
clean aggregates, it is not sensible to put an
excessive quantity of fines back in the form of dirty water.
Large-volume settlement tanks are normally required. They do not
need
to be particularly deep but should have a large surface area
and,
ideally, the water should pass through a series of tanks, becoming progressively
cleaner at each stage. Alternatively, the water
may be chemically treated, particularly where space is
limited, in order to make it suitable for re-use or for discharge
into drains in a condition that conforms to statutory requirements.
Measuring the quantity
of water
Mixing water is usually measured
by volume but, in some plants, it
may
be more convenient for it to be batched
by weight. One liter of
clean water weighs exactly one kilogram
and so the quantity of water remains numerically the same regardless of whether it is measured by volume or by mass, but corrections should be applied
when
water contains fines.
Regardless of the method of gauging the mixing water, it is recommended that the concrete is finally checked by a batcher and/or
driver to see that it has the specified consistence and a
uniform appearance. This is the most effective way of ensuring that
the
concrete is thoroughly mixed and has the designed free water content. When the free water content is closely
monitored and cement content accurately weighed, the free water/cement ratio is
controlled and, therefore, strength, durability and many other essential properties of the concrete are assured.