Although
underwater concreting has been in use for a long time, development of the
technique has mainly proceeded in the areas of concrete placing method and
improvements to the construction machinery. The prepacked concrete method,
tremie method, concrete pump method, and others are now the representative
underwater concreting methods. With all these concreting methods, the essential
aim of technological development has been to improve how the concrete is placed
and to minimize contact between the water and mortar so as to prevent the
concrete from segregating under water.
On the other
hand, antiwashout underwater concrete is quite different in
concept from the methods mentioned above; the developmenta1aim in this case was
improved performance of the fresh concrete. That is, the viscosity of the
concrete was increased and its resistance to segregation under the washing
action of water was enhanced by mixing an antiwashout admixture with the
concrete. The effect of this is not only to greatly improve the reliability of
Concrete placed underwater, but it also has remarkable effects on environmental
preservation in the construction area. In addition, the earlier tremie and
concrete pump placing methods can be adopted for construction.
The specific
advantages of antiwashout underwater concrete include the
following:
- Compared with ordinary concrete, antiwashout underwater concrete is highly resistant to the washing action of water, and rarely separates even when dropped under water
- Its yield value is small and viscosity high, so the concrete components never segregate and it displays high fluidity.
- As a result of the high fluidity, filling property and self-leveling ability are improved.
- Almost no bleeding occurs.
These
qualities are taken full advantage of in work which would be difficult to
handle using conventional underwater concrete. This includes
work where high reliability is required, work in flowing water, work where
water turbidity is restricted due to environmental considerations, and work
where construction stretches over a considerable area and good flatness is
necessary. On the other hand, however, handling is more difficult than with
ordinary concrete, and in order to produce concrete of the required quality and
a structure of the required performance, careful consideration of mix
proportion, mixing, transport, and placing, etc. is necessary when antiwashout
underwater concrete is used.
In
particular, when producing the underwater antiwash concrete it
is necessary to mix it for longer than ordinary concrete in a mixer large
enough to uniformly disperse the antiwashout admixture. Also, when using
concrete pumps for placement, it is necessary to design a pumping plan with
care as regards pumping equipment, pumping distance, etc., because the pumping
resistance is increased by the higher viscosity.
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