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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.
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.
