1. The antiwashout underwater concrete shall have adequate fluidity and the required antiwashout properties underwater.
During the placing of concrete, segregation is liable to occur as the concrete is dropped at such times as: when starting placement, when replacing the placing tool, when the placing tool rises and falls with the motion of the flat barge or similar, when a current crosses the placing location, and when the concrete has to be dropped with out inserting the placing tool to the bottom. To obtain good quality concrete despite these conditions, the most important factors the antiwashout property of the concrete. The antiwashout underwater concrete should be able to withstand these washing actions of the water and suffer only minor reductions in quality.
Antiwashout underwater concrete is widely used in revetments, slope coverings, the fabrication of horizontal members, and filling into narrow gaps. Since the concrete needs a dry consistency to prevent it from flowing when used to cover a slope and the ability to permeate thoroughly between reinforcing bars and structural steel when used in small space sand in the fabrication of horizontal members, it should have a fluidity suitable for each of these uses. However, if the underwater flow distance is increased in anticipation of greater fluidity, there is a danger of coarse aggregate settling at the tip of the flow and increasing the mortar ratio. Therefore, it is necessary to carefully examine the underwater flow distance in advance to prevent loss of quality.
2. The antiwashout property is expressed as the ratio of air to underwater strength.
The antiwashout property is actually the ability of constituent materials to withstand the washing action of water (obtained by giving the concrete sufficient viscosity) but for practical purposes it is expressed as the air/underwater strength ratio. Also, depending on the application, the amount of suspended solids, pH, and other factors may be taken into consideration.
When a concrete is mixed with the proper proportion of antiwashout admixture, thus giving it adequate antiwashout properties under water, quality requirements can be met perfectly satisfactorily even if it is placed with some drop under water Judging from past experience, the air/underwater strength ratio of concrete with adequate antiwashout properties is generally greater than 0.70. Where concrete of greater quality is called for, as in the case of reinforced concrete, then it is desirable to use an antiwashout underwater concrete with a higher strength ratio.
Where antiwashout underwater concrete is used to control water pollution, the quantity of suspended solids and pH, etc. should be taken into account in addition to the strength ratio. In this case, thorough attention should be paid to the dilution ratio in the area of construction to ensure that water quality standards in the surrounding environment, as provided for in the related laws, are not exceeded. The laws in question include the Water Pollution Prevention Act, the Environmental Pollution Prevention Act, and the Sewerage Law.
3. The fluidity is expressed as the slump flow
Although concrete consistency is expressed in terms of slump in conformity with JIS A 1101 "Testing Method for Concrete Slump" for concretes in general, it is not proper to express it in this way in the case of antiwashout underwater concrete, because of its peculiar flow properties. Therefore, the fluidity of antiwashout underwater concrete should be expressed as a slump flow, in conformity with "Testing Method for Concrete Slump Flow" of the Standards of the Japan Society of Civil Engineers.
4. The antiwashout underwater concrete shall have the required filling and self-leveling properties as required
Since the fluidity of the antiwashout underwater concrete can be varied without leading to segregation, good filling and self-leveling properties can be obtained. The filling property is useful when placing into the corners of structures and in portions with complex slopes, etc., while the self-leveling property is advantageous when the surface of the concrete is required to be of good flatness over a wide area. Although good filling may be displayed with a slump flow above 45-50 cm, it is necessary to examine the effects of the maximum size of coarse aggregate and shape of portions where the concrete is being placed. Where the antiwashout underwater concrete is expected to display superior self-leveling properties, as in the case where an installation is to be loaded after the concrete hardens, the slump flow should be about 55-60 cm. However, since concrete never has perfect self-leveling properties, it is necessary to examine fluidity in terms of its relationship with allowable unevenness.
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