Anti-washout admixture of concrete for underwater concreting is produced as a viscosity modifying admixture to enhance the rheological properties of cement paste. It mainly composed of microbial polysaccharides for example gum or polysaccharide derivatives for instance hydroxyethyl cellulose and hydroxypropyl methyl cellulose.
It is demonstrated that, the Antiwashout admixture is substantially influential in enhancing the cohesiveness of concrete that is poured underwater and in danger of washout or segregation due to surrounding water.
Moreover, it is by far the most utilized admixture in large repairing and placement applications. The Antiwashout admixture is also called viscosity improving admixture, and it is occasionally employed to create self compacting concrete.
Finally, the advantages, disadvantages, classification, and practical consideration of Antiwashout admixture will be discussed in the following sections.
Fig.1: Anti-washout Admixture
Advantages of Anti-Washout Admixture for Underwater Concreting
It is added to concrete mixture to be able to place it underwater without facing segregation and bleeding. As the amount of anti-washout admixture is increased the rate of dispersion of concrete is decreased which is an advantage during concreting underwater.
Disadvantages of Anti-Washout Admixture
The most outstanding disadvantages of anti-washout admixture is decreasing strength and modulus of elasticity based on concrete batch design, water to cement ratio, amount of added anti-washout admixture and its type.
It is reported that, the compressive strength and modulus of elasticity of concrete containing anti-washout admixture ranges between 75-100% and 80-100% of that of concrete without anti-washout admixture, respectively. Consequently, small amount of reinforcement bars may be increased in reinforced concrete member.
Dosage of Anti-Washout Admixture for Underwater Concreting
The amount of Antiwashout admixture which is required to be added to concrete mixture is specified based on required flowability, depth of the underwater placement, horizontal flow distance, water to cementitious materials ratio and the quantity of cementitious materials to be utilized.
Classification of Anti-Washout Concrete Admixtures
It can be divided into the following classes:
Class-A Anti-Washout Admixtures
Water soluble synthetic and natural organic admixture which improve the viscosity of the mixing water. The ranges of this class applied are between 0.2 to 0.5% solid by mass of cement.
Anti-washout admixtures containing cellulose ether, pregelatinized starches, carageenans, polyacrylamides, polyethylene oxides, alignates, carboxyvinyl polymers, and polyvinyl alcohol are examples of the Class A.
Class-B Anti-Washout Concrete Admixtures
It is organic flocculants which can dissolve in water and absorbed by cement particles, and consequently it enhances viscosity by increasing attractions between cement particles.
The dosage is between 0.01 and 0.10% solid by mass of cement. Examples of Class B are Styrene copolymers with carboxyl groups, synthetic polyelectrolytes, and natural gums.
Class-C Anti-Washout Concrete Admixtures
It is emulsions of different organic material that not only improve attractions between particles but also provide extremely fine particles in the cement paste. The amount of Class C anti-washout admixture that is usually added it ranges from 0.10 to 1.50% solid by mass of cement.
Paraffin-wax emulsions that are unstable in the aqueous cement phase, acrylic emulsions, and aqueous clay dispersions are examples of Class C anti-washout admixture.
Class-D Anti-Washout Concrete Admixtures
These are large surface area inorganic materials which rise mixture capacity to retain water. The dosage range employed is 1-25% solid by mass of cement. Examples include bentonites, pyrogenic silicas, silica fume, milled asbestos, and other fibrous materials.
Class-E Anti-Washout Concrete Admixtures
It is inorganic materials which provide extra fine particles to the mortar pastes. The mount of the Class E that is added is between 1 to 25% solid by mass of cement.
Fly ash, hydrated lime, kaolin, diatomaceous earth, other raw or calcined pozzolanic materials, and various rock dusts are examples of Class E Antiwashout admixture.
Principal Considerations for Anti-Washout Concrete Admixtures
There are number of practical considerations which need to be considered when anti-washout admixture is added to concrete mixture.
The most significant consideration may be the amount of concrete mixture need to be dealt with and the capacity of the mixer that is utilized. This is because of the anti-washout admixture increase viscosity of fresh concrete substantially and thus the load of fresh concrete increases by 25 to 50 percent.
In many circumstances, high range water reducing admixture is added in combination with anti-washout admixture to decrease the water demand of concrete mixture, which is increased due to the addition of anti-washout admixture, and hence maintaining flowability.
The high range water reducing admixture increases the setting time that is why types and dosages of high range water reducing admixtures need to be considered.
Capacity of the pump is another consideration that needs to be dealt with because on one hand, high dispersion resistance blockage is possible to occur in pump lines if issues are came across within the pressure transmission tube during pumping pressure period.
On the other hand, because of large viscosity of concrete mixture, pressure transmission resistance is increased by 2 to 4 times that of conventional concrete.