29-06-2013, 03:43 PM
ADMIXTURES
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Why is admixture used?
Over decades, attempts have been made to obtain concrete with certain desired characteristics such as high compressive strength, high workability, and high performance and durability parameters to meet the requirement of complexity of modern structures.
The properties commonly modified are the heat of hydration, accelerate or retard setting time, workability, water reduction, dispersion and air-entrainment, impermeability and durability factors.
Admixtures are used to
1. Reduce cost of concrete construction
2. Achieve specific concrete properties more effectively
3. Ensure quality of concrete during mixing,
transporting, placing, and curing in adverse weather condition
4. Overcome emergencies during operations
So the admixtures are used.
Chemical admixtures
1.Water-reducing admixture / Plasticizers:
These admixtures are used for following purposes:
1. To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix.
2. To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.
3. To increase the workability so as to ease placing in accessible locations
4. Water reduction more than 5% but less than 12%
5. The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.
6. Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry
Actions involved:
Dispersion:
Surface active agents alter the physic chemical forces at the interface. They are adsorbed on the cement particles, giving them a negative charge which leads to repulsion between the particles. Electrostatic forces are developed causing disintegration and the free water become available for workability.
Lubrication:
As these agents are organic by nature, thus they lubricate the mix reducing the friction and increasing the workability.
Retardation:
A thin layer is formed over the cement particles protecting them from hydration and increasing the setting time. Most normal plasticizers give some retardation, 30–90 minutes
Hardening Accelerators,
Which increase the strength at 24 hours by at least 120% at 20ºC and at 5ºC by at least 130% at 48 hours. Hardening accelerators find use where early stripping of shuttering or very early access to pavements is required. They are often used in combination with a high range water reducer, especially in cold conditions.
Calcium chloride is the most effective accelerator and gives both set and hardening characteristics. However, is limited due to acceleration of corrosion of steel reinforcement and decrease resistance of cement paste in a sulfate environment. For this reason, it should not be used in concrete where any steel will be embedded but may be used in plain unreinforced concrete.
Chloride-free accelerators are typically based on salts of nitrate, nitrite, formate and thiocyanate. Hardening accelerators are often based on high range water reducers, sometimes blended with one of these salts.
Silica Fume
By-product of semiconductor industry
The terms condensed silica fume, microsilica, silica fume and volatilized silica are often used to describe the by-products extracted from the exhaust gases of silicon, ferrosilicon and other metal alloy furnaces. However, the terms microsilica and silica fume are used to describe those condensed silica fumes that are of high quality, for use in the cement and concrete industry.
Silica fume was first ‘obtained’ in Norway, in 1947, when environmental restraints made the filtering of the exhaust gases from the furnaces compulsory.
Silica Fume consists of very fine particles with a surface area ranging from 60,000 to 150,000 ft²/lb or 13,000 to 30,000 m²/kg, with particles approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, Silica Fume is a highly effective pozzolanic material. Silica Fume is used in concrete to improve its properties. It has been found that Silica Fume improves compressive strength, bond strength, and abrasion resistance; reduces permeability of concrete to chloride ions; and therefore helps in protecting reinforcing steel from corrosion, especially in chloride-rich environments such as coastal regions.