CONCRETE ADMIXTURES

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Transcript of CONCRETE ADMIXTURES

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Admixtures are those ingredients in concrete other than cement, water, and aggregates that are added to the mixture immediately before or during mixing as to obtain the required properties.

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1.AIR-ENTRAINING ADMIXTURES1.AIR-ENTRAINING ADMIXTURES2. PLASTISIZERS2. PLASTISIZERS3. RETARDING ADMIXTURES3. RETARDING ADMIXTURES

4. ACCELARATING ADMIXTURES4. ACCELARATING ADMIXTURES 5.CORROSION INHIBITING ADMIXTURES5.CORROSION INHIBITING ADMIXTURES 6.WATER-PROOFING ADMIXTURE6.WATER-PROOFING ADMIXTURE 7.GROUTING ADMIXTURE7.GROUTING ADMIXTURE 8.MINERAL ADMIXTURES8.MINERAL ADMIXTURES

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Air-entraining admixtures are used to introduce and stabilize microscopic air bubbles in concrete.

 These bubbles are introduced by an air entraining agent, a type of chemical that includes detergents.

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The primary ingredients used in air-The primary ingredients used in air-entraining admixtures are salts of Vinsol entraining admixtures are salts of Vinsol resin, synthetic detergents and salts of resin, synthetic detergents and salts of petroleum acids.petroleum acids.

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  In cold weather, water in concrete converts into ice . As ice occupies more volume than liquid water so it exerts stress on surrounding concrete as a result concrete might crack . But air bubbles inside the concrete provide space for the enhanced volume of ice and prevents cracks in the concrete.

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Plasticizers or water reducers, and superplasticizer or high range water reducers, are chemical admixtures that can be added to concrete mixtures to improve its workability.

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In order to produce stronger concrete, less water is added which makes the concrete mixture less workable and difficult to mix, necessitating the use of plasticizers.

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The action of plasticizers is mainly to fludify the mix and improve the workability of concrete , mortar or grout .

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Plasticizers are also often used when pozzolanic ash is added to concrete to improve strength.

Adding 1-2% plasticizer per unit weight of cement is usually sufficient. Adding an excessive amount of plasticizer will result in excessive segregation of concrete which is not advisable.

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Plasticizers are commonly manufactured from pop lignosulfonates which is a by-product from the  paper  industry.

 In ancient times,the Romans used animal fat, milk and blood to improve workability of concrete mixes.

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Retarding admixtures are used to slow down the speed of the reaction between cement and water by affecting the growth of the hydration products or reducing the rate of water penetration to the cement particles. The use of a retarder will increase the setting time and may delay strength development of the concrete.

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Retarders can be used:

in hot weather to prevent early stiffening.

to allow the placing of a large pour of concrete over several hours.

to place concrete in layers without cold joints. to extend the time between mixing and

placing. to prevent setting of the concrete in the truck

in case of delay.

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The typical materials used as retarders are:The typical materials used as retarders are: Lignin, Lignin, Borax, Borax, Sugars, Sugars, Tartaric acid and salts.Tartaric acid and salts.

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 The admixture that causes an increase in the rate of hydration of the hydraulic cement and thus shortens the time of setting, increases the rate of strength development.

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 The admixture may allow a concrete worker to remove forms, get onto a concrete surface for finishing, and sometimes even put loads on it earlier, such as when diverting foot traffic to do patching.

Accelerators counteract the influence of cold weather, which slows down the curing and setting process.

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Calcium chloride is a common accelerator, used to accelerate the time of set and the rate of strength gain.

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 The function of this type of admixture is to enhance the long term durability of reinforced concrete by enhancing passivation to the cathodic and/or anodic areas of embedded steel. 

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Corrosion-inhibiting admixtures can significantly reduce maintenance costs of reinforced concrete structures throughout a typical service life of 30 to 40 years. 

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Structures built in the 1970s with calcium nitrite still contain the original level of nitrite and show no signs of deterioration.

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Water is essential to concrete production, placement, and curing. But once it fulfills its role in those processes, water is no longer concrete’s friend. Depending on its function and the nature of its exposure, concrete can of course perform well in wet environments. As a naturally porous material, though, and one that is prone to cracking, concrete is vulnerable to water infiltration. The unfortunate results can be freeze/thaw damage and deterioration due to corrosion of embedded steel reinforcement.

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One method that can simplify the protective process is to make concrete with admixtures that reduce its permeability—in effect to make the concrete itself waterproof. 

 These consists of hydrophobic, or water-repellent, chemicals derived from soaps or fatty acids, vegetable oils, and petroleum.

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It is a powder admixture which can be used for making neat cementitious grouts. It comprises a water reducing / plasticising agent and a gas producing expansion medium.

ADVANTAGES

● Higher Strength● Higher fluidity● Lower permeability● Reduced bleeding

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APPLICATIONS● Grouting joints of precast-concrete members● Non-shrink infilling and jointing● Grouting of post-tensioned cable ducts● Repair of honeycombed and void-filled concrete structural● Bed grouting

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Mineral admixtures are “inorganic” materials that also have pozzolanic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete, or even as a replacement for portland cement. 

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FLY ASH BLAST-FURNANCE SLAGSILICA FUMESRICE HUSK

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Fly ash is a by-product of the combustion of pulverized coal in thermal power plants.

Concrete using fly ash is generally reported to show reduced segregation and bleeding and to be more satisfactory than plain concrete when placed by pumping.

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Blast furnace slag is a by-product of pig iron manufacture.

Concrete containing slag as a mineral admixtures generally offers better chemical resistance due to improved watertightness

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Silica fume is an industrial by-product consisting of ultrafine particle. It is recovered from electric furnace by means of dust collectors from the waste gas emitted during the production of ferro-silicon metal.

It can be used as an water reducing admixture.

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Mineral admixtures are generally used in concrete to help make it stronger as well as to make it more economical. Another very important benefit is that it is an environmentally responsible and effective way to recycle industrial and other waste by-products.

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