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Friday, 10 February 2012

Concrete Admixtures

Admixtures are ingredients other than water, aggregates, hydraulic cement, and fibers that are added to the concrete batch immediately before or during mixing, in nominal quantities. A proper use of admixtures offers certain beneficial effects to concrete, including improved quality, acceleration or retardation of setting time, enhanced frost and sulphate resistance, control of strength development, improved workability, and enhanced finishability. Admixtures vary widely in chemical composition, and many perform more than one function. Two basic types of admixtures are available: chemical and mineral. All admixtures to be used in concrete construction should meet specifications; tests
should be made to evaluate how the admixture will affect the properties of the concrete to be made with the specified job materials, under the anticipated ambient conditions, and by the anticipated construction procedures.

Materials used as admixtures included milk and lard by the Romans; eggs during the middle ages in Europe; polished glutinous rice paste, lacquer, tung oil, blackstrap molasses, and extracts from elm soaked in water and boiled bananas by the Chinese; and in Mesoamerica and Peru, cactus juice and latex from rubber plants. The Mayans also
used bark extracts and other substances as set retarders to keep stucco workable for a long period of time.

AIR ENTRAINMENT

It is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air-entraining agent, a surfactant. The air bubbles are created during mixing of the plastic concrete, and most of them survive to be part of the hardened concrete. The primary purpose of air entrainment is to increase the durability of the hardened concrete, especially in climates subject to freeze-thaw; the secondary purpose is to increase workability of the concrete while in a plastic state. A water: cement ratio (w/c) of approximately 0.25 is required for all the cement particles to hydrate. Water beyond that is surplus and is used to make the plastic concrete more workable or flowable. Most concrete has a w/c of 0.45 to 0.60, which means there is substantial excess water that will not react with cement. Eventually 5 the excess water evaporates, leaving little pores in its place. Environmental water can later fill these voids. During freeze-thaw cycles, the water occupying those pores expands and creates stresses, which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually the concrete breaks off. The failure of RCC is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel. Steel expands when it rusts, and these forces create even more cracks, letting in more water. These air bubbles that are created improve the resistance of the concrete structure against Freeze and Thaw cycles.

WATER REDUCERS or PLASTICIZERS

Water-reducers generally reduce the required water content of a concrete mixture for a given slump. These admixtures disperse the cement particles in concrete and make more efficient use of cement. This increases strength or allows the cement content to be reduced while maintaining the same strength. The basic role of water reducers is to deflocculate the cement particles agglomerated together and release the water tied up in these agglomerations, producing more fluid paste at lower water contents. Water-reducers are used to increase slump of concrete without adding water and are useful for pumping concrete and in hot weather to offset the increased water demand. Some water - reducers may aggravate the rate of slump loss with time. Water-reducing admixtures are used to improve the quality of concrete and to obtain specified strength at lower cement content. They also improve the properties of concrete containing marginal- or low-quality aggregates and help in placing concrete under difficult conditions. Water reducers have been used primarily in bridge decks, low-slump concrete overlays, and patching concrete.

RETARDERS or SET-RETARDERS

They are chemicals that delay the initial setting of concrete by an hour or more. Retarders are often used in hot weather to counter the rapid setting caused by high temperatures. Most retarders also function as water reducers. Retarders should meet the requirements for Type B or D in ASTM C 494.

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