Published on Feb 14, 2016
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; testsshould 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.
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.
EFFECT ON CONCRETE PROPERTIES AND APPLICATION
Because of retarding action, the 1 day strength of the concrete is reduced. How ever, ultimate strength is reported to be improved by using set controlling admixtures. One of the most important applications of retarding admixtures is hot weather concreting, when delay between mixing and placing operation, may result is early stiffening. Another important application is in prestressed concrete, where retarders prevent the concrete that is in contact with the strand from setting before vibrating operations are completed. Set retarders also allow use of high temperature curing in prestressed concrete production without affecting the ultimate strength of the concrete.
ACCELERATING ADMIXTRES or ACCELERATORS
Accelerating admixtures are added to concrete to shorten the setting time and accelerate the early strength development of concrete.
Some widely used and effective chemicals that accelerate the rate of hardening of concrete mixtures, including calcium chloride, others chlorides, triethanolamine, Silicates, fluorides, alkali hydroxide, nitrates, formats, bromides, and thiocyanates.
The earliest setting time and increased early strength gain of concrete brought about by an accelerating admixtures will result in a number of benefits, including reduced bleeding, earlier finishing, improved protection against early exposure to freezing and thawing, earlier use of structure, and reduction of protection time to achieve a given quality. Accelerators do not act as anti freeze agents therefore protection of the concrete at early ages is required when freezing temperatures are expected.
Although calcium chloride is the most effective and economical accelerators for concrete, its potential to cause corrosion of reinforcing steel limits its use. ACl Committee 318 suggests that the water soluble chloride ion content should be limited to the following levels for the conditions described.
1. Prestressed concrete – 0.06% by mass of cementations material; and
2. Reinforced concrete –0.15% by mass of cementitious material. Note that the amount of calcium chloride that may be used is based on the cement content of concrete mixture.
The following guidelines should be considered before using calcium chloride or chloride bearing admixture.
1. It should not be used in prestressed concreted because of its potential for causing corrosion.
2. The presence of chloride ion has been associated with corrosion of galvanized steel such as when this material is used as permanent forms for roof decks.
3. Where sulfate resisting concrete is required, calcium chloride should not be used.
4. Calcium chloride should be dissolved in a portion of mixing water before batching because undissolved lumps may later disfigure concrete surface.
5. Calcium chloride should be avoided in reinforced concrete in a moist condition. In non reinforced concrete, the level of calcium chloride used should not exceed 2% by weight of cementitious materials.
6. Calcium chloride precipitates most air entraining agents. So it must be dispensed separately into the mixture, and
7. Field experiences and laboratory tests have demonstrated that the use of uncoated aluminium conduct in reinforced concrete containing 1% or more of calcium chloride may lead to sufficient corrosion of the aliminium to collapse the conduct or crack the concrete.
Non chloride accelerating admixtures containing salts of formats, nitrates, nitrites and thiocyanates are available from admixtures manufactures. These non chloride accelerators are effective for set acceleration and strength development. Hour ever the degree of effectiveness of some of these admixtures is dependent on the ambient temperature and concrete temperature of the time of placement.
Some formulations will give protection against freezing to concrete placed in sub freezing ambient temperatures. These non chloride accelerating admixtures offer years round versatility because they are available to be used for acceleration purpose in cool weather and for sub freezing protection.
The role water reducing set controlling admixtures play in achieving control of concrete quality continues to grow as the admixtures are improved. They are used is all types of concrete construction to achieve a wide range of benefits.
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