22-03-2014, 04:14 PM
SEMINAR ON SELECTION AND SPECIFIC APPLICATION OF CONCRETE ADMIXTURES
ABSTRACT:
Admixtures are ingredients other than water, aggregates, hydraulic cement,fibres
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.
INTRODUCTION
Admixtures are the chemical compounds in concrete other than hydraulic cement(OPC),water and aggregates and mineral additives that are added to the concrete mix immediately before or during mixing to modify one or more of the specific propeties of concrete in the fresh or hardened state.The use of admixtures should offer aan improvement not economically attainable by adjusting the properties of water,cement and aggregates and should not adversely affect the performance of concrete.
In ACI 212.3R it is stated that chemical admixtures are used to enhance the properties of concrete and mortar in the plastic and hardened state. These properties may be modified to increase compressive and flexural strength at all ages, decrease permeability and improve durability, inhibit corrosion, reduce shrinkage, accelerate or retard initial set, increase slump and workability, improve pumpability and finishability,increase cement efficiency, and improve the economy of the mixture. An admixture or combination of admixtures may be the only feasible means of achieving the desired results. In certain instances, the desired objectives may be best achieved by mixture changes in addition to proper admixture usage. Chemical admixtures are materials that are added to the constituents of a concrete mixture, in most cases, specified as a volume in relation to the mass of the cement or total cementitious materials. The admixtures interact with the hydrating cementitious system by physical and chemical actions, modifying one or more of the properties of concrete in the fresh and/or hardened states.
AIR ENTRAINERS:
AIR ENTRAINMENT 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 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
and patching concrete.
Water-reducers should meet the requirements for Type A in ASTM C 494 Specification
for Chemical Admixtures for Concrete.
Effect on Concrete Properties and Application:
Because of retarding action, the 1-day strength of the concrete is reduced. However,
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 delays between mixing and placing operation, may result in 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 ADMIXTURES OR ACCELERATORS
They are added to concrete either to increase the rate of early strength development
or to shorten the time of setting, or both. Chemical compositions of accelerators
include some of inorganic compounds such as soluble chlorides, carbonates, silicates,
fluosilicates, and some organic compounds such as triethanolamine.
Among all these accelerating materials, calcium chloride is the most common accelerator used in concrete. It should meet the requirements of ASTM D 98.Excessive amounts of calcium chloride in concrete mix may result in rapidstiffening, increase in drying shrinkage and corrosion of reinforcement. In colderclimates, calcium chloride should not be used as an anti-freeze. Large amount of calcium chloride is required to lower the freezing point of the concrete, which may ruin the concrete
However, growing interest in using "chloride-free" accelerators as replacement for
calcium chloride has been observed because they form the root cause for the steel
corrosion in RCC. Calcium nitrite accelerates the hydration of cement, as shown by the
larger amounts of heat developed in its presence.