What are the Concrete Admixtures?
Concrete admixtures are the natural or manufactured additive added in the concrete during concrete mixing. Admixtures are used to enhance or modify some specific property of concrete-like Workability, durability and hardening time, etc.
Types of Concrete admixtures
Air-entraining admixtures may be interground as additives with the cement at the mill or added separately at the concrete mixing, plant, or both. Where quality control is provided, it is preferable to add such admixtures at the concrete plant so that the resulting air content can be controlled for changes in temperature, sand, or project requirements.
The use of entrained air is recommended for all concrete exposed to weathering or deterioration from aggressive chemicals. Air entrainment Requires for all concrete subject to freezing temperatures while wet.
One common misconception relative to air entrainment is the fear that it has a deleterious effect on concrete strength. Air entrainment, however, improves workability. This will usually permit some reduction in water content. For lean, low strength mixes, the improved workability permits a relatively large reduction in water content, sand content, and water-cementitious materials ratio, which tends to increase concrete strength. The resulting strength gain offsets the strength-reducing effect of the air itself and a net increase in concrete strength is achieved. For rich, high-strength mixes, the relative reduction in the ratio of water to cementitious materials, water-cementitious materials ratio, is lower and a small net decrease in strength results, about on the same order of the air content (4 to 7%). The improved durability and reduction of segregation in handling, because of the entrained air, usually make air entrainment desirable, however, in all concrete except extremely high-strength mixtures, such as for lower-story interior columns or heavy-duty interior floor toppings for industrial wear.
Calcium chloride for accelerating the rate of strength gain in concrete is perhaps the oldest application of admixtures. Old specifications for inter concreting or masonry work commonly required the use of a maximum of 1 to 3% CaCl2 by weight of cement for all concrete. Proprietary admixtures now available may include accelerators, but not necessarily CaCl2. The usual objective for use of an accelerator is to reduce curing time by developing 28-day strengths in about 7 days In spite of users’ familiarity with CaCl2, a number of misconceptions about its effect persist.
It has been sold (sometimes under proprietary names) as an accelerator, a cement replacement, an ‘‘antifreeze,’’ a ‘‘waterproofer,’’ and a ‘‘hardener.’’
It is simply an accelerator; any improvement in other respects is pure serendipity. Experience, however, indicates corrosion damage from indiscriminate use of chloride-containing material in concrete exposed to stray currents, containing dissimilar metals, containing prestressing steel subject to stress corrosion, or exposed to severe wet freezing or saltwater.
Unless proper precautions are taken, hot-weather concreting may cause ‘‘flash set,’’ plastic shrinkage, ‘‘cold joints,’’ or strength loss. Admixtures that provide controlled delay in the set of a concrete mix without reducing the rate of strength gain during subsequent curing offer inexpensive prevention of much hot weather concreting problems.
These (proprietary) admixtures are usually combined with water-reducing admixtures that more than offset the loss in curing time due to delayed set. See ‘‘Hot Weathering Concreting,’’ for further details on retarders, methods of cooling concrete materials, and limiting temperatures for hot-weathering concreting.
These admixtures, which are technically known as ‘‘high-range water reducers,’’ produce a high-slump concrete without an increase in mixing water. Slumps of up to 10 in. for a period of up to 90 min can be obtained. This greatly facilitates placing concrete around heavy, closely spaced reinforcing steel, or in complicated forms, or both, and reduces the need for vibrating the concrete.
It is important that the slump of the concrete be verified at the job site prior to the addition of the superplasticizer. This ensures that the specified water-cementitious materials ratio required for watertight impermeable concrete is in fact being achieved. The superplasticizer is then added to increase the slump to the approved level.
A number of substances, such as stearates and oils, have been used as masonry-mortar and concrete admixtures for ‘‘waterproofing.’’ Indiscriminate use of such materials in concrete without extremely good quality control usually results in disappointment. The various water-repellent admixtures are intended to prevent capillarity, but most severe leakage in concrete occurs at honeycombs, cold joints, cracks, and other noncapillary defects.
Concrete containing water repellent admixtures also requires extremely careful continuous curing, since it will be difficult to rewet after initial drying. Waterproof concrete can be achieved by use of high-strength concrete with a low water-cementitious materials ratio to reduce segregation and an air-entraining agent to minimize crack width. Also, good quality control and inspection is essential during the mixing, placing, and curing operations. Surface coatings can be used to improve resistance to water penetration of vertical or horizontal surfaces.
The term ‘‘cement replacement’’ is frequently misused in reference to chemical admixtures intended as accelerators or water reducers. Strictly, a cement replacement is a finely ground material, usually weakly cementitious, which combines into a cementlike paste replacing some of the cement paste to fill voids between the aggregates. The most common applications of these admixtures are for low-heat, low-strength mass concrete or for concrete masonry. In the former, they fill voids and reduce the heat of hydration; in the latter, they fill voids and help to develop the proper consistency to be self-standing as the
machine head is lifted in the forming process. Materials commonly used are fly ash, silica fume, ground granulated blast-furnace slag, hydraulic lime, natural cement, and pozzolans.
The list of materials used from the earliest times as admixtures for various purposes includes almost everything from human blood to synthetic coloring agents. Admixtures for coloring concrete are available in all colors. The oldest and cheapest is perhaps carbon black. Admixtures causing expansion for use in sealing cracks or under machine bases, etc., include powdered aluminum and finely ground iron.
Special admixtures are available for use where the natural aggregate is alkali reactive, to neutralize this reaction. Proprietary admixtures are available that increase the tensile strength or bond strength of concrete. They are useful for making repairs to concrete surfaces. For special problems requiring concrete with unusual properties. For all these special purposes, a thorough investigation of admixtures proposed is recommended. Tests should be made on samples containing various proportions for colored concrete. Strength and durability tests should be made on concrete to be exposed to sunlight, freezing, salt, or any other job condition expected, and special tests should be made for any special properties required, as a minimum