The ACI 318 Building Code requires prior approval by the engineer of admixtures to be used in concrete.
Air Entrainment. Air-entraining admixtures (ASTM C260) 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.
Use of entrained air is recommended for all concrete exposed to weathering or deterioration from aggressive chemicals. The ACI 318 Building Code requires air entrainment for all concrete subject to freezing temperatures while wet. Detailed recommendations for air content are available in ‘‘Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete,’’ ACI 211.1, and ‘‘Standard Practice for Selecting Proportions for Structural Lightweight Concrete,’’ ACI 211.2.
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, lowstrength 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.
Accelerators. Calcium chloride for accelerating the rate of strength gain in concrete (ASTM D98) is perhaps the oldest application of admixtures. Old specifications for winter concreting or masonry work commonly required 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 (ASTM C494).
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 salt water. The ACI 318 Building Code prohibits the use of calcium chloride or admixtures containing chloride from other than impurities from admixture ingredients in prestressed concrete, in concrete containing embedded aluminum, or in concrete cast against stay-in-place galvanized forms. The Code also prohibits the use of calcium chloride as an admixture in concrete that will be exposed to severe or very severe sulfate-containing solutions. For further information, see ‘‘Chemical Admixtures for Concrete,’’ ACI 212.3R.
Retarders. 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 many hotweather 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 (ASTM C494). See ‘‘Hot Weathering Concreting,’’ ACI 305R, for further details on retarders, methods of cooling concrete materials, and limiting temperatures for hot-weathering concreting.
Superplasticizers. 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 jobsite 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.
Waterproofing. 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 waterrepellent 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. For detailed information on surface treatments, see ‘‘Guide to Durable Concrete,’’ ACI 201.2R.
Cement Replacement. 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 (Art. 9.1), 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.
Special-Purpose Admixtures. The list of materials used from 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, detailed recommendations of ‘‘Chemical Admixtures for Concrete,’’ ACI 212.3R, and references it contains, may be helpful.
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 precaution.