Part 7 - Admixtures for Concrete


An admixture is "a material other than water, aggregate and hydraulic cement that is used as an ingredient of concrete or mortar and is added to the batch immediately before or during mixing." A functional addition is an admixture added by the cement manufacturer at the cement plant and not at the job site. There are four categories of admixtures:

  • Air-entraining agents -- added to improve frost resistance of concrete.
  • Chemical admixtures -- used to control setting and hardening of concrete or reduce concrete water requirements.
  • Mineral admixtures -- fine solids used to improve workability, durability, and provide additional cementing properties.
  • Miscellaneous admixtures -- all other admixtures not in the above categories.

** Admixtures should not be used when similar results can be obtained by following good design and concrete practices.

When using admixtures, the following precautions should be considered: 1) admixture should conform to relevant ASTM specifications, 2) follow manufacturers instructions regarding dosage, 3) ensure that reliable procedures are established for batching of admixture, 4) consider the effects of the admixture on other concrete properties.

Air-Entraining Admixtures

One major disadvantage of concrete is its susceptibility to damage by single or multiple freeze-thaw cycles when it is in a saturated or near-saturated state. Without some admixture, concrete could not be used in pavements, dams, foundations, or other major applications. However, concrete can be made frost-resistant by using air-entraining admixtures. Concrete is routinely air-entrained in the Northern U.S. and Canada.

  • Entrained Air-Void System -- The volume of air for optimum frost protection is about 9% by volume of the mortar. The easiest quantity to measure in the field is the amount of air as a percentage of concrete volume. The air content should be in the 4 to 8% range for good frost protection. Some air is naturally entrained in the cement paste, therefore air-entraining admixtures increase the air voids 3 to 4% by volume of concrete. Air-entraining admixtures cause the mixing water to foam resulting in literally millions of tiny air bubbles to be uniformly spaced throughout the paste. These tiny voids are not visible with the naked eye, but are observable with a microscope. The spacing of the voids is a critical measure of the effectiveness of the admixture. Also, small voids do not easily fill with water even when the concrete is saturated.

  • Air-Entraining Materials -- A good air-entraining compound will promote the formation of small stable bubbles during agitation. Surface-active agents concentrate at the water-air interface, lowering the surface tension of the water allowing bubble formation. A typical dose of admixture is 0.0005 to 0.05% of active ingredient by weight of cement - often requiring predilution before batching.

  • Testing of Air-Entrained Agents -- One way to test the effectiveness of air entrainment is by counting bubbles in a polished section of concrete under a microscope and calculating the spacing factor. A more routine test is a performance specification in which an admixture concrete is tested in rapid freeze-thaw cycles.

  • Factors Affecting Air Entrainment -- Increasing admixture dosage will increase air content and decrease spacing factors. Finely ground cements entrain less air than do coarsely around ones. Therefore, the addition of fines into the mix will reduce the air content. The use of other admixtures can affect the air-entraining potential of surface-active agents. Low cement content concretes entrain more air than do rich mixtures. In addition, low w/c ratio mixes entrain less air than do concretes with high w/c ratios.

  • Mixing and Consolidation -- Air entrainment occurs during mixing. There are several factor which can effect the air content: type of mixer, rate of mixing, amount of concrete being mixed, time of mixing, consolidation, temperature, and slump.

  • Effects on Concrete Properties -- Air entrainment increases the workability of fresh concrete. The tiny bubbles in the cement act like fine aggregates and reduce the interactions between solid aggregates. The improvement in workability leads to use of air entrainment admixtures even when freeze-thaw is not a problem. In general, air entrainment will produce a uniform well-compacted concrete. Air entrained concrete is generally 10 to 20% weaker than non-air-entrained mixtures. Excessive air will lower strength and reduce freeze-thaw resistance.

Chemical Admixtures

This class of admixtures encompasses all soluble chemicals which affect setting times and reduce water requirements of concrete mixes. They are classified as follows:

  • Type A. Water-Reducing Admixtures

    • Lowers the water required to obtain a given slump. ASTM classifies an admixture as water-reducing if it reduces water requirements by 5%. Under this specification, many air-entraining admixtures are also classified as water-reducing. Most water-reducing admixtures reduce water requirements by 5 to 10%. Newer admixtures called "superplasticizers" achieve reductions of 15 to 30% in water requirements. Reducing water requirements while maintaining cement contents effectively lowers the w/c ratio with an accompanying gain in strength.

    • The main reaction of all water-reducing admixtures is at the solid-water interface. In general, solid particles carry a residual surface charge, which may be positive or negative. This causes the particles to collect together trapping and attracting water. Water-reducing admixtures neutralize the surface charge so that all surfaces carry a uniform charge of like sign. The particles now repel each other instead of attracting one another. The water is free to reduce the viscosity of the paste and improve workability. Most conventional water-reducing admixtures will also act as retarding admixtures.

    • By lowering the water requirements using water-reducing admixtures, an increase in compressive strengths up to 25% greater than those anticipated from an equivalent mix with a decrease in the w/c ratio is obtained. This can be attributed to more uniformity throughout the cement paste structure.

    • Superplasticizing Admixtures -- A linear polymer which can reduce water requirements by 15 to 30%. They are used to produce flowing concretes with very high slumps (7 to 9 in.) and high strength concretes with w/c ratios in the range of 0.3 to 0.4. ASTM refers to these admixtures as "high-range water-reducing admixtures." If added dosages equivalent to normal water-reducing admixtures are used the results are similar (5 to 10%). However, at higher dosages, the water reduction increases. The effect of this admixture is that undesirable side-effects like air-entrainment and retardation are much reduced. High strength can be obtained for concretes with w/c ratio below 0.4 (incomplete hydration). Superplasticizers can modify Type I cement resulting in strength gain in excess of Type III. The accompanying lower cement content reduces the rate of heat generation. Also, lower w/c ratios lead to better durability, lower creep, and dry shrinkage.

  • Type B. Set-Retarding Admixtures

    • Admixtures which prolong the plasticity of concrete. Useful to counter the effects of high temperature, eliminate cold joints, and reduce cracking associated with form deflections. Basically, retarding admixtures increase the dormant stage in the C3S hydration process. However, subsequent hydration in stages 3 and 4 will be more rapid. Too large a dose of retarding admixture will cause the reaction never to proceed beyond stage 2 resulting in a cement that will never set. Over-retarding of concrete has helped many a ready-mix truck driver out of trouble. The addition of sugar or carbonate drinks to concrete that may have set up in the truck will return it to a useable form.

    • The effective of the retarder depends on the amount of C3A in the concrete. Retarder is removed from the solution by the C3A reaction, so less is available to retard C3S hydration. Less retarder is removed if its addition to fresh concrete is delayed. Even though this admixture extends setting times, a side effect is loss of workability. Set-retarding admixtures have been reported to increase ultimate compressive strengths. Dry shrinkage and creep rate are increased, but ultimate values are unaffected.

  • Type C. Set-Accelerating Admixtures

    • There are two types of set-accelerators, those that accelerate normal setting and strength development and those that provide rapid-setting concretes by means not associated with normal hydration. There are many application; shotcreting, plugging leaks under pressure, rapid emergency repair, or when rapid development of rigidity is required.

    • Conventional accelerators increase the rate of hydration of C3S by shortening the dormant period and increasing the rate of hydration in stages 3 and 4.

    • Set-accelerators do not generally affect air entrainment. However, handling time is reduced and additional water or a water-reducing admixture may be necessary to control workability. Early strength gain can be observed however, ultimate strength is reduced. Effects on dry shrinkage and creep are similar to those resulting from set-retarding admixtures.

    • Some set-accelerating admixtures have a chloride component which will cause corrosion on reinforcing bars. As discussed earlier in the course, chloride should never be used in any prestress application. Alternative admixtures require larger dosages and will be more expensive. Another solution is to use Type III cement and a concrete with a higher cement content in order to get early strength.

  • Type D. Water-Reducing and -Retarding Admixtures

  • Type E. Water-Reducing and -Accelerating Admixtures

Mineral Admixtures

Mineral admixtures are used to improve workability and durability and to harden concrete. This can be accomplished by introducing finely ground minerals, generally divided into three groups:

  • Materials of Low Reactivity -- Improve the workability of concrete deficient in fines. Generally, cementitious or pozzolanic materials are preferred due to an additional increase in strength and durability.

  • Cementitious Materials -- Materials that have hydraulic reactions off their own, like hydraulic limestones and blast-furnace slags. Most common admixture of this category.

  • Pozzolanic Materials -- A material that reacts with calcium hydroxide (CH) to form C-S-H. The reaction improves workability and lowers heat of hydration while causing a more impermeable cement. This reaction is comparable to that of C2S hydration. Type I cement can be turned into Type IV cement with a pozzolan admixture. Therefore, Type IV cement is rarely manufactured. A low early strength is obtained similar to type IV cements.

Miscellaneous Admixtures

Consumption of these admixtures added together is less than the amount used in any of the single types discussed so far.

  • Bonding Admixtures -- Bonding between old and new concrete or concrete and other materials.

  • Corrosion Inhibitors -- Generally, an accelerating admixture which is not corrosive to reinforcing bars.

  • Damp-proofing Admixture -- prevent penetration of rain into porous concrete; provide water-repellent characteristic.

  • Expansion-Producing Admixtures -- convert ordinary cement into expansive cement.

  • Grouting Admixtures -- A variety of admixtures for concrete-based grouts which prevent bleeding and segregation, increase cohesion and retention of water during pumping, and increase set times.

This website was originally developed by Charles Camp for his CIVL 1101 class.
This site is maintained by the Department of Civil Engineering at the University of Memphis.
Your comments and questions are more than welcome.