other than water, cement, and aggregate. Their purpose is to alter some property of the concrete such as: improve workability, reduce aggregate separation; add entrained air; accelerate the hydration process; retard the hydration process;
or make the concrete more waterproof.
Admixtures should be used with caution for the following reasons:
• In many instances, simply adjusting the ratios of cement, aggregate, or water can more economically and predictably perform the desired function of an admixture.
• Admixtures do not always react predictably with all cements, even those of the same type.
• Many admixtures affect more than just one property of the concrete; sometimes to a deleterious result.
• Admixtures can behave unpredictably due to such variables as wetness and richness of mix, aggregate gradation, type of mixing, and length of mixing.
Those cautions being said, there are several basic types of admixtures commonly used.
They are discussed below.
6.9.1. Air-entrainment. Of all admixtures, this is probably the most prevalent. It has many well documented benefits, and is required by many engineers in all concrete. It consists of a foaming agent that is added either directly to the cement during the manufacturing process (resulting in Types IA, IIA, and IIIA cements), or as an admixture during batching. Regardless of which method of addition is used, the result is the same: innumerable microscopic voids are formed in the cement paste. Several benefits of air-entrainment follow:
• Improved workability without added water
• Improved durability
• Protection against freezing and frost action
• Improves resistance to salts, such as those used for snow and ice removal
• Improved water-tightness
• Reduces ‘bleeding’ (excess water which leaches out during curing)
The amount of air-entrainment is expressed as a percentage of volume of concrete. In general, the smaller the aggregate, the more air-entrainment may be used. For 3/8” maximum aggregate, the recommended amount of air-entrainment is between 6 – 10%. For 1-1/2”
maximum aggregate, the recommended amount of air-entrainment is 3 – 6%.
6.9.2. Accelerators. These either accelerate the set time of freshly poured concrete, or impart high early strength to the concrete, or both. The purposes generally include:
• Reduces pressure on forms, particularly in cold weather. This is useful in cases where forms are tall (such as walls), which if filled would result in the fresh concrete exerting huge outward pressures on the forms, potentially causing bursting
• Permits early removal of forms
• Reduces the amount of time for curing and protection of the green concrete, permitting early finishing
• Compensates for the effect of low temperature on strength development
• Permits placing the structure in service sooner than would otherwise be allowed
• Greatly improves the concrete’s resistance to abrasion and erosion at early stages of curing
The principle accelerator ingredient most commonly used is calcium chloride. The maximum amount of calcium chloride that should be used is 2% by weight of the cement.
Concrete with calcium chloride added is approximately twice as strong as without at 3 days, and approximately 70% stronger at 7 days. The difference is still evident at 28 days, but tapers off after that. Setting time is typically reduced by about a third to a half. Early
hydration heat is increased, but calcium chloride must never be considered an antifreeze. There are several warnings that go with this admixture;
they follow:
• Increased drying shrinkage
• Increased corrosion potential of regular reinforcement steel, and must not be used at all in prestressed concrete
• Increased corrosion potential if there is embedded aluminum (such as conduits), particularly if the aluminum is in the presence of steel
• It may defeat air-entrainment unless added to the batch separately
• Reduces concrete’s resistance to sulfate attack, so don’t use it in concrete exposed to alkaline soils
• Should not be used in hot environments 6.9.3. Water Reducers. This type of
admixture is used to reduce the amount of water in the mix, while maintaining good workability.
Recall that the less water used results in greater strength concrete (assuming the amount of cement does not change). Water reducers should not be used to lower the amount of cement used (while keeping a constant
water-cement ratio). Water reducers are commonly used for very high strength concrete, because they allow the water-cement ratio to stay low, thereby helping to ensure high strength.
Chemicals commonly used for water reduction are the lignosulfonates (calcium, sodium or ammonium) and salts of hydroxy-carboxylic acids. It should be noted that these chemicals may also act as retarders (see below). A water reducer should be used only after field tests have demonstrated it’s effect on the concrete, particularly if used in conjunction with air-entrainment or other admixtures.
Mid-range water reducers are similar to normal water reducers, except that they can reduce water requirements slightly more (approximately 8 – 12% reduction in added water) than
conventional water reducers. More importantly, mid-range water reducers also reduce the
‘stickiness’ of mixes with high cement content, which makes finishing easier.
Superplasticizers, plasticizers, or high-range water reducers are a relatively new admixture (mid – 1970’s) in the water reducer line. They can reduce added water by up to 30%. These are commonly used as follows:
• As a water reducer to give concrete a very low water-cement ratio, but normal
workability and high strength
• To provide normal workability and strength at a reduced cement content but normal water-cement ratio.
• As a plasticizer to produce very workable concrete; that is, a flowing, self-leveling concrete with a high slump and high compressive and flexural strength.
Consolidation (vibration) effort becomes less
critical. More fines (such as fly-ash) are typically added for this type of application.
• In cases where closely spaced rebar is used, and / or in cases where pumping of concrete is required. The increased
flowability helps ensure that concrete fills all the voids and tight places in and around the rebar.
Superplasticizers have minimal effects on the other properties of cement, but as always, should be field tested prior to service use.
6.9.4. Retarders. This type of admixture acts to slow the hydration process, thus increasing the time that the concrete remains plastic and workable. They are the opposite of accelerating admixtures. Once the concrete starts to set, however, strength gain is at approximately the normal rate.
Retarders are used primarily when placing concrete in hot weather, especially if the pours are in small increments. Recall that hydration occurs faster with an increase in ambient temperature. Retarders are not generally needed for small to normal sized pours as long as concrete placement occurs at temperatures less than 75 degrees F. Retarders are
commonly used for mass concrete pours to help keep the heat of hydration low.
Retarding agents include inorganic compounds such as borax, boric acid, calcium borate, sodium bicarbonate and certain phosphates.
Extreme care must be used with this class of retarders because they can have erratic and unreliable results. Some of the same
admixtures used for water reduction (metallic salts of lignosulfonic acid or salts of organic hydroxy-carboxylic acid) are also used as
retarders. As with other admixtures, the effects of retarders depends on other mix properties of the concrete, as well as temperature. For these reasons, field tests should be made to
determine the actual results of the admixture.
6.9.5. Damproofers, Waterproofers, Permeability Reducing Agents. This class of admixture is intended to reduce water
permeability through hardened concrete. There are a myriad of proprietary products available for this purpose. Some are excellent and some are nearly useless - some will actually make the concrete less water resistant if not used exactly per the manufacturer’s recommendations; so beware.
Plain concrete has been used successfully for many years in the construction of water storage tanks and vessels. In these cases, it is
important to use a rich mixture, and ensure that there are enough fines in the fine aggregate.
Leaks are almost always due to poor workmanship, or bad mix design.
It is interesting that there are many products available that are excellent in sealing hardened concrete against water intrusion. Some of these are silica based compounds that are applied in an aqueous solution which penetrate the pores then harden thus forming an impenetrable barrier. Some products can actually
permanently plug flowing leaks through concrete from the inside.
6.9.6. Pozzolans. This is an admixture that mimics (to some degree) the cement portion of concrete, and thus is commonly used to reduce the amount of cement added. A common definition of a pozzolan is as follows: “A siliceous or siliceous and aluminous material, which in itself possesses little or no cementitous
value, but will in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.” The word ‘pozzolan’ comes from the town of Pozzuoli, Italy, situated near the source of volcanic ash (a pozzolan) used by the Romans in the construction of many of their structures.
Pozzolans are either naturally occurring:
volcanic tuff, volcanic ash, pumicite and obsidian; calcined or burnt clays or shales, or man-made: crushed and ground blast furnace slag, fly ash or silica fume.
Pozzolans react with the hydrated lime or calcium hydroxide (byproducts of hydration) in hydrated concrete. This reaction increases the tensile and compressive strength, lowers the permeability, reduces leaching and improves the sulfate resistance of concrete (all good things).
Summarized below are the general effects of pozzolans.
• Can be used to reduce the amount of cement required for equivalent strength
• Reduced heat of hydration due to less cement used
• Increased workability
• Reduced segregation of aggregates and bleeding
• Durability of concrete varies depending on the pozzolan used. Check with your supplier
• Strength is usually improved with lean mixes
• Drying shrinkage – little effect
• Improved sulfate resistance, particularly those with high silica contents. Not a substitute for Type V cement, however
• Slightly reduced resistance to scaling from de-icers
The amount of pozzolan used varies commonly from 10 to 30 percent replacement of the cement. No pozzolan should be used without complete understanding of it’s character or without trial testing.
6.10. Shrinkage. As concrete cures, it shrinks. The amount of shrinkage depends mostly on the amount of water used in the mix;
the more water used, the greater the shrinkage.
In dry climates, most of the shrinkage will occur within the first 2 – 3 months. In humid, moist climates, most of the shrinkage will occur within the first year. Shrinkage does not generally depend on the stress level of the concrete, but rather on moisture conditions and time.
Shrinkage causes internal stresses within concrete; however, code compliant reinforced concrete will have enough rebar in it to adequately resist these stresses.
Slabs on grade will crack due to shrinkage (note use of the word ‘will’, not ‘may’). Such cracking can be controlled, however, through the correct placement of contraction and isolation joints (see the following section on slabs on grade for more on these joints).