Concrete Admixtures

Admixtures may be used to modify and control specific characteristics of concrete. 

 Major types of admixtures include set accelerators, water reducers, air entrainers, and waterproofing compounds. In general, admixtures are helpful in improving concrete workability. Some admixtures, if not administered properly, could have undesirable side effects. Hence, every engineer should be familiar with admixtures and their chemical components as well as their advantages and limitations. Moreover, admixtures should be used in accordance with manufacturers’ recommendations and, if possible, under the supervision of a manufacturer’s representative. Many admixtures are covered by ASTM specifications.

Accelerating admixtures are used to reduce the time of setting and accelerating early strength development and are often used in cold weather, when it takes too long for concrete to set naturally. The best-known accelerator is calcium chloride, but it is not recommended for use in prestressed concrete, in reinforced concrete containing embedded dissimilar metals, or where progressive corrosion of steel reinforcement can occur. Nonchloride, noncorrosive accelerating admixtures, although more expensive than calcium chloride, may be used instead. 

Water reducers lubricate the mix. Most of the water in a normal concrete mix is needed for workability of the concrete. Reduction in the water content of a mix may result in either a reduction in the water-cement ratio (w/c) for a given slump and cement content or an increased slump for the same w/c and cement content. With the same cement content but less water, the concrete attains greater strength. As an alternative, reduction of the quantity of water permits a proportionate decrease in cement and thus reduces shrinkage of the hardened concrete. An additional advantage of a waterreducing admixture is easier placement of concrete. This, in turn, helps the workers and reduces the possibility of honeycombed concrete. Some water-reducing admixtures also act as retarders of concrete set, which is helpful in hot weather and in integrating consecutive pours of concrete.

High-range water-reducing admixtures, also known as superplasticizers, behave much like conventional water-reducing admixtures. They help the concrete achieve high strength and water reduction without loss of workability. Superplasticizers reduce the interparticle forces that exist between cement grains in the fresh paste, thereby increasing the paste fluidity. However, they differ from conventional admixtures in that superplasticizers do not affect the surface tension of water significantly, as a result of which, they can be used at higher dosages without excessive air entrainment.

Air-entraining agents entrain minute bubbles of air in concrete. This increases resistance of
concrete to freezing and thawing. Therefore, airentraining agents are extensively used in exposed concrete. Air entrainment also affects properties of fresh concrete by increasing workability.

Waterproofing chemicals may be added to a concrete mix, but often they are applied as surface treatments. Silicones, for example, are used on hardened concrete as a water repellent. If applied properly and uniformly over a concrete surface, they can effectively prevent rainwater from penetrating the surface. (Some silicone coatings discolor with age. Most lose their effectiveness after a number of years. When that happens, the surface should be covered with a new coat of silicone for continued protection.) Epoxies also may be used as water repellents. They are much more durable, but they also may be much more costly. Epoxies have many other uses in concrete, such as protection of wearing surfaces, patching compounds for cavities and cracks, and glue for connecting pieces of hardened concrete.

Miscellaneous types of admixtures are available to improve properties of concrete either in the plastic or the hardened state. These include polymerbonding admixtures used to produce modified concrete, which has better abrasion resistance, better resistance to freezing and thawing, and reduced permeability; dampproofing admixtures; permeability-reducing admixtures; and corrosioninhibiting admixtures.