Admixtures are materials other than cement, aggregate and water that are added to concrete either before or during its mixing to alter its properties, such as improved retention of workability; higher early strengths; higher ultimate strengths; reduced shrinkage; improved durability of reinforced structural concrete; and enhanced quality of surface finishes. Some admixtures have been in use for a very long time, such as calcium chloride to provide a cold-weather setting concrete. Others are more recent and represent an area of expanding possibilities for increased performance.
The chemistry of concrete admixtures is a complex topic requiring in-depth knowledge and experience. A general understanding of the options available for concrete admixtures is necessary for acquiring the right product for the job, based on climatic conditions and job requirements. Based on their functions, admixtures can be classified into the following five major categories:
- Air-entraining admixtures
- Retarding admixtures
- Water reducing/plasticizing admixtures
Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction and damp proofing. The following paragraphs provide details on the above-mentioned categories of concrete admixtures.
Air-entraining agents entrain small air bubbles in the concrete. The major benefit of this is enhanced durability in freeze-thaw cycles, especially relevant in cold climates. While some strength loss typically accompanies increased air in concrete, it generally can be overcome by reducing the water/cement ratio via improved workability (due to the air-entraining agent itself) or through the use of other appropriate admixtures. The use of air-entraining agents also reduces bleeding and segregation of fresh concrete.
Retarding admixtures (retarders) slow down the hydration of cement, lengthening set time. Retarders are beneficially used in concrete to offset the effect of high temperatures, which decrease setting times, or to avoid complications when unavoidable delays between mixing and placing occur. Because most retarders also act as water reducers, they are frequently called water-reducing retarders.
Water reducing/plasticizing admixtures
Water-reducing admixtures are groups of products that are added to concrete to achieve certain consistencies (slump) at lower water/cement ratios than that of control concrete. Water-reducing admixtures are used to improve the quality of concrete and to obtain specified strength at lower cement content. They also improve the properties of concrete containing marginal- or low-quality aggregates and help in placing concrete under difficult conditions. However, water-reducing admixtures can entrain air into the concrete mix via its effect on water’s surface tension, thereby also, obtaining some of the benefits of air-entrainment.
Superplasticizers can be used in the same three ways as a conventional plasticizer:
- To impart extreme workability (beyond that obtainable with a conventional plasticizer)
- To permit a large water reduction to be made beyond the limits of normal plasticizing admixtures
- To achieve economic and environmental benefits (e.g. reduction of the cement content) whilst maintaining performance.
The mode of action of superplasticizers is similar to conventional plasticizers; the admixture particles are adsorbed onto the cement particles, causing them to become mutually repulsive and thus having a dispersing effect.
The first generation of superplasticizers were commercially launched in the early 1960’s and had an effective working life of less than one hour. Superplasticizers were first used in the United Kingdom in 1973. The original application of superplasticizers was to produce flowing concrete to be used in heavily reinforced structures and in placements where adequate consolidation by vibration cannot be readily achieved. However, they now have far wider applications including the production of high-strength concrete (compressive strengths greater than 100 MPa) at water/cement ratios ranging from 0.3 to 0.4.