Nobel Corporation

Construction 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 workability, curing temperature range, set time or color. 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. Not all admixtures are economical to employ on a particular project. Also, some characteristics of concrete, such as low absorption, can be achieved simply by consistently adhering to high quality concreting practices.

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:

Among other important admixtures that do not fit into these categories are admixtures whose functions include bonding, shrinkage reduction, damp proofing and coloring.

Most organic chemical-type admixtures are affected by cement type and brand, water-cement ratio, aggregate grading, and temperature. Damp proofing and waterproofing admixtures still have uncertain value and hazards. These are just two cases that point to the learning curve required of anyone working with admixtures. In some cases, if exacting directions are not followed, including addition of supplemental materials to balance the negative or undesirable side effects of an admixture, the resulting concrete mix may be compromised. For example, retarding admixtures generally have a possibility of rapid concrete stiffening, resulting in difficulty in concrete placement and finishing. Therefore, an in-depth knowledge of the potentially complex interrelated effects, besides specifications, is required to successfully use a number of admixtures.

This is even more critical when a number of parties are involved in the manufacture of the concrete, for example the producer, the placing contractor and the builder, where the finished concrete is a combined result of a number of individual decisions. Choosing an appropriate admixture for a specific job should be the responsibility of an experienced expert. Alternatives to the use of admixtures should always be considered.

Finally, admixtures cannot compensate for bad practice and low quality materials.