Admixtures are materials, mainly liquids, that are added, to the concrete at the batching stage, and modify the properties of the plastic or hardened concrete. They are usually added in small concentrations. There are a number of different types of admixtures, i.e., water reducing, plasticising, air entrainment, retarders, accelerators, shrinkage compensation, gas generation, water-proofing.

Some background information on current admixture use is given in Concrete Society Current Practice Sheet 149, Admixtures current practice. Part 1 was published in Concrete in September 2006, pp. 35 & 36 and Part 2 in November 2006, pp. 57 to 59.

(Note that Concrete Society Current Practice Sheets can be downloaded from the Members Area of the Concrete Society web site.)