Admixtures can be added to the concrete to speed up or slow down the hydration process of concrete. These should only be used when the specifications call for them or by the request or approval of the architect or field representative. Precision should be exercised in the addition of admixtures because if not carefully controlled, the side effects become more significant as the dosage increases. Admixtures should not be used in lieu of temperature control. Heating or cooling the materials is effective and involves none of the dangers associated with chemical control. Substituting ice for water in the mix is the best method of cooling concrete and is highly recommended. Heating the concrete is usually done by running steam pipes near the forms or by placing electrical or propane heaters near the forms.
Using fly ash in the concrete mix is not recommended because of its tendency to bond with the forms.