The Use of Chemical Admixtures in Concrete. Part II: Admixture-Admixture Compatibility and Practical Problems

Abstract

Different combinations of admixtures can produce concretes having high flow, good cohesion, and self-leveling properties; they can be placed underwater, can expand and induce compressive stresses or reduced shrinkage, and can be placed and cured at sub-zero temperatures. Thus, correct combinations of admixtures produce custom-made concrete for particular jobs. This article, the second in a series, explores admixture-admixture compatibilities; the first article in this series discussed admixture-cement compatibilities. Topics include compatibility problems with viscosity-enhancing admixtures, compatibility problems with water reducing admixtures (WRA) or damproofing admixture combinations, shrinkage compensating concretes for flat slab applications, accelerated setting in concrete containing calcium nitrite and superplasticizers, and decreased air contents in silica fume concrete. The authors also consider problems caused by admixture side effects and poor practice, including surface finishing problems, plastic shrinkage, and the effect of retempering of concrete on the air void system. The authors conclude that it is vital to understand both admixture-cement and admixture-admixture interactions so that the best use of materials can be achieved, field problems are avoided, and more accurate prediction of concrete properties is made possible.