Water-reducing admixtures are groups of products that are added to concrete to achieve certain workability (slump) at a lower w/c than that of control concrete (Rixom and Mailvaganam 1986). 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 (ACI Comm. 212 1963). Water reducers have been used primarily in bridge decks, low-slump concrete overlays, and patching concrete.
Composition. Water-reducing admixtures can be categorized according to their active ingredients. There are the following:
The basic role of water reducers is to deflocculate the cement particles agglomerated together and release the water tied up in these agglomerations, producing more fluid paste at lower water contents.
Effect of Water-reducing Admixtures on Concrete. Use of water reducers usually reduces water demand 7-10%. A higher dosage of admixtures leads to more reduction; however, excess retardation may be encountered (Admixtures and ground slag 1990). Many of the water-reducing admixtures tend also to retard the setting time of the concrete. This effect is counteracted in Type A and Type E chemical admixtures by adding other acceleration chemicals such as calcium chloride (Admixtures and ground slag 1990) or triethanolamine (TEA). HC admixtures tend to increase bleeding and should be used with care in high-slump concrete. Lignosulfonate-based admixtures perform better in this regard because they entrain air; normal dosages of lignin admixtures may add 1-2% of entrained air to the concrete. All water-reduced concretes usually lose slump more quickly than do equivalent concretes without the admixtures. However, this loss generally does not create problems when conventional water reducers (HC, PC, and lignins types) are used (Admixtures and ground slag 1990; Previte 1977; Collepardi 1984).
It is well known now that using water-reducing admixtures increases concrete strength. Increases in compressive strength are as much as 25% greater than would be anticipated from the decrease in w/c (Mindess and Young 1981). For flexural strength, an increase of 10% for concrete at 7 days to 1 year has been reported for lignosulfonate and hydroxycarboxylic admixtures (Collepardi 1984). Freeze-thaw resistance and other durability aspects can also be improved when water-reducing admixtures are properly used in concrete.
Although using admixtures in concrete improves concrete's properties, misusing any kind of admixtures will negatively affect these properties. It is therefore important to follow the manufacturer's recommendations whenever admixtures are used.
Sections of this document were obtained from the Synthesis of Current and Projected Concrete Highway Technology, David Whiting, . . . et al, SHRP-C-345, Strategic Highway Research Program, National Research Council.
ACI Committee 212. 1963. Admixtures for concrete. ACI Journal Proceedings 60 (11):1481-524.
Admixtures and ground slag for concrete. 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council.
Collepardi, M. 1984. Water reducers/retarders. In Concrete admixtures handbook. Properties, science, and technology, ed. V. S. Ramachandran, 116 210. Park Ridge, N.J.: Noyes Publications.
Previte, R. 1977. Concrete slump loss. ACI Journal Proceedings 74 (8):361-67.
Rixom, M. R., and N. P. Mailvaganam. 1986. Chemical admixtures for concrete. Cambridge, England: The University Press.