Admixtures are those ingredients in concrete other than portland cement, water, and aggregates that are added to the mixture immediately before or during mixing.
For basics on chemical admixtures, click here.
A wide range of admixtures are available. The table below provides a list of common types of chemical admixtures. The effectiveness of an admixture in concrete depends upon many factors including cementitious materials properties, water content, aggregate properties, concrete materials proportions, mixing time and intensity, and temperature.
Air-entraining admixtures are used to stabilize microscopic air bubbles in concrete. Proper air-entrainment, with appropriate volume and spacing factor, will dramatically improve the durability of concrete exposed to moisture during cycles of freezing and thawing. Entrained air also improves concrete’s resistance to surface scaling caused by chemical deicers.
Air-entrained concrete contains minute air bubbles that are distributed uniformly throughout the cement paste. Entrained air can be produced in concrete by use of an air-entraining cement, by introduction of an air-entraining admixture, or by a combination of both methods. An air-entraining cement is a portland cement with an air-entraining addition interground with the clinker during manufacture. An air-entraining admixture, on the other hand is added directly to the concrete materials either before or during mixing.
Specifications and methods of testing air-entraining admixtures are given in ASTM C260 and C233 (AASHTO M 154 and T 157). Both the total volume of air and the spacing factor (average distance between a point in the paste and the edge of an air void) are important to providing resistance to freezing and thawing. Air-entrainment is affected by concrete materials selection, mixing, and placement methods as summarized in the following table. Applicable requirements for air-entraining cements are given in ASTM C150 and AASHTO M 85. See Manual on Control of Air Content in Concrete for more information.
Air-entraining admixtures stabilize air bubbles in concrete by reducing the surface tension at the air-water interface. The shearing action of the mixer and aggregates will entrain and divide air into finely divided bubbles surrounded by the stabilizing agent. It is critical that sufficient mixing time be allowed for the air bubbles to be generated and stabilized. Trial mixtures or prior experience with the job materials and job equipment is necessary to determine the proper dosage and minimum mixing time. Fresh concrete testing (ASTM C231 or C173; AASHTO T 152 or T 196) during construction will confirm the proper volume of air. A relatively new technique, known as the air-void analyzer (AVA), shows promise as a tool for determining the spacing factor in fresh concrete. (For AVA references, see below.)
Air voids in concrete improve durability by reducing stresses associated with freezing water in pores. The expansion as water converts from liquid to solid upon freezing creates a pressure on the remaining liquid. Entrained air provides relief for this pressure, by providing space for the water to flow into. Without air voids, the pressure creates stress on the concrete, creating cracks which cumulatively begin disrupting the concrete.
Generally, for every one percent entrained air, concrete loses about five percent of its compressive strength (Whiting and Nagi,1998). However, in addition to providing saturated freeze-thaw protection and improved salt scaling resistance, entrained air provides several other benefits: increased workability, reduced water demand, decreased segregation and bleeding, and reduced permeability.
Additional information on air in concrete:
"Benefits of Air Entrainment in High-Performance Concrete"
"Control of Air Content in Concrete"
"Guidelines for Control of Air Content in Concrete" "It's Only Air-But It's Really Important"
"Measuring the Air Content of Low-Slump Concrete"
"The Air-Void System in Concrete"