The use of air entrainment in concrete for freeze thaw protection has been known and used for almost a hundred years. The first time air was introduced into concrete it was an accident. During the time of the Romans, in order to keep the work site clean, food scraps would be thrown into the concrete mix. The Roman cement, much like cement today, produced a concrete with a high pH. In this alkaline environment the fat in the food scraps would be converted into soap and, during the mixing of the concrete, this soap would produce bubbles, becoming dispersed in the cement paste. This process is called the air entrainment of concrete.
With the fall of the Roman Empire, the art of concrete was lost. In 1756 John Smeation, a British engineer, rediscovered hydraulic cement and built the first concrete structure since the end of the Roman Empire. It was not until the early 1900's that air entrainment was rediscovered. This rediscovery was also an accident. During the manufacturing process fatty acid lubricants from nearby equipment leaked into the cement. The resulting cement was air entrained. The concrete made from this cement was found to be very durable in freeze thaw conditions and, as they say, the rest is history.
Fig. 1. Relationship between temperature, slump, and air content of concrete. PCA Major Series 336 and Lerch 1960
The percent of air in the concrete is determined by the amount of soap (air entraining agent) used and a number other factors. For example,the temperature of the concrete plays a very important role: as the temperature increases the percent of air will decrease and therefore, more air-entraining agent will have to be used. Also, air content increases as the slump increases, up to about 6 inches, and then decreases with further increased slump. (See figure 1)
Normal variations in cement and admixtures can have a profound effect on air entrainment. As the fineness (Blaine) of the cement increases, more air-entraining agent is needed to maintain the required percent of air. On the other hand, many of the ASTM C 494 admixtures can increase the efficiency of the air-entraining agent and, as a result, when used together, less will be required. In many cases the amount of air-entraining agent can be reduced by as much as 1/2 when a water-reducing admixture is used.
Compatibility with fly ash is not the problem that it once was. Fly ash is a by-product of the burning of coal in power plants. In the mid 70's much of the fly ash produced in this country had a very high carbon content-as much as 6 percent. High carbon content fly ash can reduce the air content to 1 - 2 percent. Carbon absorbs organic material and air-entraining agents are organic: thus, the problem. However, with the improvements made on the fly ash side and the new improved air-entraining agents, that problem has been brought under control.
Entrained air should not be confused with entrapped air.
The normal amount of entrapped air in most concrete mixes will fall within the 1 - 2 percent range. Entrapped air is not found as bubbles but rather appears as irregular shaped voids found in the cement paste. The entrapped air found in concrete is the result of the cement paste's inability to close with the aggregate. Entrapped air will not protect the concrete from freeze-thaw damage.
The fine aggregate (sand) in concrete has a key role to play in the generation of air in concrete. Aggregate particles passing the #30 sieve and retained on the # 100 sieve entrain more air than either finer or coarser particles. For optimum results, a good range for fine aggregates is 25 to 65 percent of the particles passing the #30 sieve and retained on the #50 sieve with 10 to 30 percent retained on the #100 sieve. An appreciable amount of particles passing the #100 sieve will significantly reduce air entrainment, therefore the maximum allowable amount passing the #100 should be 10 percent.
The amount of air entrainment in a given mix decreases appreciably as the mixer blades become worn or if hardened concrete is allowed to accumulate in the drum or on the blades of the mixer. Generally speaking, more air is entrained as the speed of the mixer drum is increased, with optimum air being generated at approximately 11 rpm. Concrete that is mixed at approximately 11 rpm will reach maximum air content with a given air-entraining agent in about 20 minutes, after which air will be gradually lost from the mix.
A key reason for controlling air in concrete is strength. If a concrete mix is designed to produce a given strength at a given percent of air and the air is allowed to increase one percent, a 5 percent reduction in strength can the expected at mid-range cement factors of approximately 500 pounds of cement per cubic yard of concrete. At cement factors below mid-range, the strength loss will be less. At cement factors above mid-range, the loss will be greater.
This article has only touched on the subject of controlling air in concrete. There have been many fine articles written on air entrainment. The Portland Cement Association now has in print its 14th edition of "Design and Control of Concrete Mixtures." This publication goes into great detail on the subject of air entrainment, as well as many other subjects concerning concrete mixes. I have found this publication to be a very good source for practical information on the subject of concrete and recommend it to all concrete industry professionals.
NOTE: NOTE: Material contained herein was derived, in part, from PCA literature.
© 2004 L&M Construction Chemicals, Inc. | ConcreteNews Spring 2004.