Fallacies and Misconceptions Related to Air Entrained Concrete (Part 1)

A popular saying among admixture producers is that "air accounts for 10% of our sales and 90% of our headaches..."

It is common practice within the engineering and construction industry to think about concrete in terms of three fundamental properties: slump, air content, and compressive strength. This article addresses air content, specifically air entrainment, and the common fallacies and misconceptions related to the use of entrained air in concrete. Fundamentally, this article asks the question “Are we as an industry thinking about air entrainment in concrete correctly?” What follows constitutes the basis that the answer to this question is all too often “no.”

Although air entrainment imparts several favorable concrete properties, including improved pumpability and cohesion in lean cement mixtures, the overwhelming majority of the time, air entrainment is used in concrete to improve the freeze-thaw durability of concrete subjected to saturated conditions or in the presence of deicing chemicals.

One of the most common misconceptions is that the freeze-thaw durability of concrete is a function of the total air content. In reality, the freeze-thaw durability of concrete is related to (1) the size of the entrained air bubbles, (2) the distance the entrained air bubbles are spaced, and (3) the surface area of the entrained air bubbles (for more information, refer to ASTM C457). Depending on the constituent materials and proportioning of the concrete mixture, particularly with respect to the type of air entraining admixture used, it can easily be demonstrated that, keeping all else equal except the type of air entraining admixture used, concrete consisting of a lower total air content can have superior freeze-thaw durability compared to concretes having a higher total air content. Many types of air entraining admixtures used today produce bubble systems that are smaller and more closely spaced and can offer equal or better freeze-thaw durability at a lower total air content. Different types of air entraining admixtures impart different mixing requirements to ensure that a proper and stable system of bubbles is produced. Industry representatives that can corroborate this fact include technical representatives of companies that manufacturer air entraining admixtures, expert consultants knowledgeable in the field of concrete materials engineering, and knowledgeable producers of ready-mixed, precast, and prestressed concrete.

Based on the above-stated paragraph, there exists a significant need in the concrete industry to modify the total recommended air content tables to recognize the fact that the type of air entraining admixture used significantly affects the three above-stated important properties (size, spacing and surface area) of air entrained concrete. If such modifications are instituted within the concrete industry, less problems associated with air entrained concrete will occur. Currently, the total recommended air content is based on the nominal maximum aggregate size and the severity of the weathering region without recognizing the air entrainer type. The concrete industry utilizes outdated tables that are based on vinsol-resin AEA's which produce large, farther spaced bubbles and tend to be less stable than newer-generation AEA's.

It should also be recognized that, in addition to freeze-thaw durability, the properties of a concrete consisting of a system of entrained air also significantly affects the finishability of fresh concrete and many mechanical properties, particularly compressive strength, because the strength of concrete in uniaxial compression is by far the most common mechanical test performed in the industry.

With respect to finishability, excess air entrainment can reduce the hard-trowel finishability of concrete, which can result in several very objectionable problems, such as blistering and delamination's. In general, interior horizontal surfaces should never require air entrainment, even if the structure involved is subjected to one or two years of winter exposure prior to completion. Among other things, finishing involves proper timing, and finishers inexperienced with the adjustments needed when hard troweling air entrained concrete can experience surface blistering, sometimes very severe blistering. In general, as the amount of entrained air increases, hard troweling will become more difficult. Mixtures that are both rich in cement and containing entrained air can be very vulnerable to surface blistering or delaminating when troweled. In addition, exterior exposed horizontal surfaces should never be troweled, as troweling will only open the door to a multitude of problems.

Air entrainment can impart a very significant effect on higher strength concretes or concretes rich in cementitious materials. The relationship between air content and compressive strength is not linear in nature. For example, when the air content changes from 4% to 7% in a concrete mixture with a design compressive strength of 4000 psi (28 MPa), this could result in a nominal strength loss of 200 to 300 psi (1.4 to 2.1 MPa, or 5 to 7.5%). Conversely, a similar change in air content in a 10,000 psi (69 MPa) mixture could result in a severe strength loss of 1500 to 2000 psi (10.3 to 13.8 MPa, or 15 to 20%). Reduced strength attributable to objectionably high air contents can also detrimentally influence creep, another important mechanical property of high-strength concrete.

Take note that these are only a few examples of how air entrainment can affect the properties of concrete. Entrained air can affect a multitude of concrete properties in both the fresh and hardened state.

A common misconception exists that air entrainment is required simply because a vertical concrete element is subjected to exterior exposure in a severe weathering region. Periodic exposure to cyclical wetting and drying on the face of a vertical concrete element does not constitute saturated conditions. In modern times, the use of higher strength concrete in vertical structural elements is increasing, therefore, careful attention should be paid as to whether or not entrained air is truly technically justifiable. In more cases than not, the need for entrained air in exterior exposed vertical elements is not needed. This can be corroborated in the findings of condition surveys conducted on exterior exposed vertical elements for structures that have been in service for 50+ years. Such elements usually show no-to-negligible freeze-thaw damage, and this includes structural elements installed with ready-mixed, precast or prestressed concrete.

Lastly, the role that the water/cement ratio plays in the freeze-thaw durability of structural concrete should never be overlooked. It is recommended that structural concretes exposed to cyclical freezing and thawing while saturated or in the presence of deicing chemicals in a severe weathering region should never have a w/c ratio exceeding 0.45.