By Alex Morales, M.Ed.
Editor’s Note: This is the third article in a year-long series explaining common raw materials used in precast.
Air-entrainment of concrete refers to the intentional creation of tiny air bubbles in a fresh concrete mix. In an industry that consolidates concrete to drive out air, it may seem counterintuitive to want air in concrete. However, entrained air differs greatly from entrapped air. Entrapped air – large, irregularly sized pockets of air caused by the mixing and placement of concrete – must be minimized because it is detrimental to concrete’s performance, reduces compressive strength, increases permeability and decreases watertightness. Conversely, entrained air is the result of adding an air-entraining agent during batching which develops an evenly distributed network of stable, microscopic voids which provide many fresh and hardened concrete benefits and remain in the mix as it cures.
In its plastic state, air-entrained concrete exhibits better workability. The air voids act like ball bearings, reducing friction during the fresh concrete’s movement, allowing it to flow better during placement.
Although better workability is a desirable property of fresh concrete, it is considered a value-added benefit of air-entrainment. The primary purpose of air-entrainment is to provide hardened concrete better resistance to freeze-thaw cycles.
History of air-entrainment
The benefits of air-entrainment were first noticed in the 1930s.1 Engineers found certain highway sections performed better in freeze-thaw environments than others and conducted research to figure out why. Studies found highway sections exhibiting better freeze-thaw resistance were made with cement milled at plants using beef tallow as a grinding agent. Further scrutiny revealed the beef tallow was an unintended air-entraining agent which contributed to the improved durability of concrete. By 1938, several state highway departments conducted studies that, “indicated definitely the possibilities of utilizing the air-entraining characteristic of certain materials to improve durability.”2
Mixing’s impact on entrained air bubbles
As researchers continued their work, they discovered as the entrained air bubbles collide repeatedly as a result of longer mixing times, the collisions can cause the individual air bubbles to join (a function of reduced interfacial tension) creating bubbles of larger sizes. Significantly larger entrained air bubbles are undesirable because they possess a greater buoyant force, which can cause them to rise to the surface and escape the mix. As a result, longer mixing times were associated with a reduction in entrained air content.
Throughout the 1940s and 1950s, studies of the behavior of entrained air voids helped researchers conclude that some intervention would be required to prevent the bubbles from joining together during the mixing process. In addition to the reduction in entrained air voids seen during the mixing process, the air void system also saw deterioration when driving out entrapped air pockets during concrete consolidation.3 Researchers postulated that some film at the interface of the air bubbles would be required to prevent them from joining and escaping during both placement and consolidation processes.
Impact of dilatancy
Earlier research from the Bureau of Public Roads, now the Federal Highway Administration, tested various commercially available admixtures for potential air-entrainment properties.4 The studies identified admixtures that created entrained air systems, but not all admixtures produced air bubbles that provided freeze-thaw resistance.5 Studying the air-inducing effect of then-common admixtures, early researchers identified the difference between entrained air that provided no dilatancy and entrained air that could sustain short-term loads within fresh concrete.
The film surrounding entrained air bubbles exhibited dilatancy by reducing the hydrophilic quality of the surface of each bubble rendering it hydrophobic. In a hydrophobic state, researchers noted the entrained air bubble would cling to cement and aggregate particles in the plastic mix while it resisted coalescence with surrounding air voids during the mixing, placing and consolidation processes. This phenomenon contributes to both workability and increases in slump.
The ideal air-entraining admixture was one that created microscopic air voids, each surrounded by a film strong enough to resist coalescence during agitation and prevent their buoyant expulsion from the mix.
Air-entrainment’s impact on freeze-thaw durability
Early researchers also sought to understand the mechanism by which air-entrainment improved freeze-thaw resistance of hardened concrete. Research showed that the value of the air void system in hardened concrete was two-fold:
- Size of the individual entrained air bubbles.
- Spacing within the system.
The impact of the air-entrainment system on the freeze-thaw resistance of hardened concrete is directly related to the distance from any point in the paste to the nearest air void. This is the distance water that has penetrated the concrete would have to travel as it expands during the freezing process in order to reach an air void. The closer together the entrained air voids are, the less hydraulic pressure is created during the freezing process because the freezing water would have a nearby air void within which to expand.
“The principal purpose of entraining air in concrete is to protect the paste from the potentially destructive forces generated during the freezing process. This protection is derived from the cellular paste structure produced by the randomly dispersed air bubbles.”6
Air-entraining admixtures today
Today’s air-entraining admixtures are primarily liquids, typically produced from byproducts of wood resins.7 This is no surprise since studies from the 1950s stated, “One agent in powder form was found to develop a system of voids significantly different from that developed by the same agent used in solution form,” pointing to the benefit of liquid air-entraining admixtures.8 That was an important discovery that has implications still today in the precast concrete industry.
The nature of an air-entraining agent impacts the air void system it produces in a particular mix at a particular w/c. Modern day air-entraining admixtures will produce air void structures with varying bubble size (although they tend to be about less than 1 millimeter in diameter) and spacing, depending on the type of cement used, chemical makeup of the water batched into the system, gradation of the aggregate used, use of other admixtures and more.
Interaction with raw materials
Today’s admixtures take advantage of the lessons learned from early research and preserve the entrained air bubbles in the concrete paste throughout the production process. Controlling the consistency of concrete ingredients (as well as consistency in batching, mixing, placing and finishing practices) is key to ensuring the reliable and predictable performance of an air-entraining admixture. This is also another major benefit of producing concrete in a quality-controlled plant environment.
An increase in cement’s Blaine fineness can result in less air-entrainment for the dosage rate. Therefore, a change from Type I cement to Type III cement may require an increased air-entraining admixture dosage to keep the same percentage of entrained air.
Conversely, as the cement’s soluble alkali content increases, so does the air-entrainer’s potential to entrain air. An increase in cement content will also decrease the efficacy of an air-entrainer. Consequently, a lower w/c may require an increased air-entrainment dosage.
Like cement Blaine, as the fineness of fly ash increases, the air-entraining admixture’s ability to entrain air decreases. Fly ash with higher carbon contents also tends to result in decreased entrained air potential. Using fly ash at higher replacement percentages of portland cement may require an increase in air-entrainment dosage.
As with cement and fly ash, finer particles (those passing the #100 sieve) tend to necessitate a higher dosage of air-entrainment to achieve the same entrained air content as a mix using only larger aggregate sizes. Maintaining the aggregate’s fineness modulus from batch to batch is critical to ensure a consistent level of entrained air in these concrete mixtures.
Additionally, aggregates in an oven-dry or air-dry condition tend to soak up liquid air-entraining admixtures, reducing their dispersion and overall effectiveness. Consider wetting aggregates prior to batching or consider alternative batching sequences when batching dry aggregates to prevent a decrease in the air-entraining admixture’s effectiveness.
Dirty aggregates coated with dust and other extremely fine contaminants will also reduce the effectiveness of air entrainers.
Air-entraining admixtures have also been shown to be more effective when used with rounded aggregate particles as opposed to irregularly shaped or rough-textured aggregates like crushed stone.
Hard water, such as well water, tends to reduce the amount of entrained air achieved by a particular dosage of air-entraining admixture. Water softeners, on the other hand, tend to have the opposite effect.
Batching and mixing considerations
As with any raw material, the batching sequence and mixing duration is crucial to the performance of air-entraining admixtures. Air-entrainers are often batched into the mixer along with the aggregate. However, precasters should rely on the admixture supplier for guidance specific to their mix design. Additionally, mixing beyond two minutes in most mixer types will begin to reduce the admixture’s air-entraining potential.
Fresh concrete considerations
Many current-day studies prove air-entraining admixtures significantly improve the workability of fresh concrete, which, in turn, can increase the slump or slump flow as compared to the same mix without air-entrainment. Additionally, as slump increases up to about 6 inches, so does the air-entraining admixture’s ability to entrain air.9
The structure of the entrained air void system, including the number, size and distribution of bubbles along with the dilatancy of each individual bubble, can be impacted by seasonal changes in production processes:10
- Hot concrete temperatures can reduce an air-entrainer’s effectiveness by up to 25%.
- Cool concrete temperatures (less than 75 F) can increase an air-entrainer’s effectiveness by up to 40%.
Therefore, with all other factors remaining the same, precasters may notice a reduction in a mix’s entrained air content in warmer months compared to cooler months.
Excessive vibration can also reduce the entrained air content of a mix. In addition, evidence shows that the bleeding rate decreases as a mix’s entrained air content increases.11 Watch for these effects when adding air-entrainment to a known mix.
Many concrete mix designs use more than one admixture. Even when the dosing of an air-entrainer is perfected and necessary adjustments are made to account for the chemical makeup of raw materials, it is important to note the mix design’s performance may change when a new admixture is introduced. Any change in raw material, proportions or production processes can have an impact on the chemistry of the mix and, ultimately, on the performance of an air-entrainer. Each mix design and its source for each raw material must be assessed to determine the right air-entraining admixture at the right dose for its individual application. Precasters are advised to work closely with their admixture supplier to run trial batches to determine the best solution for each particular plant.
Alex Morales, M.Ed., is NPCA’s director of workforce development.
2. Jackson, Frank H., “Concretes Containing Air-Entraining Agents, “Journal of the American Concrete Institute,” Vol. 40, 1944, pp. 509-515.
3. Mielenz, R. C., Wolkodoff, V. E., Backstrom, J. E., and Flack, H. L., “Origin, Evolution and Effects of the Air Void System in Concrete. Part 1-Entrained Air in Unhardened Concrete,” Proceedings, American Concrete Institute, Vol. 55, 1958, pp. 95-121.
4. U.S. Bureau of Public Roads, “Evaluation of Air-entraining Admixtures for Concrete,” Public Roads, Vol. 27, No. 12, February 1954, pp. 259-267.
5. Powers, T. C., “The Air Requirement of Frost Resistant Concrete,” Proceedings, Highway Research Board, Vol. 29, 1949, pp. 184-202.
8. Backstrom, J. E., Burrows, R. W., Mielenz, R. C., and Wolkodoff, V. E.., “Origin, Evolution and Effects of the Air Void System in Concrete. Part 2- Influence of Type and Amount of Air-entraining Agent,” Proceedings, American Concrete Institute, Vol. 55, 1958, pp. 261-272.
10. More information found at https://precast.org/2019/05/troubleshooting-hot-weather-concreting and https://precast.org/2019/11/troubleshooting-cold-weather-concrete
11. Yang, Quan-Bing; Zhu, Peirong; Wu, Xueli; Huang, Shiyuan, “Properties of concrete with a new type of saponin air-entraining agent,” Cement and Concrete Research, Vol 30, 2000/08/01.