Entrained Air: Now You See It, Now You Don't

Exciting new research for the concrete pumping industry finds air void loss after pumping to be a temporary condition.

Although concrete pumps place more than 60 percent of ready mixed concrete in the U.S., the method’s prevalence hasn’t inoculated it against criticism during its 60-year history. One common concern when pumping concrete is the modification—especially the loss—of air content. However, exciting new research from Tyler Ley, professor at Oklahoma State University, challenges the long-held belief that pumping concrete reduces the air voids and therefore the performance of finished concrete.

Since entrained air is critical to finished concrete’s performance, it’s no surprise field personnel keep a close eye on air content. However, researchers are questioning whether the industry is taking the correct measurements at the correct point in time to predict finished concrete’s strength and performance accurately. Ley and his team at Oklahoma State have conducted experiments suggesting that different collection points and methodologies may provide better guidelines when pumping and placing concrete.

Passionate about his research for the concrete pumping industry, Ley has varied experience in the field as well as in the classroom and research lab. Before entering academia, Ley worked for the Texas Department of Transportation and for Zachary Construction as a field engineer. As he became familiar with pump issues and the common belief that pumping caused air to disappear from the concrete, he became interested in researching the topic. Ley has studied concrete and air entrainment for almost 16 years, including running experiments at Oklahoma State using a test pump that provides real-world simulations. He examines how to design mixtures to make them easier to pump.


Air entrainment is usually obtained by incorporating a chemical admixture while mixing concrete on the job site. The chemical agents used are categorized as surfactants or soaps. These chemicals help stabilize bubbles within the concrete mix. The network of bubbles reduces internal stresses within the concrete by providing spaces for water to expand into during freeze-thaw cycles.

Air volume alone does not equate to freeze-thaw performance of finished concrete. Small, evenly distributed bubbles are more effective than large, concentrated volumes of air when it comes to providing freeze-thaw resistance. Yet, current practice does not identify the size or distribution of bubbles within fresh concrete. Being able to measure the spacing factor (one-half of the average distance of an average-sized void uniformly distributed in the paste) would likely lead to more predictable outcomes in finished concrete. The desired value is less than 0.008 inches, per the “Guide to Concrete Durability” provided by ACI Committee 201.

Ley and his team at Oklahoma State University developed a proprietary device known as a Super Air Meter (SAM). This modified ASTM C231 type B pressure meter can hold larger-than-usual pressures. It has a digital gauge instead of a dial, and its reading—the SAM number—delivers information about bubble size and distribution in fresh concrete. The meter yields data similar to that of hardened air void analysis and rapid freeze-thaw testing, but it can be completed before the concrete hardens. This means information about concrete quality and performance can be understood before the concrete sets, and insights can be gained into how different mix design variables or construction practices impact air voids.


It is commonly believed that the pumping process causes air volume to be lost from the concrete; such concerns are fed by the fact that pressure, vacuum conditions and impact all act upon the concrete while it is being pumped. Frequently, concrete is sampled and tested after the pumping process and results often suggest that air has been lost.

Oklahoma State researchers conducted lab tests on concrete both before and after it was pumped, investigating air volume, air void spacing per AASHTO TP 118 (equivalent to the SAM number), spacing factor (petrographic analysis) per ASTM C457 and freeze-thaw performance per ASTM C666. Results for fresh concrete showed air loss of approximately 20 percent after one circulation through the concrete pump. SAM numbers indicated that pumping resulted in the loss of small bubbles. Analysis of the pumping operation showed that the air seemed to change right after the pump was activated and it stayed almost constant throughout the pipe network.

Concrete was examined again after it had hardened and, while low air contents and high SAM numbers had been recorded in fresh concrete after pumping, the freeze-thaw performance of the hardened concrete was satisfactory. Furthermore, the air content was higher once the concrete hardened. This means the measurements taken while the concrete was fresh did not represent the hardened properties.

Tests were duplicated in the field, using different pump configurations (flat, arched and A-frame boom configuration), as well as different equipment and materials. Results were similar to those in the lab. The findings suggest that pumping concrete compresses the air bubbles and causes them to dissolve. The bubbles do not immediately re-form when pressure is decreased, so testing fresh concrete gives the appearance of having a reduced air content.

However, the dissolved air seems to reform before the concrete hardens—and it is again well-dispersed, with a spacing like it had when it went into the pump. The findings were confirmed by good performance in the petrographic analysis, freeze-thaw testing and SAM numbers taken over time as the concrete set. This video summarizes the study’s findings: https://youtu.be/38H6yXi_of8.

Air dissolving under pressure is common and widely-known. This can be seen when opening a carbonated beverage or filling a glass of water from a water faucet and allowing it to sit. This is also a danger to deep-water divers that rise quickly. While it is a common area of study, it has never been applied to pumping concrete.

If the apparent loss of air in fresh concrete is only a temporary condition, it’s important for the industry to stop basing decisions on high or low air measurements taken after the concrete has been pumped. Instead, the concrete should be sampled prior to pumping and this will provide a good indicator of the air void system in the hardened concrete. Researchers also found air loss was more common with high pumping pressures, small diameter lines, mixtures with poor aggregate gradations, lower slump mixtures and with A-frame and arch boom configurations.


Ley has collected three years of data specific to issues of air entrainment related to pumping, but he’s looking for more data. Data collected under a variety of field conditions and using additional admixtures would be particularly valuable.

“Because we’re challenging an existing belief, we need to prove beyond a doubt that air entrainment returns after pumping and before setting. That will require many, many field measurements,” says Ley. “We’d like to enlist contractors in the field to help us collect that data—taking measurements and hardened air void analysis on concrete both before and after pumping. If we can establish a pattern in the results, in different parts of the country and in different situations, we’ll know we can make solid recommendations on how to better design concrete mixtures to make them easier to pump and place.”

While Ley’s team will conduct petrographic analysis, he needs images of batch sheets, photos of pump configuration and details about pump makes and models. More data will help Ley and his team further challenge the long-held perception of air entrainment in concrete pumping. If you’d like to learn how you can contribute, contact Ley at tyler.ley[at]okstate[dot]edu.