The Effects of Pumping Concrete on Air Entrainment

Concrete Pumping Basics

• Basic concrete is a mixture of water, aggregate and cement. Air entrainment agents, or chemical surfactants, are added to the concrete for creating widely dispersed air voids as the mixture mixes within a concrete mixer. The air voids create a buffer for trapped moisture, reducing the impact of freezing and thawing cycles for preserving the concrete's hardened structure over time. Typically, the concrete pumps out from the mixer, down a long pipe and into the intended frame for hardening. However, this pumping action against the concrete mixture can alter the air voids.

Air Loss

• Concrete pumping typically involves dropping the mixture from the piping and into the intended framing. This violent movement can cause as much as 4 percent of the air to escape from the mixture, reports the Iowa Department of Transportation. This loss of air reduces the amount of air voids within the concrete mixture. As a result, the final hardened concrete is more susceptible to cracking from freezing and thawing cycles.

Air Void Shape Alterations

• Effective air voids must be numerous and small to support the concrete mixture successfully over many years. However, concrete pumping actions can alter the air void shapes. The concrete's forceful movement during pumping moves the air voids, possibly creating large air pockets from a conglomeration of small voids. The large pockets destabilize the concrete mixture, making the entire concrete composition subject to cracking after hardening.

Considerations

• Avoiding air entrainment problems requires careful movement of the concrete mixture. Place the concrete's piping at a close angle toward the intended framing, instead of vertically above the area. The concrete will move down the piping slowly rather than splattering downward through the piping and into the framing. The concrete should remain as undisturbed as possible to preserve the air voids.