Entrained air refers to a system of microscopic, spherical air pockets intentionally introduced into a mixture, most commonly concrete. These tiny voids are stable and evenly distributed throughout the material matrix, typically ranging in diameter from 10 to 100 micrometers. The purpose of this fine, uniform network is to enhance the material’s performance and durability in specific environmental conditions.
The Critical Difference Between Entrained and Entrapped Air
The air content within a construction material like concrete is composed of two distinct types: entrained air and entrapped air. Entrained air is deliberately incorporated using specialized additives to achieve a specific, beneficial effect. These bubbles are microscopic, spherical, and uniformly spaced, necessary for their function as internal pressure relief valves.
Entrapped air, by contrast, is an accidental byproduct of the mixing and placement process. It forms naturally when consolidation is insufficient and is generally considered detrimental to the material’s integrity. Entrapped air voids are irregular, unstable, and significantly larger than entrained air, often exceeding one millimeter in diameter and frequently visible to the naked eye. Because entrapped air is large and unevenly distributed, it creates weaknesses, reducing the concrete’s strength and increasing its permeability.
Why Entrained Air is Necessary in Construction Materials
The primary purpose of entrained air is to provide durability, particularly resistance to damage from repeated freeze-thaw cycles. When water absorbed into the capillary pores of concrete freezes, it expands in volume by approximately nine percent. This volumetric increase generates hydraulic pressure that can exceed the tensile strength of the concrete, causing micro-cracking and eventual disintegration.
The microscopic air voids act as empty chambers into which excess water is forced when it begins to freeze. They serve as safety valves that absorb the internal stress created by the expanding water, preventing the buildup of destructive pressure. For this mechanism to be effective, the spacing between the air voids must be tightly controlled, typically no greater than 0.2 millimeters.
Beyond freeze-thaw protection, entrained air provides secondary benefits that improve the handling and long-term performance of the material. The spherical air bubbles act as a type of ball bearing, which reduces internal friction in the fresh mixture. This effect significantly improves the workability, making the concrete easier to place, pump, and finish without adding excessive water. Improved workability also helps prevent problems like segregation and bleeding.
The Role of Air-Entraining Agents
The intentional creation of entrained air is accomplished by adding specialized chemical compounds known as Air-Entraining Agents (AEAs). These agents are surfactants that reduce the surface tension of the water within the concrete mix. AEAs are molecules with a dual structure, possessing both a hydrophilic (water-attracting) end and a hydrophobic (water-repelling) end.
During the mixing process, mechanical action creates air bubbles, and the AEA molecules immediately orient themselves around them. The water-attracting end dissolves into the mixing water, while the water-repelling end points inward toward the air bubble. This molecular arrangement forms a stable, protective film around the air pocket, which prevents the small bubbles from collapsing or merging, a process known as coalescence.
By stabilizing the bubbles, the agents ensure that the air remains finely dispersed and uniformly distributed throughout the cement paste. This chemical action is crucial because it transforms the large, unstable air pockets that would naturally form into the minute, stable, and well-spaced air voids necessary for long-term durability. Common AEAs include compounds derived from wood resins and synthetic detergents, all selected for their ability to generate and maintain this stable air-void system.