Both Air-Entrained concrete & Entrapped Air Concrete (i.e non-air entrained concrete) possess an air-void system.The term “air-void system” refers to collective presence of discrete air bubbles distributed throughout the cement paste matrix of concrete.

However, Entrained air should not be confused with Entrapped air.

The entrapped air found in concrete is the result of the cement paste's inability to close with the aggregate. In a non-air entrained concrete , the voids or the bubbles get entrapped in the concrete primarily because of inadequate consolidation. Voids of this type are present in all types of concrete, including air entrained concrete. They are often distinguished by their non spherical /distorted shape and their considerable greater size (usually larger than 1mm).The normal amount of entrapped air in most concrete mixes will fall within the 1 - 2 percent range. Entrapped air is not found as bubbles but rather appears as irregular shaped voids found in the cement paste.  Entrapped air does NOT provide the concrete reliable protection against frost action.

Air-entrained concrete contains minute air bubbles  which are smaller than the the voids of Entrapped concrete & are and more frequently occurringmore uniformly distributed .  

Entrained air can be produced in concrete by use of an air-entraining cement, by introduction of an air-entraining admixture, or by a combination of both methods.

An air-entraining cement is a portland cement with an air-entraining addition interground with the clinker during manufacture. An air-entraining admixture, on the other hand is added directly to the concrete materials either before or during mixing.

The bubbles are generated during mixing of fresh concrete and, in concrete with intentionally entrained air, are stabilized by the use of air entraining admixtures incorporated into the concrete mix. Air-entraining agents stabilize miccroscopic air bubbles in concrete by reducing the surface tension at the air-water interface. The shearing action of the mixer and aggregates will entrain and divide air into finely divided bubbles surrounded by the stabilizing agent. It is critical that sufficient mixing time be allowed for the air bubbles to be generated and stabilized. 

Air entraining admixture	Air entrained concrete
Reduction of stresses in Air entrained concrete (Source: PCA)

Proper air-entrainment will dramatically improve the durability of concrete exposed to moisture during cycles of freezing and thawing. Entrained air also improves concrete’s resistance to surface scaling caused by chemical deicers.In addition to providing saturated freeze-thaw protection and improved salt scaling resistance, entrained air provides several other benefits: increased workability, reduced water demand, decreased segregation and bleeding, and reduced permeability.

Air voids in concrete improve durability by reducing stresses associated with freezing water in pores. The expansion as water converts from liquid to solid upon freezing creates a pressure on the remaining liquid. Entrained air provides relief for this pressure, by providing space for the water to flow into. Without air voids, the pressure creates stress on the concrete, crreating cracks which cumulatively begin disrupting the concrete.

Both classes of air voids have an impact on the frost resistance of concrete but the smaller more frequently occurring and more uniformly distributed entrapped-air voids are the ones relied upon to provide permanent frost protection. The total volume of air required is a function of maximum aggregate size.