Air entrainment in concrete is the intentional creation of tiny air bubbles in a batch by adding an air entraining agent during mixing. A form of surfactant (a surface-active substance that in the instance reduces the surface tension between water and solids) it allows bubbles of a desired size to form. These are created during concrete mixing (while the slurry is in its liquid state), with most surviving to remain part of it when hardened.

Air entrainment makes concrete more workable during placement, and increases its durability when hardened, particularly in climates subject to freeze-thaw cycles.

In contrast to the foam concrete, that is made by introducing stable air bubbles through the use of a foam agent, which is lightweight (has lower density), and is commonly used for insulation or filling voids, air entrained concrete, has evenly distributed tiny air voids introduced through admixtures to enhance durability, workability, and resistance to freeze-thaw cycles without significantly reducing its overall density, and without negative impact on its mechanical properties, allowing to use it in objects such as bridges or roads built using roller compacted concrete. Another difference is manufacturing process: foam concrete involves the creation of a foam mixture separately, which is then mixed with cement, sand, and water to form the final product, while air entrained concrete is produced by adding specialized admixtures or additives directly into the concrete mix during mixing to create small air bubbles throughout the mixture.

Approximately 85% of concrete manufacturing in the United States contains air-entraining agents, which are considered the fifth ingredient in concrete manufacturing technology.

Air entrainment is beneficial for the properties of both fresh and hardened concrete. In fresh concrete, air entrainment improves workability and makes it easier to handle and pump. It also helps prevent bleeding and segregation, unwanted processes that can occur during mixing. In hardened concrete, air entrainment strengthens the material by making it better able to withstand freeze-thaw cycles. It also increases its resistance to cracking, improves durability against fire damage, and enhances overall strength. Therefore, adding air to concrete when it's being made makes it easier to handle at first, but then later helps it stay strong even under tough conditions like freezing temperatures or fire exposure.

Tiny air bubbles in air entrained concrete act as internal cushioning, absorbing energy during impact and increasing resistance to physical forces such as shock or vibration. This improved impact resistance helps minimize surface damage and prevent the propagation of cracks or breaks, thereby increasing overall durability. Additionally, the air voids, acting as pressure relief zones, allow water or moisture expansion during freeze-thaw cycles without causing internal stresses and subsequent cracking.

Though hardened concrete appears as a compact solid, it is actually highly porous (typical concrete porosity: ~ 6 – 12 vol.%), having small capillaries resulting from the evaporation of water beyond that required for the hydration reaction. A water to cement ratio (w/c) of approximately 0.38 (this means 38 lbs. of water for every 100 lbs. of cement) is required for all the cement particles to hydrate. Water beyond that is surplus and is used to make the plastic concrete more workable or easily flowing or less viscous. To achieve a suitable slump to be workable, most concrete has a w/c of 0.45 to 0.60 at the time of placement, which means there is substantial excess water that will not react with cement. When the excess water evaporates it leaves little pores in its place. Environmental water can later fill these voids through capillary action. During freeze-thaw cycles, the water occupying those pores expands and creates tensile stresses which lead to tiny cracks. These cracks allow more water into the concrete and the cracks enlarge. Eventually the concrete spalls – chunks break off. The failure of reinforced concrete is most often due to this cycle, which is accelerated by moisture reaching the reinforcing steel, causing it to rust, expand, create more cracks, let in more water, and aggravate the decomposition cycle.

Air entrainment is a process that should be tightly controlled to avoid naturally occurring entrainment, which means the unintentional or undesirable presence of air voids in concrete, caused by factors such as improper mixing or insufficient consolidation, which may lead to reduced strength and durability due to inconsistent sizes and placement of air voids, making it less desirable for achieving specific concrete performance properties.