**7. Mechanism of action of the air-entraining admixtures**

For adequate resistance of concrete to the freezing/thawing cycles; air voids should not have a spacing factor (the maximum distance between two air voids in concrete) larger than 0.2 mm or 0.008 in. For a given air content, the size of the air voids cannot be too large if the proper spacing factor is to be achieved. The specific surface should be greater than 24 mm2/mm3 [32], where the air void system consists mainly of three parameters: the void's size, the space factor, and the void size distribution [33, 34]. As previously mentioned, to produce freeze/thaw resistant concrete; a proper air-void system is required in the concrete to accommodate the volume extension of water when it freezes without causing damages.

Artificially, the use of effective air-entraining admixtures can ensure the stabilization of the air-void system and produce air-entrained concrete which is one of the greatest advancements in concrete technology [35]. The term "air-entrainment" refers to the air deliberately introduced into concrete by adding air-entraining admixture. In the case of using air-entraining admixtures, the entrained air voids reduce the hydraulic pressure by behaving as expansion chambers despite the volume increment when water turns into ice [36, 37]. The schematic mechanism of the performance of concrete (with and without air-entraining admixture) exposed to freezing is shown in **Figure 12**. Šelih [39] and Wang et al. [40], investigated the freeze/thaw resistance of concrete with air-entraining admixture and low water content and they experienced a better performance of 10 times lower mass loss comparing to the normal one.

Air-entraining admixture is either surfactant that reduces the surface tension of water, or substances that produces a water-repellent precipitate which is required to produce the air voids dispersed throughout the concrete and that ultimately provides durability in freezing/thawing situations. Surfactant air-entraining admixture secured the best overall air void characteristics, followed by salt-type air-entraining admixture containing tall oil, and then, salt type air-entraining admixture containing Vinsol resin and wood rosin. However, some types of air-entraining admixtures, including vinsol resin, sodium adipate, sodium oleate, do not reduce the surface tension of water [16, 41]. The fresh state of concrete seriously affected its freeze/ thaw resistance like the flowability and setting time of concrete. Air voids have a lifetime and should not be unstable, where they could be collapsed due to different fundamental physical mechanisms. Such as the diffusion of air from a void to a larger one, the void coalescence due to capillary flow, or the rapid hydrodynamic drainage of liquid between voids [33].

Other types of chemical admixtures are usually used to control the flowability and viscosity of concrete such as superplasticizers and viscosity modifying admixtures, which also affect the hardened properties of concrete and generally they affected the
