Uses and effects of air entrainment
Extensive laboratory testing and field investigation concluded that the formation of minute air bubbles dispersed uniformly through the cement paste increased the freeze-thaw durability of concrete. This formation can be achieved through the use of organic additives, which enable the bubbles to be stabilized or entrained within the fresh concrete (Whiting 1983, ACI Comm. 212 1963). These additives are called air-entraining agents.
Besides the increase in freeze-thaw and scaling resistances, air-entrained concrete is more workable than non-entrained concrete. The use of air-entraining agents also reduces bleeding and segregation of fresh concrete (Whiting 1983; ACI Comm. 212 1963; Rixom and Mailvaganam 1986).
Materials and Specifications. The most commonly used chemical surfactants can be categorized into four groups: 1) salts of wood resins, 2) synthetic detergents, 3) salts of petroleum acids, and 4) fatty and resinous acids and their salts (Dolch 1984; Whiting 1983).
Until the early 1980s, the majority of concrete air entrainers were based solely on salts of wood resins or neutralized Vinsol resin (Edmeades and Hewlett 1986), and most concrete highway structures and pavements were air entrained by Vinsol resin. Today, a wider variety of air-entraining agents is available and competes with Vinsol resins.
Requirements and specifications of air-entraining agents to be used in concrete are covered in ASTM C 260 and AASHTO M 154. According to these specifications, each admixture to be used as an air-entraining agent should cause a substantial improvement in durability and none of the essential properties of the concrete should be seriously impaired. This provides a means to evaluate air entraining admixtures on a performance basis.
Factors Affecting Air Entrainment
The air-void system created by using air-entraining agents in concrete is also influenced by concrete materials and construction practice. Concrete materials such as cement, sand, aggregates, and other admixtures play an important role in maintaining the air-void system in concrete. It has been found that air content will increase as cement alkali levels increase (Pomeroy 1989; Whiting 1983) and decrease as cement fineness increases significantly (ACI Comm. 212 1963).
Fine aggregate serves as a three-dimensional screen and traps the air; the more median sand there is in the total aggregate, the greater the air content of the concrete will be (Dolch 1984). Gradation has more influence in leaner mixes. Median sand ranging from the No. 30 sieve to the No. 100 is the most effective at entraining air . Excessive fines, minus No. 100 material, causes a reduction in air entrainment.
Because the use of chemical and mineral admixtures in addition to air-entraining agents has become common practice, concrete users are always concerned about the effects of these admixtures on the air-void system and durability of concrete. Effects of water reducers, retarders, and accelerators were widely investigated by many researchers. In regards to gross air content obtained when water-reducing and retarding admixtures are used in concrete, numerous studies have shown that for most of the materials, less air-entraining agent is needed to achieve a given specified air content (Whiting 1983). Chemical admixtures should be added separately from air entraining additives.
When lignosulfonate water reducers are used, less air-entraining agent is required because the lignosulfonates have a moderate air-entraining capacity, although alone they do not react as air-entraining agents (Dolch 1984; Rixom and Mailvaganam 1986). For a fixed amount of air-entraining agent, the effect of added calcium chloride is to slightly increase the air content (Edmeades and Hewlett 1986). The effect is more pronounced as amounts greater than 1% of the weight of cement are used. Some HRWR (superplasticizers) interact with cements and air-entraining agents, resulting in reductions in specific surfaces and increases in air-void spacing factors (Whiting 1983; Whiting and Stark 1983; Whiting and Dziedzic 1990).
Mineral admixtures such as fly ash and silica fume also affect the formation of void systems in concrete. Gebler and Klieger (1983) showed, in their study on the effect of fly ash on air-void stability of concrete, that concretes containing fly ash produced relatively stable air-void systems. However, the volume of air retained is affected by fly ash types. In mixtures containing fly ashes, the amount of air-entraining agent required to produce a given percentage of entrained air is higher, and sometimes much higher, than it is in comparable mixtures without fly ash (Gebler and Klieger 1983). In a series of papers, researchers presented the results of a study on factors that affect the air-void stability in concretes (Pigeon, Aitcin, and LaPlante 1987; Pigeon and Plante 1989). They found that silica fume has no significant influence on the production and stability of the air-void system during mixing and agitation. Bunke (1988) also indicated that silica fume has no detrimental effects on the air-void system.
Temperature can also have a significant effect on air entrainment. Air entrainment varies inversely with temperature. The same mix will entrain more air at 50 F (10C) than at 100F (38C).
Air Content Control. Measurement of air content is an important checking “sensor” for the concrete user to know whether concrete will resist freeze-thaw damage. Because average void spacing decreases as air content increases, an “optimum” air content at which void spacing will prevent the development of excessive pressure due to freezing and thawing will exist.
It is important to check the air content of fresh concrete regularly for control purposes. Air content should be tested not only at the mixer but also at the point of discharge into the forms, because of losses of air entrainment due to handling and transportation.