Air-entraining admixtures

Water-reducing admixture Air-entraining admixture Slump (mm) Air content (%) Sand-aggregate ratio ml cm of surface % total mix water... 

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

A change in the supply situation of Vinsol resin with a commensurate price increase as well as the more widespread use of supplementary materials led almost all admixture companies to reformulate their air-entraining agents using synthetic or alternative natural products such as tall oil or rosins. In a period of about 2 years, a product that had been in dominant use for 40 years was relegated to a lesser role in the industry. 

Air-entraining water-reducing admixtures containing lignosulfonates can be based on impure lignosulfonate raw materials, as stated earlier, where only 2-3% additional air is required. However, this air may not be of the amount, type, and stability required, therefore additions of surfactants are made. Several different types can be used but in the majority of cases they are based on alkyl-aryl sulfonates (e. g. sodium dodecyl benzene sulfonate) or fatty-acid soaps (e.g. the sodium salt of tail-oil fatty acids). Additions of these types will allow incorporation of sufficient stable air of the correct bubble size to meet durability requirements under freeze-thaw conditions. 

The presence of a water-reducing admixture can alter the air content of concrete, either as a deliberate measure (the air-entraining water-reducing admixtures) or as a side effect of the material in lowering the surface tension of the aqueous phase. 

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

Water-reducing admixture Dosage Air-entraining agent type Water-cement ratio Freeze-thaw data Durability factor (%) Weight loss (%)... 

It can be concluded from the assessment of the data in this section that inclusion into a concrete mix of a water-reducing admixture of the lignosulfonate, hydroxycarboxylic acid and air-entraining type should not lead to any deterioration in the durability of that concrete to freeze-thaw cycling. Indeed there are strong indications that, when used either as a means of reducing the water-cement ratio or, alternatively, of reducing the cement content, more durable concrete may result. 

Concrete containing No admixture Neutralized wood resin Air-entraining water-reducing agent... 

Table 1.27 Freeze-thaw resistance of corresponding mixes containing water-reducing admixtures and air-entraining agents (after Mielenz)...

Series Mix Air- entraining agent Water-reducing admixture Cement content (kg m - ) Water- cement ratio 28-day compressive strength (N mm-2) Air content (%) Slump (mm) Durability factor ASTM C290 1967... 

Coarse aggregate was crushed limestone with a maximum size of 19 mm. Fine aggregate was natural sand. Air-entraining admixture was a sulfonated hydrocarbon. 

The air-entraining admixtures are organic materials, usually in solution form, which when added to the gauging water of a concrete mix, entrain a controlled quantity of air in uniformly dispersed microscopic bubbles. This type of air should not be confused with entrapped air which is often present in concrete in the form of irregularly shaped cavities and which can be due to inadequate compaction or flaky aggregates. 

The effect that air-entraining agents have on the rheology of fresh cement pastes can be considered from the point of view of changes due to the admixture itself, and those due to the presence of entrained air. 

When the viscosities of air-entrained pastes were measured by the same means, the results shown in Fig. 3.7 were obtained for sodium abietate at 0.05% by weight of cement. It can be seen that the magnitude of the effect due to the presence of the admixture itself is small in relation to the effect of the air it causes to be entrained. 

A study of the foaming capacities and stabilities [10] of a variety of air-entraining agents in a solution of cement extracts showed that commonly used anionic air-entraining agents, such as sodium dodecyl sulfate and sodium resinate (1) were visually precipitated from solution, (2) retained their ability to form stable foams after precipitation with only minor amounts of admixture left in solution, and (3) lost the major part of their ability to form stable foams after filtration. It was further shown from studies in cement pastes firstly that the admixture should be adsorbed on the solid particles of the paste with the non-polar ends of the molecule pointed towards the water phase, imparting a hydrophobic character to the cement... 

The fineness of cement is a major factor in determining the quantity of air-entraining admixture required to incorporate a given amount of entrained... 

The temperature of the concrete has a significant influence on the amount of air entrained in concrete by a standard addition level of admixture the higher the temperature, the lower the air content. A typical set of results is shown in Fig. 3.19 [23]. The effect is more marked at higher slump values. 

The air-entraining admixture, at the manufacturer s recommended addition level and premixed in part of the gauging water, is added to the mix, and sufficient additional gauging water included to achieve the desired workability. 

---