Volume of air entrained

These air bubbles are of the order of 0.05 to 1 mm in size. Air bubbles can only avoid generation of stresses in the capillaries when water freezes, if they are not too far apart. Experience has shown that good freeze-thaw resistance of concrete requires distances between bubbles less than 0.1-0.2 mm. The volume of air entrained in concrete is of the order of 4-7 % with respect to the volume of the concrete [10] however, for every mixture there is a minimum content of entrained air below which the presence of air bubbles is not effective. 

A fireball is assumed to bum with a constant temperature in the isothermal fireball model of Lihou and Maund (1982). Combustion is controlled by the supply of air and ceases after a time which is correlated empirically with the mass of flammable gas in the initial vapor sphere. It is assumed that a fraction (1 — /c) of the fuel is used to form soot, and the remaining fractionbums stoichiometrically, producing an increase of /ij moles per mole of flammable gas. The stoichiometric molar ratio of air to flammable gas is p, and dVIdt is the volumetric rate of air entrainment. The rate of increase of volume can now be written as ... 

The effect of the entrainment of a minor amount (4-6% by volume) of air on the ability of concrete to withstand freeze-thaw cycling is remarkable [3] and is illustrated in Fig. 3.1. 

Table 3.11 Cement type and source can influence the volume of air obtained in both plain and air-entrained mixes...

Determine the proper volume of air to achieve adequate mixing and particle movement in the bowl. Avoid excessive volumetric airflow so as to entrain the particles into the filters. 

The use of an air entrainer increases the yield of concrete by the volume of the entrained air. This has a significant effect on the apparent cost of using an air-entraining admixture. 

Too high addition of plasticizer does not involve such negative effects as in the case of air entraining agents [360]. The air bubbles are isolated and irregularly distributed therefore they do not improve the paste freeze-thaw resistance. Moreover, they have somewhat higher volume as the voids formed in the case of air entraining admixtures. 

It is noted that the addition of certain additives slightly decreases the concrete strength, despite its positive effects. Early studies found that the addition of air-entraining admixture reduces the concrete s compressive and tensile strength (HRB 1971 Teychenne et al. 1975). The average reduction on the concrete s compressive and tensile strength was found to be at the 5% and 4% level, respectively, for every 1% by volume of additive in the mixture (Teychenne et al. 1975). 

Many lignosulfonates, especially the less pure types, entrain a certain volume of air in concrete. This may be desirable to improve the durability of concrete against freezing and... 

Twenty percent of that internal volume of air in the degreaser is entrained, pushed, or pulled out into the work area every time a basket of parts is removed from the top of the freeboard area, and... 

Air Content. The volume of air voids in cement paste, morter or concrete, excluding the pore space in the aggregate. Air entrained by mixing or gases generated chemically may be included. (ASTM C-125). 

The air-entrainment effects are related to the workability of the fresh mix with higher slump the volume of air is increased, but for high slump (al50-175 mm) the mix may be too fluid to retain the air bubbles before hardening. The proper amount and nature of an air-entraining admixture is needed to obtain the required effects and this is usually established by testing. 

The total volume of air voids is not the most important parameter, but it gives certain indication as to the freeze-thaw resistance. It is measured in a fresh mix and in hardened material, but between these two measures large differences may occur during casting, vibration, etc. When the total air volume in the fresh mix is below 4% it indicates that there is not enough air and probably L > L . When the air volume is greater than 4%, then with appropriate air bubbles distribution a satisfactory situation may be expected, that is, that T s and other entrained air parameters may have satisfactory values also in hardened concrete all this should be verified, for example, by testing cores. 

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