Concrete air entrained

Air that is acddentally entrapped in concrete does not improve frost resistance of concrete, since it is distributed in voids which are relatively large, few in number, and unevenly distributed. Conversely, by introducing air-entraining admixtures to the concrete, it is possible to incorporate a system of very tiny and uniform bubbles inside the cement paste. 

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. 

Informative examples where exposure classes may occur 

5 - Freeze-thaw attack with or without de-idng agents 

Where concrete is exposed to significant attack by freeze-thaw cycles whilst wet, the exposure shall be classified as follows  

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In the early days of the use of superplasticized concrete, some concerns were aired regarding the resistance of air-entrained concrete containing superplasticizers to freeze-thaw cycling. However, more recent research has indicated the following ... 

Air entrainment of concrete carriageways, aircraft runways, and parking garages is now accepted as normal practice and several hundred thousands of cubic meters of this type of concrete have been placed. In North America the use of air-entrained concrete is universally accepted and therefore finds greater use than in Europe. 

The characteristics and quantity of cement used in the production of air-entrained concrete can have a pronounced effect on the air content and/or... 

A relationship exists between the size of bubbles that can be accommodated in a system containing fine particles of 300-600 pm diameter varies from about 30 to 100 pm. Since a large proportion of the bubbles in air-entrained concrete are less than 100 pm diameter, it is clear that this particle size is of considerable importance in determining the amount of air entrained. Therefore, even at constant sand content, an increase in the properties of particles of this size will lead to an increase in air content. This effect is shown for a number of sand contents in Fig. 3.21 [24]. 

Fig. 3.25 Change in void size distribution on vibration of air-entrained concrete (Johansen). 

In order to minimize air losses, air-entrained concrete should be compacted with the minimum of vibration, 5-15 s usually being sufficient. 

Table 3.14 Bleeding of plain concrete, air-entrained concrete and air-entrained concrete containing a water-reducing agent...

This is American data and aithough usefui to show the comparative strengths for plain and air-entrained concrete, in practice, with higher cement contents and lower workability used in the UK, values should not be used as the basis for mix design. 

Table 3.17 Factors to be added to the water-cement ratio to calculate the anticipated strength of air-entrained concrete...

Fig. 3.28 The relationship between the flexural and compressive strengths of plain and air-entrained concretes (after Shacklock). ...

Fig. 3.32 Deterioration of plain and air-entrained concrete in sulfate solutions (Wright). 

Some carbonation tests have been reported on plain and air-entrained concretes using two aggregate types, where the depth of carbonation on indoor and atmospheric exposure for 5 years have been measured [40]. The results are given in Table 3.24. In all cases, the air-entrained mixes have shown less carbonation than similar controls, suggesting that air-entrained concrete should provide a better reinforcement protection in the long term. 

Fig. 3.36 The increase in compressive strength of plain and air-entrained concrete up to a period of 1 year. Table 3.25 Drying shrinkage of air-entrained concrete is no greater than that of plain concrete...

Fig. 3.38 The creep of plain and air-entrained concretes at 21°C and a stress-strength ratio of 45% (Nasser).

Sutherland, A. (1974). Air Entrained Concrete, Publication 45022, Cement and Concrete Association. 

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

When added at dosage of 2% by weight of cement to a concrete mixture with 460 kg m of cement without adjustment for the volume of the water introduced by the admixture, the concrete s slump and porosity are increased. However, when substituted for an equal volume of water, the SRA has little or no effect on concrete slump. It does have a slight retarding effect on the rate of hydration and may extend the setting time up to about an hour. The admixture also affects the air content of fresh concrete and therefore when used in air-entrained concrete, the air-entraining admixture dosage must be increased to achieve a specified air content. 

Other fine materials which tend to inhibit air entrainment include pigments, particularly carbon black. This is of concern to the ready-mixed operator supplying integrally colored concrete for exterior exposure [6]. Accelerating admixtures which are used to reduce down time and in cold weather concreting can be used successfully in air-entrained concrete, but should be added separately and in solution form to the mix. Direct contact of these admixtures with some types of air-entraining agents mixed in the same water phase may adversely affect both admixtures. 

The use of superplasticizers in air-entrained concrete has caused much debate. Two main problems are associated with superplasticized air-entrained concrete (1) a decrease in air content by 1-3% when slump is increased from 75 mm to 220 mm after the addition of the superpiasticizer to create flowing concrete, and (2) a change in the air void system to less desirable values. However, most investigators [10-11, 12] have shown that, although the air-void spacing factor required for adequate frost resistance is altered, the change did not necessarily affect the freeze-thaw durability of... 

Because of the essential role that air-entrained concrete has played in reducing freeze-thaw susceptibility, its use in North America has been predominantly associated with this property. Consequently, other favorable modifications have often been ignored. In Europe, Australia and Africa where freeze-thaw action seldom or rarely affects the concrete, the secondary modifications produced by air-entraining admixtures have been realized to their full potential, when used as a composite water-reducing and air-entraining admixture. The increased use of this type of admixture is due to the following reasons ... 

Table 7.2 Air-entrained concrete has a higher standard deviation that plain concrete, but this can be minimized by the use of an air-entraining, water-reducing agent...

In North America almost all air-entrained concrete contains a water-reducing admixture. There is some reluctance to use a composite type of admixture because of the concern that it reduces flexibility in use due to the variation in concrete materials and job site conditions. Therefore, the use of... 

Table 7.3 Comparative porperties of plain concrete and air-entrained concrete produced with neutralized wood resins and a water-reducing air entraining agent...

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