Air Entrained Concrete

In concrete, formation of air bubbles is inevitable due to agitation during mixing, kneading, toppling, turning, etc. Normally these air bubbles escape, or coalesce to form bigger bubbles which partly dissipate during placement and consolidation. The bubbles or pockets that do not escape are called entrapped air.

Air entrainment is the intentional creation of tiny air bubbles in concrete. The bubbles are introduced into the concrete by the addition to the mix of an air entraining agent, a surfactant (surface-active substance, a chemical that includes detergents). As we all know detergent used for washing of clothes, does not produce air bubbles in the water all by itself but is formed after agitating the water for sometime. Similarly, the mixing operation is essential to let bubbles form in concrete after addition of air entraining agent. Air entraining agents help to stabilise smaller diameter air voids by modifying the property of cement paste, i.e. reducing the surface tension of water. These air bubbles are created during mixing of the plastic (flowable, not hardened) concrete, and most of them survive to be part of the hardened concrete. These are called entrained air.
The main purpose of air entrainment is to protect the concrete from damage due to freezing and thawing cycles. A hardened concrete always has minute cracks and capillary pores. Whenever it comes in contact with water, the fissures/ pores absorb and retain water due to forces of capillary action. It is an inherent property of water to expand during transition from liquid stage to ice at freezing temperature. So, during sub-zero temperature the water present in the fissure & pores within concrete changes into ice which is much larger in volume and consequently exerts disruptive internal pressure within the concrete. The solution to this problem has been found in artificially creating some quantity of air voids with closer spacing between them (usually referred as air entrainment), which acts as chamber for the extra volume of frozen water.

Obviously the potential of damage is dependent on how much of the voids are filled with absorbed water. If there is enough space left for accommodating the expanded volume of ice, no internal pressure would be generated during freezing. It is therefore necessary to understand the degree of saturation that is safe, which is expressed by the term ‘Critical Saturation’. The ratio of volume of water present in these voids of concrete to the total volume of voids expressed as percentage is known as saturation in concrete. The term Critical saturation has been coined to explain the role of percentage saturation, in concrete, on aspects related to service life and durability of structure. It is defined as that percentage of saturation beyond which volume of water on freezing expands beyond the capacity of voids present in concrete and consequently damages the concrete. It has been studied, that water can expand up to 9% on freezing. Critical saturation thus, is considered as 91% saturation, since beyond this limit, chances of damage due to freezing are increased.
It is normally recommended to have air content of 3 to 7 % to prevent damage of concrete due to freezing and thawing cycles. But it is not only the volume of entrained air that influences the durability against freezing and thawing, but also the pattern of distribution of these voids. When there is expansion of water due to freezing, the air voids should be
1) Close enough so that water expands and travels to reach another void without damaging the concrete,
2) Enough in volume so that the expanded volume of frozen water does not exceed the total volume of voids.
3) Not forming clusters around aggregates or in the paste,
4) Not more than the volume specified in the specifications.
Spacing of air voids in practical applications is not constant as can be seen in Fig.1, and generalization of spacing is impossible, though spacing between air voids is said to be the most important factor in deciding durability of a structure. Consequently a measurement method has been used widely to check if spacing of air voids in the air void system is acceptable which is often referred to as ‘Spacing Factor’. Spacing factor is calculated based on certain assumptions. The calculation is done by first assuming that all air voids are of same size, and spaced equally as corners of a cubic lattice. To do so, the volume of cube lattice is calculated by setting ratio of unit cube to unit air void, equal to, ratio of total air voids volume to total paste volume (paste volume=

Cement volume + water volume + admixture volume). Spacing factor is not the distance between centre of one air void to another but is the distance of periphery of any air void from centre of the cube (Ĺ).


In other words Spacing factor is the thickness of imaginary shell which makes up an air bubble in the cement paste. This is the distance that frozen water has to travel from one void to enter into another. ASTM/ ACI specifies a maximum spacing factor of 300 microns, and average 200 microns as requirement for durable concrete in addition to the Air content for different categories of exposure and Maximum Aggregate Size. ASTM 457 has various methods to calculate the spacing factor of Air Voids.
Since shearing action of aggregates and mixer cause air bubbles to be formed and divided into smaller sizes, it becomes critical to give ample time for mixing of concrete at the batching plant so that enough air voids are formed and stabilised. Air entrainment is affected by following factors:
1) Higher carbon content in Fly Ash reduces the efficiency of Air entraining admixtures, thereby creating hindrance in consistency of air content, when carbon content in fly ash keeps on varying,
2) 0.1% dosage of admixture w.r.t. cement weight increases the air content by about 2-3%. Though this should be confirmed during trial mix, with proper tests for air content.
3) Pumping of Air entrained concrete, usually leads to decrease in air content, since pressure of pumping leads to expulsion of air voids from the system.
4) Excessive Vibration also leads to loss of air voids in concrete. Care should be taken to impart not more than few seconds of vibration to concrete, so that not much of entrained air is lost.
Usually it is perceived that air content and size of air voids dictate the spacing factor in concrete, which holds true in many cases. But there are cases of clustering of air voids at one location, which not only disturbs the spacing consistency of air voids, but also leads to remarkable loss in strength of concrete (Fig. 5). This usually is seen around coarse aggregates. As a thumb rule each 1% of void is responsible for 5% loss of strength, whereas a clustering similar to rating 2 (fig. 5) can reduce the strength by 20%.





Clustering of air voids is a consequence of re-tampering of concrete (addition of water to concrete at site), and must always be avoided especially in Air-entrained concrete.
Following table shows the recommended Air content (Entrapped + Entrained) for various conditions of exposure with different sizes of aggregates.



Air entrained concrete has some additional advantages over Non Air- entrained concrete like 1) Improved workability, 2) Better Cohesiveness.

Admixtures for air entrainment
Admixtures used for Air entrainment into concrete as discussed previously are surfactants, like anionic Sodium Vinsol resin. When other admixtures are also used in concrete in addition to air entraining agent, their interaction and their combined interaction with cement become important for compatibility reasons. Calcium Chloride added to concrete in cold weather to overcome freezing damages should not be mixed with air entraining agent rather should be added separately. Some Air entraining Agents are also known to retard the strength gain rate.
There is a new Air entraining agent developed, with a commercial name of Micro Air, which has capacity to produce ultra stable Air voids of small size and are closely spaced, it can be used in concretes containing high carbon fly ash. The Air entraining admixtures are also available in Powder form, which find their use in production of dry mix air entrained shotcrete, since liquid air entraining admixtures pose lot of problems for addition into Dry mix shotcrete.