Air Entrained Concrete

             A major advance in concrete technology in recent years is the introduction of tiny disconnected air bubbles into concrete. The process which involves the introduction of tiny air bubbles into concrete is called as air entertainment. And the concrete formed through this process is called air entrained concrete. Using air entertaining Portland cement or air entertaining agents such as admixture, air entertainment is done in concrete. 

            The amount of air in such concrete is usually between 4 to 7 % of the volume of concrete. It is measured by galvanometric method, volumetric method and pressure method. The air bubbles relieve internal pressure on the concrete by providing chambers for water to expand when it freezes.

Adding entrained air to concrete provides important benefits in both plastic and hardened concrete, such as resistance to freezing and thawing in a saturated environment. Air entrapped in non air-entrained concrete fills relatively large voids that are not uniformly distributed throughout the mix.

Development of Air Entrained Concrete :-

                   As we know, volume of ice is about 10% greater than corresponding volume of water.
This causes a lot of problems in areas with extreme weathers. During the winters, water in the pores of concrete freezes. The ice formed occupies larger volume and hence exerts pressure. During the summer, the ice melts, and in the next winter, the pore water freezes again. This cycle is called freezing and thawing of concrete. 

              The pressure developed usually causes surface scaling. To overcome the ill effects of freeze thaw cycles, air entertainment in concrete was developed. Microscopic air bubbles were introduced in concrete. These bubbles give room for the ice formed to expand without exerting pressure on the concrete.

The dark areas in the picture above are air bubbles, the lighter areas are coarse aggregates, suspended in a cement mortar

Air entrainment has the following effects on concrete:

• Resistant to freezing and thawing

• Improved workability and durability

• Decrease in strength 

Properties of  Air Entrained Concrete :-

The following are properties of air entrained concrete:

1. Workability

The improved workability of air entrained concrete greatly reduces water and sand requirements, particularly in lean mixes and in mixes containing angular and poorly graded aggregates. In addition, the disconnected air bubbles reduce segregation and bleeding of plastic concrete.

2. Freeze-thaw durability

The expansion of water as it freezes in concrete can create enough pressure to rupture the concrete. However, entrained air bubbles serve as reservoirs for the expanded water, thereby relieving expansion pressure and preventing concrete damage.

3. De-icers resistance

Because entrained air prevents scaling caused by de-icing chemicals used for snow and ice removal, air-entrained concrete is recommended for all applications where the concrete contacts de-icing chemicals.

4. Sulphate resistance

Entrained air improves concrete’s resistance to sulphate. Concrete made with a low W/C ratio, entrained air, and cement having low tricalcium-aluminate content is the most resistant to sulphate attack.

5. Strength

The voids to cement ratio basically determines air-entrained concrete strength. For this ratio, voids are defined as the total volume of water plus air (both entrained and entrapped). When the air content remains constant, the strength varies inversely with the W/C ratio. As the air content increases, you can generally maintain a given strength by holding the voids to the cement ratio constant. 

To do this, reduce the amount of mixing water, increase the amount of cement, or both. Any strength reduction that accompanies air entrainment is minimized because air-entrained concrete has lower W/C ratios than non air-entrained concrete having the same slump.

However, it is sometimes difficult to attain high strength with air-entrained concrete, such as when slumps remain constant while the concrete’s temperature rises when using certain aggregates.

6. Abrasion resistance

Air-entrained concrete has about the same abrasion resistance as that of non air-entrained concrete of the same compressive strength. Abrasion resistance increases as the compressive strength increases.

7. Water tightness

Air-entrained concrete is more watertight than non air-entrained concrete since entrained air prevents interconnected capillary channels from forming. Therefore, use air-entrained concrete where water tightness is a requirement.

Advantages :-

1. Workability of concrete increases.
   
2. Use of air entraining agent reduces the effect of freezing and thawing.
3. Bleeding, segregation and laitance in concrete reduces.
4. Entrained air improves the sulphate resisting capacity of concrete.
5. Reduces the possibility of shrinkage and crack formation in the concrete surface.

Disadvantages :-

1. The strength of concrete decreases.
2. The use of air entraining agent increases the porosity of concrete thereby reducing the unit weight.
3. Air-entrainment in concrete must not be done if the site control is not good. This is due to the fact that the air entrained in a concrete varies with the change in sand grading, errors in proportioning and workability of the mix and temperatures.