9 AIR ENTRAINING CEMENT Properties You Need to be Aware of

Air Entraining Cement is essentially ordinary portland cement in which air entraining agent is added during grinding of cement clinkers.

Hence, it is also called Air Entraining Portland Cement as it is modified portland cement.  There is no dedicated IS Standard for air entraining cement.

Let us get to know about its different aspects in this post.

What is Air Entraining Cement?

Air Entraining Cement Definition: Air entraining portland cement is a cement containing minute air bubbles which provide space for expansion of water during freeing and thawing.

Air entraining Type I A portland cement is ordinary portland cement in which air entraining agents are added.

Air entraining agents were introduced in the late 1930s proving to be advancement in concrete technology. Air entraining cement was then recommended for concreting where freezing occurs.

Common Terminologies

Air Entrainment

Inclusion of minute air bubbles in concrete or cement with the help of some chemicals is called air entrainment.

Air Entrained Cement

Ordinary portland cement in which air entrainment is done to introduce small air bubbles in the cement is called air entrained cement.

Air Entrained Concrete

Air entrained concrete is a concrete in which billions of microscopic air bubbles are introduced with the help of air entrained cement or air entraining admixtures. The volume may be 4-7 % of the volume of concrete.

Entrained Air

The microscopic air bubbles, ranging from 5 microns to 80 microns in size, introduced carefully and distributed throughout the mass of concrete uniformly are termed entrained air.

Entrapped Air

Voids left in the concrete due to insufficient compaction, which are larger in size (about 10-1000 microns) and are not distributed evenly in concrete.

Manufacture of Air Entraining Cement

The manufacturing process of air entraining portland cement is same as the manufacturing process of ordinary portland cement. 

The only difference is that before mixing, air entraining agents are added to the cement clinkers.

Small, discrete, detached, and tough air bubbles are produced in concrete due to these agents. These air bubbles modify the properties of plastic and hardened concrete. 

Air Entraining Agents

As mentioned above, air entraining agents are to be added during the manufacture of air entraining cement. Air entraining agents are normally in liquid state.

Air entraining agents are listed below:

• Alkali salts of wood resins like vinol resin
• Synthetic detergents having alkyl- aryl sulphonate group
• Calcium lignosulphate (which are obtained as by-products during the manufacture of paper from sulphite process)
• Calcium salts of glues & other proteins, which can be attained from animal hide treatment
• Salts of sulphonated hydrocarbons
• Wetting agents
• Aluminium powder
• Hydrogen peroxide
• Other air-entraining additives like animal & vegetable fats, oil, & their acids

The specifications for air entraining agents are covered in the standard ASTM C 260.

Size of Air Voids

Generally speaking, all concrete have voids having diameter of 1 mm or more. Their volume is about 0.2-3.0 % of the concrete volume approximately.

In air entraining cement, the diameter of air bubbles may be 0.01 – 1.00 mm. Majority of air voids in this cement are less than 0.1 mm in diameter.

The air voids are not connected in this cement. The air bubbles should be spaced at less than 0.01 inches in concrete. 

The total volume of these air voids may range from 1 to 7.5 % of total concrete volume.

Effect of Void Size in Concrete: Small voids give better frost resistance. Besides this, decreasing the size of air voids reduces the strength reduction in concrete.  

Measure of Air Voids

The measure of fineness of air voids is specific surface index.

Specific surface index is the total surface area of voids in a unit volume of cement paste.

To provide frost resistance, specific surface index should be more than 23,600 m²/m³, i.e. 600 in²/in³.

How Air Entraining Agents Works?

One of the mechanisms in which air entraining agents work is described below:

• Air entraining agents reduce the surface tension of the water used for mixing
• The air entraining agents have negative charge and are hydrophobic
• The negative charge of anionic air entertainers attracts the positively charged cement particle
• This is how tough, elastic, bubbles filled with air are formed 
• As mixing process is carried out, the bubbles are dispersed throughout the paste
• The air bubbles are held in place due to the grid formed by sand

Air Entraining Cement Properties

Properties of air entraining cement are discussed below:

Properties of plastic concrete-

• Workability
• Unit weight
• Segregation & bleeding
• Permeability

Properties of hardened concrete-

• Resistance to frost action
• Resistance to sulphate attack
• Resistance to de-icing
• Resistance to abrasion
• Reduction in strength

Properties of plastic concrete that are modified are-

1. Workability

Air bubbles act as lubricants between the aggregates. This increases the workability of plastic concrete.

Increased workability facilitates placing of concrete even with lower water cement ratio.

2. Unit Weight

Air entrained cement has comparatively lower unit weight. Because about 5 % air is introduced where otherwise solid particles may have been present.

3. Segregation & Bleeding

Properly dispersing the air bubbles can reduce the harmful segregation and bleeding of plastic concrete.

Segregation is a harmful phenomenon which reduces the strength of concrete. To know what is
segregation and its other harmful effects, Read Segregation of Concrete.

4. Permeability

Entrained air breaks the capillaries formed in the concrete made by air entraining cement. Thus, it improves permeability. 

Properties of hardened concrete modified by air bubbles-

1. Resistance to frost action

The tiny air bubbles formed in air entraining cement provides space to the water for expansion upon freezing

Freezing of moisture in the concrete pores gives rise to development of internal stress in concrete as follows-

Critical saturation: The volume of water increases by 9 % upon expansion. Increase in saturation beyond 91.7 % produces stress in concrete.

Hydraulic Pressure: Freezing water causes the unfrozen water to move into it. As the unfrozen water traverses through the pores in concrete, stress is again generated. The magnitude of the stress depends upon the length of flow, rate of freezing permeability, and salt concentration in the pores. 

Osmotic pressure: To equalise the alkali concentration and accomplish thermodynamic equilibrium, water moves from gel to the capillaries. The water from the interlayer hydration space and capillaries moves into air bubbles via voids. Here, it gets room for freezing and hence expansion is not caused in concrete. 

Effect of Internal Stresses on Concrete

Internal stresses reduce the durability of concrete. The effect is magnified if the cycle of freezing and thawing is repeated many times.

Interconnected tiny voids can be provided in hardened concrete to reduce the effect of freezing and thawing. Air entraining cement can fulfil this purpose.

2. Resistance to Sulphate Attack

Sulphate attack is more if the water cement ratio is high. By using air entraining cement, water cement ratio is to be reduced. Thus, air entraining cement can increase the resistance to sulphate attack.

3. Resistance to De-icing

De-icing is needed on a sloped surface where snow accumulates. The chemicals used for removing this snow causes scales in concrete. Using air entrained cement in concrete can prevent the formation of these scales.

4. Resistance to Abrasion

Abrasion resistance of air entraining cement is not affected much. It is almost same as that of non-air entrained cement.

5. Reduction in Strength & Solution

Increase in air content reduces the ultimate strength of the cement. Increase in air content by 1 % reduces the strength by 5 % approximately.

The reduction in strength can be counterbalanced by lowering the w/c ratio and increasing the cement content.

Advantages of Air Entraining Cement

• Frost resistance is increased
• Resistance to freeze-thaw cycles is increased
• Resistance to sulphates and salts is increased
• Reduction in alkali-silica reactivity can be achieved
• Reduction in harmful phenomena like bleeding and segregation of concrete can be attained
• Increased workability can be achieved on employing more cement content and lowering w/c ratio

Disadvantages of Air Entraining Cement

• Porosity of concrete is increased with the introduction of air bubbles
• Unit weight is reduced as porosity is increased
• Durability of concrete is reduced as air entrainment reduces strength and increases porosity
• High strength concrete is difficult to attain with air entraining cement
• Ultimate strength of concrete is reduced
• Strict control at site is required as little error in proportions or workability or temperature varies the content of air entrainment, which greatly varies the concrete properties

However, the reduction in strength of concrete can be compensated by decreasing the water/cement ratio and increasing the cement content.

Air Entraining Cement Uses

Uses of air entraining cement are listed below:

• Air entraining cement may be used in the regions exposed to freezing and thawing.
• It can be used as cement with air entraining for countertops
• It can be used for construction prone to chemical attacks
• It can be used for construction in water
• It is suitable for marine construction

Air Entraining Cement Applications

• To produce air entrained concrete
• Marine construction
• Regions prone to chemical attack
• Areas exposed to water

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