What Is Air-Entrained Concrete?
Air-Entrained Concrete contains billions of microscopic air pockets and these air pockets relieve internal pressure on the concrete by creating a small air void for water to expand into when it freezes. Air-entrained concrete is made by using air-entraining portland cement or by use of the air-entraining agent in the concrete mix.
The method of forming a small tiny bubble in the concrete is known as Air entrainment. During the concrete-making process, some air entrain admixtures are added to the concrete which forms air bubbles in it.
Bubbles are created in the concrete when the concrete is in the plastic stage, most of the bubbles are surviving in the hardened stage of concrete. The primary purpose of air entrainment in the concrete is to increase the durability of the hardened concrete where the climates are subject to freeze and thaw.
The main secondary purpose is to increase the workability of concrete at its plastic stage. Due to air stickier being produced, it gives fewer results of segregation and a good attractive surface of the concrete.
How to Make Air-Entrained Concrete
How to make air-entrained concrete? following materials are used to make concrete,
- Air entraining cement
- Air entraining agents
1. Air Entraining Cement
Air-entrained Portland cement is one of the special types of cement that creates air bubbles in the cement or concrete. The Air bubbles provide the space for the Expansion of the concrete due to freezing and thawing and protect concrete from cracks and damage.
2. Air Entraining Agents
Air entraining agents or material including the following things:-
- Using gas-forming material:-
- Aluminum powder
- Zinc powder
- Hydrogen peroxide
- Lowering Surface tension agents:-
- Natural wood resins
- Animal fats
- Wetting agent
- Water-soluble soaps of certain acids
All materials given above are mixed with other concrete constitutes and act as a foaming agent. Due to the lowering of the surface tension of water, the stable foam creates small tiny microbubbles in the air-entrained concrete.
- Using cement dispersing agents
- Calcium lignin-sulfate and calcium salts of glues
Properties of Air-Entrained Concrete
- Freeze-thaw durability
- De-icers resistance
- Sulfate resistance
- Abrasion resistance
- Water tightness
If the workability of air-entrained concrete is improved then reduce water and sand requirements, particularly in lean mixes of concrete. If the air bubbles are disconnected then it reduces segregation and bleeding of plastic concrete.
2. Freeze-Thaw and Durability
In the freezing process, the water expands and creates enough pressure for a rupture in the concrete. Air bubbles work in the concrete as reservoirs to protect the concrete from the expansion water and protect the damage to concrete.
3. De-icers Resistance
Air-entrained concrete is mostly used where concrete is in contact with de-icing chemicals because entrained air prevents scaling caused by de-icing chemicals use for snow and ice removal.
4. Sulphate Resistance
If concrete is made with a low w/c ratio, entrained air and cement having low tri-calcium aluminate content are useful to resist the sulfate attack. Entrained air is very useful to resist the sulfate attack in concrete.
The strength of air-entrained concrete is basically determined by the voids to the cement ratio. In this ratio, voids are defined as the total volume of water plus air.
If the air content in concrete remains constant the strength varies inversely with the w/c ratio. If the air content increases then we can generally maintain a given strength with the help of holding the voids to the cement ratio constant.
6. Abrasion Resistance
The abrasion resistance in air-entrained concrete is the same compressive strength as in non-air-entrained concrete. If abrasion resistance increases then the compressive strength also increases.
7. Water Tightness
The water tightness of air-entrained concrete is more than that of non-air-entrained concrete. Entrained air prevents the interconnection of capillary channels from forming. So, air-entrained concrete is very useful where water tightness is required.
Effect of Air Entrainment on the Strength of Concrete
The main thing that is affected by air-entrained concrete is its workability and compressive strength of concrete.
The air-entrained concrete has increased the workability of concrete without increasing the water-cement ratio.
If the workability of the concrete is increasing then compressive strength automatically decreases because workability and compressive strength are always inversely proportional to each other.
- Effect of air entrainment on compressive strength of concrete
- Effects of air entrainment on flexural strength of concrete
1. Effect of Air Entrainment on Concrete Compressive Strength
Air entraining admixture is commonly introduced to increase concrete workability without affecting the concrete strength very much. The study says that the air-entrained concrete having a 7.5 cm slump is better than the slump of 12.5 cm of the non-air-entrained concrete.
If we use air-entrained admixture in large or huge quantities then the compressive strength of concrete is reduced. The common factors that affect the reduction of the strength of concrete are mix proportions, type of concrete, grading of concrete, type of cement, and entraining agent.
The strength of concrete reduced with the use of air-entrained admixtures is varied from 3 to 7%. The variation of this 3 to 7% is considered in the concrete mix design. So the desired compressive strength is achieved with the required amount of admixture and workability.
Before the design trial mix designs are done and find the exact variation of strength with the use of the admixtures and then suitable corrections are done in the mix design.
Generally assumed that a loss of 5% of the compressive strength of concrete occurs due to each 1% volume of entrained air in the concrete mix.
For estimating the water-cement ratio required for an air-entrained concrete which allows for strength reduction is incorporated in the mix design and assumes the high target mean strength.
The high target mean strength for an air-entrained mix is given by:-
Target mean strength = fc + m/(1-0.055a)
f = specified characteristic strength
m = the margin
a = % by volume of entrained air
2. Effect of Air Entrainment on Flexural Strength of Concrete
The reduction of flexural strength of the concrete due to air entrainment is generally not determined as in the case of compressive strength.
To the report, if the air content is 4% in concrete then maximum flexural strength.
From both cases, it concludes that the compressive and flexural strength of lean concrete mix design increases provided that, maximum water reduction is considered and a small maximum aggregate size is employed.
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