It is one of the most advanced and useful research made in 1930 in concrete technology was the discovery of air-entrained concrete which is made by adding Air-Entraining Admixtures or agent which generate air bubble in concrete.
Since, 1930 the use of air-entrained concrete all over the world especially, in the United States and Canada. After recognizing the advantages of air-entrained concrete, about 85 percent of concrete manufactured in America contains one or the other type of air-entraining admixture or agent.
After that, the air-entraining admixture agents have almost necessarily considered as ‘fifth ingredient’ in concrete making.
Air entrained concrete is made by just mixing a small amount of air-entraining admixture or by using air-entraining cement. These air-entrained agents in concrete generate millions of non-coalescing air bubbles, These air bubbles act as flexible ball bearings and will modify the properties of plastic concrete regarding workability, segregation, bleeding, and finishing quality of concrete.
These changes also affect the properties of hardened concrete regarding its resistance to frost action and permeability.
The air bubble or voids available in concrete can classified as under two groups:
(a) Entrained air (b) Entrapped air.
Air entrained or air bubble generating agents are intentionally incurred in the concrete mix which large produce air bubble in the entire mass of concrete. The entrapped air is defined as the voids present in the concrete mix due to insufficient compaction.
These entrapped air voids or bubbles may have any shape and size normally embracing the contour of aggregate surfaces. Their size air bubble generally ranges from 10 to 1000 microns or more and they are not uniformly distributed throughout the concrete mass.
Read More: Air entrained Concrete – Method & Properties
Following is the list of air-entraining admixtures or agents that are used for making air entrained concrete.
a) Natural wood resins
b) vegetable and Animal fats and oils, such as tallow, olive oil, and fatty acids such as stearic and oleic acids.
c) Different types of wetting agents such as alkali salts or sulphated and sulphonated organic compounds.
d) Soup resin acids that soluble in water, and animal and vegetable fatty acids.
e) Miscellaneous materials such as the sodium salts of petroleum sulphonic acids, hydrogen agents here are a number of air-entraining admixtures or agents available in the market.
The common air-entraining admixtures are T peroxide and aluminium powder, etc. out gents United States are Vinsol resin, Darex, N lair, Airalon, Orvus, Teepol, Petrosan, and Cheecol. Out of the above air-entraining admixture or agent, the most important worldwide used agents are Vinsol resin and Darex.
In India, Air entrained concrete is not widely practiced, primarily due to the fact that frost scaling of concrete is not a serious problem in our country so far. However, the advantages of the use of air-entrained concrete have been realized for the construction of multi-purpose dams.
Air entrained concrete in past used in the construction of Hirakud dam, Koyna darn, Rihand da, etc. In these dams, to start with, American air-entraining admixtures agents such as Vinsol resin, Darex, etc. used. Later on, in the 1950s some Indian origin air-entraining admixtures or agents were developed.
Factors Affecting the Amount of Air-Entraining Admixture:
The making of air-entraining concrete is somehow little complicated considering that the amount of air-entraining admixtures in a mix is affected by many factors; the important ones are:
1) The types and quantity of air entraining agent used.
2) Water/cement ratio of the mix.
3) Type and grading of aggregate.
4) Mixing time.
5) The Temperatures.
6) Types of Compaction.
7) Influence of compaction.
8) Admixtures other than air entraining agent used.
Various types of air-entraining admixtures or agents in concrete produce different amounts of air bubbles, depending upon the elasticity of the film of the bubble produced, and the extent to which the surface tension is reduced. Similarly, different quantities of air-entraining admixtures or agents will result in different amounts of air entrainment.
The water/cement ratio of concrete mix is one of the important factors considered which affects the quantity of air. At a very low water/cement ratio, water film around cement is insufficient to produce adequate foaming action.
At an intermediate water/cement ratio (viz. 0.4 to 0.6) abundant air bubbles will be produced. But at a higher water/cement ratio although to start with, a large amount of air entrainment is produced, a large proportion of the bubbles will be lost progressively with Arne.
The grading of aggregate also has an important effect on the quantity of air entrainment. It was established that the quantity of air increased from the lowest fineness modulus 07 sand to a peak at about F.M. of 2.5, and, thereafter, decreased sharply.
Sand having a size of about 300 and 150 microns showed a significant effect on the quantity of air entrainment. The higher the amount of these particle size fractions resulted in more air entrainment.
The amount of air bubble increase with the increase in mixing time up to a certain time and thereafter with prolonged mixing the air entrainment gets reduced. The Concrete temperature during mixing also shows a significant effect on the amount of air entrainment. It is concluded that the air entrainment decreases as the concrete temperature increases.
The cement constituent especially alkali plays an important role in the entrainment of air in concrete. Similarly, cement fineness is also an important factor. With compaction of concrete, forcing the movement of air bubbles to the surface and destruction which cause reduction of air bubbles in concrete.
Experiments show that the air bubble amount may reduce up to 50 percent lost after vibration for 2 1/2 minutes and as much as 80 percent may be lost by vibration for 9 minutes.
During the construction Hirakud dam experiment conducted which indicated that an air content of 10.5 percent after 30 sec of vibration came down to 6 percent after 180 sec of vibration.
Read More: 15+ Types of Admixtures used in Concrete
The Effect of Air Entraining Admixture on the Properties of Concrete:
Air entrainment will effect directly the following three properties of concrete:
- Increased resistance to freezing and thawing.
- Improvement in workability.
- Reduction in strength.
Incidentally addition of air entrainment also effects the properties of concrete in the following ways:
- Reduces the tendencies of segregation.
- Reduces bleeding and laitance.
- Decreases the permeability.
- Increases the resistance to chemical attack.
- Permits reduction in sand content.
- Improves placeability, and early finishing.
- Lower the cement content, cost, and heat of hydration.
- Reduces the unit weight.
- Permits a reduction in water content.
- Reduces the alkali-aggregate reaction.
- Reduces the modulus of elasticity.
1) Resistance to Freezing and Thawing:
The most important advantage of the use of air-entrained concrete is the high resistance of hardened concrete to scaling due to freezing and thawing. Experiments show that when normal concrete subject to a temperature below the freezing point, the water contained in the pore of the concrete freezes.
It is well known that the ice volume is generally 10 times more than that of water volume. Hence, water that is freeze and becomes ice in hardened concrete exerts pressure. The combined effect of pressure becomes considerable, with the result that surface scaling and disruption of concrete at the weaker section takes place.
Sometimes, the concrete surface scaling and disruption also takes place in plain concrete when subjected to the action of salt used for deicing purposes.
A similar, failure pattern is also observed in concrete structures at the tidal zone and spray zone. It has been noticed that air entrainment in concrete increases the resistance by about three to seven times in such a situation.
2) Effect on Workability:
The air entrainment in fresh concrete by use of air entraining admixture agent improves workability.
It was observed that the air entrainment concrete having a 7.5 cm slump is superior to that of non-air entrained concrete having a 12.5cm slump. This easier place ability of lower slump should be recognized by the people concerned with concrete construction in difficult situations.
Better place-ability of air-entrained concrete results in more homogeneous concrete with less segregation, bleeding, and honeycombing. The concrete which has air entrained or air bubble is more plastic and ‘fatty’ and can be easily handled than ordinary concrete. The pump-ability of the mix also increases enormously
3) Effect on strength:
As an increased air content in concrete, it is generally observed the reduction of the compressive strength of concrete. But when the process is applied properly, taking advantage of the benefits accrued on account of air-entrainment, little or no loss of strength should take place and gain of strength may be possible in certain circumstances.
It is clear fact that the water/cement ratio, an increase in air content results in loss of strength, but the air entrainment enables reduction of water/cement ratio and sand content, for given workability, thereby regaining most if not all the lost strength.
4) Effect on Segregation, Bleeding, and Laitance:
Segregation and Bleeding are two different ways of loss of homogeneity. Segregation is defined as the separation of coarser aggregate from mortar or separation of cement paste from aggregates.
Bleeding is the natural flow of mixing water within, or its emergence to the surface from freshly placed concrete, usually, as a result of sedimentation of the solids due to compaction and self-weight of the solids.
Bleeding in concrete sometimes result in the formation of a series of the small channel which may extend to the surface of the concrete. A layer of water will emerge at the surface of the concrete, often bringing some quantity with it. The formation of this layer of neat cement particles is called laitance.
Air entrainment in concrete greatly helps in the reduction of segregation, bleeding, and consequent formation of laitance. This process results from physical phenomena due to the incorporation of a system of air bubbles. Firstly, the air bubble lifted up aggregate and cement and hence reduce the rate at which sedimentation occurs in the freshly placed concrete.
Secondly, the air bubble generated in concrete decreases the effective area through which the differential movement of water occurs. Thirdly, the air bubble in concrete significantly increases the abrasion resistance between cement and aggregate. Lastly, Plastic concrete surface area due to air bubbles is sufficiently large to retard the rate at which water separates from the paste by drainage.
5) Effect on Permeability:
The entrainment of air does not appear to have much effect on the permeability of concrete.
Air entrained concrete offered greater workability due to its increased workability, modified pore-structure of the air-entrained concrete, reduction of water channel due to reduction in bleeding, which is some important reason behind improving permeability characteristics of air-entrained concrete.
The cement which is used to make air-entrained concrete stored in silos has been found to show no caking of cement, whereas, cement stored in silos made of ordinary concrete revealed caking along the periphery of the silos.
The minute disconnected air bubble offers a better barrier to the passage of water. The reduced water/cement ratio also is one of the factors for reduced permeability.
6) Effect on Chemical Resistance:
In view of lower permeability and absorption and absorption, the air-entrained concrete will have greater resistance for chemical attack than that of normal concrete. In the Road Research Laboratory, U.K., the experiment is done by preparing the ordinary and air-entrained concrete specimen samples which have been immersed in 5 percent solution of magnesium sulphate, and the deterioration in quality has been assessed by measuring the decrease in the velocity of ultrasonic waves through the specimen. Its result shows that air-entrained concrete showed less deterioration than ordinary concrete.
7) Unit Weight of Air-Entraining Admixtures:
While making entrainment concrete never ignore the unit weight factor is due to a reduction in density of the air-entrained concrete.
Comparing two mixes, one ordinary concrete and the other air-entrained, which have the same workability and strength, air entrainment concrete contains 5 percent less solid materials in it, and hence will be lower in weight.
Incidentally, this will result in an economy of about 5 percent in the cost of cement and aggregate, less the cost of air-entraining admixture or agent, and the cost of extra supervision.
8) Alkali-Aggregate Reaction:
Much research shows that air entrainment in concrete considerably reduces the alkali-aggregate reaction. The use of air-entrainment admixture agents has frequently been recommended as a means for controlling expansion due to alkali-aggregate reaction in mortar and concrete.
9) Abrasion Resistance:
It is observed that the concrete with 6 percent air entrainment has almost the same resistance to abrasion as normal concrete when cement contents of the comparable concrete are constant.
However, with an increase of air-entraining admixtures or agents in concrete abrasion resistance with further increase in air content. In concrete, added about 10 percent of air-entraining agents, abrasion resistance is markedly low.
Since concrete is used in the pavement is generally specified to have not more than 3 to 6 percent of entrained air the abrasion resistance should be satisfactory.
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