The factors that influence the amount of air entrainment are: 1. Type and Amount of Air Entrain Agent 2. Water Cement Ratio of the Mix 3. Type and Grading of Aggregates 4. Mixing Time 5. Temperature at the Time of Mixing 6. Type of Cement 7. Compaction 8. Influence of Other Admixtures 9. Effect of Hard Mixing Water 10. Effect of Rotation of Mixer.
Factor # 1. Type and Amount of Air Entrain Agent:
It has been observed that different air entraining agents produce different amount of air entrainment depending upon the elasticity of film of bubbles produced and the extent to which the surface tension is reduced. Similarly different quantities of air entraining agents produce different air entrainment. Hence the production of air entrained concrete is very difficult.
Factor # 2. Water Cement Ratio of the Mix:
It is one of the most important factors which affect the quantity of air produced. At very low w/c ratio i.e., about (0.35 to 0.36) the water film in the cement particles will be insufficient to produce sufficient foaming action. At water/cement ratio 0.4 to 0.6 abundant air bubbles will be produced. At more than 0.6 water/cement ratio large amount of air bubbles will be produced in the beginning but large proportion of these bubbles will be lost progressively with time. A more workable mix holds more air than dry mix.
Factor # 3. Type and Grading of Aggregates:
It has been observed that grading of aggregates has a marked influence on the quantity of air entrainment. It has been established that the quantity of air increases from the lowest fineness modulus of sand and attains the peak value at fineness modulus (F.M) of about 2.5 and afterwards decreases sharply. The sand fraction of 300 and 150 micron has shown a marked effect on the quantity of air entrainment. Higher the quantities of 300 and 150 micron fractions more will be the air entrainment. Excess of very fine particles reduces the amount of air entrained.
Factor # 4. Mixing Time:
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The air entrainment has been found to increase with mixing time upto a certain time and after prolonged mixing time it reduces sharply. Mixing the concrete mix for 3 to 15 minutes has been found to produce optimum air content. A very fast rotation of the mixer increases the amount of air entrained. Agitating of the mix upto 300 revolution seems to decrease the loss of air entrained but after 2 hours a loss upto 20% of the original air content has been found.
Factor # 5. Temperature at the Time of Mixing:
The temperature at the time of concrete mixing has been found to have a significant effect on the air entrainment. It has been found that with the increase in temperature, the amount of air entrainment decreases. At normal temperature the reaction starts at the time of mixing and may continue for 1.5 to 4.0 hours. At temperature above 38°C the reaction may be completed within 30 minutes and at 4°C the reaction may not even start for several hours. An increase in temperature from 10°C to 32°C may result in the loss of about 50% air entrained.
Factor # 6. Type of Cement:
The type of cement, i.e. the constituents of cement, especially the alkali content has a marked influence on the entrainment of air in concrete. Alkali content more than 0.8% increases the amount of entrained air. Similarly fineness of cement also influences the air entrainment in concrete. An increase in fineness of cement decreases the effectiveness of air entrains. For example if the surface area of cement is increased from 3000 cm 2/g to 5000 cm2/g the amount of air entrain agent will be required double of the quantity for the same amount of air entrained.
Factor # 7. Compaction:
Compaction of concrete reduces the air entrainment. During the process of compaction the bubbles move to the surface and also get destroyed resulting in the decrease of air entrainment. It has been found the 2½ minute vibration of concrete, air entrainment is lost by 50 percent and 9 minutes vibration of concrete resulted in 80% loss of air entrainment. Experiments carried out at Hirakud dam showed that 10.5% air contents reduced to 6% after 30 sec vibration.
Factor # 8. Influence of Other Admixtures:
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The use of other admixtures along with air entraining agents also has been found to have significant effect on the air entrainment. The use of fly ash and calcium chlorides have been found to reduce the amount of air entrainment i.e. the increase in carbon content decreases the amount of air entrain. The increase of carbon content of fly ash from 1 to 12% needs the air entrain agents 3 times more of the original.
Factor # 9. Effect of Hard Mixing Water:
Hard mixing water reduces the entrained air content.
Factor # 10. Effect of Rotation of Mixer:
Very fast rotation of mixer increases the amount of entrained air.