One of the greatest achievements in field of concrete technology is development of air entrained concrete. It is used where the concrete is vulnerable to freezing and thawing action.
It is used where the concrete is vulnerable to freezing and thawing action. It is prepared by adding the air entraining admixture.
The air entrainment in concrete does the following functions:.
It lowers the surface tension of water and thus bubbles are created.
Secondly the air entraining agents prevents coalescing i.e. the combining of bubbles. The diameter of these bubbles ranges form 10 micrometer to 1000 micrometer and in entrapped air the diameter of bubble is greater than 1mm.
Air entraining agents OR air entrained admixtures are used for the purpose of making entrained air in concrete.
FREEZING AND THAWING:
There are two phenomenons regarding the freezing and thawing action on concrete.
when water inside concrete mass freezes it expands 9-10% due to this increase in the size it exerts pressure on its surrounding and thus creating a tensile force due to which micro cracks appear in the concrete. Due to freezing these micro cracks develop into fissures which results in disruption of concrete.
When the air entrained agents are present, extra amount of air is there as water expands these air bubble provide them thin space and the exertion of pressure is prevented.
Second is of osmotic pressure: In a concrete structure there are two parts, frozen and unfrozen. As the water content is higher in the frozen part, the osmotic pressure is developed and water tends to flow towards the low water concentration part. If capillaries are not available, the water develops cracks.
Normal concrete can not sustain 3-4 cycles of freezing and thawing where as the AEA concrete can sustain 100 cycles of it.
DRAW BACKS of Air Entrained Concrete:
It has low strength as compare to normal concrete.