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Why is there air in concrete?
Small, well-dispersed air voids in concrete can improve workability, reduce bleeding and segregation, and improve resistance to freezing and thawing.
Air is always present in concrete mixes. It is intentionally
or unintentionally trapped in fresh concrete as a result of mixing and placing.
About the only way to avoid trapping some air would be to mix, transport, and
place concrete in a vacuum side, just like the addition of sand makes the total
aggregate gradation finer. These chemical admixtures stabilize and retain more
of the smaller bubbles trapped during mixing and agitating. These finer bubbles
become the more desirable part of the air void system. The coarser bubbles
randomly trapped during mixing and placing is generally present with or without
an air-entraining admixture.
Air bubbles and air voids:
Because air and water do not mix, air trapped in fresh
concrete normally is in the form of gas bubbles surrounded by a thin liquid
film and suspended in the mix w a t e r. These bubbles vary in size and shape
from microscopic, hollow spheres the size of cement grains to large,
irregularly shaped gas pockets the size of coarse aggregate particles or larger.
All these bubbles can move in fresh concrete. They can change size and shape,
expand or contract, merge or rupture, or be removed from fresh concrete through
vibration. Once concrete hardens, however, the air bubbles are fixed in place.
The hollow space formed by the last position and shape of a bubble is an air
void. T h e whole collection of these hollow spaces in a sample of hardened
concrete is the air void system.
Effects of air voids:
When the air voids in hardened concrete are mostly in the
form of large voids and pockets, the effects are generally detrimental. Failure
to remove larger voids results in porous, poorly consolidated concrete in the
forms and around re-bars and other inserts, with honeycombing and reduced
in-place strength. Fresh concrete should be consolidated during placement to
remove as many of the larger voids as possible. Consolidation will be discussed
in Part 2 of this series. When an air- entraining admixture is used, concrete
needs to be handled so as to preserve as many of the smaller voids as possible.
Workability improves when an air- entraining admixture is used to increase the
number of small voids. Small, well-dispersed bubbles act as air cushions
between aggregate particles, reducing friction and interlocking. This benefit often
allows a reduced water content for concretes with an air-entraining admixture.
As a large number of microscopic bubbles work themselves in between cement
grains and fine aggregate particles, bleeding and segregation are reduced.
Increased cohesion caused by the air-entraining admixture helps the mix resist
segregation, but can sometimes make its ticky. Further, when the void size
gradation shifts toward finer sizes, there is less reduction in compressive
strength for a given total air content, and it is possible to offset strength
reduction with a lower required water content.
Volume of air required:
An air void system having a total volume of about 18% to 20%
of the cement paste volume can generally accommodate expanding ice and water.
More air volume is required to accommodate ice expansion for porous and
saturated concrete. Paste volume is difficult to measure, so it is more
convenient to describe air content as a fraction of total concrete volume. Air
contents of 18% to 20% of paste volume usually are 4% to 7% of concrete volume.
However, since air protects only the paste, richer mixes with greater paste
volumes need higher air contents to protect the paste. For the same reason,
leaner mixes may need less air. Aggregate size, shape, and gradation also can
affect required air volume because paste contents are different. For example, a
topping mix with pea gravel usually will require more air to achieve frost
resistance than a pavement mix with 11⁄2inch stone.
Void size and spacing:
Expansion pressure caused by freezing is reduced only if ice
or water do not have to travel too far to the closest air void. Greater
pressure is required to push ice or water long distances to a void space. If
this pressure is too great, the cement paste will crack. This means that the
cement paste will have a critical distance beyond which ice and water cannot
flow without causing damage.
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