Frequently Asked Questions
Q: On what basis is air content for concrete specified?
A: Air content is specified on the basis of 2 variables: maximum coarse aggregate
size and exposure environment.
Air content, percent*
37.5 (1 1/2)
* Project specifications often allow the air
content of the concrete to be within -1 to +2 percentage points
of the table target values.
** Concrete exposed to wet-freeze-thaw conditions, deicers, or other
† Concrete exposed to freezing but not continually moist, and not
in contact with deicers or aggressive chemicals.
†† Concrete not exposed to freezing conditions,
deicers, or aggressive agents.
‡ These air contents apply to the total mix, as for the preceding
aggregate sizes. When testing these concretes, however, aggregate
larger than 37.5 mm (1 1/2 in.) is removed by handpicking or sieving
content is determined on the minus 37.5 mm (1 1/2 in.) fraction
of mix. (Tolerance on air content as delivered applies to this value.)
As the maximum aggregate size increases for any given concrete
mixture the amount of paste in the overall mixture decreases. For
this reason the required air content decreases. In a concrete mixture
with a maximum 3/4" aggregate the required target air content
for a severe exposure would be 6%; this would equate to a paste
to air content of about 9% which is the approximate change of volume
that water undergoes when it freezes hard.
As the aggregate size increases the quantity of paste decreases
so a lower air content will maintain the same relationship. If
the maximum aggregate size were decreased the overall paste content
would increase and require an increased air content to maintain
the 9% air to paste relationship. Depending on the structure a wide
variety of maximum coarse aggregates may apply. Typically in a range
from 3/8" to 3", however in massive structures aggregates
of up to 6" have been used.
The second variable is exposure: extreme, moderate, and mild. For
extreme exposures the air content ranges from 7 1/2 % for 3/8"
aggregate to 4 1/2 % for 3/8" aggregate. Extreme exposure is
defined as,"an environment in which concrete is exposed to
freeze-thaw conditions, de-icers, or other
The air content range for moderate exposure is from 6 % for 3/8"
aggregate to 3 1/2 % for 3" aggregate.
Moderate exposure is defined as,"an environment in which concrete is exposed to freezing
but will not be continually moist, not exposed to water for long
periods before freezing, and will not be in contact with deicers
or aggressive chemicals".
The air content range for mild exposure is from 4 1/2 % for 3/8"
aggregate to 1 1/2 % for 3" aggregate. Mild exposure is defined
as,"an environment in which concrete is not exposed to
freezing, deicers, or aggressive agents".
The quotes are taken from ACI 201.1R, Guide to Durable Concrete.
See Table for total target air content for concrete