Frequently Asked Questions
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 Q:
Can we add air-entraining admixture at the jobsite?
When an air content test shows that entrained air
content is below a specified lower limit, we commonly add an air
entraining agent at the jobsite (Photo), turn the drum an additional
30 revolutions (or more if necessary) at mixing speed and re-check
the air content. If the air and slump are within limits, we use
the concrete. Is there a publication that describes this procedure
or a similar one, so we can use it as backup when questioned about
the practice?
A: ASTM C 94, Standard
Specification for Ready Mixed Concrete, section 7.3 provides a method
for field adjustment of air content when the tested value falls
outside of the acceptable tolerance range (plus or minus 1.5%).
Air entraining admixture may be added to achieve the proper air
content, followed by a minimum 30 drum revolutions at mixing speed.
At no time are the total revolutions of the drum to exceed 300 revolutions
for the load in question. ACI 301-99, Specifications for Structural
Concrete in Buildings, nor AASHTO M 157,
Standard Specification for Ready Mixed Concrete address this issue.
The Canadian Standards Publication A23.1-00, Concrete Materials
and Methods of Concrete Construction, also refers to a procedure
for field control of air content. Section 18.4.3.4 says that: “The
air content of the concrete shall, if necessary, be brought up to
the specified range by the addition of an air-entraining admixture
in the field. Mixing shall follow to ensure proper dispersion. The
air content shall be retested. ... The amount of admixture added
shall be recorded on the delivery slip.”
As an added control check, it might be advisable
to measure air content at the point of discharge into the forms.
This is recommended in Section 2.10 of ACI 212.3R-91, Chemical Admixtures
for Concrete.
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