Get the Air Out?

I
n 1957 and 1958,Armand Gustaferro
was the manager of a plant that
produced prestressed concrete
beams and girders for 50 bridges
on the Illinois Tollway. At that
time, the maximum design strength
given in ACI 318 was 3750 psi. Gusta-
ferro, however, needed a compressive
strength of 4000 psi at strand release
and 5000 psi at 28 days. The tollway
engineers were concerned that high-
strength, air-entrained concrete could
not be economically achieved on a daily
basis.So theyspecified non-air-entrained
concrete.
Would that happen today? Many
producers of both prestressed and
ready-mixed concrete are facing speci-
fied average total air contents that are
6% to 7%, or even higher, for freeze/
thaw resistance in exterior vertical mem-
bers. Engineers seem to believe that if a
little air is good, a lot is better. There
also seems to be confusion in the
industry as to the correct exposure cat-
egory for vertical concrete members. If
air contents really need to be in the
4% to 7% range, Gustaferro’s non-
air-entrained concrete should have
already been replaced. But it hasn’t.
Even today, Gustaferro is proud to say
that the non-air-entrained beams and
girders his plant made are still per-
forming well.
Twenty-five years of service
In the summer of 1982, Gustaferro,
Hillier, and Janney (Ref. 1) visually
inspected 20 bridges on the Illinois
Tollway. An item of particular interest
was the durability of the prestressed
concrete. The report indicated that, in
general, the durability of the bridge
girders had been excellent. The girders
had been essentially maintenance free,
and only minimal freeze/ thaw deterio-
ration or rusting of reinforcement
occurred. Most girders looked very
much as they did when the bridges
were first built in 1957.
Girders on one bridge in particular
had suffered slight corrosion damage
due to the penetration of deicing salts.
The authors noted that there were lay-
ers of salt on the sides of some of the
girders. In some areas the salt layer was
as thick as 1/8 inch. Although there was
corrosion damage, the authors noted
no freeze/thaw deterioration in these
girders.
Using a spinning process that pre-
cluded incorporation of entrained air,
cylindrical prestressed concrete piles
that served as the bridge piers were
cast. The authors report that these piles
also were in generally good condition
and required little or no maintenance.
After 25 years,however, some piers suf-
fered freeze/thawdeteriorationin thezone
where passing vehicles had splashed deic-
ing-salt solutions onto the piers. This
would seem to confirm current ACI 318
requirements for air entrainment in areas
in contact with deicing salts.
Portland Cement Association tests
At the Portland Cement Association,
researchers made precast panels in their
lab and tested them in a simulated out-
door environment. They also tested
high-strength concretes using a more
standard freeze/thaw test in water. Both
test results indicated good performance
for non-air-entrained concrete.
Panel tests. Isberner (Ref.2) investi-
gated the resistance to freezing and
thawing of precast panels with facing
mixes consisting of white quartz aggre-
gates and white portland cement. The
cement content of these concretes ranged
from 840 to 900 pounds per cubic yard,
and the water-cement ratio was 0.40.
Some of the concrete contained no air-
GettheAirOut?

entraining agent. Other concrete panels
contained an air-entraining agent,
which in normal concretes would pro-
vide the recommended amount of air.
But since the panel contained stiff
mixes, the air-entraining agent pro-
duced no increase, or only a very small
increase, in air content.
After a 7-day moist cure and 21 days
of air drying, the panels were immersed
face down for 24 hours in water 1/8 inch
deep. They were then mounted vertical-
ly and subjected to alternate freezing at
0° F and thawing in air at 73° F. Before
each freezing cycle, technicians thor-
oughly wetted the exposed-aggregate
facing concrete. None of the panels
showed any sign of deterioration of the
facing concrete after 125 cycles of freez-
ing and thawing.
Freeze/thaw tests. PCA funded
studies in 1960 and 1978 on low water-
cement-ratio (0.30 to 0.40) moist-cured
concretes (Ref. 3), and on low water-
cement-ratio (0.33) simulated steam-
cured concrete and moist-cured con-
crete (Ref. 4). These tests showed that
even non-air-entrained concretes were
very frost-resistant when air-dried
before freezing and thawing in water.
For the non-air-entrained con-
cretes, petrographers microscopically
measured the air-void system. The
maximum distance of any point in the
paste to an air void (spacing factor) was
measured as 0.02 inch. This is dramat-
ically higher than the generally accept-
ed 0.008-inch criterion for an adequate
air-void system. The authors wrote,
Field surveys and lab tests show
that non-air-entrained concrete
performs well, but ACI 318
requires a minimum air content
By Bruce A. Suprenant and Ward R. Malisch
Air entraining low-water-cement-ratio concrete for structures located
outside of splash zones probably raises production costs needlessly .
The good condition of 70-year-old non-air-entrained precast panels on
this entryway column at Grant Park in Chicago—arguably the harshest
freeze-thaw environment in the nation—attests to that.

“These results indicate that, at water-
cement ratios of 0.40 or less and for
freezing and thawing in water, the
usual requirements of the air-void sys-
tem do not apply, probably due to the
greatly reduced freezable water con-
tent, and to a lesser degree, to the
increased tensile strengths of such
high-quality concretes. The results are
of considerable practical significance,
particularly to the precast, prestressed
industry, which sometimes experiences
difficulty in obtaining the high strengths
usually specified while providing the
required air contents specified for dura-
bility.”
How much air?
ACI 318-99, “Building Code Re-
quirements for Structural Concrete”
(Ref. 5), requires concrete exposed to
freezing and thawing or deicing chem-
icals to be air-entrained, with an air
content as shown below for moderate
exposures. ACI 318-99, however, indi-
cates that for specified compressive
strengths greater than 5000 psi, the air
content can be reduced by 1.0%. And
in the same section, the code also pro-
vides a tolerance of 1.5%. The table
shows the minimum total air content
required for moderate exposures.
As the table shows, the minimum
permissible ACI 318 total air contents
for moderate exposures to freezing and
thawing are low. Typical air contents of
non-air-entrained concrete are about
0.5% lower than the permissible mini-
mums shown in the far right column in
the table. For instance, the air content
for a 3/4-inch maximum-size-aggregate,
non-air-entrained concrete will general-
ly be about 2%. The table shows that
2.5% is the permissible minimum. If a
water reducer is used, which is quite
likely, the measured total air content of
the nominally non-air-entrained fresh
concrete is likely to exceed the mini-
mums. The air content that results
from using a water reducer, however,
may not provide the air-void-system
properties needed for adequate freeze/
thaw resistance.
The 1983 revision of ACI 318 was
the first to specify total air content
required for frost resistance, based on a
classification of severe or moderate
exposure. The accompanying com-
mentary indicated that the required air
contents were based on the recommen-
dations in ACI 211.1,“Standard Practice
for Selecting Proportions for Normal,
Heavyweight, and Mass Concrete” (Ref.
6). The ACI 211 definitions are as fol-
lows.
Moderate exposure: Service in a cli-
mate where freezing is expected but
where the concrete will not be continu-
ally exposed to moisture or free water
for long periods prior to freezing and
will not be exposed to deicing agents or
other aggressive chemicals. Examples
include exterior beams, columns,walls,
girders, or slabs that are not in contact
with wet soil and are located such that
they will not receive direct applications
of deicing salts.
Severe exposure: Concrete that is
exposed to deicing chemicals or other
aggressive agents or where the concrete
may become highly saturated by contin-
ued contact with moisture or free water
prior to freezing.Examples include pave-
ments, bridge decks, curbs,gutters,side-
walks, canal linings, or exterior water
tanks or sumps.
ACI 211 also indicates that if a
member is not continually wet and will
not be exposed to deicing salts, lower
air content values such as those for a
moderate exposure are appropriate
even though the concrete is exposed to
freezing and thawing.
The ACI 318 commentary defini-
tion of moderate exposure (from 1983
until 1999), reads slightly differently:
“A moderate exposure is where the
concrete in a cold climate will be only
occasionally exposed to moisture prior
to freezing, and where no deicing salts
are used. Examples are certain [empha-
sis ours] walls, beams, girders, and slabs
not in direct contact with soil.” Certain
Some ground-level non-air-entrained concrete at Chicago’s Grant Park reveals distress that may be
related to freeze/thaw cycles. But note the generally good condition of the precast balusters and the
rail beneath them. Settlement of the structure caused the cracking seen at the right.
Total air content for frost-resistant concrete
Nominal maximum Moderate With 1% reduction With (1.5%
aggregate size, in. Exposure due to f’c > 5000 psi tolerance
3⁄8 6.0% 5.0% 3.5%
1⁄2 5.5% 4.5% 3.0%
3⁄4 5.0% 4.0% 2.5%
1 4.5% 3.5% 2.0%
11⁄2 4.5% 3.5% 2.0%

is a rather ambiguous term that could
lead a conservative designer to assume
severe instead of moderate exposure.
We agree with the ACI 211 defini-
tion that places vertical concrete in the
moderate-exposure category unless
it’s in direct contact with wet soil or
deicing salts. However, the abbreviated
definition of moderate exposure in
ACI 318 doesn’t make it clear that if
there is no exposure to wet soils or
deicing salts, moderate exposure is the
correct category.
ACI 301 is confusing
Unfortunately, both ACI 301-96 and
301-99 (Ref. 7) have the same language
for specifying the total air content for
concrete. Section 4.2.2.4 of ACI 301-99
states the following.
“Air content: Unless otherwise spec-
ified, concrete shall be air-entrained.
Unless otherwise specified, air content
at the point of delivery shall conform to
the requirements for severe exposure.”
ACI 301 contains the definitions for
moderate and severe exposures in the
Optional Requirements Checklist to
the Architect/Engineer at the end of the
document. This checklist is not includ-
ed as part of the specification.
Many engineers and architects refer-
ence ACI 301 without additional air
content information. Section 4.2.2.4
then requires all concrete (interior and
exterior) to be air-entrained at the
amount required for severe exposure.
Obviously, it would be preferable to
include more detailed air-content
information in the specification that
would allow interior concrete to be
non-air-entrained and exterior verti-
cal concrete to have air contents that
fit the moderate-exposure category.
The high cost of air
We usually hear that air is free. But
is it? ACI 211.1-91 states that “the use
of normal amounts of air entrainment
in concrete with a specified strength
near or about 5000 psi may not be pos-
sible due to the fact that each added
percent of air lowers the maximum
strength obtainable with a given com-
bination of materials.” As the air con-
tent increases, therefore, producers add
more cement to offset the strength
reduction.
For strand release, most prestressed
concrete producers need 4000- to
5000-psi compressive strengths in 24
hours.The 28-day compressive strengths
range from 6000 to 8000 psi. As a gener-
al rule, 1 percent of air reduces 28-day
compressive strength by about 5.0%.
The 1.5% increase in air content needed
to meet requirements for severe instead
A walk in the park proves our point
T
hanks to its location in
the low Upper Midwest
and its proximity to
Lake Michigan, which helps
cause temperatures to fluc-
tuate wildly, you’d be hard-
pressed to find a harsher
freeze/thaw environment for
concrete than Chicago. It’s
such a harsh environment
that you’d think precasters
would want to air-entrain all
exterior concrete, even if it
weren’t required, just for lia-
bility protection.
However, the condition of
70-year-old downtown con-
crete structures attests to
non-air-entrained concrete’s
durability in non-splash
zones. Leo Schlosberg,
owner and president of
Cary (Ill.) Concrete
Products, requested a petro-
graphic analysis of circa
1927 architectural concrete
at Grant Park as his compa-
ny prepared to do some
renovation work there in the
mid-1990s. The analysis
revealed that the non-air-
entrained exposed-aggregate
concrete New York pro-
ducer Benedict Stone
used to cast walkway rail-
ings and entryway
columns has a probable
water-cement ratio of
0.40 and about 760
pounds of cement per
yard.
Last summer, we took
Schlosberg back to Grant
Park to visually examine
several hundred of the
1920s railing balusters and
entryway columns. In the
rare cases where balusters
are deteriorating, the lower
railings invariably reveal
cracks from structural
stresses, most likely due to
settlement. (The park, locat-
ed on what used to be the
bottom of Lake Michigan,
now sits on lake fill.) “Once
stress cracks the concrete,
water gets in and damages
it,” notes Schlosberg. When
we examined the entryway
column panels, any cracking
we found was limited strict-
ly to corners, again proba-
bly due to stresses from
settlement. A couple of
panels had popouts and a
couple of inches of exposed
rebar, but the cover was
less than 1 inch thick in all
cases.
The unblemished surfaces
of both the balusters and the
entryway columns support
the belief that architects
need not automatically spec-
ify air entrainment for exteri-
or precast concrete.
— Don Talend
Leo Schlosberg, Cary Concrete Products: Rare cases of deterioration at
Grant Park did not originate from freeze/thaw damage.

of moderate exposure produces a
strength loss of about 450 psi. It takes
about 50 pounds of cement to avert
this loss.
For higher-strength concretes, the
strength loss due to unnecessarily high
air-content requirements increases, as
does the cost of increasing the cement
content to offset that loss.One produc-
er estimates that it costs an extra $2 per
cubic yard for concrete that meets the
air-content requirement for severe,
instead of moderate, exposure.
Don’t pay if there’s no problem
What happens when the air content
for vertical concrete is lower than spec-
ified? On behalf of the owner, the engi-
neer or architect may pursue one or
more of the following remedies:
s An extended warranty
s Application of sealers or coatings
to prevent saturation of concrete by
water
s A payment reduction to cover
potential future maintenance or repair
In the worst case, the engineer may
insist on removal and replacement of
the concrete.
These remedies may sometimes be
justified when the air-content require-
ments don’t meet those for moderate
exposure since the concrete may not
perform satisfactorily. However, if the
engineer has specified air contents for
severe exposure, when air content for a
moderate exposure is appropriate, a
warranty or pay reduction may not be
necessary or reasonable. The field and
laboratory data we’ve cited show that
vertical concrete with a low air content
can perform satisfactorily in a freeze-
thaw environment.
References
1. Armand Gustaferro, Marc A. Hillier, and
Jack R. Janney,“Performance of
Prestressed Concrete on the Illinois
Tollway After 25 Years of Service,”
Journal of the Prestressed Concrete
Institute, January–February, 1983.
2. A.W. Isberner, “Durability Studies of
Exposed Aggregate Panels,” Journal, PCA
Research and Development Laboratories,
Portland Cement Association, Skokie,
Ill., May 1963.
3. W.F. Perenchio and P. Klieger,“Some
Physical Properties of High Strength
Concrete,” PCA Bulletin RDO56,
Portland Cement Association,1978.
4. P. Klieger,“Some Aspects of Durability
and Volume Change of Concrete for
Prestressing,” Journal,PCA Research and
Development Laboratories, September
1960.
5. ACI 318-99,“Building Code
Requirements for Structural Concrete
and Commentary,” American Concrete
Institute, Farmington Hills, Mich.,1999.
6. ACI 211.1-91, “Standard Practice for
Selecting Proportions for Normal,
Heavyweight,and Mass Concrete,”
American Concrete Institute, 1999.
7. ACI 301-99, “Specification for Structural
Concrete,” American Concrete Institute,
1999.
Publication #J00J040
Copyright © 2000 Hanley-Wood, LLC
All rights reserved

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