Air-Entrained Concrete - Design and Control of Concrete Mixtures - page 142

The air-void characteristics of hardened concrete can
be determined by ASTM C 457 methods. This test is used
to determine void spacing factor, specific surface of voids,
and number of voids per length of traverse.

Air-Void Analysis of Fresh Concrete

The conventional methods for analyzing air in fresh con-
crete, such as the pressure method noted above, measure
the total air content only; consequently, they provide no
information about the parameters that determine the
quality of the air-void system. These parameters—the size
and number of voids and spacing between them—can be
measured on polished samples of hardened concrete
(ASTM C 457); but the result of such analysis will only be
available several days after the concrete has hardened.
Therefore, test equipment called an air-void analyzer
(AVA) has been developed to determine the standard
ASTM C 457 air-void parameters in fresh samples of air-
entrained concrete (Fig. 8-23). The test apparatus deter-
mines the volume and size distributions of entrained air
voids; thus an estimation of the spacing factor, specific
surface, and total amount of entrained air can be made.
In this test method, air bubbles from a sample of fresh
concrete rise through a viscous liquid, enter a column of
water above it, then rise through the water and collect
under a submerged buoyancy recorder (Fig. 8-24). The vis-
cous liquid retains the original bubble sizes. Large bubbles

Fig. 8-24. Air bubbles rising through liquids in column.
(67962)

rise faster than small ones through the liquids. The change
in buoyancy is recorded as a function of time and can be
related to the number of bubbles of different size.
Fresh concrete samples can be taken at the ready mix
plant and on the jobsite. Testing concrete before and after
placement into forms can verify how the applied methods
of transporting, placing, and consolidation affect the air-
void system. Since the samples are taken on fresh con-
crete, the air content and air-void system can be adjusted
during production.
Currently, no standard exists for this method. The
AVA was not developed for measuring the total air-con-
tent of concrete, and because of the small sample size, may
not give accurate results for this quantity. However, this
does not mean the AVA is not useful as a method for
assessing the quality of the air-void system; it gives good
results in conjunction with traditional methods for meas-
uring air content ( Aarre 1998 ).

RECOMMENDED AIR CONTENTS

The amount of air to be used in air-entrained concrete
depends on a number of factors: (1) type of structure,
(2) climatic conditions, (3) number of freeze-thaw cycles,
(4) extent of exposure to deicers, and (5) the design life of
the structure.
The ACI 318 building code states that concrete that
will be exposed to moist freezing and thawing or deicer
chemicals shall be air entrained with the target air content
of Table 8-6 for severe exposure. Furthermore, the water
to cementitious materials ratio should not exceed 0.45.
ACI 318 allows a one percentage point reduction in target
air contents for concretes with strengths over 34 MPa
(5,000 psi) and presumably very low water-cement ratios.

Fig. 8-23. Equipment for the air-void analyzer. (67961)