Civil Engineering Reference
In-Depth Information
Fig. 16-6. Volumetric air meter. (69886)
Fig. 16-5. Pressure-type meter for determining air content.
(69766)
normal-weight aggregates are relatively constant and,
though small, should be subtracted from the pressure meter
gauge reading to obtain the correct air content. The instru-
ment should be calibrated for various elevations above sea
level if it is to be used in localities having considerable
differences in elevation. Some meters utilize change in pres-
sure of a known volume of air and are not affected by
changes in elevation. Pressure meters are widely used
because the mix proportions and specific gravities of the
concrete ingredients need not be known. Also, a test can be
conducted in less time than is required for other methods.
The volumetric method (Fig. 16-6) outlined in ASTM
C 173 (AASHTO T 196) requires removal of air from a
known volume of concrete by agitating the concrete in an
excess of water. This method can be used for concrete con-
taining any type of aggregate, including lightweight or
porous materials. An aggregate correction factor is not
necessary with this test. The volumetric test is not affected
by atmospheric pressure, and specific gravity of the
concrete ingredients need not be known. Care must be
taken to agitate the sample sufficiently to remove all air.
The addition of 500 mL (1 pt) of alcohol accelerates the
removal of air, thus shortening test times; it also dispels
most of the foam and increases the precision of the test,
including those performed on high-air-content or high-
cement-content concretes.
The gravimetric method utilizes the same test equip-
ment used for determining the density (unit weight) of
concrete. The measured density of concrete is subtracted
from the theoretical density as determined from the abso-
lute volumes of the ingredients, assuming no air is present
(see ASTM C 138 or AASHTO T 121). This difference,
expressed as a percentage of the theoretical density, is the
air content. Mixture proportions and specific gravities of
the ingredients must be accurately known; otherwise
results may be in error. Consequently, this method is suit-
able only where laboratory-type control is exercised.
Significant changes in density can be a convenient way to
detect variability in air content.
A pocket-size air indicator (AASHTO T 199) can be
used as a quick check for the presence of low, medium, or
high levels of air in concrete, but it is not a substitute for
the other more accurate methods. A representative sample
of mortar from the concrete is placed in the container. The
container is then filled with alcohol and rolled with the
thumb over the open end to remove the air from the
mortar. The indicated air content is determined by
comparing the drop in the level of the alcohol with a cali-
bration chart. The test can be performed in a few minutes.
It is especially useful in checking for the presence of air in
concrete near the surface that may have suffered reduc-
tions in air because of faulty finishing procedures.
With any of the above methods, air-content tests
should be started within 5 minutes after the final portion of
the composite sample has been obtained.
Studies into the effect of fly ash on the air-void stability
of concrete have resulted in the development of the foam-
index test. The test can be used to measure the relative air-
entraining admixture requirements for concrete mixtures
containing fly ash. The fly ash to be tested is placed in a
wide mouth jar along with the air-entraining admixture
and shaken vigorously. Following a waiting period of 45
seconds, a visual determination of the stability of the foam
or bubbles is made (
Gebler and Klieger 1983
).
