Civil Engineering Reference
In-Depth Information
FIGURE 7.11
Consolidating con-
crete with an internal vibrator.
increase the amount of mixing water in the concrete beyond that which is
specified in the mix design.
■
7.2.7
Measuring Air Content in Fresh Concrete
Mixing and handling can significantly alter the air content of fresh concrete.
Thus, field tests are used to ensure that the concrete has the proper air con-
tent prior to placing. Air content can be measured with the pressure, volu-
metric, gravimetric, or Chace air indicator methods.
The pressure method (ASTM C231) is widely used, since it takes less
time than the volumetric method. The pressure method is based on Boyle's
law, which relates pressure to volume. A calibrated cylinder (Figure 7.12) is
filled with fresh concrete. The vessel is capped and air pressure is applied.
The applied pressure compresses the air in the voids of the concrete. The vol-
ume of air voids is determined by measuring the amount of volume reduced
by the pressure applied. This method is not valid for concrete made with
lightweight aggregates, since air in the aggregate voids is also compressed,
confounding the measurement of the air content of the cement paste.
The volumetric method for determining air content (ASTM C173) can be
used for concrete made with any type of aggregate. The basic process in-
volves placing concrete in a fixed volume cylinder, as shown in Figure 7.13.
An equal volume of water is added to the container. Agitation of the con-
tainer allows the excess water to displace the air in the cement paste voids.
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