PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 264

FIGURE 7.14 Chace air indicator.

the specific gravity of each ingredient. This method requires very accurate
specific gravity measurements, and thus is more suited to the laboratory
rather than the field.
The Chace air indicator test (AASHTO T199) is a quick method used to
determine the air content of freshly mixed concrete. The device consists of
a small glass tube with a stem, a rubber stopper, and a metal cup mounted
on the stopper, as shown in Figure 7.14. The metal cup is filled with cement
mortar from the concrete to be tested. The indicator is filled with alcohol to
a specified level, and the stopper is inserted into the indicator. The indica-
tor is then closed with a finger and gently rolled and tapped until all of the
mortar is dispersed in the alcohol and all of the air is displaced with alco-
hol. With the indicator held in a vertical position, the alcohol level in the
stem is read. This reading is then adjusted using calibration tables or figures
to determine the air content. The Chace air indicator test can be used to
rapidly monitor air content, but it is not precise, nor does it have the re-
peatability required for specification control. It is especially useful for mea-
suring the air content of small areas near the surface that may have lost air
content by improper finishing.
These methods of measuring air content determine the total amount of
air, including entrapped air and entrained air, as well as air voids in aggre-
gate particles. Only minute bubbles produced by air-entraining agents im-
part durability to the concrete. However, the current state of the art is unable
to distinguish between the types of air in fresh concrete.

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7.2.8 Spreading and Finishing Concrete

Different methods are available to spread and finish concrete, depending on
the nature of the structure and the available equipment. Tools and equip-
ment used for spreading and finishing concrete include hand floats, power
floats, darbies, bullfloats, straightedges, trowels, vibratory screed, and slip
forms. (See Figures 7.8, 7.15-7.18).