Humboldt H-2795 Roll-a-Meter - Manual (Page 3)

The Solution
The accuracy of Roll-a-Meter results is not dependent on the correctness of all
these factors, but gives directly the percent air in the sample. It is unnecessary
to know anything about the weight or physical characteristics of the ingredients
which are supposed to be in the mix.
In contrast to other methods commonly used to determine the percentage
of entrained air, this method is unaffected by changes in water cement ratio,
sand cement ratio, sand to gravel ratios, inaccuracies of specific gravity
determinations, and uncertainties as to absorbed or free water content of
the aggregates used in the mix.
The use of the Roll-a-Meter has eliminated practically all of the above listed
work, together with the arduous computations and uncertainties Involved.
Even the extremely accurate measurement of the sample to be tested is
not as important with this new meter, as the resulting error would be only about
1/20 as great in using the meter as the same error would be when
using other methods or meters. Only a small percentage of the original error
is involved in the air-meter result.
Used as a pycnometer, the Roll-a-Meter has been found to be excellent for
other tests, such as determination of specific gravities of cement, sand, gravel
and admixtures, and for quickly obtaining the percentage of free water in damp
sand and gravel.
Practical Control of Entrained Air
Held within well-established limits, air entrainment is highly beneficial In many
ways. With ordinary highway or building construction using 1-1/2" to 2"
maximum aggregate, a maximum of about 4% air is usually desirable.
Consequently, it is fundamentally important that an accurate method of
determining the percentage of air be available. Having this, the amount of
air entraining agent to be used, under any of the infinitely variable mix
combinations and placing conditions can be quickly determined and
effectively controlled.
Why the unit weight or gravimetric
method is unsatisfactory
The unit weight method of determining the volume of entrained air is tedious
and often impractical and unreliable. It involves technical manipulations and
computations, which can readily lead to serious errors.
General information
The accuracy of the unit weight method is dependent on extreme accuracy of
several Coronary technical determinations. Among these are:
• specific gravity of cement.
• specific gravity of sand and graver.
• absorption of sand and gravel.
• average free moisture in sand and gravel.