Humboldt H-2795 Roll-a-Meter - Manual (Page 5)

Important
Care should be taken to have approximately the same proportions of mortar and
course aggregate as are used in the mix. Larger than 2-inch aggregate should be
discarded and air determinations made on the balance. After proper agitation, the
air, accompanied by some foam, rises to the top. This should be allowed to stand,
with occasional light agitation until the bubbles practically cease rising. This may
take 3 to 5 minutes, although an immediate reading will tell whether there is any
material excess or deficiency of air. Following this, far closer results if desired,
two general steps are possible:
Method A
The foam may be dispelled by adding 23ml. of IsoPropyl alcohol (rubbing alcohol)
in a special brass cup provided with each meter. This 23ml. is sufficient to reduce
the air reading 1% (decreased air due to the added alcohol) will be the correct
percentage of the air in the test batch.
Method B
Numerous tests made by Method A indicate that usually the true reading should
be 85 to 90 percent of the primary reading before defoaming. This is often
sufficiently accurate for routine control purposes, but may be confirmed or
modified by a few (A) tests. The water, the kind of air entraining agent used,
as well as the brand of cement and the type of sand, gravel or admixture, may
alter the above factor.
Value of Entrainment
This is probably the greatest new development in concrete in this generation.
Tests indicate that correctly controlled air entrainment will increase the durability
of concrete under severe exposure several hundred percent. The careful control
of the air to about 4% of the volume is considered ideal for average 1-1/2" to 2"
maximum concrete. Beyond this point the concrete strength is rapidly reduced.
Air entrainment increases the placability of concrete, and aids in preventing
segregation. When air is entrained, the water may be reduced, which aids in
keeping the strength high.
With air entrainment, coarser sand my be used or less of the regular sand, which
again helps to lower the water and maintain high strengths. Sand may be in-
creased in coarseness from .3 to .5 above normal fineness. Modulus when proper
air entrainment is used. Tests indicate that the troubles caused by premature
stiffening of cement may be materially decreased by the use of the ideal amount
of air. Resistance to the deleterious action of sulphate waters may be increased
with air entrainment. Many contractors are voluntarily using air entraining agent
because of the great improvement in placability, prevention of bleeding, and
reduction in segregation accompanying its use, even where the extra durability is
not required.
Where is air entrainment desirable?
Any job where the concrete is to be exposed to weathering will benefit by the
use of an air entraining agent. To avoid loss in strength due to too much air, which
may be serious if over 5 or 6 percent, frequent routine tests of entrained air will