Evening times-Republican. [volume] (Marshalltown, Iowa) 1890-1923, 1907-06-20

INST be filed nv the cement.

must be filled by the cement.
This is done by taking a cubic-foot
box and filling it with broken stone in
a thoroughly wet Estate. The box is
then filled with as much water as is
required to completely fill it in addi-
ton to the stone, which, upon being
poured off, gives the relation between
the volume of the voids and the vol
ume of the stone. The required amount
of local sand thus determined is then
measured out and placed in' the box
with the stone in damp state. Water
is then used to determine the percent
age of voids left in the sand, which
gives the approximate amount of ce-
ment required, altho an excess of ce-'
ment is invariably used. Engineers
everywhere differ regarding the best
proportion to be used, but in general
the above test, roughly made, will de-
termine it well enough.
The proportions which are most uni
versally used are as follows: l ce-
ment, 2 sand. 4. broken stone, where
extremely strong work is desired.
Tests show that a 6-inch thickness Of
1-2-4 concrete properly made is water
proof up to about so pounds to the
square inch. This concrete is fire.
quently used for facing dams; 1876
is the proportion generally used for the
interior of dams and large structures.
1t is entirely suitable for large foun-
dations: 1-4-8 is frequently used for
foundation work, and when properly
mixed makes good concrete, altho it
is about the limit Of what is consid-
cred good work, and would not be suit
able for very important structures. 1t
is well-known fact that the vol
Tune of concrete, when MIXED with wa
ter, is somewhat less than the volume
of the aggregates and cement before
mixing. The contractors rule is. that
the volume of mixed concrete is equal
to the volume of stone plus onehalf,
to one third the volume of the sand.
There has been much discussion
among engineers and others as to the
amount of water that should be added
to the aggregates and cement for max
ing the best concrete. Tie old way
was to add just enough water so that
when all the concrete was in the form
and tamped, it would show moisture
on the surface. The tamping is a very
Important part of the operation, and
the quality of the work is dependent
upon how we'll this is superintendent,
as unless it is we'll and thoroughly
done the concrete is liable to be honey
combed and imperfect, especially near
the forms.
With the growth of the use of con
crete the old method of putting it in
the forms nearly dry and depending on
tamping to consolidate 1t has been
more or less abandoned and the more
modern way Is to put the concrete in
quite wet, as less tampins is required
and much labor and expense saved
One Of the. great objections to this
scheme is that if care is rot taken the
water will tend to wash the cement
from the stone and sand: in other
words. unmix it. However, it may be
said that it is now generally UNDER
stood that rather wet concrete properly
handled makes better work. The
amount of water to be added to the
aggregates and cement varies from 1
water to cement by measurement, to
IL per cent of water by weight.
Quite frequently salt water is used
in mixing concrete in cold weather to
prevent freezing, and it seems to have
no i'll effects on the resulting mixture.