The Minneapolis journal. [volume] (Minneapolis, Minn.) 1888-1939, 1906-04-15

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stick to the work until he has learned
it thoroly, as I am sure can be done by
the earnest worker.
..-. Questions and Answers.
~ MIXING CONCRETE FOR BLOCKS.
In small plant where tamping is done by
hand, is continuous mixing machine the best
and most economical method Of preparing the
concrete for the workmen? Il. NJ. j.,
Shell Lake, Wis.
Answer-Thoro mixing and use Of an
exact and uniform proportion Of water
are necessary for the production Of
good block, and a continuous mixer is
not suited to the purpose. Batch mixers,
supplied with measured quantities of ma
terials, mixed the required time and pro-
vided with the proper percentage Of
water, will give much better results.
Batch mixers are very much to be pre-
ferred to hand mixing by the use Of
hoe or shovel, since the latter must be
the full effect Of the cement.
In calculating the percentage of water neces-
sary to form good block, is the quantity taken
by measure by weight? Also a good
block be made by pouring into the mold and
dispensing with the tamping? H. H. s.,
Answer-The use of a proper amount
Of water is essential to produce good
block, and on this point many block
makers are careless. It is not a dis
puted point that fairly wet concrete
is far better than dry mixture, and
too much water is better than too little.
AS to how much water should be used,
the percentage should be 8 or 9 per
cent of the mixture by weight, and not
by measure. It will vary slightly &C.
oor1ircr To ho TO As tho nht
Of one gallon of water is about eight
pounds, the quantity is easily. calcu-
lated. a.,.',. X. :.
AS to pouring concrete into molds,
cannot see why anyone should want to
do this in the making Of blocks, for they
would present poor appearance when
the forms were removed. Pouring is all
in order to fill all voids around the
iron in the wall, and which could not
we'll be filed by tamping dry concrete
into the forms. The best plan for the
blockmakers is to avoid the extremes of
wet and dry processes, and get his mix
ture just right in all respects. 49,.
How soon may blocks he placed in wall after
tamping7 1t is said time may be saved by lay
ing up in the wall at the end of ten days. and
allowing the curing to take place there. =.
Twentieth Ave. S., City.
Answer-For good work blocks must
be kept under roof for at least a. week
or ten days. Sun and wind having free
access to freshly made blocks during
the first week, even tho frequently
allowing them to cure in the wall at the
expiration of week after making, and
before seasoning is completed, it would
result in opening the joints, for block
will shrink not less than one-sixteenth
Of an inch in one months seasoning,
and the shrinkage will show in the YER
tical joints.
What should be the crushing strength Of
two-story building of nine-inch blocks for the
first story? $. p. F.:
Carson. Iowa.
Answer-It is not easy to obtain re-
liable crushing tests when the actual
block is objected to pressure, as the
load cannot be applied uniformly over
the whole surface. Good concrete in
stfengh Fl 266 f pounds
per square inch when one month old,
and when one year old it will resist q
pressure of more than 5000 pounds. We
can approach the matter in this way:
A well cured block 9x9x24 inches weighs
about eighty-fve pounds and has sur
face Of about 1919 inches, excluding hol-
low spaces. If the crushing strength
at the end of one month be 1000 pounds
per square inch, or, to simplify the cal
culation, each square inch will sustain
should carry 100 tons before crushing
| under the weight, and this would be
| the equivalent Of 3,035 blocks resting
upon the lower block, or 1924 feet high.
| Allowing a. factor of safety Of five, a
| height of 405 feet could be reached,
| which shows that the strength far ex.
ceeds any demands which could be made
upon it. -.:.t -...-... - ...~.. 81, -t...
! Can I build a foundation wall Of solid con
crete, twelve inches thick, in which the porocity
or permeability is SO reduced as to make the
wall damp-proof and thus secure dry cellar?
! ..,.. .,.-. III. "A. c.,
| - Nora Springs, Iowa.
Answer-This question brings out
once more the confusion Of terms which
have a widely different meaning, and
using them as tho they are Synonymous.
The "porocity' Of concrete is the per-
centage of voids or empty spaces within
the mass, while the permeability is the
rate of speed with which water, under
certain pressure, will pass thru it. All
concretes are more or less porous, and
! all are, to some extent, .permeable by
water under heavy pressure. With the
same proportions Of cement, mixtures Of
fine sand are more porous than those of
coarse sand. The permeability Of the
/ fine sand mixture is also much greater
/ than the coarse sand mixture. Porocity
means the total amount of voids con-
/ tanned in block or wall; permeability
depends upon the size Of the individual
openings. 1t is easy to see that the
| least porous concrete may be the most
permeable, and the reverse of this will
| also be true.
i This is the reason why we advise GER
/ tain proportions of fine and coarse sand
/ with the still cOarser gravel or crushed
/ monolith. The best concrete is that
which is most dense, provided suffi-
| Gent amount of fine grains Of sand are
i present.
It is safe to assume. therefore, that