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

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possessed of sufficient persistence to
stick to the work until he has learned
it thOroly, as / am sure can be done by
the earnest worker.
| <I~. Questions and Answers.
| 49, 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 workmen2 Il. M j.
Shell Lake, Wis.
Answer-Thoro mixing and use Of an
exact and uniform proportion of water
are necessary for the production Of a
good block, and continuous mixer il
not suited to the purpose. Batch mixers,
supplied with measured quantities of ma
ter1als, 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 a
hoe or shovel, since the latter must be
vigorous and long-continued to obtain
sary to form good block, is the quantity taken
by measure by weight? Also can good
block be made by pouring into the mold and
dispensing with the tampng? H. H. s.,
Eureka, S. D.
Answer-The use of proper amount
of water is essential to produce good
block, and on this point many block
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.
cording to the aggregate. AS the weight
OF one gallon Of water is about eight
pounds, the quantity is easily calcu-
lated.
AS to pouring concrete into molds, I
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
right enough in reinforced construction
in order to fill all voids around the
iron in the wall, and which could not
well be filled by tamping dry concrete
into the forms. The, best plan for the
sma a1 is to AVOID the extremes Of
ture just right in all respects. s..-...
How soon may blocks be placed in a wall after
tamping? 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. H..
Il. L. O.,
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
sprinkled, will prove Injurious. AS for
allowing them to cure in the wall at the
expiration Of a 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.
teal joints.
What should be the crushing strength Of
good concrete block and will it be safe to erect
a two-story building Of nine-inch blocks for the
first story? s. f. f.
Carson, Iowa.
Answer-It is not easy to obtain re-
liable crushing tests when the actual
block is subjected to pressure, as the
load cannot be applied uniformly over
the whole Surface. Good concrete in
proportions Of one to five will have q
compression strength Of 1000 pounds
per square inch when one month old,
and when one year old it will resist a
pressure of more than 5000 pounds. We
can approach the matter in this way:
A well-cured block 8x9x24 inches weighs
about eighty-6ve 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
each square inch will sustain
q pressure of one ton,s then this block
should carry 129 tons before crushing
under the weight, and this would be
the equivalent Of 3,O35 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. D.. g....... d.
.rete. twelve inches thick, In which the porocity
n permeability Is 30 reduced as to make the !
wall damp-proof and thus secure dry cellar? |
~ --. ,-..F zs.- III. A. c.,
Nora Springs, Iowa.
Answer-This question brings out
once more the confusion Of terms which
nave a widely different meaning. and
Using them as tho they are synonymous.
The "porocityo of concrete is the per-
eentage of voids or empty spaces within
The mass, while the permeability is the
'ate Of speed with which water, under
! certain pressure, will pass thru it. All
,oncretes are more or less porous, and
LII are, to some extent, permeable by
water under heavy pressure. With the
zane proportions Of cement, mixtures Of
inc sand are more porous than those Of
coarse sand: The permeability Of the
inc sand mixture is also much greater
han the coarse sand mixture. Porocity
means the total amount Of voids con-
aimed in a block or wall; permeabiHty L
depends upon the size Of the individual
openings. 1t is easy to see that the
east porous concrete may be the most
,ermeable, and the reverse Of this will Il
slso be true.
This is the reason why we advise GER
ain proportions of fine and coarse sand
With the still .coarser gravel or crushed y
tone in making a good block, or
aonolith. The best concrete is that
which !. is most dense, provided suff-
lent amount of fine grains Of sand are
resent
4 is safe to assume. therefore, that