kept in view in adjusting the proportions of materials for block
concrete—strength, permeability, and cost. So far as strength goes, it
may easily be shown that concretes very poor in cement, as 1 to 8 or 1
to 10, will have a crushing resistance far beyond any load that they
may be called upon to sustain. Such concretes are, however, extremely
porous, and absorb water like a sponge. The blocks must bear a certain
amount of rough handling at the factory and while being carted to
work and set up in the wall. Safety in this respect calls for a much
greater degree of hardness than would be needed to bear the weight of
the building. Again, strength and hardness, with a given proportion of
cement, depend greatly on the character of the other materials used;
blocks made of cement and sand, 1 to 3, will not be so strong or so
impermeable to water as those made from a good mixed sand and gravel,
1 to 5. On the whole, it is doubtful whether blocks of satisfactory
quality can be made, by hand mixing and tamping, under ordinary factory
conditions, from a poorer mixture than 1 to 5. Even this proportion
requires for good results the use of properly graded sand and gravel or
screenings, a liberal amount of water, and thorough mixing and tamping.
When suitable gravel is not obtainable, and coarse mixed sand only is
used, the proportion should not be less than 1 to 4. Fine sand alone is
a very bad material, and good blocks cannot be made from it except by
the use of an amount of cement which would make the cost very high.
The mixtures above recommended, 1 to 4 and 1 to 5, will necessarily be
somewhat porous, and may be decidedly so if the gravel or screenings
used is not properly graded. The water-resisting qualities may be
greatly improved, without loss of strength, by replacing a part of the
cement by hydrate lime. This is a light, extremely fine material, and
a given weight of it goes much further than the {693} same amount of
cement in filling the pores of the concrete. It has also the effect
of making the wet mixture more plastic and more easily compacted by
ramming, and gives the finished blocks a lighter color.
The following mixtures, then, are to be recommended for concrete
blocks. By “gravel” is meant a suitable mixture of sand and gravel, or
stone screenings, containing grains of all sizes, from fine to 1/2
inch.
1 to 4 Mixtures, by Weight.
Cement, 150 parts; gravel, 600 parts.
Cement, 125 parts; hydrated lime, 25 parts; gravel, 600 parts.
Cement, 100 parts; hydrated lime, 50 parts; gravel, 600 parts.
1 to 5 Mixtures, by Weight.
Cement, 120 parts; gravel, 600 parts.
Cement, 100 parts; hydrated lime, 20 parts; gravel, 600 parts.
«Proportion of Water.»—This is a matter of the utmost consequence, and
has more effect on the quality of the work than is generally supposed.
Blocks made from too dry concrete will always remain soft and weak,
no matter how thoroughly sprinkled afterwards. On the other hand, if
blocks are to be removed from the machine as soon as made, too much
water will cause them to stick to the plates and sag out of shape. It
is perfectly possible, however, to give the concrete enough water for
maximum density and first-class hardening properties, and still to
remove the blocks at once from the mold. A good proportion of coarse
material allows the mixture to be made wetter without sticking or
sagging. Use of plenty of water vastly improves the strength, hardness,
and waterproof qualities of blocks, and makes them decidedly lighter in
color. The rule should be:
Use as much water as possible without causing the blocks to stick to
the plates or to sag out of shape on removing from the machine.
The amount of water required to produce this result varies with the
materials used, but is generally from 8 to 9 per cent of the weight
of the dry mixture. A practiced blockmaker can judge closely when
the right amount of water has been added, by squeezing some of the
mixture in the hand. Very slight variations in proportion of water make
such a marked difference in the quality and color of the blocks that
the water, when the proper quantity for the materials used has been
determined, should always be accurately measured out for each batch. In
this way much time is saved and uncertainty avoided.
«Facing.»—Some blockmakers put on a facing of richer and finer mixture,
making the body of the block of poorer and coarser material. As will
be explained later, the advantage of the practice is, in most cases,
questionable, but facings may serve a good purpose in case a colored or
specially waterproof surface is required. Facings are generally made
of cement and sand, or fine screenings, passing a 1/8-inch sieve.
To get the same hardness and strength as a 1 to 5 gravel mixture, at
least as rich a facing as 1 to 3 will be found necessary. Probably 1
to 2 will be found better, and if one-third the cement be replaced by
hydrate lime the waterproof qualities and appearance of the blocks will
be improved. A richer facing than 1 to 2 is liable to show greater
shrinkage than the body of the block, and to adhere imperfectly or
develop hair-cracks in consequence.
«Poured Work.»—The above suggestions on the question of proportions of
cement, sand, and gravel for tamped blocks apply equally to concrete
made very wet, poured into the mold, and allowed to harden a day or
longer before removing. Castings in a sand mold are made by the use of
very liquid concrete; sand and gravel settle out too rapidly from such
thin mixtures, and rather fine limestone screenings are generally used.
«Mixing.»—To get the full benefit of the cement used it is necessary
that all the materials shall be very thoroughly mixed together. The
strength of the block as a whole will be only as great as that of its
weakest part, and it is the height of folly, after putting a liberal
measure of cement, to so slight the mixing as to get no better result
than half as much cement, properly mixed, would have given. The poor,
shoddy, and crumbly blocks turned out by many small-scale makers owe
their faults chiefly to careless mixing and use of too little water,
rather than to too small proportion of cement.
The materials should be mixed dry, until the cement is uniformly
distributed and perfectly mingled with the sand and gravel or
screenings; then the water is to be added and the mixing continued
until all parts of the mass are equally moist and every particle is
coated with the cement paste.
«Concrete Mixers.»—Hand mixing is always imperfect, laborious, and
slow, {694} and it is impossible by this method to secure the thorough
stirring and kneading action which a good mixing machine gives. If a
machine taking 5 or 10 horse-power requires 5 minutes to mix one-third
of a yard of concrete, it is of course absurd to expect that two men
will do the same work by hand in the same time. And the machine never
gets tired or shirks if not constantly urged, as it is the nature of
men to do. It is hard to see how the manufacture of concrete blocks
can be successfully carried on without a concrete mixer. Even for a
small business it will pay well in economy of labor and excellence of
work to install such a machine, which may be driven by a small electric
motor or gasoline engine. In work necessarily so exact as this,
requiring perfectly uniform mixtures and use of a constant percentage
of water, batch mixers, which take a measured quantity of material,
mix it, and discharge it, at each operation, are the only satisfactory
type, and continuous mixers are unsuitable. Those of the pug-mill
type, consisting of an open trough with revolving paddles and bottom
discharge, are positive and thorough in their action, and permit the
whole operation to be watched and controlled. They should be provided
with extensible arms of chilled iron, which can be lengthened as the
ends become worn.
«Concrete Block Systems.»—For smaller and less costly buildings,
_separate blocks_, made at the factory and built up into the walls
in the same manner as brick or blocks of stone, are simpler, less
expensive, and much more rapid in construction than monolithic work.
They also avoid some of the faults to which solid concrete work, unless
skillfully done, is subject, such as the formation of shrinkage cracks.
There are two systems of block making, differing in the consistency of
the concrete used:
1. Blocks tamped or pressed from semi-wet concrete, and removed at once
from the mold.
2. Blocks poured or tamped from wet concrete, and allowed to remain in
the mold until hardened.
«Tamped Blocks from Semi-Wet Mixture.»—These are practically always
made on a block machine, so arranged that as soon as a block is formed
the cores and side plates are removed and the block lifted from the
machine. By far the larger part of the blocks on the market are made in
this way. Usually these are of the one-piece type, in which a single
block, provided with hollow cores, makes the whole thickness of the
wall. Another plan is the _two-piece_ system, in which the face and
back of the wall are made up of different blocks, so lapping over each
other as to give a bond and hold the wall together. Blocks of the
two-piece type are generally formed in a hand or hydraulic press.
Various shapes and sizes of blocks are commonly made; the builders of
the most popular machines have, however, adopted the standard length
of 32 inches and height of 9 inches for the full-sized block, with
thickness of 8, 10, and 12 inches. Lengths of 24, 16, and 8 inches are
also obtained on the same machines by the use of parting plates and
suitably divided face plates; any intermediate lengths and any desired
heights may be produced by simple adjustments or blocking off.
Blocks are commonly made plain, rock-faced, tool-faced, paneled, and of
various ornamental patterns. New designs of face plates are constantly
being added by the most progressive machine makers.
«Block Machines.»—There are many good machines on the market, most
of which are of the same general type, and differ only in mechanical
details. They may be divided into two classes: those with vertical
and those with horizontal face. In the former the face plate stands
vertically, and the block is simply lifted from the machine on its
base plate as soon as tamped. In the other type the face plate forms
the bottom of the mold; the cores are withdrawn horizontally, and by
the motion of a lever the block with its face plate is tipped up into
a vertical position for removal. In case it is desired to put a facing
on the blocks, machines of the horizontal-face type are considered
the more convenient, though a facing may easily be put on with the
vertical-face machine by the use of a parting plate.
«Blocks Poured from Wet Concrete.»—As already stated, concrete made
too dry is practically worthless, and an excess of water is better
than a deficiency. The above-described machine process, in which
blocks are tamped from damp concrete and at once removed, gives blocks
of admirable hardness and quality if the maximum of water is used. A
method of making blocks from very wet concrete, by the use of a large
number of separable molds of sheet steel, into which the wet concrete
is poured and in which the blocks are left to harden for 24 {695} hours
or longer, has come into considerable use. By this method blocks of
excellent hardening and resistance to water are certainly obtained.
Whether the process is the equal of the ordinary machine method in
respect of economy and beauty of product must be left to the decision
of those who have had actual experience with it.
The well-known cast-stone process consists in pouring liquid concrete
mixture into a sand mold made from a pattern in a manner similar to
that in which molds for iron castings are produced. The sand absorbs
the surplus water from the liquid mixture, and the casting is left in
the mold for 24 hours or longer until thoroughly set. This process
necessitates the making of a new sand mold for every casting, and
is necessarily much less rapid than the machine method. It is less
extensively used for building blocks than for special ornamental
architectural work, sills, lintels, columns, capitals, etc., and for
purposes of this kind it turns out products of the highest quality and
beauty.
«Tamping of Concrete Blocks.»—This is generally done by means of hand
rammers. Pneumatic tampers, operated by an air compressor, are in use
at a few plants, apparently with considerable saving in time and labor
and improvements in quality of work. Hand tamping must be conscientious
and thorough, or poor work will result. It is important that the mold
should be filled a little at a time, tamping after each addition; at
least four fillings and tampings should be given to each block. If
the mixture is wet enough no noticeable layers will be formed by this
process.
«Hardening and Storage.»—Triple-decked cars to receive the blocks from
the machines will be found a great saving of labor, and are essential
in factories of considerable size. Blocks will generally require to
be left on the plates for at least 24 hours, and must then be kept
under roof, in a well-warmed room, with frequent sprinkling, for not
less than 5 days more. They may then be piled up out of doors, and
in dry weather should be wetted daily with a hose. Alternate wetting
and drying is especially favorable for the hardening of cement,
and concrete so treated gains much greater strength than if kept
continuously in water or dry air.
Blocks should not be used in building until at least 4 weeks from the
time they are made. During this period of seasoning, blocks will be
found to shrink at least 1/16 inch in length, and if built up in a
wall when freshly made, shrinkage cracks in the joints or across the
blocks will surely appear.
Efflorescence, or the appearance of a white coating on the surfaces,
sometimes takes place when blocks are repeatedly saturated with water
and then dried out; blocks laid on the ground are more liable to show
this defect. It results from diffusion of soluble sulphates of lime and
alkalies to the surface. It tends to disappear in time, and rarely is
sufficient in amount to cause any complaint.
«Properties of Concrete Blocks»—Strength.—In the use of concrete blocks
for the walls of buildings, the stress to which they are subjected is
almost entirely one of compression. In compressive strength well-made
concrete does not differ greatly from ordinary building stone. It
is difficult to find reliable records of tests of sand and gravel
concrete, 1 to 4 and 1 to 5, such as is used in making blocks; the
following figures show strength of concrete of approximately this
richness, also the average of several samples each of well-known
building stones, as stated by the authorities named:
Limestone, Bedford, Ind. (Indiana Geographical Survey) 7,792 pounds
Limestone, Marblehead, Ohio (Q. A. Gillmore) 7,393 pounds
Sandstone, N. Amherst, Ohio (Q. A. Gillmore) 5,831 pounds
Gravel concrete, 1:1.6:2.8, at 1 year (Candlot) 5,500 pounds
Gravel concrete, 1:1.6:3.7, at 1 year (Candlot) 5,050 pounds
Stone concrete, 1:2:4 at 1 year (Boston El. R. R.) 3,904 pounds
Actual tests of compression strength of hollow concrete blocks are
difficult to make, because it is almost impossible to apply the load
uniformly over the whole surface, and also because a block 16 inches
long and 8 inches wide will bear a load of 150,000 to 200,000 pounds,
or more than the capacity of any but the largest testing machines.
Three one-quarter blocks, 8 inches long, 8 inches wide, and 9 inches
high, with hollow space equal to one-third of the surface, tested at
the Case School of Science, showed strengths of 1,805, 2,000, and {696}
1,530 pounds per square inch, respectively, when 10 weeks old.
Two blocks 6 x 8 x 9 inches, 22 months old, showed crushing strength
of 2,530 and 2,610 pounds per square inch. These blocks were made of
cement 1 1/4 parts, lime 1/2 part, sand and gravel 6 parts, and
were tamped from damp mixture. It is probably safe to assume that the
minimum crushing strength of well-made blocks, 1 to 5, is 1,000 pounds
per square inch at 1 month and 2,000 pounds at 1 year.
A block 12 inches wide and 24 inches long has a total surface of 288
square inches, or, deducting 1/3 for openings, a net area of 192
inches. Such a block, 9 inches high, weighs 130 pounds. Assuming a
strength of 1,000 pounds and a factor of safety of 5, the safe load
would be 200 pounds per square inch, or 200 × 192 = 38,400 pounds for
the whole surface of the block. Dividing this by the weight of the
block, 130 pounds, we find that 295 such blocks could be placed one
upon another, making a total height of wall of 222 feet, and still the
pressure on the lowest block would be less than one-fifth of what it
would actually bear. This shows how greatly the strength of concrete
blocks exceeds any demands that are ever made upon it in ordinary
building construction.
The safe load above assumed, 200 pounds, seems low enough to guard
against any possible failure. In Taylor and Thompson’s work on
concrete, a safe load of 450 pounds for concrete 1 to 2 to 4 is
recommended; this allows a factor of safety of 5 1/2. On the other
hand, the Building Code of the city of Cleveland permits concrete to
be loaded only to 150 pounds per square inch, and limits the height of
walls of 12-inch blocks to 44 feet. The pressure of such a wall would
be only 40 pounds per square inch; adding the weight of two floors at
25 pounds per square foot each, and roof with snow and wind pressure,
40 pounds per square foot, we find that with a span of 25 feet the
total weight on the lowest blocks would be only 52 pounds per square
inch, or about one-twentieth of their minimum compression strength.
Blocks with openings equal to only one-third the surface, as required
in many city regulations, are heavy to handle, especially for walls
12 inches and more in thickness, and, as the above figures show, are
enormously stronger than there is any need of. Blocks with openings of
50 per cent would be far more acceptable to the building trade, and if
used in walls not over 44 feet high, with floors and roof calculated as
above for 25 feet span, would be loaded only to 56 pounds per square
inch of actual surface. This would give a factor of safety of 18,
assuming a minimum compression strength of 1,000 pounds.
There is no doubt that blocks with one-third opening are inconveniently
and unnecessarily heavy. Such a block, 32 inches long, 12 inches wide,
and 9 inches high, has walls about 3 1/2 inches thick, and weighs
180 pounds. A block with 50 per cent open space would have walls and
partitions 2 inches in thickness, and would weigh about 130 pounds.
With proper care in manufacture, especially by using as much water as
possible, blocks with this thickness of walls may be made thoroughly
strong, sound, and durable. It is certainly better for strength and
water-resisting qualities to make thin-walled blocks of rich mixture,
rather than heavy blocks of poor and porous material.
Filling the voids with cement is a rather expensive method of securing
waterproof qualities, and gives stronger concretes than are needed.
The same may be accomplished more cheaply by replacing part of the
cement by slaked lime, which is an extremely fine-grained material, and
therefore very effective in closing pores. Hydrate lime is the most
convenient material to use, but nearly as costly as Portland cement
at present prices. A 1 to 4 mixture in which one-third the cement
is replaced by hydrate lime will be found equal to a 1 to 3 mixture
without the lime. A 1 to 4 concrete made from cement, 1; hydrate
lime, 1/2; sand and gravel, 6 (by weight), will be found fairly
water-tight, and much superior in this respect to one of the same
richness consisting of cement, 1 1/2; sand and gravel, 6.
The cost of lime may be greatly reduced by using ordinary lump lime
slaked to a paste. The lime must, however, be very thoroughly hydrated,
so that no unslaked fragments may remain to make trouble by subsequent
expansion. Lime paste is also very difficult to mix, and can be used
successfully only in a concrete mixer of the pug-mill type. Ordinary
stiff lime paste contains about 50 per cent water; twice as much of it,
by weight, should therefore be used as of dry hydrate lime.
«Waterproof Qualities.»—The chief fault of concrete building blocks,
as ordinarily made, is their tendency to absorb water. In this respect
they are generally no {697} worse than sandstone or common brick; it
is well known that stone or brick walls are too permeable to allow
plastering directly on the inside surface, and must be furred and
lathed before plastering, to avoid dampness. This practice is generally
followed with concrete blocks, but their use and popularity would be
greatly increased if they were made sufficiently waterproof to allow
plastering directly on the inside surface.
For this purpose it is not necessary that blocks should be perfectly
waterproof, but only that the absorption of water shall be _slow_,
so that it may penetrate only part way through the wall during a
long-continued rain. Walls made entirely water-tight are, in fact,
objectionable, owing to their tendency to “sweat” from condensation
of moisture on the inside surface. For health and comfort, walls must
be slightly porous, so that any moisture formed on the inside may be
gradually absorbed and carried away.
Excessive water absorption may be avoided in the following ways:
1. Use of Properly Graded Materials.—It has been shown by Feret and
others that porosity and permeability are two different things;
porosity is the total proportion of voids or open spaces in the mass,
while permeability is the rate at which water, under a given pressure,
will pass through it. Permeability depends on the _size_ of the
openings as well as on their total amount. In two masses of the same
porosity or percentage of voids, one consisting of coarse and the other
of fine particles, the permeability will be greater in the case of the
coarse material. The least permeability, and also the least porosity,
are, however, obtained by use of a suitable mixture of coarse and fine
particles. Properly graded gravel or screenings, containing plenty of
coarse fragments and also enough fine material to fill up the pores,
will be found to give a much less permeable concrete than fine or
coarse sand used alone.
2. Use of Rich Mixtures.—All concretes are somewhat permeable by water
under sufficient pressure. Mixtures rich in cement are of course much
less permeable than poorer mixtures. If the amount of cement used is
more than sufficient to fill the voids in the sand and gravel, a very
dense concrete is obtained, into which the penetration of water is
extremely slow. The permeability also decreases considerably with age,
owing to the gradual crystallization of the cement in the pores, so
that concrete which is at first quite absorbent may become practically
impermeable after exposure to weather for a few weeks or months. There
appears to be a very decided increase in permeability when the cement
is reduced below the amount necessary to fill the voids. For example,
a well-mixed sand and gravel weighing 123 pounds per cubic foot, and
therefore containing 25 per cent voids, will give a fairly impermeable
concrete in mixtures up to 1 to 4, but with less cement will be found
quite absorbent. A gravel with only 20 per cent voids would give about
equally good results with a 1 to 5 mixture; such gravel is, however,
rarely met with in practice. On the other hand, the best sand, mixed
fine and coarse, seldom contains less than 33 per cent voids, and
concrete made from such material will prove permeable if poorer than 1
to 3.
3. Use of a Facing.—Penetration of water may be effectively prevented
by giving the blocks a facing of richer mixture than the body. For
the sake of smooth appearance, facings are generally made of cement
and fine sand, and it is often noticed that these do not harden well.
It should be remembered that a 1 to 3 sand mixture is no stronger and
little if any better in water absorption than a 1 to 5 mixture of
well-graded sand and gravel. To secure good hardness and resistance to
moisture a facing as rich as 1 to 2 should be used.