_Bricks._—In considering the essential qualities of bricks for the
small house it must be appreciated that those bricks which are used on
the exterior must be able to resist the effects of weather and produce
the best artistic results, while those which are in the interior of
walls or chimney need not be held up to such rigid standards. The
determination of the resistance of bricks to frost and weather action
is quite simple. A brick which struck by a hammer gives a clear ring is
one which has been well burned and has no soft spots, cracks, or weak
places. Such a brick can be said to be satisfactory for exterior use,
provided that it has the proper form and color desired and is not so
overburned as to be twisted and warped. Another requirement sometimes
specified is that the face brick made from soft clay should not show
a percentage of absorption in excess of 15 per cent, and for the
stiff-moulded or dry-pressed bricks not more than 10 per cent. This,
however, cannot be a hard-and-fast rule, due to the variation of clays.
Certain red bricks, unless they are burned very hard, show, when
built into the wall, a very ugly white surface discoloration, called
“whitewash” or efflorescence. This is not entirely due to the brick,
since the mortar that is used may sometimes produce it. If it is due to
the brick it can be discovered before the brick is used in the wall, by
placing a sample brick on edge in a pan containing one inch of either
rain or distilled water. As the water is absorbed by the brick, the
white discoloration will develop on the top surface after several days
of standing if it contains the salts which will cause the whitewash.
Those bricks which have been very hard-burned will not discolor under
any circumstances. If after passing this test the brick wall should
develop whitewash, it can be laid to the mortar. In order to prevent
any such occurrence it is necessary to waterproof the joints around
window-sills and between the foundations and the wall, so that the
minimum amount of water will be soaked up into the wall when it rains.
An expensive addition of 2 per cent of barium carbonate to the mortar
will tend to fix the soluble salts which cause this efflorescence.
[Illustration: Method of testing a sample brick to see whether it will
have a tendency to whitewash]
_Hollow Tiles._—Hollow terra-cotta tiles covered with stucco or brick
veneer are being used more extensively than ever, due to the cheaper
cost of laying them, since they are larger units, and also to the fact
that they build a cellular wall. Wherever these tiles are used for
bearing walls it is important that they be hard-burned, but the softer
ones may be permitted in non-bearing partitions. Tiles for use in outer
walls should be hard-burned, free from cracks, straight, and should not
show a greater absorption of water than 10 per cent. As these tiles
are intended to support loads from floor-joists, it is essential that
they should have the correct proportion of voids to solid shells and
webs. The maximum width of any voids should not exceed 4 inches and the
thickness of any shells or webs should not be less than 15 per cent
of this measurement. In tests it has been shown that tiles laid with
webs vertical are stronger than those with webs horizontal, but this
difference in strength is not of very great importance in the small
house, where the loads are very light. The chief thing to avoid in the
setting of tile, when they are vertical webbed, is the dripping of
mortar to the bottom and the insufficient spreading of it over the ends
of the webs and shells. This can be overcome by laying wire lath over
each course, and then buttering the mortar on the inside and outside
edges. The mortar is prevented from falling out of place by the lath,
and because it is not continuous through the wall, any penetration of
moisture through it is stopped.
[Illustration: Showing the use of metal lath in the joints of
vertically webbed hollow-tile, to prevent the dropping of the mortar
into the voids and also allow the separation of mortar joint]
_Cementing Materials_
The most important cementing materials which enter into the
construction of the small house are lime, cement, gypsum, and their
various mixtures, as mortar, plaster, and concrete.
The various technical requirements for good lime and cement are very
strict and detailed, and for the small house it is customary to cover
their qualities in the briefest manner by referring to the standard
specifications of the American Society for Testing Materials.
Slaked lime should be made from well-burned quicklime, free from ashes,
clinker, and other foreign materials.
Dry hydrated lime should be the finely divided product resulting from
mechanically slaking pure quicklime at the place of manufacture.
The specifications of the American Society for Testing Materials
covering the quality of cement should be followed where large purchases
are made. Where small quantities are to be used, the reliability of the
dealer must be the basis of purchase.
As mortars and concretes made from these materials are as important as
the cements or limes, it is essential to have definite standards for
them.
_Lime mortar_ should be made of 1 part by volume of slaked lime putty
or dry hydrated lime and not more than 4 parts by volume of sand. The
use of hydrated lime is recommended, since the poor qualities which are
apt to develop from careless slaking of quicklime are thus avoided. It
also comes in smaller packages, and if the entire quantity is not used
at once it may be stored without deterioration. It is only necessary to
mix the hydrated lime with water until it becomes a paste, and then add
the necessary sand. The purpose of adding sand is to increase the bulk
and to reduce the shrinkage which pure lime paste will develop as it
hardens. Pure lime paste, without sand, will shrink, crack, and develop
very little strength. By introducing sand this contraction is reduced,
but the addition of too much will decrease the strength slightly.
However, this decrease of strength is very little. A mortar made of 1
part lime to 6 parts sand is nearly as strong as one made from 1 part
lime and 3 parts sand. The maximum amount of sand to be used is
generally governed by the ease of working, and not so much by the
strength. A lime which is too sandy will not spread easily on the
trowel.
_Cement mortar_ is, of course, a stronger material and can be used in
damp places where lime mortar would deteriorate. The theory of mixtures
of both cement mortar and concrete is to proportion the materials
so that they produce the most compact substance. For instance, in
the cement mortar the cement should just fill the voids between the
particles of sand, and in concrete this cement mortar should just fill
the voids in between the larger aggregate, and this larger aggregate
should be so graded in size that it makes the most compact body. It
used to be thought that certain definite numerical proportions, as laid
down by theory, of the various ingredients would hold true for all
kinds of sands and aggregates. For instance, the proportion of 1 part
of cement, 3 parts of sand, and 6 parts of aggregate was thought to be
the best for ordinary use under all conditions. But extensive tests
by the government have shown that the only real way to determine the
correct proportions of mixtures is to experiment with the particular
sand and gravel that will be used, and to test them to see what
ratios give the most compact mass. It has also been found that round
aggregates, like pebbles, produce the strongest concrete, since the
particles flow into place better than the sharper aggregates, which
formerly were considered necessary because of the supposed idea that
they made a better mechanical bond with one another. The proportion of
water is also important, a quaking mixture producing the best results.
It is customary in small work, however, where no experiments can be
made on various mixtures to determine their proper proportions, to
follow the old rules of thumb for amounts.
Cement mortar should be made of cement and sand in the proportions of 1
part of cement and not more than 3 parts of sand by volume.
[Illustration:
Good. Very compact
Bad. Not compact because of poor grading of aggregate