108 THE BUILDING NEWS. Fes. 9, 1872.
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Where a better appearance is required than
a drawn joint presents, the mortar is raked
ont with a sharp-pointed tool, each end of
which is bent down at a right angle in a con-
trary direction to the other, and the joint is
refilled with hydraulic lime mortar, or with
mortar made with smith’s ashes; and for flat-
joint pointing—that is, when the joint is
pointed flush with the face of the brickwork,
it is marked of an even thickness with the
trowel. Fertuck-pointing a slight projection
is given to the joint with putty.
Some time ago it was suggested in a letter
to the Burtpin@ News that it would enhance
the pleasing effect of a large surface of brick
wall if the joints were to be recessed. It
would probably do so, and if the joints were
but slightly recessed, say din. or 3in., it
would not materially reduce the strength of the
work, but the difficulty would be, with or-
dinary mortar, that it would require a longer
time for the mortar to set, for not only would
the surface of the joint be less exposed to the
air, but the bed joints would retain water to
some small amount after each shower of rain,
aud so the effect might be to create dampness
in the wall; but if the bricks were very
good and non-absorbent, and the mortar
were made from hydraulic lime, the effect. of
this kind of joint might be tried. It would
have to be done by placing a strip of wood,
or a bar of iron of the intended depth of the
recess and of the thickness of the joint,
along the outer edge while spreading the
mortar and setting the next course of bricks.
Ornamental brickwork requires additional
appliances to those necessary for plain brick-
work. Itis thought by some to require a
higher degree of skill, but we do not think
so. The bricklayer does not design the or-
nament any more than he designs the form
and dimensions of plain work. The work is
closer jointed and of smoother surface, but
the chief skill and judgment required in
bricklaying is to bend the mass firmly to-
gether, and this requires strict attention to
the bond, to the consistency of the mortar,
to the closeness and regularity of the joints,
and to the true building in line and level;
and no ornamental work requires greater
skill and judgment than are required for
these. But for gauged arches and similar
work additional appliances are required, viz,
a brick axe, which is a double-ended chisel
with hand-hold between the two; a chopping
block, upon which to dress the bricks that
are to be rubbed to a gauge; a banker or
table, upon which is place a coarse-grained
stone for rubbing the bricks upon; the
square, the level, and the mould, by the edges
of which the outline of the brick is scribed or
marked with a sharp-pointed spike; the tin
Saw, to cut. a short depth into the brick along
the scribed lines, to prevent the arrisses
spalling in using the brick axe.
‘The inclined surface upon which is bedded
the first course of an arch is called the skew-
back. Contrary to the practice in larger
arches the whole underside of a small ganged
arch—as over windows—is called the soffit.
(It may be unnecessary to say that in the
pronunciation of this word, the accent is.
placed on the second syllable.)
There are what are cailed straight arches,
that is, an arrangement of bricks in such a
manner that the so-called arch acts really as
a beam, but to give them a lighter appear-
ance, they are slightly cambered—more so on
the underside than the upper. ‘The camber-
’ slip is placed from side to side of the arch,
and serves to steady the bricks until the
mortar or cement has had time to set.
It may be observed that when a straight
line approaches a curve, there is an illusion
of the sight in the appearance of the straight
line when seen from a distance. It seems to
dip to the arch.
and to make the straight line appear truly
horizontal, it should be slightly inclined up-
wards asit approaches the arch. One inch
in 20ft. has been found sufficient to correct
this apparent error.
To correct this appearance, |
HOW TO BUILD SCIENTIFICALLY WITH THE AID OF MODERN IN- VENTIONS.—HOl. WALLS, ONCRETE.—The consideration of this subject, would be incomplete without treating of thisnew application of old materials For along time past the construction of walls of cottages and farm buildings in this country has been in lime concrete, the great objections to the use of common chalk lime preventing the more extended use of this class of con- crete. The common method adopted has been the use of boards, fixed to regulate the thick- ness of the walls; and these, owing to the slow-setting nature of the lime, could only be remoyed at considerable intervals of time, with the liability, when being changed, to cause fracture of the wall. It should be re- membered that many of the buildings of the kind I have described were constructed at a time when Portland cement was unknown, and the price of Roman cement was too high to admit of its use for this purpose. The only material now recommended is the Portland cement concrete. Portland cement, it may be well to mention, is manufactured from chalk (white chalk) and clay, and as the use of this material is so much extending, it may be desirable to give Mr. Reid’s analysis of what is considered a good and suitable quality of clay :—
Dilicayaticaccedeeseeouent teckiewe es mOGrED
Alumina.... 11-64
@amlimeseeccacese 0-75
Oxide of iron .. 14°80
Soda and kali 40
The result of a series of experiments on
this material, has shown that Portland cement
has more than doubled in strength in twelve
months’ use.
‘There are two systems of building cement
walls—the one in which the walls are con-
structed in their position by filling concrete
into moulds which are made to shifc to requi-
site heights as the work proceeds, and the
other in which the blocks are cast separately
and put together when dry, according to the
usual methods of construction ; but the system
that can be adopted with most advantage is
the former.*
Points to which Attention should be Directed.—
The quality of the cement; and this is so im-
portant that I feel I should not do justice to
this part of my subject if I did not give that
method of testing samples of cement which
Mr. Henry Reid, in his exhaustive treatise on
Portland cements, considers the most simple
and safe :—
Take at regular intervals during the day a quan-
tity of the semi-liquid overflowing from the wash-
mill, say from thirty to forty gallons at a time, and
place it on the hot plates near the sample kiln. This
may be dried in four or five hours, according to the
temperature of the plates. When dry enough it
should be at once put into the sample kiln and burnt ;
no time should be lost in doing this, so as to have
the earliest evidence of the condition of the washing.
The sample should be moderately-well burnt, and
when quite cool pounded in a mortar, from which it
is sifted in a fine-meshed sieve, of about 2,000 meshes
to the square inch. Make a sample from the powder
with the least possible quantity of water, which
divide into two circular pats, three or four incbes in
diameter and half an inch thick. Place one of them,
when sufficiently set, into a basin of water, leaving
the other in a dry place—the first with the object of
proving the hydraulicity of the mixture, and the
other the colour. After an interval of twenty-four
hours the samples should be carefully examined in
a good light, and if the water sample is free from
cracks or fissures, it may be passed as sound, or at
least may be considered safely mixed with the proper
proportion of carbonate of lime. If, on examination,
the air or dry pat appears of a blue-gray colour,
without any stains or brown specks, you may safely |
continue the proportions of chalk and clay represen-
ted by the duplicate samples. But if, on the con-
trary, the water sample gives way, cracking and fly-
ing, as it is technically called, no time must be lost
in reducing your measure of chalk or increasing the
- Should the building with blocks for any reason be
adopted, the size of the blocks most convenient will be lft. Gin. x 9in. x 44in. A block this size is equal in cubical capacity to eight ordinary building bricks, and is not too large or too heavy for one man to lift and lay,
quantity of clay. Again, should the water sample
continue sound in appearance after twenty-four
hours’ immersion, having set quickly when being
worked up into the pat, and the air-sample of a
brown colour, you may consider the mixture oyer-
clayed, and instant steps must be taken to alter the
proportions. The sampling should be intrusted to a
reliable workman, who must keep a register of all
his samples, marking each, when soft, with the date
and distinguishing marks between water and air
samples. No samples should be destroyed until after
the lapse of one month, so that they may be fre-
quently examined in case any exceptional phenome-
non should be displayed in the interval. Sometimes,
through carelessness or by allowing the knives to
revolve at too high a speed, the particles of chalk
are passed out of the mill in too large pieces, and
consequently, after being burnt, become developed in
the sample as slaked lime. This state of things
should, however, be considered as very exceptional,
and such a circumstance ought not to occur in any
well-conducted manufactory.
But, it may be urged, this test only applies
where one has the power of going on the
works; I want to know how to test any cask
of cement when delivered at the buildings
where it will be used. Mr. Grant’s ma-
chine will do this, the cost of which is about
£50, and he, ina paper read before the In-
stitution of Civil Engineers on 12th Decem-
ber, 1865, says, in recommending its use, that
beyond the cost of the machine, the annual
charge for labour will be about £80; that
therefore, in all works of magnitude, en-
gineers and architects should insist upon the
cement being thoroughly tested before being
used. He then showed, that the expense of
testing by this method, eighteen miles of the
Southern High Level Sewer, the cost of which
was £1,250,000, and which took 70,000 tons
of Portland cement; was only five farthings
per ton of cement. The extract from the
specification will be useful: ‘‘ Of the very
best quality, ground extremely fine, weighing
not less than one hundred and ten pounds to
the striked bushel, and capable of maintaining
a breaking-weight of 400Ib. on an area
din. square, equal to 21 square inches,
seven days after being made in an iron
mould, of the form and dimensions shown
on drawing, and immersed six of these days
in water.”
The moulds above referred to were made
of bell metal having a sectional area at the
breaking point of ljin. X 1}in. = 24 square
inches, with templates of thin iron, the exact
size of the mould, the bricks, or briquettes,
being afterwards, and in a short time, pressed
out by the machine.
T have gone fully into this test, because [
want specially to recommend to all intending
to use concrete walls, to put in their specifi-
cation some such test as the above, as other-
wise, I much fear many serious accidents may
occur. Nor are the risks limited only to the
cement. We have next to consider the
Crushing-weight—The result on Portland
cement blocks, the size of ordinary bricks,
after three months, is 65 tons; after six
months, 92 tons; after twelve months, 102
tons.
Matrix.—It may be, as has been stated,
fairly assumed that every locality commands
the material to combine with the cement
—the sand, grayel, or shingle. If sand, it
should be as rough and coarse as possible,
and is preferable if made by the various
natural or physical influences, from sand-
stone, limestone, or other similar rocky for-
mations. In competition they should be
selected in preference to those sands derived
from flints, porphyritie or volcanic rocks, for
the reason that they are more porous and
consequently better able to absorb the soluble
silicates of the cement when being mixed.
Avoid a rotten or friable material. If this
cannot be done you should increase the
quantity of cement. The sand, gravel, or
shingle should not be too wet, for if this is
the case the matrix is unable to imbibe the
fluid portion of the mixture, and conse-
quently it is thrown off as waste from the
concrete. If clay abounds in a district, and
there is an absence of other suitable material,
the clay will make excellent material if burnt
