Page:The Building News and Engineering Journal, Volume 22, 1872.djvu/124

 108 THE BUILDING NEWS. Fes. 9, 1872. ooo ee EEE 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 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
 * Should the building with blocks for any reason be