Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/508

Rh 460 BUILDING [BRICKWORK. both obtained from France. In using asphalt for paving, a bed of concrete, made of the best hydraulic lime, is first prepared, and made fair at top by a rendering of similar mortar. The asphalt will not dissolve with heat by itself, but will calcine in the caldron. A small quantity of pure mineral pitch is therefore first put in ; when this is hot the asphalt is added, and soon dissolves ; a quantity of powdered stone-dust is then stirred in, and a small portion of quicklime. The mixture in its melted state is then laid on the bed of concrete (which must be quite dry), and spread close and fair, some sand being sprinkled over the top and well trowelled in. The best proportions are said to be about 2 pints of mineral pitch to 10 Ib of asphalt and one-fourth bushel of stone-dust. Another method of forming a paving is to place on the concrete a layer about 3 inches in thickness of hot asphalt in powder, and then to ram it down with hot iron rammers, until it has come to its proper consistency ; it is then finished as usual. This has been lately much used for roadways in the city of London and elsewhere. The same material has been com pressed into tiles about 6 inches square, and these laid on a good foundation. A very inferior imitation is made by mixing a quantity of sharp sand with gas-tar, heated in a caldron, and then adding some qxiicklime. This may do for rendering walls, &c., to keep out wet, but it is of very little use as paving. Gravel coated with tar, and then laid and set in tar, rammed down, and sanded over, makes a very good pavement for ordinary footpaths. Mortar As before noticed, particular attention must be paid to making, cleansing the sand to be used for mortar of every particle of clay or mud that may adhere to or be mixed up with it. Sea-sand is objectionable for two reasons : it cannot be perfectly freed from a saline taint, and the particles are moreover generally rounded by attrition, caused by the action of the sea, which makes it less efficient for mortar than if they retained their natural angular forms. Lime should not be slaked until the moment it is to be mixed up with the sand in mortar, but the sooner that is done after it is burnt the better. The proportion of lime to sand generally taken, and the best, is one to three ; but if both the materials be of good quality, that is, if the lime slake freely, and become a fine pungent impalpable powder, perfectly clear from argillaceous or any other foreign matter, and the sand be clean and sharp, one part to four is enough ; more is injurious. The ingredients should be well mixed together, and with just as much water as will suffice to make the compound consistent and paste-like. Of late years, in lieu of sand, burned clay, as above noticed, has been much used in localities where it is diffi cult to obtain the former material. This is ground up with lime in a mill, but unless very great care is taken in its manufacture the result is a very poor substitute for sand and lime mortar ; and brick and lime rubbish have also been used in like manner, with an equally inferior result. Rain or other soft water should be used for the pur pose of making mortar, and not spring or hard water, though any other may be preferred to what is brackish even in the slightest degree. Higgins recommended that lime-water should be used in preference to pure water. A quick-setting cement, such as those which are commonly used in building in England, and known as Parker s or Roman cement, and Portland cement, can only be mixed or gauged as it is required for use. A bricklayer will keep a labourer fully employed in gauging cement for him alone. It is mixed with sand in the proportion of from about two or three to about five or six of sand, to one of the cement, according to the quality of the latter ; and the labourer as he gauges on one board supplies the mixture to the bricklayer fit for use on another board, a spadeful at a time ; it must then be applied within half a minute, or it sets and is spoiled and wasted, for it should never be worked up again. The average size of bricks in England is a fraction Brick under 9 inches long, 4 inches wide, and 2i- inches walls - thick ; and in consequence of this uniformity of size, a wall of this material is described as of so many bricks in thickness, or of the number of inches which result from multiplying 9 inches by any number of bricks ; a 9-inch or one-brick wall ; a 14-inch or one-brick-and-a-half wall (13 J inches would be more correct, in fact, for although a joint of mortar must occur in this thickness, yet the fraction under the given size of the brick is enough to form it); an 18-inch or two brick wall, and so on. The great art in bricklaying is to preserve and maintain Bricklay- a bond, to have every course perfectly horizontal, both il! S- longitudinally and transversely, and perfectly plumb (which last, however, may not mean upright, though that is the general acceptation of the term, for the plumb-rule may be made to suit any required inclination, as inward against a bank, for instance, or in a tapering tower) ; and also to make the vertical joints recur perpendicularly ovei each other, which is vulgarly and technically called keeping the perpends. By bond in brickwork is intended that Bonding, arrangement which shall make the bricks of every course cover the joints of those in the course below it, and so tend to make the whole mass or combination of bricks act as much together, or as dependency one upon another, as possible. The workmen should be strictly supervised as they proceed with it, for many of the failures which have occurred may be referred to their ignorance or carelessness in this particular. The object of bonding will be under stood by reference to the diagram, fig. 1, Plate XX. Here, Plate XX it is evident, from the arrangement of the bricks, that any weight placed on a would (supposing, as we are obliged to suppose, that every brick bears equally, throughout its whole length, a stress laid on every part of it) be carried down and borne alike in every course from b to c ; in the same manner the brick d is upborne by every brick in the line ef, and so throughout the structure. But this forms a longitudinal bond only, which cannot extend its influence beyond the width of the brick ; and a wall of one brick and a half or two bricks thick, built in this manner, would, in effect, consist of three or four half-brick-thick walls, acting independently of each other, as shown in the plan at i in the diagram under fig, 1. If the bricks were turned so as to show their short sides or ends in front, instead of their long ones, certainly a compact wall of a whole brick in thickness would be produced ; but the longitudinal bond would be shortened one-half, as at y c h, and a wall of any greater thickness, in the same manner, must be composed of so many independent one- brick walls, as at Jc in the plan before referred to. To obviate this, to produce a transverse, and yet preserve a true longitudinal bond, the bricks are laid in alternate courses of headers and stretchers, or of ends and sides, as shown in fig. 2, thus combining the advantages of the two modes of arrangement a b c and g c li fig. 1, in a It c fig. 2. Each brick in fig. 2 showing its long side in front, or being a stretcher, will have another lying parallel to it, and on the same level, on the other side, to receive the other ends of the bricks showing as headers in front, which in their turn bind, by covering the joint between them, as shown in the end of such a wall at d. Thus a well-bonded 9-inch or one-brick wall is produced. The end elevations of the same wall at e and / show how the process of bonding is pursued in walls of one and a half and two bricks thick, the stretcher being abutted in the same course by a header,- thus, in a 14-inch wall, inverting the appear ance on the opposite sides, as seen at e, and producing the same appearance in an 18-inch wall, as at/. In the diagram