STONES.

262. The varieties of this material most commonly used in engineering operations are granites, limestones, sandstones, slates, brick, and artificial stones; the latter being made by compounding clays, limes, and cements.

Rock taken from the surface, which has been exposed to the atmosphere, is of an inferior quality to that found at a depth where it has been exposed to a strong pressure; and is consequently denser. Therefore, in opening a quarry it is advisable to excavate upon a hill-side and come at once to the sound stone. Rock is generally found in beds, divided by joints or seams, at which the natural adhesion is broken and the layers are easily separated. When the quarry shows no natural line of separation, one may be produced by drilling a line of holes at equal distances from each other, into which conical steel pins are driven, and the stone splits; the pins being placed in the plane of the required seam.

263. Stone is used almost entirely to resist a compressive strain; as in the voussoirs of an arch, or in the courses of a pier. The resistance of stone to crushing, is as follows:—

264. When stone cannot be found, brick forms an excellent substitute; being made from clay earths, which can be found in almost any locality. Bricks are well fitted for nice work, are cheap, and easy of transport. The French, at Algiers, have used concrete, rammed in boxes so as to make large cubes and other shapes. The structures built of this material are found to be very nearly if not quite as strong as those of natural rock.

LIMES, CEMENTS, MORTARS, AND CONCRETES.

265. Nothing is more important in the construction of masonry than good cement; and generally, no part of construction is intrusted to more ignorant persons. Under the above head are to be considered limes, cements, sands, common mortar, hydraulic mortar, and concrete.

266. Lime is obtained by burning off the carbonic acid from the pure limestones; when it is put up in air tight barrels and is unslacked lime. Natural cements are composed of pure lime mixed with argil, magnesia, iron, and manganese. Artificial cements are prepared by mixing with pure lime, calcined clay, forge scales, powdered bricks which are underburnt, and other materials of like nature. Cements made thus artificially, are as good as those naturally hydraulic.

Lime is termed rich, poor, hydraulic, and eminently hydraulic, according to its properties.

Rich or fat limes are those which double their volume in slacking and dissolve in fresh water to the last particle. They absorb about 300 per cent. of their weight of water.

Poor limes do not much increase their volume, do not dissolve completely, and absorb 200 per cent. of water.

Hydraulic limes set in fifteen or twenty days after immersion, and continue to harden as they grow older. After one year their consistency is about that of hard soap.

Eminently hydraulic limes set in five or six days, and continue to harden.

Limes are said to set when they will bear, without depression, a rod of 1
20 of an inch diameter loaded with ten or twelve ounces.

Note.—The following test was applied to every tenth cask of Rosendale cement used upon the masonry of the United States Dry Dock at the Brooklyn (N. Y.) Navy Yard. Cakes two inches in diameter and three fourths of an inch in thickness, after being immersed five days, were required to bear a rod of one twenty-fourth of an inch diameter loaded with fifty lbs. Two bricks united with the cement and immersed five days, were required to resist one hundred lbs. before separating. The following shows the progress of hardening. The force required to thrust a rod one twenty-fourth of an inch in diameter through a cake three fourths of an inch in thickness, was, after

SAND.

267. Sand is the product of the decomposition of granitic and schistose rocks, and weighs, per unit of bulk, somewhat less than one half of the rock producing it; owing to the spaces between the grains. The amount of lime necessary to fill these spaces must be known before we can form a solid mass with the least lime. The amount of void may be found by filling a measure with sand, and then pouring in water: the volume of water is that of the spaces. In pebbles of one half inch in diameter the void amounts to about one half, in gravel about five twelfths, in common sand two fifths, and in very fine sand, one third. Clean sharp sand obtained from the beds of rivers is the best for mortars.

268. In mixing the ingredients for mortar, the lime is first spread on a platform and wet by sprinkling with water, which causes it to give off a great deal of heat and vapor, and fall into a powder. The sand is then applied, and the whole brought with water to a consistent paste.

The proportions for common mortar for dry work are

It is well always to use a small quantity of cement; the parts which have in practice been found perfectly satisfactory are

For hydraulic mortar the following proportions have been used with success:—

269. Concrete is made by mixing broken stone, brick, or shells, with cement mortar; it is used for foundations, backing of arches, and for making artificial stone. The common proportions are

The cement and sand are first mixed as for cement mortar; the broken stone is added and the whole well mixed and immediately applied before it has time to set. Both concrete and cement mortar should be made as required for use, and in no case applied after standing over three hours.

FLASHING MORTAR.

270. Flashing consists of a thin coat of cement mortar made with a very large part of cement. It is used to protect the face of walls exposed to the wet; such as the top of arches. Stone liable to disintegration may be protected by flashing.

POINTING MORTAR.

271. Pointing is used to protect the joints of masonry, and is made by mixing cement and sand with a minimum of water. The joint is first cut out to the depth of from one half to one inch, carefully brushed clean, moistened with water, and filled with the mortar, which is well rubbed with a steel tool. To give architectural effect, plaster of Paris (Gypsum) is sometimes used in pointing.

GROUT.

272. Grout is thin-tempered mortar, composed almost entirely of cement and water. It is run into the joints, and is useful in filling crevices in masonry which cannot be filled with mortar.

273. The foundations being secured, and the piers and abutments being carried up to the springing line of the arch, the centres are carefully adjusted to their places and the arch is commenced. When the voussoirs begin to bear upon the centre (which is when the angle of the joint with the horizontal is greater than the angle of repose of one stone upon another), the frame is liable to change of form, (particularly when the arch is flat,) which must be provided for by counter loading the centre in various points as the work proceeds. Great care should be taken to make each stone point in the direction of the radius of the arch. To do this effectually, their thickness should be marked upon the outer rib of the centre. The line of the joint may then be fixed by a straight-edge placed both on the centre and the rib mark, or by a template so cut that when one side is level the other shall stand at the proper angle. Excess of weight upon one side of the centre causes a depression at that point, and a corresponding rise at the opposite side of the arch. Both sides being loaded, the haunches settle, and the crown rises. The point where the centre is first loaded will determine the point where the frame is to be temporarily weighted. Such precautions, however, need only to be taken in arches of over fifty feet span, unless the curve is quite flat. The keystone should be put into the proper place, but not driven until the rest is finished. The back joints are then closely wedged and cemented with thin tempered mortar, and the whole is left to set. The masonry of the spandrels is brought up to about one fourth the height of the arch, or enough to prevent by their weight any change of form of the curve. The centres are then struck and the soffit and voussoir joints cleaned and pointed. The facing and road-way may next be carried up; the parapet coping and drains finished off; and the whole pointed. Parapets are shown in figs. 127 and 128. The spandrels, fig. 129, may be carried up solid or hollow; their weight must be enough to stiffen sufficiently the arch. It should, at least, be carried up solid to the line c c c; the shaded mass being of well-cemented rubble. Above this the filling may be of masonry, solid or arched, or even of well-rammed layers of earth. The road-way should, in all cases, be well drained, that the water may not sink through to the masonry.

The apparatus for handling stone (cranes, lewises, and derricks) is much better understood by inspection than by description.

Wherever walls support masses of earth, the thrust may be somewhat lessened by ramming the earth behind the wall in layers inclining backward. In laying up the courses each should be well cleaned and moistened before the mortar is laid upon it. When a stone has been once placed upon the mortar bed, it should not be moved at all laterally, but may be gently mauled on top.

CULVERTS AND DRAINS.

274. Small culverts are made by covering two side walls with large flat stones; the bottom being paved with stone at least nine inches deep, laid dry. The general dimensions of such structures depend somewhat upon the class of masonry, but as this is generally the third or fourth, will not vary much.

Figs. 130, 131, and 132, show plans for culverts of from 5 to 25 feet span.

RETAINING WALLS.

275. A wall made to sustain a mass of earth or water, to resist overthrow, requires a certain thickness. A body of earth assumes what is termed the natural slope, the inclination of which depends upon the adhesion of the soil, but may be taken as one and one half horizontal, to one vertical, (1½ to 1), as an average.

The problem is, knowing the height of the wall and the form of the mass of earth to be supported, to find the thickness of the wall.

Let A B 6 F, represent the thickness of the wall. Its centre of gravity is at O, and is horizontally projected at m. The centre of gravity of the thrusting triangle of earth, B 4 6, is C, (found by the cutting of lines joining any two angles to the centre of the opposite sides,) is horizontally projected at C^a, and the horizontal component of the thrust is exerted at 2, tending to overthrow the wall with a leverage, 6 2.

The overthrowing power is, then, the area of the triangle B46 × the weight of the unit of area × the leverage 6 2. And the resisting power, the area AB × B6 × the weight of a unit of area by one half breadth, or m 6; or, calling w the weight of the wall, and w′ that of the triangle, B 4 6, and L and L′ the leverage respectively of the wall to resist and of the earth to overthrow; we must have at least

and to insure stability,

or,

and as L = half base finally, the thickness, or

276. Example.—Let the height of wall be twenty feet, slope one and one half to one; if a cubic foot of earth weighs one hundred lbs., and of masonry, one hundred and sixty lbs., we have the overthrowing force,

and the resisting force, (assuming the thickness as eight feet, in order to get the area),

Or performing the operations,

If the wall in place of retaining only the mass B 4 6, retains the bank B E F^a, the pressure will evidently be increased. The centre of gravity of the trapezoid B E F^a 6, is at C′, which is horizontally projected at C′^a, and the horizontal component of the thrust acts at 3 with the leverage 63.

Any superincumbent load, as a train of cars at E F^a, will again increase the pressure, not only by reason of weight, but from shocks and vibration.

For resisting lateral pressure, the beds of masonry are best when rough dressed. For vertical loads, hammer dressed beds are the best.

The leverage of resistance is very much increased by battering the wall in front, as at A D. The centre of gravity is then horizontally projected at m′, but the distance D m′ is much greater than F m.

The amount of masonry remaining the same, by decreasing the top, and increasing the base, the strength is very much increased.

When retaining walls are exposed to shocks or pressures special directions, they may be very much aided by buttresses opposing directly such forces, as in fig. 134.

The increase of strength thus made by a small bulk of masonry is very great.

All abutments, wing-walls, and side walls of culverts, come under the head of retaining walls.

When the face of the wall does not by its position admit of buttresses, as in fig. 134, it may be dovetailed into the earth; the latter being firmly rammed around the masonry, as in fig. 135.

277. The weight of the different earths and stones are shown in the following table.