It is especially

surface of the finished pavement is to receive. necessary that the surface be free from all inequalities, elevations as well as depressions. Work must not begin on the construction of the foundation until the inspector has definitely stated that the work on the roadbed has been finished in the manner prescribed in the regulations.

514. Specifications for Concrete (New York).-One part of American cement, equal to the best quality of freshly burned Rosendale cement, two parts of clean, sharp, washed sand, free from clay, to be thoroughly mixed dry and then made into mortar with the least possible amount of water; to this shall be added three parts of sound stone, broken with a hammer, the largest of which will pass through a 2-inch ring, the broken stone to be wet before being added to the mortar. The whole mass shall then be shoveled over until it is thoroughly mixed before it is put in place; it shall then be put in place and rammed until it is thoroughly compacted and has a clean mortar surface.

The whole operation of mixing and laying each batch will be performed as expeditiously as possible, by the employment of a sufficient number of skilled men.

The upper surface will be made exactly parallel with the pavement when laid, and, if necessary, will be protected from the action. of the sun and wind until set.

No concrete will be allowed to be used which has been mixed more than three hours.

The concrete shall be laid to a depth of 6 inches.

515. Concrete for Foundations, as used in Paris.-The proportions by bulk are:

The concrete is mixed on a large mortar-board, the mixers moving the board ahead as the work advances, and never being more than a few feet from the spot where the concrete is to be placed.

A square wooden form is placed on the mortar-board; into this is dumped successively, in the order named, 2 barrows of gravel,

sack of cement, 1 barrow of gravel, sack of cement, 2 barrows of sand. The form is then removed, and the mass turned over dry with the shovel by two men working side by side. It is then turned a second time by the two men, while a third sprinkles on the water from a pot. The mass is then turned over a third time, and shoveled from the board directly into place.

This concrete sets quickly, and every evening the surface of that laid during the day is covered with a thin coat of pure cement.

516. Specifications for Preparation of Roadbed.-The subsoil or other matters (be it earth, rock, or other material) shall be excavated and removed to a depth of inches below the top line of the proposed pavement. Should there be any spongy material, vegetable or other objectionable matter, in the bed thus prepared, all such material must be entirely removed, and the space filled with clean gravel or sand carefully rammed.

The roadbed shall be truly shaped and trimmed to the required cross-section and grade, and rolled to ultimate resistance with a roller weighing not less than ten tons; such portions of the roadbed as cannot be reached by the roller shall be consolidated with hand rollers or tampers.

Note. The employment of ashes, garbage, or other objectionable matter should not be permitted for filling on the streets of cities and towns.

Rock shall be excavated to a depth of 2 feet below the level of the finished grade, and the space so excavated shall be refilled to subgrade level with gravel, steam ashes, or other approved material, and thoroughly consolidated.

CHAPTER X.

RESISTANCE TO TRACTION.

517. The resistance to traction on highways is occasioned (1) by the want of uniformity in the surface of the road, the weight of the load having to be lifted over the projecting points and out of hollows and ruts, thus diminishing the effective load which the horse may draw to such as it can lift.

(2) The want of strength of the roadbed itself, however free its surface may be from asperities or cavities, if its substructure be of such a nature that it will yield to the pressure of the wheels, adds another impediment to the movement of a load over it, with the additional disadvantage that while the horse is endeavoring to lift the load from a cavity or hollow, the fulcrum, which in the first case was supposed to be fixed and rigid, is in the latter yielding and variable, subjecting the horse to the constant effort of lifting, instead of simply drawing.

518. Want of Uniformity in the Surface. The power required to draw a wheel over a stone or any obstacle, such as S in Fig. 36, may be thus calculated. Let P represent the power sought, or

투

B

S

FIG. 36.

that which would just balance the weight on the point of the stone, and the slightest increase of which would draw it over.

This power acts in the direction CP with the leverage of BC or DE. Gravity, represented by W, resists in the direction CB with the leverage of BD. The equation of equilibrium will be PX CB WX BD, whence

Let the radius of the wheel CD = 26 inches, and the height of the obstacle = AB 4 inches. Let the weight W = 500 pounds, of which 200 pounds may be the weight of the wheel and 300 pounds the load on the axle. The formula then becomes.

The pressure at the point D is compounded of the weight and the power, and equals

and therefore acts with this great effect to destroy the road in its collision with the stone, in addition there is to be considered the effect of the blow given by the wheel in descending from it. For minute accuracy the non-horizontal direction of the draught and the thickness of the axle should be taken into account. The power required is lessened by proper springs to vehicles, by enlarged wheels, and by making the line of draught ascending.

519. Resistance of Penetration. This resistance is that of a medium distributed over the submerged portion of the circumference of a wheel, in advance of the perpendicular line drawn from the centre of the wheel to the plane of the road. The following investigation furnishes a formula for calculating, with sufficient degree of accuracy, the resistance of gravel, loose stones, soft earth, or clay.

Let AOB, Fig. 37, be a wheel drawn over the horizontal surface CDE of the road, in the direction OF, and let the road be of such a consistency that the wheel penetrates to the depth

The arc

DB below the surface, leaving a track BG behind it. BC is the submerged portion of the circumference, and it may be assumed to be identical with the chord of the arc BC. Now the resistance is distributed over the surface BC, and it may be taken

as acting on this surface perpendicularly to the plane of the road, or vertically and directly opposed to the gross weight, consisting of the weight of the wheel and the load upon it. To simplify the investigation, let it be supposed that the upper portion of the road is homogeneous, as clay or sand; then the resistance to penetration is nothing at the surface, and it increases as the depth; and the upward resistance along the line of submersion, BC, is a maximum at B and it vanishes at C, and the varying intensity of the graduated pressure may be represented by an isosceles triangle, of which the centre of gravity, H, situated at one third of its length, BH, from the base, B, is also the centre of resistance, and therefore also the centre of pressure under the load; and the radial line OH is the resultant of the pressure of the load, measured in force and direction by the vertical OI, and the tractive force, measured by the horizontal line HI or OK. But the vertical OI may be taken as equal to the radius OB, and the horizontal HI may be taken as one third of the semi-chord of submersion CD; whence the proportion

Load tractive force :: OB: CD :: radius of wheel: semichord;