cushion coat should be laid directly on the binder course, or on the concrete foundation when no binder course is used, and should be of such depth as to give a thickness of in. when consolidated by rolling. The materials for the surface coat, which is laid on the cushion coat, should be delivered on the pavement in carts, at a temperature of about 250° F.; when the temperature of the air is below 50°, each cart should be equipped with a suitable heating apparatus that will prevent the paving material from cooling below the proper temperature.

The material of the surface coat should be carefully spread on the cushion coat to such a depth as will give a uniform surface and a thickness of 2 in. after being consolidated; hot iron rakes should be used for this purpose. The material should first be moderately compressed by hand rollers; a small amount of hydraulic cement should then be spread lightly over it, after which it should be thoroughly compacted by continued rolling with a heavy steam roller for not less than 5 hr. for each 1,000 sq. yd. of surface.

One Coat. When the pavement is given only one coat of asphaltic material, it is laid in much the same manner as just described for the surface coat. The material should be delivered in carts, at a temperature not below 250° nor above 310° F.; while in the carts, it should be protected with canvas covers when the temperature of the air is below 50° F. It should be spread on the foundation to such depth as will give a uniform surface and a thickness of 2 in. after being consolidated. The material should first be moderately compressed by hand rollers, and a small amount of hydraulic cement should be spread lightly over it, the same as described for the surface coat, after which it should be thoroughly compacted by rolling with a steam roller weighing not less than 5 T., followed by a second roller weighing not less than 10 T.; the rolling should be continued for not less than 10 hr. for each 1,000 sq. yd. of surface.

Bitulithic Pavements.-A bitulithic pavement is composed of broken stone ranging in size from 2 in. to dust, mixed in the necessary proportions to reduce the voids to about 10%, and cemented together by a bituminous cement manufactured either from coal tar, from asphalt, or from a combina

tion of both. The pavement is constructed in much the same manner as an asphalt pavement. The foundation is composed of a 4-in. layer of broken stone compacted by rolling. The interstices are filled and the surface is covered with bituminous cement. The material for the wearing surface is heated to about 250° F., spread while hot, and compacted by rolling with a 10-T. roller to a thickness of about 2 in. The surface is then covered with a liquid bituminous cement, on which, while it is in a sticky condition, there is spread a layer of sand or stone dust to a depth of about in. The rolling is then repeated, after which the pavement is ready for use.

CITY STREETS

Width. The roadway of a city street should be of such a width as to accommodate the traffic. For business streets, a width of roadway from 40 to 80 ft. is required, and for residence streets it should generally be from 24 to 36 ft. The sidewalks on business thoroughfares usually extend from the curbing to the building line, and on residence streets the width is about one-fifth to one-sixth the width of the roadway. The outer edges of the sidewalks on residence streets are commonly placed about 2 ft. from the fence line.

Height of Crown.-Let w be the width of the roadway, in feet; p, the per cent. of grade; and q, a coefficient given in the table on page 411. Then the height of crown in feet is

When the grade is comparatively level, the height of crown is determined in the same manner as for highways, previously given. Expressed by a formula

c = qw

Form of Crown.-For laying out a curving crown, Fig. 1, the method given under Highways may be used, or the following formula may be employed:

in which x and y are, respectively, the abscissa and ordinate to any point in the surface line of the cross-section with reference to the origin o.

to

-p

w

FIG. 2

For a sloping crown, Fig. 2, the portions tg and t'g' have a uniform slope of

in which b is the width of the parabolic portion tt'. This parabolic portion may be constructed by the formula

sxc2

Ус

in which x, and y, are the coordinates of any point with reference sb 4' and the coordinates to any point p along the straight slope line t g are related by the formula

to o as an origin. The ordinate at the tangent point t is y=

y=s x

Grades. In order that the surface water may be promptly and effectually removed from a roadway, the rate of grade for the street should never be less than one-fourth of 1%, that is, .25 ft. per 100 ft.; the grade should not be as flat as this except in extreme cases and with first-class pavements, such as brick or asphalt. A minimum grade of one-half of 1%, is as flat as should generally be used, and a grade as steep as 1% is very

desirable. Where the grade line has the same elevation at the intersecting streets at both ends of a block, instead of making the grade level between those streets, it should be elevated in the center of the block sufficiently to cause the water to flow in each direction toward the intersecting streets. If the street is sewered, the grade may be depressed at the center of the block by locating catch basins there; generally, however, it is better to elevate the grade at the center of the block.

Lateral Slopes of Sidewalks. For the purpose of drainage, sidewalks should have a slight lateral slope toward the curb. On business streets that are closely built up, in which the entire width between the curb and the building line is occupied by the sidewalk, this lateral slope of the sidewalk will fix the elevations on the building line. The edge of the sidewalk adjacent to the curb will be placed at the elevation of the curb, that is, at the street grade, and the edge of the sidewalk adjacent to the building line will be higher or above grade an amount equal to the width of the sidewalk in feet multiplied by the lateral slope per foot. In some cities, a lateral slope of 21%, or 1 in 40, is given to the sidewalks; a slope of 2%, or 1 in 50, however, is generally very satisfactory for this purpose. All that portion of the street between the curb and the property line should have this uniform lateral slope, whether wholly occupied by the sidewalk or not.