CURBS AND GUTTERS.
A curb is a plank or slab of stone set at the edge of the roadway to protect the sidewalk or tree space and to form the side of the gutter. Curbs are not usually set except where the street is paved, but they greatly improve the appearance of an unpaved street and protect the glass plats at the side of the street, particularly during the muddy season.
Curbs are usually formed of natural stone, although concrete curb, usually combined concrete curb and gutter, are increasing very rapidly in recent years—partly because of the decrease in the price of Portland cement. Granite is the best natural stone, but it is usually very expensive. Limestone and sandstone are frequently used, but they are generally too easily chipped or broken. Con crete, unless made with unusual care or protected by steel on the edge, is too friable for a business street where heavy loads fre quently back up against the curb.
The thickness should be sufficient to give strength to resist the blows of wheels and to prevent the frost in the earth back of the curb from breaking it off at the top of the gutter. They are 4 to 8 inches thick, usually 4 to 6 inches, depending upon the quality of the stone and the locality. The depth must be
sufficient to prevent the thrust of the earth behind the curb from overturning it, and is usually 18 to 24 inches. If the section are too short, it is difficult to keep them in place and the general appearance is not good; and if they are too long, it is difficult to handle and set them, and nearly impossible to get a firm bearing on the bottom. They usually vary from 3 to 8 feet.
The exposed face of the curb should be bush-hammered or axed; and where the sidewalk extends to the curb, the back also should be smoothly dressed so the sidewalk may fit closely against the curb. The upper face should be cut to a slight bevel with the front face, say inch to the foot, so that when the face of the curb is set with a little inclination backward, the top face will be level or slope downward and to the front a trifle. The pavement slopes toward the gutter, and therefore a wagon wheel inclines toward the curb; hence the curb is set leaning back a little to prevent a wheel from striking the face when running at the inner edge of the gutter and also to secure increased stability. The curb is usually cut with a square corner at the outer upper edge; but it would be better if this corner were rounded off slightly, say to a radius equal to one third of the thickness of the curb, to decrease the tendency to chip. The ends of the sections should be smoothly dressed to the ex posed depth, and the part not exposed should be knocked off so as to permit the dressed ends to come into close contact. The ends should fit closely for appearance and to prevent the earth, partic ularly if sand, from running from behind the curb between the sec tions into the gutter, or to prevent the sand cushion of a brick pavement from running from under the bricks into these cracks and possibly through them into holes behind the curb. In a number of European cities, notably Brussels, the curb is cut with a tongue in one piece which fits into a groove in the next piece, to aid in keeping the curb to line.