Some Types of Roads and Their Qualities.
—Earth Roads.—The good qualities are: low first cost, not slippery, noiseless, easy on horses’ feet and on rubber tires, comfortable when in first-class condition. The poor qualities are: high tractive resistance, not durable, high cost of maintenance when traffic becomes dense, requiring constant attention to be kept in good condition, difficult to clean, muddy in wet weather, dusty in dry weather, choppy when dust blows away, rut easily, wear down rapidly under heavy traffic especially in windy localities, uncomfortable except when in prime condition. Adaptability: Satisfactory for light or medium traffic when properly drained and constantly maintained. It will probably pay to put in better roads when the traffic amounts to more than 400 vehicle-tons per day.
Sand-clay Roads.—The good and poor qualities are about the same as for earth roads. In fact they are earth roads with a selected mixture of sand and clay. They are more durable, harder and smoother than the ordinary earth road. They are appropriate for a light or medium traffic and are especially adaptable for sandy stretches or over clay or gumbo soils. The cost will depend upon the availability of materials; the cost of maintenance should be no more or very little more than earth roads. They should be good up to 800 vehicle-tons per day.
Gravel Roads.—The good qualities are: moderately hard, compact, and smooth, not slippery, noiseless, easy on horses’ feet, and not very hard on tires, not muddy, are comfortable, and low in first cost. Poor qualities: rut rather easily and require constant attention to keep them in first-class condition, dusty in dry weather. Gravel sometimes becomes loose on top and rolls under fast moving vehicles, causing skidding. When not thoroughly compacted gravel roads have high tractive resistance. They are particularly well adapted to country roads under medium traffic, especially where gravel may be obtained at a reasonable cost near at hand. At the present time more miles of gravel roads than of any other type of surface are being constructed in the United States. This is because of their low first cost and general satisfactory character for medium traffic.
Macadam Roads.—Moderate first cost and when well compacted smooth but not slippery. They require new dust continually to keep the stones cemented together. Under rubber tires the dust is not worn off the stones and what little there is on the roadway is picked up and spread to the winds. If covered with tar or asphaltic oil the stones cement together and form excellent roadways under medium traffic, where there are no extremely heavy trucks to cut through the surface. Traffic up to 1200 vehicle-tons per day is accommodated well by these roads.
Bituminous macadam roads are ordinary macadam roads impenetrated with bituminous materials. When well made they are excellent roadways, and unless extremely heavy trucking comes upon them ought to prove satisfactory for medium to moderately heavy traffic.
Bituminous Concrete Roads are made of broken stone mixed with a bituminous cement before laying and rolling. They, like bituminous macadam, are smooth, non-slippery, easy riding, have small tractive resistance and the first cost and cost of maintenance are moderate. Such roads have proven very satisfactory where the traffic is dense but not composed of real heavy units. On account of their dustlessness and general sanitary character as well as for their durability they are deservedly popular.
Brick Roads.—Vitrified paving brick give a hard durable surface, reasonably smooth and not slippery. The cost of maintenance is low and the appearance is good. Brick roads are expensive as a heavy concrete foundation is necessary, and they are noisy. They are well adapted for heavy hauling.
Concrete Roads.—This type of roadway is rapidly forging to the front. With the exception of gravel it leads in mileage of hard-surfaced roads. When made of good concrete sufficiently thick it has proven itself to be durable, hard, smooth, of small tractive resistance, comfortable, and not particularly expensive in first cost or maintenance.
With horse-drawn iron-tired vehicles it is doubtful if it would prove as durable as some other types but for rubber tired motorized vehicles it seems to be extremely well adapted. There is no doubt but that this type will continue to be popular. It has a tendency to crack under the action of temperature and moisture. It is customary to fill these cracks with tar, pitch or asphalt, giving an appearance which some people think not pleasing. The pavement is rigid and noisy, therefore objectionable for some localities.
Creosoted Wood Block Roads.—Wood blocks treated with creosote to preserve them from decay make an excellent pavement. They are smooth, durable, noiseless and sanitary, have small tractive resistance and are comfortable to ride upon. The principal objection is their habit of “bleeding” in the summer time. The sticky oil tar that oozes out is very objectionable, as it adheres to shoes and is tracked into houses. The first cost is considerable, but maintenance is low for many years after laying. Wood block roadways seem well adapted for bridge floors, for stable and shop floors, and for heavy teaming when placed on a substantial concrete foundation. They seem to last better for a moderate or semi-heavy use; when left idle they are more subject to decay.
Asphalt Block Roads have proven satisfactory for both country and city roads where the traffic is reasonably heavy. They are laid on both cement concrete and asphaltic concrete bases. They are smooth, easy riding, have light tractive resistance, are not very noisy, and are sanitary. The dark color is rather pleasing.
Sheet Asphalt Roads and Streets, considering their cost, durability, smoothness, ease of riding, low tractive resistance, and general acceptability, are among the most popular roads. What has been said of sheet asphalt will apply to asphaltic concrete of the Topeka specification and bitulithic types. The road is better for use. The asphalt and sand surface has the habit of swelling and cracking when not used. The proportioning and laying of a sheet asphalt surface is a particular job and requires a person of technical knowledge and experience to do it properly. Sheet-asphalt pavements seem well adapted for city streets and roads where there is a medium or dense traffic. With a firm foundation it stands up well under the heaviest traffic. Its popularity is truly deserved. The pavement under some conditions of moisture is inclined to be slippery but when dry is not. Neither is it very noisy.
Miscellaneous.—There are numerous other types of roads that have their proper uses in many localities. Burned clay, shell, furnace slag, coal slack, cinders, plank, corduroy, hay, bagasse, and possibly other materials have and will continue to be used with more or less success. The proper places for their use will depend upon local conditions which every good engineer always takes into account before deciding upon a type of roadway.