DURABILITY OF VARIOUS SURFACES.
The durability of a road or pavement is dependent upon so many circumstances connected with local con ditions, the nature of the traffic, methods of con struction, and efficiency of maintenance, that any comparison of the various kinds of pavement in this respect is difficult and likely to be misleading.
The qualities which especially affect the durability of the road may be partially enumerated as follows: (I) The hardness and toughness of the material com posing the surface, upon which depends the resistance of the surface to the abrading action of the wheels and horses' feet passing over it.
(2) The firmness of the foundation, which serves to distribute the loads over the road-bed and keep the surface uniform.
(3) The drainage of the road-bed, which can only properly sustain the loads which come upon it when it is dry.
(4) The permeability of the surface, which should form a water-tight covering to serve the purpose of keeping the foundation and road-bed in a dry con dition.
(5) The resistance of the materials of the pavement to the disintegrating influences of the atmosphere and to the action of the weather.
The relative importance of these various factors, in any particular case, depends largely upon the nature and extent of the traffic which is to pass over the pavement.
The amount of traffic to which a street is subjected is usually estimated in terms of tons per foot of width of street, by observing the number of teams passing a given point during certain times, classifying them, and assigning an average value of load to each class. The wear of the surface will naturally be somewhat propor tional to the amount of traffic. The life of a pave ment is, however, affected by other conditions, and hence cannot always be inferred from the amount of traffic.
Traffic may also be classified according to its nature as heavy or light, depending upon the weight of indi vidual loads which are carried. It is the heavy loads borne upon narrow wheel-tires that do the greatest damage to a pavement, and hence the nature rather than the amount of traffic determines the character of pavement necessary.
Granite blocks, where a firm unyielding foundation is employed, give the hardest and most durable surface of any of the common pavements. This is epecially the case under very heavy loads.
The durability of wood-block pavements under wear varies widely for the different types of construction. The better grades of treated wood-block pavement seem to have given results, in some instances, second only to granite blocks, and they are being used under some of the heaviest traffic in the larger cities. The older and cheaper types of wood pavement are inferior in wearing qualities to brick or asphalt.
Asphalt and brick pavements when well constructed are satisfactory under any but the heaviest traffic. The relative durability under wear of brick and asphalt is a matter of doubt, both materials being subject to considerable variations in quality, and showing varying results in different localities, due both to differences in the quality of the material and in the methods of construction. Bitulithic may be classed with asphalt as to durability, although it seems in some instances to have shown greater resistance to wear than ordinary asphalt.
Broken stone wears rapidly under moderately heavy traffic, and should be employed only on suburban streets or country roads used mainly for light driving or a small amount of traffic.