It has been recognized since 1836 that the bitumen which forms the greater part of natural asphalts can be separated into two substances, which have been commonly known as petrolene and asphaltene, the former of which possesses the cementitious qualities essential to the making of a successful pavement. Instead of the arbitrary names—petrolene and asphaltene—these substances are sometimes more aptly designated as active and inert bitumen. It has been found that of the bitumen extracted from asphalts which have given the most satisfactory results in making street pavements, sixty-nine per cent or more is soluble in petroleum naphtha having a specific gravity of 72° Beaumé.
An asphalt pavement can not be economically kept in good condition unless every defect which may develop is immediately repaired. When the smooth, hard surface is once broken, disintegration proceeds very rapidly, and a large hole is soon formed. The more general distribution of smooth pavements, however, will tend to distribute the traffic more evenly, and the increasing use of rubber tires and rubber shoes for horses, to say nothing of the probably quite general use of motor vehicles, within the next decade will result in the elimination of the forces at present most destructive to pavements.
Much regret is often expressed that asphalt pavements should be so frequently opened for the purpose of laying or obtaining access to subsurface pipes and conduits, and thereby mutilated. As a matter of fact, there is no pavement at present in use which can be so effectively and satisfactorily restored as asphalt. When skillfully done, almost no trace of such an opening can be found.
The first question to arise, when it has been determined to pave a street, will be the selection of material, or the kind of pavement to be laid. In determining this, the governing considerations will be the traffic to be sustained, its density and character, the rate of grade, and the presence or absence of railroad tracks.
If the traffic be very heavy and the street given up wholly to business, ease of traction, durability, and economy of maintenance are of first importance, while quiet, comfortable riding, and beauty can be sacrificed to them. Many efforts have been made to determine the relative force required to draw a load over different kinds of surface under similar conditions. The following is from a table compiled by Mr. Rudolph Hering, from different authorities, the force being that necessary to move one ton on a level grade at a speed of three miles an hour:
| Kind of Road. | Pounds. | ||
| Ordinary dirt road | 224 | ||
| Ordinary cobblestone | 140 | ||
| Good cobblestone | 75 | ||
| Common macadam | 64 | ||
| Very hard, smooth macadam | 46 | ||
| Good stone block | 45 | ||
| Best stone block (London) | 36 | ||
| Asphalt | 17 | ||
| Granite tramway | 12½ to | 13 | ½ |
| Iron railway | 8 to | 11 | ½ |
The question of durability occurs next, and the different kinds of pavement which may be considered for city streets may be rated as follows, it being assumed that the traffic is not excessively heavy:
| Kind of Pavement. | Life in years. |
| Best granite block on concrete | 30 |
| Granite block laid on sand | 20 |
| Belgian trap | 20 |
| Cobblestone | 18 |
| Asphalt | 15 |
| Best wood—rectangular block | 10 |
| Vitrified brick | 12 |
| Macadam | 8 |
| Cedar block—round—on sand | 5 |
No class of municipal work comes so near to the daily life of an urban population—both the business and the home life—as the surface improvement of city streets, and no expenditure is too great (provided the work is skillfully and honestly done) to make them smooth, clean, sanitary, and beautiful.