as before it was disturbed, though usually the product of several mines is mixed in order to obtain the best percentage of bitumen, but nothing is added to or taken from the bituminous rock. In the pavement usually laid in America, on the other hand, only a small proportion of the material is brought from the asphalt deposits, the principal part of it (sand) being obtained near at hand. In the one case the cost of long ocean or rail transportation has to be paid on the entire mass forming the pavement, while in the other this expense attaches to but from twelve to fifteen per cent of the material. This, of course, is a great advantage, and at recent prices it is scarcely possible for the European rock asphalts to compete with the artificial ones.
The making of a pavement from one of the standard asphalts may be briefly described as follows: The material as found in Nature has this composition:
|Organic matter, not bitumen||7.63||"|
This is cooked until the water has been driven off, and some of the mineral matter has settled.
The above analysis is of Trinidad Pitch Lake asphalt, and is a particularly favorable result. This material is too hard for use in making a pavement, and it has to be softened or fluxed by the addition of something which will accomplish this purpose. In order to do this there is usually added to each one hundred pounds of refined asphalt about eighteen pounds of heavy petroleum oil. After this addition we have the asphaltic cement ready to combine with mineral matter, which is so selected that when asphaltic cement is added at the rate of about seventeen pounds of the cement to eighty-three pounds of the other all the particles will be coated, and more could not be added without making the pavement too soft. What is found to accomplish this best is fine stone dust and sand.
The asphaltic cement and sand are heated separately to about 300° F. The stone dust is then added to and mixed with the hot sand in the proportion of from five to eighty in the case of fine, well graduated sand, to fifteen to sixty-seven for coarse sands, having less variation in size. The asphaltic cement is then added, and the materials are mixed to a homogeneous mass, which is ready to be taken to the street. It should reach there at a temperature not less than 250°, and is spread with hot iron rakes so as to give usually a thickness of two inches after consolidation. After spreading, it is rolled with a hand roller, after which a small amount of hydraulic