CONSTRUCTION - The asphaltic rock is quarried, and then crushed to about egg size by toothed rollers. These pieces are first reduced to powder and then sifted to uniform fineness. The powder is dropped through a hopper into a revolving cylinder like a coffee roaster, which is about 61 feet in diameter, and is sur rounded by a chamber the air in which is heated by a movable furnace placed just below it. The cylinder itself revolves and, since it is provided with blades arranged in screw form, the pow dered rock is well mixed with hot air and is thus thoroughly heated to a temperature between 300° and 350° F. Specifications frequently permit the rock asphalt to be heated to but 200° to 250° F.
When the powder is hot enough, the furnace is removed from under the heater and a cart replaces it, into which the asphalt powder is discharged and hauled upon the work. The powder will retain its heat for several hours and so admits of being carted long distances without losing its heat, thus doing away with the necessity of having roasters at the point where the to be laid, as was at one time the practice. For the best results. the mixture should be delivered upon the street at a temperature of not less than 250° F., although specifications sometimes permit a temperature of but 190° F.
The heated powder is spread upon the concrete base to a uni form thickness about 40 per cent greater than that required for the finished pavement. This must be done with great care in order
that the material, which while hot has a great tendency to consoli date, may not be denser in one spot than another. The material is compacted by rolling and ramming in much the same way as is described for the artificial asphaltic paving compound (see § 629), except that as a rule the natural rock asphalt is not consolidated to so great an extent as is customary in laying the artificial mix ture. The evidence of this is that a rock asphalt pavement will continue to shrink in thickness under traffic for a year or two, while the artificial mixture shrinks but little, if any, after com pletion.
The general appearance of the completed pavement is much the same as that of the pavement made of the artificial mix ture, except that the European rock pavements are lighter in color. The claim is that European natural rock asphalt pave ments are less slippery and less susceptible to changes in tempera ture than are American artificial asphalt pavements.
Not infrequently the term rock asphalt pavement is inappro priately applied to a pavement made of an artificial mixture of sand and of asphalt extracted from a natural rock.