THE WEARING COAT - The asphaltic cement and the sand (or more properly, the mineral matter) are mixed to form the wear ing coat. Enough cement should be used to fill the voids in the compacted sand, as otherwise the mineral matter will not be held together with the maximum force; but more cement than enough to fill the voids is objectionable, since the expense is increased, an excess of asphalt causes the wearing coat to flow under traffic during warm weather, and also causes the surface to chip and flake off during cold weather.
In a sense, the purity of the asphalt is not a matter of impor tance, since the more mineral matter in the asphalt the less the amount of sand and pulverized limestone required. The mineral matter in the asphalt is usually an impalpable powder and is thor oughly incorporated with the bitumen; and therefore it is more desirable than new material. However, since different asphalts differ materially in the amount of contained mineral matter, it is customary to specify the amount of bitumen soluble in carbon bisul phide which the completed pavement shall contain. In southern cities having long hot summers, the soluble bitumen is usually limited to from 9 to 12 per cent; and in northern cities to from 12 to 15 per cent. The exact quantity varies with the amount and character of the traffic, the climate, the fineness of the mineral matter, etc.
Formerly it was customary to state the composition of the wearing surface about as follows: asphaltic cement, from 12 to 15 per cent; sand, from 70 to 83 per cent; limestone dust, from 3 to 15 per cent. The proportions differ considerably with the locality, the contractor, and the kind of asphalt used. The method of stating the composition in terms of the per cent of bitumen is more exact, and is the form now generally employed.
The proportions of the wearing coat are usually stated by weight, while the method of finding the per cent of voids given in § .622 determines the voids in volumes. To determine the propor tions by weight proceed as follows: Assume, for example, that it has been found, as described in § 622, that the voids in a mixture consisting of 50 pounds of sand and 5 pounds of limestone dust contain 6 pounds of water; and assume further that the specific gravity of the asphaltic cement is 1.25. The weight of the cement
required to fill the voids in the above quantity of sand and dust then is 6 X 1.25 — 7.5 pounds. The proper proportions by weight then are: Mixing the Cement and Sand. The asphaltic cement and the sand rather the sand and the limestone dust thoroughly mixed) are separately heated to 275° to 325° F. The proper amount of cement and sand (§ 626) are weighed out and simultaneously poured into a mechanical mixer consisting of two sets of interlocking revolving blades which thoroughly mix the materials. Usually about 800 pounds of cement and sand are mixed in one batch. When the mixing is completed, a process which ordinarily requires at least 1 to lf minutes, sliding doors in the bottom of the mixer are opened, and the material drops out into the carts or wagons which carry it to the street. In this condition, it is a loose pulverulent mass, each grain of sand being very completely coated with the asphaltic cement.
Usually the temperature of each load is taken, and a canvas is thrown over it. The operation of mixing the cement and sand requires care (1) in heating the ingredients to secure a uniform temperature and not to overheat the asphalt, (2) to proportion the mixture accurately, and (3) to mix the materials thoroughly.
There is no accurate method of determining whether the asphaltic cement and the mineral matter have been mixed in the best proportions. The only method in use for this purpose con sists in compressing a sample of the hot mixture by hand between two sheets of manilla paper or smooth brown wrapping paper, and in noting the mark left on the paper. If the mixture is too rich, the stain will be distinct and blotchy, and some of the cement will stick to the paper; if the cement is just sufficient to fill the voids, the stain will be distinct but not blotchy; and if the mixture is too lean, there will be almost no stain. The test is inexact, since the result depends not only upon the proportion of the cement present, but also upon the temperature and the amount of the pressure. This so-called test is scarcely any better than judging by the general appearance of the mixture. The men employed at the plant soon learn to judge quite accurately of the proportions by the appear ance of the material in the mixer.