Although asphaltum is a bad conductor of heat, and the cement retains its plasticity for several hours, occasions may and do arise through which the composition before it is spread has cooled; its con dition when this happens is analogous to hydraulic cement which has taken a "set," and the same rules which apply to hydraulic cement in this condition should be respected in regard to asphaltic cement.
The proportions of the ingredients in the paving mixture are not constant, but vary with the climate of the place where the pavement is to be used, the character of the sand, and the amount and character of the traffic that will use the pavement. The range in the proportion is as follows: Formula for Asphaltic Paving Mixture Asphalt cement 12 to 15 per cent.
Sand 70 to 83 " " Stone-dust 5 to 15 " " A cubic, yard of the prepared material weighs about 4,500 pounds, and will lay the following amount of wearing-surface: 21 inches thick .. 12 square yards.
One ton of refined asphaltum makes about 2,300 pounds of asphalt cement, equal to about 3.4 cubic yards of surface material.
Foundation. A solid, unyielding foundation is indispensable with all asphaltic pavements, because asphalt of itself has no power of offering resistance to the action of traffic, consequently it is nearl:, always placed upon a bed of hydraulic cement concrete. The concrete must be thoroughly set and its surface dry before the asphalt is laid upon it; if not, the water will he sucked up and converted into steam, with the result that coherence of the asphaltic mixture is prevented, and, although its surface may be smooth, the mass is really honey combed, so that as soon as the pavement is subjected to the action of traffic, the voids or fissures formed by the steam appear on the surface, and the whole pavement is quickly broken up.
Advantages of Asphalt Pavement. These may be summed up as follows: (1) Ease of traction.
(2) It is comparatively noiseless under traffic.
(3) It is impervious.
(4) It is easily cleansed.
(5) It produces neither mud nor dust.
(6) It is pleasing to the eye.
(7) It suits all classes of traffic.
(8) There is neither vibration nor concussion in traveling over it.
(9) It is expeditiously laid, thereby causing little inconvenience to traffic.
(10) Openings to gain access to underground pipes are easily made.
(11) It is durable.
(12) It is easily repaired.
Defects of Asphalt Pavement. These are as follows: (1) It is slippery under certain conditions of the atmosphere. The American asphalts are much less so than the European, on account of their granular texture derived from the sand. The difference is very noticeable; the European are as smooth as glass, while the Ameri can resemble fine sandpaper.
(2) It will not stand constant moisture, and will disintegrate if excessively sprinkled.
(3) Under extreme heat it is liablo to become so soft that it will roll of creep under traffic and present a wavy surface; and under ex treme cold there is danger that the surface will crack and become friable.
(4) It is not adapted to grades steeper than 21 per cent, although it is in use on grades up to 7.30 per cent.
(5) Repairs must be quickly made, for the material has little coherence, and if, from irregular settlement of foundation or local vio lence, a break occurs, the passing wheels rapidly shear off the sides of the hole, and it soon assumes formidable dimensions.
The strewing of sand upon asphalt renders it less slippery; but in addition to the interference of the traffic while this is being done, there are further objections—namely, the possible injury by the sand cutting into the asphalt, the expense of labor and materials, and the mud formed, which has afterwards to be removed.
Although pure asphaltum is absolutely impervious and insoluble in either fresh or salt wafer, yet asphalt pavements in the continued presence of water are quickly disintegrated. Ordinary rain or daily sprinkling does not injure them when they are allowed to become per fectly dry again. The damage is most apparent in gutters and adja cent to overflowing drinking fountains. This defect has long been recognized; and various measures have been taken to overcome it, or at least to reduce it to a minimum. In some cities, ordinances have been passed, seeking to regulate the sprinkling of the streets; and in many places the gutters are laid with stone or vitrified brick (see Figs.