Page:Popular Science Monthly Volume 61.djvu/225

Rh, or still, where it is slowly melted in order to drive oft' the water and light oils. Such oils make the work on the street dangerous from liability of fire. The refined pitch or asphaltum, from which an excess of mineral matter has settled or any sticks or other organic matter has been skimmed off, is drawn into barrels.

A fluid residuum of heavy petroleum or mineral tar. which has also been deprived of water and light oils, is mixed with the asphalt in the proportion of 20 parts by weight of oil to 100 parts of asphalt. It is of the highest importance that this mixture should be very complete. If the blending is imperfectly done, the oil will wash out from the asphalt after the street is laid and the surface will dry out, crack and disintegrate. This asphaltic cement should be plastic and very tenacious and should preserve these properties through a wide range of temperature. If it becomes brittle at zero or nearly fluid at or about 100° Fahr., it, will not answer, as it will be brittle and break up in winter and will soften and flow in summer.

To prepare the surface mixture, an asphaltic cement possessing the proper qualities is mixed with sand and pulverized rock in such proportions that when finished it will contain about eleven per cent, of bitumen. The proportions depend upon the kind and quality of the crude bitumen, the locality in which the street is to be laid and the traffic to which it will be subjected. The sand should be clean and sharp and should consist of both fine and coarse particles in such proportions that the fine particles will fill the voids between the coarse particles, leaving the bitumen to hold all of the particles together.

Every element in the construction of an asphalt-surfaced street is important if the result is to be in every respect a durable and satisfactory street. No part can be slighted or neglected in either materials or workmanship. If the street is to be first class each and all of these must be first class of its kind.

The material and drainage of the sub-soil is of the highest consequence. Water is a great enemy to asphalt streets, particularly to those constructed of Trinidad pitch. It is therefore of the greatest imports nee that the subsoil on which the concrete foundation is laid should be as solid and dry as possible. All excavations made in the subsoil should be puddled and rammed, in order that the rolling may result in a perfectly uniform and unyielding surface.

Of equal importance is a proper concrete foundation. It should not only be sufficiently strong, but it should be as impervious to water as possible. Soft, spongy foundations of natural cement or Portland cement of inferior quality are not only an inadequate support for the yielding surface, but they are easily penetrated by water from below and act as conveyors of water, while a sound and firm foundation of the best Portland cement concrete not only presents an unyielding support