A MODERN STREET. 219 



tie, 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 disin- 

 tegrate. 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 pro- 

 portions 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 pro- 

 portions that the fine particles will fill the voids between the coarse par- 

 ticles, 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 satisfac- 

 tory street. Xo 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 conse- 

 quence. Water is a great enemy to asphalt streets, particularly to those 

 constructed of Trinidad pitch. It is therefore of the greatest impor- 

 ts 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 luryielding surface. 



Of equal importance is a proper concrete foimdation. 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 



