HIGHWAY CONSTRUCTION 111 



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- surf ace : 



1\ inches thick 12 square yards. 



2 " " 18 " 



\\ " " 27 " 



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 nearly 

 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 w r ater will be 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. 



