90 HIGHWAY CONSTRUCTION 



employed are: (1) The tar produced in the manufacture of gas, 

 which, when redistilled, is called distillate, and is numbered 1, 2, 3, 4, 

 etc., according to its density; this material under the name of paving- 

 pitch is extensively used, both alone and in combination with other 

 bituminous substances; (2) Combinations of gas tar or coal tar with 

 refined asphaltum; (3) Mixtures of refined asphaltum, creosote, and 

 coal tar. 



The formula for the bituminous joint filling used in New York 

 City, is: 



Refined Trinidad asphaltum 20 parts. 



No. 4 coal-tar distillate ". 100 parts. 



Residuum of petroleum 3 parts. 



In Washington, D. C., coal tar distillate No. 6 is used alone. 



In Europe a bituminous cement much used is composed of coal- 

 tar, asphaltum, gas tar, and creosote oil, in the proportion of 100 

 pounds of asphaltum to 4 gallons of tar and 1 gallon of creosote. 

 These proportions are varied somewhat, according to the quality of the 

 asphaltum employed. The mixture is melted, and is boiled from 

 one to two hours in a suitable boiler, being then poured into the joints 

 in a boiling state. This mixture is impervious to moisture, and pos- 

 sesses a degree of elasticity sufficient to prevent it from cracking. 



The mode of applying the bituminous cement is as follows : After 

 the blocks are rammed, the joints are filled to a depth of about two 

 inches with clean gravel heated to a temperature of about 250 F.; 

 then the hot cement is poured in until it forms a layer of about one inch 

 on top of the gravel ; then more gravel is filled in to a depth of about 

 two inches; then cement is poured in until it appears on top of the 

 gravel, more gravel being next added until it reaches to within half an 

 inch of the top of the blocks; this remaining half-inch is filled with 

 cement, and then fine gravel or sand is sprinkled over the joints. 



In some cases the joints are first filled with heated gravel; the 

 cement is poured in until the sand beneath and the gravel between 

 the blocks will absorb no more, and the joints are filled flush with the 

 top of the pavement. This method is open to objection; for, if the 

 gravel is not sufficiently hot, the cement will be chilled and will not flow 

 to the bottom of the joint, but, instead, will form a thin layer near the 

 surface, which under the action of frost and the vibration of traffic, 

 will ta quickly cracked and broken up; the gravel will settle, and the 



