HO HIGHWAY CONSTRUCTION 



impurities ; its grains should be angular and range from coarse to fine. 



The stone-dust is used to aid in filling the voids in the sand and 

 thus reduce the amount of cement. The amount used varies with the 

 coarseness of the sand and the quality of the cement, and ranges from 

 5 to 15 per cent. (The voids in sand vary from .3 to .5 per cent.) 



As to the quality of the stone-dust, that from any durable stone is 

 equally smitable. Limestone-dust was originally used, and has never 

 been entirely discarded. 



The paving composition is prepared by heating the mixed sand 

 and stone-dust and the asphalt cement separately to a temperature of 

 about 300 F. The heated ingredients are measured into a pug-mill 

 and thoroughly incorporated. When this is accomplished, the mix- 

 ture is ready for use. It is hauled to the street and spread with iron 

 rakes to such depth as will give the required thickness when compacted 

 (the finished thickness varies between l and 2J inches). The re- 

 duction of thickness by compression is generally about 40 per cent. 



The mixture is sometimes laid in two layers. The first is called 

 the binder of cushion=coat ; it contains from 2 to 5 per cent more cement 

 than the surface-coat; its thickness is usually \ inch. The object of the 

 binder course is to unite the surface mixture with the foundation, which 

 it does through the larger percentage of cement that it contains, which, 

 if put in the surface mixture, would render it too soft. 



The paving composition is compressed by means of rollers and 

 tamping irons, the latter being heated in a fire contained in an iron 

 basket mounted on wheels. These irons are used for tamping such 

 portions as are inaccessible to the roller namely, gutters, around man- 

 hole heads, etc. 



Two rollers are sometimes employed; one, weighing 5 to 6 tons 

 and of narrow tread, is used to give the first compression; and the 

 other, weighing about 10 tons and of broad tread, is used for finishing. 

 The amount of rolling varies; the average is about 1 hour per 1,000 

 square yards of surface. After the primary compression, natural 

 hydraulic or any impalpable mineral matter is sprinkled over the sur- 

 face, to prevent the adhesion of the material to the roller and to give 

 the surface a more pleasing appearance. When the asphalt is laid 

 up to the curb, the surface of the portion forming the gutter is painted 

 with a coat of hot cement. 



Although asphaltum is a bad conductor of heat, and the eement 



