HIGHWAY CONSTRUCTION 83 



upper face becomes rounded, thus making the pavement very 

 rough. 



The stone is sometimes tested to determine its strength, resistance 

 to abrasion, etc.; but, as the conditions of use are quite different from 

 those under which it may be tested, such tests are seldom satisfactory. 

 However, examination of a stone as to its structure, the closeness of its 

 grain, its homogeneity, porosity, etc., may assist in forming an idea of 

 its value for use in a pavement. A low degree of permeability usually 

 indicates that the material will not be greatly affected by frost. 



Materials. Granite. Granite is more extensively employed for 

 stone block paving than any other variety of stone; and because of this 

 fact, the term "granite paving" is generally used as being synonymous 

 with stone block paving. The granite employed should be of a tough, 

 homogeneous nature. The hard, quartz granites are usually brittle, 

 and do not wear well under the blows of horses' feet or the impact of 

 vehicles; granite containing a high percentage of feldspar will be inju- 

 riously affected by atmospheric changes; and granite in which mica 

 predominates will wear rapidly on account of its laminated structure. 

 Granite possesses the very important property of splitting in three 

 planes at right angles to one another, so that paving blocks may readily 

 be formed with nearly plane faces and square corners. This property 

 is called the rift or cleavage. 



Sandstones of a close-grained, compact nature often give very 

 satisfactory results under heavy traffic. They are less hard than 

 granite, and wear more rapidly, but do not become smooth and slip- 

 pery. Sandstones are generally known in the market by the name 

 of the quarry or place where produced as "Medina," "Berea," 

 etc. 



Trap rock, while answering well the requirements as to durability 

 and resistance to wear, is objectionable on account of its tendency to 

 wear smooth and become slippery; it is also difficult to break into 

 regular shapes. 



Limestone has not usually been successful in use for the construc- 

 tion of block pavements, on account of its lack of durability against 

 atmospheric influences. The action of frost commonly splits the 

 blocks; and traffic shivers them, owing to the lamination being 

 vertical. 



