ROADS AND PAVEMENTS 403 



and brick. For the foundations, hydraulic-cement concrete, 

 bituminous concrete, brick, broken stone, gravel, sand, and 

 plank are employed. 



The selection of the paving material depends on the character 

 of the expected traffic, on the cost, and to a certain extent on 

 the grade of the street. The maximum grade on which the 

 different materials may be used is about as follows: Asphalt 

 and wood, 4% ; brick, 7% ; stone blocks, 15%. The width of a 

 street, too, influences the selection. For instance, it would not 

 be advisable to place wood on a narrow street lined with high 

 buildings, because, owing to the exclusion of light and air, the 

 pavement would decay rapidly. 



SYSTEMS OF CONSTRUCTION 



Broken-Stone Pavement; Macadam. Macadam's system 

 of broken-stone pavement is generally found very satisfactory 

 for roadways in suburban districts. The construction of broken- 

 stone roads is treated under the heading Highways. 



Stone Pavements. The stone used for pavements is gener- 

 ally obtained from the granitic, sandstone, and limestone rocks. 

 Among the varieties of granite, those containing a large per- 

 centage of feldspar or mica are unsuitable for paving. The 

 feldspar rapidly decays in consequence of the action of the air 

 and water. The micaceous stones are too easily laminated. 

 The limestones, when used for paving, wear unevenly, and 

 under the action of frost are quickly split and broken. 



The most enduring pavements are made of granite or sand- 

 stone blocks. The best material for the foundation of such 

 pavements is hydraulic-cement concrete from 4 to 9 in. in 

 thickness, according to the nature of the traffic. When suffi- 

 cient time has been allowed for the concrete to set and dry, a 

 cushion coat of suitable material is spread over it to receive the 

 paving blocks. For this purpose, a J- to 1-in. layer of fine clean 

 and dry sand for granite blocks and somewhat deeper for sand- 

 stone blocks is very appropriate. A still better cushion coat 

 is afforded by a ^-in. layer of asphaltic cement. 



The paving blocks should be rectangular in form and of 

 uniform dimensions. A depth of 7 in. is generally considered 

 suitable; in which case the width should be from 3 to 4 in. and 



