402 ROADS AND PAVEMENTS 



In all types of concrete pavements, care must be taken to 

 prevent cracks that are liable to result from expansion and 

 contraction of the concrete. This is usually done by providing 

 expansion joints, which should be arranged transversely at 

 intervals of about 50 ft., and longitudinally between the gutter 

 and the roadway proper. The expansion joints are usually 

 made about 1 in. wide and are filled with tar paper or bitu- 

 minous cement. 



Care must also be taken to prevent the surfaces of concrete 

 roads from being too smooth and slippery. This is usually 

 accomplished by roughening the finished surface with a stiff 

 broom or a brush before the mortar has set. 



CITY PAVEMENTS 



GENERAL EXPLANATIONS 



A good pavement should be: (1) impervious, in order 

 to retain water or surface liquids, but to facilitate their discharge 

 into the side gutters; (2) such as to afford a secure foothold for 

 horses, and not to become polished and slippery from use; 

 (3) hard, tough, and durable, so as to resist wear and disinte- 

 gration; (4) adapted to the grade; (5) suited to the traffic; 

 (6) smooth and even, so as to offer the minimum resistance to 

 traction; (7) comparatively noiseless; (8) such as to yield very 

 little dust or mud; (9) easily cleaned; and (10) economical 

 with regard to first cost and maintenance. 



It is also desirable that the pavement should be of such 

 material and construction that it can be readily taken up in 

 places and quickly and substantially relaid, in order to give 

 access to water, gas, and sewer pipes. 



A pavement consists of two more or less distinct parts; 

 namely, the wearing surface, and the foundation by which the 

 wearing surface is supported. The wearing surface receives 

 and sustains the traffic, but is not of itself capable of distributing 

 the weight of the traffic over a sufficient area of yielding ground, 

 which office is performed by the foundation. 



Pavement Materials. The materials commonly used for 

 the wearing surfaces of pavements, are stone, wood, asphalt, 



