PORTLAND CEMENT CONCRETE ROADS. 27 



to any appreciable extent. The amount of abrasive traffic on country 

 roads is steadily decreasing and a well-constructed concrete pave- 

 ment no longer shows any marked deterioration through abrasive 

 action. The chief advantage of a bituminous surface treatment lies 

 in the fact that cracks are automatically bridged over as they appear 

 find surface water is prevented from reaching the subgrade through 

 these cracks. The difficulty of securing proper adhesion of the bitu- 

 minous surface to the concrete, its cost, and the necessity for continu- 

 ous maintenance of the surface constitute its greatest disadvantages. 

 It is believed that these disadvantages greatly outweigh any possi- 

 ble advantages which might be obtained through its use. 



THE CROSS-SECTION. 



Typical cross sections of pavements based upon the foregoing dis- 

 cussion of design are shown in Figure 7. 



CONSTRUCTION. 

 GRADING. 



The grading requirements for concrete pavements are essentially 

 the same as for other types of pavement. The shoulders may either 

 be roughly built at the time the heavy grading is done or be con- 

 structed after the pavement has been placed. If the shoulders are 

 roughly built before the pavement is placed, frequent drainage open- 

 ings must be left in them to insure the rapid drainage of the sub- 

 grade during periods of rainfall. This is very essential if the pave- 

 ment operations are not to be delayed by a poor subgrade. 



DRAINAGE. 



Surface drainage is secured by means of the pavement crown, the 

 slope of the shoulders to the ditches, and frequent outlets for the 

 water from the side ditches through culverts and bridges. In addi- 

 tion to surface drainage, soil conditions are sometimes such as to 

 require subdrainage. Subdrainage is usually desirable over low, 

 swampy ground and at points where ground water is encountered on 

 hillsides or in deep cuts. Subdrainage may be effected by the use of 

 drain tile, laid in trenches back filled with stone, gravel, or other 

 porous material, or by the use of V-drain foundations in which large- 

 sized stone is used and outlets are provided at all low points in the 

 grade. The use of a V-drain foundation or any other form of pre- 

 pared porous foundation under concrete pavements serves only to 

 lower the point of support of the pavement. A somewhat wider dis- 

 tribution of pressure is secured by the use of these foundations ; but 

 on soils requiring this wider distribution of pressure it is believed it 

 can be more cheaply obtained by reinforcement than by the use of 

 the prepared foundation. The most effective subdrainage for con- 

 crete pavements is obtained by the use of tile laid under the outer 



