PORTLAND CEMENT CONCRETE ROADS. H 



be required for one-course construction, but it is not believed that this 

 practice is justifiable unless the thickness is correspondingly increased. 

 With the development of modern traffic, the load-carrying capacity of 

 the pavement is an important consideration, and the requirements of 

 strength should govern the proportions of the lower course in two- 

 course construction to the same extent as in one-course construction. 

 The construction of a two-course pavement involves construction diffi- 

 culties in mixing and handling two kinds of concrete and usually in 

 securing two kinds of coarse aggregate, especially if one kind is 

 shipped by rail, and therefore usually costs correspondingly more to 

 build than the one-course pavement. 



Under modern traffic conditions the amount of abrasive traffic on 

 main roads is rapidly decreasing and observations of concrete pave- 

 ments built with aggregates of average wearing qualities that have 

 been in service from 6 to 8 years fail to show any serious wear from 

 abrasion. Except under unusual conditions, therefore, it would. not 

 appear necessary to resort to two-course construction. 



Besides the two general types of concrete pavement described 

 above, there are several patented types, but so far as is known these 

 do not possess any particular advantages and will not be discussed in 

 detail. The one-course pavement is believed to be better adapted to 

 most ordinary conditions than any other type of concrete pavement 

 and will be principally considered in the following discussion. 



WroXH OF PAVEMENT. 



The width of pavement necessary will depend upon the frequency 

 with which vehicles are expected to pass each other, the character of 

 the vehicles, and their speed. For single-track roadways a width of 

 10 feet is usually adopted. This width is ample for a single line of 

 traffic, but passing vehicles will be forced to use the shoulders of the 

 road which consequently will require considerable maintenance. The 

 frequency with which vehicles pass each other has made it necessary 

 in some instances to construct shoulders of broken stone or gravel. 



It is believed that all trunk-line roads and roads of primary State 

 systems should be constructed to accommodate two lines of traffic, 

 whether the necessity for such a width exists at the time of con- 

 struction or not. The history of highway improvement shows that 

 there is always a tremendous increase in traffic upon the completion 

 of the improvement. This potential increase usually justifies the 

 double-track road. Where funds are the controlling factor in the 

 construction of the primary system, it may be desirable to construct 

 a single-track pavement in certain sections and make provision for 

 widening the pavement at a later date when the volume of traffic 

 justifies the expense. In doing this the road should be graded the 



