PORTLAND CEMENT CONCRETE ROADS. 11 
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. 
WIDTH 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 
