14 BULLETIN 1077, U. S. DEPARTMENT OF AGRICULTURE. 
ment by making the concrete thicker at the center than at the sides. 
The flat subgrade was adopted originally, no doubt, for the reason 
that it was simpler to construct than any other form. For a double- 
track pavement, however, where two lines of traffic are accommodated, 
the use of a flat subgrade imposes the maximum wheel load on prac- 
tically the thinnest part of the pavement. Under heavy traffic con- 
ditions this has often led to complete breakdowns of the edges of the 
pavement. This action is greatly accentuated where diagonal trans- 
verse cracks occur. For a double-track pavement where the volume 
of traffic confines the limits of travel in each direction, it is essential 
that the edges be of the same thickness as the remainder of the pave- 
ment. This can be secured by using a crowned subgrade and a uni- 
form thickness of pavement. 
On a sandy or sandy-loam soil, where the traffic consists mainly 
of horse-drawn vehicles and passenger automobiles, with compara- 
tively few trucks, a thickness of pavement of 6 inches will often 
prove satisfactory. As the volume of truck traffic and the weight 
per truck load increase, the pavement should be made correspond- 
ingly thicker. A greater thickness should also be used on soils of 
poor bearing quality which are difficult to drain than on soils of 
good bearing quality which are easily drained. 
For the average condition of soil under traffic conditions up to 
and including 150 trucks per day, a thickness of 8 inches is believed 
desirable. In the neighborhood of large cities where a large volume 
of heavily loaded truck traffic is to be expected, the thickness should 
preferably be 9 inches, and under very unusual conditions a thick- 
ness of 10 inches may be necessary. A failure of a thin concrete 
pavement is shown in Figure 2, Plate X. 
CROWN OF PAVEMENT. 
A concrete pavement lends itself readily to the construction of 
low crowns. A low-crowned road is very desirable for the traffic. 
Water does not damage the surface of a concrete road and under 
present traffic conditions the wear of the surface is comparatively 
small, so the necessity for a high crown does not exist in this type. 
The amount of crown need not be any more than is necessary to 
shed the water from the surface, taking into consideration the small 
imperfections and depressions which exist in it. A crown of one- 
eighth to one- fourth inch per foot is sufficient. In the operations of 
finishing a concrete pavement surface a slight amount of crown will 
be lost, so that if the tamper is cut to a true 2-inch crown, the re- 
sulting crown in the pavement will closely approximate If inches. 
This fact should be taken into consideration in specifying the amount 
