14 BULLETIN 1077, U. S. DEPAETMENT 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, liOTreTer, 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 

 I)avement. 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- 

 tivel}' few trucks, a thickness of pavement of 6 inches will often 

 prove satisfactory. As the voliune 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 



