PORTLAND CEMENT CONCRETE ROADS. 
13 
pavement, the speed of the vehicle, the character of the tires, and 
the percentage of the total load which is carried above the springs 
of the vehicle. Under very unfavorable conditions it may be as high 
as five times the amount of the static load. 
The pavement itself depends upon the subgrade for support, and 
this support is extremely nonuniform in character. The supporting 
power of a subgrade depends upon the type of soil, its capillarity, the 
Fig. 1. 
TRUCK PASSIN6 TRUCK 
-Width of road required for safe passage of vehicles. 
proximity of ground water, the condition of surface drainage, the 
amount of sustained rainfall, and the extent of freezing and thawing. 
All of these factors are extremely variable, and in combination are 
almost indeterminate, so that it is almost impossible to reduce the 
determination of pavement thickness to a simple mathematical com- 
putation. The behavior of concrete pavements of known thickness 
under known soil conditions and known conditions of traffic is the 
most satisfactory index of the thickness of pavement required. 
It has been more or less customary in the past to use a flat subgrade 
for concrete pavements, and obtain the necessary crown in the pave- 
