PORTLAND CEMENT CONCRETE PAVEMENTS. 9 
jacent land. The nature of the soil, the character of the topography, 
and the amount and rate of rainfall must all be taken into consider- 
ation, if a system of drainage is to be properly planned. 
The second requirement, firmness, can be secured only after the 
road has been properly drained. Soils which readily absorb moisture 
will not remain firm in wet weather and therefore should not be per- 
mitted to form a part of the roadbed, especially if they occur in the 
subgrade. This requirement also makes it necessary that the road- 
bed be thoroughly compacted. In forming embankments the ma- 
terial should be put down in layers not more than about 12 inches 
thick, and each layer should be thoroughly rolled. (See PI. VI.) 
The subgrade in both excavation and embankment should be brought 
to its final shape by means of picks and shovels and rolling. 
The cross section of the subgrade may be either flat or shaped to 
conform with the finished surface of the pavement. The fiat cross 
section involves the use of a slight additional quantity of concrete, 
but gives an increased thickness at the center, where maximum 
strength is required. It has been observed that longitudinal cracks 
occur less frequently in concrete pavements laid on a flat subgrade 
than where the subgrade is curved to conform to the surface of a 
crowned pavement. 
In either case the subgrade when completed should be uniform in 
grade, cross section, and firmness, not only to prevent a waste of con- 
crete in filling up depressions but in order to facilitate the necessary 
movement of the pavement due to contraction and expansion and 
thus reduce its tendency to crack. The subgrade should be rolled 
and reshaped until the specified shape is secured. The forms, which 
should be set before the final shaping, may be made to serve as a 
guide for this work. 
USE OF SUB-BASE. 
Where old pavements which have been constructed on a sub-base 
are replaced by concrete pavements, it is frequently convenient to 
place the new pavements on the old sub-base. Furthermore, soil 
conditions are sometimes such as to make the use of a sub-base very 
desirable. This is especially true of soils which do not compact 
readily under the roller or which can not be effectively drained at a 
reasonable cost. 
A satisfactory sub-base may be constructed of gravel, broken stone, 
telf ord, cinders, or any other similar material. The essential features 
in every case are firmness, smoothness, and uniformity in grade and 
cross section. Telford is seldom employed as a sub-base for concrete 
pavements, except when old macadam roads having such sub-bases 
are being repaved with concrete. When this is the case it would seem 
advisable to spread a layer of sand or other fine material over the sub- 
' 92759°— Bull. 249— 15- 2 
