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 PL 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 flat 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 



