8 BULLETIN 249, U. S. DEPARTMENT OF AGRICULTURE. 



concrete pavement, and this general form is suitable for either one- 

 course or two-course work. The one-course pavement is somewhat 

 simpler to construct than the two-course type. It possesses the ad- 

 vantages that there is no possibility for the wearing surface to sep- 

 arate from the rest of the pavement, and that the resistance to wear 

 should be uniform throughout the life of the pavement. Notwith- 

 standing these advantages, local conditions may sometimes justify 

 the two-course type of construction. For example, if the only ma- 

 terials locally available for use as aggregate were of very inferior 

 quality, it might be more economical to use them for aggregate in 

 the lower course of a two-course pavement and import aggregate for 

 the wearing course than to employ a one-course pavement and import 

 all the aggregate. The two-course pavement also requires slightly 

 less cement per square yard than the one-course type if different 

 proportions are used in the top and bottom courses; but this factor 

 alone would seldom, if ever, justify a preference for the former type, 

 especially in view of the objections to this method of construction, 

 already noted. 



Besides the two general types of concrete pavement described 

 above, there are several special patented types, but so far as is 

 known these do not possess any particular advantages and will not 

 be discussed in detail. The one-course pavement is believed to be 

 better adapted to most ordinary conditions than any other type of 

 concrete pavement and will be given principal consideration in the 

 following discussion. 



Plates I to IV are arranged in logical sequence, to show the vari- 

 ous steps in the construction of a one-course concrete pavement and 

 are intended to supplement the descriptions of construction methods 

 given below. 



GRADING AND PREPARING THE SUBGRADE. 



In forming a roadbed upon which a concrete pavement is to be con- 

 structed, the features which should receive primary consideration are 

 (1) adequate drainage, (2) firmness, and (3) uniformity in grade 

 and cross section. 



It is impracticable to prescribe definite methods for securing 

 thorough drainage which would be applicable to every location. The 

 local conditions which affect the accumulation and " run-off " of both 

 surface and ground water vary considerably even in the same locality, 

 and it is only by means of a careful study of these conditions that a 

 satisfactory system of drainage can be devised. For example, if the 

 material composing the roadbed consists of springy earth, either tile 

 or French drains would probably be necessary. In another case ex- 

 tremely flat topography may make it necessary to elevate the grade, 

 by means of an embankment, considerably above the level of the ad- 



