8 BULLETIN 249, U. S. DEPAETME^T 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 s}^stem 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- 
