10 BULLETIN 1077, IT. S. DEPARTMENT OF AGRICULTURE. 
QUANTITIES OF MATERIALS REQUIRED. 
The quantities of materials theoretically required for concrete pave- 
ments of various proportions, thicknesses, and widths are given in 
the appendix, pages 61 and 62. The quantities of aggregates are given 
in cubic yards. To correct to an approximate tonnage basis, the fine 
aggregate quantities should be multiplied by one and one-half and the 
coarse aggregate quanties multiplied by one and one-third. In prac- 
tice an allowance must also be made for waste or loss in handling 
these materials. This allowance should be approximately 2 per cent 
for cement, from 2 to 4 per cent for fine aggregate, and from 3 to 7 
per cent for coarse aggregate, depending upon the method used in 
handling the material. 
DESIGN OF CONCRETE ROADS. 
There are two general types of concrete pavement, known, respec- 
tively, as one-course and two-course pavement. The former con- 
sists of one course of concrete, all of which is mixed in the same pro- 
portion and composed of the same kind of materials, while the latter 
consists of two courses, usually mixed in different proportions and 
containing different kinds of aggregate. The one-course pavement 
is much simpler to construct than the two-course type. For the one- 
course construction it is customary to employ a coarse aggregate of 
average wearing qualities, which can readily be obtained from com- 
mercial sources. 
Where a very large volume of steel-tired traffic is anticipated, how- 
ever, it is sometimes desirable to provide a surface of exceptionally 
good wearing quality to resist the abrasive action of this particular 
kind of traffic. Inasmuch as aggregates having high resistance to 
wear, such as granite and trap, frequently have to be imported from 
long distances at great cost, the cost of a road composed entirely of 
this aggregate would be almost prohibitive. This has led to the 
development of the two-course type of construction in which local 
coarse aggregate of average or low wearing qualities is used in the 
lower course and imported aggregate with high resistance to wear 
is used in the top course. For example, if the only materials locally 
available for use as aggregate are of inferior quality, it would usually 
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 coarse aggregate in the top course is somewhat smaller than in 
one-course construction and the thickness of the top course is usually 
about 2 inches. 
In the two-course construction it has been somewhat general 
practice to permit leaner proportions for the lower course than would 
