10 BULLETIN 1077, IT, S. DEPAETME2TT 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 



