DEAINAGE METHODS FOR COUNTY ROADS. 67 



under even the most favorable circumstances, while 8 inches is suffi- 

 cient for the worst conditions, except in the case of stone curbs where 

 the stone used is not of very good quality. In the latter case a thick- 

 ness of as much as 12 inches has been employed. 



STEEL REINFOKCEMENT. 



In a few instances concrete foundations have been reinforced with 

 steel in order to secure the required strength with less depth. But, 

 in general, the use of steel reinforcement has been confined to anchor 

 rods for tying flange curbs on to the foundation. For this purpose 

 one-half -inch rods spaced about 18 inches apart should be sufficient. 

 The rods should extend about 1 foot back into the foundation and 

 should be bent so as to project- up into the curb near its inner face. 



MATERIALS. 



The materials composing Portland-cement concrete are: (1) Port- 

 land cement, (2) sand, (3) gravel, crushed stone, or crushed slag, 

 and (4) water. To secure high-grade concrete it is necessary that 

 each of the constituent materials shall possess certain definite qual- 

 ities. Inferiority in the quality of a single constituent may easily 

 be responsible for complete failure of the concrete in which it is 

 used, and can not be overcome by the relative excellence of the other 

 constituents. While the cement and sand are the two materials 

 most frequently responsible for failures, the coarse aggregate usually 

 composes the bulk of the concrete, and if it is defective a correspond- 

 ing weakness necessarily must be apparent in the concrete. 



In selecting materials for concrete availability is, of course, a 

 very important consideration, but preference for local materials 

 should be exercised only in so far as it can be controlled by such 

 requirements regarding the quality of the materials as are known 

 to have produced satisfactory results. Wherever any doubt exists 

 as to the suitability of some particular material proposed for use in 

 the concrete it should be subjected to such tests as will determine 

 the points in question before its use is permitted. The following 

 brief discussions of the different concrete materials should serve at 

 least as a partial guide in making proper selections. 



Cement. — Portland cement is manufactured in nearly every section 

 of the country, and most plants are equipped to turn out a satisfac- 

 tory product. But the facts that poor cement sometimes may be 

 produced by the most carefuly operated plants, and that good cement 

 is damaged easily by age or exposure make it advisable to subject 

 the cement to very careful investigation before it is used. Suitable 

 specifications for Portland cement have been isued by the United 

 S-tates Bureau of Standards in their Circular No. 33 and by the 



