DEAINAGE METHODS FOR COUNTY EOADS. 69 



but it is doubtful if the difference would warrant any very expensive 

 discrimination against gravel so far as concrete foundations are 

 concerned. 



No matter what kind of coarse aggregate is used it should consist 

 of sound particles of at least moderate toughness, and should be 

 practically free from clay, loam, and other foreign matter. In 

 general, the specification for coarse aggregate should be drawn with 

 a view to securing the best material available, and not merely to 

 exclude material of a known unsatisfactory character. 



The difficulties experienced in securing satisfactory coarse aggre- 

 gate frequently are due to a lack of facilities for preparing the 

 natural materials available locally, rather than to any lack of natural 

 materials. There are few gravel pits, for example, which furnish 

 gravel suitable for use as concrete aggregate without washing, and 

 this process, to be carried on economically, requires a rather ex- 

 pensive plant. It is necessary also that stone be quarried, and slag 

 must be selected carefully; and, moreover, both must be crushed 

 before they are suitable for use as concrete aggregate. 



The size of the particles composing the coarse aggregate is an 

 important consideration and usually is fixed by specifying upper 

 and lower limiting screens which' the coarse aggregate shall pass and 

 be retained upon respectively. Experience has shown that for con- 

 crete fundations the largest particles of coarse aggregate should be 

 such as just pass a screen having circular openings equal in diameter 

 to not more than one-third or less than about one-fourth the depth 

 of the foundation. The lower limiting screen should have openings 

 of from one-quarter to one-half inch in diameter according to the 

 amount of fine particles present in the unscreened material. It is 

 very desirable that the particles be graded in size between the upper 

 and lower limits specified so as to secure a dense mass. This per- 

 haps may be best assured by providing that not less than a certain 

 percentage of the coarse aggregate shall pass and not less than a 

 certain percentage be retained upon a properly selected intermediate 

 screen. 



Water. — The water used in mixing concrete should be reasonably 

 clear and practically free from alkalies, acids, vegetable matter, or 

 other injurious materials. Ordinarily very little difficulty is ex- 

 perienced in obtaining suitable water. 



PEOPOETIONING. 



The proportion in which the materials should be mixed for con- 

 crete to be used in road foundations depends, theoretically, upon the 

 percentages of voids in the sand and coarse aggregate. In theory, 

 the amount of cement should be just sufficient to fill the voids in the 

 sand and coat the individual sand grains with cement paste, and the 



