274 Journal of Agricultural Research voi. x, no. s 



practice. This would seem to be allowable, especially in view of the 

 large excess of cement that is always used. When one considers that a 

 large proportion of the wearing surface of a concrete pavement is com- 

 posed of mortar, the danger of using a fine sand, with subsequent weaken- 

 ing of the matrix, is apparent. 



In the foregoing discussion it is realized that but few naturally occurring 

 concrete sands are as coarse as those making the strongest mortars, ac- 

 cording to these tests. Neither has the fact been overlooked that the best 

 mortar, when combined with stone or gravel, without reference to its 

 grading, will not necessarily produce the best concrete. A poorly graded 

 coarse aggregate will unquestionably require more mortar than will a 

 well-graded one. Likewise, a coarse aggregate containing a large amount 

 of small stone will allow the use of a somewhat finer sand than when the 

 larger-sized stones predominate. When it is considered, however, that 

 the mortar in concrete forms a matrix by which the larger stones are held 

 in place, that this matrix occupies nearly one-half the total volume of the 

 concrete, and, finally, that its strength and toughness are undoubtedly 

 influenced to a large degree by the grading of the sand, the discussion 

 becomes of practical value. So important does it become that it might 

 even be considered practical to use a graded rather than a naturally 

 occurring concrete sand in such important work as concrete-road con- 

 struction if by so doing the life of the pavement can be prolonged. 



ADDITIONAL COPIES 



OF THIS PUBUCATION MAY BE PROCTJRED FROM 



THE SUPERINTENDENT OF DOCUMENTS 



GOVERNMENT PRINTING OFFICE 



"WASHINGTON, D. C. 



AT 



25 CENTS PER COPY 



Subscription Price, $3.00 Per Year 



A 



