12 



BULLETIN 370, U. S. DEPAETMENT OF AGRICULTURE. 



PORTLAND CEMENT CONCRETE AND STONE BLOCK. 



The most desirable limitations for broken stone to be used as coarse 

 aggregate in Portland cement concrete wearing surfaces has not as 

 yet been ascertained. In general, however, it would seem that the 

 low limit for hardness should be no less than the hardness of the 

 mortar which binds the rock fragments together. At the present 

 time a minimum hardness of 12 for moderate and 16 for heavy traffic 

 would appear reasonable. In consideration of the type of traffic to 

 which concrete roads are subjected, a minimum toughness of 8 is 

 suggested. 



Stone blocks are usually manufactured from granite or sandstone, 

 although other rocks may also be used. Specifications for granite 

 block adopted in 1914 by the American Society of Municipal Improve- 

 ments ^ call for a toughness of not less than 9 and a crushing strength 

 of not less than 20,000 pounds per square inch. It would appear wise 

 to also require that the hardness be not less than 16. 



APPENDIX. 



The results of all of the physical tests made on rock samples in the 

 laboratory of the Office of Public Roads and Rural Engineering 

 from the date of its installation in 1902 up to January 1, 1916, are 

 included in Table V, together with the results obtained by Logan 

 Waller Page for the Massachusetts State Highway Commission 

 previous to 1902. 



The rocks are classified according to their location, so that this 

 table shows the availability and character of the materials, as far as 

 they have been tested, throughout the United States. 



Table IV shows the number of samples of material tested in the 

 different States. 



Table IV. — Geographical distrihution of samples tested. 



state. 



Number 



of 

 samples 

 tested. 



State. 



Number 



of 

 samples 

 tested. 



State. 



Number 



of 

 samples 

 tested. 





29 



3 



14 



101 



21 



43 



30 



9 



157 



9 



122 



151 



23 



11 



41 



7 



72 

 116 



Massachusetts 



179 



84 



16 



11 



33 



4 



11 



22 



72 



136 



137 



138 



50 



14 



599 



38 



26 



South Dakota 



11 





Michigan 



Tennessee 



61 



Arkansas 



Minnesota 



Texas.. . . 



62 





Mississippi 



Utah 



13 



Colorado 



Missouri 



Vermont 



32 



Connecticut 



Montana 



Virginia . . 



404 



Delaware 



Nebraska. 



New Hampshire 





212 



Florida 





139 



Georgia 





139 



Idaho 



New York 





3 



Illinois 



North Carolina 



Ohio 



Canada 





Indiana 



3,605 



Iowa 



Oklahoma . .... 



49 



Kansas 



Oregon 





12 



Kentucky 



Pennsylvania 



Cuba 



4 







Total 





Maine 



South Carolina 



3 670 



Maryland 











^ Proceedings of the 1914 Convention of the American Society of Municipal Improve- 

 ments, p. 511. 



