12 
BULLETIN 370, U. S. DEPARTMENT 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 1911 by the American Society of Municipal Improve- 
ments 1 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. 
Tablf 
IV. — Geographical distribution of samples tested. 
State. 
Number 
of 
samples 
tested. 
State. 
Number 
of 
samples 
tested. 
State. 
Number 
of 
samples 
tested. 
Alabama 
29 
3 
14 
101 
21 
43 
30 
9 
157 
9 
122 
151 
23 
11 
41 
7 
72 
116 
Massachusetts 
Michigan 
179 
84 
16 
11 
33 
4 
11 
22 
72 
136 
137 
138 
50 
14 
599 
38 
26 
South Dakota... 
11 
61 
Arkansas 
Minnesota 
62 
California 
Mississippi 
Utah 
13 
Colorado 
32 
Connecticut 
Montana 
404 
Delaware 
Nebraska 
212 
Florida 
New Hampshire 
New Jersey 
139 
Georgia 
139 
Idaho 
3 
North Carolina 
Ohio. . 
Indiana. . . 
3,605 
49 
Iowa 
Kansas 
Oregon 
12 
Kentucky 
Cuba.... 
4 
Total 
Maine 
South Carolina 
3,670 
Maryland 
1 Proceedings of the 
ments, p. 511. 
1914 Convention of the American Society of Municipal Improve- 
