RELATION BETWEEN THE PROPERTIES OF HARD¬ 
NESS AND TOUGHNESS OF ROAD-BUILDING ROCK 
By Provost Hubbard, Chemical Engineer , and F. H. Jackson, Jr., Assistant Test¬ 
ing Engineer y Office of Public Roads and Rural Engineering 
It has for some time past become increasingly evident to engineers 
interested in the testing of road materials that from the standpoint of 
the road builder some of the most important physical properties of rock 
are not independent, but are more or less definitely related to each other. 
In 1913, Mr. L. W. Page, 1 Director of the United States Office of Public 
Roads and Rural Engineering, called attention to some of these points, 
and suggested that, as the volume of data relating to the subject became 
greater, it might be possible to determine the dependent variable by 
reference to suitable curves showing the relative values of tests for 
thousands of individual cases, and thus dispense with one or more of the 
tests now in use. The large amount of additional data which have 
accumulated since that time makes it possible to take up the subject 
again, with a view to determining just what physical tests are necessary 
in order to judge properly the fitness of a rock for use in road construction. 
It is now generally recognized that any stone, to be suitable for use in 
macadam construction, must possess to a certain degree, depending on 
circumstances such as character of traffic and method of construction, 
three distinct physical properties, which may be briefly defined as 
follows: 
(1) Hardness, the resistance which a rock offers to the displacement of 
its surface particles by abrasion; 
(2) Toughness, the resistance which a rock offers to fracture under 
impact; 
(3) Binding power, the ability which the dust from the rock possesses, 
or develops by contact with water, of binding the larger rock fragments 
together. 
Of these, the first two are of particular interest from the standpoint of 
the present discussion, and they may be very briefly described as follows: 
The degree of hardness of a rock is determined by what is known as the 
Dorry method. It consists essentially of subjecting a cylinder, 25 mm. 
in diameter, of the material to be tested, to the abrasive action of crushed 
quartz sand fed upon a revolving steel disk, against which the test 
1 Page, L. W. Relation between the tests for the wearing qualities of road-building rocks. In Amer. 
Soc. Testing Materials, Proc. 16th Ann. Meeting, 1913, v. 13. P- 983-992, 7 fig-> 1913. Discussion, p. 993- 
995 - 
-Tests of materials used in the construction of macadamised roads. Permanent Intemat. Assoc. 
Road Cong., 3d Cong. London, 1913, Rpt. 76, 27 P-, 15 fig- * 9 i 3 - 
Journal of Agricultural Research, Vol. V, No. 19 
Dept, of Agriculture, Washington, D. C. Feb. 7, 1916 
cf D 3 
22532°—16 - 3 
(903) 
