70 BULLETIX 724, U. S. DEPARTMENT OE AGRICL T LTURE. 
amount of sand and cement mortar should be just sufficient to fill the 
voids in the coarse aggregate and coat the individual particles of 
aggregate with mortar. In practice, the percentage of voids varies 
considerably, especially in the coarse aggregate, even with materials 
from the same source, and it is impracticable to vary the proportions 
to correspond with the voids. It is customary, therefore, to fix a 
definite proportion, by bulk, in which the materials shall be mixed, 
without further reference to the voids than to consider the general 
nature of the aggregates it is purposed to use. This arrangement 
works out satisfactorily, -provided the proportion be chosen intelli- 
gently. 
The percentage of voids in concrete materials average about as 
follows : 
Per cent. 
Sand 30 to 40 
Gravel 30 to\ 45 
Crushed stone 35 to 50 
Crushed slag 40 to 50 
The proportions most commonly used for road foundations are, 
where the coarse aggregate consists of gravel, 1 part of cement. 2J 
parts of sand, and 5 parts of gravel, or 1 part of cement, 3 parts of 
sand, and 6 parts of gravel; where crushed stone or slag is used as 
coarse aggregate, 1 part of cement, 3 parts of sand, and 5 to 6 parts 
of coarse aggregate. Sometimes, when it is purposed to use espe- 
cially well graded coarse aggregate and great strength for the con- 
crete is not deemed necessary, it may be permissible to specify a pro- 
portion of 1:3:7, but in general, no concrete leaner than 1:3:6 
should be used in road foundations without first making a series of 
tests to demonstrate whether satisfactory results may be secured by 
using a smaller proportion of mortar. 
CONSISTENCY. 
The two considerations that should determine the consistency of 
freshly mixed concrete to be used in road foundations are: (1) It 
should be sufficiently wet to be spread, shaped, and compacted with 
facility. (2) It must not be mixed so thin as to lose its crown or to 
flow on grades after it is struck off and shaped. It has been claimed 
frequently that a rather dry mixture, provided it is properly tamped, 
makes stronger concrete than wet mixture, and this claim no doubt 
is partly true. But the additional tamping necessary to secure a 
dense mass, together with the extra difficulties involved in striking 
off and shaping the dry mixture, add considerably to the cost of 
doing the Avork, and it is doubtful if the advantage gained is com- 
mensurate with this extra cost. When the concrete will hold a crown 
of at least one-quarter inch to the foot immediately after it is placed 
