70 BULLETIN 724^ U, S, DEPARTMENT OF AGRICULTURE. 



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, especiall}^ 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 j)ercentage of voids in concrete materials average about as 

 follows : 



Per cent. 



Sand 30 to 40 



Gravel : 30 toi 45 



Crushed stone 35 to 50 



Crushed slag 40 to 50 



The j)i'oportions most commonly used for road foundations are, 

 where the coarse aggregate consists of gravel, 1 part of cement, 2^ 

 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, wh.en 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 work, 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 imnue^iately after it is placed 



