42 The Colorado Experiment Station. 
This will require a working force of four men. After setting over 
night they may be raised and filled again the next day. Four men 
will be required to accomplish this labor of raising and filling. 
Since four men can fill the forms, no time is gained by increasing 
the size of the force. A day’s time is lost, however, when the forms 
are not filled each day. 
Material .—The selection of proper sand and gravel and deter¬ 
mining the proportions in which cement shall be mixed with the 
sand and gravel are vital factors in concrete silo building. 
The sand and gravel commonly used is found in mixed form in 
banks or creek bottoms. The fine sand and coarser gravel may be 
present in this mixture in the right proportion for making good 
concrete, but there is generally quite an excess of sand. This mix¬ 
ture may be tested by passing it over a * 4 -inch screen. There should 
be five parts of the coarse material to three of the finer. When this 
proportion prevails generally throughout the gravel bank, the two 
materials need not be separated but may be mixed with the cement 
at the rate of one part of cement to five of the bank mixture. 
If the proportion is undesirable as it comes from the bank, the 
sand and gravel should be separated and remixed. It should be 
borne in mind that one part of the cement is to be used for each 
three parts of sand, regardless of whether the coarser aggregate is 
present or not. Good walls can be made when there is not sufficient 
gravel to make the full five parts, provided the one part to three 
relation is maintained between cement and sand. Care should be 
taken that the sand does not contain dirt or clay. 
The desirable concrete mixture for concrete silos consists of 
one part cement, three parts sand and five parts crushed rock or 
gravel. 
It should be remembered that when one cubic foot of cement, 
three cubic feet of sand and five cubic feet of crushed rock or gravel 
are mixed together, the total is not equal to nine cubic feet. The 
cement fills the voids between sand particles and the sand particles 
fill the voids between the coarse aggregate or gravel. A strong 
mortar is made by mixing one part cement and three parts clean, 
coarse sand. This mortar will fill the voids between and cover five 
parts of rock or gravel. If the five parts of gravel are not present 
one part of cement must still be used for each three parts of sand. 
A cheaper wall is obtained when the full five parts of gravel or rock 
are present, since in this case less cement will be used. In certain 
cases, however, the gravel or rock are not obtainable. 
Sand. —Sand for concrete work should be coarse and free from 
clay, dirt and vegetable matter. If it is dirty it must be washed 
over a screen before being used. 
