136 ENGINEERING ON THE FARM 



are screened as they come from the bank and then remixed 

 in the proper proportions. The natural mixture of bank-run 

 gravel may be determined by screening of a cubic foot or a 

 cubic yard and measurement of the quantities which pass 

 through the screen. Gravel seldom occurs uniform in the 

 bank; coarse layers alternate with fine, while other strata 

 may be intermixed with silt and clay. The user of bank- 

 run gravel should be on the alert to detect any change in 

 grading that will affect the quality of the concrete. Much 

 poor concrete construction can be traced to natural mixed 

 gravel. 



MIXTURE 



Ideal concrete. In ideal concrete all of the materials 

 are graded and proportioned in size from the smallest to 

 the largest so that the voids are practically zero, and suffi- 

 cient cement is added to coat all of the individual particles. 

 The result is a mass similar to solid rock. It is not practical 

 to obtain the ideal. Where large quantities of concrete are 

 being used, it is possible, by a series of tests, to secure more 

 satisfactory results along this line than in smaller work. 

 The small user of cement will have to depend, in mixing 

 his materials for various purposes, upon formulas which 

 are accepted by custom as being satisfactory for each 

 purpose. 



Designation by volume. The mixture of cement, fine, and 

 coarse gravel is defined by numbers which represent volume 

 of each material used. A 1:2:4 mixture means that one 

 part, by volume, of cement has been mixed with two parts of 

 sand or screenings and four parts of gravel or crushed rock. 

 This may be measured in any convenient way, by half 

 cubic feet, by cubic feet, or by the cubic yard. If the 

 basis is 1 cubic foot of cement, it takes 2 cubic feet of sand 

 and 4 cubic feet of gravel to go with it; or if the basis is 

 yi cubic foot of cement, it takes 1 cubic foot of sand and 

 2 cubic feet of gravel to go with it. Either of these gives 



