FARM MANAGEMENT 



57 



is in most cases accepted as indicating the strength of the cement. 

 (F. B. 235, 461.) 



Proportions of Mixture. For an accurate determination of 

 the best and most economical proportions where maximum strength 

 is required, it is well to proceed in the following way: First, pro- 

 portion the cement 'and sand so that the cement paste will be 10 

 per cent in excess of the voids in sand; next, determine the voids 

 in the aggregate and allow sufficient mortar to fill all voids, with an 

 excess of 10 per cent. 



The proportions are always measured by volume. A 1:2:4 

 mixture means one part of cement, twice as much sand, and four 

 times as much stone or gravel, so that the whole mixture consists 

 of seven parts. A 1 :2Vfc :5 mixture means one part of cement, two 

 and one-half times as much sand, and five times as much stone or 

 gravel, so that the whole mixture consists of eight and one-half 

 parts. It should be noticed that the dimensions given for the 

 measuring boxes for sand and stone or gravel are inside measure- 

 ments. These boxes are made with straight sides of any kind of 

 rough boards and have no top or bottom. 



Variations in Mixture. If the sand is very fine, the cement 

 should be increased from 10 to 15 per cent. When the mixture 

 does not have a uniform color, but looks streaky, it has not been 

 fully mixed. If the mixture does not work well, and the sand andi 

 cement do not fill the voids in the stone, the percentage of stone 

 should be reduced slightly, but the concrete should first be properly 

 mixed. Concrete that is poorly mixed may present features that 

 are entirely eliminated by turning it over once or twice more. 



Determination of Quantities. The number of cubic feet of 

 concrete that will be required for the work in question should first 

 be calculated. By multiplying this number by the number under 

 the proper column in the following table the amount of cement, 

 sand, and stone or gravel can be found. 



Quantities of Materials in 1 Cubic Foot of Concrete. 



(F. B. 461.) 



Equipment for Mixing Concrete. When the proper materials 

 .have been selected, the next step is to mix them properly and with 

 dispatch. On large jobs it is more economical to mix concrete by 

 machine, but for small jobs, using even as much as several hundred 

 cubic yards of concrete, it is much cheaper and more expedient to 

 mix by hand. This is, of course, especially true when only two or 

 three men are available and the work is often interrupted. There are 

 many ways of mixing by hand, all of which have the same good re- 



