ADVANTAGES OF CONCRETE ON FARMS 153 



tained, and it is frequently used on the farm and in small 

 jobs of concrete work just as it comes from the pit or 

 creek. Occasionally this gravel contains nearly the right 

 proportions of sand and gravel, but in the majority of 

 sand pits and gravel banks there is a great variation in 

 the sizes of the grains and pebbles or gravel and in the 

 quantities of each. This is due to the fact that all the 

 deposits are formed in seams or pockets that make it im- 

 possible to secure anything like uniformity. Therefore, 

 to get the best and cheapest concrete, it is advisable to 

 screen the sand and gravel and to remix them in the cor- 

 rect proportions. 



Dirty sand makes a weak concrete. Crushed rock is 

 much better than screened gravel because of the rougher 

 edges. 



As a test of sand, rub it in the hand and if there is 

 much dirt left on the hand discard that sand. 



If, when a large handful of the same is thrown into a 

 pail of water, it leaves the water muddy, discard it. 



Following are the four recognized mixtures for con- 

 crete : 



Rich mixture — One part Portland cement, two parts 

 of clean, coarse sand, four parts of crushed rock. This 

 is used for floors, fence posts, and the like. 



Medium mixture — One-half and one and two-fifths 

 parts respectively of cement, sand and crushed rock. 

 This mixture is used for walks and thin walls. 



Ordinary mixture — 1-3-6 for heavy walls, piers, abut- 

 ments, etc. 



Lean mixture — 1-4-8 for footings and in places where 

 volume and not great strength is needed. 



When gravel is used, the proportions are one part of 

 cement and from six to nine parts of gravel, according to 

 the amount of sand in the gravel. 



To make one cubic yard of concrete the following 

 respective amounts of cement are required: Rich mix- 



