WASHING AND GRADING SAND FOR CONCRETE, XXI. 
2. Measure the amount of contained sand in the aggre- 
gate, and this having been removed, determine the 
proportion of voids; observe also the quantity of water 
absorbed. 
3. Choose the quality of cement to be employed in the 
work. 
4, Calculate how much cement, sand, and water will be 
required for some definite amount of the aggregate—say 
a cubic yard—making allowance for the sand in the aggre- 
gate and the water absorbed. 
do. Have boxes or measures made to contain the calcu- 
lated amounts of cement and sand and water. Inthe case 
of water a margin, of say 10 to 20 per cent. overplus, 
should be allowed. 
The following is a short description of a method for 
determining the voids, etc., in an aggregate :— 
Obtain some water-tight receptacle with an arrangement 
at the bottom for letting water drain off. The size is 
immaterial, but the larger it is the more accurate will be 
the experiment, but also more difficult will be the apparatus 
to handle. Somewhere in the neighbourhood of half a cubic 
yard will be convenient. Call this the “ tank.”’ 
Now find how many times (say 7) some smaller receptacle, 
which may be called the bucket, is contained in the tank, 
which is most conveniently done by filling the tank with 
water, one bucketfulatatime. The capacity of the bucket 
need not be determined, but it can be called q. Hence 
the capacity of the tank isuq. After letting off the water 
fill the tank with dry aggregate, without removing the 
contained sand. Now fill up the tank with water, and 
suppose it holds just w bucketsful before overflowing, 
sufficient time being allowed for the aggregate to absorb 
the water. Let off the water and fill up again, and say 
that the number of bucketsful is x. 
