48 



displacement apparatus. The mix has been specified 

 in an office before any material was received on the 

 work and is consequently little more than an assump- 

 tion. So many conditions, such as loading, crush- 

 ing, etc., enter into the grade of the aggregate. Per- 

 haps it runs too fine, and the specified mix, say 1-2-4, 

 runs too high in sand. Make a mixture without ce- 

 ment of 2 parts of sand to 5 parts of stone, and see 

 whether or not this weighs more or displaces more 

 water than a mixture of 2 parts of sand to 4 parts of 

 stone. If the stone is large and unusually clean a 2 to 

 4 combination might not be nearly as dense (i. e., 

 displace as much water, or weigh as much) as a 2 to 

 3 combination. 



By all means use the proportion between sand and 

 stone, which is the most dense. This means a combi- 

 nation in such proportions to weigh the most, or dis- 

 place the most water for a given volume. 



Here we simply have touched upon an elemental 

 field-method of determining "voids by water displace- 

 ment." Using a box 10 in. x 10 in. x 10 in. would give 

 1,000 cu. in. and the ratio could be figured direct. For 

 active field work, however, experimenting with differ- 

 ent proportions and determining which of these are 

 the most dense, is undoubtedly the best way. If a 

 1-2-4 mix is specified and the stone is too large and 

 clean to give you a well graded concrete, yon are, we 

 believe, on consultation with the office, justified in the 

 interests of good work, in interpreting this to mean I 

 part of cement to 6 parts of sand and stone, and de- 

 termining the ratio between the sand and stone by ex- 

 periment. 



Mixing Plants. The layout of the mixing plant is 

 a question which cannot be derided on paper or in 

 any book, no matter what its size. Each piece of work 

 is a new problem, and it behooves the field superin- 

 tendent to study well the conditions, analyze the com- 

 ponent factors and realize the results to be obtained. 



