6 BULLETIN 1077, U. S. DEPARTMENT OF AGRICULTURE. 
and water carrying sewage or manufacturing wastes should not be 
used until tests have shown that it will not impair the strength of 
the concrete. For a description of a test to determine the quality of 
water, see United States Department of Agriculture Bulletin 949. 
REINFORCEMENT. 
Wire mesh, expanded metal, or steel rods may be used to reinforce 
the pavement. In any case the reinforcement should be reasonably 
free from rust, or other coatings, and should be so handled prior to 
use that it will not be coated with mud or clay when placed in the 
pavement. 
PROPORTIONING. 
The physical characteristics of the concrete are determined not 
only by the quality of the several materials which enter into it, but 
also, and perhaps to a greater degree, by the proportions in which the 
materials are mixed, especially by the amount of water used. A 
number of theories are offered concerning the proportions required 
to produce strong and economical concrete. All are based on experi- 
mental data, but at present no particular one is generally accepted, 
and a great deal of investigation is being carried on in the attempt to 
evolve a theory which will be generally acceptable. 
The theory most generally accepted in the past is called the maxi- 
mum density theory 2 and is based on the assumption that with a 
given amount of cement the strongest concrete is secured with aggre- 
gates graded and mixed so as to have the least amount of voids, 
without an excess of fine material. It has been found from a lar<re 
number of tests that the average ratio of fine to coarse aggregate 
for maximum density is approximately 1 to 2, a fact which accounts 
for the rule-of-thumb mixes, as, for example, the 1:2:4 mix, which 
means 1 part of cement to 2 parts of sand and 4 parts of coarse ag- 
gregate. If greater strength is required a 1 : 1| : 3 mix is used ; if less 
strength is needed the proportions 1 : 2| : 5 or 1 : 3 : 6 may be adopted, 
but the practice is to maintain the ratio of 1 to 2 between the volumes 
of the two aggregates. A large amount of concrete has been mixed 
according to these rules, but objection is made to them on the ground 
that the particular aggregates used may differ materially from the 
average. Good aggregate and cement mixed according to the maxi- 
mum density theory with a proper amount of water will produce a 
good concrete, but the theory itself does not take into account the 
amount of water to be used. Lately the amount of water has been 
found to exert a most important influence on the strength of the 
concrete. Excess, it has been found, invariably brings about a de- 
crease in strength. 
2 " Concrete, rialn and Reinforced," by Taylor and Thompson. 
