6 BULLETIN 1077, U. S. DEPAETMEXT 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- 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 large 

 nimiber 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 diifer 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. 



-' " Concrite. Plain and Reinforced," by Taylor and Thompson. 



