CONCRETE 



17 



a variation in compressive strength at the end of 1 month, for 

 concrete of ordinary proportions, from 1500 to 4000 Ib. per square 

 inch. Under reasonably good conditions as to character of 

 material and workmanship, an average strength of 2000 Ib. per 

 square inch may be expected of a 1:2:4 concrete at the end of 

 1 month; and for a 1:3:6 mixture, a strength of 1600 Ib. per 

 square inch. 



Taylor and Thompson present a practical working formula of 

 sufficient accuracy to compare the compressive strength of 

 mixtures of the same materials in different proportions. The 

 values in the following table 1 have been obtained from this for- 

 mula based on cube specimens and medium consistency: 



Note. Proportions are based on a barrel of 3.8 cu. ft. Values are for average ultimate 

 strength, which must be divided by a factor of safety for working loads. Quality of mate- 

 rials and methods of mixing may affect the strength by 25 per cent in either direction, while 

 the relative values for different proportions are not materially changed. 



1 From Taylor and Thompson's "Concrete, Plain and Reinforced," 2nd edition, page 360. 



2 Use 50 per cent columns for crushed stone screened to uniform size. 



3 Use 45 per cent columns for average conditions and for crushed stone with dust screened 

 out. 



* Use 40 per cent columns for gravel or mixed stone and gravel, 

 6 Use these columns for graded mixtures. 



