CONCRETE 15 



mate rule devised by William B. Fuller gives the quantities 

 of packed cement, loose sand, and loose stone required, to make 

 a cubic yard of concrete. Let c, s, and g be the number of parts 

 by volume of cement, sand, and stone, respectively. Also, let 

 C, S, and G be the required number of barrels of packed cement, 

 the required number of cubic yards of loose sand, and the required 

 number of cubic yards of loose stone, respectively. Then 



c+s+g 



~ 



If the stone is of nearly uniform size, about 5 per cent should be 

 added to all quantities computed by the above rule and, on the 

 other hand, if the stone is well-graded, about 5 per cent should 

 be deducted. Although stone is sometimes screened to approxi- 

 mately one size, it is only a waste of labor and material, for the 

 screened stone makes a weaker concrete, and requires more 

 cement. 



10. Compressive Strength. The compressive strength of 

 concrete varies within wide limits, due to the fact that there are 

 so many reasons for variation. The principal factors which 

 affect compressive strength are: (1) the quality of cement used; 

 (2) the quantity of cement in a unit volume of the concrete; (3) 

 the character and size of the aggregates; (4) the density of the 

 concrete; (5) the care taken in mixing; (6) the age of the mixture; 

 and (7) the conditions under which the concrete seasons. 



Because of the different conditions met with in practice, it is 

 somewhat misleading to present average values for the compres- 

 sive strength of concrete. Where possible, the strength of any 

 given concrete should be determined by actual tests and, if the 

 results are too low, the ingredients or proportions should be 

 changed until a satisfactory result is obtained. 



The two chief factors which determine the compressive strength 

 of concrete, considering the materials as satisfactory, are age 

 and proportions of ingredients. The relative amount of increase 

 in strength of concrete, from 7 days to 6 months, for two common 

 mixtures, is shown approximately in the table which follows. 

 The table gives average values of the compressive strength 

 in pounds per square inch based on tests made for the Boston 



