PROPORTIONS OF INGREDIENTS 129 



otherwise the setting may have proceeded undesirably far before the 

 workmen have had time to deposit the concrete in place. 



If the concrete is to set properly so as to form a strong mass, it must 

 be kept moist for at least ten days after laying. This may be done either 

 by spraying the concrete with water twice a day, or by covering it 

 with wet sacks which are re-wet whenever they begin to dry. In the 

 case of more or less level surfaces, after the concrete has become a little 

 hard, the wet sacks may be replaced by a layer of moist sand. The sacks 

 are thus relieved for duty elsewhere. These precautions also serve to 

 protect the " green " concrete from the injurious effects of exposure to 

 hot sun, drying winds, and frost. 



Growth in Strength. — The strength of concrete continues to increase 

 for years after it has been laid. Up to an age of two or three months the 

 strength increases rapidly, but after that age the rate of increase in 

 strength is very slow. For this reason concrete should not be subjected 

 to stress too soon. Thus a reinforced concrete tank or silo should not be 

 filled for at least two months after the last concrete has been deposited. 

 Neither should earth be filled against a " green " concrete wall. 



Proportions of Ingredients. — For important works careful experiments 

 are carried out, with the object of so proportioning the ingredients as to 

 obtain a concrete of maximum density, maximum density having been 

 found to correspond to maximum strength. 



For small works, however, such elaborate experiments are dispensed 

 with, and arbitrary proportions are adopted according to the class of 

 work, the ordinary rule being to use twice as much broken stone as sand. 



A " rich " concrete is one in which the percentage of cement is high. 

 When the percentage of cement is low, the concrete is spoken of as 

 " lean." 



The following mixtures differ only in the relative quantity of cement 

 in each: — 



1. Proportions of 1 : 1£ : 3, i.e. 1 barrel packed Portland cement, to 



1£ barrels loose sand, to 3 barrels loose broken stone, is called 

 a rich mixture, and is employed for parts requiring exceptional 

 strength, or water-tightness, or both. 



2. What might be called a standard mixture is given by the pro- 



portions 1:2:4. It is suitable for engine foundations, 

 reinforced concrete work, tanks, conduits, and other water- 

 tight work. 



