PLAIN CONCRETE 237 



A serious objection to dry concrete is that it cannot be 

 rammed to so dense a mass as wet concrete. Therefore, 

 if a waterproof concrete is desired, it should be mixed wet. 



PROPERTIES OF CONCRETE 



GENERAL CHARACTERISTICS 



Corrosion of Steel in Concrete. It has been conclusively 

 proved by experience and test that steel or iron completely 

 embedded in the usual mixture of concrete will not corrode 

 seriously. Portland cement contains free alkali, and steel 

 or iron will not rust in the presence of an alkali. Corrosion 

 will occur only where the concrete has been carelessly placed 

 and where voids in the concrete have exposed the metal. 



Wet concrete offers more protection to iron or steel than 

 a dry mixture, because the metal is better coated with the 

 cement mixture. Cinder concrete, when of a rich mixture 

 at least one sufficiently rich in cement to coat every particle 

 of the cinder will protect ironwork or steelwork as well 

 as stone concrete; but, if it is not properly mixed and particles 

 of the uncoated coal or cinder come in contact with the steel, 

 rapid corrosion is likely to take place. 



Effect of Fire on Concrete. Concrete is essentially a fire- 

 proof material. All the ingredients of which it is composed 

 are of a highly refractdry nature, the aggregates being the 

 elements of the mixture that are most quickly affected by 

 intense heat. This is especially true of granite and lime- 

 stone aggregates, the former being likely to crack or burst 

 when heated, and the latter liable to calcine. After cement 

 has set, the chemical union of its particles is liable to destruc- 

 tion by fire, because intense heat robs the cement of the 

 water of crystallization, or dehydrates the cement, thus 

 softening the material and making it crumbly. If concrete 

 in a mass is subjected to intense heat, this action of dehydra- 

 tion extends into the concrete for a depth of only J to ^ men, 

 and is not likely to penetrate farther. 



Considere, a French concrete expert, has found by experi- 

 ment that a 1-3 mortar will shrink from about .05 to .15% 



