236 PLAIN CONCRETE 



in the foundations for asphalt pavements, and for similar 

 purposes. A richer mixture, 1-2-4 is used in piers, in 

 dams, in important reinforced-concrete work, and in other 

 places where great strength is desired. 



Water for Concrete. The wetter the concrete is, the easier 

 it will be put in place, but mixtures that are too wet are 

 not so strong as medium mixtures. The amount of water 

 that will make the best mixture is such that after the con- 

 crete has been put in place and rammed it will quake like 

 jelly when struck with a spade, and water will come to the 

 surface. If the concrete is wetter than this, the water will 

 have a slight chemical effect on the cement, and, moreover, 

 the sand and cement will tend to separate from the broken 

 stone. 



In cinder concrete, owing to the porosity of the cinders, 

 it is necessary to use a little more water, so that the cement 

 will be liquid enough to fill the little cavities in each cinder. 

 This precaution is indispensable when the concrete is to 

 be used with steel, as otherwise the steel will be rapidly 

 corroded by the action of air reaching it through the pores 

 in the cinder. 



Dry Concrete. With the advent of the concrete block, 

 a great deal is heard about dry concrete. This name is given 

 to concrete in which as little water as possible is mixed. 

 In the concrete-block manufacturing business, the mold 

 in which each block is made is required as soon as possible, 

 so that it can be used over again and thus increase the 

 capacity of the machine to which it 'belongs. For this 

 reason, the concrete-block manufacturers use, often, dry 

 concrete, and attempt to supply the remainder of the water 

 required for the complete crystallization, or setting, of the 

 cement by curing the blocks; that is, by sprinkling them 

 with water for a week or so. The results of recent tests 

 seem to indicate that dry concrete will show higher com- 

 pression values for a limited time after it is made, but that 

 the rate of increase of strength is not so great as with wet 

 concrete. After 1 yr. or 6 mo., the strength of the wet con- 

 crete will be found to have attained, and perhaps surpassed, 

 that of the dry mixture. 



