PLAIN CONCRETE 235 



a dry place, is probably as dense a concrete, and as good for 

 waterproofing qualities, as can be made. 



When a concrete is made of cement, sand, and stone, and 

 the stone is of such a size that it will pass through a 3-in. 

 ring, but will not pass through a 2$-in. ring, the concrete is 

 weaker and requires more cement than one made with 

 graded stone from 3-in. down. When the stone is graded 

 in size, the stones of smaller size fill the voids between the 

 larger stones and thus reduce the quantity of cement and 

 sand required. 



Proportioning by Weight. A method of proportioning 

 the materials, that is simple and fairly accurate, is as fol- 

 lows: A batch of concrete is mixed in known proportions. 

 The same quantity of water is used that it is proposed to use 

 on the work, and the mixture is rammed and tamped in the 

 receptacle in a uniform manner. The receptacle should pre- 

 ferably be of metal; a tin wash tub, or a short section of 

 12-in. pipe, capped at one end, will answer. When the 

 receptacle is full, it is weighed, and if the weight of the recep- 

 tacle itself has previously been found, the weight of the con- 

 crete may be obtained. Various other mixtures of concrete 

 are tried in the same manner, and since the denser the mixture 

 the stronger it will be, the heaviest concrete is the strongest 

 for the particular work. Each batch of concrete must be 

 weighed and taken out of the receptacle before it has time 

 to set; otherwise, some difficulty might be experienced in 

 getting.it loose. 



Usual Proportions of Materials. The strongest concrete 

 does not always have to be used, as it may be required to 

 withstand only slight stresses and be simply used for its 

 weight. The strongest concrete would then be unnecessarily 

 expensive. Therefore, the foregoing method for proportion- 

 ing concrete is seldom employed. The engineer usually 

 specifies a mixture from his own experience without testing 

 the aggregates in any way, except to see that the stone is 

 under the specified maximum size and that the sand is in 

 large grains and free from dirt and loam. A common pro- 

 portion for unimportant work is 1-3-6. This proportion 

 may be used for foundations below ground, in engine bases, 



