246 PLAIN CONCRETE 



WORKING OF CONCRETE 



Mixing of Concrete. Concrete may be mixed either by 

 hand or by machine. For small work, the concrete is mixed 

 by hand in small batches, such as would be made up from 

 1 of 2 bags of cement. In mixing, hand work should be 

 performed on a flat, water-tight platform. The sand, after 

 it has been measured, is spread over the platform in an even 

 layer. Upon the sand is placed the cement, and these two 

 materials are turned over with shovels at least three times, 

 or until the uniform color of the mixture indicates that they 

 are thoroughly incorporated. The stones, or aggregates, 

 having previously been well wetted, are then placed on the 

 top of the mixture of sand and cement and these materials 

 are also turned at least three times, water being added after 

 the first turning. The water should always be added in 

 small quantities. If a hose is used for this purpose, it should 

 be fitted with a sprinkling nozzle, as otherwise much of the 

 cement is liable to be washed out of the mixture. The 

 concrete, when ready for placing, should be of uniform con- 

 sistency, either mealy for a dry mix or mushy for a wet mix. 



In large work, the mixing should be done by machine. 



Retempering of Concrete. If the cement of the concrete 

 has attained its initial set before being placed that is, if 

 the concrete has commenced to harden remixing with 

 water, or retempermg of concrete, as it is called, should not 

 be allowed; and if concrete treated in this manner has been 

 deposited in the forms, it should be taken out and removed 

 from the site of the operation, because concrete cannot be 

 retempered properly, except in small quantities for labor- 

 atory tests. 



Concreting at High Temperatures. If the weather is 

 extremely warm, the stone and sand are liable to become 

 heated to a high temperature. Then, in mixing the materials, 

 the water necessary for the crystallization of the cement is 

 rapidly absorbed by the stone and the sand, or else rapidly 

 evaporated by contact with them. Again, the extreme 

 heat will hasten the setting of the cement, and this tends 

 to cause the concrete to cake in the mixing machine, pro- 

 ducing lumpy and inferior concrete. In order to overcome 



