128 CEMENT AND CONCRETE 



enough water to permit its flowing from the shovel or convey- 

 ors from elevators to various points in the construction of 

 large buildings. There is a tendency on the part of some 

 contractors to make the mixture very wet so as to make it 

 flow more easily. This will cause the separation of the coarse 

 materials from the finer and reduce the quality of the concrete. 

 One main point to remember in connection with the proper 

 consistency is that the materials must not be too dry nor too 

 wet; either condition will cause the separation of the coarse 

 material from the mortar. 



122. Placing of Concrete. No time should elapse be- 

 tween the "mixing" and the "placing." One's judgment 

 must be used in placing; the method adopted will depend on 

 the particular job. The essential feature in placing is to pre- 

 vent the separation of the stone from the mortar. 



123. Three Methods of Placing Concrete. 



1) A dry mixture of concrete is placed by thoro tamping or 

 by pressure. The density and the final strength of a dry mix- 

 ture will depend on the extent of tamping. This method of 

 placing concrete is used in making concrete products that are 

 not reinforced, such as blocks, bricks and jardinieres. The 

 material must be carefully tamped as the mold is being filled, 

 either by hand or by power machines. 



2) A quaky mixture can be placed by agitation or slight 

 tamping. This method is used in making reinforced prod- 

 ucts, such as posts, large tile and tanks; also for slab work, 

 such as floors and sidewalks. Some forms are designed so 

 they can be vibrated to settle the concrete into place. 



3) A wet mixture is simply deposited into place, and re- 

 quires no tamping. A spade or board should be used for 



