142 ENGINEERING ON THE FARM 



spread out the mixture and place the measuring box on top. 

 Measure out 4 cubic feet of rock or gravel. Spread this 

 over the surface of the sand and cement and mix as before. 

 Two or three times will do. Now shovel the mixture into 

 a ring-shaped pile and place the water in the center of the 

 ring, not all at once, however. Start with the inner edges 

 of the ring and mix the center first, adding more water as 

 required and being careful that the water does not break 

 through in large quantities and wash the cement out with it. 

 (See Figs. 137 and 138.) 



Mechanical mixers. Contractors or others engaged in 

 cement construction ordinarily use mechanical mixers. 

 These are of two kinds. The continuous mixer is one in 

 which the materials are placed in the hopper and concrete 

 is continually being delivered, ready mixed for spreading. 

 A poor mixture is frequently secured from this machine for 

 the reason that the materials are not put into it at a uniform 

 rate. The batch mixer avoids the fault of the continuous 

 mixer by having the materials measured into the hopper 

 and then the entire batch is mixed and run out before a new 

 one is placed in the machine. Consequently the batch 

 mixer is the more satisfactory. These machines are made 

 in sizes to suit the various kinds of work and are usually 

 operated by a gas engine. 



Consistency. The quantity of water added depends on 

 how the concrete or mortar is to be used. Three classes of 

 consistency are in use: moderately wet, that is, just wet 

 enough to run off the shovel or pour easily from a wheel- 

 barrow or bucket; quaking or jelly-like, so that it will 

 tremble when spaded; and dry as damp earth, so that it 

 will hold its shape when squeezed in the hand. The moder- 

 ately wet mix is used for pouring into forms such as walls, 

 columns, etc., the quaking mix for such class of work as 

 floors, and pavements, and the dry mix is tamped into forms 

 where no reinforcement is used, such as curbing and gutters. 

 The dry mix should never be used when reinforcement is 



