CONCRETE CONSTRUCTION 255 



of rich concrete, troweling it up the sides 

 as far as the tar has been placed. When an 

 absolutely watertight job is required it may 

 be necessary to coat the entire outside sur- 

 face of the walls with tar and then bank up 

 with earth. 



Several so-called waterproofing materials 

 designed to be placed in the concrete when 



Fig. 131. A small power machine for mixing concrete 



it is mixed are on the market, but as a rule 

 they are not fully satisfactory. 



Concrete Blocks. Blocks made of con- 

 crete in special molds or forms are sometimes 

 employed for, walls. They are usually hol- 

 low and for that reason make a warmer and 

 somewhat dryer wall than does solid, poured 



