FOEMS 131 



cement is taken as the unit. A bag of Pretoria cement contains 188 lbs. 

 nett of cement, or just half as much as the above size of cask. Examples 

 of the use of the table will be found in Chapter XXVI. 



Mixing. — In this country it is advisable to mix concrete rather wet 

 especially if water-tight work is called for. In work of the latter class, 

 if the concrete runs off the shovel unless handled quickly, it will be of 

 about the correct consistence. The materials may be mixed on a 

 platform of iron sheets, or of wood having a raised border formed by a 

 strip of 2" by 3" timber nailed round the edge, to prevent waste of 

 water and materials. On this platform, the correct proportions of 

 sand and cement should be measured out, the former by the use of a 

 cask or measuring box (without top or bottom), and should be turned 

 over three times dry, or until the mixture assumes a uniform colour 

 throughout. 



The measured quantity of broken stone is now added, being 

 measured by the box or cask that was used to measure the sand. 



The mixture is then wetted with water applied by a watering can 

 with a rose, and is turned over three times wet. Care should be 

 taken that none of the fine cement is washed away while water is 

 being added. To prevent this the mixing platform should be more or 

 less water-tight. 



In turning the mixture, either dry or wet, it should be shovelled from 

 one pile into a distinct and separate new pile, and then shovelled back 

 again on to the site of the original pile ; by proceeding in this way we 

 can be quite sure that every particle has been turned over the required 

 number of times. 



As concrete begins to set in from 30 to 60 minutes (in the case of 

 ordinary slow-setting cement) after it has been mixed wet, it is most 

 important that it should be deposited in place and rammed before the 

 initial set commences. Concrete should therefore be mixed in small 

 quantities as required, and deposited at once, so as to catch the initial 

 set of the cement in the wall or work under construction. 



Forms. — The great advantage possessed by concrete lies in the 

 facility with which it can be moulded, or formed, to almost any desired 

 shape. 



For a plain wall, the forms may be made of horizontally placed, 

 tongued and grooved boards called sheeting, nailed to verticals called 

 studs. Flooring boards are suitable for the sheeting, and in many cases 



