CONCRETE 



137 



a 1:2:4 mixture, but a batch of different size in each case. 

 A common error is to think that this combination of one 

 unit of cement, two units of sand, and four units of gravel 

 will give a mixture 1 + 2 + 4 = 7 units in volume. It will 

 not, when mixed, give seven units in volume as a result, 

 because the cement fills m the voids in the sand, and the 

 sand plus the cement fills in the voids in the gravel, thus 

 combining to give, when all mixed together, but very little 

 more than four units of gravel (perhaps 15 per cent more). 

 Figure 134 illustrates this point. 



Another common error is to consider a so-called 1 : 6 

 mixture, meaning a combination of one unit (by volume) 

 of cement with six units of bank-run gravel, as giving the 

 same result as this 1:2:4 mixture. It is evident that we 

 obtain a weaker mixture in the 1 : 6 combination than in the 

 1:2:4, because in the finished product we have one unit of 

 cement mixed with six units of aggregate in the first case as 

 against one unit of cement mixed with about four units 

 of aggregate in the second case. 



;<"?!?■;;■:■:••. ■-'.; 





fa/.f) 'cement 



2cu.ft.jana 1 



•4<u/y.fra/e/ 



•••».'.•«■ -t ■*. 

 ..•>.j-vv '':> * 



/. £. 4 Concrete 



Fig. 134. Diagram showing total volume made by combining i part cement, 

 2 parts fine aggregate, and 4 parts coarse aggregate 



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