42 



FIELD MANUFACTURE OF CONCRETE. 



Too much importance cannot be attached to the care- 

 ful selection and grading of the inert materials enter- 

 in.? into combination with cement to produce concrete. 



Natural sand is, so far, the most commonly used 

 granular inert. The bulk is supplied by the use of 

 gravel, crushed stone of various kinds, crushed furnace 

 slag, etc. What demands our earnest and constant at- 

 tention is a practical method or methods of securing 

 data upon which to determine the ratio between the 

 quantities of the different sized inerts. 



Proportioning and Grading Aggregate. The vol- 

 umes of the materials entering into concrete are sub- 

 ject to variations due to handling, conditions of load- 

 ing, piling and so forth. 



The weight is a constant factor. In laboratory work 

 proportioning is generally, if not always, done in trns 

 way. Quantities using volume units of measure are 

 not exact, although as conditions are at present, suf- 

 ficiently accurate to meet the demands of the field. 



The ideal concrete is one in which the amount of 

 water is inst sufficient to supply all that is needed for 

 the crystallisation of the cement: where the amount of 

 this neat cement pnste is sufficient to fill every void in 

 the particles of sand and where this mortar of sand 

 and cement is sufficient to fill all the voids in the stone 

 or slacr. or whatever asrcrreeate is used. This. then, 

 would he your ideal concrete, dense and wnternroof. 

 the maximum strength and quality for the priven ma- 

 terial. What methods ami means can we adopt in 

 actual field construction to secure as nearly as possible 

 this quality of concrete? 



For discussion, let us assume the use of a crushed 

 limestone ard hank sand. We wish to combine these 

 two materials in such proportions 'as to secure the 

 most den c e combinations. 



One method is to determine what is termed the 



