904 PROCEEDINGS OF SECTION J. 



For concrete making the poor or hydraulic limes are best 

 adapted. In England the bine lias varieties are mnch used. 



It is necessary here to explain the advantage which arises 

 in treating the lime before it is mixed with the gravel or 

 stones. Beton differs from concrete in its being subject to 

 two operations. First, the hydraulic lime or cement is mixed 

 with sand and treated as a mortar, to which afterwards is 

 added the required quantity of aggregates. Concrete, how- 

 ever, as originally prepared in England, only consists of one 

 clumsy operation of mixing 'the matrix and aggregate 

 together. Hence it is presumably more correct to say that 

 beton is essentially a French process, and concrete the some- 

 what analogous one in England. In both cases the mixture 

 is accomplished with the same object, although with a 

 difference of detail. There can be no question that the 

 beton process is the more yterfect one, and especially when 

 the concrete is made into blocks or frames, offers great advan- 

 tages over the other. 



When however, it is used as in engineering works in large 

 masses or trenches, it involves a double operation ; first, the 

 preparation of the mortar, which is followed by its incorporation 

 with the larger ingredients, such as gravel, broken bricks, or 

 stone. When moulding the concrete the mortar can be used 

 simultaneously with the gravel, and under such circumstances 

 with beneficial effect. Its use in this way secures a solid mass, 

 having a minimum of interstitial space. In all concretes it is 

 necessary to adjust the proportions of the hydraulic lime, 

 sand, and gravel so that no vacuities will occur in the mixture. 

 The larger the size of the aggregate the more necessary it is 

 that attention should be paid to this point. With an aggregate 

 of an average size of say two inches it will be found that in 

 every cubic yard there will be vacuities equal to about IJ 

 cubic feet, so that the mortar should be equal in quantity to 

 the interstitial space. This vacant space will, of course, vary 

 with the size or particles of the aggregate, and the amount of 

 shrinkage will also fluctuate accordingly. When in a dry 

 state it will shrink less than when wet in proportion to its 

 specific gravity. A silicious or quartzose sand has a specific 

 gravity of 2-6, and a solid cubic foot of it would therefore 

 weigh about 162| lbs. — a cubic foot of water weighing 1000 

 ozs. Sand of this kind, without being specially dried, when 

 filled into a measure of a cubic foot, however, weighs only 

 75 lbs., showing that the space between the grains was nearly 

 equal to their own bulk. The weight of sand of the above 



