WATERPROOFING 343 



and broken stone. This most waterproof mixture is also 

 the densest mixture. Therefore, in searching for the most 

 waterproof mixture of concrete, the engineer really finds 

 the densest mixture. For many grades of sand and broken 

 stone, a 1-1^-3 mixture is used. To be impervious to water, 

 concrete must be placed in a wet condition and be well 

 rammed into place 



Concrete, particularly that which is very dense, becomes 

 more impervious to water as it grows older. The first water 

 that penetrates it carries particles of clay and other material 

 that stops up its pores and gradually makes it more water- 

 proof. 



Adding of Lime or Clay. By hydrated lime is meant lime 

 that has been slaked in water. It can be bought com- 

 mercially in the form of a dry powder. The purpose of 

 adding hydrated lime to concrete is to fill mechanically the 

 voids in the latter. A small quantity added to the concrete 

 when it is being mixed does not greatly reduce the strength 

 of the latter. After the cement and lime are mixed until 

 the color of the mixture is uniform, the sand, broken stone, 

 and water are added as usual. The concrete should be made 

 wet, and great care must be exercised in bonding old and 

 new work. The amount of lime to be employed is usually 

 given as a percentage, by weight, of the cement. Under 

 ordinary conditions, for 1-2-4 concrete, 8% of lime will be 

 found sufficient, and for 1-3-6 concrete, 17% of lime will 

 be required. 



In place of hydrated lime, concrete is often waterproofed 

 by mixing with it finely ground colloidal clay. It is recom- 

 mended that clay equal to about 10% of the weight of the 

 cement be used in a mixture. The clay must be thoroughly 

 dry and well mixed with the cement. 



Sylvester Process. The Sylvester process of waterproofing 

 concrete consists in adding powdered alum and sojt soap to the 

 concrete. The sand and cement are mixed together dry, 

 as usual. To this mixture is added alum equal to 1 % of the 

 weight of the mixture. To the water to be used is added 

 1%, by weight, of soft soap, and this soap is then thoroughly 

 dissolved. The mortar is made wet, and the broken stone 



