MATERIALS. 115 



In depositing lime concrete for under-water foundations, it 

 is sometimes merely thrown into the water from baskets carried 

 upon the head, timber frames and boarding being used to give 

 it the requisite shape, and to retain it in place till hard. 



Although, as has already been said, lime concrete is but little 

 used in this country for sea-works, it is largely employed in 

 dock works, and makes excellent work. 



In the Liverpool Docks and London Docks, also at Glasgow, 

 Fleetwood, and many other places, hydraulic limes have been 

 used with marked success. 



Proportions of Ingredients for Concrete. In determining the 

 proportions of the ingredients for concrete, the mass should be 

 looked upon as so much rubble masonry. 



It is evident that the strength of the mass will depend upon 

 that of the matrix, or mortar, in which the broken stone or 

 pebbles are embedded. Therefore, if it be decided that concrete 

 shall have the strength of, say, 2 to 1 cement mortar, sand and 

 cement should be mixed in these proportions, and a trial be 

 made to see how much broken stone or shingle can be mixed 

 with it, so as to ensure all the interstices being well filled, and 

 a thoroughly good pasty mass formed. This will vary according 

 to the size of the broken stone or shingle used, and it will also 

 be affected by the incorporation, or otherwise, of large pieces 

 of rubble stone in the mass. If these be introduced, it will be 

 found necessary, in order to ensure their being thoroughly well 

 bedded and surrounded by the concrete an important point 

 to attend to to make the concrete more pasty and slightly 

 wetter than if no rubble were incorporated. 



Later on I will give the proportions which were adopted 

 for concrete in several sea-works as actually executed. 



In the preparation of concrete, great care should be exercised 

 in selecting proper materials. These should be as hard and heavy 

 as possible, and perfectly free from all earthy or soluble matter. 



They should be thoroughly mixed in their dry state, and 

 again after adding the water, which should be quite clean, and 

 only sufficient to give the concrete a moderately thick pasty 

 and slimy consistency. 



It is not easy to determine the exact quantity of water except 

 by trial with the materials in use. Even with the same materials, 

 if exposed to the weather, the quantity of water required will 

 vary much, seeing that in wet weather the materials are already 



