MATERIALS. 119 



formed either by means of cast-iron tapered core-boxes, which 

 leave the blocks when lifted, or by rough wooden boxes, which 

 are allowed to remain in the blocks. Upon the top of these 

 boxes whichever kind may be adopted pieces of evenly and 

 finely dressed hard stone, hard wood, or other unyielding material 

 are placed and built into the blocks, for the T-heads to take 

 their bearing upon. 



In order to ensure the sides of the moulds leaving the concrete 

 freely and cleanly, they should be painted over with a mixture 

 of boiled soap and water, of the consistency of a jelly. 



The floor upon which the blocks are made is usually coated 

 with fine sifted sand. 



Blocks should be allowed ample time to harden before being 

 lifted or the moulds removed, otherwise they are apt to become 

 strained, and to develop incipient cracks. Under ordinary condi- 

 tions the moulds may be removed on the fourth day ; and the 

 blocks maybe lifted and stacked when from three weeks to a month 

 old. The exact period will, however, depend much upon the 

 proportion and setting properties of the cement which they 

 contain, and also upon the state of the weather. In warm 

 weather they will be found to harden much more quickly than 

 in cold. 



The following information respecting concrete in its various 

 forms, as used in the construction of several important works, 

 may be found useful. 



The proportions adopted for concrete blocks used in making 

 the breakwaters, or moles, at Marseilles were 



2 parts hard broken limestone. 



1 part mortar, composed of 3 parts Theil lime, 5 parts sand. 



These were all mixed and formed into concrete by cylindrical 

 mixing-machines. 



The moulds were taken off the blocks after a lapse of not less 

 than three days. The blocks for pierre perdue work were allowed 

 at least three months to harden before being put into the work, 

 those for use in foundation work being allowed six months. 



These blocks are said to have stood well, having been but 

 little affected by the action of the waves, whereas some of those 

 used in constructing breakwaters at other Mediterranean ports 

 Algiers and Leghorn, for example are said not to have 

 behaved quite so satisfactorily. 



