128 HARBOUR CONSTRUCTION. 



proportioned and mixed a sufficient quantity of water being 

 used to ensure its not being porous it will not go to pieces or 

 give any cause for anxiety when put into the sea. 



I am aware that, in the opinion of some, cement cooled to the 

 extent just described would be considered spoilt; but I am 

 guided by experience, and I am convinced that a very large 

 proportion of the trouble which is experienced with concrete 

 structures in the sea is attributable to the use of unslaked 

 cement. When once concrete becomes porous, whether by 

 expansion caused by hydration or otherwise, the sea-water, 

 gaining admission, takes up the lime, deposits magnesia, and thus 

 renders the work rotten and honeycombed. 



A good Portland cement for sea-works should fulfil the 

 following conditions : 



(1) It should be of a uniform dark grey colour. 



(2) Its specific gravity should not be less than 314 when 



fresh, or 3*125 a month after being ground. (The 

 weight-per-bushel test may be disregarded as mislead- 

 ing and useless.) 



(3) It should be finely ground, at least 92 per cent, by 



weight, passing a sieve with 2,500 meshes per square 

 inch, and the whole passing a sieve with 1,600 meshes 

 per square inch, the sieves being made of wire No. 37 

 B.W.G. (= '007 inch). 



(4) The time of setting should be from half an hour to an 



hour and a half, when the temperature of the air is 

 about 60 Fahr. and the cement fresh. 



Warmth hastens the setting of cement, while cold 

 retards it. 



In making briquettes for testing, the cement should 

 be mixed with sufficient water to form a tough paste, 

 such as will fill the moulds without ramming. 



(5) Pats or slabs, J inch in thickness, after remaining in water 



for twenty-four hours, on being heated to a tempera- 

 ture of 160 Fahr., should not show any signs of " flying." 



(6) Briquettes, with a minimum section of 1J" X 1J", made 



with the cement when fresh, should stand the following 

 tensile strains : 



Ibs. Ibs. 



Briquette 2 days old, from 450 to 600 

 4 700 900 

 7 950 1100 



