VARIOUS TYPES OF BREAKWATERS. 



189 



steam hopper barge, the concrete for filling one of these being 

 mixed in about 20 minutes by a large " Carey-Latham " machine. 

 Large bag-blocks were, I believe, first used by Mr. Dyce 

 Cay, M. Inst. C.E., in constructing the Aberdeen breakwaters. 

 Some of these, which form the "apron" on the seaward side of 

 the south breakwater, weigh about 100 tons each. They were 

 dropped a distance of from 40 feet to 50 feet from a movable 

 hopper box, bracketed out from the side of the breakwater, there 

 being no machine upon the works available for depositing them 

 otherwise. In the north breakwater they were placed by means 

 of a hopper barge, which was afterwards used for similar work 

 at Fraserburgh. 



SIDE 



END VIEW 



FIG. 33. Concrete bag-box. 1 



It is important that concrete in bag-work should be deposited 

 in place as soon after mixing as possible, otherwise setting 

 commences, any interference with which impairs the strength of 

 the concrete, rendering it friable and very liable to disintegrate. 



In placing bag-blocks under water, the box should be lowered 

 over the site where the bag is to be deposited, only sufficient 

 room being allowed, as in the case of mass- work, to give clearance 

 for the box flaps to open. The distance through which the bags 

 have to fall is thus reduced to a minimum, and the rending of 

 the canvas by the weight of the concrete, which sometimes 

 occurs, is obviated. 



The system of dropping large bags of concrete through a 



1 Min. Proc. Inst. C.E., vol. cxiii. 



