i 7 a THE IMPROVEMENT OF RIVERS. 



be fine rather than coarse. If made in tin- mixer, the box must be first cleaned from 

 any stone or gravel that may have remained in it. 



The insides of the culverts need not be faeed, as the plain concrete is sufficiently 

 strong to withstand the abrasion of the water. 



The coping is usually finished off by taking some of the facing mortar, mixed a little 

 more wet than for the walls, and spreading it on the surface of the concrete before the 

 latter has begun to set. This mortar is then floated over with a straight-edge and 

 brought to a level finish. This method secures a good bond between the surfaces, but 

 the top lacks the smooth appearance to be seen on good cement sidewalks, and which 

 it is very desirable to secure. To obtain it, the work should be put in the hands of an 

 experienced sidewalk mason, or if such is not available, an ordinarily intelligent mason, 

 acting under instructions, can produce fair results. The mortar is spread as before 

 mentioned, and just before it attains the final set it is carefully troweled over, causing 

 the cement next the top to rise to the surface and giving a smooth finish. Division 

 lines, if such are used, are cut with a special tool, something like a plasterer's float, but 

 having a curved V-shaped iron on its under side, with which the lines arc struck. 

 The work should be carefully protected from the sun until hardened. 



If the workman is inexperienced, a few sample blocks should be made and fin- 

 ished off before he is allowed to attempt the main walls. 



If the coping finish were put on after the concrete had set, the operation would 

 be much simplified, but it may be doubted if the bond would be lasting. We are 

 acquainted with cases, however, where new copings have been put on walks and have 

 lasted without sign of failure. In such cases the old surfaces are well roughened and 

 washed over with a grout of neat cement before the finishing coat is put on. 



As the coping of the different blocks is usually not ready for finishing at the same 

 time, and as it is often not possible to secure a competent mason at any time his ser- 

 vices may be required, the blocks can be built up to within an inch or two of the finished 

 height and be left low in the center, so as to provide a good base of fresh concrete on 

 which to lay the finish without having to leave or put up forms again. Thus if the 

 edges of the wall are to be rounded to a 2 -inch radius, the facing or concrete may 

 be built up and carefully leveled off at 2 inches below the coping. From this level 

 the concrete is made to slope down at about 45 degrees to a depth of a foot or more, 

 and then carried flat to meet the opposite slope, leaving a trough in which a body of 

 fresh concrete can be placed whenever the coping is to be put on. By this method the 

 coping when commenced can be finished without interruption, the troughs being filled 

 up as required. 



Monolithic Concrete. In some locks the concrete has been built in courses a few 

 feet in height, placed at different times; in others it has been built in one block, of 

 the full height of the wall, and from 20 to 30 feet long. The former method, as 

 shown where concrete work has been removed, does not possess the strength of the 

 latter, and requires a constant moving of men and appliances from one point to an- 



