166 THE IMPROVEMENT OF RIVERS. 



The stone should be thoroughly wetted before being put in the mixer, but the 

 sand should be dry, or not more than moist, as when it is wet it is more difficult to 

 incorporate it thoroughly with the cement, and the varying amount of water in it will 

 cause a constant variation in the wetness or dryness of the concrete. 



Of slow-setting and quick-setting cements we believe the former to be preferable, 

 as with the latter, if delays occur, the concrete may be spoiled, besides which, the feet 

 of the men ramming keep the surface in the forms more or less disturbed, and if it has 

 begun to set before the next layer has covered it, the bond will be destroyed. A 

 cement which will not take an initial set in less than an hour, at a temperature of 70 

 Fahrenheit, is well suited for this class of work. 



The use of different qualities of cement, or different proportions of the same 

 cement, in the same section of a wall, is not a satisfactory practice from the point of 

 view of construction. It has been done sometimes to save expense, a Portland 

 cement being used for the outside 2 feet or so of thickness, and a natural cement 

 for the interior of the walls, or using a richer and poorer mixture respectively of the 

 same cement. In the former case it requires a constant attention from the cement 

 house till the concrete is in place, in order to see that the proper cement is put into the 

 mixer when called for, and in either case careful watch must be kept to see that when 

 mixed it is placed in its special zone in the wall, and even with the best of care the 

 batches will sometimes be misplaced, leading to delay. It is claimed by some, more- 

 over, that as Portland and -natural cements usually have different speeds of setting, the 

 bond between them is not as strong as is desirable. In certain walls built with such 

 combined concrete a parting between the two kinds has actually occurred. 



The practice of embedding stones in the walls for the purpose of saving cement 

 is rarely an economical one, unless they can be set without causing delays in the 

 concreting. 



Forms.* The forms, or timber work inside which the concrete is placed, and which 

 give the shape to the finished work, vary considerably in design. They are of two 

 general types, one where planks are laid horizontally, held apart at the proper dis- 

 tances by tie-rods and struts, and without outside posts or supports, arid the other 

 where posts are used to support these planks, or lagging, as they are technically 

 called, well braced to the ground or to other supports, and without any tie-rods, 

 except perhaps at the top. 



In the former type the plank are placed one or two at a time, and raised up when 

 the concrete has hardened, and placed ready for the next layer above. After a section 

 is finished the ends of the rods are cut off and covered with cement, or small turn- 

 buckles or gas-pipe sleeves are used, allowing the ends to be unscrewed and with- 

 drawn, the turnbuckles remaining in the wall. This type, although effecting a con- 

 siderable economy in the cost of the forms, has not met with much favor, because it 



* See also specifications for a lock "Forms," Appendix B 



