i?o THE IMPROVEMENT OF RIVERS. 



In some cases it has taken the entire time of an inspector, besides the time of employees 

 of the contractor, to watch and correct changes in this portion of the work Once the form 

 has been set up and brought to line it should need little more attention, and whrre 

 proper experience has been brought to the design this result can be secured with but 

 small extra expense. 



Mixing and Placing. In mixing the concrete, which should always be done by 

 machinery, the materials are usually dumped into the mixing box all together, the 

 water poured in on top, and the box revolved. In one type of mixer the water is intro- 

 duced after the box has commenced to revolve, the shaft being made hollow and piercnl 

 with holes, and connected by a pipe with the water-tank. While this method is pref- 

 erable in theory, it has drawbacks in practice, as the holes become clogged with mortar 

 or spalls, resulting in an uneven mixture, and a necessity for cleaning them out every 

 few hours. Fewer turns are required if the materials are first revolved dry and the 

 water then introduced, but this is rarely done in practice, and equally good results are 

 obtained by the method usually followed. 



No special order need be observed in putting the materials into the box, except 

 that the water should be put in last, nor is it necessary first to mix the sand and 

 cement. From eight to sixteen turns will be required, depending on the size of the 

 box, which should not be more than about one-half full. The amount of water will 

 depend on whether "wet" or "dry" concrete is to be used, and also on the dryness 

 and temperature of the air. By dry concrete is meant concrete which requires long 

 and hard ramming before any moisture shows on the surface, while Wet concrete shows 

 it with much less ramming, and soon begins to quake or shake like jelly, showing that 

 it is worked into a mass. If the water runs in trickles over the surface it is too wet. 



While dry concrete, carefully mixed and placed, appears from samples to give a 

 stronger result than wet concrete, engineers seem to be coming to the conclusion that 

 such a result depends on conditions hardly realizable in practice. For a dry concrete 

 the amount of water must be gauged almost to a quart, and as the total needed per 

 charge will sometimes vary 25 per cent between 6 A.M. and 6 P.M. on a hot day, 

 owing to the evaporation, it is very difficult to obtain the exact amount required. 

 Moreover, when the concrete is exposed in the forms, evaporation from it is very rapid, 

 and if the water dries out before the concrete has set, the latter is ruined. It is not 

 practicable to keep it covered while the men are working, and if water is sprinkled over 

 it the cement is liable to be washed in, exposing the sand and stone. Dry concrete can thus 

 be easily spoiled, while the same causes will affect wet concrete to a very much less 

 degree. The latter also packs more closely in ramming, and with less work, and judg- 

 ing by experiments made in the same wall, is less porous when subjected to water 

 under pressure. 



When the concrete is mixed it is taken to the forms by dump-cars, or in dump- 

 boxes handled by derricks. Frequently a track is laid in the lock pit on which a trav- 

 eling derrick runs, taking the boxes from stationary derricks or from cars, and moving 



