PORTLAND-CEMENT CONCRETE 71 



and rapid mixing of at least two batches of concrete 

 at the same time. Batches shall not exceed 1 cubic 

 yard each, and smaller batches are preferable, . based 

 upon a multiple of the number of sacks to the barrel. 



(6) Spread the sand evenly upon the platform, then the 

 cement upon the sand, and mix thoroughly until of 

 an even color. Add all the water necessary to make 

 a thin mortar, and spread again; add the gravel if used, 

 and finally the broken stone, both of which, if dry, 

 should first be thoroughly wet down. Turn the mass 

 with shovels or hoes until thoroughly incorporated, 

 and until all the gravel and stone is covered with mor- 

 tar, which will probably require the mass to be turned 

 four times. 



(c) Another method, which may be permitted at the option 

 of the engineer in charge, is to spread the sand, then 

 the cement, and mix dry; then the gravel or broken 

 stone, add water, and mix thoroughly as above. 



313. Mixing by Machine. A machine mixer shall be used 

 wherever the volume of work will justify the expense of installing 

 the plant. The necessary requirements for the machine shall 

 be that a precise and regular proportioning of materials can be 

 controlled, and the product as delivered shall be of -the required 

 consistency and be thoroughly mixed. 



314. Consistency. The concrete shall be of such consistency 

 that when dumped in place it will not require much tamping. 

 It shall be spaded down, and be tamped sufficiently to level it 

 off, after which the water should rise freely to the surface. 



315. Forms: 



(a) Forms shall be well built, substantial and unyielding, 

 properly braced or tied together by means of wire or 

 rods, and shall conform to the lines given. 



(6) For all important work, the lumber used for face work 

 shall be dressed on one side and both edges, and shall 

 be sound and free from loose knots, secured to the 

 studding or uprights in horizontal lines. 



(c) For backing and other rough work, undressed lumber 



may be used. 



(d) Where corners of the masonry and other projections 



liable to injury occur, suitable moldings shall be 



