274 



BRIDGE ABUTMENTS AND PIERS. 



CHAP. VI 



24. Proportions. The proportions of materials for the different classes of concrete shall be 

 as follows: 



Note: This blank to be filled for each contract. 



25. For plain concrete, a proportion of i : 9 (unless otherwise specified) shall be used, i. e., 

 one part of cement to a total of nine parts of fine and coarse aggregates measured separately; for 

 example, I cement, 3 fine aggregate, 6 coarse aggregate. 



26. For reinforced concrete a proportion of I : 6 (unless otherwise specified) shall be used, 

 i. e., one part of cement to a total of six parts of fine and coarse aggregates measured separately; 

 for example, I cement, 2 fine aggregate, and 4 coarse aggregate. 



27. Mixing. The ingredients of concrete shall be thoroughly mixed to the desired con- 

 sistency, and the mixing shall continue until the cement is uniformly distributed and the mass 

 is uniform in color and homogeneous. 



28. Measuring Proportions. The various ingredients, including the water, shall be measured 

 separately, and the methods of measurement shall be such as to secure the proper proportions at 

 all times. 



29. Machine Mixing. A machine mixer, preferably of the batch type, shall be used, wher- 

 ever the volume of the work will justify the expense of installing the plant. The requirements 

 demanded are that the product delivered shall be of the specified proportions and consistency 

 and thoroughly mixed. 



30. Hand Mixing. When it is necessary to mix by hand, the mixing shall be on a watertight 

 platform of sufficient size to accommodate men and materials for the progressive and rapid mixing 

 of at least two batches of concrete at the same time. Batches shall not exceed one-half cubic 

 yard each. The mixing shall be done as follows: The fine aggregate shall be spread evenly upon 

 the platform, then the cement upon the fine aggregates, and these mixed thoroughly until of an 

 even color. The water necessary to mix a thin mortar shall then be added and the mortar spread 

 again. The coars'e aggregates, which, if dry, shall first be thoroughly wetted down, shall then 

 be added to the mortar. The mass shall then be turned with shovels or hoes until thoroughly 

 mixed and all the aggregate covered with mortar. Or, at the option of the engineer, the coarse 

 aggregate may be added before, instead of after, adding the water. 



31. Consistency. The materials shall be mixed wet enough to produce a concrete of such 

 consistency that it will flow into the forms and about the metal reinforcement, and which, on 

 the other hand, can be conveyed from the place of mixing to the forms without separation of the 

 coarse aggregate from the mortar. 



32. Retempering. Retempering mortar or concrete, i e., remixing with water after it has 

 partially set, will not be permitted. 



33. Placing of Concrete. Concrete after the completion of the mixing shall be handled 

 rapidly to the place of final deposit and under no circumstances shall concrete be used that has 

 partially set before final placing. 



34. The concrete shall be deposited in such a manner as will prevent the separation of the 

 ingredients and permit the most thorough compacting. It shall be compacted by working with 

 a straight shovel or slicing tool kept moving up and down until all the ingredients have settled in 

 their proper place and the surplus water is forced to the surface. In general, except in arch work, 

 all concrete must be deposited in horizontal layers of uniform thickness throughout. 



35. In depositing concrete under water, special care shall be exercised to prevent the cement 

 from floating away and to prevent the formation of laitance. 



36. Before depositing concrete the forms shall be thoroughly wetted (except in freezing 

 weather) or oiled, and the space to be occupied by the concrete cleared of debris. 



37. Before placing new concrete on or against concrete which has set, the surface of the latter 

 shall be roughened, thoroughly cleansed of foreign material and laitance, drenched and slushed 

 with a mortar consisting of one part Portland cement and not more than two parts fine aggregate. 



38. The faces of concrete exposed to premature drying shall be kept wet for a period of at 

 least three days. 



