26. Water. The water used in mixing concrete should he free from oil, acid, strong alkalis, or vegetable matter.
27. Metal Reinforcement. The Committee recom mends that mild steel such as fills the specifications fol structural steel of the American Railway Engineering and Maintenance of Way Association is a suitable material.
28. If steel of higher elastic limit is used, it must be of first-class quality and subjected to special tests even if used at the same working stress as mild steel. It should be required to pass the following test: 29. Test specimens for bending shall be bent cold under the following conditions without fracture on the outside of the bent portion : Around twice their own diameter : 1 in. diameter, 80 degrees ; in. diameter, 90 degrees; 1/2 in. diameter, 110 degrees.
Around their own diameter: 1/4 in. diameter, 130 de grees; in. diameter, 140 degrees; in. diameter or less, 180 degrees.
The reinforcement should be free from rust, scale, or coatings of any character which would tend to reduce or destroy the bond.
30. Proportions. The materials to be used in con crete should be carefully selected, of uniform quality, and proportioned with a view to securing as nearly as possible a maximum density.
31. The unit of measure should be the barrel, which should be taken as containing 3.8 cubic feet. Four bags containing 94 pounds of cement each should be considered the equivalent of one barrel. Fine and coarse aggregate should be measured separately as loosely thrown into the measuring receptacle.
32. If the coarse aggregate contains sand or other fine material, that which passes a sieve with 1/4-inch round holes should be considered as sand in measuring propor tions. In general, the concrete on the work should con tain enough and only enough mortar to cover all particles of stone and fill the voids without an appreciable excess of mortar.
33. The proportions of cement to sand and stone should be chosen after a very careful study of the local conditions and the available materials. For small and unimportant structures, the following list is presented to be used as a rough guide to the selection of proportions for different classes of work. The relative proportions of
fine to coarse aggregate may be varied to suit the materials: (a) A rich mixture, for columns and other structural parts subjected to high stresses or requiring exceptional water-tightness : Proportions 1 :3.
(b) A standard mixture, for reinforced floors, beams, and columns, for arches, for reinforced engine or machine foundations subject to vibrations, for tanks, sewers, con duits, and other water-tight work: Proportions 1:2:4.
(c) A Medium mixture, for ordinary machine foun dations, retaining walls, abutments, piers, thin foundation walls, building walls, ordinary floors, sidewalks, and sewers with heavy walls: Proportions 1 :5.
(d) A lean mixture, for unimportant work in masses, for heavy walls, for large foundations supporting a sta tionary load, and for backing for stone masonry: Pro portions 1 :3 :6.
34. Mixing of Concrete. Methods should be devised to make always positive the proper proportioning and therefore the accurate measuring of the concrete mate rials, including water. Accurate measurements are of the greatest importance.
35. The concrete materials should be mixed until the cement is uniformly distributed throughout the mass, and the concrete therefore uniform in color and homo geneous. Sufficient water must be added during the mix ing to produce a concrete that will flow when agitated, but not so wet as to permit a separation of the materials in transferring from mixer to the work. Mixing by machine should always be preferred, since the most thor ough and uniform consistency can thus be obtained.
36. The type of mixer should be used which will insure thorough and accurate proportioning of the mate rials throughout the mass.
37. Where hand-mixing is necessary, the mixing should be done on a water-tight platform, and special precautions taken to turn the materials until they are homogeneous in appearance and color. Concrete that has started to set or has been out of the mixer more than thirty minutes, should not be used. Re-tempering of either mortar or concrete which has partially hardened should not be permitted.