36 BULLETIIsr 801^ U. S. DEPARTMENT OF AGEICULTUEE. 



Reinforcement sliould consist of steel of medium hardness and for 

 general use should be in the form of bars or rods, as they are adaptable 

 alike for walls, columns, beams, and slabs. These bars may be of any 

 suitable shape, of Trhich there are a great variety on the market. The 

 plain, round or square bars are entirely satisfactory for all ordinary 

 sizes. Many companies make a specialty of supplying steel for this 

 use; the rods may be had cut to the proper length and bent to the 

 exact shape desired. Usuall}^ it is best to use regular stock lengths 

 and do the bending locally. A very simple bending machine for 

 hand operation may be had for this work. Reinforcement for use in 

 the slab may consist in whole or in part of woven-wire reinforcing 



not over 2 inches in diameter, and preferably should be of varying sizes, ranging 

 from li inches down. It may consist of hard stone, such as trap rock, 

 granite, limestone, and aggregates which are hard and do not decompose when 

 subjected to severe heat. (The U. S. Bureau of Standards may supply re- 

 ports, at a very small charge, on materials submitted.) The sand should 

 be clean and free from loam, clay, or organic matter. The gi-ains should be 

 fairly coarse or a mixture of fine and coarse with the coarse grains predom- 

 inating. The Portland cement should be of a good quality. While analytic 

 tests have been fixed as a required standard (see U. S. Bureau of Standards 

 Circular No. 33, United Slates Government Specification for Portland Cement, 

 1912; also Standard Test Specifications for Portland Cement recommended 

 by the American Society for Testing Materials), it is usually a safe policy 

 to use a brand of cement which hasi been on the market and in satisfactory 

 use. for five years and is m^de by a reliable company. Water for mixing 

 should be free from alkali or injurious salts! in damaging qualities. (See 

 TJ. S. Bureau of Standards Bulletin No. 12 : Action of the Salts in Alkali 

 Water and Sea Water on Cement.) 



Slag from blast furnaces, if burned free from coal, may be used as the 

 coarse aggregate for concrete. It makes a concrete which is light, fairly 

 strong, and withstands fire well. However, there are but few localities where 

 slag is available. 



Crushed terra cotta, tile, or brick are good substitutes for crushed stone 

 for concrete, provided the clay material is hard-burned and used properly. 

 After crushing, the dust should be screened out and the crushed brick should 

 be wet thoroughly before mixing, for if the material is used dry it will absorb 

 too rapidly the water used iu mixing the concrete. Care should be exercised 

 to see that brick used are hard and well-burned and that they are free from? 

 mortar; otherwise the finished concrete will be unreliable in strength. 



Gravel should not be used for concrete work which may be subjected to 

 fire, as such use has been found exceedingly dangerous. The use of quartz 

 gravel in the reinforced concrete of a warehouse at Far Rockaway, N. Y., 

 caused serious damage fi'om a fire of comparatively short duration. A detailed 

 report of this fire, written by Mr. Ira H. Woolson, may be had from the 

 National Board of Fire Underwriters, New York City. A valuable test of 

 gravel as compared with other stone for concrete has been made by the Bureau 

 of Standards and the results have been recorded in an illuminating way by 

 Mr. Walter A. Hull. (See Proceedings American Concrete Institute. Vol. 

 XIY, 1918.) There is no objection to gravel, however, for concrete paving 

 and for footings of the foundation. 



