and indicated in the Scientific Structures Index in these specifications. 5-10. PROPORTIONING OF CONCRETE: @. Coral Aggregate Concrete: Shall be proportioned to yield the strengths for the various types to the satis- faction of the Engineer. Prior to placing any concrete in the structures, a sufficient number of trial design batches shall be prepared and tested to determine the mix to be used for each type of concrete. b. Limonite Aggregate Concrete: Limonite aggregate concrete shall be proportioned to yield a minimum strength of 3500 p.s.i. at 28 days. following per cubic yard: Proportions not to exceed the 1. Cement: 940 pounds. 2. Graded Scrap Steel: 3. Iron Ore: 4, Water: 4100 pounds. 1880 pounds. 55 gallons. Resulting unit weight per cubic foot - 273 pounds ec. Measurements: 1. Cement: 2. Water: A one-cubic foot bag of Portland cement will be considered as 94 pounds ered as 8,33 pounds. in weight. One gallon of water will be consid- No increase in the maximum water con- tent as determined in the approved trial mixes will be per- mitted, 3. Aggregate: The maximum amount of the maximum size of coarse aggregate economically available and placeable, as compatible with the type and character of the structure, shall be used. The exact ratio of the aggregate shall be determined and modified as required by the Engineer, Ingredients shall be measured separately for each batch, d. Corrective Additions: e. Control: Corrective additions, to remedy the deficiencies in the aggregate gradations, may be used, subject to approval by the Engineer. The proportions and exact amounts of 5-7