. Engineering and Contracting . y Commission are given inTable II. Three Methods of Using Pit Run Gravel.—Pit run gravelmay be used in each of three different ways: 1. Enough cement may be used to make concrete of thedesired class from the gravel without change. 2. The gravel may be separated into sand and coarse ag-gregate by screening, and then remixed in predeterminedproportions. 3. The gravel may be made the equivalent of the propor-tion in Case 2, by adding coarse aggregate or sand as mayfce required. Case 2 will yield the most uniform concrete. There is no ^^/c - C/oss 1 gravel, by weigh
. Engineering and Contracting . y Commission are given inTable II. Three Methods of Using Pit Run Gravel.—Pit run gravelmay be used in each of three different ways: 1. Enough cement may be used to make concrete of thedesired class from the gravel without change. 2. The gravel may be separated into sand and coarse ag-gregate by screening, and then remixed in predeterminedproportions. 3. The gravel may be made the equivalent of the propor-tion in Case 2, by adding coarse aggregate or sand as mayfce required. Case 2 will yield the most uniform concrete. There is no ^^/c - C/oss 1 gravel, by weight and the relative volumes of sand and peb-bles a unit volume of the pit run will yield, and the weightper cubic foot of the pit run. a will depend upon localconditions, b may be estimated as shown in the Iowa High-way Commission table of proportions, c must be deter-mined by test. If a unit volume of pit run gravel be separated into twoiizes it will be found that the sum of the volumes of the fiona forP/f/funGnormf Cartere^e. oportioning Pit Run Gravel for Class 1Concrete. choice between Case 1 and Case 3 on the score of uniform-ity. In general, the method to use will depend upon thecost. Comparison of Methods of Using Pit Run Gravel.—To makea comparative estimate of the costs of the three methodsthe following information will be required: (a) Unit pricesof cement, pit run gravel, screened gravel or crushed stone,and cost of screening; (b) Quantities of materials requiredfor 1 cu. yd. of concrete; (c) Percentage of sand in the TABLE I—STANDARD CO>rCRETE PROPORTIONS. IOWAHIGHWAY COMMISSION. Maximum Bbl. cementProportion. Description. size of a&- per cu. yd. gregate. In. of :2:4 Class A bridge concrete for con-crete arch, slab and girder bridgesthroughout, reinforced abutmentsand wing walls and coping of grav-ity sections IH 1:2^:5 Class B bridge concrete for bodyof gravity section pier and abut-ment 2% 1:2:3^4 Class C bridge concrete for thin sectio
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