LABOR The following brief specifications for mate rials and labor in bridge and culvert construe tion, issued by the Corrugated Bar Company, of St. Louis, may be regarded as typical of stand ard modern practice: Specifications for Materials and Labor Cement. Only Portland cement conforming to the re quirements of the specifications adopted by the American Society for Testing Materials shall be used.
Sand. Sand to be clean and coarse, and free from or ganic matter; a graded sand, with coarse grains pre dominating, is to be preferred.
Coarse Aggregate. Broken stone to be hard, durable limestone, or its equivalent, free from dust and foreign materials; maximum-sized particles to pass through a one-inch ring; fines to be removed by passing over a one quarter-inch screen. The fines may replace part of the sand. Under special conditions making for uniformity, crusher-run may be used; but this is not desirable.
Gravel shall be clean and of graded sizes; the sand carried to be removed by screening as for broken stone.
Proportions of Mix. Concrete for columns, beams, and slabs to be mixed in the proportion of one part cement to six parts aggregate; proportions by volume taking one bag containing not less than 94 pounds of cement, equal to one cubic foot of cement.
The proportions of fine and coarse aggregate used shall be chosen so as to give a concrete of maximum density; in no case, however, may the amount of fine aggregate be less than 50 per cent of the coarse.
Reinforcing Steel. All reinforcing steel used in bridge and culvert construction shall be rolled to such form that it has a positive mechanical bond with the concrete. Adhesive bond will not be considered sufficient ly reliable for this class of structures.
Steel may be made by either the Bessemer or open hearth process; bars to be rolled from billet stock. Re rolled material will be accepted under conditions insur ing rigid inspection.
The elastic limit and percentage of elongation shall be determined by tests on accurately-machined speci mens, and shall conform to the following requirements: Elastic limit to be from 50,000 to 60,000 pounds per square inch, ultimate strength not less than X elastic limit.
The percentage of elongation in 8 inches must not be less than given by the formula : 000400, Percentage of elongation = 1,— 5.0.
Ultimate strength Bending Test. Bars as rolled shall bend cold, 90 de grees, to a radius equal to three times the least diameter of the specimen, without sign of fracture.
Forms. Forms must be smooth and true to dimen sions, with close joints to prevent leakage, and must be of sufficient strength to carry the load without appreci able deflection.
Removal of Forms. The time of removal of forms should be left to the discretion of the engineer, as the time required for the concrete to gain sufficient strength is dependent upon temperature and weather conditions and on the particular cement used. Forms for slabs should not be removed in less than two weeks under the most favorable conditions; girder forms should not be removed in less than three weeks.
Mixing. Machine mixing is to be preferred in all cases—hand mixing to be allowed at the discretion of the engineer. Concrete for reinforced work shall be mixed wet, sufficient water being used to make a mass that will flow readily and be of such consistency that the reinforc ing steel will become coated with a protective coat of fluid mortar. Excess of water should be avoided, as it tends to a separation of the parts.
Placing. In girder bridges and in all T-beam designs, the concrete must be placed the full depth (to the top of the slab) at one operation. If possible, the work should be carried on continuously to completion.