The short sections of the plain beams are not suspended for zero deformations in the gage length, and therefore the deformations calculated for these are those due to the applied load only. Form L (p. 57) is used for reporting the results of the tests of reinforced concrete beams. The percentages of steel recorded in the batch report are given in terms of the section of concrete above a line drawn through the centers of the rods, the lower layer being taken when there is more than one layer. The position of the neutral axis is calculated as in the plain beams, except that instead of using the deformation of the lower fiber, the deformation of the steel is used, thus obtaining the percentage of the depth below the top in terms of the distance from the top of the beam to the center of the lower layer of rods. The position of the neutral axis is calculated for several loads up to the maximum, and curves are drawn in order to show the variation in the position with the increase in the load. The values under this general heading are obtained from the load, deformations, and deflections at the last full set of micrometer readings before the maximum load. After the location of the neutral axis has been found, the final deformeter values at the top of the beam are corrected so as to give the deformations at the extreme top. It should be noted that the lower micrometers are clamped directly over the steel, and therefore no correction of the micrometer readings is necessary to allow for the fact that the fiber whose elongation is required is not the fiber upon which the micrometers are clamped. All the calculations made for the plain beams are repeated here except the one giving the modulus of rupture, for which a special formula must be used. The maximum values are obtained from the load, lower micrometer readings, and deflections when the beam has reached its maximum resistance, or from the last full set of deformeter readings before failure. TESTS OF CYLINDERS AND CUBES. Method. A cylinder and a cube are made from the same batch of concrete from which each beam is molded, and all are tamped by hand with a tamper weighing 7 pounds and having a circular face 3 inches in diameter. The same molds (shown in Pl. VIII, B, p. 30) are used for these specimens as for those tested in the constituent-materials section. The method of testing is also the same (pp. 31-36). The compressometers for measuring the deformations of the cylinders are shown in Pl. IX, C (p. 32). The micrometers used on these compressometers measure directly to T inch. Form M is used for recording the results of tests. inches; inches. Lab. No. Gage length, pounds. Area, mate strength, value, machine, pounds per cubic foot. Ultimate load, pounds per square inch. Initial coefficient of elasticity, pounds per square inch. Probable ultimate unit deformation, CYLINDER inches. ...... cubic pounds. UltiRange of linear Bedding in For character of concrete and corresponding test piece see Batch report Bm... Cylinders and cubes brought from damp closet at ......; weighing, measuring, and capping finished at... time......; test of cylinders started at ......; test of cylinders completed at ......, ......; sheet handed to office at ......, time......; delay, due to... Remarks.-. .., time Computations. The compressive strengths of the cubes and the cylinders are calculated in pounds per square inch. For the cylinders the modulus of elasticity is determined by drawing a curve showing the values at different loads. A tangent to the curve at or near its origin is assumed to represent the initial modulus of elasticity. BOND TEST PIECES. Method. The schedule of bond tests is shown in the table on pages 40-47. A bond test piece in the machine ready for testing is shown in Pl. XVIII, B. The concrete cylinder is placed on top of the machine with the embedded rod projecting downward. The lower end of the rod is gripped in the jaws of the machine. The lower surface of the concrete cylinder is embedded in plaster of Paris on a 1-inch plate with a hole in its center one-sixteenth inch greater in diameter than the rod which passes through it. The instrument for measuring the slip of the rod is shown at the top of the test piece in the figure. During the test the micrometer and load are read at intervals of about 500 pounds until the slip of the rod amounts to about one-tenth inch. The load in all cases is applied continuously until failure. The bond pieces are tested at the ages of 30, 90, 180, and 360 days. Computations.-Form N is used for recording results of the bond tests. Form N. Bond reg. No. of rod, ....... UNITED STATES GEOLOGICAL SURVEY. ....... Lab. No. inches. Embedded surface, pounds per square in h. Yield point, pounds. Unit bond stress at first slip, . Maximum unit bond stress,. } BOND TEST. Diameter of embedded rod....... inches. Embedded length square inches. Reg. No. of rod ....... Elastic limit. pounds per square inch. Load at first slip, .... pounds per square inch. Maximum load, pounds. square inch. Unit stress in stecl at first slip,.. pounds. Unit stress in steel at maximum load. pounds. Condition of surface of embedded steel, Condition of surface of steel when pulled from conercte, For character and proportions of concrete see Batch report Bm....... Bedding of test piece Remarks.-... pounds per The unit bond at any load is found by dividing the load by the surface area of the rod in contact with the concrete. TESTS OF STEEL. Method. As may be seen in the tables (pp. 38, 40-47), every rod used for reinforcement is tested. The tests include the determination of the yielding point as seen from the drop of the beam, the elastic limit obtained by the divider method, and the ultimate strength, elongation, reduction of area, and breaking strength. The elastic limit is determined on a gage length of 8 inches, one point of the dividers resting in a punch mark and the other marking on chalk rubbed on the surface of the test piece. The elongation is measured on the gage length of 8 inches. In addition to the above, the modulus of elasticity is determined on every tenth bar, using the Henning extensometer with electric contact. The percentage of carbon, phosphorus, and sulphur in every bar is determined in the chemical laboratory. A view of the steel ready for testing is shown in Pl. XVIII, A. Computations. A record of steel tests is kept on a log sheet similar to Form E (p. 34). After the computations have been made the results are entered in Form O. The unit stresses at different loads are calculated by dividing the gross load by the area of the test piece. The percentage elongation in 8 inches and the reduction of area at fracture are also calculated. The modulus of elasticity is determined by dividing any unit stress below the elastic limit by the unit elongation at that stress. The elongations as obtained by the two micrometers on either side of the test piece are averaged and divided by the gage length to obtain the unit elongation. |