The New York Rapid-transit Subway

Proceedings.] PARSONS ON NEW YORK RAPID-TRANSIT SUBWAY. 45 did not indicate rupture, as is shown in every case by the very much higher ultimate load required to break down the beam. Thus in the test of plain rods at the age of 3 months, the first crack appeared at 5,500 lbs. as against 7,950 in another similar beam at the age of 1 month. Yet the former beam carried an ultimate load of 11,050 lbs. as against 10,800 lbs. for its fellow. The two were equally strong. Allowing beam No. 10 to give as high unit- stresses as beam No. 1, it will be noted that the beams with smooth rods of plain sections, Nos. 1, 2, 3, 10, 11 and 12, gave the highest steel-stresses, and higher stresses at the time of tlie first crack than the elastic limit of the steel reinforcement, as shown in Table VII. The rods in the Hennebique reinforcement were the only rods that broke when the beams finally failed, indicating that adhesion between concrete and steel exceeded the strength of the latter, as should be the case. In the tests under examination this was due, not to the method of reinforcement, but to the small cross section of the rods, which had a larger surface in proportion to area than the rods in any other test. The manner of failure further indicated the lack of virtue in the variable sections. In all cases the beams with plain rods, including the Hennebique tests, broke squarely across in the middle. Ilie beams with twisted, expanded and corrugated rods not only broke in the middle, but also split along the rods, indicating the wedge- like action of the expansion, as had been also noted in the adhesion tests. The only exceptions were the tests with twisted rods supple- mented witli stirrups. The latter, by giving vertical reinforcement, prevented the beams from splitting along a horizontal plane. Specifications always prohibit the use of any cement that has taken an initial set, though experiments have shown that cement can be retempered without injurious result. As concrete formed such a very important part of the subway, and as careful inspection could not be absolutely relied on to ensure every batch of mortar being free from set, it was decided to repeat experimental tests on the effect of retempering. Cubes of 6-inch edge were made of concrete in the proportion of 1 cement, 2 sand, and 4 gravel. One set was allowed to stand for 1 month after being made from freshly-mixed mortar. The second set was broken at the end of 24 hours, retempered and remoulded, and allowed to stand 1 month. The third was retempered at the end of 48 hours and then allowed to stand 1 month. On testing, the following results were obtained, showing that concrete broken and retempered at the end of as long a period as 24 hours was not injured.