POSH BARS IN CONCRETE.
622
Compression Test oe Yellow Pine Push Bars Embedded in
Concrete.
Material submitted by Mr. H. A. Carson, chief engineer, Boston transit commission.
The push bars were embedded in a cylindrical mass of concrete 10 feet long by 28 inches diameter in the following manner:
A cylindrical mold 10 feet long, the interior diameter of which was 28 inches, was placed on end, the bottom being closed with a bulkhead having a central aperture 5" x 5," through which the end of the first yellow pine push bar was placed. This first push bar was centered within the mold and a section of concrete tamped about it, filling the mold a length of 2' 6". At the expiration of 8 hours a second push bar was placed upon the first bar and connected therewith by means of a cast-iron ribbed coupling and a second section of concrete 2' 6" long tamped in the mold. In like manner a third section of concrete and its push bar were added 16 hours after the first section was made, and a fourth section and push bar 24 hours after the first section was made. The upper end of the mold was closed with a bulkhead having a central opening 5" x 5" to accommodate the upper end of the fourth push bar. The concrete was then lowered into a horizontal position and adjusted in the testing machine, and about 1 hour later loads were applied to the column of push bars in the axis of the mold.
Under a load of compression of 102,500 pounds the fourth push bar, at the bulkhead or outer end, crushed at two knots about 4" from the end of the stick. The knots were 1J" and f" in diameter. This load was the maximum resistance displayed. As the fibers were crushed, the load fell to about 17,600 pounds, at which time the test was discontinued and the material allowed to rest over night.
Renewing the load 14 hours later the end push bar continued to yield under the load of 77,600 pounds. The test was then discontinued and the material removed from the testing machine.
The mold was stripped from the concrete and the latter removed from the fourth and the third push bars. A slight movement of the coupling below the fourth bar appeared to have taken place in the direction of the axis of the specimen. The coupling below the third bar did not appear to have moved during the test.
A supplementary test was made with the fourth push bar. The injured portion was sawed off and the remaining part subjected to endwise compression, sides free. The bar was now 21f" long, and it was immersed in water for 5 hours immediately preceding this test. Its ultimate strength was now found to be’ dll,600 pounds. Failure occurred at a knot f" diameter, which was located 2>\" from the end of the bar.
The composition of the concrete was Vulcanite cement 1, stone dust 2|-, crushed stone 4i. The push bars had been soaked in water before being used in the test cylinder.
A photograph showing the test cylinder in the mold as it appeared in the testing machine, also a view of the original fractured end of the fourth push bar are shown herewith.