Bond stress distribution in concrete beams and eccentric pullout specimens



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The distribution of steel stress along the embedded length of a reinforcing bar is significant in studying the distribution of bond stress, inasmuch as bond stress is caused by the difference in steel stress between two points in a reinforcing bar. In order to get the true distribution of steel stress found in reinforced concrete, a technique which does not disturb the bond between the steel and concrete is required. In this investigation, a technique introduced by R. M. Mains was used with minor alterations. An eccentric pullout specimen was used as the basic specimen for this investigation. A study of the correlation between the stress distribution in the pullout specimen and the stress distribution measured from a crack in a reinforced concrete beam was one of the major objectives of this project. It is believed that there is some similarity between the bond stress distribution adjacent to a crack in a reinforced concrete beam and the bond stress distribution in the eccentric pullout test. As a second objective, a study was made of the correlation of the stress distribution in the eccentric pullout test and the stress distribution at cut-off points in reinforced concrete beams. The author did not do any laboratory investigation of the stress distribution at cut-off points, but used the findings of Larry T. Brooks for the correlations. Although some investigations have been made on the distribution of steel stress and bond stress in pullout specimens and related to beam action, most of these investigations were concerned with concentric pullout specimens. The concentric pullout test seems entirely inadequate to represent beam action. Furthermore, the concentric pullout specimen cannot represent the horizontal shearing stresses which exist in considerable magnitude at the level of the bar in a beam. The eccentric pullout test seems much improved in studying beam action. For this investigation three pullout specimens and three beams were cast and tested. In an effort to obtain a wider range for interpretation, three different concrete strengths were used; one for each test. High early strength cement was used for all tests and all specimens were cured dry. Two # 7 hard grade reinforcing bars were instrumented with eight foil electric strain gages mounted on the inside. One bar was used for the pullout specimen and one for the beam specimen. After testing each specimen the bar was removed from the concrete and reused in the next specimen