Research of the resistance of pulling out steel bars from concretes of different classes

Abstract

The article gives the results of the experimental researches of the bond between steel and concretes of different classes. The basic test samples were produced in the form of concrete prisms of square cross section, sides of which equal 15 сm, and the height corresponds to the planned length of anchorage. Steel bars were located in the prisms in such a way that their longitudinal axis matched together, and parts of bars cantilevering out of prisms allowed to fasten in the grip of the hydraulic press on the one side, and on the other (free) side of the bars to measure their displacement relative to the end of the prisms. The analysis of the impact of the concrete cover on the boundary stresses of the bond between steel and concrete was carried out. The analysis of regression equation structures shows the absence of the linier impact of the bar diameter on the boundary stresses of the bond f_b. The obtained mathematical model allows analyzing the impact of factors, that were investigated on the boundary stresses on the contact surface of steel and concrete. The analysis of the results of the experimental researches speaks  that the depth of the concrete cover  has a significant impact on the boundary bond stresses. A similar relative reduction of the bond stresses is observed in samples with steel bars of 16 і 20 mm in diameter. Thus, the maximum bond occurs at the largest concrete cover, while the minimum bond being at the least. The character of destruction of the samples also depends on the depth of the concrete cover. At с = 3d the destruction occurred in three-four planes with the appearing of cracks on the lateral area, the prism was divided into separate elements. However, in the prisms with the concrete cover l_b = 10d there was a stiffening of the steel bar and appearance of a split in the direction toward all sides. At с = 1,5d cracks were developed within the concrete cover, but immediately before the complete destruction – in the direction toward the cheeks. In the samples with с = 1d, the destruction took place in the plane which crossed the concrete cover. It should be noted that the longitudinal cracks were formed along the longitudinal actions of the bars as a result of the wedging moment of the transverse actions, located at the angle to the direction of pulling out the bars. It has been experimentally proved that the depth of the concrete cover substantially affects the maximum stresses of the bond between concrete and steel and the character of destructing prisms.