The article presents the studies’ results of the oblique sections strength of 24 reinforced concrete beams, reinforced by three-sided U-shaped carbon fiber collars. The impact of the span section is estimated as 1.5h0 2,0h0 and 2,5h0 on the beams’ composite reinforcement efficiency in the initial inclined cracks presence and absence.
Abstract. The increase of high-rise construction volume or «High-Rise Construction» requires the use of high-strength concrete and that leads to the reduction in section size of structures and to the decrease in material consumption. First of all, it refers to the compressed elements for which, when the transverse dimensions are reduced, their flexibility and deformation increase but the load bearing capacity decreases. Growth in construction also leads to the increase of repair and restoration works or to the strengthening of structures. The most effective method of their strengthening in buildings of «High-Rise Construction» is the use of composite materials which reduces the weight of reinforcement elements and labour costs on execution of works. In this article the results of experimental research on strength and deformation of short compressed reinforced concrete structures, reinforced with external carbon fiber reinforcement, are presented. Their flexibility is λh=10, and the crosssection dimensions ratio b/h is 2, that is 1,5 times more, than recommended by standards in Russia. The following research was being done for three kinds of strained and deformed conditions with different variants of composite reinforcement. The results of the experiment proved the real efficiency of composite reinforcement of the compressed elements with sides ratio equal to 2, increasing the bearing capacity of pillars till 1,5 times. These results can be used for designing the buildings of different number of storeys.
In the theory of reinforced concrete, the issue on strength of the oblique beam sections is more complicated than that on the standard sections, since it depends on many factors. The change of at least one of them leads to a significant change in the carrying capacity and in the structural damage pattern. This is due to the fact that at the operating level of the load, all conventional reinforced concrete structures work with cracks, which must be considered in the calculation. However, in the existing regulatory documents and public sources, this issue is not specified. This paper considers the effect of initial cracks on the strength of oblique cross sections of the reinforced concrete beams strengthened with carbon fiber. The experimental studies results obtained through the transverse force testing of forty-two prototypes made of heavy concrete of B30 design grade are presented. The test samples had initial oblique cracks of 0.6-0.9 mm width and were reinforced with three composite U stirrups from the fabric based on unidirectional carbon fibers in the shear span. Initial cracks in the beams were formed at three values of the shear span – 1.5h0, 2h0 and 2.5h0. The test data show the impact of initial cracks on the efficiency of composite reinforcement of oblique cross sections of the prototypes at various values of shear spans.
The test results on deformation and rigidity of short compressed reinforced concrete pillars with various types of external transverse and longitudinal composite reinforcement are given. The samples from heavy concrete with design strength class B30-35 were tested, having the same cross-section 250x125 (h) mm and length 1200mm with flexibility λh = 10. The pillars were reinforced with 4Ø12A500 in the longitudinal direction and with tied clamps Ø6B500, installed with the step of 180 mm - in the transverse direction. The purpose of the experiment was to determine the effect of the rigidity of reinforced elements on the deformability of short experimental samples. It was necessary to determine how the eccentricity of the load application influences on the variation in the rigidity of the reinforced elements. The purpose was also to obtain data on the deformability of pillars loaded with small eccentricities, i.e. when e0 = 0.16h. It was found that the most effective type for short pillars reinforcement is a three-layer holder, which has maximum rigidity and minimal deformability. However, its efficiency gradually decreases when the eccentricity of the load application increases.
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