Mechanical Properties of Microsteel Fiber Reinforced Concrete and Its Gradient Design in the Partially Reinforced RC Beam

Effects of two kinds of microsteel fibers were employed in reinforced concrete (RC) with different fiber volumes fraction. The RC beam was partially reinforced by microsteel fiber reinforced concrete (MSFRC) based on the idea of gradient design. Flexural performances were specially investigated. Results show that microsteel fiber highly strengthened and toughened the concrete matrix. With the same fiber volume content, the concrete reinforced by Type I fiber was generally better in strength compared with that of Type II, while the bending toughness was substantially improved. The bending strength of the concrete reinforced by microsteel fiber in partial section of tensile region was comparable to that in whole section. Based on the traditional strength theory, the critical MSFRC layer depth of in the partially reinforced RC beam was about 0.3 times of the beam depth, which possessed the same crack resistance ability with the beam composed of MSFRC in the whole section. Compared with that of the reference beam, the cracking load of the partially reinforced beam was enhanced by 119%, and the ratio of the cracking moment to ultimate moment improved by 91%. Moreover, the width and height of the cracks in the partially reinforced beam developed much slower than those in the reference beam, and the steady state in which all cracks emerged appeared later; meanwhile, the crack spacing in the pure bending region was smaller, and the number of cracks in the bending-shear region was less, which means that the partially reinforced beam is of excellent properties to resist cracking and bending. Finally, the calculation formula of the bearing capacity of the partially reinforced beam was proposed, which was in good agreement with experimental results.

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