Study On Uniaxial Compressive Constitutive Relationship And Uniaxial Tensile Behavior Of Steel-Polypropylene Hybrid Fiber Reinforced Concrete

As a high performance construction material, fiber reinforced concrete is highly concerned by engineers and researchers due to excellent tensile strength, resistant to cracking, penetration, impact and fatigue, which lead to a wide range of practical application in civil engineering during the last couple of years.However, it is commonly known that concrete material is inherently a heterogeneous and inhomogeneous material, the performance might be partially developed by inclusion of only one kind of fiber, thus researchers commence to investigate better enhancements by way of mixing different kinds and types of fibers into concrete.In microscopic view, the presence of micro-cracks at the mortar aggregate interface is responsible for the inherent weakness of concrete and results in brittle performance and poor ductility, while this weakness can be remedied by the inclusion of hybrid fibers in the mix at different stages and levels. Fibers help to transfer loads, bridge the gaps at the internal micro-cracks and relieve the stress concentration. As a consequence, the strength as well as toughness and durability are enhanced. However, the research referring to hybrid fiber reinforced concrete is still at the beginning stage, few systematic theories of fiber enhancing mechanism of fibers can be found, particularly in terms of constitutive relation which is of essential significance to structural design and numerical simulation. As aforementioned, the steel-polypropylene hybrid fiber reinforced concrete is experimentally and theoretically studied in this research as shown in following aspects:(1) The slump test, cubic compressive strength and splitting tensile strength test of 34 groups of steel-polypropylene hybrid fiber reinforced concrete specimen are conducted by using orthogonal experiment method. The influences of fiber volume fraction as well as aspect ratio are investigated through variance analysis. The equations for calculating the cubic compressive strength and splitting tensile strength of hybrid fiber reinforced concrete as well as the relation between compressive and tensile strength are developed.(2) The uniaxial compression tests of steel-polypropylene hybrid fiber reinforced concrete prisms are finished, and the complete load-deformation curves are obtained. The influence of hybrid fibers with respect to uniaxial compressive strength, peak strain, elastic modulus, Possion’s ratio, compressive toughness and volumetric strain are comprehensively analyzed, meanwhile, the stress-strain curves between hybrid fiber reinforced concrete and conventional concrete are compared and the enhancing effect of hybrid fibers are detailedly demonstrated. Besides, the equations of uniaxial compressive stress-strain equation are proposed by Origin software corresponding to the test results. In order to verify the reliability of the uniaxial compressive stress-strain equation, the nonlinear finite element simulations of the shear behavior of hybrid fiber reinforced concrete deep beams are implemented by using proposed uniaxial constitutive relationship.(3) The uniaxial tension test is conducted with the use of self-built moulds due to lack of systematic standard of testing uniaxial tensile strength throughout. In this study, the influence of fibers to uniaxial tensile strength, fracture strain, tensile modulus as well as the ratio of tensile strength and compressive strength are intensively discussed. Furthermore, according to test data, the equations of uniaxial tensile strength, fracture strain, tensile modulus and the ratio of tensile strength and compressive strength are proposed.(4) On the basis of test results, the mechanical behavior of steel-polypropylene hybrid fiber reinforced concrete is analyzed, in which the effect of hybrid fibers can be divided into four stages:initiation of micro cracks, stable development of micro cracks, propagation of micro cracks and sudden extension of macro cracks. Besides, the hybrid effect of hybrid fiber is analyzed and quantified via defining the synergetic coefficient. The optimized mix proportion of fibers is recommended after comprehensively consideration of each index of mechanical behavior in this study which is A3B3C2D1, where the volume fraction of steel fiber and polypropylene fiber are 1.5% and 0.15% respectively, the aspect ratio of steel fiber and polypropylene fiber are 60 and 167 respectively.