Influential Research On The Steel Fiber Properties On The Tensile Properties Of UHPC

Ultra-high performance concrete(UHPC)is a new structural material with great development prospects and application value,and tensile properties are one of the important properties of this material,which directly affect the crack resistance and durability of UHPC structures.Unlike ordinary concrete,due to the presence of steel fibers in UHPC,the fiber properties(such as fiber content,fiber shape and fiber size)have a significant impact on the tensile properties of UHPC.Improper selection may reduce the tensile properties of UHPC,so it is necessary to study the influence of steel fiber properties on the tensile properties of UHPC.In this paper,UHPC specimens with different fiber aspect ratios(59~100),different fiber types(hooked-end or straight),single blending or mixed blending fibers and steel-free fibers were tested by direct tensile test,small splayed tensile test and four-point bending tensile test respectively under the same fiber content(2%).The constitutive curves of UHPC under direct tensile test were fitted,and the two indirect tensile test results were compared with the direct tensile test results.The main conclusions are as follows:(1)For UHPC specimens with a certain fiber aspect ratio,the three tensile peak strengths,direct tensile toughness and four-point bending tensile toughness increase with the increase of fiber aspect ratio.Compared with direct tensile properties,the increase of fiber aspect ratio has a more obvious effect on the UHPC small splayed tensile properties and four-point bending tensile properties,but the change of fiber aspect ratio does not affect the hardening type of UHPC.(2)Under the same fiber aspect ratio,the direct tensile peak strength and the small splayed tensile peak strength of the straight fiber specimens are slightly higher than those of the hooked-end fiber specimens,but the high-strain hardening phenomenon and the low-strain hardening phenomenon can occur respectively in the hooked-end and straight fiber direct tension specimens,and the stresses of the two fiber types of specimens are higher than 7MPa when the direct tensile strain reaches 3000??;however,the four-point bending tensile peak strength of the hooked-end fiber specimens is slightly higher than that of the straight fiber specimens,and flexural hardening phenomenon can occur in the four-point bending tensile specimens of both types of fibers,and the strength of the two fiber types of specimens is still higher than 16.4MPa when the mid-span deflection of four-point bending tensile specimens reaches L/150.(3)The fiber blending has little effect on the UHPC initial crack strength under the direct tensile test and the four-point bending tensile test,and the flexural hardening type of UHPC can not be changed,while the strain hardening type of UHPC can be changed,that is,low strain hardening phenomenon and high strain hardening phenomenon occur respectively in specimens with single and mixed fibers;moreover,the fiber blending has different effects on the UHPC peak strength under the three tensile tests,among which: the peak strength of the specimens with mixed blending fibers under the direct tensile test is significantly higher than that of the specimens with single blending fibers,while the peak strength under the four-point bending tensile test shows the opposite trend,and the peak strength of mixed fiber-blended specimens is roughly equivalent to that of single fiber-blended specimens.(4)The presence or absence of steel fiber has little effect on the the initial crack strength of UHPC under direct tensile test and four-point bending tensile test,but compared with the steel-free fiber specimens with brittle failure,the steel-containing fiber specimens can show more obvious hardening characteristics.At the same time,UHPC specimens containing steel fibers under three kinds of tensile tests can show different degrees of post-crack reinforcement effect,and the order is: four-point bending tensile test> small splayed tensile test > direct tensile test.(5)The fitting curves of UHPC direct tensile stress-strain multi-fold form and stress-crack width double-fold form are generally consistent with the corresponding measured curves,and the two fitting curves are simple in form;the estimation of UHPC direct tensile peak strength by small splayed tensile test is not suitable for specimens with excessive fiber aspect ratio;however,the modified initial crack bending tensile strength considering the size effect can be used to estimate the initial crack direct tensile strength,and after deducting the influence of fiber orientation,the input direct tensile constitutive curves by ABAQUS finite element software is basically consistent with the measured one.