Experimental Method And Properties Of Ultra-High Performance Concrete Under Uniaxial Tension

Ultra-high performance concrete(UHPC)has ultra-high compressive strength and excellent durability.Steel fibers were added to enhance toughness and overcome brittleness.Therefore,UHPC also has good tensile performance.The uniaxial tensile experiment is the most effective method to explore the relationship between the tensile performance and the constitutive relationship of UHPC.However,the existing uniaxial tensile experimental methods are not uniform,and the experimental success rate is low.This situation affects the comparability of the experimental results and restricted the research and communication on the tensile performance of UHPC.In response to the problems of uneven distribution of tensile stress in specimens and inconvenient experimental operations in existing uniaxial tension experiments,the numerical simulation and experimental research were combined in this paper,and a new pull-up uniaxial tension optimized specimen and uniaxial tension experimental method were designed.Subsequently,the optimized uniaxial tensile experimental method was used to study the effects of steel fiber volume fraction and shape on the tensile performance of UHPC.Then,a UHPC uniaxial tensile constitutive model was proposed based on experimental data.The main conclusions are as follows:(1)Three stress uniformity indicators,including unit’s position-stress curve,stress uniformity coefficient η,and relative length of equal stress zone γ were proposed to indirectly evaluate the success rate of uniaxial tension experiments.Through numerical simulation,the influence of specimen types(non-notched column,notched column,dog-bone type),specimen size,connection methods(in-plane clamping,out-of-plane clamping,adhesive,embedded),and local reinforcement methods(rigid reinforcement,flexible reinforcement,embedded reinforcement)on the success rate was systematically analyzed.The results indicated that the curved dog-bone specimen is the best.The in-plane clamping connection device is the most reliable,but it will exacerbate the stress concentration of the specimen.The method of flexible reinforcement(pasting carbon fiber reinforced plastic sheet,CFRPs)can effectively improve the additional stress concentration caused by in-plane clamping connection devices,increase the success rate.(2)Based on the comprehensive comparison of the advantages and disadvantages of existing uniaxial tension experimental methods,a new pull-up uniaxial tension optimized specimen and uniaxial tension experimental method were proposed.By conducting validation experiments,it has been proven that the optimized experimental method has a higher success rate than existing experimental methods when the steel fiber volume fraction is greater than2.0%.After the local strengthening treatment of the specimen,the success rate of the validation experiments reached 97%.Moreover,main cracks were located in the middle of the uniform cross-section area of most specimens,which increases the success rate and facilitates crack observation.(3)Using the uniaxial tension optimization experimental method proposed in this research,uniaxial tension experiments were conducted on 6 groups of UHPC with 3 shapes and 2 volume fractions.The experimental results showed that when the volume fraction of steel fibers is2.0%～3.0%,the failure mode of UHPC uniaxial tensile specimens exhibits obvious pseudostrain hardening characteristics.As the volume fraction of steel fibers increases,the initial crack strength,initial crack strain,tensile strength,and peak tensile strain of UHPC all increase.Comparing the effects of different fiber shapes,the long-hooked fiber has the best enhancement effect on the tensile performance of UHPC,followed by the corrugated fiber,and the straight fiber is the weakest.(4)Based on the experimental data of the UHPC uniaxial tensile stress-strain curve,the plastic damage theory was used to establish the uniaxial tensile constitutive relationships of UHPC with different steel fiber volume fractions and shapes,which are in good agreement with the measured curves.By analyzing the constitutive equation,it is found that when the fiber shape is the same,the larger the fiber volume fraction is,the smaller the control coefficient n is,the steeper the linear rising section of the constitutive curve is,the more obvious the strainhardening section is,the more obvious the multi crack phenomenon of the specimen is,and the more obvious the pseudo-hardening phenomenon is.When the volume fraction is the same,compared with the constitutive curves of UHPC with different fiber shapes,the control coefficient n of the straight fiber is the largest,followed by the corrugated fiber,and the longhook fiber is the smallest.The constitutive curve of long-hook fiber UHPC has the steepest linear rise segment,and the phenomenon of multiple cracks is the most obvious,indicating that its reinforcement effect is the best.The downward curve of UHPC for both types of profiled fibers is relatively flat,indicating that the toughening effect of profiled fibers is better than that of straight fibers.