IntroductionIn modern society, the electricity is the foundation of all intelligent facilities. To realize the intelligent development of cementitious materials, electrical conductivity is one of the important directions for the future research of engineering structures and building materials. Of which, ultra-high performance concrete (UHPC) has been widely used in large-span building structures, tunnel and bridge structures, and repair and reinforcement projects, etc. If the conductive properties of UHPC are realized, and its self-sensing characteristics are given to achieve the demand of intelligent monitoring, it is of great theoretical significance and practical value to ensure the service safety of the building structure. It has reached a consensus at present that UHPC is modified by adding conductive materials to reduce the resistivity. However, conductive materials are expensive and difficult to achieve large-scale engineering applications. The use of waste carbon fibers (WCFs) can not only meet the performance requirements, but also reduce the preparation cost, improve the utilization rate of resources, and avoid environmental pollution. But at this stage, there is a lack of research on the influence of WCFs on the electrical conductivity of UHPC, and the relationship between electrical conductivity and load-deflection needs to be explored. Therefore, this paper took the electrical conductivity and bending performance of UHPC as the research point, and analyzed the bending sensitivity and load-deflection relationship of C-UHPC under different WCFs content. And the variation law between stress, strain and resistance was explored.MethodsThe UHPC with conductive properties was prepared by WCFs and steel fiber (SF), and the final mix ratio was determined by combining theoretical analysis and pre-experimental research. The water-binder ratio of C-UHPC was 0.16, and the content of WCFs was 0%, 0.5%, 1.0%, 1.5%, and 2% (in volume), respectively. Due to some differences between WCFs and virgin carbon fibers (VCFs), a comparative study of the two fibers was carried out by SEM, Raman spectroscopy, and contact angle. The compressive strength tests of C-UHPC cured for 1, 3, 7 d, and 28 d were carried out. The conductivity of C-UHPC was tested by four-electrode method (28 d). The bending sensitivity of C-UHPC mainly studied the relationship between stress, strain, and resistance under bending load. During the test, the load was automatically recorded by the stress sensor of the test machine, the deflection was measured by the bottom displacement meter of the specimen, and the resistance was measured by the external resistance box. After loading, the change values of load, deflection, and resistance were collected synchronously. Finally, SEM was used to observe the microscopic morphology of fibers and fiber-matrix interface in C-UHPC.Results and discussionCompared with VCFs, the surface of WCFs was smoother, but the wettability and graphitization degree were relatively poor. Compared with the reference group (WCFs content of 0%), the compressive strength of C-UHPC decreased after the addition of WCFs, which was attributed to the smaller bond force between the interface of WCFs and matrix. In addition, WCFs and SF were easily entangled with each other, and the positive synergism between fibers was weak. The resistivity of C-UHPC decreased with the increase of WCFs content, but when the content of WCFs was greater than or equal to 1%, C-UHPC reached the conductive threshold, that is, the effective lap probability between fibers satisfied the conductive path of electronic transmission. The main conductive pathways of WCFs in C-UHPC included overlapping, connecting cracks, and micropores. When C-UHPC was subjected to bending load, the specimen underwent uncracked, critical cracks, macroscopic cracks, and finally failure. Of which, the macro cracks were mainly axial cracks, which expanded from bottom to top.A three-stage constitutive equation applicable to the C-UHPC bending load-deflection model was established with high prediction accuracy. When C-UHPC was subjected to bending load, the resistance changed, and this phenomenon indicated that C-UHPC had a bending sensitive property. Taking the peak load as the change node, the resistance curve of the pre-peak load decreased slightly, and the resistance curve of the post-peak load increased rapidly. This was because when C-UHPC was subjected to external loading, the internal microcracks and micropores were closed, resulting in a temporary increased and more complete conductive path, and the resistance decreased. After exceeding the peak load, the matrix gradually cracked, and the axial cracks expanded, interrupting most of the conductive paths, resulting in an increase in resistance. The stress sensitivity could be divided into two stages: first, the stress sensitivity decreased significantly first and then tends to be gentle with the increase of stress; second, the stress began to decrease after reaching the peak value, and the sensitivity increases. The Poisson's ratio was introduced to improve the formula for strain sensitivity, and the strain sensitivity perpendicular to the loading direction was obtained. The strain sensitivity decreased first and then increases with the increase of strain.Results and discussionThe addition of WCFs could reduce the compressive strength of C-UHPC, but improved its conductivity. Under bending stress, the load-deflection curve of C-UHPC could be divided into elastic stage, bending hardening stage, and failure stage. After mixing with WCFs, C-UHPC took the peak load as the change node, and the resistance curve of the pre-peak load basically did not fluctuate, and the resistance curve of the post-peak load increased rapidly. When WCFs content was 1%, C-UHPC reached the conductive threshold, and continuing to increase fiber content had no significant effect on the resistivity. The sensitivity of C-UHPC decreased first and then increases with the increase of stress and strain.