Carbon fiber reinforced ultra-high performance concrete and its preparation and application

Abstract

This invention relates to a carbon fiber reinforced ultra-high performance concrete (UHPC) and its preparation and application. This concrete constructs a multi-level coupled conductive path, "trunk-secondary," by synergistically introducing magnetite and carbon fiber into a UHPC matrix. The carbon fiber provides a highly sensitive piezoresistive response framework, while the magnetite particles, through their semiconductor interface properties, are dispersed within the fiber network, effectively bridging local breakpoints and suppressing resistance abrupt changes caused by uneven fiber dispersion or microcrack propagation. This significantly improves the overall stability and repeatability of the conductive network and enhances the mechanical properties of the concrete. By adjusting the carbon fiber content, the resistivity, flowability, and mechanical properties of the concrete can be flexibly controlled, allowing the material to adapt to the needs of different engineering scenarios. It is particularly suitable for fields with stringent requirements for antistatic properties, conductivity, and structural strength, such as electronic factories, electromagnetically sensitive areas, explosion-proof locations, and intelligent structures, demonstrating high engineering practical value.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a carbon fiber reinforced ultra-high performance concrete and its preparation and application. Background Technology

[0002] As water conservancy, nuclear power, and transportation infrastructure develop towards larger scale and greater complexity, there is an urgent need for life-cycle health monitoring of engineering structures. Real-time perception of internal stress-strain distribution and damage evolution is key to improving the intelligent perception capabilities of infrastructure.

[0003] Existing structural health monitoring mainly relies on external embedded sensors, such as fiber optic sensors, piezoelectric ceramic sensors, and resistance strain gauges. However, these traditional monitoring technologies have significant limitations: First, the physical and mechanical properties of traditional sensors are incompatible with those of the concrete matrix, resulting in poor interfacial compatibility. These sensors are prone to detachment or failure during long-term service, potentially affecting the durability of the concrete structure. Second, while distributed fiber optic sensors can provide multi-dimensional mapping, they are expensive, structurally complex, and require stringent installation processes, making large-scale deployment difficult. Finally, once external sensors are damaged, repair or replacement costs are extremely high, and continuous monitoring throughout the entire lifecycle is difficult to achieve.

[0004] To overcome the aforementioned shortcomings, researchers have proposed self-sensing cement-based composite materials (smart concrete), which incorporate conductive fillers to imbue concrete with piezoresistive properties. Ultra-high performance concrete (UHPC), with its ultra-high strength, high toughness, and excellent durability, has become an ideal matrix for smart concrete. Meanwhile, magnetite aggregate, due to its high density, high mechanical properties, and excellent radiation shielding capabilities, is often used to prepare radiation-shielding concrete. Introducing magnetite into UHPC to prepare magnetite ultra-high performance concrete (MUHPC) can simultaneously meet the requirements of high load-bearing capacity and radiation protection.

[0005] However, the following technical challenges still exist in the existing technology: First, the high density of magnetite aggregate and its weak interfacial bonding with the cement matrix pose a challenge in maintaining excellent mechanical properties while imparting electrical functionality to concrete. Although the incorporation of carbon fiber can construct a conductive network, the relationship between carbon fiber content and material properties exhibits a complex nonlinearity: too low a carbon fiber content cannot form an effective conductive path, while too high a content severely degrades the workability and mechanical properties of concrete due to the large specific surface area of ​​carbon fiber and its adsorption of free water.

[0006] Second, existing research lacks systematic optimization of carbon fiber content in magnetite ultra-high performance concrete, and fails to reveal the comprehensive influence of carbon fiber content on the material's workability, mechanical properties, and electrical properties. In particular, it fails to achieve decoupled control of mechanical and electrical properties, making it difficult to select the formula according to specific needs in practical engineering applications.

[0007] Third, the pressure-sensitive properties (i.e., the ability of resistivity to change with load) of magnetite ultra-high performance concrete have not been fully developed, and its quantitative application in structural health monitoring still lacks a reliable material design and performance evaluation system.

[0008] Therefore, how to precisely control the carbon fiber content to endow magnetite ultra-high performance concrete with stable and sensitive pressure-sensitive characteristics while ensuring its excellent mechanical and workability, and to reveal the control mechanism of carbon fiber content on multiple performance indicators of the material, has become a key technical problem that urgently needs to be solved in this field.

[0009] The information disclosed in this background section is intended only to enhance the understanding of the background technology of this disclosure and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0010] This invention addresses the problem that existing magnetite ultra-high performance concrete often fails to balance mechanical, workability, and electrical properties when carbon fiber content is adjusted, making it impossible to achieve functional synergy optimization according to engineering requirements. By systematically controlling a specific range of carbon fiber content (0.15%~0.6%) and introducing a fiber factor to establish a quantitative relationship with workability, this invention achieves decoupled control of mechanical properties (flexural strength increased by 31.75%) and electrical properties (resistivity reduced to below 5000 Ω·mm). This endows the material with stress / strain self-sensing capabilities, providing a smart concrete solution for the life-cycle health monitoring of major engineering structures, where mechanical and electrical properties can be designed on demand and the material itself acts as a sensor.

[0011] The inventors discovered through preliminary research that magnetite (Fe3O4) exhibits high density, high hardness, and excellent mechanical reinforcement potential, as well as unique electrical properties. Carbon fibers, on the other hand, can directly contact and construct primary conductive channels within a cement matrix. Due to the semiconductor properties of the surface oxide layer (Fe2O3) of Fe3O4 particles, a secondary electron transport network is formed within the fiber gaps, suppressing local resistance abrupt changes caused by uneven fiber dispersion and creating multi-level conductive path coupling.

[0012] Based on this, the present invention proposes a novel non-metallic intelligent ultra-high performance concrete: by synergistically introducing magnetite and carbon fiber into the UHPC matrix, a multi-level coupled conductive path of "main trunk-secondary" is constructed; wherein, carbon fiber provides a highly sensitive piezoresistive response framework, while magnetite particles are dispersed in the fiber network through their semiconductor interface characteristics, effectively bridging local breakpoints and suppressing resistance abrupt changes caused by uneven fiber dispersion or microcrack propagation, thereby significantly improving the overall stability and repeatability of the conductive network.

[0013] According to one aspect of this disclosure, a carbon fiber reinforced ultra-high performance concrete is provided, comprising a cementitious material, fine aggregate, carbon fiber, water, and admixtures; the fine aggregate comprises magnetite sand, which partially or completely replaces river sand; the carbon fiber is dispersed in a matrix formed by the cementitious material, fine aggregate, and water, and the carbon fiber and magnetite sand together construct a conductive network, enabling the concrete to possess piezoresistive sensing characteristics; the water-cement ratio of the concrete is 0.15 to 0.25; and the amount of carbon fiber is 0.1% to 0.6% of the concrete volume.

[0014] In some embodiments of this disclosure, the cementitious material includes cement and active mineral admixtures; the active mineral admixtures are selected from one or more of silica fume, slag powder, fly ash, or limestone powder.

[0015] In some embodiments of this disclosure, the volume replacement rate of the magnetite sand is 50% to 70% of the volume of the river sand; the river sand is composed of two gradations with particle sizes ranging from 0 to 0.6 mm and 0.6 to 1.18 mm, respectively, and the mass ratio of the two is 1:(0.35 to 0.38); the magnetite sand is composed of two gradations with particle sizes ranging from 0 to 0.6 mm and 0.6 to 1.18 mm, respectively, and the mass ratio of the two is 1:(0.35 to 0.42).

[0016] In some embodiments of this disclosure, the amount of carbon fiber is adjusted according to engineering requirements: When the focus is on mechanical reinforcement, the carbon fiber content is 0.1% to 0.3%; When the focus is on the pressure-sensitive sensing sensitivity and conductivity, the carbon fiber content is 0.45%~0.6%.

[0017] In some embodiments of this disclosure, the concrete satisfies the fiber factor χ f The fiber factor ranges from 1.29 to 5.14. χ f Defined as χ f = V f × l f / d f ,in V f This refers to the carbon fiber volume content. l f The length of the carbon fiber. d f The diameter is the carbon fiber.

[0018] In some embodiments of this disclosure, the admixture includes a water-reducing agent and a dispersant; the dosage of the water-reducing agent is 0.5% to 2.0% of the mass of the cementitious material; and the dosage of the dispersant is 0.1% to 0.5% of the mass of the cement.

[0019] According to another aspect of this disclosure, a method for preparing the carbon fiber reinforced ultra-high performance concrete is provided, comprising the following steps: S1. Material preparation: Weigh the cementitious materials, magnetite aggregate, fine aggregate, carbon fiber, water and additives according to the preset mix proportion, and set aside; S2. Preparation of pre-dispersion liquid: Dissolve the dispersant in 1 / 2 of the mixing water, add carbon fiber and mix evenly to obtain the dispersant-carbon fiber pre-dispersion liquid; dissolve the water-reducing agent in the remaining mixing water to obtain the water-reducing agent pre-dispersion liquid; S3. Dry mixing: First, mix the river sand and magnetite evenly; then add silica fume, cement, and fly ash and dry mix evenly to obtain the dry-mixed material; S4. Wet mixing: Add the dispersant - carbon fiber pre-dispersion liquid and water-reducing agent pre-dispersion liquid to the dry mixed material, and stir to form a uniform concrete mixture; S5. Molding and curing: The mixture is poured into shape and then cured.

[0020] According to another aspect of this disclosure, a method for monitoring the health of concrete structures is provided, wherein structural components are prepared using the aforementioned carbon fiber reinforced magnetite ultra-high performance concrete. At least one pair of electrodes is provided on the structural member for applying current and / or measuring voltage; During the service life of the structure, the electrical parameters of the structural components are measured in real time or periodically. Based on the changes in the electrical parameters, the stress-strain state and / or damage evolution process inside the structure can be inverted.

[0021] According to another aspect of this disclosure, the carbon fiber reinforced magnetite ultra-high performance concrete is provided for application in at least one of the following scenarios: ① Nuclear power plant nuclear waste disposal facilities; ② Water conservancy dams and hydraulic structures; ③Bridge structure and bridge deck pavement; ④ Military protective works and bunkers; ⑤ Core tube of high-rise and super high-rise buildings; ⑥ Anti-static flooring and grounding electrode; ⑦ Reinforcement and repair of existing structures.

[0022] In some embodiments of this disclosure, the carbon fiber content range is selected according to different engineering requirements: when it is necessary to prioritize flexural toughness, a carbon fiber content range of 0.15% to 0.3% is selected; when it is necessary to prioritize electrical conductivity or electrostatic conductivity, a carbon fiber content range of 0.4% to 0.6% is selected.

[0023] In the mix design of this invention, two types of river sand with particle size ranges (0~0.6 mm and 0.6~1.18 mm) and two types of magnetite with different particle sizes (0~0.6 mm and 0.6~1.18 mm) are selected, mainly based on the following technical considerations: ① In ultra-high performance concrete systems, if only fine aggregates of a single particle size are used, it is difficult to achieve effective filling between particles, and local accumulation or void concentration areas are easily formed in the paste; while using river sand with two-stage continuous gradation can be closer to the ideal compact packing state, reducing the need for ineffective filling of the cement paste.

[0024] ② Magnetite has a significantly higher density than ordinary siliceous aggregates (approximately 5.0 g / cm³). 3 vs. 3.2 g / cm 3 Magnetite is prone to sedimentation and segregation during mixing, especially when its particle size distribution is too narrow. The lack of interlocking between particles further exacerbates the difficulty of homogeneity control. Simultaneously, if magnetite is added only as a single ultrafine powder, its excessively large specific surface area may lead to over-adsorption of water-reducing agents, resulting in a sharp decline in workability. Therefore, introducing both coarse and fine magnetite particles can alleviate the processing challenges posed by high-density materials to some extent. This is achieved by utilizing the coarser particles to participate in framework construction and the finer particles to fill micro-voids.

[0025] Furthermore, considering that conductive phases such as carbon fibers tend to agglomerate and are randomly distributed in cement matrices, if the particle size and distribution of functional fillers (such as magnetite) are also highly concentrated, it will be difficult to form effective spatial dispersion in the fiber gaps, thus limiting their potential regulatory effect on the conductive network. Using multi-scale magnetite particles helps to achieve a more uniform spatial distribution in the matrix, providing the necessary microscopic basis for the subsequent construction of stable force-electric coupling structures.

[0026] In the carbon fiber and magnetite co-doped system, there is a triple synergistic degradation effect: (1) Dispersion failure: Due to its hydrophobicity, high aspect ratio and extremely large specific surface area, coupled with the high friction and strong adsorption properties of magnetite, carbon fiber forms dense agglomerates when directly fed into the material. The internal slurry wetting is hindered, resulting in original pores and weak interfaces. The fiber is suspended, leading to the failure of the reinforcement mechanism. (2) Workability collapse: When the dosage is >0.3%, the slurry spread drops sharply and loses fluidity, appearing as "dry flocs"; the sedimentation of magnetite and the floating of fibers cause vertical stratification and segregation, and the high specific surface area intensifies the adsorption of water and water-reducing agent, and the construction window period is shortened sharply. (3) Mechanical properties decline: Agglomeration areas become stress concentration sources and defect cores. The 28-day compressive strength decreases instead of increasing. Increased internal porosity induces brittle failure. The random distribution of fibers interrupts the stress transmission path. Toughness indicators such as flexural strength and fracture energy deteriorate significantly.

[0027] The aforementioned problems are interconnected, forming a vicious cycle of "agglomeration → stratification → adsorption → performance degradation," severely restricting the engineering applicability of high-performance filler concrete. To solve this problem, this invention employs a process of "precise interface control—rheological structure optimization—full-process reconstruction," namely: (1) Interface regulation and multi-scale enhancement: Carbon fiber is pretreated with a high-efficiency dispersant liquid phase. The dispersant molecules are directionally anchored to the active sites on the fiber surface, competitively blocking the non-functional adsorption of water and water-reducing agent, inhibiting adsorption and agglomeration, and ensuring the deflocculation of single filaments. Ultrafine silica fume is hydrated to generate highly active CSH gel, which is directionally deposited at the three-phase interface of carbon fiber-magnetite-cement matrix to construct a dense transition zone, significantly improving the interfacial bonding strength and microstructure continuity. With the help of high-frequency vibration and vacuum negative pressure composite process, residual air bubbles and original pores inside the fiber clusters are efficiently discharged, weak interface defects are eliminated, and full-scale structural densification from nano-interface to macro-sample is achieved.

[0028] (2) Rheological reshaping: Incorporating ultrafine silica fume to fill the gaps in magnetite, reducing the yield stress of the slurry, improving suspension stability, and alleviating sedimentation and floating caused by density difference.

[0029] (3) Complete process reconstruction: Innovative integration of "four-stage precise feeding" and "three-stage pre-dispersion of carbon fiber" dual-track process: Three-stage pre-dispersion (fiber pretreatment): ① Liquid-phase deflocculation: High-speed shearing in the dispersant aqueous solution achieves monofilament dispersion; ② Dry mixing and coating: Dry mixing with fine magnetite aggregate to physically break down residual agglomerates through particle friction; ③ Gradual introduction: Sprinkle the slurry at a low speed, and use a "slow → medium → fast" stirring program to avoid secondary entanglement.

[0030] Four-stage precise feeding: I. River sand + magnetite (premixed to construct a stable aggregate skeleton); II. Silica fume + cement + fly ash (uniformly coated cementitious system); III. Pre-dispersed carbon fiber suspension (uniformly introduced into space); IV. Residual water + water-reducing agent (final adjustment to target workability).

[0031] In summary, this invention effectively overcomes the technical dilemma of "high dosage inevitably leads to deterioration" in carbon fiber-magnetite co-doped systems through the systematic integration of interface control, rheological optimization, and process reconstruction. Experimental verification shows that although the spread of the freshly mixed slurry decreases with increasing carbon fiber dosage, the slurry remains uniform and stable, without segregation, bleeding, or "dry flocculent" agglomeration. Crucially, the mechanical properties of the hardened concrete are precisely controlled, with both 28-day compressive and flexural strengths remaining near benchmark levels (Example 4: compressive strength 105.15 MPa, only slightly lower than the fiber-free benchmark by 7.1%; flexural strength 11.90 MPa, still slightly higher than the benchmark value), avoiding the sharp drop in strength and toughness degradation caused by fiber agglomeration and interface defects in traditional processes. This invention achieves stable and controllable structural performance while reasonably accepting moderate changes in fluidity.

[0032] One or more technical solutions provided in the embodiments of this application have at least one of the following technical effects or advantages: 1. Controllability and Predictability of Workability: By introducing fiber factor as an evaluation parameter for workability, a quantitative relationship between carbon fiber content and concrete fluidity was established. Experiments show that when the carbon fiber content increases from 0% to 0.6%, the fiber factor increases from 0 to 5.14, and the concrete spread decreases from 230 mm to 156 mm (a decrease of 32.2%). When the carbon fiber volume content is 0.15%, 0.30%, 0.45%, and 0.60%, the spread decreases by 21.91%, 24.95%, 29.72%, and 32.32%, respectively. Based on this, this invention reveals a negative correlation between fiber factor and spread: as the fiber factor increases, the spread of MUHPC decreases. This finding allows for the prediction of concrete fluidity at different carbon fiber contents in practical engineering through fiber factor calculation, providing a basis for formula design for applications such as pumping construction and complex structure casting, and resolving the contradiction between conductivity and workability in existing technologies where carbon fiber content is difficult to balance.

[0033] 2. Selective enhancement of mechanical properties This invention discovers that the carbon fiber content has a selective enhancing effect on the mechanical properties of magnetite ultra-high performance concrete, specifically manifested as follows: ① Significant Improvement in Flexural Strength: Carbon fiber significantly enhances flexural strength in the low-dosage range (0.15%~0.3%). Compared to the blank group without carbon fiber, the flexural strength of MUHPC with carbon fiber dosages of 0.15%, 0.30%, 0.45%, and 0.60% increased by 31.75%, 19.11%, 0.24%, and 1.17%, respectively. This indicates that the reinforcing effect is optimal when the carbon fiber dosage is less than 0.3%, and the reinforcing effect weakens when the dosage exceeds 0.3%. This finding provides an optimal formulation range (carbon fiber dosage 0.15%~0.3%) for engineering structures requiring high flexural toughness (such as bridge deck pavement and blast-resistant structures).

[0034] ② Stability of compressive strength: While imparting electrical properties to the material, the compressive strength of the carbon fiber reinforced magnetite ultra-high performance concrete of this invention remains stable. The compressive strength reaches a maximum of 117.75 MPa when the carbon fiber content is 0.15%, an increase of 3.9% compared to the control group; even when the content increases to 0.6%, the compressive strength loss is only 7.14%, far lower than the conventional performance loss caused by functional modification. This fully demonstrates that this invention achieves an excellent balance between functional modification and the maintenance of mechanical properties.

[0035] 3. Gradient Regulation of Electrical Properties and Conductive Percolation Effect: This invention systematically reveals for the first time the gradient regulation law of carbon fiber content on the electrical properties of magnetite ultra-high performance concrete. As the carbon fiber content increases from 0 to 0.6%, the concrete resistivity decreases successively from 16332 Ω·mm to 6918 Ω·mm, 6827 Ω·mm, 4986 Ω·mm, and 4332 Ω·mm, representing an overall reduction of more than an order of magnitude. Particularly important, this invention clarifies the critical dosage range for the conductive percolation zone: when the carbon fiber content reaches 0.45%~0.6%, the resistivity drops below 5000 Ω·mm, and the material transforms from an insulator to a conductor, entering a stable conductive region. This discovery provides clear technical indicators and formulation guidelines for applications requiring electrostatic conductivity (such as anti-static flooring and grounding electrodes).

[0036] 4. Decoupled Design and Engineering Applicability of Mechanical and Electrical Properties: Based on extensive experimental data, this invention achieves for the first time a "decoupled design" of the mechanical and electrical properties of carbon fiber reinforced magnetite ultra-high performance concrete. This means that the appropriate carbon fiber dosage range can be selected according to different engineering requirements. ① For applications requiring high bending toughness (such as structures subjected to dynamic loads and impact-resistant components): Selecting a formulation with a carbon fiber content of 0.15% to 0.3% can achieve a bending strength improvement of up to 31.75%, while maintaining a basic improvement in electrical properties (resistivity reduced to around 7000 Ω·mm).

[0037] ②For applications requiring high conductivity / static conductivity (such as antistatic flooring, grounding electrodes, and electromagnetic shielding structures): Selecting a formulation with a carbon fiber content of 0.45% to 0.6% can ensure that the material resistivity is stably below 5000 Ω·mm, entering the static conductivity functional region, while keeping the compressive strength loss within an acceptable range (<10%).

[0038] This "one material, multiple functions, and on-demand selection" design strategy greatly improves the applicability of materials in different engineering fields such as water conservancy, nuclear power, military, and civil use, and has important engineering application value.

[0039] 5. Realization of Self-Sensing Function and Application in Health Monitoring: Under uniaxial compressive load, the resistivity of the carbon fiber reinforced magnetite ultra-high performance concrete of this invention exhibits a regular change with the load, demonstrating stress / strain self-sensing function. The conductive network constructed from carbon fibers deforms and reconstructs under load, resulting in detectable changes in resistivity, and these changes correspond well to the stress state of the material. Based on this, structural components prepared using the material of this invention can have their resistivity changes measured in real time using the Kelvin four-wire detection method by setting electrodes (preferably conductive adhesive tape electrodes) on the component surface, thereby inverting the stress-strain state and damage evolution process inside the structure. This provides an innovative solution for the life-cycle health monitoring of major engineering structures such as water conservancy dams, nuclear power plants, and bridges, eliminating the need for external sensors and using the material itself as a sensor. This effectively overcomes the technical difficulties of traditional embedded sensors, such as poor compatibility with the concrete matrix, insufficient durability, and complex installation processes. Attached Figure Description

[0040] Figure 1 The image shows the morphology of the raw materials used in one embodiment of this application, where A is cement; B is fly ash; C is silica fume; D and E are magnetite with particle sizes of 0-0.6 mm and 0.6-1.18 mm, respectively; F and G are river sand with particle sizes of 0-0.6 mm and 0.6-1.18 mm, respectively; H is carbon fiber; I is water-reducing agent; and J is dispersant.

[0041] Figure 2 The following is a schematic diagram of a concrete performance testing device in an embodiment of this application; wherein, A: concrete flowability testing device; B: concrete flowability testing state diagram; C: concrete flexural strength testing device; D: concrete compressive strength testing device; E: schematic diagram of concrete electrical conductivity testing principle; F: concrete electrical conductivity testing device and sample connection diagram.

[0042] Figure 3 This is a comparison chart showing the scalability test results and their relative changes for different embodiments of this application.

[0043] Figure 4The graph shows the test results of bending strength (A) and compressive strength (B) of different embodiments of this application.

[0044] Figure 5 This is a graph showing the conductivity characteristics of an embodiment of this application. Detailed Implementation

[0045] Unless otherwise specified, the instruments and equipment involved in the following embodiments are all conventional instruments and equipment; the industrial raw materials involved are all commercially available conventional industrial raw materials; and the processing and manufacturing methods involved are all conventional methods unless otherwise specified.

[0046] To better understand the technical solution of this application, the above technical solution will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0047] I. General methods for specimen preparation and curing: S1. Material Preparation: Weigh each component by mass according to the mixing ratio and set aside; the chemical composition or performance indicators of each raw material are as follows: (1) Cement: PO 52.5 grade cement (e.g.) is used. Figure 1 A), produced by China Tianrui Group Zhengzhou Cement Co., Ltd., has its main chemical components and performance indicators shown in Tables 1 and 2.

[0048]

[0049]

[0050] (2) Fly ash: Grade I fly ash is used, such as Figure 1 B, provided by Henan Rongsong Construction Engineering Co., Ltd. The density of this fly ash is 2.53 g / m³. 3 Its main chemical components are shown in Table 3. The darker the color, the finer the particle size of the fly ash and the higher the carbon content.

[0051]

[0052] (3) Silica fume: Due to its nanoscale particle size and high silicon content, it has a high specific surface area, thus playing a filling role and exerting a pozzolanic effect in UHPC. For example Figure 1 c, whose main component is silicon dioxide, is gray in color. The microsilica powder used in this experiment is produced by Henan Dingnuo Purification Materials Co., Ltd., and its chemical composition is shown in Table 4.

[0053]

[0054] (4) Magnetite: To ensure the magnetite met the test requirements, a jaw crusher was used for crushing and screening, ultimately yielding 0-0.6mm ( Figure 1 D) and 0.6-1.18mm ( Figure 1 E) Magnetite of two particle sizes. The main chemical composition of the magnetite used was obtained by X-ray fluorescence (XRF) analysis, as shown in Table 5. It can be seen that the main component of magnetite is Fe, followed by Si.

[0055]

[0056] (5) River sand: The sand used in this experiment was natural river sand, which was obtained by screening by Runzhou District Yilin Internet Sales Department, with a particle size of 0-0.6mm (see Figure 1 F) and 0.6-1.18mm (see F) Figure 1 G) Two types of river sand.

[0057] (6) Carbon fiber: It has superior properties such as high strength (3-7 GPa), high modulus (200-400 GPa), corrosion resistance, electromagnetic shielding, electrical conductivity, and thermal conductivity. The carbon fiber used is produced by Suqian Nakaite New Material Technology Co., Ltd., mainly using 6mm short-cut carbon fiber. Its physical morphology and such Figure 1 As shown in H, the main performance indicators are shown in Table 6.

[0058]

[0059] (7) Water-reducing agent: A high-efficiency polycarboxylate water-reducing agent produced by Jiangsu Subote New Material Co., Ltd. is used. This water-reducing agent is a colorless, transparent, viscous liquid, such as... Figure 1 As shown in Figure I, the water-reducing agent has a water reduction rate of 30% and a solid content of 30%. It is used to adjust the fluidity of UHPC mixtures and has good compatibility with cementitious materials.

[0060] (8) Dispersant: Methylcellulose, as a surfactant, is an important way to improve the hydrophobicity of carbon fiber surfaces and can promote fiber dispersion in cement paste. It can reduce the surface tension of conductive materials and the surface energy of the cement matrix. The dispersant used in this experiment is as follows: Figure 1 As shown in J, the performance indicators are shown in Table 7.

[0061]

[0062] S2. Preparation of pre-dispersion liquid: Dissolve the dispersant in 1 / 2 of the mixing water, add carbon fiber and mix evenly to obtain the dispersant-carbon fiber pre-dispersion liquid; dissolve the water-reducing agent in the remaining mixing water to obtain the water-reducing agent pre-dispersion liquid; S3. Dry mixing: Mix river sand and magnetite evenly, dry mix at low speed for 50s to 60s; add silica fume, cement and fly ash, and mix for 50s to 60s; S4. Wet mixing: Add dispersant - carbon fiber predispersant and water-reducing agent predispersant to the mixture obtained in S3, and stir for 60s to 120s; S5. Molding: Pour into the mold, vibrate to compact, and then smooth the surface; the vibration time is 10-15 seconds. S6 Curing: Cover the molded specimen with plastic wrap and cure for 24 hours before demolding. Then cure in a humid environment for 28 days to complete the process.

[0063] II. Test methods for the obtained specimens: (1) Workability: According to the "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (GB / T 50080-2016), the flowability of UHPC was determined using an NLD-3 type tumbling table: the mixture was placed into a truncated cone mold in two layers, and each layer was tamped evenly from the edge to the center; after tamping, it was leveled, and the mold was lifted vertically and the tumbling table was started immediately, completing 25 tumbling cycles within 25 s ± 1 s; the two mutually perpendicular diffusion diameters on the bottom surface were measured with a steel ruler, and the average value was the flowability. The test process is shown in [see details]. Figure 2 As shown in A and 2B.

[0064] (2) Bending strength test: According to the "Test Method for Strength of Cement Mortar" (GB / T 17671-2021), the specimen cured to the specified age is placed on the bending test machine with one side as the bearing surface, ensuring that the fracture point is located between two concentrated loads. The specimen is uniformly loaded at a rate of 50±10 N / s until fracture, and the peak load is recorded and converted into bending strength (accurate to 0.01 MPa). The concrete specimen loading is as follows... Figure 2 As shown in Figure C, the flexural strength of the concrete specimen should be calculated using the following formula:

[0065] in, f f denoted as flexural strength of concrete (MPa); F is the load applied to the middle of the prism at break (N); L is the support spacing (mm); and b is the side length of the square cross-section of the prism (mm).

[0066] (3) Compressive strength test: According to the requirements of "Test Method for Strength of Cement Mortar" (GB / T 17671-2021), the specimen is placed at the center of the upper and lower pressure plates of the testing machine, with the molded side as the compression surface, and protective barriers are added to prevent cracking. A continuous and uniform load is applied at a rate of 2400±200 N / s until the specimen fails, and the peak load P during the failure process is recorded. max And the failure load P (peak load accurate to 0.01 MPa), 2 specimens were tested in each group, for a total of 24 groups, and the average value was taken as the compressive strength of that group. Figure 2 As shown in D, the compressive strength of the concrete specimen should be calculated using the following formula:

[0067] Among them, f c P is the cube compressive strength (MPa); P is the specimen failure load (N); A is the specimen bearing area (mm²). 2 Each formulation was tested using 6 test blocks, and the average value was taken.

[0068] (4) Electrical performance: The electrical performance of MUHPC was characterized by resistivity. Resistivity (ρ) is a material property that is affected by the length and cross-sectional area of ​​the material. Since traditional copper mesh electrodes can damage the in-situ strength of concrete, conductive adhesive tape (LX-LGD12, China) was used instead of traditional copper mesh electrodes in this study. Its performance parameters are shown in Table 8.

[0069]

[0070] The Kelvin four-terminal sensing method is used, such as... Figure 2 As shown in E and 2F. Before testing, the specimen was smoothed with an angle grinder and sandpaper to achieve the required dimensions. The surface was cleaned with anhydrous ethanol and allowed to air dry. Then, conductive tape was used to fix the wires to the electrodes and create leads. The electrodes were located on a cross-section perpendicular to the height direction during fabrication. The resistivity of the UHPC was measured using a DC power supply (maximum voltage 60V). For safety, the output voltage was kept constant at 30V. The current (I) of the two outer electrodes and the voltage (V) of the two inner electrodes were collected using a Keysight Digit Multimeter (34465A, USA). To avoid polarization effects, the specimen was powered on for 20 minutes before testing. The test environment was room temperature 20℃.

[0071] like Figure 2 As shown in Figure E, a voltmeter is connected between electrodes B and C, measuring a voltage of V; an ammeter is connected between electrodes A and D, measuring a current of I. According to Ohm's law, the resistance between electrodes B and C can be calculated using the following formula:

[0072] In the formula: ρ is the resistivity of concrete, Ω·mm; U(t) is the voltage difference between the two inner electrodes B and C, V; I is the current intensity through the sample, A; L is the distance between the two inner electrodes B and C, mm; A is the cross-sectional area of ​​the test section, mm². 2 . Example

[0073] The following embodiments are intended to illustrate the implementation process and technical effects of the present invention. All embodiments are prepared and tested according to the above-described general method, with the main difference being the different raw material ratios, as shown in Table 9:

[0074] Note: (1) The water-cement ratio is uniformly 0.19; (2) Magnetite 1 is 0.0-0.6mm magnetite, and magnetite 2 is 0.6-1.18mm magnetite; River sand 1 is 0.0-0.6mm river sand, and river sand 2 is 0.6-1.18mm river sand.

[0075] Comparative Example 1: Except for the absence of carbon fiber, it is the same as Example 1.

[0076] Comparative Example 2: Contains 450 kg / m³ of cement 3 Standard sand 1350 kg / m³ 3 225 kg / m³ of water 3 The experimental procedure was the same as in Example 1.

[0077] Results analysis: 1. Performance characteristics: such as Figure 3 As shown, the spreadability of ultra-high performance concrete (UHPC) decreases significantly with increasing carbon fiber content. This phenomenon is mainly due to the high specific surface area and strong hydrophilicity of carbon fibers: during mixing, carbon fibers adsorb a large amount of free water and water-reducing agent molecules in the system, resulting in a reduction in the liquid phase available for lubricating cement particles; simultaneously, the fibers form a spatial network structure in the paste, further increasing the flow resistance. These two factors combined significantly increase the viscosity of the matrix-fiber composite system, thereby reducing the fluidity of fresh concrete.

[0078] 2. Mechanical properties: such as Figure 4As shown in Figure A, the flexural strength variation of different carbon fiber content examples is illustrated. Compared to Comparative Example 1 (without carbon fiber), the flexural strength of Examples 1-4 increased by 31.75%, 19.11%, 1.17%, and 0.24% respectively. The results indicate that in the magnetite ultra-high performance concrete system described in this invention, the enhancement effect on flexural strength is significant when the carbon fiber volume content does not exceed 0.3%; however, the enhancement effect tends to weaken when the content exceeds 0.3%. As shown in Figure B, the magnetite ultra-high performance concrete system described in this invention exhibits excellent compressive strength. Compared to the 28-day compressive strength of approximately 60 MPa of ordinary Portland cement (PO52.5), even Comparative Example 1, which contains only magnetite without carbon fiber, achieved a compressive strength of 113.24 MPa. This demonstrates that magnetite, as a high-density aggregate, can significantly improve the matrix density and mechanical properties, fundamentally enhancing the strength performance of concrete, leaping from ordinary strength grade to ultra-high strength grade, thus verifying the feasibility of magnetite as a component of high-performance concrete. Based on this, Examples 1 to 4 introduced 0.15%, 0.3%, 0.45%, and 0.6% carbon fiber, respectively. The results showed that when the carbon fiber content was 0.15%, the compressive strength further increased to 117.75 MPa (3.98% higher than Comparative Example 1); when the content increased to 0.3%, the strength remained basically the same (114.00 MPa); however, when the content continued to increase to 0.45% and 0.6%, the compressive strength decreased to 109.00 MPa and 105.15 MPa, respectively, a decrease of 3.74% and 7.15%. The above results indicate that in this concrete system, the effect of carbon fiber on compressive strength has a significant threshold effect: low content (≤0.15%) can slightly strengthen the matrix, while high content leads to decreased density due to fiber agglomeration, increased interface defects, or deterioration of workability, thus weakening the strength. However, after the innovative process of this invention, the decline in mechanical properties of high-dosage fibers is controlled, and high strength can still be maintained, avoiding the problems of sudden drop in strength and toughness degradation caused by fiber agglomeration and interface defects in traditional processes.

[0079] 3. Electrical properties:

[0080] like Figure 5As shown, the resistivity of magnetite UHPC matrix exhibits typical percolation behavior after the addition of carbon fiber, which can be divided into an insulating zone, a percolation zone, a conductive zone, and a hyperpercolation zone. When the carbon fiber content is below the percolation threshold, the material is in an insulating state; when the content exceeds the threshold, the resistivity drops sharply and tends to stabilize, entering the conductive zone. Experimental results (Table 10) show that as the carbon fiber content increases from 0 to 0.6%, the resistivity of the concrete material gradually decreases from 16332 Ω·mm to 4332 Ω·mm, a reduction of approximately one order of magnitude. This fully demonstrates that the introduction of carbon fiber effectively constructs a stable conductive network and significantly improves the electrical properties of the material. This result is consistent with the expectations of percolation theory, indicating that the incorporation of carbon fiber effectively reduces the resistivity of the magnetite UHPC matrix, transforming it from a highly insulating material into a conductive material, thus providing its potential as an antistatic, conductive, or electromagnetic shielding material.

[0081] In summary, this invention effectively integrates antistatic properties and ultra-high mechanical properties by incorporating carbon fibers into a magnetite UHPC matrix without introducing any metal components. Experiments show that when the carbon fiber content is 0.6%, the material resistivity can be reduced to approximately 4.3 × 10⁻⁶. 4 The strength (Ω·mm) meets the technical requirements for antistatic materials in standards such as GB 50944-2013 "Code for Construction and Quality Acceptance of Antistatic Engineering" and GB 50611-2010 "Code for Antistatic Design of Electronic Engineering". Although the introduction of carbon fiber has some impact on flowability and some mechanical properties, the compressive strength remains above 105 MPa, the flexural strength increases by up to 31.75%, and the overall mechanical properties remain at a high level. This technical solution provides a non-metallic solution for intelligent structures in electromagnetically sensitive environments that combines reliable antistatic capabilities with structural load-bearing performance.

[0082] Although some preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention.

[0083] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this application and their equivalents, this invention is also intended to include these modifications and variations.

Claims

1. A carbon fiber reinforced ultra-high performance concrete, characterized in that, The concrete comprises cementitious materials, fine aggregates, carbon fibers, water, and admixtures; the fine aggregates include magnetite sand, which may partially or completely replace river sand; the carbon fibers are dispersed in a matrix formed by the cementitious materials, fine aggregates, and water, and the carbon fibers and magnetite sand together construct a conductive network, enabling the concrete to possess piezoresistive sensing characteristics; the water-cement ratio of the concrete is 0.15 to 0.25; the amount of carbon fibers is 0.1% to 0.6% of the total volume of the concrete.

2. The carbon fiber reinforced ultra-high performance concrete according to claim 1, characterized in that, The cementitious material includes cement and active mineral admixtures; the active mineral admixtures are selected from at least one of silica fume, slag powder, fly ash or limestone powder.

3. The carbon fiber reinforced ultra-high performance concrete according to claim 1, characterized in that, The volume replacement rate of the magnetite sand is 50% to 70% of the volume of the river sand; the river sand is composed of two gradations with particle sizes ranging from 0 to 0.6 mm and 0.6 to 1.18 mm, respectively, and the mass ratio of the two is 1:(0.35 to 0.38); the magnetite sand is composed of two gradations with particle sizes ranging from 0 to 0.6 mm and 0.6 to 1.18 mm, respectively, and the mass ratio of the two is 1:(0.35 to 0.42).

4. The carbon fiber reinforced ultra-high performance concrete according to claim 1, characterized in that, The amount of carbon fiber is adjusted according to engineering requirements: When the focus is on mechanical reinforcement, the carbon fiber content is 0.1% to 0.3%; When the focus is on the pressure-sensitive sensing sensitivity and conductivity, the carbon fiber content is 0.45%~0.6%.

5. The carbon fiber reinforced ultra-high performance concrete according to claim 1, characterized in that, The concrete satisfies the fiber factor χ f The fiber factor ranges from 1.29 to 5.

14. χ f Defined as χ f = V f × l f / d f ,in V f This refers to the carbon fiber volume content. l f The length of the carbon fiber. d f The diameter is the carbon fiber.

6. The carbon fiber reinforced ultra-high performance concrete according to claim 1, characterized in that, The admixtures include water-reducing agents and dispersants; the dosage of the water-reducing agent is 0.5% to 2.0% of the mass of the cementitious material; the dosage of the dispersant is 0.1% to 0.5% of the mass of the cement.

7. A method for preparing carbon fiber reinforced ultra-high performance concrete as described in any one of claims 1 to 6, characterized in that, It includes the following steps: S1. Material preparation: Weigh the cementitious materials, magnetite aggregate, fine aggregate, carbon fiber, water and additives according to the preset mix proportion, and set aside; S2. Preparation of pre-dispersion liquid: Dissolve the dispersant in 1 / 2 of the mixing water, add carbon fiber and mix evenly to obtain the dispersant-carbon fiber pre-dispersion liquid; dissolve the water-reducing agent in the remaining mixing water to obtain the water-reducing agent pre-dispersion liquid; S3. Dry mixing: First, mix the river sand and magnetite evenly; then add silica fume, cement, and fly ash and dry mix evenly to obtain the dry-mixed material; S4. Wet mixing: Add the dispersant - carbon fiber pre-dispersion liquid and water-reducing agent pre-dispersion liquid to the dry mixed material, and stir to form a uniform concrete mixture; S5. Molding and curing: The mixture is poured into shape and then cured.

8. A method for monitoring the health of concrete structures, characterized in that, Structural components are prepared using carbon fiber reinforced magnetite ultra-high performance concrete as described in any one of claims 1 to 6; At least one pair of electrodes is provided on the structural member for applying current and / or measuring voltage; During the service life of the structure, the electrical parameters of the structural components are measured in real time or periodically. Based on the changes in the electrical parameters, the stress-strain state and / or damage evolution process inside the structure can be inverted.

9. The application of the carbon fiber reinforced magnetite ultra-high performance concrete according to any one of claims 1 to 6 in at least one of the following scenarios: ① Nuclear power plant nuclear waste disposal facilities; ② Water conservancy dams and hydraulic structures; ③Bridge structure and bridge deck pavement; ④ Military protective works and bunkers; ⑤ Core tube of high-rise and super high-rise buildings; ⑥ Anti-static flooring and grounding electrode; ⑦ Reinforcement and repair of existing structures.

10. The application according to claim 9, characterized in that, Choose the carbon fiber content range according to different engineering needs: when it is necessary to prioritize flexural toughness, choose a carbon fiber content range of 0.15% to 0.3%; when it is necessary to prioritize electrical conductivity or electrostatic conductivity, choose a carbon fiber content range of 0.4% to 0.6%.

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