Steel fiber reinforced magnetite ultra high performance cementitious composites and preparation and application thereof

Abstract

The application discloses a kind of steel fiber reinforced magnetite ultra-high performance cement-based composite material and preparation and application, propose a kind of only by steel fiber and magnetite constitutes dual conductive reinforcement system: secondary electron transmission path is constructed by magnetite semiconductor interface, and the main channel formed by direct contact with steel fiber is coupled with each other, and jointly constitute multistage conductive structure, effectively inhibit the resistance mutation caused by uneven dispersion or local gap of steel fiber, significantly improve the overall connectivity and stability of conductive network. With the increase of steel fiber content, the mechanical properties of the material are continuously enhanced, the electrical properties are significantly optimized, and the fluidity only decreases slightly and still meets the construction requirements, truly realizing the synchronous improvement and high unification of mechanical properties, electrical functions and working performance, breaking through the technical limitations of single function optimization leading to other performance degradation, with outstanding technical advantages and broad engineering application value.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a steel fiber reinforced magnetite ultra-high performance cement-based composite material and its preparation and application. Background Technology

[0002] Structural health monitoring (SHM) is a key technology for ensuring the safe operation of infrastructure such as hydraulic engineering projects, nuclear power plants, bridges, and high-rise buildings. Traditional structural health monitoring mainly relies on embedded or external sensors, such as fiber optic sensors, piezoelectric ceramic sensors, and resistance strain gauges. Although these traditional sensors are effective in specific scenarios, they have significant drawbacks in practical engineering applications: First, the sensors have poor material compatibility with the concrete matrix, and the interface can easily become a weak point, affecting the durability and mechanical properties of the structure; second, the installation process is complex and demanding, making it difficult to promote on a large scale in large and complex civil structures; third, fiber optic sensors are expensive, and if damaged, they are difficult to repair; finally, traditional sensors usually only provide point-based monitoring and cannot achieve multi-dimensional distribution mapping of internal structural damage.

[0003] To overcome these shortcomings, researchers have proposed the concept of self-sensing cement-based composite materials (smart concrete), which involves incorporating conductive fillers (such as carbon fibers, steel fibers, and carbon nanotubes) into the cement matrix to construct a conductive network, thereby endowing the concrete with piezoresistive properties. However, existing smart concrete technologies still face the following challenges: 1. The contradiction between high percolation threshold and cost: A single conductive fiber often requires a high volumetric dosage (percolation threshold) to form an effective conductive network. While high dosage can improve pressure sensitivity, it significantly weakens the workability (such as fluidity) and mechanical properties of concrete, and leads to a substantial increase in material costs.

[0004] 2. Functional limitation: In special projects such as nuclear power plants and military facilities, concrete not only needs to have self-sensing capabilities, but also often needs to have high density and radiation protection (such as shielding gamma rays). Although traditional magnetite concrete has high density and shielding properties, it is prone to cracking, leading to performance failure, and lacks self-sensing capabilities, thus failing to meet the needs of full life cycle health monitoring.

[0005] 3. Insufficient stability of the conductive network: In ordinary aggregate concrete, conductive fibers mainly rely on direct contact for conductivity. Under long-term loads or environmental influences, the contact points between fibers are prone to slippage or breakage, leading to drift in the resistance signal and insufficient monitoring stability.

[0006] While ultra-high performance concrete (UHPC) possesses excellent mechanical properties and durability, its application in intelligent monitoring faces the same challenges as mentioned above regarding the synergistic effect between conductive fillers and the matrix. This is particularly true when high-density magnetite aggregates are introduced, as the weak interfacial characteristics of the aggregates further exacerbate the difficulty of fiber-matrix synergy. Therefore, there is an urgent need to develop a novel concrete material that can utilize the synergistic effect of aggregates and fibers to simultaneously enhance both mechanical and electrical properties while reducing the amount of conductive fibers required, and also provide special functions such as radiation protection.

[0007] 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

[0008] To address the technical problems of existing self-sensing concrete conductive fiber permeation threshold leading to contradictions between mechanical and electrical properties, difficulty in simultaneously achieving radiation protection functions, and high costs, this invention adopts a method of replacing part of river sand with magnetite sand in ultra-high performance concrete and utilizing its semiconductor properties to construct a multi-level conductive path coupling system with steel fibers. Combined with the technical means of "quantitative regulation of working performance by fiber factors," this invention achieves the technical effect of reducing fiber usage, synergistically enhancing mechanical and electrical properties, and simultaneously providing structural health monitoring and radiation shielding functions.

[0009] Previous systematic studies have revealed that magnetite (Fe3O4) not only effectively enhances the wear resistance, thermal stability, and electromagnetic properties of concrete as a high-density aggregate, but also possesses unique semiconductor characteristics: the Fe2O3 oxide layer naturally formed on the surface of magnetite particles exhibits stable electron transport capabilities, and its overall resistivity is significantly lower than that of ordinary siliceous aggregates. Based on this, this invention proposes a dual-element conductive reinforcement system composed of steel fibers and magnetite: when the steel fiber content is low and it is difficult to form a continuous main conductive network, the magnetite particles dispersed in the matrix can act as "conductive bridges" between adjacent steel fibers, constructing secondary electron transport pathways through their semiconductor interfaces. This secondary network couples with the main channel formed by direct contact with the steel fibers, jointly constituting a multi-level conductive structure, effectively suppressing resistance abrupt changes caused by uneven steel fiber dispersion or local gaps, and significantly improving the overall connectivity and stability of the conductive network. Therefore, without introducing other conductive components, by controlling the synergistic effect of steel fibers and magnetite, precise control of resistivity can be achieved at a low steel fiber content, simultaneously meeting the engineering requirements of ultra-high mechanical properties and reliable anti-static / self-sensing functions.

[0010] According to one aspect of this disclosure, a steel fiber reinforced magnetite ultra-high performance cementitious composite material is provided, comprising cementitious materials, fine aggregates, steel fibers, water, and admixtures; the fine aggregates comprise magnetite sand, which partially or completely replaces river sand; the water-cement ratio is 0.15~0.25; and the amount of steel fiber is 0.5%~2.0% of the concrete volume. In this system, the steel fibers overlap in the cement-based composite matrix to form the main conductive channel, and the magnetite sand particles fill the gaps between the steel fibers. Based on their semiconductor properties, they form a secondary electron transport network between adjacent steel fibers, which together with the main conductive channel constitute a multi-level conductive path coupling system.

[0011] In some embodiments of this disclosure, the cementitious material includes cement, fly ash, and silica fume, with a mass ratio of 1:(0.2-0.3):(0.1-0.15).

[0012] In some embodiments of this disclosure, 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).

[0013] In some embodiments of this disclosure, the fiber content of the cement-based composite material χ f The value is 0.3~1.3, corresponding to an extension range of 230mm~201.5mm, and the fiber factor is... χ f Defined as χ f = V f × l f / d f V f For steel fiber volume content, l f d represents the length of the steel fiber. f The diameter is the steel fiber.

[0014] In some embodiments of this disclosure, the admixture includes a water-reducing agent and a dispersant; the water-reducing agent is a polycarboxylate water-reducing agent, and its dosage is 0.5% to 2.0% of the mass of the cementitious material.

[0015] In some embodiments of this disclosure, the mass substitution rate of the magnetite sand for the river sand is 50% to 70%.

[0016] According to another aspect of this disclosure, a method for preparing the steel fiber reinforced magnetite ultra-high performance cement-based composite material as described above is provided, comprising the following steps: S1. Material preparation: Weigh the cementitious materials, magnetite aggregate, fine aggregate, steel fiber, water and additives according to the preset mix proportion, and set aside; S2. Preparation of pre-dispersion: Dissolve the water-reducing agent in the mixing water to obtain the water-reducing agent pre-dispersion; S3. Dry mixing: First, mix the river sand and magnetite sand evenly; then add silica fume, cement, and fly ash and dry mix evenly to obtain the dry-mixed material; S4. Wet mixing: Add water-reducing agent pre-dispersion liquid to the obtained dry mixture and stir thoroughly; then add steel fiber, stir at low speed for 25-35 seconds and then stir at high speed for 25-35 seconds to form a uniform mixture; S5. Molding and curing: The mixture is poured into shape and then cured.

[0017] According to another aspect of this disclosure, an engineering structure health monitoring system is provided, including a structural component made of the aforementioned steel fiber reinforced magnetite ultra-high performance cement-based composite material, and an electrical signal acquisition and analysis module; the electrical signal acquisition and analysis module is connected to the structural component and is used to monitor the resistivity change of the structural component in real time, and to evaluate the stress, strain or damage state of the structure based on a preset force-electric mapping relationship.

[0018] According to another aspect of this disclosure, the steel fiber reinforced magnetite ultra-high performance cementitious composite material is used in at least one of the following scenarios: ① Nuclear waste storage facilities at nuclear power plants; ②Military protective engineering and bunkers; ③ Construction of bridges in high-intensity earthquake zones; ④ Water conservancy dams and hydraulic structures; ⑤ Reinforcement and repair of existing structures.

[0019] In some embodiments of this disclosure, the pressure-sensitive properties of the cement-based composite material are used for structural health monitoring, and / or the high-density properties of the magnetite sand are used for radiation shielding or impact protection.

[0020] One or more technical solutions provided in the embodiments of this application have at least one of the following technical effects or advantages: 1. A multi-level coupled conductive network was constructed, significantly reducing the percolation threshold and material cost of the conductive filler: Utilizing the semiconductor properties of magnetite sand, it is filled into the gaps between steel fibers to form a "secondary electron transport network," which couples with the "main conductive channel" of the steel fibers. Compared to ordinary aggregate concrete, magnetite particles act as "conductive bridges" between fibers, allowing the conductive network to be formed with a lower steel fiber content (starting from 0.5%). Experiments show that this system can achieve the same conductivity effect by reducing the steel fiber content by 20%~30% compared to ordinary aggregate concrete. This not only significantly reduces raw material costs but also reduces the performance degradation caused by high fiber content, solving the technical challenge of "high conductivity depending on high fiber content."

[0021] 2. Synergistic enhancement of mechanical and electrical properties: Ultra-high performance concrete matrix is ​​used, combined with 0.5%–2.0% volumetric steel fiber and a 50%–70% magnetite substitution rate. The bridging effect of the steel fiber significantly improves the compressive strength of the concrete (28-day compressive strength ≥120 MPa, reaching up to 137 MPa or more), while avoiding the brittle fracture of plain UHPC. Simultaneously, the resistivity decreases by orders of magnitude with increasing steel fiber content (resistivity as low as approximately 1050 Ω·mm at 2.0% content). This invention overcomes the contradiction between the mutual constraints of mechanical and electrical properties in a single-fiber system, achieving a simultaneous improvement in "high strength and high sensitivity."

[0022] 3. Improved stability and sensitivity of the pressure-sensitive response: In the multi-level conductive path coupling system (fiber contact + magnetite bridging), the secondary network formed by magnetite particles suppresses local resistance abrupt changes caused by uneven fiber dispersion or stress slippage. Under uniaxial compressive load, the resistivity change rate of concrete exhibits a good linear or piecewise linear relationship with strain. This stable force-electric response characteristic allows the material to more accurately reflect the stress / strain state and damage propagation within the structure, making it suitable for structural health monitoring throughout its entire life cycle.

[0023] 4. Combining radiation protection and high density, it expands its application scenarios in special engineering projects: The introduction of high-density magnetite as fine aggregate (with a replacement rate of 50%~70%) endows concrete with excellent gamma-ray shielding capabilities and wear resistance, making it particularly suitable for nuclear power plant waste treatment facilities, military nuclear facilities, and bridges in high-intensity earthquake zones. While meeting structural health monitoring requirements, it eliminates the need for an additional radiation protection layer, simplifying construction processes, improving space utilization, and achieving integrated "structure-function-intelligence" functionality.

[0024] 5. A quantitative control relationship between material composition and workability was established, enabling the design and prediction of concrete workability. The study found that as the steel fiber content increases, the fiber factor increases, and the concrete spread decreases accordingly, showing a clear negative correlation. When the fiber factor is in the range of 0.3 to 1.3, the spread decreases from 230 mm to 201.5 mm, with a controllable reduction. This established relationship allows for precise design of the fiber factor in practical engineering by adjusting the fiber content, length, and diameter according to the conveying method and pouring requirements, thereby predicting and controlling the workability of concrete. Furthermore, by optimizing the gradation of magnetite and river sand (0.0~1.5 mm continuous gradation) and the use of admixtures, the negative impact of fiber incorporation on fluidity was further effectively mitigated, ensuring good spreadability of the concrete mixture. Attached Figure Description

[0025] 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 steel fiber; and I is water-reducing agent.

[0026] 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.

[0027] Figure 3 This is a comparison chart showing the scalability test results and their relative changes for different embodiments of this application.

[0028] Figure 4 The diagram shows the test results of compressive strength (A) and flexural strength (B) of different embodiments of this application.

[0029] Figure 5 This is a graph showing the conductivity characteristics of an embodiment of this application. Detailed Implementation

[0030] 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.

[0031] 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.

[0032] I. Raw material preparation: The physical properties or chemical composition of the raw materials used in the following examples 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.

[0033] .

[0034] .

[0035] (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.

[0036] .

[0037] (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.

[0038] .

[0039] (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.

[0040] .

[0041] (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 1G) Two types of river sand.

[0042] (6) Steel Fiber: The UHPC matrix itself is insulating, and steel fiber is an ideal conductive medium. After being incorporated, it forms a conductive network in the matrix. Steel fiber not only effectively improves the conductivity of UHPC, but also enhances its mechanical properties, significantly improving compressive strength, toughness, flexural strength, and impact resistance. Copper-plated micro-steel fibers produced by Yutian County Zhitai Steel Fiber Manufacturing Co., Ltd. are used. The fibers are hooked at the ends, and their physical morphology and... Figure 1 As shown in H, the main performance indicators are shown in Table 6.

[0043] .

[0044] (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.

[0045] II. General methods for specimen preparation and curing: S1. Material preparation: Weigh each component by mass according to the mixing ratio and set aside; S2. Preparation of pre-dispersion: Dissolve the water-reducing agent in the mixing water to obtain the water-reducing agent pre-dispersion; 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 the pre-dispersion liquid of water-reducing agent to the mixture obtained in S3 and stir for 60s to 120s; then add steel fiber, stir at low speed for 25 to 35s and then stir at high speed for 25 to 35s; 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 for 28 days in an environment with humidity ≥95%.

[0046] III. 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.

[0047] (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:

[0048] 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).

[0049] (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:

[0050] 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.

[0051] (4) Electrical properties: The electrical properties of MUHPC were 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 7.

[0052] .

[0053] 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℃.

[0054] 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:

[0055] 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

[0056] The following embodiments are intended to illustrate the implementation process and technical effects of the present invention. All embodiments were prepared and tested according to the above-described general method, with the main difference being the amount of steel fiber incorporated in each embodiment, as shown in Table 8:

[0057] 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; (3) Based on density calculation, with 1m 3 Taking concrete as an example, the volumetric content of steel fiber in Examples 1-4 is 0.5%, 1%, 1.5%, and 2%, respectively.

[0058] Results analysis: 1. Performance characteristics: such as Figure 3As shown, Comparative Example 1, without added steel fibers, had a spread of 230 mm, serving as the baseline group. In Examples 1 to 4, as the steel fiber content gradually increased, the spread of the concrete decreased sequentially to 225.8 mm, 215 mm, 208 mm, and 201.5 mm, representing decreases of 2.04%, 6.72%, 9.76%, and 12.58% relative to Comparative Example 1, respectively.

[0059] The aforementioned fluidity variation patterns can be further analyzed using the fiber factor analysis method proposed by Markovich: the fiber factor is defined as...

[0060] in, χ f For fiber factor; V f I is the fiber volume content; f d represents fiber length. f The fiber diameter is given. As the steel fiber content increases, the fiber volume content V... f Increased size, leading to increased fiber factor χ f The corresponding increase is shown in Table 9. According to this theory, the fiber factor is inversely proportional to the workability of the mixture; that is, the larger the fiber factor, the lower the fluidity. The underlying mechanism is that the steel fibers form a three-dimensional network structure in the slurry, and its specific surface area increases significantly with the increase of the admixture amount. This consumes more cementitious slurry to coat the fiber surface, resulting in a reduction in the amount of freely flowing slurry. At the same time, the overlap and interweaving between fibers enhance the internal frictional resistance, physically hindering the slurry flow, ultimately manifesting as a continuous decrease in the spread.

[0061]

[0062] 2. Mechanical properties: Figure 4 A provides the compressive strength of the high-performance concrete after 28 days of curing. Compared with ordinary P.O52.5 cement (compressive strength approximately 60 MPa), the compressive strength of Comparative Example 1 and each embodiment of this application is several times higher, exhibiting excellent ultra-high strength characteristics. The compressive strength of Comparative Example 1 (containing only magnetite) is 113.24 MPa, which is approximately 1.89 times that of ordinary P.O52.5 cement; as the steel fiber content gradually increases in Examples 1 to 4, the compressive strength continues to increase to 119.60 MPa, 125.75 MPa, 131.40 MPa and 137.68 MPa, with the highest increase reaching 21.58%, which is approximately 2.29 times that of ordinary P.O52.5 cement.

[0063] Figure 4B provides the flexural strength of the high-performance concrete after 28 days of curing. Compared with ordinary PO 52.5 cement (flexural strength approximately 9 MPa), the flexural strength of Comparative Example 1 and each embodiment of this application is significantly improved, exhibiting excellent toughness and crack resistance. The flexural strength of Comparative Example 1 (containing only magnetite) is 11.76 MPa, which is approximately 1.31 times that of ordinary PO 52.5 cement; as the steel fiber content gradually increases in Examples 1 to 4, the flexural strength continues to increase to 14.72 MPa, 15.11 MPa, 20.10 MPa, and 26.65 MPa, with the highest increase reaching 126.61%, which is approximately 2.96 times that of ordinary PO 52.5 cement.

[0064] The above results indicate that the introduction of magnetite effectively optimizes the density and hydration product structure of the concrete matrix, while the incorporation of steel fibers constrains the propagation of microcracks by forming a three-dimensional network. The synergistic effect of the two significantly improves the load-bearing capacity and flexural toughness of the material, achieving a synergistic unity of ultra-high compressive strength and excellent flexural toughness.

[0065] (3) Electrical properties: such as Figure 5 As shown, after adding steel fibers to the magnetite-based high-strength concrete matrix, its resistivity exhibits typical percolation behavior as the fiber content changes, which can be divided into an insulating zone, a percolation zone, a conductive zone, and a hyperpercolation zone. When the steel 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.

[0066]

[0067] Table 10 shows the resistivity (ρ) and relative resistivity (Dr) of UHPCs with different steel fibers. Comparative Example 1 has a resistivity of 16332 Ω·mm, indicating it is in an insulating state and cannot meet the requirements of multifunctional engineering. With the gradual increase of fiber B content, the resistivity of Examples 1 to 4 decreased sequentially to 6669 Ω·mm, 4284 Ω·mm, 4032 Ω·mm, and 3269 Ω·mm, with relative resistivity changes of 59.17%, 85.51%, 95.98%, and 97.20%, respectively, with the highest reduction reaching 80.0%. The steel fibers form a continuous conductive network in the matrix, effectively reducing the material resistivity. Furthermore, the resistivity of all examples is stably controlled within the range of 3000–7000 Ω·mm, achieving both electrostatic dissipation and stray current protection while retaining excellent electrical response sensitivity. This allows for structural health self-sensing, ultimately achieving synergistic optimization of mechanical, electrical, and operational properties.

[0068] In summary, the cement-based composite material of this invention, through the synergistic design of magnetite and steel fiber, successfully overcomes the technical bottleneck of existing cement-based composite materials that struggle to simultaneously achieve workability, ultra-high mechanical strength, and controllable electrical properties. In terms of workability, although the increased steel fiber content slightly reduces workability, it still meets construction requirements. Mechanically, the compressive strength is >119 MPa and the flexural strength is >14 MPa, reaching a maximum of 137.68 MPa and 26.65 MPa respectively, far exceeding ordinary PO 52.5 cement. Electrically, the resistivity is precisely reduced from 16332 Ω·mm to 3000–7000 Ω·mm, simultaneously achieving electrostatic dissipation and structural self-sensing functions. The synergistic optimization of the above-mentioned multi-dimensional performance verifies the scientific nature and feasibility of the technical solution of this application. The prepared concrete material takes into account the feasibility of construction, heavy load bearing capacity, crack resistance and toughness and multi-functional electrical properties. It can adapt to the application needs of high-end engineering scenarios such as intelligent structural health monitoring, electrified rail transit, and anti-static high load bearing flooring, and has important engineering practice value and industrialization prospects.

[0069] 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.

[0070] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, this application is also intended to include such modifications and variations if they fall within the scope of the claims of this application and their equivalents.

Claims

1. A steel fiber reinforced magnetite ultra-high performance cement-based composite material, characterized in that, The material comprises cementitious materials, fine aggregates, steel fibers, water, and additives; the fine aggregates include magnetite sand, which partially or completely replaces river sand; the water-cement ratio is 0.15~0.25; the steel fiber content is 0.5%~2.0% of the total volume of the cement-based composite material. In this system, the steel fibers overlap in the cement-based composite matrix to form the main conductive channel, and the magnetite sand particles fill the gaps between the steel fibers. Based on their semiconductor properties, they form a secondary electron transport network between adjacent steel fibers, which together with the main conductive channel constitute a multi-level conductive path coupling system.

2. The steel fiber reinforced magnetite ultra-high performance cement-based composite material according to claim 1, characterized in that, The cementitious material includes cement, fly ash, and silica fume, with a mass ratio of 1:(0.2-0.3):(0.1-0.15).

3. The steel fiber reinforced magnetite ultra-high performance cement-based composite material according to claim 1, characterized in that, 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 steel fiber reinforced magnetite ultra-high performance cement-based composite material according to claim 1, characterized in that, The fiber factor of the cement-based composite material χ f The value is 0.3~1.3, corresponding to an extension range of 230mm~201.5mm, and the fiber factor is... χ f Defined as χ f = V f × l f / d f ,in V f This refers to the volumetric content of steel fibers. l f For the length of the steel fiber, d f The diameter is the steel fiber.

5. The steel fiber reinforced magnetite ultra-high performance cement-based composite material according to claim 1, characterized in that, The admixtures include water-reducing agents and dispersants; the water-reducing agent is a polycarboxylate water-reducing agent, and its dosage is 0.5% to 2.0% of the mass of the cementitious material.

6. The steel fiber reinforced magnetite ultra-high performance cement-based composite material according to claim 1, characterized in that, The mass substitution rate of the magnetite sand for the river sand is 50% to 70%.

7. A method for preparing a steel fiber reinforced magnetite ultra-high performance cementitious composite material as described in any one of claims 1-6, characterized in that, It includes the following steps: S1. Material preparation: Weigh the cementitious materials, magnetite aggregate, fine aggregate, steel fiber, water and additives according to the preset mix proportion, and set aside; S2. Preparation of pre-dispersion: Dissolve the water-reducing agent in the mixing water to obtain the water-reducing agent pre-dispersion; S3. Dry mixing: First, mix the river sand and magnetite sand evenly; then add silica fume, cement, and fly ash and dry mix evenly to obtain the dry-mixed material; S4. Wet mixing: Add water-reducing agent pre-dispersion liquid to the obtained dry mixture and stir thoroughly; then add steel fiber, stir at low speed for 25-35 seconds and then stir at high speed for 25-35 seconds to form a uniform mixture; S5. Molding and curing: The mixture is poured into shape and then cured.

8. A structural health monitoring system, characterized in that, The invention includes a structural component made of steel fiber reinforced magnetite ultra-high performance cement-based composite material as described in any one of claims 1-6, and an electrical signal acquisition and analysis module; the electrical signal acquisition and analysis module is connected to the structural component and is used to monitor the resistivity change of the structural component in real time, and to evaluate the stress, strain or damage state of the structure based on a preset force-electric mapping relationship.

9. The application of the steel fiber reinforced magnetite ultra-high performance cementitious composite material as described in any one of claims 1-6 in at least one of the following scenarios: ① Nuclear waste storage facilities at nuclear power plants; ②Military protective works and bunkers; ③ Construction of bridges in high-intensity earthquake zones; ④ Water conservancy dams and hydraulic structures; ⑤ Reinforcement and repair of existing structures.

10. The application according to claim 9, characterized in that, Structural health monitoring can be performed using the pressure-sensitive properties of the cement-based composite material, or / and radiation shielding or impact protection can be performed using the high-density properties of the magnetite sand.

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