A buried salinity monitoring structure based on conductive concrete and a preparation method and application thereof

Abstract

This invention relates to an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application, belonging to the field of concrete structure durability monitoring technology. The embedded salinity monitoring structure includes a concrete matrix, carboxylated MWCNTs conductive phase filler, a conductive copper mesh connector, a water-reducing agent, a dispersant, and water. The concrete matrix includes cement and aggregate; the dispersant includes silica fume and fly ash; the conductive copper mesh connector is embedded in the concrete matrix; the embedded salinity monitoring structure is placed vertically within the concrete structure to be monitored, enabling in-situ, quantitative, and continuous monitoring of the salinity within the concrete structure. Compared with existing technologies, the structure prepared by this invention possesses good structural compatibility, signal stability, and long-term reliability, solving problems commonly found in existing monitoring methods such as large voltage drop, significant signal drift, inability to achieve continuous dynamic monitoring, and severe environmental interference affecting monitoring results.

Description

Technical Field

[0001] This invention relates to the field of concrete structure durability monitoring, and in particular to an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. Background Technology

[0002] Traditional concrete, with its excellent mechanical properties and cost advantages, is widely used in transportation infrastructure such as roads, airports, and bridges. However, as an insulator, this material is difficult to use directly for autonomous monitoring of structural health, especially under complex environments and long-term high loads. Projects such as airport runways in marine areas are subjected to a combination of factors including alternating hot and cold temperatures, wet and dry cycles, and chloride erosion, making them prone to cracking, spalling, and other defects, affecting their safe service performance. Current monitoring of internal salt corrosion in concrete mainly relies on manual inspections or discrete point sensors, which cannot meet the requirements for real-time, comprehensive, and continuous monitoring. Furthermore, existing methods generally suffer from limited measurement accuracy, high implementation costs, and susceptibility to environmental interference.

[0003] With the goal of functional modification of concrete, the development of self-sensing intelligent concrete has become a research hotspot. Its core strategy lies in introducing conductive fillers to obtain electrical signal responses to the internal state of the structure. By introducing fillers such as steel fibers, carbon fibers, or nano-conductive particles, electrical signal responses to stress, strain, and damage states within concrete can be achieved, providing a new approach for structural health monitoring. However, traditional conductive fillers often require high dosages to form conductive pathways, which may affect the original mechanical and construction properties of concrete, and also suffer from poor compatibility with the cement matrix and susceptibility of electrical signals to interference from temperature and humidity fluctuations. Among these, carbon nanotubes have become a research hotspot due to their excellent conductivity and mechanical strengthening effects, but their uniform dispersion in the cement matrix still faces technical challenges. Existing dispersion methods mostly employ physical stirring or conventional dispersant processes, which are difficult to effectively suppress the agglomeration of carbon nanotubes, resulting in discontinuous conductive networks and insufficient electrical signal stability, failing to meet the requirements of practical engineering for the accuracy and long-term reliability of salinity monitoring.

[0004] Especially in high-salt environments such as airport pavements and port terminals, chloride ion corrosion is a major cause of durability degradation in concrete structures. Current monitoring of salinity corrosion mainly relies on damaged sampling analysis or predictive models, which cannot accurately reflect the dynamic evolution of structures in actual service environments. While some studies have attempted in-situ monitoring using fiber optic or electrode sensors, these methods generally suffer from high costs, significant signal drift, and weak anti-interference capabilities, making it difficult to balance monitoring continuity, long-term stability, and engineering feasibility. Therefore, developing an embedded salinity monitoring structure with good sensitivity, environmental adaptability, and long-term reliability to achieve real-time, continuous, and quantitative monitoring of salinity within concrete structures has become a critical issue urgently needing breakthroughs in the field of structural health monitoring.

[0005] Taking an airport runway in southeastern coastal China as an example, on-site exposure tests showed that chloride ion deposition occurred on the surface of the concrete structure, resulting in a chloride ion concentration exceeding 0.05 wt% at a depth of 20-30 mm within the protective layer. At this point, the concrete appearance remained intact, with no visible cracks or spalling, but the passivation film of the internal reinforcing bars had begun to deteriorate. Existing non-destructive testing methods (resistivity method, half-cell potential method, etc.) are insufficient for continuous, in-situ quantitative monitoring of this early and concealed chloride ion infiltration process. Embedded sensors typically only output effective signals after macroscopic damage to the structure, making it difficult to issue early warnings in the early stages when chloride ions have infiltrated but have not yet caused obvious damage. Furthermore, they are susceptible to interference from factors such as signal drift and interface debonding, thus missing the optimal warning window. Therefore, there is an urgent need for an embedded structure capable of long-term, stable, and highly sensitive monitoring of internal salinity even when the concrete structure is intact. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the existing technology by providing an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application, for in-situ, real-time, and quantitative monitoring of chloride ion erosion. The structure prepared by this method possesses good structural compatibility, signal stability, and long-term reliability, solving problems commonly found in existing monitoring methods such as large voltage drop, significant signal drift, inability to achieve continuous dynamic monitoring, and severe environmental interference affecting monitoring results. This provides an effective technical means for long-term health monitoring of the durability of concrete structures.

[0007] The objective of this invention can be achieved through the following technical solutions: One of the technical solutions of the present invention is to provide an embedded salinity monitoring structure based on conductive concrete. The embedded salinity monitoring structure includes a concrete matrix, a carboxylated multi-walled carbon nanotube (carboxylated MWCNTs) conductive phase filler, a conductive copper mesh connector, a water-reducing agent, a dispersant, and water.

[0008] Furthermore, the concrete matrix includes cement and sand / gravel.

[0009] Furthermore, the dispersant includes silica fume and fly ash.

[0010] Furthermore, the carboxylated multi-walled carbon nanotubes (carboxylated MWCNTs) conductive phase filler is uniformly distributed in the concrete matrix.

[0011] Furthermore, the conductive copper mesh connector is partially embedded (partially embedded) in the concrete substrate. Specifically, half of the conductive copper mesh connector is embedded in the concrete substrate, while the other half extends out of the concrete substrate.

[0012] Furthermore, the embedded salinity monitoring structure is placed vertically within the concrete structure to be monitored, so as to achieve in-situ, quantitative, and continuous monitoring of the salinity inside the concrete structure.

[0013] Furthermore, the water-reducing agent is a polycarboxylate water-reducing agent, designed to enhance the dispersion effect of carboxylated MWCNTs.

[0014] Furthermore, the cement is silicate cement.

[0015] Furthermore, the sand and gravel include sand and gravel.

[0016] Furthermore, the sand is standard quartz sand.

[0017] Furthermore, the embedded salinity monitoring structure comprises the following components in parts by weight: 3-10 parts water-reducing agent, 1-5 parts carboxylated MWCNTs conductive phase filler, 10-100 parts silica fume, 10-100 parts fly ash, 2000-3000 parts cement, 4000-6000 parts sand and gravel, and 600-1000 parts water.

[0018] The second technical solution of the present invention provides a method for preparing an embedded salinity monitoring structure based on conductive concrete, comprising the following steps: S1: Mix the water-reducing agent with water to obtain a water-reducing agent solution; S2: Add carboxylated MWCNTs (as conductive phase fillers of carboxylated multi-walled carbon nanotubes (carboxylated MWCNTs)) and silica fume and fly ash as dispersants to the water-reducing agent solution, stir and mix to obtain a mixed suspension containing carbon nanotubes. S3: The mixed suspension is subjected to ultrasonic dispersion treatment to obtain a carbon nanotube dispersion with good dispersibility; S4: Inject the carbon nanotube dispersion into the premixed dry concrete and mix evenly to obtain a conductive concrete mixture. S5: Inject the conductive concrete mixture into the mold, and pre-embed conductive copper mesh connectors in the mold; S6: The conductive concrete mixture in the mold is vibrated and compacted, and then cured to obtain the embedded salinity monitoring structure.

[0019] Further, in step S1, the solid content of the water-reducing agent is controlled at 40-50 wt%, the water reduction rate is 20-50%, and the amount of water-reducing agent solution added is 0.1-1 wt% of the cement mass. Further, in step S2, the carboxylated MWCNTs have an outer diameter of 10-100 nm, a length of 10-100 μm, a carboxyl content of 1-2 wt%, and a carboxylated multi-walled carbon nanotube doping amount of 0.05-0.25 wt% of the cement weight.

[0020] Further, in step S2, the mass ratio of the carboxylated MWCNTs to the water-reducing agent in step S1 is 1:(2-4), and the mass ratio of the carboxylated MWCNTs to the water in step S1 is 1:(100-300).

[0021] Furthermore, in step S2, the mass ratio of silica fume to carboxylated MWCNTs is controlled at (5-20):1.

[0022] Furthermore, in step S2, the mass ratio of fly ash to carboxylated MWCNTs is controlled at (5-20):1.

[0023] Furthermore, in step S3, the ultrasonic vibration frequency is 5-25 kHz, the ratio of output power to the volume of liquid processed in a single session is 1 kW:(0.5-0.8 mL), and the ultrasonic duration is 7-15 min.

[0024] Further, in step S4, the concrete dry material includes cement and sand and gravel, and the mass ratio of cement, sand and gravel to carbon nanotube dispersion is (20-30):(40-60):(2-3).

[0025] Furthermore, in step S5, the mold is a cuboid mold, and conductive copper mesh connectors are pre-embedded at the four equal division points on the left and right sides of the length direction of the mold. Furthermore, the conductive concrete mixture is poured into a mold to form a concrete test block; the width of the conductive copper mesh connector in the mold should be slightly smaller than the width of the concrete test block, and the embedment depth should be slightly higher than the bottom of the concrete test block.

[0026] Furthermore, in step S6, the curing conditions are: temperature (20±2)℃, relative humidity ≥95%, the structure must be kept in a closed humid environment during the curing process, and the curing time is 28 days.

[0027] Furthermore, the rectangular mold should correspond to a concrete test block size of 4×4×16 cm.3 Furthermore, the two conductive copper meshes should be 4 cm away from both sides of the concrete test block.

[0028] The third technical solution of the present invention is to provide an application of an embedded salinity monitoring structure based on conductive concrete, wherein the embedded salinity monitoring structure is used for in-situ, quantitative and continuous monitoring of the salinity inside the concrete structure to be monitored.

[0029] Furthermore, the application includes the following steps: The embedded salinity monitoring structure is pre-embedded in the concrete structure to be monitored. When chloride ions invade, the electrical properties of the conductive network inside the embedded salinity monitoring structure change. This change outputs an electrical signal in real time through the conductive copper mesh connector, realizing in-situ, quantitative, and continuous monitoring of the salinity inside the concrete structure to be monitored.

[0030] Furthermore, the embedded salinity monitoring structure is placed vertically in the concrete structure to be monitored, with a spacing of 0.1-2 m. The embedment depth should not exceed the range of the structural protective layer of the concrete structure to be monitored, and it should not be in direct contact with the main reinforcement of the concrete structure to be monitored.

[0031] Furthermore, the electrical signal output is impedance or conductivity, as well as the change in impedance or conductivity, with a monitoring frequency range of 10 Hz-10 MHz and a measurement voltage ≤5 V.

[0032] Furthermore, the operating parameters of the embedded salinity monitoring structure are as follows: the salinity monitoring range is 0-100‰ practical salinity standard, and the operating temperature range is -20℃ to 80℃.

[0033] The technical concept of this invention includes: This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application, specifically relating to the field of concrete structure durability monitoring. This embedded salinity monitoring structure utilizes an ultrasonic and dispersant-assisted dispersion process to uniformly distribute the conductive filler within the concrete matrix, forming a stable and continuous conductive network. When chloride ions penetrate the concrete, the electrical properties of the conductive network change, and this change in electrical signal is transmitted through the conductive copper mesh, thereby achieving in-situ, real-time, and quantitative monitoring of chloride ion salinity.

[0034] The embedded salinity monitoring structure includes a concrete matrix, carboxylated multi-walled carbon nanotubes (carboxylated MWCNTs) conductive phase filler, conductive copper mesh connectors, water-reducing agent, dispersant, and water; wherein the concrete matrix includes silicate cement, quartz standard sand, and crushed stone; and the dispersant includes silica fume and fly ash.

[0035] The preparation method of the salinity monitoring structure is as follows: First, the water-reducing agent is dissolved in water, then carboxylated MWCNTs and dispersant are added, and the carbon nanotube suspension is obtained by ultrasonic treatment. Then, the suspension is thoroughly mixed with pre-mixed dry concrete to obtain concrete material with stable conductivity. During pouring, the conductive concrete is injected into a cuboid mold, and conductive copper mesh is pre-embedded at two quarter points on the left and right sides as electrical signal connectors. After compaction by vibration, it is cured for 28 days according to standard, and finally a salinity monitoring structure that can be directly embedded in the concrete structure is obtained.

[0036] This structure possesses an autonomous salinity sensing function within the structure, overcoming the shortcomings of traditional methods in terms of continuity, accuracy, and stability, and can be used for long-term dynamic assessment of structural durability.

[0037] Compared with the prior art, the present invention has the following beneficial effects: 1) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method and application. The embedded salinity monitoring structure is used for structural health monitoring. The embedded salinity monitoring structure is used to monitor salinity and mainly includes marine environment structures and salt-rich environment structures. By detecting changes in impedance, conductivity or capacitance signals, the degree of salt corrosion and damage inside the structure can be sensed, thereby realizing self-sensing monitoring of the structure's salinity.

[0038] 2) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. Unlike traditional horizontal embedding methods, this embedded salinity monitoring structure employs a vertical embedding method, ensuring that the main chloride ion intrusion path aligns with the axial direction of the embedded salinity monitoring structure, thereby improving the sensitivity and accuracy of monitoring the ion penetration process. The embedded salinity monitoring structure can be placed at key locations such as the center, four corners (edges), and the center of the four sides (midpoints of the edges) of the target concrete structure (the concrete structure to be monitored), forming a distributed monitoring network. This enables real-time, multi-point, and three-dimensional monitoring of the salt erosion process within the concrete structure, achieving three-dimensional and real-time tracking of the salt erosion gradient, and providing long-term, stable, and comprehensive data support for structural durability assessment.

[0039] 3) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. The prepared embedded salinity monitoring structure uses conductive concrete as the functional matrix, and its material composition is highly consistent with the main structural concrete (the concrete structure to be monitored), effectively avoiding the interface weakening problem caused by material heterogeneity. After the structure is embedded, it can form a good physical and chemical bond with the surrounding concrete, improving the overall durability of the structure, while ensuring that the monitoring signal is not interfered with by factors such as interface debonding or stress concentration.

[0040] 4) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. By adding silica fume and fly ash, a synergistic dispersion system with polycarboxylate superplasticizer is formed. Combined with ultrasonic treatment, this effectively improves the macroscopic distribution uniformity and microstructural stability of carboxylated MWCNTs in cement paste. The polycarboxylate superplasticizer inhibits carbon nanotube agglomeration through electrostatic repulsion and steric hindrance; silica fume, with its ultrafine particle size, fills the gaps between carbon nanotubes, enhancing the steric barrier effect; and fly ash acts as a ball-bearing lubricant, further improving dispersion fluidity. This method can construct a continuous and stable three-dimensional conductive network at low dosages, avoiding the negative impact of traditional dispersants on concrete performance. While improving conductivity, this invention also considers the mechanical properties and durability of concrete. The incorporation of silica fume optimizes the microstructure, enhancing density and erosion resistance; fly ash improves workability and long-term volume stability. Conductive concrete, while possessing self-sensing capabilities, maintains good construction adaptability and economy, making it suitable for large-scale engineering applications.

[0041] 5) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. The embedded salinity monitoring structure pre-embeds a conductive copper mesh connector inside the concrete (the concrete structure to be monitored), forming an integrated signal acquisition unit. It eliminates the need for external reference electrodes, avoiding problems such as excessive voltage drop and signal drift. Its robust structure and strong resistance to physical impact make it suitable for long-term installation in harsh environments. This embedded salinity monitoring structure can achieve a continuous, reversible, and quantitative response to chloride ion concentration by monitoring changes in impedance, conductivity, or capacitance signals, overcoming the limitations of traditional methods that can only detect threshold concentrations or irreversible chemical reactions. Crucially, this salinity monitoring structure can identify and quantify changes in chloride ion concentration inside the concrete structure during its early service stage when the concrete structure is externally intact and without any macroscopic damage. At this stage, conventional non-destructive testing methods (such as half-cell potential and resistivity methods) cannot provide reliable readings due to large signal fluctuations and insufficient sensitivity. This ability to monitor salinity in the early, "non-destructive" stage of concrete structures is one of the differences between this invention and existing conductive concrete patents (which focus more on cracks and stress-induced electrical signal abrupt changes).

[0042] 6) This invention provides an embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application. The preparation process is simple, compatible with conventional concrete production processes, requires no complex equipment or significant adjustments to the mix proportions, and is easily applicable to practical engineering projects. This salinity monitoring structure combines self-sensing, durability, and economy, enabling long-term, real-time, and in-situ monitoring of infrastructure salt corrosion. This helps in early warning of damage, optimizing maintenance decisions, and improving the safety and economy of the structure throughout its entire life cycle. Attached Figure Description

[0043] Figure 1 This is a schematic diagram illustrating the process of the embedded salinity monitoring structure based on conductive concrete, its preparation method, and its application according to the present invention.

[0044] Figure 2 This is a schematic diagram of the embedded salinity monitoring structure based on conductive concrete of the present invention.

[0045] Figure 3 This is a cross-sectional view of the embedded salinity monitoring structure based on conductive concrete of the present invention.

[0046] Figure 4 This is a schematic diagram of the embedded salinity monitoring structure based on conductive concrete of the present invention, which is placed vertically in the concrete structure to be monitored. Detailed Implementation

[0047] The present invention will now be described in detail with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. Component models, material names, connection structures, control methods, and other features not explicitly stated in this technical solution are considered to be common technical features disclosed in the prior art.

[0048] Unless otherwise specified, the reagents used in the following examples and comparative examples are commercially available products, and the methods used are those known in the art.

[0049] The cement used is PO42.5 grade ordinary Portland cement produced by Taicang Conch Cement Co., Ltd.

[0050] The sand is ISO standard quartz sand produced by Xiamen Aisiou Standard Sand Co., Ltd., with a particle size range of 0.08-2 mm.

[0051] The crushed stone is limestone crushed stone used in the Pudong Airport reconstruction project, with good gradation.

[0052] The polycarboxylate superplasticizer is a high-performance polycarboxylate superplasticizer, purchased from Shanxi Feike New Material Technology Co., Ltd., with a water reduction rate of 30%.

[0053] Carboxylated multi-walled carbon nanotubes (carboxylated MWCNTs) were purchased from Chengdu Zhongke Times Nanotechnology Co., Ltd., with a purity greater than 95 wt%, a carboxyl content of 1.43% wt, an outer diameter of 20-40 nm, a length of 10-30 μm, and a specific surface area greater than 80 m². 2 / g, ash content less than 5.0wt%. Grade A fly ash was selected, purchased from Jiewei Environmental Materials Co., Ltd., with a density of 2.55 g / cm³. 3 The loss on ignition was 3.2%, the water requirement ratio was 92%, and the residue on the sieve was 13%.

[0054] The silica fume used was highly reactive micro silica fume, purchased from Henan Yixiang New Materials Co., Ltd., with a specific surface area of ​​19.1 m². 2 / g, loss on ignition is 3.92%, chloride ion content is 0.07%, water requirement ratio is 112%, and activity index at 28d is 105%.

[0055] Example 1: This embodiment provides an embedded salinity monitoring structure based on conductive concrete, and its specific mix proportions are shown in Table 1.

[0056] Table 1. Specific mix proportions of the embedded salinity monitoring structure based on conductive concrete. The above-mentioned method for preparing an embedded salinity monitoring structure based on conductive concrete, such as... Figure 1 As shown, the specific steps are as follows: S1. Preparation of water-reducing agent solution: Take 6.9 g of polycarboxylate water-reducing agent (solid content 45%, water reduction rate 30%) and add it to 500 g of water. Stir at 1200 rpm for 5 min on a magnetic stirrer until the agent is completely dissolved to obtain a clear solution.

[0057] S2. Preparation of carbon nanotube composite slurry: To the clear solution obtained in step S1 above, 2.3 g of carboxylated MWCNTs (as a conductive phase filler for carboxylated multi-walled carbon nanotubes) (purchased from Chengdu Zhongke Times Nanotechnology Co., Ltd., purity >95%, specifications: outer diameter 30±10 nm, length 20±10 μm, carboxylation degree 1.43 wt%), 46 g of silica fume, and 46 g of fly ash were added sequentially. The mixture was stirred at 1200 rpm for 5 min to allow the carbon nanotube powder (carboxylated MWCNTs) to be initially impregnated and suspended in the liquid phase, forming a composite slurry.

[0058] S3. Ultrasonic Dispersion Process: The composite slurry obtained in step S2 is divided into two equal portions (approximately 250 mL each), and ultrasonically dispersed sequentially. The process parameters are set as follows: frequency 20 kHz, power 380 W, and single treatment time 10 min. The carbon nanotube aggregates are exfoliated by the cavitation effect of ultrasound to obtain a stable nano-dispersion system (carbon nanotube dispersion).

[0059] S4. Mixing process of conductive concrete (mixing stage): Place 2300 g of cement, 2500 g of sand, and 1800 g of crushed stone in a concrete mixer and dry mix for 1 min to ensure thorough mixing. Then, add the carbon nanotube dispersion prepared in step S3 to the mixer in three portions, rinsing the inner wall of the container several times with the remaining 270 g of water before adding it all at once. After each addition of liquid, run the mixer at a low speed of 59 rpm for 1 min, then at a high speed of 198 rpm for 2 min, until a conductive concrete mixture with good workability and uniform composition is formed.

[0060] S5. Pouring and Electrode Installation: The conductive concrete mixture prepared in step S4 is poured into a container with dimensions of 4×4×16cm. 3 In the rectangular mold, during the casting process, a conductive copper mesh with a width slightly smaller than the width of the specimen (about 3.8 cm) and a height slightly higher than the bottom of the specimen (about 1 cm) is pre-embedded at two quarter points on the left and right sides of the mold length direction (i.e., 4 cm from each side) as electrical signal measurement connectors (conductive copper mesh connectors).

[0061] S6. Vibration and Curing: Place the mold containing the embedded electrodes (conductive copper mesh connectors) on a vibrating table and vibrate to ensure the concrete fills the mold and removes air bubbles. After vibration, smooth the surface with a trowel and cover the concrete specimen with plastic wrap to retain moisture. Then, transfer the specimen to a standard curing chamber and cure for 28 days at a temperature of (20±2)℃ and a relative humidity ≥95%, thus obtaining the embedded salinity monitoring structure.

[0062] S7. Application and Monitoring: The prepared embedded salinity monitoring structure is removed from the mold and placed on the concrete structure to be monitored (in this example, an airport runway slab in a sea area in southeastern China; the service environment characteristics of this runway are: average annual temperature 22~28℃, relative humidity 75%~85%). The average annual chloride ion deposition is measured to be 0.3 g / m² using the on-site hanging plate method. 2 •d, Concrete design strength C40, structural protective layer thickness 50 mm, design service life 50 years) Before pouring, the embedded salinity monitoring structure is vertically embedded within the structural protective layer of the concrete structure to be monitored, ensuring that it does not directly contact the main structural reinforcement of the concrete structure to be monitored. Figure 4 As shown, a conductive copper mesh connector is connected to a data acquisition system via wires (using existing technology; this invention does not improve data acquisition). When external chloride ions penetrate the concrete structure and reach the monitoring location, the uniformly distributed carbon nanotube conductive network inside the structure undergoes changes in electrical properties (such as impedance) due to ion adsorption and transport. This change is output in real time through the conductive copper mesh. By monitoring the impedance change within the frequency range of 10 Hz-10 MHz, in-situ, real-time, continuous, and quantitative monitoring of salinity inside the structure can be achieved.

[0063] Comparative Example 1: This comparative example provides a plain concrete specimen without conductive filler and its preparation method. The main operation process is similar to that of Example 1, but steps S2 and S3 are omitted. In step S4, during the mixing stage, carboxylated MWCNTs, silica fume and fly ash are removed, and only ordinary concrete mix proportions are used to prepare the plain concrete specimen without conductive filler.

[0064] Comparative Example 2: This comparative example provides a conductive concrete specimen and its preparation method, which is basically the same as Example 1. The difference is that all materials (including carboxylated MWCNTs, silica fume, and fly ash) are directly mixed and stirred in one go using a common mechanical mixer without pre-dispersion treatment and ultrasonic dispersion steps (corresponding to steps S2 and S3), resulting in a conductive concrete specimen that is dispersed by mechanical stirring for only 5 minutes.

[0065] Comparative Example 3: This comparative example provides a conductive concrete specimen with a dry-mixed dispersant and its preparation method, which is basically the same as Example 1. The difference is that silica fume and fly ash are not added in step S2, but are added as dry materials together with cement, sand and crushed stone in step S4 to obtain a conductive concrete specimen with a dry-mixed dispersant.

[0066] Comparative Example 4: This comparative example provides a conductive concrete specimen using unmodified carbon nanotubes and its preparation method, which is basically the same as Example 1, except that in step S2, the carboxylated MWCNTs are replaced with the same mass of ordinary unmodified MWCNTs (ordinary unmodified multi-walled carbon nanotubes were purchased from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd., with specifications: purity greater than 95%, diameter of 5-15 nm, and length of 10-30 μm) to obtain a conductive concrete specimen using unmodified carbon nanotubes.

[0067] Comparative Example 5: This comparative example provides a conductive concrete specimen and its preparation method, which is basically the same as Example 1, except that the dispersion treatment of carboxylated MWCNTs with polycarboxylate superplasticizer solution in step S1 and the pre-dispersion stirring treatment in step S2 are omitted. The carboxylated MWCNTs, silica fume, and fly ash are added to an aqueous solution and mixed without stirring at 1200 rpm before proceeding to the ultrasonic dispersion process in step S3. The remaining steps are consistent with the example.

[0068] Comparative Example 6: This comparative example provides a conductive concrete specimen and its preparation method, which is basically the same as that in Example 1, except that the ultrasonic dispersion process in step S3 is omitted. After obtaining the carbon nanotube composite slurry in step S2, no ultrasonic treatment is performed, and the slurry is directly used for mixing in step S4. The remaining steps are the same as in the example.

[0069] Comparative Example 7: This comparative example provides a conductive concrete specimen and its preparation method, which is basically the same as Example 1, except that silica fume is not added in step S2, and only fly ash is added as a dispersion medium. Only 2.3 g of carboxylated MWCNTs and 92 g of fly ash are added to the solution obtained in S1, without adding silica fume; the amounts of other materials and all operating steps remain unchanged.

[0070] Comparative Example 8: This comparative example provides a conductive concrete specimen and its preparation method, which is basically the same as Example 1, except that fly ash is not added in step S2, and only silica fume is added as a dispersion medium. Only 2.3 g of carboxylated MWCNTs and 92 g of silica fume are added to the solution obtained in S1, without adding fly ash; the amounts of other materials and all operating steps remain unchanged.

[0071] Comparative Example 9: This comparative example provides an application method for an embedded salinity monitoring structure based on conductive concrete, which is basically the same as that in Example 1, except for the placement method in step S7. The salinity monitoring structure prepared according to the method of Example 1 is placed in a traditional horizontal embedding method during the pouring of the concrete structure to be monitored, i.e., the axial direction of the structure is parallel to the concrete surface and perpendicular to the main direction of chloride ion intrusion (perpendicular to the surface). The remaining monitoring steps are the same as in the example.

[0072] Performance testing: Tests were conducted on the specimens (4×4×16 cm) prepared in the above embodiments and comparative examples. 3 The following performance tests were performed: Resistivity and stability testing: After the specimens were cured, a Keithley DMM6500 benchtop multimeter was connected via a pre-embedded conductive copper mesh connector using the two-electrode method to measure the DC resistance of the specimens at 22°C. To reduce the influence of polarization, the specimens were electrically polarized for 2 hours before testing. Five specimens were tested for each example and comparative example, and the average resistance and standard deviation were calculated to evaluate the conductivity and its uniformity (stability). The test results are shown in Table 2.

[0073] Table 2 Comparison of resistivity values ​​of concrete specimens from the examples and comparative examples As shown in Table 2, Comparative Example 1 (without conductive filler) exhibits extremely high resistance, acting as an insulator. Comparative Example 2 (mechanical stirring only) shows some conductivity, but due to uneven dispersion of carbon nanotubes, its resistance is high and highly variable. Comparative Example 3 (dry-mixed with dispersant) and Comparative Example 4 (unmodified carbon nanotubes) show improved conductivity and uniformity, but still fall short of Example 1. Comparative Example 5 (alone without pre-dispersion) has a slightly higher resistance and increased standard deviation than the example, indicating that pre-dispersion treatment helps in the initial de-agglomeration of carbon nanotubes. Comparative Example 6 (alone without ultrasonication) shows a significantly increased resistance and greater dispersion, demonstrating that ultrasonic dispersion is a key step in constructing a continuous conductive network. Comparative Example 7 (without silica fume only) and Comparative Example 8 (without fly ash only) both have higher resistance and standard deviation than Example 1, indicating that the synergistic effect of silica fume and fly ash is indispensable for optimizing the conductive network. Comparative Example 9 (horizontal embedment method) and the Example 1 used the exact same conductive concrete material. The difference between the two lies in the salinity monitoring sensitivity. The resistance value of Comparative Example 9 was slightly higher than that of Example 1, and the data dispersion was larger, indicating that the vertical embedment method was superior. The specimen prepared in Example 1 had the lowest resistance value and the smallest standard deviation, indicating that a uniform and stable conductive network was formed inside, laying a solid foundation for subsequent high-sensitivity and high-stability salinity monitoring.

[0074] Salinity response test: After drying, the specimens prepared in Example 1 were immersed in NaCl solutions of different concentrations (0‰, 10‰, 30‰, 50‰) to simulate different salinity environments. After adsorption equilibrium was reached, the change in AC impedance at a frequency of 1 kHz was measured. The results showed that as the salinity increased from 0‰ to 50‰, the impedance value of the specimen exhibited a regular decreasing trend, and the rate of change had a good linear relationship with salinity (R0). 2 The value > 0.98 indicates that the structure can effectively respond to changes in external chloride ion concentration and achieve quantitative monitoring of salinity.

[0075] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. An embedded salinity monitoring structure based on conductive concrete, characterized in that, The embedded salinity monitoring structure includes a concrete matrix, a carboxylated multi-walled carbon nanotube conductive phase filler, a conductive copper mesh connector, a water-reducing agent, a dispersant, and water. The concrete matrix includes cement and sand / gravel; The dispersant includes silica fume and fly ash; The carboxylated multi-walled carbon nanotube conductive phase filler is uniformly distributed in the concrete matrix. The conductive copper mesh connector is partially embedded in the concrete substrate; The embedded salinity monitoring structure is placed vertically within the concrete structure to be monitored, enabling in-situ, quantitative, and continuous monitoring of the salinity inside the concrete structure.

2. The embedded salinity monitoring structure based on conductive concrete according to claim 1, characterized in that, The water-reducing agent is a polycarboxylate water-reducing agent; The cement is silicate cement; The aggregate includes sand and gravel; The sand is standard quartz sand; The embedded salinity monitoring structure comprises the following components in parts by weight: 3-10 parts water-reducing agent, 1-5 parts carboxylated multi-walled carbon nanotube conductive phase filler, 10-100 parts silica fume, 10-100 parts fly ash, 2000-3000 parts cement, 4000-6000 parts sand and gravel, and 600-1000 parts water.

3. A method for preparing an embedded salinity monitoring structure based on conductive concrete as described in any one of claims 1-2, characterized in that, Includes the following steps: S1: Mix the water-reducing agent with water to obtain a water-reducing agent solution; S2: Add carboxylated multi-walled carbon nanotubes, silica fume and fly ash as dispersants to the water-reducing agent solution, stir and mix to obtain a mixed suspension; S3: The mixed suspension is subjected to ultrasonic dispersion treatment to obtain a carbon nanotube dispersion; S4: Inject the carbon nanotube dispersion into the premixed dry concrete and mix evenly to obtain a conductive concrete mixture. S5: Inject the conductive concrete mixture into the mold, and pre-embed conductive copper mesh connectors in the mold; S6: The conductive concrete mixture in the mold is vibrated and compacted, and then cured to obtain the embedded salinity monitoring structure.

4. The preparation method according to claim 3, characterized in that, In step S1, the solid content of the water-reducing agent is controlled at 40-50 wt%, the water reduction rate is 20-50%, and the amount of water-reducing agent solution added is 0.1-1 wt% of the cement mass. In step S2, the carboxylated multi-walled carbon nanotubes have an outer diameter of 10-100 nm, a length of 10-100 μm, a carboxyl content of 1-2 wt%, and an adsorption amount of 0.05-0.25 wt% of the cement weight. In step S2, the mass ratio of the carboxylated multi-walled carbon nanotubes to the water-reducing agent in step S1 is 1:(2-4), and the mass ratio of the carboxylated multi-walled carbon nanotubes to the water in step S1 is 1:(100-300). In step S2, the mass ratio of silica fume to carboxylated multi-walled carbon nanotubes is controlled at (5-20):

1. In step S2, the mass ratio of fly ash to carboxylated multi-walled carbon nanotubes is controlled at (5-20):

1. In step S3, the ultrasonic vibration frequency is 5-25 kHz, the ratio of output power to the volume of liquid processed in a single session is 1 kW:(0.5-0.8 mL), and the ultrasonic duration is 7-15 min. In step S4, the dry concrete material includes cement and sand and gravel, and the mass ratio of cement, sand and gravel to carbon nanotube dispersion is (20-30):(40-60):(2-3). In step S5, the mold is a cuboid mold, and conductive copper mesh connectors are pre-embedded at the four equal division points on the left and right sides of the length direction of the mold. The conductive concrete mixture is poured into a mold to form a concrete test block; The width of the conductive copper mesh connector in the mold is smaller than the width of the concrete test block, and the embedment depth is higher than the bottom of the concrete test block; In step S6, the curing conditions are: temperature 18~22℃, relative humidity ≥95%, and curing time is 28 days.

5. The preparation method according to claim 4, characterized in that, The rectangular mold corresponds to a concrete test block with dimensions of 4×4×16 cm. 3 Furthermore, the two conductive copper meshes should be 4 cm away from both sides of the concrete test block.

6. An application of an embedded salinity monitoring structure based on conductive concrete as described in any one of claims 1-2, characterized in that, The embedded salinity monitoring structure is used for in-situ, quantitative, and continuous monitoring of salinity inside the concrete structure to be monitored.

7. The application according to claim 6, characterized in that, Includes the following steps: The embedded salinity monitoring structure is pre-embedded in the concrete structure to be monitored. When chloride ions invade, the electrical properties of the conductive network inside the embedded salinity monitoring structure change. The electrical signal is output in real time through the conductive copper mesh connector, so as to realize the in-situ, quantitative and continuous monitoring of the salinity inside the concrete structure to be monitored.

8. The application according to claim 7, characterized in that, The embedded salinity monitoring structure is placed vertically in the concrete structure to be monitored, with a spacing of 0.1-2 m. The embedment depth does not exceed the structural protective layer of the concrete structure to be monitored, and it does not directly contact the main reinforcement of the concrete structure to be monitored.

9. The application according to claim 7, characterized in that, The electrical signal output is either impedance or conductivity, as well as the change in impedance or conductivity. The monitoring frequency range is 10 Hz-10 MHz, and the measured voltage is ≤5 V.

10. The application according to claim 7, characterized in that, The operating parameters of the embedded salinity monitoring structure are as follows: the salinity monitoring range is 0-100‰ practical salinity standard, and the operating temperature range is -20℃ to 80℃.

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