Concrete pad for measuring concrete working stress and its preparation and testing methods

Abstract

This invention discloses a concrete pad block for measuring the working stress of concrete and its preparation and testing method. Several carbon fiber cement mortar sensors are fabricated, and the sensors are shaped, wired, and waterproofed and insulated. The sensors are numbered; the temperature resistance curve of each sensor is fitted to determine a temperature correction coefficient; the pressure sensitivity curve of each sensor is calibrated to obtain a pressure sensitivity curve fitting formula; and the stress value is calculated based on the given pad block pressure sensitivity fitting formula and temperature correction coefficient. Suggested dimensions, electrode design, and wire connection methods for the carbon fiber cement mortar sensors are designed and provided. Three potential influencing factors on the carbon fiber cement mortar sensors during use are analyzed: external insulation, humidity, and temperature. Insulation and waterproofing methods, as well as temperature correction methods, are provided.

Description

Technical Field

[0001] This invention relates to a process for manufacturing concrete pads capable of measuring concrete working stress and a method for detecting concrete working stress, with the matrix of carbon fiber cement-based composite materials being cement mortar. The purpose of this invention is to study the properties and applicability of carbon fiber cement mortar (CFRM) in order to manufacture sensors for stress detection of bridge components. The research focuses on three aspects: mix optimization, sensor manufacturing, and engineering application. Background Technology

[0002] The working stress of reinforced concrete structures under working conditions is the most direct and intuitive basis for determining the safety of concrete structures. This is especially true for prestressed structures, where the working state of the prestressed steel bars and concrete embedded in the structure is difficult to verify due to limitations in early prestressed concrete construction and structural damage detection and health monitoring technologies. If the actual working state of the prestressed tendons in a prestressed concrete structure designed according to prestressing conditions does not meet or is far below the requirements, this will pose a significant safety hazard to the structure itself. Therefore, developing a low-cost sensing system that operates under the same conditions and with the same lifespan as the structure under test for tracking and monitoring the working stress of concrete is of great significance for the development of health monitoring for reinforced concrete structures.

[0003] However, research on using carbon fiber cement mortar to fabricate sensors for bridge prestressing detection faces three key challenges. First, the impact of carbon fiber content on the workability, mechanical properties, and pressure sensitivity of cement-based carbon fiber composite materials is not fully understood, making it difficult to determine a optimal mix ratio. Second, there is no standardized procedure for fabricating sensors from carbon fiber cement mortar, including procedures for dimensional setting, insulation and waterproofing, and temperature-related corrections. Finally, there are few practical applications of carbon fiber cement mortar sensors in engineering projects. Summary of the Invention

[0004] The purpose of this invention is to provide a concrete spacer block capable of measuring the working stress of concrete, and its preparation and testing method. This invention achieves the testing requirement for the working stress of concrete structures while ensuring the basic function of the concrete spacer block, thus solving the technical problem that traditional methods are insufficient for testing the working stress of concrete. The technical solution of this invention to achieve its purpose is as follows:

[0005] The first aspect of this application provides a concrete pad block for measuring the working stress of concrete, comprising upper and lower electrode plates with electrode ribs and a mold surrounding the upper and lower electrode plates with electrode ribs. A carbon fiber cement-based composite material is poured into the space formed between the upper and lower electrode plates with electrode ribs and the mold. After the carbon fiber cement-based composite material and the upper and lower electrode plates with electrode ribs solidify together, the block is demolded to obtain a solidified carbon fiber cement-based composite material with electrodes. The solidified carbon fiber cement-based composite material has an upper wall, a lower wall, and side walls. The electrode ribs of the upper and lower electrodes extend into the upper and lower walls of the solidified carbon fiber cement-based composite material, respectively. A through-hole is formed at the axial center of the upper and lower electrode plates with electrode ribs and the solidified carbon fiber cement-based composite material. A plurality of wire holes are formed circumferentially along the inner edge of the through-hole. A wire is bolted to the upper and lower electrode plates with electrode ribs. A waterproof insulating layer is wrapped around the solidified carbon fiber cement-based composite material, the upper and lower electrode plates with electrode ribs, and the wires.

[0006] In some embodiments, the carbon fiber cement-based composite material is prepared from water, cement, sand, water-reducing agent, defoamer, silica fume, dispersant, and carbon fiber. The mixing ratio of the carbon fiber cement-based composite material is as follows: water, cement, and sand are in a ratio of 0.5:1:1.5; the water-reducing agent accounts for 0.80% of the total mass of water, cement, and sand; the defoamer accounts for 0.20% of the total mass of water, cement, and sand; the silica fume accounts for 10% of the total mass of water, cement, and sand; the dispersant accounts for 0.20% of the total mass of water, cement, and sand; and the carbon fiber accounts for 0.60% of the total mass of water, cement, and sand.

[0007] The carbon fiber is 4-6mm PAN-based short-cut carbon fiber.

[0008] The second aspect of this application provides a method for preparing a concrete pad block capable of measuring the working stress of concrete, comprising the following steps:

[0009] Step 1.1: Preparation of dispersant solution: Measure the dispersant according to the mixing ratio of the carbon fiber cementitious composite material, dissolve it in cold water with half the total water volume, and stir thoroughly;

[0010] Step 1.2: Carbon fiber dispersion: Measure the carbon fiber according to the mixing ratio of the carbon fiber cement-based composite material and pour it into the dispersant solution. After stirring thoroughly, the carbon fiber is in a monofilament state and there is no agglomeration on the solution surface.

[0011] Step 1.3: Dry mixing: Measure the cement, standard sand, and silica fume according to the mix proportion of the carbon fiber cement-based composite material, pour them into the mixing pot, and dry mix evenly;

[0012] Step 1.4: Stirring: Pour the carbon fiber solution into a mixing pot and stir, then add the remaining half of the water and water-reducing agent, and stir until well mixed;

[0013] Step 1.5: After mixing, pour the mixture into the mold. Make the mold according to the actual application size. Place the bottom ribbed electrode plate at the bottom of the mold and the top ribbed electrode plate on the top surface. Vibrate the mold, remove the mold after molding, and place it in the curing room for curing.

[0014] Step 1.6: After curing, remove the material, place it in an oven to dry, and obtain a solidified carbon fiber cement-based composite material with electrodes, then connect the wires.

[0015] Step 1.7: Immerse the solidified carbon fiber cement-based composite material with electrodes after connecting the wires into epoxy resin solution. After the epoxy resin completely coats the solidified material, remove it and let it dry. Finally, a concrete pad block that can measure the working stress of concrete is obtained.

[0016] The third aspect of this application provides a method for testing the working stress of concrete, based on a concrete pad capable of measuring the working stress of concrete, comprising the following steps:

[0017] Step 1: Number several concrete pads that can measure concrete working stress.

[0018] Step 2: Fit the temperature resistance curve of each measurable concrete working stress concrete pad to determine the temperature correction coefficient.

[0019] Step 3: Calibrate the pressure sensitivity curve of each concrete pad block with measurable concrete working stress and obtain the pressure sensitivity curve fitting formula;

[0020] Step 4: Calculate the stress value based on the given fitting formula for the pressure sensitivity of the concrete pad block with measurable concrete working stress and the temperature correction coefficient.

[0021] In some embodiments, step 2 includes the following specific steps:

[0022] Step 2.1: Fit the temperature resistance curve of each concrete pad block with measurable concrete working stress, and take the commonly used temperature range of 0~50℃, with the temperature of 20℃ when the concrete pad block with measurable concrete working stress was calibrated as the reference value R0.

[0023] Step 2.2: Correct the temperature system error by testing the temperature and the resistance at the corresponding temperature. Compare the resistance values ​​measured at different temperatures with the reference value R0, and the result is the temperature correction coefficient kt.

[0024] Step 2.3: After measuring the resistance value on site, multiply the measured resistance value by the corresponding correction factor according to the real-time temperature to obtain the corrected resistance value.

[0025] In some embodiments, step 3 specifically involves: calibrating the pressure-sensitive curve of the concrete pad block for measuring concrete working stress. Before use, each concrete pad block for measuring concrete working stress is subjected to 5 linear stage cyclic loads to eliminate microcracks, and at the same time, the concrete pad block for measuring concrete working stress is calibrated to obtain the calculation formula.

[0026] The advantages and effects of this invention are:

[0027] (1) The proportion of carbon fiber cement mortar was optimized based on pressure sensitivity, while also taking into account mechanical properties, workability of the mixture, and dispersion properties.

[0028] (2) The recommended dimensions, electrode design, and wire connection method of the carbon fiber cement mortar sensor are designed and given. The three influencing factors that the carbon fiber cement mortar sensor may be affected by during use are analyzed: the external insulation of the sensor, the influence of humidity, and the influence of temperature. Insulation and waterproofing treatment methods and temperature correction methods are given. Attached Figure Description

[0029] Figure 1 This is an exploded view of the concrete pad block for measuring concrete working stress according to the present invention.

[0030] Figure 2 A schematic diagram of the concrete pad block wire connection for measuring concrete working stress according to the present invention.

[0031] Figure 3 Schematic diagram of the stainless steel electrode of this invention;

[0032] In the diagram: 1. Electrode plate, 2. Through hole, 3. Stainless steel plate for electrode ribs, 4. Through hole, 5. Concrete pad. Detailed Implementation

[0033] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings;

[0034] The working stress of reinforced concrete structures under working conditions is the most direct and intuitive basis for determining the safety of concrete structures. Research on using carbon fiber cement mortar to fabricate sensors for bridge prestressing detection faces three key challenges. First, the impact of carbon fiber content on the workability, mechanical properties, and pressure sensitivity of cement-based carbon fiber composite materials is not fully understood, making it difficult to determine a suitable mix ratio. Second, there is no standardized procedure for fabricating sensors from carbon fiber cement mortar, including procedures for dimensional setting, insulation and waterproofing, and temperature correction. Finally, there are few cases of applying carbon fiber cement mortar sensors to actual engineering projects.

[0035] Based on the above, this invention firstly investigates the effects of different dispersants and their dosages on the dispersibility, workability, mechanical properties, and pressure sensitivity of carbon fiber cement mortar, as well as the effects of different carbon fiber dosages on the mechanical properties and pressure sensitivity of carbon fiber cement mortar, and provides an optimized mix proportion for carbon fiber cement mortar; it also presents a complete manufacturing process for carbon fiber cement mortar sensors, and provides protective measures and corrections for external environmental factors affecting sensor properties, such as insulation, temperature, and humidity; it calibrates the sensors before they are put into use, determines the pressure sensitivity formula and temperature correction coefficient, and comprehensively develops a resistance / temperature stress comparison table;

[0036] In this embodiment of the invention, the concrete pad block for measuring concrete working stress is a carbon fiber cement mortar sensor, comprising upper and lower electrode plates with electrode ribs and a mold surrounding the upper and lower electrode plates with electrode ribs; exemplaryly, the upper and lower electrode plates with electrode ribs include an upper electrode plate and a lower electrode plate, and a 5mm high stainless steel plate 3 with electrode ribs is welded vertically along the radial direction of the upper and lower electrode plates on the opposite surfaces of the upper and lower electrode plates; it should be noted that the upper electrode plate and the lower electrode plate are stacked vertically, have the same size and have a certain gap between them; the mold surrounding the upper and lower electrode plates with electrode ribs is cylindrical or rectangular; it can be understood that a cavity is formed between the upper and lower electrode plates with electrode ribs and the surrounding mold;

[0037] In this embodiment of the invention, carbon fiber cement-based composite material is cast into the space formed between the upper and lower electrode plates with electrode ribs and the mold. After the carbon fiber cement-based composite material and the upper and lower electrode plates with electrode ribs solidify together, the mold is demolded to obtain a solidified carbon fiber cement-based composite material with electrodes. The solidified carbon fiber cement-based composite material has an upper wall, a lower wall, and a side wall. The electrode ribs of the upper and lower electrodes extend into the upper and lower walls of the solidified carbon fiber cement-based composite material, respectively. Through holes are opened at the axial center positions of the upper and lower electrode plates with electrode ribs and the solidified carbon fiber cement-based composite material. Four through holes are opened at 90° intervals along the circumferential direction at the inner edge of the through holes. Wires are connected to the upper and lower electrode plates with electrode ribs by bolts. The solidified carbon fiber cement-based composite material, the upper and lower electrode plates with electrode ribs, and the wires are all wrapped with a waterproof insulating layer.

[0038] In some embodiments, the carbon fiber cement-based composite material is prepared from water, cement, sand, water-reducing agent, defoamer, silica fume, dispersant, and chopped carbon fibers. The mixing ratio of the carbon fiber cement-based composite material is: water, cement, and sand in a ratio of 0.5:1:1.5; the water-reducing agent is 0.80% of the total mass of water, cement, and sand; the defoamer is 0.20% of the total mass of water, cement, and sand; the silica fume is 10% of the total mass of water, cement, and sand; the dispersant is 0.20% of the total mass of water, cement, and sand; and the chopped carbon fibers are 0.60% of the total mass of water, cement, and sand. The chopped carbon fibers are 4-6 mm PAN-based chopped carbon fibers.

[0039] In some embodiments, the carbon fiber cement-based composite material mix proportion is determined through experimental research to take into account the processability, mechanical properties, and pressure-sensitive properties of the carbon fiber cement mortar sensor, as shown in Table 1.

[0040]

[0041] In this embodiment of the invention, five carbon fiber cement mortar sensors are fabricated. The specific fabrication method includes the following steps:

[0042] Step 1.1, Preparation of dispersant solution: Measure the dispersant according to the mixing ratio of carbon fiber cementitious composite material, dissolve it in cold water with half the total water volume, and stir thoroughly;

[0043] Step 1.2, Carbon fiber dispersion: Measure the carbon fiber according to the mixing ratio of carbon fiber cement-based composite material and pour it into the dispersant solution. After stirring thoroughly, observe with the naked eye that the carbon fiber is in a monofilament state and there is no agglomeration on the solution surface.

[0044] Step 1.3, Dry mixing: Measure the cement, standard sand, and silica fume according to the mix proportion of carbon fiber cement-based composite material, pour them into the mixing pot, and dry mix for 1 minute;

[0045] Step 1.4, Stirring: Pour the carbon fiber solution into a mixing pot and stir, then add the remaining half of the water and water-reducing agent, and stir for 3 minutes;

[0046] Step 1.5: After mixing, pour the mixture into a mold. Make the mold according to the actual application size; for example, a circular mold can be made with a diameter of φ100mm, and a square mold can be made with a diameter of 100×50mm. Place the lower electrode plate at the bottom of the mold and the upper reinforcing rib electrode plate on the top. Vibrate the mixture, remove the mold after 48 hours, and place it in a curing room for 28 days. The dimensions and electrode arrangement of the carbon fiber cement mortar sensor are as follows: Figure 1 and Figure 3As shown, the electrodes of the carbon fiber cement mortar sensor are made of stainless steel. Based on the size of the pad and the wire connection requirements, the radius is set to 50mm. The diameter of the central opening of electrode plate 1 is 20mm, and four 4mm radius wire-passing holes 4 are set on the edge of the inner hole for connecting wires. A 5mm high stainless steel electrode rib plate 3 is vertically welded to the middle of the electrode for anchoring. Each pad uses two electrodes: an upper electrode plate and a lower electrode plate.

[0047] Step 1.6: After curing, remove the test block, dry it in an oven to obtain a solidified carbon fiber cement-based composite material with electrodes, and then weld wires onto it. The wire connection method is as follows: Figure 2 As shown, the wires are connected to the stainless steel electrodes using 6mm long M3 screws. Each electrode is connected by two bolts to ensure the reliability of the connection.

[0048] Step 1.7 Immerse the solidified carbon fiber cement-based composite material with electrodes after connecting the wires into epoxy resin solution. After the epoxy resin completely coats the pad, remove it and let it dry. The waterproof and insulating method of this invention uses the external epoxy resin coating method. Epoxy resin generally refers to organic compounds containing two or more epoxy groups in their molecules. Their relative molecular mass is generally not high. Cured epoxy resin has good physical and chemical properties. It has excellent adhesion strength to the surfaces of metals and non-metals, good dielectric properties, low deformation shrinkage, good dimensional stability of the product, high hardness, and good flexibility.

[0049] The specific process is as follows: First, the solidified carbon fiber cement-based composite material with electrodes connected to the wires is completely immersed in epoxy resin, so that the epoxy resin fully adheres to all parts of the sensor; then, a thin layer of epoxy resin is applied to the bottom of the prepared specimen mold; epoxy resin is placed into the mold, and more epoxy resin is poured in to ensure that all gaps are filled; after being placed in a cool and dry place for 24 hours, the mold is removed, and the carbon fiber cement mortar sensor is finally obtained.

[0050] Step 2: Number the 5 carbon fiber cement mortar sensors as follows:

[0051] (1) Insulation method: HYSZ - outer epoxy resin coating, SJHYSZ - outer epoxy resin coating followed by mortar coating;

[0052] (2) CFRM dimensions: length × width × height, unit mm;

[0053] (3) Electrode: Embedding method + electrode width / mm. M is anchored type;

[0054] (4) Production date: Month + Day;

[0055] (5) Number: The number in the same batch.

[0056] For example, HYSZK-100-M20-0728-1 indicates: the insulation method is modified epoxy resin particles—the pad size is 20×20×30mm—the electrode is an anchored 20mm wide electrode—the manufacturing date is July 28—the number is No. 1 in the same batch;

[0057] The five carbon fiber cement mortar sensors in this embodiment are numbered as follows:

[0058] HYSZK-100-M20-0728-1, HYSZK-100-M20-0728-2, HYSZK-100-M20-0728-3, HYSZK-100-M20-0728-4, HYSZK-100-M20-0728-5;

[0059] Step 3: Fit the temperature resistance curve of each carbon fiber cement mortar sensor to determine the temperature correction coefficient; the specific steps are as follows:

[0060] Step 3.1: Fit the temperature resistance curve of each carbon fiber cement mortar sensor, and take the commonly used temperature range of 0~50℃, with the temperature of 20℃ when the carbon fiber cement mortar sensor is calibrated as the reference value R0.

[0061] Step 3.2: Correct the temperature system error by testing the temperature and the resistance at the corresponding temperature. Compare the resistance values ​​measured at different temperatures with the reference value R0. The resulting value is the temperature correction coefficient kt. To facilitate field application, it is made into a table, as shown in Table 2.

[0062] Step 3.3: After measuring the resistance value on site, multiply the measured resistance value by the corresponding correction factor according to the real-time temperature to obtain the corrected resistance value;

[0063]

[0064] Step 4: Calibrate the pressure sensitivity curve of each carbon fiber cement mortar sensor to obtain the pressure sensitivity curve fitting formula. Specifically, the pressure sensitivity curve calibration of the carbon fiber cement mortar sensor involves subjecting each sensor to 5 linear stage cyclic loading cycles before use to eliminate micro-cracks and calibrate the sensor, thus deriving the calculation formula. In this project, the sensor's operating range is 1-15MPa. Five sensors were monotonically loaded to 6kN at a loading rate of 20N / S for 5 cycles. The result of the last cycle was used as the calibration curve for sensor calibration. Given that the sensor's operating range is 1-15MPa, a monotonically loaded load of 6kN at a loading rate of 20N / S was applied, and curve fitting was performed to obtain the pressure sensitivity curve fitting formula, as shown in Table 3.

[0065]

[0066] Where y=k t △R / R0=(R-R0) / R0, where R is the measured resistance, R0 is the resistance at 0℃, and y is the relative rate of change of resistance, x is the force (kN), and k t This is the temperature correction factor. The slope of the fitting formula. k Defined as sensitivity, its physical meaning is the rate of change of resistivity under unit force. b In the formula for fitting x When the value is 0, its physical meaning is the change in initial resistivity of the test piece, which can be calculated from the initial resistance measured for each test block.

[0067] Step 5: Based on the fitting formula, the magnitude of the applied force x can be deduced. The ratio of the obtained force value to the pressure area of ​​the sensor is the stress value (k). t (R-R0) / R0–b) / kS (k-slope, S-sensor pressure area), unit is MPa.

[0068] Based on the pressure sensitivity fitting formula and temperature correction coefficient, stress comparison tables for five sensors were created as shown in Tables 4-8 (partial).

[0069]

[0070]

[0071]

[0072]

[0073]

Claims

1. A method of testing the working stress of concrete, based on a concrete pad of measurable working stress of concrete, characterized in that: It comprises the following steps: Step 1: numbering a plurality of concrete pads of measurable concrete working stress; Step 2: fitting the temperature-resistance curve of each concrete pad of measurable concrete working stress to determine the temperature correction coefficient; Step 3: calibrating the pressure-sensitive curve of each concrete pad of measurable concrete working stress to obtain a pressure-sensitive curve fitting formula; Step 4: calculating the stress value according to the given pressure-sensitive fitting formula of the concrete pad of measurable concrete working stress and the temperature correction coefficient. The concrete pad of measurable concrete working stress comprises upper and lower electrode ribbed electrode plates and a mold surrounding the periphery of the upper and lower electrode ribbed electrode plates, and a carbon fiber cement-based composite material is poured into the space formed between the upper and lower electrode ribbed electrode plates and the mold, and after the carbon fiber cement-based composite material and the upper and lower electrode ribbed electrode plates solidify together, the electrode ribbed carbon fiber cement-based composite material solid body is demolded, the carbon fiber cement-based composite material solid body has an upper wall surface, a lower wall surface and a side wall surface, the electrode ribs of the upper and lower electrodes extend into the upper wall surface and the lower wall surface of the carbon fiber cement-based composite material solid body, respectively, a through-hole is formed at the axial center position of the upper and lower electrode ribbed electrode plates and the carbon fiber cement-based composite material solid body, a plurality of wire holes are formed along the circumference at the inner hole edge of the through-hole, the upper and lower electrode ribbed electrode plates are connected by a wire through a bolt, and the exterior of the carbon fiber cement-based composite material solid body, the upper and lower electrode ribbed electrode plates and the wire are wrapped with a waterproof insulation layer.

2. The method of testing the working stress of concrete according to claim 1, characterized in that: In step 2, the specific steps are as follows: Step 2.1: fitting the temperature-resistance curve of each concrete pad of measurable concrete working stress, and taking the commonly used temperature interval 0~50℃, and taking the temperature 20℃ at the time of calibration of the concrete pad of measurable concrete working stress as the reference value R0; Step 2.2: correcting the temperature system error by testing the temperature and the corresponding temperature resistance, comparing the resistance values measured at different temperatures with the reference value R0, and the obtained number is the temperature correction coefficient kt; Step 2.3: after measuring the resistance value in the field, the measured resistance value is multiplied by the corresponding correction coefficient according to the measured real-time temperature, and then the corrected resistance value is obtained.

3. The method of testing the working stress of concrete according to claim 1, characterized in that: In step 3, the specific steps are as follows: the calibration of the pressure-sensitive curve of the concrete pad of measurable concrete working stress, before use, each sensor is subjected to 5 times of linear stage cyclic load to eliminate microcracks and calibrate the sensor, and a calculation formula is obtained.

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