Method for nondestructive testing of carbonization depth of cement-based material
The internal nuclear magnetic signals of cement-based materials are detected through unilateral low-field nuclear magnetic resonance technology, combined with data processing, and the non-destructive and accurate measurement of carbonization depth is achieved, solving the problem of insufficient destructiveness and accuracy of traditional methods on samples.
Patent Information
- Application Number
- CN202510425695.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-07
- Publication Date
- 2025-06-27
AI Technical Summary
The existing carbonization depth measurement methods are destructive to the samples, and the carbonization transition zone cannot be clearly observed, and rely on the alkalinity gradient, which has problems of accuracy and reliability.
The single-sided low-field nuclear magnetic resonance technology is used to detect nuclear magnetic signals at different depths inside the sample, and combine pre-processing and first-order derivative processing to determine the carbonization depth and achieve non-destructive testing.
Accurate non-destructive measurement of the carbonization depth of cement-based materials is achieved, sample damage is avoided, measurement accuracy is improved, and alkalinity gradient is not limited.
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Figure CN120214003A_ABST
Abstract
Description
Technical Field
[0001] The present invention belongs to the field of material testing, and relates to a method for non-destructively detecting the carbonation depth of cement-based materials based on unilateral low-field nuclear magnetic resonance technology. Background Art
[0002] Cement is one of the most widely used building materials at present. Cement undergoes a hydration reaction and hardens by mixing with water, and thus serves as a binder to combine with sand and gravel to form cement-based materials such as concrete. However, calcium hydroxide and calcium silicate hydrate generated by cement hydration in cement-based materials will react with CO2 to form calcium carbonate, and the reaction equations are as follows: Ca(OH)2 + CO2 → CaCO3 + H2O xCaO·SiO2·yH2O + xCO2 → xCaCO3 + SiO2 + yH2O This process is generally the diffusion of CO2 in the air into the cement-based material, which will cause the pH value of the pore solution to decrease, and thus may damage the steel structure of the building and affect the durability of the material. With the diffusion of CO2, the carbonation reaction gradually spreads from the outer surface of the material to the inside. The boundary between the carbonated area and the non-carbonated area is the carbonation boundary, and the thickness of the area where the carbonation reaction occurs is the carbonation depth. In engineering practice, it is necessary to avoid the carbonation depth from extending to the surface of the steel bars as much as possible to prevent steel bar corrosion and cause a decrease in the bearing capacity due to material cracking. Therefore, the test for the carbonation depth is an important part of the durability evaluation of cement-based materials.
[0003] Currently, the most commonly used method for measuring the carbonation depth in experimental research or engineering practice is the phenolphthalein method. Referring to the "Standard Test Method for Long-Term Performance and Durability of Ordinary Concrete (GB / T50082 - 2009)", the specific method is as follows: (1) Use appropriate tools to break or drill holes in the cement-based sample.
[0004] (2) Use a brush or the like to remove the powder and debris on the cross-section or in the hole.
[0005] (3) Spray a 1% - 2% phenolphthalein alcohol solution on the cross-section or the inner wall of the hole with a spray bottle. The non-carbonated area will turn purple due to the reaction of the alkaline environment with phenolphthalein, while the carbonated area will not change color due to the decrease in alkalinity.
[0006] (4) When the boundary between the carbonated and non-carbonated areas is clear, use a vernier caliper, depth gauge or other measuring tools to measure the vertical distance from the carbonated and non-carbonated interface to the surface of the material, which is the carbonation depth.
[0007] As a conventional method for measuring carbonation depth, the phenolphthalein method often requires breaking or drilling the sample. Given the possible errors caused by the sampling location, this method even needs to drill multiple holes to reduce uncertainty, which undoubtedly increases the damage to the sample. Especially under laboratory conditions, the damage caused by breaking the sample is irreversible. In addition, the carbonation depth is not a sharp interface but has a slow transition part. The accuracy of the phenolphthalein method in positioning is significantly affected by the alkalinity gradient, and it is difficult to observe the transition zone. For some cement-based materials or alternative cementitious materials with a high alkalinity ratio, if the alkalinity of the pore solution is still higher than the transition point of the phenolphthalein indicator at 8.2 pH value after carbonation, the phenolphthalein method may even fail. Summary of the Invention
[0008] Aiming at the problems of the existing carbonation depth measurement methods, such as being destructive to the sample, unable to clearly observe the carbonation transition zone, and relying on the alkalinity gradient, the present invention provides a method for non-destructively detecting the carbonation depth of cement-based materials.
[0009] The unilateral low-field nuclear magnetic resonance technology can measure the amount of nuclear magnetic resonance signals at a certain depth inside the sample. By combining the unilateral low-field nuclear magnetic resonance instrument (also known as the unilateral low-field nuclear magnetic resonance testing machine) with a lift, the nuclear magnetic resonance signals at different depths inside the sample can be measured. Since the total porosity of cement-based materials generally decreases after the carbonation reaction, the water content in the carbonated layer is lower under the saturated water state. At the same time, the nuclear magnetic resonance signals collected by low-field nuclear magnetic resonance come from the hydrogen nuclei of water molecules in the sample, so the nuclear magnetic resonance signals of the carbonated layer are also lower than those of the non-carbonated layer.
[0010] The method of the present invention is a non-destructive measurement method for carbonation depth using the unilateral low-field nuclear magnetic resonance technology. Based on the principle that the pore structure of cement-based materials changes after the carbonation reaction, the area where the nuclear magnetic resonance signal begins to show a decreasing trend is found to locate the carbonated layer, and the carbonation boundary is located without damaging the sample.
[0011] To achieve the above object, the present invention provides the following technical solutions.
[0012] A method for non-destructively detecting the carbonation depth of cement-based materials, which is realized based on a unilateral low-field nuclear magnetic resonance instrument, a lift, and a PC; the main body of the unilateral low-field nuclear magnetic resonance instrument is fixed on the lift; the PC is connected to the unilateral low-field nuclear magnetic resonance instrument, and the PC runs a preprocessing module and a carbonation depth determination module, including the following steps: Step 1: Saturate the sample with water. Step 2: After the saturation treatment, place the sample with the carbonated surface facing down above the probe of the unilateral low-field nuclear magnetic resonance instrument, and start measuring the nuclear magnetic resonance signal from a preset initial depth from the carbonated surface. Step 3: Start the elevator carrying the main body of the unilateral low-field nuclear magnetic resonance instrument, and gradually move the main body of the unilateral low-field nuclear magnetic resonance instrument downward at a preset step length until the nuclear magnetic signal cannot be detected. Measure the nuclear magnetic signal once for each step to obtain the nuclear magnetic signal data at different depths inside the sample; Step 4: After the data acquisition is completed, the preprocessing module processes the nuclear magnetic signal data at different depths obtained in Step 3 to obtain the relative nuclear magnetic signal, so as to obtain the attenuation data of the relative nuclear magnetic signal with the test depth; the relative nuclear magnetic signal is calculated according to formula (1): (1) In the formula, I 0 is the nuclear magnetic signal at the initial depth, regarded as the reference nuclear magnetic signal, I n is the depth n where the nuclear magnetic signal is located, k n is the depth n where the relative nuclear magnetic signal is located; Step 5: Based on the attenuation data of the relative nuclear magnetic signal with the test depth obtained in Step 4, the carbonation depth determination module further processes the data and determines the carbonation depth. Specifically: Perform a first-order derivative processing on the relative nuclear magnetic signal data according to formula (2) to obtain the first-order derivative of the attenuation data of the relative nuclear magnetic signal with the test depth k’ n , (2) The first-order derivative k’ n The depth at which the mutation occurs is the carbonation depth.
[0013] The beneficial effects of the present invention are: The present invention uses unilateral low-field nuclear magnetic resonance technology to detect the carbonation depth of cement-based materials. Compared with the traditional carbonation depth measurement method, the method of the present invention will not cause damage to the material, is not limited by the alkalinity inside the material, and the test results are more accurate, having important application potential in the field of durability evaluation of cement-based materials. For some scenarios requiring sample integrity, continuity or higher precision requirements, using unilateral low-field nuclear magnetic resonance technology to non-destructively measure the carbonation depth has unique advantages. Description of the Drawings
[0014] Figure 1 is a schematic diagram of the experimental device for measuring the carbonation depth by unilateral low-field nuclear magnetic resonance.
[0015] Figure 2 is a graph of the attenuation data of the relative nuclear magnetic signal of the sample with the test depth obtained by the unilateral low-field nuclear magnetic resonance instrument in the application example.
[0016] Figure 3 It is the first derivative curve of the relative nuclear magnetic resonance signal of the carbonized sample with respect to the attenuation data of the test depth in the application example.
[0017] Figure 4 It is the first derivative curve of the relative nuclear magnetic resonance signal of the control sample with respect to the attenuation data of the test depth in the application example.
[0018] Reference signs: Test bench 1, lift 2, unilateral low-field nuclear magnetic resonance instrument main body 3, unilateral low-field nuclear magnetic resonance instrument probe 4, measurement sample 5, carbonized layer 6, measurement area 7. Specific implementation mode
[0019] The technical solution provided by the present application will be further described below in conjunction with specific embodiments and their accompanying drawings. In combination with the following description, the advantages and features of the present application will be clearer.
[0020] A method for non-destructively detecting the carbonation depth of cement-based materials, which uses unilateral low-field nuclear magnetic resonance technology for non-destructive measurement of the carbonation depth. As Figure 1 shown, this method is based on a unilateral low-field nuclear magnetic resonance instrument, a lift 2, and a PC; the main body of the unilateral low-field nuclear magnetic resonance instrument is fixed to the lift 2; the PC is connected to the unilateral low-field nuclear magnetic resonance instrument, and the PC runs a preprocessing module and a carbonation depth determination module. The specific implementation steps are as follows: Step 1: Saturate the measurement sample with water. Select the normal-pressure water immersion method or the vacuum saturation method according to the specific situation of the measurement sample. The normal-pressure water immersion method requires soaking in deionized water for no less than 24 h; the vacuum saturation method requires a vacuum degree of -0.08 to -0.09 Mpa to be maintained for 1 to 3 h, and continue to maintain for 6 h after adding water.
[0021] Step 2: After the saturation treatment, place the measurement sample 5 with the carbonated surface facing down on the test bench 1 and above the probe 4 of the unilateral low-field nuclear magnetic resonance instrument. The carbonated layer 6 above the probe of the unilateral low-field nuclear magnetic resonance instrument is the measurement area 7. Start the nuclear magnetic resonance signal test from a certain depth from the carbonated surface. The initial depth should not be less than 10 mm to avoid being unable to detect the carbonation boundary. The measurement sample cannot move during the test.
[0022] Step 3: Start the lift 2 carrying the main body 3 of the unilateral low-field nuclear magnetic resonance instrument, and gradually move the main body of the unilateral low-field nuclear magnetic resonance instrument downward at a certain step length until the nuclear magnetic resonance signal cannot be detected. Measure the nuclear magnetic resonance signal once for each step of movement to obtain the nuclear magnetic resonance signal data at different depths inside the measurement sample. To ensure the accuracy of the carbonation depth test and at the same time take into account the test efficiency, it is required that the single-step length of the lift is between 0.2 and 1 mm.
[0023] Step 4: After the data acquisition is completed, the preprocessing module processes the nuclear magnetic resonance signal data at different depths obtained in Step 3 to obtain the relative nuclear magnetic resonance signal, thereby obtaining the attenuation data of the relative nuclear magnetic resonance signal with the test depth: Specifically, the nuclear magnetic resonance signal data is processed according to formula (1) to obtain the relative nuclear magnetic resonance signal: (1) In the formula, I 0 is the nuclear magnetic resonance signal at the initial depth, regarded as the reference nuclear magnetic resonance signal, I n is the depth n at which the nuclear magnetic resonance signal is located, k n is the depth n at which the relative nuclear magnetic resonance signal is located.
[0024] Step 5: Based on the attenuation data of the relative nuclear magnetic resonance signal with the test depth obtained in Step 4, the carbonation depth determination module further processes the data and determines the carbonation depth.
[0025] Specifically, after obtaining the relative nuclear magnetic resonance signal k n varying with the test depth n in order to accurately locate the depth at which the turning point of the nuclear magnetic resonance signal begins to appear, the carbonation depth determination module performs a first-order derivative processing on the relative nuclear magnetic resonance signal data according to formula (2): (2) According to the principle that the pore structure changes after the carbonation reaction of the cement-based material, the depth at which the first-order derivative k’ n has a mutation is the carbonation depth.
[0026] Application example: The unilateral low-field nuclear magnetic resonance testing machine used in this example is the NMR-MOUSE PM25 produced by Magritek Company of Germany, with a maximum measurement depth of 25 mm, a coil diameter of 40 mm, and a gradient strength of 310 kHz / mm.
[0027] (1) Prepare the sample and perform water saturation treatment on the sample.
[0028] The cement used is the 52.5-grade white Portland cement produced by Jiangxi Yingshan White Cement Co., Ltd., and the production of this cement complies with the GB / T 2015-2017 standard. Two cylindrical samples were prepared with a water-cement ratio of 0.4, and the dimensions were 5 cm in diameter and 9 cm in height. One of them was used as the carbonated sample, which was subjected to accelerated carbonation for 3 days at 23 °C, 95% humidity, and 20% CO2 concentration to obtain a certain carbonation depth; the other was used as the control sample, which was not subjected to accelerated carbonation treatment and was cured conventionally under the same environmental conditions except for the CO2 concentration. After the curing was completed, the two samples were saturated with water by soaking them in deionized water for 24 h.
[0029] (2)Use a unilateral low-field nuclear magnetic resonance testing machine to test the nuclear magnetic signals on the carbonated surface of the sample to obtain the nuclear magnetic signal data at different depths inside the sample.
[0030] After the water saturation treatment, dry the outer moisture, place the carbonated surface of the sample downward above the probe of the unilateral low-field nuclear magnetic resonance testing machine, and start the layer-by-layer nuclear magnetic signal testing from a depth of 14 mm from the carbonated surface. The parameters of the magnetic probe are PM25-10 mm.par, and the number of echoes is 32. According to the expected carbonation depth, taking into account the accuracy and measurement efficiency, the step lengths are selected as 1 mm, 0.5 mm, and 0.2 mm. The step length in the depth range of 4-14 mm is selected as 1 mm, the step length in the depth range of 2-4 mm is selected as 0.5 mm, and the step length in the depth range below 2 mm is selected as 0.2 mm. Further, to avoid the sample debris or liquid dropping and contaminating the nuclear magnetic probe, a glass slide with a thickness of 1 mm is placed below the carbonated surface of the sample in this example. When calculating the actual carbonation depth later, the actual carbonation depth needs to be obtained by subtracting 1 mm from the depth of the turning point of the first derivative curve.
[0031] (3)Based on the collected nuclear magnetic signal data, obtain the attenuation data of the relative nuclear magnetic signal with the test depth.
[0032] After the data acquisition is completed, the PC runs the preprocessing module. According to formula (1), taking the nuclear magnetic signal at the initial depth as the reference, calculate the ratio of the nuclear magnetic signal of each layer downward to the reference nuclear magnetic signal, so as to obtain the attenuation data of the relative nuclear magnetic signal with the test depth, as Figure 2 shown.
[0033] (4)Perform the first derivative processing on the obtained attenuation data of the relative nuclear magnetic signal with the test depth to determine the carbonation depth.
[0034] Based on the attenuation data of the relative nuclear magnetic signal with the test depth, the PC runs the carbonation depth determination module. According to formula (2), the first derivative of the attenuation curve of the relative nuclear magnetic signal with the test depth is calculated, and the depth at which the first derivative changes abruptly is the carbonation depth. The first derivative curves of the attenuation data of the relative nuclear magnetic signals of the carbonated sample and the control sample with the test depth are respectively as Figure 3 、 Figure 4 shown.
[0035] (5) The carbonation depth of the sample is measured by the phenolphthalein method.
[0036] After the unilateral low-field nuclear magnetic resonance test is completed, the sample is sawn perpendicular to the carbonation layer and the cross-section is cleaned, and the phenolphthalein indicator is sprayed to observe the carbonation depth measured by the phenolphthalein method.
[0037] (6) The carbonation depth measured by the method of the present invention is compared and analyzed with the carbonation depth measured by the phenolphthalein method.
[0038] The carbonation depth of the carbonated sample measured by the phenolphthalein method is 2.36 mm, while the carbonation depth of the control sample is difficult to observe.
[0039] Observing Figure 2 the attenuation curve of the relative nuclear magnetic signal with the test depth, it can be found that the relative nuclear magnetic signal of the carbonated sample begins to attenuate at a depth of about 3-4 mm. Subtracting the thickness of the glass slide of 1 mm, that is, at an actual carbonation depth of about 2-3 mm, but the accurate depth still needs to be further located; while no obvious attenuation is observed for the control sample except for the thickness of the glass slide.
[0040] Observing Figure 3 and Figure 4 , it can be found that the first derivative of the relative nuclear magnetic signal of the carbonated sample with the depth changes abruptly at a depth of 3.5 mm. Subtracting the thickness of the glass slide of 1 mm, the corresponding actual depth is 2.5 mm, that is, the carbonation depth of this sample is 2.5 mm. While the depth at which the first derivative of the relative nuclear magnetic signal of the control sample changes abruptly is 1.2 mm, and subtracting the thickness of the glass slide of 1 mm, the corresponding actual depth is only 0.2 mm.
[0041] The above application examples show that the result of measuring the carbonation depth by the unilateral low-field nuclear magnetic resonance technology is in good agreement with the result of the traditional phenolphthalein method test. At the same time, the unilateral low-field nuclear magnetic resonance technology avoids damaging the sample. In addition, as Figure 2As shown, the process of the gradual decrease of the nuclear magnetic signal, i.e., the transition zone of the carbonation reaction, can be clearly observed through the unilateral low-field nuclear magnetic resonance technology for carbonation depth measurement, indicating the trend of the carbonation reaction gradually diffusing into the deep part of the sample. The traditional phenolphthalein method is difficult to observe this transition zone. At the same time, since the phenolphthalein method is based on the alkalinity gradient for positioning, it may not be able to capture the weak carbonation reaction occurring deeper in the sample. The unilateral low-field nuclear magnetic resonance method is based on the influence mechanism of the carbonation reaction itself on the pore structure of cement-based materials, can capture the weaker carbonation reaction of the sample, and has higher positioning accuracy.
[0042] The above description is only a description of the preferred embodiments of the present application and does not limit the scope of the present application in any way. Any change or modification made by any ordinary person skilled in the art based on the technical content disclosed above should be regarded as an equivalent effective embodiment and fall within the scope of protection of the technical solution of the present application.
Claims
1. A method for non-destructive detection of carbonization depth of cement-based materials, characterized in that: The method is implemented based on a unilateral low-field nuclear magnetic resonance instrument, an elevator, and a PC; the main body of the unilateral low-field nuclear magnetic resonance instrument is fixed to the elevator; the PC is connected to the unilateral low-field nuclear magnetic resonance instrument, and the PC runs a pretreatment module and a carbonization depth determination module, including the following steps: Step 1: Saturate the sample with water; Step 2: After the saturated water treatment, place the sample with the carbonized surface facing downward on the probe of the single-sided low-field nuclear magnetic resonance instrument, and start the nuclear magnetic signal test from the preset initial depth from the carbonized surface. The position of the sample remains unchanged during the test; Step 3: Start the elevator carrying the main body of the single-sided low-field nuclear magnetic resonance instrument, and gradually move the main body of the single-sided low-field nuclear magnetic resonance instrument downward with a preset step length until the nuclear magnetic signal cannot be detected, and measure the nuclear magnetic signal once for each step to obtain the nuclear magnetic signal data at different depths inside the sample; Step 4: After the data acquisition is completed, the preprocessing module processes the nuclear magnetic signal data of different depths obtained in step 3 to obtain the relative nuclear magnetic signal, thereby obtaining the attenuation data of the relative nuclear magnetic signal with the test depth; wherein the relative nuclear magnetic signal is calculated according to formula (1): (1) In the formula, I 0 is the initial depth nuclear magnetic signal, which is regarded as the baseline nuclear magnetic signal. I n For Depth n The nuclear magnetic signal at k n For Depth n The relative NMR signal at Step 5: Based on the attenuation data of the relative nuclear magnetic signal with the test depth obtained in step 4, the carbonization depth determination module further processes the data and determines the carbonization depth, specifically: According to formula (2), the first-order derivative of the relative nuclear magnetic signal data is processed to obtain the first-order derivative of the relative nuclear magnetic signal attenuation data with the test depth: k’ n , (2) First Derivative k’ n The depth at which the mutation occurs is the carbonization depth.
2. A method for non-destructive detection of carbonization depth of cement-based materials according to claim 1, characterized in that: In step 1, select the normal pressure water immersion method or the vacuum water saturation method, where: the normal pressure water immersion method requires immersion in deionized water for no less than 24 hours; the vacuum water saturation method requires the vacuum degree to be maintained at -0.08~-0.09 MPa for 1~3 hours, and continue to maintain it for 6 hours after adding water.
3. The method for nondestructively testing the carbonization depth of cement-based materials according to claim 1, characterized in that: In step 2, the initial depth is selected to be greater than 10 mm.
4. The method for nondestructively testing the carbonization depth of cement-based materials according to claim 1, characterized in that: In step 3, the single step length of the elevator is between 0.2 and 1 mm.