Method for evaluating crack resistance of natural anhydrite-based putty

Abstract

This invention discloses a method for evaluating the crack resistance of natural anhydrous gypsum-based putty, relating to the field of building materials technology. The method includes: preparing a sample from natural anhydrous gypsum-based putty; sealing the sides and bottom of the sample to prevent moisture exchange, leaving only the top surface for moisture exchange and covering it with a permeable membrane for water release curing; dividing the sample into layer units along its thickness direction after curing; determining the increase in bound water content using thermogravimetric analysis; determining the delayed hydration rate based on the increase in bound water content per unit time, thus forming a delayed hydration rate distribution field; removing the permeable membrane; placing the top surface of the sample in a low-humidity environment while maintaining the sides and bottom to prevent moisture exchange, thereby reversing the direction of moisture migration and forming a moisture content gradient distribution. This invention, by first constructing a delayed hydration rate distribution field in the sample and then constructing a moisture content gradient distribution field, allows the natural anhydrous gypsum-based putty to simultaneously exhibit hydration hysteresis characteristics and moisture migration characteristics during the evaluation process.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a method for evaluating the crack resistance of natural anhydrous gypsum-based putty. Background Technology

[0002] Natural anhydrous gypsum-based putty, as a wall leveling material with natural anhydrous gypsum as the main cementing component, has the characteristics of high resource utilization value and strong construction adaptability. It has good application prospects in fine leveling of interior walls and green building materials. Compared with other cementing systems, natural anhydrous gypsum-based putty is more significantly affected by the water supply conditions during the hardening process. There is a strong coupling relationship between its hydration process, water loss process and internal structure formation process. Therefore, its cracking behavior often has certain layer differences and temporal differences. Existing methods for evaluating the crack resistance of putty materials mostly involve surface crack observation, standard test plate cracking tests, drying shrinkage deformation tests, or macroscopic mechanical property tests. These methods can reflect the cracking results or cracking tendency of materials to a certain extent and are suitable for performance comparison and quality control of some conventional material systems. Meanwhile, techniques such as acoustic emission detection, humidity change analysis, and delamination testing have also been applied in materials research and damage monitoring, providing a technical basis for cracking process identification. However, for natural anhydrous gypsum-based putty, its crack resistance formation process is not only related to the final crack morphology, but also closely related to hydration lag, changes in water migration direction, layer misalignment, and the temporal characteristics of crack signal appearance. Existing technologies rarely unify the correlation between the distribution of delayed hydration rate, the formation of moisture content gradient, and the layer location of crack signal, and rarely classify crack causes within the same evaluation framework and further construct comprehensive crack resistance performance evaluation results. Therefore, it is necessary to provide a crack resistance performance evaluation method more suitable for natural anhydrous gypsum-based putty to improve the ability of the evaluation results to reflect the internal cracking mechanism of the material. Therefore, this invention proposes a crack resistance performance evaluation method for natural anhydrous gypsum-based putty. Summary of the Invention

[0003] The purpose of this invention is to provide a method for evaluating the crack resistance of natural anhydrous gypsum-based putty, so as to solve the problems mentioned in the background art.

[0004] This invention can be achieved through the following technical solution: a method for evaluating the crack resistance of natural anhydrous gypsum-based putty, comprising: Step 1: Prepare a sample of natural anhydrous gypsum-based putty. Seal the sides and bottom of the sample to prevent moisture, leaving only the top surface for moisture exchange and covering it with a moisture-permeable membrane for water release curing. After curing, divide the sample into layer units along the thickness direction and use thermogravimetric analysis to determine the increase in bound water content. Determine the hysteresis hydration rate by the increase in bound water content per unit time, and form a hysteresis hydration rate distribution field. Step 2: Remove the moisture-permeable membrane, place the upper surface of the sample in a low-humidity environment and keep the sides and bottom moisture-proof and sealed, so that the direction of water migration is reversed and a moisture content gradient distribution is formed; after reaching the preset moisture content difference threshold, apply a semi-permeable sealing layer to lock the moisture content gradient distribution field and form a coupling correspondence with the hysteresis hydration rate distribution field on the layer unit. Step 3: Using uniformly cured samples with the same formula as reference samples, obtain the delayed hydration rate and moisture content according to the same layer unit division method, and determine the delayed hydration rate threshold and moisture content difference threshold based on the statistical mean and standard deviation. Step 4: Set up acoustic emission sensors on both sides of the sample to collect signals, obtain effective crack signals through screening rules, and locate the effective crack signals to the corresponding layer units based on the layer wave velocity measured by the same formulation calibrated sample and the sensor arrival time difference. Step 5: Based on the hysteresis hydration rate threshold and the water content difference threshold, normalize the hysteresis hydration rate difference and water content difference of the stratigraphic units corresponding to the effective crack signal, construct and classify the crack origin discrimination index, and construct a comprehensive crack resistance performance evaluation index based on the effective crack signal density, origin ratio and stratigraphic weight.

[0005] A further technical improvement of the present invention is that, after forming the delayed hydration rate distribution field in step one, the following processing steps are included: For each layer unit, during the controlled water release curing process, the delayed hydration rate and water content are obtained at preset time intervals. The delayed hydration rate difference is determined based on the change in delayed hydration rate between consecutive time points, and the water content difference is determined based on the change in water content between corresponding time points. In each layer unit, identify the time segment where the direction of change of the hysteresis hydration rate difference is consistent with the direction of change of the corresponding water content difference and the hysteresis hydration rate difference has not reached the lower limit of the reference interval of the hysteresis hydration rate difference, and determine the time segment as the hydration response hysteresis segment. Within the hydration response lag zone, the lag hydration rate differences are sorted in descending order of water content difference, and the sorted lag hydration rate differences are summed to obtain the lag hydration driving value. The ratio of the delayed hydration driving value to the average difference of delayed hydration rates in the same layer element that is not in the hydration response lag zone is calculated, and the delayed hydration driving weight is determined based on the distribution range of the ratio in all layer elements.

[0006] A further technical improvement of the present invention is that: the process of determining the misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field on the stratigraphic unit between step one and step two further includes the following steps: After forming the hysteresis hydration rate distribution field in step one, the hysteresis hydration rates of each layer unit are sorted from largest to smallest according to the thickness direction of the sample, and the corresponding hydration sequence numbers are assigned in sequence according to the sorting results. After forming the water content gradient distribution field in step two, the layers are sorted from largest to smallest according to the difference in water content along the thickness direction, and the corresponding water content sequence numbers are assigned according to the sorting results. The hydration sequence number and the aquifer sequence number of the same stratigraphic unit are compared, and the absolute value of the difference between the two sequences is used as the sequence offset of the stratigraphic unit. The direction of change of sequential offset of adjacent layer units along the thickness direction of the sample is compared. When the sequential offset of adjacent layer units changes from increasing to decreasing or from decreasing to increasing, a cross-layer transition is recorded, and the number of cross-layer transitions corresponding to each layer unit is counted. The sequential offset of each layer unit is combined with the corresponding number of cross-layer transitions, and the initial double gradient phase difference of each layer unit is determined according to the product relationship between the sequential offset and the number of cross-layer transitions. The initial dual-gradient phase difference is introduced into the construction process of the crack origin discrimination index in step five, and is used to participate in the calculation of the combined effect of the hysteresis hydration rate difference and the water content difference.

[0007] A further technical improvement of the present invention is that the water migration direction reversal regulation process in step two further includes the following steps: After forming the delayed hydration rate distribution field in step one, the water content difference of each layer unit is continuously acquired at a preset fixed time interval, and the moment when the water content difference between adjacent time points changes from positive to negative or from negative to positive is determined to be the moment when the direction of water migration reverses. Before and after the determination time, the hysteresis hydration rate difference of the corresponding layer unit is obtained, and it is determined whether the hysteresis hydration rate difference maintains the same direction of change before and after the reversal. When the hysteresis hydration rate difference still maintains the direction of change before the reversal after the direction of water migration is reversed, the corresponding time segment is determined as the hydration hysteresis locking segment. Within the hydration lag locked section, the difference between the lag hydration rate and the difference in water content are matched point by point in chronological order, and the number of time points in which the difference between the lag hydration rate and the difference in water content change in opposite directions at the same time point is used as the reverse mismatch count value. The ratio of the inversion mismatch count value to the total number of sampling points of the corresponding stratigraphic unit in the hydration hysteresis-locked section is taken as the inversion mismatch ratio value of that stratigraphic unit. The boundary reversal driving value of each layer element is obtained by multiplying the reversal mismatch ratio value with the difference in the hysteresis hydration rate of the corresponding layer element. The boundary reversal driving value is introduced into the crack origin discrimination index construction process in step five as a correction factor for the coupled calculation of the hysteresis hydration rate difference and the water content difference.

[0008] A further technical improvement of the present invention lies in: after the formation of the water content gradient distribution field in step two, the process of determining the stratigraphic misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field includes: For each layer unit, the time point when the delayed hydration rate first reaches the delayed hydration rate threshold in step one and the time point when the moisture content difference first reaches the moisture content difference threshold in step two are determined respectively, and the corresponding hydration sequence number and moisture content sequence number are assigned according to the sample thickness direction. The absolute value of the difference between the hydration sequence number and the aquifer sequence number of the same stratigraphic unit is determined as the stratigraphic offset value of that stratigraphic unit. The time interval between two time points in the same layer unit is compared with the total time from the start of step two to the application of the semi-permeable sealing layer on the upper surface, and the time lag value of the layer unit is determined by the proportion of the time interval to the total time. The time correction value of each layer element is obtained by multiplying the layer offset value with the time lag value. The time correction value is multiplied by the initial double gradient phase difference value to obtain the final double gradient phase difference value of each layer unit. The final dual-gradient phase difference value is incorporated into the construction process of the crack origin discrimination index in step five.

[0009] A further technical improvement of the present invention lies in the correction process for the delayed hydration rate threshold and the water content difference threshold determined in step three, including: After obtaining the hysteresis hydration rate and moisture content difference of each layer unit at the same time interval, the layer units in which the moisture content difference reaches the moisture content difference threshold and the hysteresis hydration rate does not reach the hysteresis hydration rate threshold are identified, and at least three consecutive layer units that meet this condition are selected in the sample thickness direction; among the selected layer units, taking the middle layer unit as the reference, when the difference between the moisture content difference and the moisture content difference threshold in the middle layer unit is greater than the corresponding difference in the adjacent layer unit, the corresponding region is identified as the hysteresis hydration enhancement section; Within the hydration lag enhancement zone, the difference between the lag hydration rate and the lag hydration rate threshold, as well as the difference between the water content difference and the water content difference threshold, are obtained, and only the layer units with the same sign for the difference between the two are retained. The retained strata units are arranged from the surface to the center layer according to the sample thickness direction. Only the strata units where the difference between the hysteresis hydration rate and the hysteresis hydration rate threshold increases layer by layer are summed to obtain the cumulative difference. The cumulative difference is multiplied by the difference between the moisture content difference and the moisture content difference threshold in the corresponding layer unit, and then multiplied by a preset coefficient that increases sequentially from the surface layer to the center layer in the sample thickness direction to obtain the threshold correction driving value. For the stratigraphic units in the hydration lag enhancement zone, the hysteresis hydration rate threshold and the threshold correction driving value are additively corrected, and the water content difference threshold is proportionally corrected. The corrected hysteresis hydration rate threshold and water content difference threshold replace the original thresholds and are introduced into the calculation process of the crack origin discrimination index in step five.

[0010] A further technical improvement of the present invention is that: after the effective crack signal is located to the layer element in step four, the following steps are included: The start time, peak time, and end time of the effective crack signal in each layer element are obtained, and the duration and peak value of the effective crack signal are determined. The start time of the effective crack signal is compared with the time point when the water content difference in the corresponding layer unit reaches the water content difference threshold and the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold. When the time difference between the start time of the effective crack signal and the corresponding time point is not greater than the preset time window, the state in which the water content difference first reaches the water content difference threshold, the state in which the hysteresis hydration rate first reaches the hysteresis hydration rate threshold, or the state in which both reach the corresponding threshold at the same time is determined. Within the same sampling period, the number of layer elements whose time difference between the start times of effective crack signals in adjacent layer elements is no greater than a preset time window is counted. The effective crack signals are then divided into single-layer occurrence type corresponding to one layer element, cross-layer transmission type corresponding to two adjacent layer elements, and multi-layer coupling type corresponding to at least three adjacent layer elements. The division results are then incorporated into the crack cause discrimination index construction process in step five.

[0011] A further technical improvement of the present invention is that the process of constructing the comprehensive crack resistance performance evaluation index in step five includes: The crack initiation discrimination index and classification results of each layer unit are arranged according to the thickness direction of the sample, and the effective crack signal density and initiation ratio of the corresponding layer unit are obtained. Obtain the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value corresponding to each layer unit, and combine the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value according to the product relationship to form a multi-factor coupled influence value. Compare the multi-factor coupling influence values ​​and classification results of adjacent layer elements. When the multi-factor coupling influence values ​​of adjacent layer elements change in the same direction, the classification results are the same, and the time difference of the corresponding effective crack signal is not greater than the preset time window, it is determined to be a continuous coupling section; otherwise, it is determined to be a coupling transition section. Based on the length of the continuous coupling section and the number of coupling transition sections, the multi-factor coupling influence value is processed by segmented accumulation, and the layer weight is adjusted by setting progressively increasing layer weights from the surface layer to the center layer in the sample thickness direction to form the layer coupling evaluation value of each layer unit. The comprehensive crack resistance performance evaluation index is obtained by summing the layer coupling evaluation values ​​of each layer unit.

[0012] Compared with the prior art, the present invention has the following beneficial effects: This invention constructs a delayed hydration rate distribution field and a moisture content gradient distribution field in the sample first, so that the natural anhydrous gypsum-based putty can simultaneously reflect the hydration hysteresis characteristics and moisture migration characteristics during the evaluation process, thereby more realistically reflecting the internal state changes of the material in the thickness direction. Compared with the evaluation method that only focuses on the surface cracking results, this invention introduces the delayed hydration behavior unique to natural anhydrous gypsum into the crack resistance evaluation process, expanding the evaluation object from a single appearance result to the internal formation mechanism. Furthermore, this invention, through acoustic emission signal acquisition and stratigraphic location, can establish a correlation between effective crack signals and the differences in hysteresis hydration rates and water content in corresponding stratigraphic units. Based on this, a crack induction discrimination index is constructed to classify cracks. This not only identifies whether the material has cracked, but also analyzes the most likely formation mechanism of the crack, thereby elevating crack resistance evaluation from result judgment to cause judgment, improving the pertinence and information content of the evaluation. On the other hand, this invention also constructs a comprehensive crack resistance performance evaluation index by integrating effective crack signal density, causal ratio, and layer weight. Furthermore, it can introduce factors such as hysteresis hydration driving weight, dual gradient phase difference, boundary reversal driving value, and threshold correction results to perform multi-factor coupling processing on the evaluation results. The resulting evaluation system takes into account the time dimension, spatial layer dimension, and causal dimension, and is suitable for the study of crack resistance performance, formula optimization, material screening, and quality control of natural anhydrous gypsum-based putty. It has good engineering application value and method promotion value. Attached Figure Description

[0013] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to the accompanying drawings.

[0014] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0015] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided.

[0016] Please see Figure 1 As shown, this invention provides a method for evaluating the crack resistance of natural anhydrous gypsum-based putty, comprising: Step 1: Prepare a sample from natural anhydrous gypsum-based putty. Seal the sides and bottom of the sample to prevent moisture exchange, leaving only the top surface for moisture exchange. Cover with a permeable membrane for water release curing. After curing, divide the sample into layers along its thickness. Use thermogravimetric analysis to determine the increase in bound water content. Determine the delayed hydration rate based on the increase in bound water content per unit time, thus forming a delayed hydration rate distribution field. Step 1, by sealing the sides and bottom of the sample to prevent moisture exchange and leaving only the top surface for moisture exchange, combined with water release curing using a permeable membrane, confines the moisture exchange path to a single direction, thereby establishing a controlled moisture migration environment along the sample's thickness. Under these conditions, due to differences in moisture supply conditions, the hydration reaction process in different layers becomes spatially uneven. By measuring the increase in bound water content using thermogravimetric analysis and determining the delayed hydration rate based on the increase in bound water content per unit time, a delayed hydration rate distribution field with layer-specific differences can be formed along the thickness, providing fundamental data support for subsequent analysis of the impact of uneven hydration on crack formation.

[0017] After forming the delayed hydration rate distribution field, the following further processing steps are included: For each layer unit, during the controlled water release curing process, the delayed hydration rate and water content are obtained at preset time intervals. The delayed hydration rate difference is determined based on the change in delayed hydration rate between consecutive time points, and the water content difference is determined based on the change in water content between corresponding time points. In each stratigraphic unit, identify time intervals in which the hysteresis hydration rate difference is consistently lower than the corresponding water content difference, and define these time intervals as hysteresis response hysteresis intervals. Within the hydration response lag zone, the lag hydration rate differences are sorted in descending order of water content difference, and the sorted lag hydration rate differences are accumulated to obtain the lag hydration driving value of the layer unit. The delayed hydration driving value is compared with the delayed hydration rate difference of the corresponding layer that is not in the hydration response lag zone, and the delayed hydration driving weight of the layer element is determined according to the proportional relationship between the two. The hysteresis driving weight is introduced into the construction process of the crack cause discrimination index in step five to adjust the value of the hysteresis factor in the discrimination.

[0018] After forming the hysteresis hydration rate distribution field in step one and the water content gradient distribution field in step two, the process of determining the misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field on the stratigraphic unit further includes the following steps: After forming the hysteresis hydration rate distribution field in step one, the hysteresis hydration rates of each layer unit are sorted from largest to smallest according to the thickness direction of the sample, and the corresponding hydration sequence numbers are assigned in sequence according to the sorting results. After forming the moisture content gradient distribution field in step two, for each layer unit, the time point when the delayed hydration rate first reaches the delayed hydration rate threshold in step one and the time point when the moisture content difference first reaches the moisture content difference threshold in step two are determined respectively, and the corresponding hydration sequence number and moisture content sequence number are assigned according to the sample thickness direction. The absolute value of the difference between the hydration sequence number and the aquifer sequence number of the same stratigraphic unit is determined as the stratigraphic offset value; The time interval between two time points in the same layer unit is compared with the total time from the start of step two to the application of the semi-permeable sealing layer on the upper surface, and the time lag value is determined by the proportion of the time interval to the total time. The time correction value of each layer element is obtained by multiplying the layer offset value with the time lag value. Subsequently, the time correction value is combined with the previously obtained initial double gradient phase difference value to obtain the double gradient phase difference value of each layer unit; The dual gradient phase difference is then correlated with the hysteresis hydration rate difference and water content difference of the corresponding layer elements, and used for coupling calculations in subsequent crack initiation determination and comprehensive crack resistance performance evaluation.

[0019] Specifically, 100 parts of natural anhydrous gypsum, 20 parts of filler, 1 part of redispersible latex powder, 0.2 parts of cellulose ether, and 0.3 parts of water-retaining agent were prepared by weight of natural anhydrous gypsum-based putty, with water added at 35% of the total solid mass. After stirring for 5 minutes, a plate-shaped sample with dimensions of 150mm × 50mm × 15mm was prepared. The sides and bottom of the sample were sealed with a 0.2mm thick moisture-barrier membrane, leaving only the top surface for moisture exchange. A permeable membrane was then used for water release and curing, with a permeability of 800g / (m²·kg⁻¹). The curing environment was 25℃ and 60% for 24 hours. After curing, the layers were evenly divided into 5 units along the thickness direction, each with a thickness of 3mm. Thermogravimetric analysis was used to determine the increase in bound water content of each unit at different time points, and the moisture content at the corresponding time points was determined by drying method, with a drying temperature of 105℃ and a constant weight determination error of no more than 0.1%. The increase in bound water content within adjacent 1 hour was used as the hysteresis hydration rate, thus forming a hysteresis hydration rate distribution field.

[0020] During the water release curing process, the delayed hydration rate and water content of each stratum unit were continuously acquired at 1-hour intervals. The change in delayed hydration rate between adjacent time points was calculated as the delayed hydration rate difference, and the change in water content between corresponding time points was calculated as the water content difference. Within each stratum unit, time intervals where the delayed hydration rate difference was lower than the corresponding water content difference for three consecutive time points were identified, and these time intervals were defined as hydration response lag intervals. Within the hydration response lag intervals, the delayed hydration rate differences were sorted in descending order of water content difference, and the sorted values ​​were then analyzed. The delayed hydration rate differences are accumulated item by item to obtain the delayed hydration driving value of the layer unit. The delayed hydration driving value is then compared with the average delayed hydration rate difference of the same layer unit that is not in the hydration response lag zone. Based on the ratio distribution of all layer units, the delayed hydration driving weight is divided according to the quantile intervals. Layer units located in the first 25% interval have a weight of 1.5, layer units located in the middle 50% interval have a weight of 1.0, and layer units located in the last 25% interval have a weight of 0.5, thus determining the delayed hydration driving weight of the layer unit. In this embodiment, the weight division is determined based on the statistical distribution of the delayed hydration driving values ​​of each layer unit.

[0021] After forming the hysteresis hydration rate distribution field in step one, the hysteresis hydration rates of each layer unit are sorted from largest to smallest according to the thickness direction of the sample, and assigned hydration sequence numbers 1 to 5 in sequence. After forming the water content gradient distribution field in step two, the layers are sorted from largest to smallest according to the water content difference, and assigned water content sequence numbers 1 to 5 in sequence. The hydration sequence number and water content sequence number of the same layer unit are compared, and the absolute value of the difference between the two numbers is used as the sequence offset. The changing trend of the sequence offset of adjacent layer units is compared along the thickness direction. When the sequence offset changes from increasing to decreasing or from decreasing to increasing, it is recorded as a cross-layer transition, and the number of cross-layer transitions of each layer unit is counted. The sequence offset and the number of cross-layer transitions are multiplied to obtain the double gradient phase difference value.

[0022] The hysteresis driving weights and dual gradient phase differences of each layer element are uniformly recorded, and a corresponding relationship is established with the hysteresis rate difference and water content difference of the corresponding layer element. This is used for multi-factor coupling analysis in the subsequent crack initiation discrimination and comprehensive crack resistance performance evaluation process.

[0023] Step 2: Remove the permeable membrane, place the upper surface of the sample in a low-humidity environment while maintaining moisture-proof sealing on the sides and bottom. This reverses the direction of moisture migration and creates a moisture content gradient distribution. After reaching a preset moisture content difference threshold, apply a semi-permeable sealing layer to lock the moisture content gradient distribution field and establish a coupling correspondence with the hysteresis hydration rate distribution field on the layer unit. Step 2, by removing the permeable membrane and placing the upper surface of the sample in a low-humidity environment while maintaining moisture-proof sealing on the sides and bottom, reverses the direction of moisture migration relative to the water release curing stage, creating a moisture content gradient distribution that develops from the surface to the interior of the sample. Applying a semi-permeable sealing layer after reaching a preset moisture content difference threshold locks this moisture content gradient distribution, thus preventing subsequent environmental changes from interfering with the moisture distribution state. It also establishes a coupling correspondence between this moisture content gradient distribution field and the hysteresis hydration rate distribution field on the layer unit, causing the hydration difference and moisture migration difference to superimpose spatially, providing coupling conditions for the analysis of the driving mechanism of crack generation.

[0024] The process of reversing the direction of water migration in step two further includes the following steps: After forming the delayed hydration rate distribution field in step one, the water content difference of each layer unit is continuously acquired at a preset fixed time interval, and the moment when the water content difference between adjacent time points changes from positive to negative or from negative to positive is determined to be the moment when the direction of water migration reverses. Before and after the determination time, the hysteresis hydration rate difference of the corresponding layer unit is obtained, and it is determined whether the hysteresis hydration rate difference maintains the same direction of change before and after the reversal. When the hysteresis hydration rate difference still maintains the original direction of change after the direction of water migration is reversed, the corresponding time segment is determined as the hydration hysteresis locking segment. Within the hydration lag locked section, the difference between the lag hydration rate and the difference in water content are matched point by point in chronological order, and the number of time points in which the difference between the lag hydration rate and the difference in water content change in opposite directions at the same time point is used as the reverse mismatch count value. The ratio of the inversion mismatch count value to the total number of sampling points of the corresponding stratigraphic unit in the hydration hysteresis-locked section is taken as the inversion mismatch ratio value of that stratigraphic unit. The boundary reversal driving value of each layer element is obtained by multiplying the reversal mismatch ratio value with the difference in the hysteresis hydration rate of the corresponding layer element. The boundary reversal driving value is introduced into the crack origin discrimination index construction process in step five to participate in the coupling calculation between the hysteresis hydration rate difference and the water content difference.

[0025] After the formation of the water content gradient distribution field in step two, the process of determining the stratigraphic misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field includes: For each layer unit, the time point when the delayed hydration rate first reaches the delayed hydration rate threshold in step one and the time point when the moisture content difference first reaches the moisture content difference threshold in step two are determined respectively, and the corresponding hydration sequence number and moisture content sequence number are assigned according to the sample thickness direction. The absolute value of the difference between the hydration sequence number and the aquifer sequence number of the same stratigraphic unit is determined as the stratigraphic offset value of that stratigraphic unit. The time interval between two time points in the same layer unit is compared with the total time from the start of step two to the application of the semi-permeable sealing layer on the upper surface, and the time lag value of the layer unit is determined by the proportion of the time interval to the total time. The time correction value of each layer element is obtained by multiplying the layer offset value with the time lag value. The time correction value is combined with the initial double gradient phase difference value to obtain the double gradient phase difference value of each layer unit. The dual-gradient phase difference is introduced into the construction process of the crack cause discrimination index in step five.

[0026] Specifically, taking the plate-shaped sample completed in the previous embodiment as the object, the moisture-permeable membrane on the upper surface was removed, and the upper surface of the sample was placed in a low-humidity environment with a temperature of 25°C, a relative humidity of 30%, and a wind speed of 0.5 m / s, while keeping the sides and bottom of the sample moisture-proof and sealed. Three sets of parallel samples of the same size were set up using a parallel measurement method. One set of parallel samples was taken out every 10 minutes after the reversal started, and five layer units were quickly separated along the thickness direction. The moisture content of each layer unit was determined by drying at 105°C to constant weight, and the difference in moisture content between adjacent layer units was used as the moisture content difference value of each layer unit. When the moisture content difference between the first layer unit and the fifth layer unit reached 2.0%, a semi-permeable sealing layer with a thickness of 0.1 mm and a moisture permeability of 300 g / (m²·24h) was applied to the upper surface of the sample to lock the moisture content gradient distribution field and establish a correspondence between the moisture content gradient distribution field and the hysteresis hydration rate distribution field formed in step one on the layer unit.

[0027] After the reversal begins, the water content difference of each stratigraphic unit is continuously acquired at fixed time intervals of 10 minutes. The moment when the sign of the water content difference change from positive to negative or from negative to positive between adjacent time points is used to determine the moment when the direction of water migration reverses. Before and after this determination moment, the hysteresis hydration rate difference of the corresponding stratigraphic unit is acquired, and it is compared whether the hysteresis hydration rate difference maintains the same direction of change before and after the reversal. When the hysteresis hydration rate difference in the same stratigraphic unit still maintains the original direction of change within three consecutive sampling points, the corresponding 30-minute time interval is determined as the hydration hysteresis lock-in interval. Within the hydration hysteresis-locked zone, the difference in hysteresis hydration rate at each sampling point is correlated with the difference in water content point by point. The number of time points at which the difference in hysteresis hydration rate and the difference in water content change in opposite directions within the same time point is counted as the reversal mismatch count value. The ratio of the reversal mismatch count value to the total number of sampling points within the hydration hysteresis-locked zone is then determined as the reversal mismatch ratio value. Finally, the reversal mismatch ratio value is multiplied by the average value of the difference in hysteresis hydration rate of the corresponding stratigraphic unit within the time period to obtain the boundary reversal driving value of each stratigraphic unit. The boundary reversal driving value is an example calculation result obtained based on the above calculation process.

[0028] After forming the hysteresis hydration rate distribution field in step one, the time point at which the hysteresis hydration rate in each layer unit first reaches the hysteresis hydration rate threshold is determined, where the hysteresis hydration rate threshold is 0.08% / h; after forming the water cut gradient distribution field in step two, the time point at which the water cut difference in each layer unit first reaches the water cut difference threshold is determined, where the water cut difference threshold is 0.80%. The values ​​of the delayed hydration rate threshold and the water content difference threshold are determined based on the statistical results of the reference sample.

[0029] Subsequently, each layer unit is assigned a hydration sequence number and a water-bearing sequence number according to the sample thickness direction, and the absolute value of the difference between the hydration sequence number and the water-bearing sequence number of the same layer unit is determined as the layer offset value; then, the time interval between two time points in the same layer unit is compared with the total time from the start of step two to the application of the semi-permeable sealing layer on the upper surface, and the time lag value is determined by the proportion of the time interval to the total time; finally, the layer offset value and the time lag value of each layer unit are multiplied to obtain the double gradient phase difference value of each layer unit, where the double gradient phase difference value is an example calculation result obtained based on the above calculation process.

[0030] Specifically, the boundary reversal driving value and the double gradient phase difference value of each layer element are recorded in the same layer element, and a corresponding relationship is established with the hysteresis hydration rate distribution field and the water content gradient distribution field, so that the various influencing factors in the subsequent crack initiation discrimination process can be coupled and analyzed on the same layer element.

[0031] Step 3: Using uniformly cured samples with the same formulation as reference samples, the delayed hydration rate and moisture content are obtained according to the same layer unit division method. The delayed hydration rate threshold and moisture content difference threshold are determined based on the statistical mean and standard deviation. Step 3 ensures that the reference sample and the test sample are consistent in material composition and spatial division, thus guaranteeing data comparability. The delayed hydration rate threshold and moisture content difference threshold determined based on the statistical mean and standard deviation can establish a benchmark range reflecting the normal hydration and moisture distribution state, which can be used to distinguish abnormal hydration lag or moisture mismatch, providing a basis for subsequent crack cause determination.

[0032] The correction process for the delayed hydration rate threshold and water content difference threshold determined in step three includes: After obtaining the hysteresis hydration rate and moisture content difference of each layer unit at the same time interval, the layer units in which the moisture content difference reaches the moisture content difference threshold and the hysteresis hydration rate does not reach the hysteresis hydration rate threshold are identified, and at least three consecutive layer units that meet this condition are selected in the sample thickness direction; among the selected layer units, taking the middle layer unit as the reference, when the difference between the moisture content difference and the moisture content difference threshold in the middle layer unit is greater than the corresponding difference in the adjacent layer unit, the corresponding region is identified as the hysteresis hydration enhancement section; Within the hydration lag enhancement zone, the difference between the lag hydration rate and the lag hydration rate threshold, as well as the difference between the water content difference and the water content difference threshold, are obtained, and only the layer units with the same sign for the difference between the two are retained. The retained strata units are arranged from the surface to the center layer according to the sample thickness direction. Only the strata units where the difference between the hysteresis hydration rate and the hysteresis hydration rate threshold increases layer by layer are summed to obtain the cumulative difference. The cumulative difference is multiplied by the difference between the moisture content difference and the moisture content difference threshold in the corresponding layer unit, and then multiplied by a preset coefficient that increases sequentially from the surface layer to the center layer in the sample thickness direction to obtain the threshold correction driving value. For the stratigraphic units in the hydration lag enhancement zone, the lag hydration rate threshold and the threshold correction driving value are additively corrected, and the water content difference threshold is proportionally corrected. The corrected threshold is then introduced into the crack origin discrimination index construction process in step five.

[0033] Specifically, after completing steps one and two in the aforementioned embodiments, a uniformly cured sample with the same formulation and size as the original sample is prepared as a reference sample. The reference sample is placed in an environment with a temperature of 25°C and a relative humidity of 60% and cured for 24 hours. During the curing process, samples are taken from each layer unit along the thickness direction of the reference sample at 1-hour intervals. The change in bound water content of each layer unit in adjacent time intervals is determined by thermogravimetric analysis, and the hysteresis hydration rate is obtained by dividing the change by the time interval. At the same time, the corresponding layer unit sample is dried in a 105°C drying oven until the mass change is less than 0.1%, and its mass loss is calculated and converted into moisture content. The moisture content difference is then obtained by the difference in moisture content between adjacent layer units. The hysteresis hydration rates corresponding to the same layer unit in multiple time points and multiple reference samples are statistically processed to calculate their average value and standard deviation. The hysteresis hydration rate threshold is determined by the sum of the average value and standard deviation. The moisture content difference threshold is determined by statistically processing the moisture content difference using the same method.

[0034] In the sample processed in step two, thermogravimetric analysis was repeated at 1-hour intervals to determine the hysteresis hydration rate of each layer unit, and the moisture content was determined by drying method, followed by calculation of the moisture content difference. At each time point, the relationship between the moisture content difference and the moisture content difference threshold, as well as the relationship between the hysteresis hydration rate and the hysteresis hydration rate threshold, were compared for each layer unit. Layer units that simultaneously met the condition of "moisture content difference greater than or equal to moisture content difference threshold and hysteresis hydration rate less than hysteresis hydration rate threshold" were marked as candidate layer units. In the sample thickness direction, consecutive candidate layer units were grouped, and at least three consecutive layer units in each group were selected as the analysis objects. In this consecutive layer unit, the difference between the moisture content difference and the moisture content difference threshold of each layer unit was calculated, and the difference between the middle layer unit and its adjacent layer units was compared. When the difference corresponding to the middle layer unit was the largest, the region where the consecutive layer unit was located was determined as the hysteresis hydration enhancement section.

[0035] After identifying the hydration lag enhancement zone, the differences between the lag hydration rate and the lag hydration rate threshold, as well as the differences between the moisture content difference and the moisture content difference threshold, were calculated for each layer within that zone at the same time point. Then, layer by layer, the signs of these two differences were checked to ensure consistency, retaining only the layers with the same sign. Among the retained layers, they were arranged in the order from the surface to the center along the sample thickness direction, and the changes in the difference between the lag hydration rate and the lag hydration rate threshold were compared layer by layer, with only the layers showing an increasing trend in the difference being evaluated. The cumulative difference is obtained by summing the corresponding differences of each layer unit in sequence. After obtaining the cumulative difference, the cumulative difference is multiplied by the difference between the moisture content difference and the moisture content difference threshold in the corresponding layer unit. At the same time, according to the position of the layer unit in the thickness direction of the sample, a coefficient distribution that gradually increases from the surface layer to the center layer is preset. This coefficient is obtained by mapping the layer number to the range of 1.0 to 1.8 in a linear proportion. The product result is multiplied by the corresponding coefficient again to obtain the threshold correction driving value of each layer unit.

[0036] For strata located within the hysteresis-enhanced hydration zone, the original hysteresis hydration rate threshold is added to the corresponding strata's threshold correction driving value to obtain the corrected hysteresis hydration rate threshold. Simultaneously, a proportional relationship is established between the original water content difference threshold and the threshold correction driving value, obtained by multiplying the water content difference threshold by "1 plus the threshold correction driving value". For strata not located within the hysteresis-enhanced hydration zone, the original hysteresis hydration rate threshold and water content difference threshold remain unchanged. Finally, a correspondence is established between the corrected hysteresis hydration rate threshold and water content difference threshold and the corresponding hysteresis hydration rate difference and water content difference for the same strata, used for crack origin determination in subsequent steps.

[0037] Step 4: Acoustic emission sensors are deployed on both sides of the sample to collect signals. Valid crack signals are obtained through a selection rule. Based on the stratum wave velocity measured from the calibrated sample with the same formulation and the sensor arrival time difference, the valid crack signals are located to the corresponding stratum unit. Step 4 achieves real-time monitoring of the crack formation process by deploying acoustic emission sensors on both sides of the sample to collect signals and obtaining valid crack signals through a selection rule. Combined with the stratum wave velocity measured from the calibrated sample with the same formulation and the sensor arrival time difference, the valid crack signals can be located to the corresponding stratum unit, thereby establishing a correspondence between crack information and spatial stratum data. This allows the crack location to be matched with the hysteresis hydration rate distribution field and the water content gradient distribution field, providing a spatial positioning basis for crack origin analysis.

[0038] After locating the effective crack signal to the layer element in step four, the process includes: The start time, peak time, and end time of the effective crack signal in each layer element are obtained, and the duration and peak value of the effective crack signal are determined. The start time of the effective crack signal is compared with the time point when the water content difference in the corresponding layer unit reaches the water content difference threshold and the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold. When the time difference between the start time of the effective crack signal and the corresponding time point is not greater than the preset time window, the state in which the water content difference first reaches the water content difference threshold, the state in which the hysteresis hydration rate first reaches the hysteresis hydration rate threshold, or the state in which both reach the corresponding threshold at the same time is determined. Within the same sampling period, the number of layer elements whose time difference between the start times of effective crack signals in adjacent layer elements is no greater than a preset time window is counted. The effective crack signals are then divided into single-layer occurrence type corresponding to one layer element, cross-layer transmission type corresponding to two adjacent layer elements, and multi-layer coupling type corresponding to at least three adjacent layer elements. The division results are then incorporated into the crack cause discrimination index construction process in step five.

[0039] Specifically, taking the sample processed in steps two and three of the aforementioned embodiment as the object, four acoustic emission sensors are symmetrically arranged along the thickness direction on both sides of the sample, with a spacing of 20 mm between adjacent acoustic emission sensors, a sampling frequency of 1 MHz, and a preamplification factor of 40 dB. At the same time, a calibration sample with the same formula and size as the sample is taken, and five excitation points are preset in the thickness direction. Acoustic emission waves are excited at each excitation point using the lead core fracture method, and the propagation time from each excitation point to the acoustic emission sensors on both sides is measured. Based on this, the layer wave velocity of each layer unit is calculated. During the formal test, the sample is continuously sampled, and the effective crack signal is obtained by using the criteria of signal amplitude not less than 45 dB, duration not less than 80 μs, and count not less than 20 times. Then, based on the arrival time difference of the same effective crack signal received by the acoustic emission sensors on both sides, combined with the layer wave velocity of the corresponding layer unit, the layer unit to which the effective crack signal belongs is determined, thereby completing the process of locating the effective crack signal to the layer unit.

[0040] Secondly, after locating the effective crack signal, temporal features are extracted and a correspondence with threshold triggering is established. For each effective crack signal located in a layer unit, the sampling times corresponding to the signal start point, peak point, and end point are read from the acquired waveform, and the start time, peak time, and end time of the effective crack signal are determined respectively. The duration of the effective crack signal is determined by the difference between the end time and the start time, and the peak value is determined by the signal amplitude corresponding to the peak sampling point. Then, the records of water content difference changes and hysteresis hydration rate changes corresponding to the same layer unit in the previous embodiment are retrieved to determine the time point when the water content difference reaches the water content difference threshold and the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold in that layer unit. In this embodiment, the preset time window is 20 minutes. When the time difference between the start time of the effective crack signal and any of the above time points is no greater than 20 minutes, that time point is regarded as the threshold trigger time corresponding to the effective crack signal. If the time point when the moisture content difference reaches the moisture content difference threshold is earlier than the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold, it is determined that the moisture content difference reaches the moisture content difference threshold first. If the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold is earlier than the time point when the moisture content difference reaches the moisture content difference threshold, it is determined that the hysteresis hydration rate reaches the hysteresis hydration rate threshold first. If both fall within the same 20-minute time window, it is determined that both reach the corresponding threshold simultaneously.

[0041] After obtaining the threshold trigger state, the distribution type of the effective crack signal in the thickness direction is identified. Using a sampling period with the same start time as a certain effective crack signal as a reference, it is searched to see if there are other effective crack signals in adjacent layer elements whose start time differs from the start time of the effective crack signal by no more than 20 minutes. If only one layer element meets this condition, the effective crack signal is determined to be a single-layer occurrence type corresponding to one layer element; if two adjacent layer elements meet this condition, the effective crack signal is determined to be a cross-layer transmission type corresponding to two adjacent layer elements; if at least three adjacent layer elements meet this condition, the effective crack signal is determined to be a multi-layer coupling type corresponding to at least three adjacent layer elements. For example, in this embodiment, the effective crack signal of the third layer unit starts 160 minutes after the inversion begins, the effective crack signal of the second layer unit starts 174 minutes after the inversion begins, and the effective crack signal of the fourth layer unit starts 168 minutes after the inversion begins. The time difference between these two time points is no more than 20 minutes. Therefore, the effective crack signal of the third layer unit is determined to be of the multi-layer coupling type. However, the effective crack signal of the first layer unit starts at a time point that is greater than 20 minutes after the inversion begins, and the effective crack signal is determined to be of the single-layer occurrence type.

[0042] The timing determination results and type classification results are recorded uniformly and used as data input for the subsequent construction of the crack origin discrimination index. Specifically, for each layer unit, the start time, peak time, end time, duration, peak size, corresponding threshold trigger state, and corresponding type classification result of the effective crack signal are recorded, and a one-to-one correspondence is established with the boundary reversal driving value, double gradient phase difference, corrected hysteresis hydration rate threshold, and corrected water content difference threshold of the same layer unit in the previous embodiment. Among them, the effective crack signal that occurs simultaneously with the state where the water content difference first reaches the water content difference threshold and the multi-layer coupling type is given priority as a candidate signal driven by water migration and has cross-layer extension characteristics in the subsequent step five. The effective crack signal that occurs simultaneously with the state where the hysteresis hydration rate first reaches the hysteresis hydration rate threshold and the single-layer occurrence type is given priority as a candidate signal dominated by local hydration hysteresis, thereby ensuring that the subsequent crack origin discrimination index construction process has a clear, continuous, and traceable data source.

[0043] Step 5: Based on the hysteresis hydration rate threshold and the moisture content difference threshold, normalize the hysteresis hydration rate difference and moisture content difference of the strata corresponding to the effective crack signal, construct and classify the crack origination discrimination index, and construct a comprehensive crack resistance performance evaluation index based on the effective crack signal density, origination ratio, and stratum weight. Step 5 normalizes the hysteresis hydration rate difference and moisture content difference of the strata corresponding to the effective crack signal, making data of different dimensions and scales comparable. On this basis, constructing and classifying the crack origination discrimination index can distinguish crack types dominated by different driving factors. Further combining the effective crack signal density, origination ratio, and stratum weight to construct a comprehensive crack resistance performance evaluation index reflects the crack resistance of the sample in the thickness direction as a whole, realizing the transformation from local crack information to overall performance evaluation.

[0044] Specifically, in step five, the differences in hysteresis hydration rate and water content of the stratigraphic units corresponding to the effective crack signal are normalized, and a crack origin discrimination index is constructed. Based on the effective crack signals recorded in each layer unit of the same sample in the aforementioned embodiments, the hysteresis hydration rate difference and water content difference corresponding to the start time of the effective crack signal for each layer unit are extracted. The hysteresis hydration rate difference is obtained by subtracting the measured hysteresis hydration rate of the layer unit from the corrected hysteresis hydration rate threshold, and the water content difference is obtained by subtracting the measured water content difference of the layer unit from the corrected water content difference threshold. Then, using the maximum and minimum values ​​of the hysteresis hydration rate differences in all layer units of the same sample as benchmarks, the hysteresis hydration rate differences of each layer unit are linearly mapped to fall within the 0-1 range. The water content difference is linearly mapped in the same way. Then, the ratio of the normalized hysteresis hydration rate difference to the normalized water content difference is calculated and multiplied by the hysteresis hydration driving weight obtained in the aforementioned embodiments to obtain the initial crack initiation discrimination index for each layer unit. For example, in this embodiment, the normalized hysteresis hydration rate difference value corresponding to the third layer unit is 0.62, the normalized water content difference value is 0.48, and the hysteresis hydration driving weight of the third layer unit is 1.5. Then, the initial crack initiation discrimination index of the third layer unit is 0.62 ÷ 0.48 × 1.5.

[0045] After obtaining the initial crack origin discrimination index, the crack origin discrimination index is classified and a single-layer evaluation basis is established. Based on the initial crack induction discrimination index distribution of all stratigraphic units, the average value was taken as 1.00, and the range above and below the average value was taken as the middle interval. When the crack induction discrimination index of a certain stratigraphic unit is less than 0.80, the effective crack signal corresponding to that stratigraphic unit is determined to be the type dominated by water content difference. When the crack induction discrimination index of a certain stratigraphic unit is greater than 1.20, the effective crack signal corresponding to that stratigraphic unit is determined to be the type dominated by hysteresis hydration rate difference. When the crack induction discrimination index of a certain stratigraphic unit falls between 0.80 and 1.20, the effective crack signal corresponding to that stratigraphic unit is determined to be the type of combined action. Then, the number of effective crack signals per unit volume of each stratigraphic unit was counted, and the effective crack signal density was determined by the number of effective crack signals per cubic centimeter. Then, the proportion of the number of effective crack signals of the same type in each stratigraphic unit to the total number of effective crack signals was counted to obtain the genetic proportion. For example, if the volume of the third layer unit is 2.25 cubic centimeters and 9 effective crack signals are recorded, then the effective crack signal density of the third layer unit is 4.0 cracks / cubic centimeter; among them, 6 cracks belong to the interaction type, so the causal ratio of the interaction type in the third layer unit is 0.67.

[0046] The process of constructing the comprehensive crack resistance performance evaluation index based on step five includes: The crack initiation discrimination index and classification results of each layer unit are arranged according to the thickness direction of the sample, and the effective crack signal density and initiation ratio of the corresponding layer unit are obtained. Obtain the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value corresponding to each layer unit, and combine the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value according to the product relationship to form a multi-factor coupled influence value. Compare the multi-factor coupling influence values ​​and classification results of adjacent layer elements. When the multi-factor coupling influence values ​​of adjacent layer elements change in the same direction, the classification results are the same, and the time difference of the corresponding effective crack signal is not greater than the preset time window, it is determined to be a continuous coupling section; otherwise, it is determined to be a coupling transition section. Based on the length of the continuous coupling section and the number of coupling transition sections, the multi-factor coupling influence value is processed by segmented accumulation, and the layer weight is adjusted by setting progressively increasing layer weights from the surface layer to the center layer in the sample thickness direction to form the layer coupling evaluation value of each layer unit. The comprehensive crack resistance performance evaluation index is obtained by summing the layer coupling evaluation values ​​of each layer unit.

[0047] Multiple factors were coupled to form the layer coupling evaluation value for each layer unit. The hysteresis hydration driving weight, dual gradient phase difference, boundary reversal driving value, and threshold correction driving value corresponding to each layer unit were retrieved and combined according to a product relationship to form the multifactor coupling influence value for each layer unit. Subsequently, the crack origin discrimination index and classification results of each layer unit were arranged according to the sample thickness direction, and the consistency of the change direction of the multifactor coupling influence value and the classification results of adjacent layer units was compared. When the multifactor coupling influence values ​​of adjacent layer units simultaneously increase or decrease, and the classification results are the same, and the time difference between the occurrence of the corresponding effective crack signals is no greater than 20 m, the multifactor coupling influence value is considered the most significant factor. When the adjacent layer is in, it is classified into the same continuous coupling segment; otherwise, it is identified as a coupling transition segment. Then, based on the length of the continuous coupling segment and the number of coupling transition segments, the multi-factor coupling influence value is processed by segmented accumulation. Simultaneously, layer weights are set progressively increasing from the surface to the center layer along the sample thickness direction. In this embodiment, the layer weights of each layer are taken as 1.0, 1.2, 1.4, 1.6, and 1.8 respectively from the surface to the center layer along the sample thickness direction. The segmented accumulation result is multiplied by the corresponding layer weight to form the layer coupling evaluation value for each layer. For example, if the hysteresis hydration driving weight of the third layer is 1.5, the double gradient phase difference value is 0.60, the boundary reversal driving value is 0.71, and the threshold correction driving value is 0.019, then the multi-factor coupling influence value of the third layer is 1.5 × 0.60 × 0.71 × 0.019.

[0048] Finally, the layer coupling evaluation values ​​of each layer unit are summed to obtain the comprehensive crack resistance performance evaluation index, which is used as the output result of crack resistance performance. Specifically, the layer coupling evaluation values ​​of all layer units are summed along the sample thickness direction to obtain the comprehensive crack resistance performance evaluation index of the sample. In this embodiment, a comprehensive crack resistance performance evaluation index less than 1.50 is considered poor, between 1.50 and 3.00 is considered medium, and greater than 3.00 is considered good. In this embodiment, the layer coupling evaluation values ​​of the five layer units are 0.32, 0.58, 0.91, 0.74, and 0.41, respectively. After summing, the comprehensive crack resistance performance evaluation index is 2.96, which indicates that the crack resistance performance of the natural anhydrous gypsum-based putty sample is at a medium-to-high level. The above-mentioned comprehensive crack resistance performance evaluation index, together with the boundary reversal driving value, dual gradient phase difference value, hysteresis hydration driving weight, and effective crack signal classification results of each layer unit in the aforementioned embodiments, constitutes a complete evaluation result, thereby realizing the quantitative judgment of the crack resistance performance of natural anhydrous gypsum-based putty.

[0049] The above formulas are all dimensionless calculations. The formulas are derived from software simulations based on a large amount of collected data to obtain the most recent real-world results. The preset parameters and thresholds in the formulas are set by those skilled in the art according to the actual situation.

[0050] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for evaluating the crack resistance of natural anhydrous gypsum-based putty, characterized in that, include: Step 1: Prepare a sample of natural anhydrous gypsum-based putty. Seal the sides and bottom of the sample to prevent moisture, leaving only the top surface for moisture exchange and covering it with a moisture-permeable membrane for water release curing. After curing, divide the sample into layer units along the thickness direction and use thermogravimetric analysis to determine the increase in bound water content. Determine the hysteresis hydration rate by the increase in bound water content per unit time, and form a hysteresis hydration rate distribution field. Step 2: Remove the moisture-permeable membrane, place the upper surface of the sample in a low-humidity environment and keep the sides and bottom moisture-proof and sealed, so that the direction of water migration is reversed and a moisture content gradient distribution is formed; after reaching the preset moisture content difference threshold, apply a semi-permeable sealing layer to lock the moisture content gradient distribution field and form a coupling correspondence with the hysteresis hydration rate distribution field on the layer unit. Step 3: Using uniformly cured samples with the same formula as reference samples, obtain the delayed hydration rate and moisture content according to the same layer unit division method, and determine the delayed hydration rate threshold and moisture content difference threshold based on the statistical mean and standard deviation. Step 4: Set up acoustic emission sensors on both sides of the sample to collect signals, obtain effective crack signals through screening rules, and locate the effective crack signals to the corresponding layer units based on the layer wave velocity measured by the same formulation calibrated sample and the sensor arrival time difference. Step 5: Based on the hysteresis hydration rate threshold and the water content difference threshold, normalize the hysteresis hydration rate difference and water content difference of the stratigraphic units corresponding to the effective crack signal, construct and classify the crack origin discrimination index, and construct a comprehensive crack resistance performance evaluation index based on the effective crack signal density, origin ratio and stratigraphic weight.

2. The method for evaluating the crack resistance of natural anhydrous gypsum-based putty according to claim 1, characterized in that, After forming the delayed hydration rate distribution field in step one, the following processing steps are included: For each layer unit, during the controlled water release curing process, the delayed hydration rate and water content are obtained at preset time intervals. The delayed hydration rate difference is determined based on the change in delayed hydration rate between consecutive time points, and the water content difference is determined based on the change in water content between corresponding time points. In each layer unit, identify the time segment where the direction of change of the hysteresis hydration rate difference is consistent with the direction of change of the corresponding water content difference and the hysteresis hydration rate difference has not reached the lower limit of the reference interval of the hysteresis hydration rate difference, and determine the time segment as the hydration response hysteresis segment. Within the hydration response lag zone, the lag hydration rate differences are sorted in descending order of water content difference, and the sorted lag hydration rate differences are summed to obtain the lag hydration driving value. The ratio of the delayed hydration driving value to the average difference of delayed hydration rates in the same layer element that is not in the hydration response lag zone is calculated, and the delayed hydration driving weight is determined based on the distribution range of the ratio in all layer elements.

3. The method for evaluating the crack resistance of natural anhydrous gypsum-based putty according to claim 2, characterized in that, The process of determining the misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field on the stratigraphic unit between step one and step two further includes the following steps: After forming the hysteresis hydration rate distribution field in step one, the hysteresis hydration rates of each layer unit are sorted from largest to smallest according to the thickness direction of the sample, and the corresponding hydration sequence numbers are assigned in sequence according to the sorting results. After forming the water content gradient distribution field in step two, the layers are sorted from largest to smallest according to the difference in water content along the thickness direction, and the corresponding water content sequence numbers are assigned according to the sorting results. The hydration sequence number and the aquifer sequence number of the same stratigraphic unit are compared, and the absolute value of the difference between the two sequences is used as the sequence offset of the stratigraphic unit. The direction of change of sequential offset of adjacent layer units along the thickness direction of the sample is compared. When the sequential offset of adjacent layer units changes from increasing to decreasing or from decreasing to increasing, a cross-layer transition is recorded, and the number of cross-layer transitions corresponding to each layer unit is counted. The sequential offset of each layer unit is combined with the corresponding number of cross-layer transitions, and the initial double gradient phase difference of each layer unit is determined according to the product relationship between the sequential offset and the number of cross-layer transitions. The initial dual-gradient phase difference is introduced into the construction process of the crack origin discrimination index in step five, and is used to participate in the calculation of the combined effect of the hysteresis hydration rate difference and the water content difference.

4. The method for evaluating the crack resistance of natural anhydrous gypsum-based putty according to claim 3, characterized in that, The process of reversing the direction of water migration in step two further includes the following steps: After forming the delayed hydration rate distribution field in step one, the water content difference of each layer unit is continuously acquired at a preset fixed time interval, and the moment when the water content difference between adjacent time points changes from positive to negative or from negative to positive is determined to be the moment when the direction of water migration reverses. Before and after the determination time, the hysteresis hydration rate difference of the corresponding layer unit is obtained, and it is determined whether the hysteresis hydration rate difference maintains the same direction of change before and after the reversal. When the hysteresis hydration rate difference still maintains the direction of change before the reversal after the direction of water migration is reversed, the corresponding time segment is determined as the hydration hysteresis locking segment. Within the hydration lag locked section, the difference between the lag hydration rate and the difference in water content are matched point by point in chronological order, and the number of time points in which the difference between the lag hydration rate and the difference in water content change in opposite directions at the same time point is used as the reverse mismatch count value. The ratio of the inversion mismatch count to the total number of sampling points of the corresponding stratigraphic unit in the hydration hysteresis-locked zone is taken as the inversion mismatch ratio of that stratigraphic unit. The boundary reversal driving value of each layer element is obtained by multiplying the reversal mismatch ratio value with the difference in the hysteresis hydration rate of the corresponding layer element. The boundary reversal driving value is introduced into the crack origin discrimination index construction process in step five as a correction factor for the coupled calculation of the hysteresis hydration rate difference and the water content difference.

5. The method for evaluating the crack resistance of a natural anhydrous gypsum-based putty according to claim 4, characterized in that, After the formation of the water content gradient distribution field in step two, the process of determining the stratigraphic misalignment relationship between the hysteresis hydration rate distribution field and the water content gradient distribution field includes: For each layer unit, the time point when the delayed hydration rate first reaches the delayed hydration rate threshold in step one and the time point when the moisture content difference first reaches the moisture content difference threshold in step two are determined respectively, and the corresponding hydration sequence number and moisture content sequence number are assigned according to the sample thickness direction. The absolute value of the difference between the hydration sequence number and the aquifer sequence number of the same stratigraphic unit is determined as the stratigraphic offset value of that stratigraphic unit. The time interval between two time points in the same layer unit is compared with the total time from the start of step two to the application of the semi-permeable sealing layer on the upper surface, and the time lag value of the layer unit is determined by the proportion of the time interval to the total time. The time correction value of each layer element is obtained by multiplying the layer offset value with the time lag value. The time correction value is multiplied by the initial double gradient phase difference value to obtain the final double gradient phase difference value of each layer unit. The final dual-gradient phase difference value is incorporated into the construction process of the crack origin discrimination index in step five.

6. The method for evaluating the crack resistance of a natural anhydrous gypsum-based putty according to claim 5, characterized in that, The correction process for the delayed hydration rate threshold and water content difference threshold determined in step three includes: After obtaining the hysteresis hydration rate and moisture content difference of each layer unit at the same time interval, the layer units in which the moisture content difference reaches the moisture content difference threshold and the hysteresis hydration rate does not reach the hysteresis hydration rate threshold are identified, and at least three consecutive layer units that meet this condition are selected in the sample thickness direction; among the selected layer units, taking the middle layer unit as the reference, when the difference between the moisture content difference and the moisture content difference threshold in the middle layer unit is greater than the corresponding difference in the adjacent layer unit, the corresponding region is identified as the hysteresis hydration enhancement section; Within the hydration lag enhancement zone, the difference between the lag hydration rate and the lag hydration rate threshold, as well as the difference between the water content difference and the water content difference threshold, are obtained, and only the layer units with the same sign for the difference between the two are retained. The retained strata units are arranged from the surface to the center layer according to the sample thickness direction. Only the strata units where the difference between the hysteresis hydration rate and the hysteresis hydration rate threshold increases layer by layer are summed to obtain the cumulative difference. The cumulative difference is multiplied by the difference between the moisture content difference and the moisture content difference threshold in the corresponding layer unit, and then multiplied by a preset coefficient that increases sequentially from the surface layer to the center layer in the sample thickness direction to obtain the threshold correction driving value. For the stratigraphic units in the hydration lag enhancement zone, the hysteresis hydration rate threshold and the threshold correction driving value are additively corrected, and the water content difference threshold is proportionally corrected. The corrected hysteresis hydration rate threshold and water content difference threshold replace the original thresholds and are introduced into the calculation process of the crack origin discrimination index in step five.

7. The method for evaluating the crack resistance of a natural anhydrous gypsum-based putty according to claim 6, characterized in that, After locating the effective crack signal to the layer element in step four, the process includes: The start time, peak time, and end time of the effective crack signal in each layer element are obtained, and the duration and peak value of the effective crack signal are determined. The start time of the effective crack signal is compared with the time point when the water content difference in the corresponding layer unit reaches the water content difference threshold and the time point when the hysteresis hydration rate reaches the hysteresis hydration rate threshold. When the time difference between the start time of the effective crack signal and the corresponding time point is not greater than the preset time window, the state in which the water content difference first reaches the water content difference threshold, the state in which the hysteresis hydration rate first reaches the hysteresis hydration rate threshold, or the state in which both reach the corresponding threshold at the same time is determined. Within the same sampling period, the number of layers whose time difference between the start times of effective crack signals in adjacent layers is no greater than a preset time window is counted. The effective crack signals are then classified into single-layer occurrence type corresponding to one layer, cross-layer transmission type corresponding to two adjacent layers, and multi-layer coupling type corresponding to at least three adjacent layers. The classification results are then incorporated into the crack origin discrimination index construction process in step five.

8. The method for evaluating the crack resistance of a natural anhydrous gypsum-based putty according to claim 7, characterized in that, Step five involves constructing the comprehensive crack resistance performance evaluation index, which includes: The crack origin discrimination index and classification results of each layer unit are arranged according to the thickness direction of the sample, and the effective crack signal density and origin ratio of the corresponding layer unit are obtained. Obtain the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value corresponding to each layer unit, and combine the hysteresis hydration driving weight, double gradient phase difference, boundary inversion driving value and threshold correction driving value according to the product relationship to form a multi-factor coupled influence value. Compare the multi-factor coupling influence values ​​and classification results of adjacent layer units. When the multi-factor coupling influence values ​​of adjacent layer units change in the same direction, the classification results are the same, and the time difference between the start times of effective crack signals in adjacent layer units is not greater than the preset time window, it is determined to be a continuous coupling section; otherwise, it is determined to be a coupling transition section. Based on the length of the continuous coupling section and the number of coupling transition sections, the multi-factor coupling influence value is processed by segmented accumulation, and the layer weight is adjusted by setting progressively increasing layer weights from the surface layer to the center layer in the sample thickness direction to form the layer coupling evaluation value of each layer unit. The comprehensive crack resistance performance evaluation index is obtained by summing the layer coupling evaluation values ​​of each layer unit.

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