Device for preparing mass concrete early-stage crack test piece and testing method

Abstract

The invention relates to the field of concrete material testing and preparation, and discloses a device for preparing a mass concrete early-stage crack test piece and a testing method.The device comprises a standard test piece mold, a crack induction device, a temperature and humidity environment curing box and a strain monitoring system.The standard test piece mold is used for preparing the concrete test piece; standardized shapes and sizes required by experiments are provided, and the consistency of all test pieces is ensured. The crack induction device is arranged at the bottom of the standard test piece mold and used for artificially guiding concrete to form cracks in the early stage. The method is used for simulating the possible crack condition of mass concrete in actual construction. The strain monitoring system is installed outside the standard test piece mold and used for monitoring the strain change of the test piece in real time. Researchers are helped to track the stress change of the concrete in the hardening process, so that the position and degree of possible cracks are predicted. The temperature and humidity environment curing box provides a stable temperature and humidity environment and simulates curing conditions of concrete in an actual construction environment.

Description

Technical Field

[0001] The present invention relates to the field of concrete material testing and preparation, and in particular to a device and a testing method for preparing early crack specimens of mass concrete. Background Art

[0002] In the water conservancy and water transportation industries, the early anti-cracking performance of mass concrete has an important impact on the construction quality. During the construction stage of mass concrete, the internal and external temperature difference effect caused by the accumulation of hydration heat, combined with the rapid water loss caused by improper early maintenance (such as the failure of temperature control measures and the delay of moisture preservation coverage), is extremely likely to form surface shrinkage cracks under restraint conditions. Such early cracks, as the penetration channels of erosion media, will significantly reduce the impermeability, freeze-thaw resistance and chemical erosion resistance of the structure, and directly threaten the durability of the building throughout its life cycle.

[0003] Although the existing technologies related to the method for preparing cracked concrete specimens (such as stress induction method, pre-embedded defect method or mechanical cutting method) can generate crack patterns, there are two major limitations: First, most methods only focus on a single mechanical cracking factor and do not reproduce the real working conditions of the multi-field coupling of temperature-humidity-restraint of mass concrete, resulting in a large deviation between the crack propagation path and the microstructure and the actual project; Second, the existing devices lack the coordinated control function of the surface temperature difference gradient (simulating the heat dissipation condition of hydration heat) and the water evaporation rate (simulating the environment lacking maintenance), and it is difficult to quantitatively study the relationship between the formation mechanism of early cracks and the deterioration of durability. For example, the traditional stress induction device cannot simulate the superposition effect of temperature stress and drying shrinkage; the crack pattern formed by the pre-embedded defect method is relatively neat and smooth, but it severs the dynamic interaction relationship between the self-shrinkage performance of the material and the external environmental conditions; the mechanical cutting method violates the crack generation principle and greatly damages the internal pore structure of the concrete specimen, affecting the subsequent test results. The above technical defects make the cracked specimens prepared in the laboratory unable to accurately reflect the characteristics of early cracks caused by out-of-control maintenance in the project, and thus lead to a significant deviation between the durability evaluation results based on such specimens and the actual structure deterioration process.

[0004] Therefore, there is an urgent need for a device and a preparation method for generating early cracks in concrete specimens that can simulate the combined action of "constraint state + temperature gradient + water migration" of mass concrete, reproduce the development process of early shrinkage cracks in concrete induced by improper construction and maintenance, and provide a standardized specimen preparation technical method for studying the quantitative relationship between early crack patterns (such as crack width, crack depth, etc.) and structural durability indicators (carbonation depth, impermeability coefficient, etc.). Summary of the Invention

[0005] To solve the above technical barriers, the present invention provides a device and a test method for preparing early crack specimens of mass concrete, so as to simulate the formation of early cracks in mass concrete under the conditions of restraint - temperature gradient - moisture migration, and lay a mature technical condition for the study of the influence of early cracks formed in mass concrete due to improper construction and maintenance on the durability performance.

[0006] The device and the test method for preparing early crack specimens of mass concrete provided by the present invention adopt the following technical solutions: In the first aspect, a device for preparing early crack specimens of mass concrete is proposed, which includes a standard specimen mold providing a placement area required for testing, a crack induction device arranged at the bottom of the standard specimen mold, a strain monitoring system arranged outside the standard specimen mold, and a temperature and humidity environmental curing box providing a placement area required for the experiment of the standard specimen mold. The temperature and humidity environmental curing box houses the standard specimen mold, and the strain monitoring system and the temperature and humidity environmental curing box cooperate to monitor the standard specimen mold.

[0007] Preferably, the bottom edge and the side edges of the standard specimen mold are connected by snap fasteners.

[0008] Preferably, the cross-section of the crack induction device is set as a T-shaped cross-section or a triangular pyramid cross-section.

[0009] Preferably, the strain monitoring system includes: a curing box covering the outside of the standard specimen mold, a PTC heater arranged at the top of the curing box, high-power fans arranged on both sides of the curing box, and a temperature and humidity monitoring device fixed on the other pair of sides of the curing box. The PTC heater, the high-power fans and the temperature and humidity monitoring device realize a periodic cycle of temperature adjustment.

[0010] Preferably, the strain monitoring system includes: a microelectronic multi-channel strain detector, strain gauges pasted on the outside of the standard specimen mold for data acquisition, and wires connecting the strain gauges and the microelectronic multi-channel strain detector.

[0011] In the second aspect, a test method for a device for preparing early crack specimens of mass concrete is proposed, which includes the following steps: S1. Place the crack induction device at the bottom of the standard specimen mold, fasten all the bolt snap fasteners on the mold, apply a layer of release oil inside the standard specimen mold for standby, and prepare several molds according to the above steps; S2. Fill the concrete mixture into the standard specimen mold at one time, place it on a vibrating table and vibrate for 30 s, and then place it in a static room for 24 h; S3. Invert the standard specimen mold, open the bolt fasteners on the side and bottom, take out the bottom and the crack induction device together, and clean the excess release agent on the upper surface of the specimen. S4. Clean the mortar adhering to the outer side of the standard specimen mold, paste a total of 4 strain gauges at a height of 1 / 2 from the ground around the mold, connect the wires, and then place them in a temperature and humidity environmental curing box. At the same time, paste 1 strain gauge at the same height on the side wall of the curing box. S5. Turn on the high-power fan and PTC heater in the curing box, set the working and shutdown times of the PTC heater through the microelectronic control system to achieve a cyclic temperature rise and fall, and observe the environmental conditions in the curing box in real time through the temperature and humidity measuring device. S6. Connect all the strain gauge wires to the strain monitoring system, turn on the strain detector for continuous monitoring, regularly observe the results of the strain monitoring data. When a sudden change is found in each group of specimens, observe the surface of the specimen. If cracks occur, record the crack generation time of the specimen. S7. After continuous curing for 28 days, turn off the curing box and remove the outer mold of the specimen to prepare for the subsequent durability test.

[0012] Preferably, to ensure the production quantity of cracked specimens, the number of prepared molds should be 3 - 5 more than the test demand.

[0013] Preferably, the high-power fan can control the wind speed through the microelectronic control system, and the adjustable range is 0.5 - 3.0 m / s.

[0014] Preferably, the sampling interval time of the strain detector is set to 30 minutes. The strain monitoring system can be set to support wireless data transmission and cloud storage. When a strain mutation is detected, the system can remotely alarm for manual inspection, and the data correlation of the early cracking time and crack parameters of the concrete can be realized.

[0015] In summary, the present invention includes the following beneficial technical effects: Through the collaborative control principle of rigid mold constraint, environmental temperature rise and fall cycle control, and accelerating the moisture migration of concrete, the "temperature gradient shrinkage - drying shrinkage - restraint stress" superposition effect caused by out-of-control curing of mass concrete can be reproduced in the laboratory, solving the problem of distorted crack morphology caused by cracking in traditional methods. The key parameters such as the mold constraint stiffness and the size of the crack induction device are determined by inversion based on actual engineering cases, ensuring a high degree of coincidence between the crack characteristics of the specimen and the actual situation on the construction site.

[0016] The linkage design of the crack induction device and the strain monitoring system realizes the accurate measurement of the crack generation time and location, significantly improves the repeatability of specimen preparation. At the same time, by adjusting the cross-section type of the crack induction device and the temperature and humidity change curve, standardized crack specimens with specific widths or curvatures can be prepared in batches. Meanwhile, the specimen preparation process completely records the time-series data of temperature and humidity-strain-crack parameters, which can be directly associated with the subsequent durability test results, providing a reliable data source for establishing a mathematical model of early crack parameters and concrete durability indicators.

[0017] The temperature and humidity environmental curing box combined with the multi-channel strain monitoring system can process 12-15 groups of specimens simultaneously, and realizes one-key setting of temperature and humidity parameters through the microelectronic control system. Meanwhile, its acrylic material is convenient for test observation, which can improve the crack production accuracy and greatly save manpower and test resources compared with traditional technical methods. Description of the Drawings

[0018] Figure 1 It is the flow chart of the preparation method of the present invention; Figure 2 It is the front view of the standard specimen mold and the crack induction device in S1 of the present invention.

[0019] Figure 3 It is the top view of the standard specimen mold and the crack induction device in S1 of the present invention Figure 4 It is the schematic diagram of the installation of the standard specimen mold and the strain gauge in S4 of the present invention.

[0020] Figure 5 It is the schematic diagram of the specimen exposure process in S5 of the present invention.

[0021] Figure 6 It is the schematic diagram of two kinds of crack induction devices.

[0022] Description of the reference numerals: standard specimen mold 1, crack induction device 2, temperature and humidity environmental curing box 3, strain monitoring system 4, bolt buckle 11, curing box body 31, PTC heater 32, high-power fan 33, temperature and humidity monitoring device 34, microelectronic multi-channel strain detector 41, strain gauge 42, wire 43, concrete specimen A. Detailed Description of the Invention

[0023] The following is a further detailed description of the present invention in conjunction with the attached Figures 1 to 6 drawings.

[0024] Example 1 The embodiment of the present invention discloses a device for preparing early crack specimens of mass concrete. Refer to Figures 2 to 6, including a standard specimen mold 1, a crack induction device 2, a temperature and humidity environmental curing box 3, and a strain monitoring system 4. The standard specimen mold 1 is used to prepare concrete specimens A, providing the standardized shape and size required for the experiment to ensure the consistency of each specimen. The crack induction device 2 is arranged at the bottom of the standard specimen mold 1 to artificially induce cracks in the concrete at the early stage. This is to simulate the crack conditions that may occur in mass concrete during actual construction. The strain monitoring system 4 is installed outside the standard specimen mold 1 to monitor the strain changes of the specimen in real time. It helps researchers track the stress changes of the concrete during the hardening process, thereby predicting the possible crack positions and degrees. The temperature and humidity environmental curing box 3 provides a stable temperature and humidity environment to simulate the curing conditions of concrete in the actual construction environment. It houses the standard specimen mold 1 inside to ensure that the concrete is cured under standard conditions. Temperature and humidity control has an important impact on the hardening process of concrete, so this component is crucial for the experimental results. The curing box ensures a constant curing environment, while the strain monitoring system 4 provides data feedback. The combination of the two can accurately monitor the development process of early cracks in concrete.

[0025] In the above embodiment, further, as Figure 2 shown, the bottom edge and side edge of the standard specimen mold 1 are connected by bolt fasteners 11; the design of the bolt fasteners 11 can make the assembly and disassembly of the standard specimen mold 1 more convenient, saving time and effort. Especially when specimens need to be frequently replaced in the experiment, this method is very efficient. Traditional bolt connections may cause uneven edges or uneven pressure on the specimen mold, while the connection of bolt fasteners 11 can reduce these problems and ensure the uniformity of the specimen.

[0026] During actual use, the material of the standard specimen mold 1 is Q235 steel, with a thickness of 3 - 5 mm. The internal dimensions are the size of the standard concrete specimen, which is 150 mm * 150 mm * 150 mm. The crack induction device 2 is made of Q235 steel, with a length the same as the inner diameter size of the standard specimen mold 1, which is 150 mm, and a height of 5 mm. It is inserted into the center position of the bottom surface of the standard specimen mold 1 through screw holes.

[0027] In the above embodiment, further, the cross-section of the crack induction device 2 can be set to a T-shaped cross-section or a triangular pyramid cross-section according to the crack type.

[0028] In the present invention, the purpose of setting the crack induction device 2 is to form a local stress concentration point at the bottom of the concrete specimen A, thereby inducing cracks to form and expand from a predetermined position.

[0029] As Figure 6As shown, when the crack induction device 2 is configured as a T-shaped cross-section, its structural features include a T-shaped cross-section with a horizontal support surface at the bottom and a vertical protrusion at the top. This cross-sectional configuration enhances stress concentration at the bottom while maintaining good contact between the specimen and the mold. The resulting cracks are generally surface microcracks with a relatively gentle and stable propagation path. This method can be used when cracks are uniform and their propagation path is stable. It is recommended for simulating crack morphology dominated by surface restraining stresses and for studying early-stage microcracks caused by surface drying shrinkage and local temperature gradients.

[0030] When the crack induction device 2 is configured with a triangular pyramid cross-section, its structure features a sharp, tapered cross-section, tapering at the top and widening at the bottom. While the bottom's contact surface with the specimen is flat, the top forms a distinct, sharp-angled stress concentration point. Due to the significant geometric discontinuity at the pyramid's tip, intense localized stress concentration occurs during specimen shrinkage, which can easily lead to rapid cracking along the pyramid's tip. The resulting cracks are deep, wide, and propagate rapidly, with a concentrated crack path and strong directionality. This device is recommended for simulations involving strong local constraints or high shrinkage stress concentrations, as well as for studying deep crack formation and cracking behavior in areas of high strain concentration. It also aligns with practical engineering requirements for simulating cracking in weak structural locations (e.g., insufficient reinforcement or formwork detachment).

[0031] like Figure 5As shown in the figure, the temperature and humidity environmental curing box 3 includes: a curing box body 31, a PTC heater 32, and a high-power fan 33. The curing box body 31 is located outside the standard specimen mold 1. The curing box body 31 has strong sealing performance, which can isolate the interference of the external environment on the experimental conditions and ensure the stability of the experimental environment. It provides a closed space for controlling the temperature and humidity for the standard specimen mold 1, enabling the concrete to be cured under predetermined conditions. The PTC heater 32 is arranged at the top of the curing box body 31. The PTC heater 32 (positive temperature coefficient PTC heater 32) can automatically adjust the heating power according to the change of the environmental temperature to ensure that the temperature inside the box is within the required range. It is used to increase or maintain the temperature inside the curing box body 31. Especially when conducting experiments in a lower temperature environment, it can ensure temperature stability and prevent the concrete from being incompletely cured or having cracks due to too low temperature. The high-power fans 33 are arranged on both sides of the curing box body 31; the high-power fans 33 can enhance the air circulation to ensure the uniform distribution of temperature and humidity inside the curing box body 31, avoiding excessive local temperature difference or humidity difference, which may lead to uneven hardening process of the concrete. The temperature and humidity monitoring device 34 is fixed on the other pair of sides of the curing box body 31; the monitoring device real-time monitors the temperature and humidity data inside the curing box and controls the working states of the PTC heater 32 and the fan through a feedback system to ensure that the curing environment remains within the set standard range. The temperature adjustment cycles of the PTC heater 32, the high-power fans 33, and the temperature and humidity monitoring device 34. Through periodic adjustment and cycling, a stable temperature and humidity environment is maintained. The periodic cycling of temperature adjustment can make the curing process more in line with the temperature and humidity fluctuations in the actual construction environment. Through the periodic adjustment of these devices, the temperature and humidity changes that the mass concrete may encounter at different time periods are simulated to ensure the accuracy and reliability of the experimental results.

[0032] In the above embodiment, further, the main body of the temperature and humidity environmental curing box 3 is made of acrylic material, with a length, width, and height of 1m * 1m * 0.5m. The PTC heater 32 is installed on the top. The PTC heater 32 is equipped with a microelectronic control system that can be used to automatically control the periodic change of the environmental temperature inside the curing box. A total of 4 high-power fans 33 with a diameter of 400mm are arranged on one pair of sides to accelerate the water loss inside the curing box. A temperature and humidity monitoring device 34 is arranged at a position 200mm from the bottom on one side of the other pair of sides to real-time monitor the environmental temperature and humidity inside the curing box.

[0033] The change of environmental temperature and humidity has a significant impact on the formation of early cracks in concrete. In the initial stage of hardening (usually within 7 days after pouring), due to the combined action of chemical reactions (hydration heat), volume changes, and external environmental factors, concrete is prone to cracking. Inside the concrete, expansion occurs due to the hydration heat release process, while on the surface layer of the concrete, shrinkage occurs due to the lower temperature, generating tensile stress. When the tensile stress exceeds the tensile strength of the concrete, cracks will occur.

[0034] When the ambient temperature is inconsistent with the hydration exotherm within the concrete, the formation and expansion of surface cracks will be accelerated. In a dry environment, the air humidity is low, and the evaporation rate of water from the concrete surface is accelerated. This rapid evaporation of surface water increases capillary tension, causing plastic shrinkage of the concrete. Plastic shrinkage cracks are the most common early cracking type and are particularly prone to occur in hot, dry, and windy environments. In environments with frequent temperature and humidity fluctuations, concrete undergoes multiple dry-wet cycles. A single dry-wet cycle can cause repeated volume contraction and expansion. This repeated contraction and expansion leads to internal stress accumulation, forming microcracks that gradually expand.

[0035] Cycling ambient temperature and humidity is primarily used to simulate the exposure of large-volume concrete to construction conditions. In high-temperature, low-humidity environments (such as sunny summer days), the dual effects of temperature and humidity can significantly exacerbate the formation of early cracks: high temperature accelerates hydration reactions, leading to rapid internal temperature rise and the formation of thermal stresses. Simultaneously, rapid surface moisture evaporation exacerbates shrinkage and creates shrinkage stresses. The combined effects of these factors increase tensile stress on the concrete surface, accelerating the formation of early cracks.

[0036] like Figure 4 As shown, the strain monitoring system 4 comprises a microelectronic multi-channel strain detector 41, strain gauges 42, and wires 43. The microelectronic multi-channel strain detector 41 is responsible for simultaneously monitoring data from multiple strain gauges 42. By simultaneously receiving data from different strain gauges 42 through multiple channels, the microelectronic multi-channel strain detector 41 can provide a more comprehensive stress distribution map, helping to analyze stress changes in different locations of concrete. Strain gauges 42 are sensitive components. Affixed to key locations on the outside of the specimen mold, they can directly sense minute deformations of concrete specimen A during the hardening process. The main task of the strain gauges 42 is to convert strain signals generated on or within the concrete surface into electrical signals and transmit these signals to the strain detector. Based on this data, researchers can accurately assess the stress state of concrete under different environmental conditions, as well as the initiation and propagation of cracks. Wires 43 are used to transmit the signals collected by the strain gauges 42 to the microelectronic multi-channel strain detector 41.

[0037] In the above embodiment, further, the strain gauge 42 is installed on the outer side of the standard specimen mold 1, and the installation height is located at 1 / 2 of the bottom surface. Four strain gauges 42 are pasted on each standard mold, and one temperature compensation sheet is pasted on the side wall of the environmental curing box.

[0038] During actual use, four strain gauges 42 are symmetrically pasted at the half-height of the outer side of each standard concrete specimen mold 1; one temperature-compensated strain gauge 42 is set at the same height on the side wall of the temperature and humidity environmental maintenance box 3 to eliminate the influence of environmental temperature changes on strain measurement. The strain monitoring system 4 uses a multi-channel microelectronic strain detector; the sampling frequency is set to once every 30 minutes, and the sampling frequency can be appropriately increased during the critical cracking stage (such as before the strain mutation); it supports wireless data transmission and cloud storage, and can remotely monitor and provide real-time feedback of data.

[0039] When the strain data of a certain specimen shows a mutation (such as a decrease exceeding 50 με), the system automatically triggers a remote alarm prompt to remind the experimenter to conduct manual observation. The experimenter observes the surface of the specimen, confirms whether cracks are formed, and records the cracking time.

[0040] Since the strain gauges 42 are arranged at symmetrical positions on the four sides of the specimen, when the strain value at a specific position undergoes a significant mutation, it can be determined that the crack appears near the position measured by the strain gauge 42; at the same time, the crack induction device 2 is set at the center of the bottom of the specimen, and the formation direction and position of the crack can be further located through the correspondence between the specimen cracking path and the strain mutation position.

[0041] During the crack formation process, the concrete is jointly affected by temperature stress, drying shrinkage stress, and restraint stress. The changes in these stresses will be reflected as a non-linear change trend in the strain curve; when the crack occurs, the strain drops sharply because the local cracking releases the accumulated tensile strain.

[0042] Example 2 Based on Example 1, a test method for a device for preparing early crack specimens of mass concrete is proposed. The early crack standard specimen of concrete (T-shaped induction) includes the following steps: S1. Place the T-shaped cross-section crack induction device 2 at the center of the bottom surface of the standard specimen mold 1, fasten all bolt buckles 11 (see Figure 2 ), apply a layer of release agent inside the standard specimen mold 1 for standby, and prepare 6 molds according to the above steps (3 for durability test requirements and 3 as redundancy).

[0043] S2. Prepare the concrete mixture according to the C3o mass concrete mix ratio (the mix ratio is shown in Table 1), and load it into all standard specimen molds 1 at one time. After vibrating on the vibrating table for 30 s, place it in the static room for 24 h; Table 1 C30 mass concrete mix ratio (unit: kg / m 3 ) S3. Invert the standard specimen mold 1, open the bolt fasteners 11 on the side and bottom, take out the bottom of the mold and the crack induction device 2 together, and clean the excess release agent on the upper surface of the specimen.

[0044] S4. Clean the mortar attached to the outside of the standard specimen mold 1. Paste 4 strain gauges 42 at a height of 1 / 2 from the ground around each mold (see Figure 3 ), connect the wires 43, and place them in the temperature and humidity environmental curing box 3. At the same time, paste 1 temperature compensation gauge 42 at the same height on the side wall of the curing box, for a total of 19 strain gauges.

[0045] S5. Turn on the PTC heater 32 and the high-power fan 33 in the curing box 3. Set the temperature cycle curve through the microelectronic control system: the heating rate is 5 °C / h to 50 °C, keep warm for 2 h, then cool down to 25 °C at a rate of 3 °C / h, and the cycle period is 12 h. Set the wind speed to 1.5 m / s, record the temperature and humidity in the box in real time, control the temperature fluctuation within ±1 °C, and the humidity control range is 30% - 60% (see Figure 4 ); S6. Connect all the wires 43 to the strain monitoring system 4, turn on the strain detector for continuous monitoring, set the sampling interval to 30 min, and trigger a remote alarm when a strain mutation (exceeding the baseline value by 50 με) is detected. Manually check the surface of the specimen and record the crack generation time and initial form; S7. After continuously curing for 28 d, turn off the curing box 3, remove the standard specimen mold 1, and select 3 groups of specimens with similar crack widths and depths from the 6 groups of cured crack standard specimens for rapid carbonation testing (the testing method refers to the rapid carbonation test steps in GB / T50082).

[0046] Example 3 Based on Example 1, a test method for a device for preparing early crack specimens of mass concrete is proposed. The early crack standard specimen of concrete (triangle induction) includes the following steps: S1. Select a crack induction device 2 with a triangular pyramid cross-section and place it at the center of the bottom surface of the standard specimen mold 1; the remaining steps are the same as those in Example 1.

[0047] The purpose of setting Example 3 is to select different types of crack induction devices to test the conclusion that the present invention induces different types or forms of cracks based on different research environments using two cross-section forms.

[0048] Example 4 Environmental gradient early crack standard specimen (T-shaped induction) The difference from Example 2 is as follows: S5. Adjust the heating rate of the PTC heater 32 to 8 °C / h and increase the wind speed of the high-power fan 33 to 2.5 m / s.

[0049] The remaining steps are the same as those in Example 2.

[0050] The purpose of setting Example 4 is to verify the conclusion that increasing the temperature gradient and the moisture loss rate can accelerate the crack induction rate by adjusting the heating rate of the heater and the fan speed to set different environmental temperature and humidity change gradients.

[0051] Example 5 Based on Example 1, a test method for a device for preparing early crack specimens of mass concrete is proposed. Different from Examples 2 to 4, the crack induction device 2 is not used, and a total of 6 groups of standard specimen molds 1 are directly installed.

[0052] The purpose of setting Comparative Example 1 is to demonstrate that the use of the crack induction device 2 in the present invention can concentrate the constraint stress and temperature stress, the generated crack positions are relatively concentrated, and the formed crack standard specimens have good uniformity, and relatively stable data results can be obtained in the durability test.

[0053] Comparative Example 2: Unconstrained early crack standard specimens of concrete The difference from Example 2 is as follows: S3. After the static setting is completed, invert the standard specimen, remove all the specimen mold 1 and the crack induction device 2, and clean the excess release agent on the upper surface of the specimen; do not paste strain gauges on Specimen A, and directly place it in the temperature and humidity environment curing box 3.

[0054] The purpose of setting Comparative Example 2 is to enable the specimen mold 1 to reflect the rigid constraint stress during the exposure process, which has a promoting effect on the generation of early cracks in concrete.

[0055] Comparative Example 3: Standard cured concrete specimens The difference from Example 2 is as follows: Use 3 cube standard molds widely used in the current technology, fill them with the concrete mixture in Example 2, demold after static setting for 24 h, place them in the standard curing room, and conduct the same rapid carbonation test as in Example 1 after curing for 28 d.

[0056] The purpose of setting Comparative Example 3 is to prepare standard cured specimens without cracks (establish a blank control group), and to compare and analyze the influence of early cracks in concrete on its durability by measuring the rapid carbonation depth of each example.

[0057] The initial cracking age, crack width, crack depth, and carbonation depth were measured for the concrete specimens from each example, and the results are presented as average values. The initial cracking age is based on the average time interval from the time of exposure to the time when the strain monitoring data of each specimen abruptly changed. Crack width and depth were measured using a concrete ultrasonic detector after the specimens were cured. Carbonation depth was measured according to the measurement procedures for the rapid carbonation test in GB / T50082.

[0058] In summary, the crack parameters and carbonization test results of the embodiment specimens are shown in Table 2.

[0059] Table 2 Performance results of the examples In the results in Table 2, Comparative Example 2 has irregular cracking locations and numbers, the cracking age cannot be calculated, and the crack width and depth vary widely, so a measurement range is given; Comparative Example 3 is a standard curing specimen with no cracking.

[0060] It can be seen from the data in the table that the comparison between Example 2 and Example 3 verifies that the present invention can achieve the ability to regulate the crack morphology by changing the cross-sectional shape of the crack inducing device, and at the same time reflects the significant influence of crack morphology on CO2 penetration; the comparison between Example 2 and Example 4 verifies that the present invention can regulate the crack formation rate by changing the environmental conditions, and at the same time further reflects the significant influence of crack morphology on CO2 penetration; the results of Example 2 and Comparative Example 1 verify the acceleration behavior of the induced cracking device on the induction of early cracks in concrete; the results of Example 2 and Comparative Example 2 verify the stability of the rigid constraint of the device on the development of early cracks; the results of Examples 2 and 3 and Comparative Example 3 reflect the adverse effects of early cracks in concrete on durability, and further verify the advanced nature of the technology of the present invention.

[0061] Finally, a few points should be explained: First, in the description of the present invention, it should be noted that, unless otherwise specified or limited, the terms "mounted", "connected", and "connected" should be understood in a broad sense, and may refer to mechanical or electrical connections, internal communication between two components, or direct connection. "Up", "down", "left", "right", etc. are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship may change. Secondly: The drawings of the embodiments disclosed in the present invention only involve structures related to the embodiments disclosed in the present invention. Other structures may refer to conventional designs. The same embodiment and different embodiments of the present invention may be combined with each other without conflict. Finally: The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0062] The above are all preferred embodiments of the present invention, and the protection scope of the present invention is not limited thereby. Therefore, all equivalent changes made according to the structure, shape and principle of the present invention shall be covered within the protection scope of the present invention.

Claims

1. An apparatus for preparing specimens of early cracks in mass concrete, characterized in that, include: Standard specimen mold, providing the placement area required for testing; A crack inducing device is provided at the bottom of the standard specimen mold; A strain monitoring system is provided outside the standard specimen mold; A temperature and humidity environment curing box, providing the required placement area for the standard specimen mold test; A standard specimen mold is housed inside the temperature and humidity environment curing box, and the strain monitoring system and the temperature and humidity environment curing box cooperate to monitor the standard specimen mold.

2. The device for preparing early crack specimens of mass concrete according to claim 1, characterized in that, The bottom edge and side edges of the standard specimen mold are connected by snap fasteners.

3. A device for preparing early crack specimens of mass concrete according to claim 1, characterized in that The cross section of the crack inducing device is set to be a T-shaped cross section or a triangular pyramid cross section.

4. A device for preparing early crack specimens of mass concrete according to claim 1, characterized in that, The strain monitoring system comprises: A curing box, which is arranged outside the standard specimen mold; A PTC heater is provided at the top of the curing box; High-power fans are arranged on both sides of the maintenance box; A temperature and humidity monitoring device is fixed on the other opposite side of the curing box; The PTC heater, high-power fan and temperature and humidity monitoring device realize temperature regulation cycle circulation.

5. A device for preparing early crack specimens of mass concrete according to claim 1, characterized in that, The strain monitoring system comprises: Microelectronic multi-channel strain detector; A strain gauge is attached to the outside of the standard specimen mold and is used for data collection; A wire is connected between the strain gauge and the microelectronic multi-channel strain detector.

6. A testing method for a device for preparing early crack specimens of mass concrete according to any one of claims 1-5, characterized in that, The following steps are involved: S1. Place the crack induction device at the bottom of the standard specimen mold, fasten all bolts and buckles on the mold, apply a layer of mold release oil inside the standard specimen mold, and prepare several molds according to the above steps. S2. Pour the concrete mixture into the standard specimen mold at one time, vibrate on a vibration table for 30 seconds, and then place it in a static room for 24 hours; S3. Invert the standard specimen mold, open the side and bottom bolts, remove the bottom and crack inducing device together, and clean the excess release agent on the upper surface of the specimen; S4. Clean the mortar attached to the outer surface of the standard specimen mold, affix four strain gauges at half the height of the ground on all four sides, connect the wires, and place it in a temperature and humidity environment curing box. At the same time, affix one strain gauge at the same height on the side wall of the curing box. S5. Turn on the high-power fan and PTC heater in the curing box, set the working and shut-off time of the PTC heater through the microelectronic control system to achieve a temperature rise and fall cycle, and observe the environmental conditions in the curing box in real time through the temperature and humidity measuring device; S6. Connect all strain gauge wires to the strain monitoring system, turn on the strain detector for continuous monitoring, and regularly observe the strain monitoring data. When a sudden change is found in each group of specimens, observe the specimen surface. If cracks occur, record the time when the cracks occur in the specimens. S7. After continuous curing for 28 days, close the curing box and remove the outer mold of the specimen to prepare for subsequent durability tests.

7. The testing method of an apparatus for preparing specimens of early cracks in mass concrete according to claim 6, characterized in that, In order to ensure the number of crack specimens produced, the number of prepared molds should be 3 to 5 more than the test requirements.

8. A test method for an apparatus for preparing specimens of early cracks in mass concrete according to claim 6, characterized in that, The high-power fan can control the wind speed through a microelectronic control system, and the adjustable range is 0.5 to 3.0 m / s.

9. A test method for an apparatus for preparing early crack specimens of mass concrete according to claim 6, characterized in that, The sampling interval time of the strain detector is set to 30 minutes. The strain monitoring system can be set to support wireless data transmission and cloud storage. When a strain mutation is detected, the system can give a remote alarm prompt for manual inspection, and the data correlation of the early cracking time and crack parameters of the concrete can be realized.

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