Cement-based material shrinkage stress strain testing device and testing method

Abstract

The invention discloses a cement-based material shrinkage stress-strain testing device and a testing method. The testing device comprises an axial constraint test rack, a displacement sensor, an end part loading device and a stress-strain measuring instrument, the axial constraint test rack comprises a first support fixed at one end of the stand column and a second support arranged on the stand column in a sliding manner and parallel to the first support, and a test piece is mounted between the first support and the second support; the displacement sensor is mounted on the test piece and used for monitoring the shrinkage deformation value of the test piece; the end loading device is used for driving the second support to slide on the stand column so as to provide preset axial constraining force. And the stress-strain measuring instrument is used for collecting the shrinkage deformation value and the tensile force / pressure value of the test piece in real time so as to test the stress and strain change of the test piece in the shrinkage process in real time. The method can simultaneously represent the shrinkage stress and strain of the axially constrained cement-based material under the dry condition, and has the advantages of real-time monitoring, simple operation, accurate measurement and the like.

Description

Technical Field

[0001] The invention relates to the technical field of cement-based material shrinkage performance testing devices, and in particular to a cement-based material shrinkage stress and strain testing device and a testing method, which are used to measure the drying shrinkage stress and strain of cement-based materials under axial constraint. Background Art

[0002] In recent years, with the increasing requirements for the construction quality of cement-based materials and the increasingly complex application environment, researchers have paid more and more attention to the cracking problem of cement-based materials. In the early stage of pouring, the tensile strength of cement-based materials is low, and cracks are easily generated on the surface due to the influence of various factors such as hydration heat, load, temperature and humidity changes. Early cracks will not only affect the appearance quality of the structure, but also the durability of the structure.

[0003] Studies have shown that 70% of cracks are caused by non-load factors, especially in the early stages, when the cracks are basically caused by shrinkage. The shrinkage of cement-based materials mainly includes plastic shrinkage, autogenous shrinkage, drying shrinkage, temperature drop shrinkage, carbonization shrinkage and chemical shrinkage. Among them, plastic shrinkage mainly occurs within a few hours after pouring. During this stage, the surface of cement-based materials loses water at a faster rate, and the uneven humidity field causes shrinkage deformation. Due to its short development time, it is not considered separately. Temperature drop shrinkage refers to the shrinkage caused by temperature reduction. When shrinkage tests are carried out in standard laboratories, the temperature is usually kept relatively constant, and the influence of temperature drop shrinkage is small, which can be ignored in early-stage shrinkage. Carbonation shrinkage refers to the process of volume reduction caused by the reaction of CO2 in the air with Ca(OH)2 in cement to generate CaCO3. This process usually occurs gradually during the service life of the material, develops relatively slowly, and is not obvious in the early stage, so it can be ignored in early-stage shrinkage. Chemical shrinkage is the shrinkage of cement caused by the volume of the product being smaller than the volume of the reactant during the hydration reaction. This shrinkage can usually be reflected by measuring autogenous shrinkage.

[0004] Drying shrinkage is an important factor leading to shrinkage cracking of cement-based materials, accounting for the majority of non-load cracks. In addition, the influence of constraint conditions on the cracking of cement-based materials cannot be ignored. The shrinkage of cement-based materials under constraint conditions will lead to the accumulation of internal stress. When the shrinkage stress exceeds the tensile strength of the material, it will lead to the generation and expansion of cracks. In summary, accurate measurement and analysis of the shrinkage stress and strain of cement-based materials under constraint conditions is the key to controlling early shrinkage cracks.

[0005] Common constrained shrinkage test methods include the flat plate method, the ring method, and the axial method. Among them, the flat plate method can only partially and unevenly constrain the shrinkage deformation of cement-based materials, the cracks produced are irregular, and the shrinkage stress and strain cannot be simultaneously characterized. During the test of the ring method, the cement-based material is in contact with the inner ring, and this contact surface cannot be used as a dry surface. Therefore, the ring method cannot simulate the shrinkage process of the component in a completely dry state. The traditional cracking test frame used in the axial constraint method is expensive, the casting and demolding process is cumbersome, and the specimen is tested with a mold during the test, which cannot accurately characterize the process of complete drying shrinkage. Summary of the invention

[0006] In view of the shortcomings of the prior art, the present invention provides a cement-based material shrinkage stress and strain testing device and testing method with simple operation, low cost and high measurement accuracy. The device can simultaneously characterize the early-age drying shrinkage stress and strain of cement-based materials under axial constraint, and provide reference data for the study of crack resistance of cement-based materials.

[0007] The invention discloses a cement-based material shrinkage stress and strain testing device, comprising: an axial constraint test frame, a displacement sensor, an end loading device and a stress and strain measuring instrument;

[0008] The axial constraint test frame comprises a plurality of parallel columns and a first support, a second support and a loading plate arranged in parallel, one end of the plurality of columns is fixed on the first support, and the other end passes through the second support and is fixed on the loading plate, and the second support can slide on the columns; grooves for clamping the end of the cement-based test piece are provided on the opposite surfaces of the first support and the second support, and the cement-based test piece is installed between the first support and the second support and the cement-based test piece is parallel to the columns;

[0009] The displacement sensor is installed on the cement-based test piece and is used to monitor the shrinkage deformation value of the cement-based test piece in real time;

[0010] The end loading device is mounted on the loading plate and acts on the second support, and is used to drive the second support to slide on the column to provide a preset axial restraining force;

[0011] The stress-strain measuring instrument is connected to the displacement sensor and the end loading device, and is used to collect the shrinkage deformation value and the tension / pressure value of the cement-based test piece in real time, so as to test the stress and strain changes of the cement-based test piece in the shrinkage process in real time.

[0012] Furthermore, the axial constraint test frame also includes a bottom plate, which is mounted on a supporting surface on the first support, and the supporting surface and the groove are located at two corresponding surfaces.

[0013] Furthermore, the material of the axial constraint test frame is metal, preferably carbon steel, stainless steel, and aluminum alloy. The bottom of the four metal columns is connected to the first support by bolts, and the top is connected to the loading plate by bolts. The second support can move axially along the metal column, and the bottom plate is connected to the first support by fixing bolts to ensure that the two do not move relative to each other. Furthermore, the axial constraint test frame is placed vertically when in use, that is, the bottom plate, the first support, the second support, the loading plate and the end loading device are arranged in sequence from bottom to top, and the second support is slidably arranged on the four vertically arranged metal columns; further, the diameter of the metal column is 10mm, the height is 1000mm, the dimensions of the first support and the second support are both 250 (length) × 150 (width) × 120 (height) mm, and the dimensions of the bottom plate are 300 (length) × 200 (width) × 40 (height) mm.

[0014] Furthermore, the cement-based test piece is formed in a dog-bone-shaped mold, and the dog-bone-shaped mold includes a detachable side mold, a bottom mold and an end mold; when the cement-based test piece reaches the final setting time point in the dog-bone-shaped mold, the side mold and the bottom mold are removed, and the cement-based test piece with the end mold at the end is installed in the dog-bone-shaped groove of the first support and the second support; during the test, the specimen is exposed to the air on all sides, and the shrinkage process of the axially constrained specimen in a dry state in an actual environment can be simulated by adjusting the ambient temperature and humidity; in addition, the disassembly of the mold can realize the measurement of drying shrinkage of the specimen at an early age, which is convenient for studying the influence of early-age drying shrinkage on the crack resistance of cement-based materials.

[0015] Furthermore, the material of the dog-bone-shaped mold can be one of the lightweight and high-strength materials such as aluminum alloy, magnesium alloy, titanium alloy, carbon fiber composite material, etc., preferably an aluminum alloy mold; lightweight and high-strength materials have certain strength and rigidity, which can ensure that they will not deform during the casting and demolding of the specimen, and ensure the integrity of the specimen shape. In addition, the lightweight and high-strength material has a small mass, which can avoid the phenomenon of fracture at the end cross-section due to the large deadweight of the two ends of the specimen during the demolding process. Furthermore, the overall lateral width of the dog-bone-shaped mold is 80mm, the lateral width of the middle straight section is 40mm, the overall longitudinal length is 600mm, and the height is 30mm.

[0016] Furthermore, the dog-bone-shaped groove is a through groove structure along the horizontal direction of the first support and the second support. When it is fixed to the end of the cement-based test piece, the cement-based test piece is placed into the groove from one side of the through groove, and after fixing, the axis of the cement-based test piece is parallel to the column.

[0017] Furthermore, the displacement sensor includes one of a linear variable differential transformer (LVDT), an eddy current sensor, a laser displacement sensor, and a capacitive displacement sensor, and the measurement accuracy is ±0.001mm; further, the displacement sensor is installed on the left and right sides of the cement-based test piece to measure the shrinkage deformation value of the cement-based material sample in a dry state, and continuous measurement can be achieved.

[0018] Furthermore, the end loading device includes an end loading mechanism, which can accurately apply load and maintain stability. The device is durable and can withstand long-term test operations; the end loading mechanism includes a support rod and a screw rod, the screw rod is placed in a sleeve and is threadedly connected to the sleeve, the sleeve is mounted on the loading plate, and the support rod is mounted on the sleeve to drive the sleeve to rotate on the loading plate; the other end of the screw rod is mounted on the second support, and based on the rotation of the sleeve, the screw rod is driven to move along the axial direction to drive the second support to slide on the column.

[0019] Furthermore, the end loading mechanism adopts a rotational loading method to load the specimen. Specifically, the support rod is rotated clockwise to apply tension to the specimen, and the support rod is rotated counterclockwise to apply pressure to the specimen. The setting of the initial tension / pressure value can be determined according to the test needs and actual conditions. The loading range of the end loading mechanism is 0 to 300 kN, and the loading accuracy is ±0.001 kN.

[0020] Furthermore, the end loading device also includes a force sensor, which is connected to the stress strain meter; the force sensor is used to monitor the initial tension / pressure value applied by the end loading mechanism to the cement-based test piece and to monitor the tension / pressure value of the cement-based test piece at any time during the shrinkage process.

[0021] Furthermore, the stress strain meter has high precision and high sensitivity, and can monitor and record the force and displacement changes during the test in real time; the sensor is stable and reliable, and can maintain accurate measurement under the environmental conditions required for the test; the stress strain meter can collect the tension / pressure value and displacement value of the specimen in real time, with a collection interval of 10 minutes / time, and the collected data is transmitted to a computer or data recording device by wireless or wired means for data recording and analysis.

[0022] The present invention also discloses a method for testing shrinkage stress and strain of cement-based materials, comprising:

[0023] pouring cement-based materials into the dog-bone mold, and removing the side mold and the bottom mold of the dog-bone mold after the cement-based materials are finally set;

[0024] The cement-based test piece with the end mold is placed on an axially constrained test frame;

[0025] One end of the cement-based test piece is fully constrained, and the other end is subjected to axial constraint force through the end loading device. The displacement sensors are fixed on both sides of the specimen. The drying speed of the specimen is controlled by adjusting the ambient temperature and humidity to simulate the shrinkage and deformation behavior of the specimen in the actual environment.

[0026] The stress and strain measuring instrument is used to monitor and record the data of the stress and strain changes of the specimen over time in real time until the specimen dries and shrinks under the constraint conditions and causes fracture, and the cracking stress and strain are recorded.

[0027] Specific testing methods include:

[0028] Step 1: Apply lubricant to the inner wall of the dog-bone mold and pour the stirred cement-based material into the dog-bone mold;

[0029] Step 2: After the cement-based material reaches the initial setting time, carefully remove the side mold and bottom mold of the dog-bone mold, keep the position of the specimen unchanged, and avoid disturbing the specimen;

[0030] Step 3, inserting the two ends of the cement-based test piece with the end mold into the grooves on the opposite surfaces of the first support and the second support to complete the installation of the cement-based test piece; one end of the cement-based test piece is fully constrained, and the other end is subjected to axial constraint force by an end loading device to simulate the constraint conditions under actual working conditions;

[0031] Step 4: Install the displacement sensors on the left and right sides of the specimen, record the data after leaving it in the required test environment for 4 hours, and take the average value of the sensors on the left and right sides as the initial reading;

[0032] Step 5: Place the test device and the test piece in the environment required by the test to cause the test piece to shrink and deform;

[0033] Step 6: During the drying process of the specimen, a stress-strain measuring instrument is used to monitor and record the tension / pressure value and displacement change of the specimen in real time;

[0034] Step 7. Continue to record stress and strain data until the specimen cracks or reaches the required age.

[0035] Further,

[0036] The calculation formula for shrinkage strain is:

[0037]

[0038] In the formula, ε s (t) is the shrinkage strain of cement-based material at age t, l0 is the initial length reading of the specimen, l t is the length reading of the specimen measured at time t, l bis the measuring gauge length of the specimen;

[0039] The calculation formula for shrinkage stress is:

[0040]

[0041] In the formula, σ s (t) is the shrinkage stress of cement-based material at age t, F0 is the initial tension / compression reading of the specimen, F t is the tensile force / compression reading of the specimen at time t, A s is the cross-sectional area of ​​the specimen.

[0042] Compared with the prior art, the present invention has the following beneficial effects:

[0043] 1. The testing device of the present invention can simultaneously measure the shrinkage stress and strain of the specimen at a specific age, thereby comprehensively evaluating the shrinkage characteristics of cement-based materials in different time periods;

[0044] 2. During the test, the cement-based material together with the end test mold is fixed on the axial constraint frame, and the end test mold can effectively protect the cement-based material from damage caused by the end constraint clamping;

[0045] 3. After casting, the specimen is placed upright with the four sides fully exposed to the environment, which can effectively simulate the actual process of drying shrinkage; when only autogenous shrinkage test is required, the surface of the specimen can be wrapped with tin foil to isolate it from environmental influences;

[0046] 4. The testing device of the present invention can accurately evaluate the influence of constraint conditions on the drying shrinkage stress and strain of cement-based materials, reveal the relationship between shrinkage stress and strain of cement-based materials under constraint conditions, and can also effectively evaluate the crack resistance of cement-based materials with different technical solutions;

[0047] 5. The testing device of the present invention is small in size, significantly saves space, is easy to operate, and is suitable for a variety of application environments in laboratories and on-site. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 It is a front view of the shrinkage stress and strain testing device for cement-based materials disclosed in the present invention;

[0049] Figure 2 for Figure 1 Schematic diagram of the central axial restraint frame;

[0050] Figure 3 for Figure 2 A top view of

[0051] Figure 4 for Figure 1 Schematic diagram of the end loading device;

[0052] Figure 5 for Figure 4 A top view of

[0053] Figure 6 A schematic diagram of a dog-bone-shaped mold disclosed in the present invention;

[0054] Figure 7 for Figure 6 Side view of

[0055] Figure 8 This is a curve diagram of the constrained shrinkage stress and strain of the test piece of the present invention changing with age.

[0056] In the figure:

[0057] 10. Axial constraint test frame; 11. Column; 12. First support; 13. Second support; 14. Loading plate; 15. Bottom plate; 16. Dog-bone groove; 17. Bolt; 20. Displacement sensor; 30. End loading device; 31. Support rod; 32. Screw rod; 40. Dog-bone mold; 41. Side mold; 42. Bottom mold; 43. End mold; 50. Stress-strain meter; 60. Cement-based test piece. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0059] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0060] like Figures 1 to 7 As shown, the present invention provides a cement-based material shrinkage stress and strain testing device, comprising: an axial constraint test frame 10, a displacement sensor 20, an end loading device 30, a dog-bone mold 40 and a stress and strain measuring instrument 50; wherein,

[0061] like Figures 1 to 3As shown, the material of the axial constraint test frame 10 of the present invention is stainless steel, including four parallel metal columns 11 and a first support 12, a second support 13 and a loading plate 14 arranged in parallel. One end of the column 11 is fixed to the first support 12 by a bolt 17, and the other end passes through the second support 13 and is fixed to the loading plate 14 by a bolt 17. The bottom plate 15 is fixed to the bottom of the first support 12 by a bolt 17, and the second support 13 can slide on the column 11 to adjust the height; dog-bone-shaped grooves 16 for clamping the ends of cement-based test pieces are provided on the opposite surfaces of the first support 12 and the second support 13, so as to facilitate the clamping and fixing of the test pieces; the column, the support and the bottom plate form a stable frame, and each part is evenly stressed to avoid test errors.

[0062] like Figure 1 , 6 As shown in Figures 1 to 7, the cement-based test piece 60 of the present invention is installed between the first support 12 and the second support 13, and the cement-based test piece 60 is parallel to the column 11; the specific installation method is: the cement-based test piece 60 is formed in a dog-bone-shaped mold 40, and the dog-bone-shaped mold 40 is made of aluminum alloy, which includes a detachable side mold 41, a bottom mold 42 and an end mold 43; when the cement-based test piece reaches the final setting time point in the dog-bone-shaped mold, the side mold 41 and the bottom mold 42 are removed, and the cement-based test piece 30 with the end mold 43 at the end is installed in the dog-bone-shaped groove 16 of the first support 12 and the second support 13; the overall lateral width of the dog-bone-shaped mold is 80 mm, the lateral width of the middle straight section is 40 mm, the overall longitudinal length is 600 mm, and the height is 30 mm.

[0063] like Figure 1 As shown, the displacement sensor 20 of the present invention adopts a linear variable differential transformer (LVDT), which is installed on the left and right sides of the specimen to accurately measure the displacement change of the specimen during the contraction process. The LVDT sensor can provide continuous and high-precision displacement measurement, and its non-contact operation and long-term stability ensure the reliability of the measurement results.

[0064] like Figure 1 , 4As shown in Figures 5 to 6, the end loading device 30 of the present invention is installed on the loading plate 14 and acts on the second support 13, and is used to drive the second support 13 to slide on the column to provide a preset axial constraint force to the cement-based test piece 60. Specifically: the end loading device 30 includes an end loading mechanism and a force sensor, the end loading mechanism includes a support rod 31 and a screw rod 32, the screw rod 32 is placed in a sleeve and is connected to the sleeve by a thread, the sleeve is installed on the loading plate 14, the support rod 31 is installed on the sleeve, and is used to drive the sleeve to rotate on the loading plate 14; the other end of the screw rod 32 is installed on the second support 13, based on the rotation of the sleeve, the screw rod 32 is driven to move along the axial direction to drive the second support 13 to slide on the column. The above-mentioned end loading mechanism uses a rotary loading method to load the specimen. Specifically, rotating the support rod clockwise applies tension to the specimen, and rotating the support rod counterclockwise applies pressure to the specimen. The setting of the initial tension / pressure value can be determined according to the test needs and actual conditions. The loading range of the end loading mechanism is 0~300kN, and the loading accuracy is ±0.001kN. The rotating support rod is made of high-strength alloy material with excellent wear resistance to ensure long-term use. The loading device is equipped with a precision force sensor that can monitor and display the loading force in real time to ensure that the force value during the loading process is accurate and controllable.

[0065] The stress strain measuring instrument 50 of the present invention adopts the DH3818 static tester. During measurement, parameter setting and related control are performed through the LCD screen and computer software to realize functions such as real-time data collection and data recovery analysis. The DH3818 static tester has high precision and high sensitivity, and can accurately capture the force value change and displacement change of the test piece during the loading process. Through its multi-channel data acquisition system, the data of multiple sensors can be monitored simultaneously, improving the measurement efficiency and data reliability.

[0066] The present invention provides a method for testing shrinkage stress and strain of cement-based materials, comprising:

[0067] S1. Apply lubricant to the inner wall of the dog-bone mold to facilitate demoulding;

[0068] S2. Pour the mixed cement mortar with the formula shown in Table 1 into a dog-bone-shaped mold with an open top. After half of it is poured in, insert a vibrator and vibrate for 10 seconds. Then pour the remaining cement mortar. After filling the dog-bone-shaped mold, vibrate for 10 seconds using the vibrator. Use a scraper to flatten the surface of the specimen, and cover the surface of the specimen with a layer of plastic wrap to prevent water evaporation.

[0069] Table 1

[0070] Jinyu Jidong Cement(PO42.5) water river sand Water-cement ratio 1 0.4 1.5 0.4

[0071] S3, after the cement mortar poured inside the dog-bone-shaped mold reaches the final setting time point, the side and bottom baffles of the mold are removed;

[0072] S4. Carefully place the specimen with the end mold on the assembled axial constraint test frame, and place the cement-based material shrinkage stress-strain test device in a room with a temperature of 20±2℃ and a relative humidity of 50±5℃, so that the specimen shrinks and deforms due to its own hydration reaction and the exchange of external temperature and humidity;

[0073] S5. Use the end loading device to apply a tensile force of 10% of its tensile strength to the specimen;

[0074] S6. Set up a linear variable differential transformer (LVDT) on the left and right sides of the specimen, read the data after standing for 4 hours, and take the average value as the initial reading;

[0075] S7. During the drying process of the specimen, the stress and strain changes of the specimen were monitored and recorded in real time using the DH3818 static tester;

[0076] S8. Use DH3818 static tester to continuously record stress and strain data until the specimen cracks.

[0077] in,

[0078] The calculation formula for shrinkage strain is:

[0079]

[0080] In the formula, ε s (t) is the shrinkage strain of cement-based material at age t, l0 is the initial length reading of the specimen, l t is the length reading of the specimen measured at time t, l b is the measuring gauge length of the specimen;

[0081] The calculation formula for shrinkage stress is:

[0082]

[0083] In the formula, σ s (t) is the shrinkage stress of cement-based material at age t, F0 is the initial tension / compression reading of the specimen, F t is the tensile force / compression reading of the specimen at time t, A s is the cross-sectional area of ​​the specimen.

[0084] Figure 8The shrinkage strain and shrinkage stress of the cement-based material measured in this embodiment are shown. It can be seen from the figure that the shrinkage strain of the cement-based material is small in the initial stage, and the shrinkage strain gradually increases with the passage of time. Especially in the early drying stage, the shrinkage strain increases rapidly, showing the obvious shrinkage characteristics of the material during the water loss process. At the same time, the change trend of the shrinkage stress corresponds to the shrinkage strain. As the shrinkage strain increases, the shrinkage stress also gradually increases. In the later stages of the test, the shrinkage stress tends to stabilize, indicating that the material gradually reaches a state of equilibrium during the stress release process. The shrinkage stress curve shows an obvious mutation at around 25d, when the shrinkage stress is 1.2MPa, and the shrinkage strain develops to 616με at around 25d, and the specimen cracks. The curve records the deformation and stress development of the specimen from shrinkage to cracking.

[0085] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A cement-based material shrinkage stress and strain testing device, characterized in that: include: Axial restraint test frame, displacement sensor, end loading device and stress and strain gauge; The axial constraint test frame comprises a plurality of parallel columns and a first support, a second support and a loading plate arranged in parallel, one end of the plurality of columns is fixed on the first support, and the other end passes through the second support and is fixed on the loading plate, and the second support can slide on the columns; grooves for clamping the end of the cement-based test piece are provided on the opposite surfaces of the first support and the second support, and the cement-based test piece is installed between the first support and the second support and the cement-based test piece is parallel to the columns; The displacement sensor is installed on the cement-based test piece and is used to monitor the shrinkage deformation value of the cement-based test piece in real time; The end loading device is mounted on the loading plate and acts on the second support, and is used to drive the second support to slide on the column to provide a preset axial restraining force; The stress-strain measuring instrument is connected to the displacement sensor and the end loading device, and is used to collect the shrinkage deformation value and the tension / pressure value of the cement-based test piece in real time, so as to test the stress and strain changes of the cement-based test piece in the shrinkage process in real time.

2. The shrinkage stress and strain testing device for cement-based materials according to claim 1, characterized in that: The axial constraint test frame further comprises a bottom plate, and the bottom plate is mounted on the first support.

3. The shrinkage stress and strain testing device for cement-based materials according to claim 1, characterized in that: The cement-based test piece is formed in a dog-bone-shaped mold, and the dog-bone-shaped mold includes a detachable side mold, a bottom mold and an end mold; when the cement-based test piece reaches the final setting time point in the dog-bone-shaped mold, the side mold and the bottom mold are removed, and the cement-based test piece with the end mold at the end is installed in the dog-bone-shaped groove of the first support and the second support.

4. The shrinkage stress and strain testing device for cement-based materials according to claim 1, characterized in that: The displacement sensor comprises one of a linear variable differential transformer, an eddy current sensor, a laser displacement sensor, and a capacitive displacement sensor, and the measurement accuracy is ±0.001 mm.

5. The shrinkage stress and strain testing device for cement-based materials according to claim 1, characterized in that: The end loading device includes an end loading mechanism, which includes a support rod and a screw rod. The screw rod is placed in a sleeve and is threadedly connected to the sleeve. The sleeve is mounted on the loading plate. The support rod is mounted on the sleeve to drive the sleeve to rotate on the loading plate. The other end of the screw rod is mounted on the second support. Based on the rotation of the sleeve, the screw rod is driven to move along the axial direction to drive the second support to slide on the column.

6. The shrinkage stress and strain testing device for cement-based materials according to claim 5, characterized in that: The end loading device also includes a force sensor, which is connected to the stress strain meter; the force sensor is used to monitor the initial tension / pressure value applied by the end loading mechanism to the cement-based test piece and monitor the tension / pressure value of the cement-based test piece at any time during the shrinkage process.

7. A method for testing shrinkage stress and strain of cement-based materials based on the cement-based material shrinkage stress and strain testing device according to any one of claims 1 to 6, characterized in that: include: pouring cement-based materials into the dog-bone mold, and removing the side mold and the bottom mold of the dog-bone mold after the cement-based materials are finally set; The cement-based test piece with the end mold is placed on an axially constrained test frame; One end of the cement-based test piece is fully constrained, and the other end is subjected to axial constraint force through the end loading device. The displacement sensors are fixed on both sides of the specimen. The drying speed of the specimen is controlled by adjusting the ambient temperature and humidity to simulate the shrinkage and deformation behavior of the specimen in the actual environment. The stress and strain measuring instrument is used to monitor and record the data of the stress and strain changes of the specimen over time in real time until the specimen dries and shrinks under the constraint conditions and causes fracture, and the cracking stress and strain are recorded.

8. The method for testing shrinkage stress and strain of cement-based materials according to claim 7, characterized in that: The calculation formula for shrinkage strain is: In the formula, ε s (t) is the shrinkage strain of cement-based material at age t, l0 is the initial length reading of the specimen, l t is the length reading of the specimen measured at time t, l b is the measuring gauge length of the specimen; The calculation formula for shrinkage stress is: In the formula, σ s (t) is the shrinkage stress of cement-based material at age t, F0 is the initial tension / compression reading of the specimen, F t is the tensile force / compression reading of the specimen at time t, A s is the cross-sectional area of ​​the specimen.

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