Apparatus and method for temperature and humidity driven cracking testing of restrained hardening cementitious materials

Abstract

The application discloses a kind of temperature and humidity driven cracking test devices of hardened cement-based material under restraint, including cracking test frame and tray, cracking test frame includes chuck and restraint arm, both ends of tray are fixed by chuck, and chuck is connected by restraint arm between;Tray includes abutment and support, the two sides of abutment not being connected with chuck are provided with abutment baffle, the space enclosed by abutment, chuck, abutment baffle is used to pour the cement-based material to be measured, and the side of abutment not being poured cement-based material to be measured is connected with support;Strain gauge for measuring the deformation of restraint arm is provided on the restraint arm.The application also discloses the testing method of the test device.The application is suitable for the cracking test of cement-based material in service period, and the test interference factor is relatively less by curing test piece to service period and then applying restraint and temperature and humidity driving;Only variable restraint is applied at the end of the device, and the test sample is exposed to air around, to ensure that the sample and the environment can be fully exchanged in temperature and humidity.

Description

Technical Field

[0001] This invention relates to a testing device and method for cement-based materials, specifically a temperature and humidity driven cracking testing device and method for constrained hardened cement-based materials. Background Technology

[0002] In regions with extremely dry climates and large temperature differences, severe cracking is common in concrete components such as bridge decks and piers. Non-load-bearing cracks account for over 70% of all cracks in these structural components. Most of these non-load-bearing cracks do not form during the early stages of cement hydration, but rather gradually develop during service life after construction due to non-load-bearing factors driven by temperature and humidity.

[0003] Under harsh environmental conditions of dryness and large temperature differences, service-hardening cementitious materials undergo drying shrinkage and temperature deformation. When this deformation is constrained at the ends, internal stress is generated. When the stress exceeds the failure limit, cracks initiate, propagate, and eventually penetrate, resulting in environmental-constraint coupled cracks. These cracks provide pathways for harmful ions, liquids, and gases to invade the material, further compromising its durability and reducing its service life. A thorough understanding of the mechanism of environmental-constraint coupled crack formation and the establishment of an environmental-constraint coupled damage model for cementitious materials are of great significance for mitigating cracking in service-hardening cementitious materials. Therefore, key parameters such as deformation, constraint stress, constraint degree, and cracking risk under service-constraint conditions are particularly important. Effective determination of these key parameters urgently requires an experimental apparatus and system that allows free deformation of the specimen during the initial casting stage, applies constraint to the specimen during the service period (e.g., after 28 days of standard curing) under specific curing conditions and times, and can flexibly control the constraint degree and adjust the specimen thickness in a timely manner.

[0004] Traditional constrained cracking test apparatuses, such as circular ring cracking tests, plate constrained cracking tests, and axial cracking frames, are widely used. However, they are only suitable for testing shrinkage cracking during the hydration process from the time of casting. After casting, cement-based materials undergo continuous changes in their internal temperature and humidity fields due to internal hydration reactions and external temperature and humidity exchanges, leading to deformation. If constraints are applied during the curing period of the specimen, it will inevitably limit the volume deformation of the specimen, further leading to stress concentration and cracking. The existence of this initial damage makes traditional constrained cracking test apparatuses unsuitable for testing the deformation and cracking of cement-based materials during their service life. Most existing testing apparatuses and methods are innovations based on the aforementioned traditional constrained cracking test apparatuses, but they all exert a constraining effect on the material from the time of casting. Furthermore, traditional constrained cracking apparatuses cannot simulate the actual constrained state in engineering projects. At the same time, the specimen cannot undergo environmental temperature and humidity transmission due to obstructions at the bottom or side walls of the apparatus, and the operation and requirements during the experiment are also very complex.

[0005] While axially constrained testing devices for drying shrinkage cracking of cement-based materials overcome the problem of temperature and humidity transmission between materials and the environment, and simulate the actual constraint state in engineering through axial constraint, their constraint effect is limited to early-stage drying shrinkage cracking testing because it begins to act as a constraint from the moment the cement-based material is poured. This limitation cannot meet the need to apply constraint and temperature / humidity driving only after the specimens have been cured to their service life. Furthermore, this device can only form specimens of a single thickness, making it unsuitable for studying the size effect response to environmental temperature and humidity, resulting in low utilization efficiency. Current testing devices for constrained cracking of service-hardened cement-based materials mostly use strong adhesive to bond the specimen to the cracking test frame. This process is complex and makes it difficult to ensure the uniformity of the adhesive application and the specimen's centered position on the test frame. These shortcomings severely interfere with the determination of key parameters. Moreover, the strong adhesive cannot be effectively removed after testing, leading to waste of the testing equipment and certain economic losses. Summary of the Invention

[0006] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a temperature and humidity driven crack testing device for constrained hardened cementitious materials that is adjustable in constraint, accurate, efficient, and easy to demold. The purpose of this invention is to provide a convenient and accurate test method for temperature and humidity driven cracking of constrained hardened cementitious materials.

[0007] Technical Solution: The present invention discloses a temperature and humidity driven cracking test device for constrained hardening cement-based materials, comprising a cracking test frame and a tray. The cracking test frame includes clamps and constraint arms. The two ends of the tray are fixed by clamps, and the clamps are connected to each other by constraint arms. The tray includes a base and a support. Base baffles are provided on the two sides of the base that are not connected to the clamps. The space enclosed by the base, clamps, and base baffles is used to pour the cement-based material to be tested. The side of the base where the cement-based material to be tested is not poured is connected to the support. Strain gauges for measuring its deformation are provided on the constraint arms.

[0008] Furthermore, the clamps and constraint arms are connected by bolts. The clamps have bolt holes and a first groove, while the constraint arms have a second groove. The bolts pass through both the bolt holes and the second groove. The width of the second groove is the same as the diameter of the bolt hole, and the length of the second groove is greater than the diameter of the bolt hole on the clamps. Therefore, even if deformation occurs during the curing of the cement-based material, causing changes in the clamp spacing, the clamps and constraint arms can still be connected by bolts during the testing phase, forming an effective constraint on the material under test.

[0009] Furthermore, the first groove is trapezoidal in shape, and the clamp fits tightly against the tray.

[0010] Furthermore, both the clamps and constraint arms are made of Invar alloy, which has a low coefficient of thermal expansion to eliminate the interference of heat generated during the hydration reaction of cement-based materials and deformation of the testing system caused by subsequent changes in ambient temperature on the measurement results. The constraint arms are the source of constraint for the cracking frame, and the constraint can be adjusted by changing the size of the constraint arms to meet the needs of different experiments.

[0011] Furthermore, the tray is made of PVC plastic, utilizing a lightweight material with good support properties, making it easy to demold. The height is pre-made as needed to control the thickness and position of the cast specimen, ensuring that the specimen is centered on the cracking test frame.

[0012] Furthermore, the support is cross-shaped to prevent warping at the bottom of the support platform, ensure the flatness of the molded sample, and provide a force point for demolding, making demolding easier.

[0013] Furthermore, a plastic film is placed on the surface of the cement-based material to be tested to isolate it from external humidity transmission. Strain gauges are evenly spaced along the circumferential direction of the constraint arms.

[0014] The test method for the temperature and humidity driven cracking test device for the above-mentioned constrained hardened cement-based materials includes the following steps:

[0015] Step 1: Secure the clamps and constraint arms together with bolts to form a stable cracking test frame;

[0016] Step 2: Select a tray of the target height and place it inside the cracking test frame. The space formed by the tray and the groove of the clamp serves as the forming mold.

[0017] Step 3: Pour the mixed cement-based material to be tested into the molding mold, vibrate and smooth it, and then cover the upper surface of the cement-based material to be tested with a plastic film.

[0018] Step 4: After the cement-based material to be poured reaches the initial setting time, pull the tray out from the bottom of the crack test frame, release the bolts connecting the clamp and the constraint arm, remove the constraint arm, and put the cement-based material to be tested in a state without axial strong constraint.

[0019] Step 5: 24 hours after the pouring is completed, place the hardened cement-based material to be tested and the clamps connected to its ends under standard curing conditions for 28 days or longer. During the curing period, the shrinkage of the material will not be affected by the constraint or the damage caused by the constraint.

[0020] Step six: Remove the cement-based material to be tested from the standard curing conditions, and reconnect it through the bolt holes of the bolted clamp and the groove of the constraint arm to reassemble the clamp and constraint arm into a stable cracking test frame.

[0021] Step 7: Place the crack test frame obtained in Step 6 into an environmental test chamber with a set temperature cycle and constant humidity, so that the test sample can undergo sufficient temperature and humidity transfer in the environment.

[0022] Step 8: Read the strain of the strain gauge as the temperature and humidity driven deformation of the cement-based material to be tested. Calculate the constraint degree and constraint stress based on the temperature and humidity driven deformation. Compare the constraint stress with the tensile strength of the measured material to obtain the cracking risk.

[0023] Working principle: This cracking test device can also be used to determine deformation and cracking parameters under the influence of a single factor such as temperature or humidity. When conducting temperature deformation constrained cracking tests on cement-based materials during service life, the influence of moisture transport should be eliminated, and a plastic film can be wrapped around the surface of the test sample. When conducting constrained cracking tests on cement-based materials during service life under extremely dry conditions, the constrained cracking frame fixing the test sample can be placed in an environmental test chamber with constant room temperature and extremely low humidity.

[0024] The calculations for constraint stress, constraint degree, and cracking risk are as follows:

[0025]

[0026]

[0027]

[0028] In the formula, σ r R(t) is the constraint stress at time t; R(t) is the degree of constraint at time t; η(t) is the cracking risk at time t; ε s (t) is the strain value measured by the strain gauge on the constraint arm at time t. The average of the strain values ​​on the two constraint arms is taken to reduce the error; E s It is the elastic modulus of the material used in the restraint arm; A s A represents the total cross-sectional area of ​​the two constraint arms. c ε is the cross-sectional area of ​​the cast specimen, that is, the cross-sectional area of ​​the region enclosed by the two baffles and the base plate; f (t) represents the free drying shrinkage deformation of the specimen at time t, measured through a matching test; f t (t) represents the tensile strength of the material at time t, which is obtained through a matching test.

[0029] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0030] 1. Applicable to crack testing of cementitious materials hardened under constraint, it can cure specimens to their service life before applying constraints and temperature and humidity driving, and the test has relatively few interference factors;

[0031] 2. The present invention applies variable constraints only at the end of the device, exposing the test sample to the air from all sides, ensuring that the sample and the environment can exchange sufficient temperature and humidity.

[0032] 3. The entire testing device is assembled with bolts, and the cross-sectional area of ​​the constraint arm can be flexibly replaced, enabling the adjustment of the constraint degree and providing the possibility of changing the constraint degree to adapt to different test requirements.

[0033] 4. The design of the constraint arm groove and the method of applying lateral constraints allow for the selection of an appropriate time to apply constraints during the service phase after maintenance, which can effectively avoid material damage caused by deformation being constrained during maintenance.

[0034] 5. By selecting a series of prefabricated matching trays, it is possible to conveniently and accurately form test samples of a specific thickness located at the center of the cracking test frame. This overcomes the shortcomings of traditional cracking test devices, which can only form samples of a single thickness. It is possible to further explore the influence of size effect as another influencing factor on temperature and humidity driven deformation and cracking of cement-based materials during service. Attached Figure Description

[0035] Figure 1 This is a top-view perspective view of the present invention;

[0036] Figure 2 This is a bottom-view perspective view of the present invention;

[0037] Figure 3 This is a top sectional view of the present invention;

[0038] Figure 4 This is a cross-sectional view of the AA plane of the present invention;

[0039] Figure 5 This is a top view of the tray 2 of the present invention;

[0040] Figure 6 This is a front view of the tray 2 of the present invention;

[0041] Figure 7 This is a top view of the chuck 3 of the present invention;

[0042] Figure 8 This is a side view of the chuck 3 of the present invention;

[0043] Figure 9 This is a front view of the constraint arm 4 of the present invention;

[0044] Figure 10 This is the temperature and humidity curve of the present invention. Detailed Implementation

[0045] like Figures 1-4The temperature and humidity driven cracking test device for constrained hardened cement-based materials consists of a metal frame 1 made of symmetrically arranged clamps 3 and constraint arms 4 tightly connected by 8mm bolts. The constraint arm 4 is 260mm long, the clamp 3 is 44mm high, and the distance between the two clamps 3 is 160mm, which is the length of the cracking test area. The space enclosed by the clamps 3 and the tray 2 is used to pour the cement-based material to be tested. Before pouring, a layer of petroleum jelly lubricant is brushed on the bottom and side walls of the enclosed space to facilitate the demolding of the material after initial setting. The clamps 3 and constraint arms 4 are both made of Invar alloy, and the tray 2 is entirely made of PVC plastic. The tray 2 includes a base 5 and a cross-shaped support 6.

[0046] like Figures 5-6 By prefabricating a specific height support plate 501 and a tray support 6, a thin sheet of cement-based material of arbitrary thickness can be formed at the center of the cracking test frame 1. When the thickness of the support plate 5 of the tray 2 is 2mm, the height of the support plate 501 is 10mm, and the height of the tray support 6 is 15mm, after initial setting, the tray 2 is pulled downwards, forming a 10mm thick thin sheet sample at the center of the cracking test frame 1. When the thickness of the support plate 5 of the tray 2 is 2mm, the height of the support plate 501 is 20mm, and the height of the tray support 6 is 10mm, after initial setting, the tray 2 is pulled downwards, forming a 20mm thick thin sheet sample at the center of the cracking test frame 1.

[0047] like Figures 7-8 The dimensions of the chuck 3 are 50mm*44mm*170mm. Bolt holes 301 with a diameter of 8mm and a depth of 10mm are opened at both ends. A trapezoidal groove 302 is opened on one side, with the angle between the waist and the bottom being 45° and the height of the trapezoid being 30mm.

[0048] like Figure 9 The dimensions of the middle part of the constraint arm 4 are 20mm*20mm*160mm, and the dimensions of the end of the constraint arm 4 are 20mm*10mm*50mm. Making the end of the constraint arm 4 thinner reduces material usage and improves the bolt connection effect. A rectangular groove 401, 20mm*8mm in diameter and 20mm from the end boundary, is made at both ends of the constraint arm 4 to ensure effective constraint application even after a curing process without constraint, even if the spacing between the clamps 3 changes due to the shrinkage of the material being tested. The clamps 3 and constraint arm 4 are fixed with bolts of 8mm diameter and 20mm length. The constraint force is transmitted through the static friction between the constraint arm 4, the clamps 3, and the bolts. The width of the groove 401 is the same as the diameter of the bolt hole 301, and the strain gauges 7 are evenly spaced along the circumference of the constraint arm 4.

[0049] Table 1 shows the formulation of cement-based materials used.

[0050] Cement (PⅠ42.5) water Standard sand water-cement ratio 1 0.4 2 0.4 1 0.45 2 0.45 1 0.5 2 0.5

[0051] The testing method of the temperature and humidity driven cracking test device for constrained hardened cement-based materials in this embodiment includes the following steps:

[0052] (a) The clamp 3 and the constraint arm 4 are tightly connected by 8mm bolts to form a stable cracking test frame 1.

[0053] (b) Select a tray 2 of the target size and place it inside the space of the crack test frame 1. The space enclosed by the tray 2 and the groove 302 of the clamp 3 serves as the forming mold.

[0054] (c) According to the formula in Table 1 above, pour the mixed mortar into the molding mold, vibrate and smooth it, and then cover the upper surface of the groove 302 of the tray 2 and the clamp 3 with a plastic film to prevent the material from transmitting humidity to the outside during the solidification process.

[0055] (d) After the cement-based material to be poured reaches the initial setting time, pull the tray 2 out from the bottom of the space of the crack test frame 1, and release the bolts connecting the clamp 3 and the constraint arm 4, and remove the constraint arm 4 so that the material to be tested is in a state without axial strong constraint.

[0056] (e) 24 hours after the pouring is completed, the hardened test sample and the clamp 3 connected to its end are placed under standard curing conditions for 28 days. During the curing period, the shrinkage of the material will not be affected by the constraint or the damage caused by the constraint.

[0057] (f) Remove the test sample from the standard curing conditions and reconnect it through the bolt hole 301 of the bolted clamp 3 and the rectangular groove 401 of the constraint arm 4, so that the clamp 3 and the constraint arm 4 can be reassembled into a stable crack test frame 1.

[0058] (g) Place the cracking test frame 1, which holds the test sample, on the pre-set surface. Figure 10 In the temperature cycling and constant low humidity environmental test chamber shown, the test sample is subjected to sufficient temperature and humidity transfer with the environment.

[0059] (h) Read the strain of strain gauge 7 as the temperature and humidity driven deformation of the cement-based material to be tested. Calculate the degree of constraint and constraint stress based on the temperature and humidity driven deformation. Compare the constraint stress with the tensile strength of the measured material to obtain the cracking risk.

[0060] The calculations for constraint stress, constraint degree, and cracking risk are as follows:

[0061]

[0062]

[0063]

[0064] In the formula, σr R(t) is the constraint stress at time t; R(t) is the degree of constraint at time t; η(t) is the cracking risk at time t; ε s (t) is the strain value measured by strain gauge 7 on constraint arm 4 at time t. The average of the two strain values ​​on constraint arm 4 is taken to reduce the error; E s It is the elastic modulus of the material used for constraint arm 4; A s A is the total cross-sectional area of ​​the two constraint arms 4; c The cross-sectional area of ​​the cast specimen is ε, which is the area enclosed by the two foundation baffles 501 and the foundation 5. f (t) represents the free drying shrinkage deformation of the specimen at time t, measured through a matching test; f t (t) represents the tensile strength of the material at time t, which is obtained through a matching test.

[0065] Remove the mortar specimen from the mold and loosen the 8mm bolts between the two clamps 3 and the two constraint arms 4. Change the cross-sectional size of the middle part of the constraint arm 4 or change the tray 2 of different heights. Repeat steps (a) to (h) to obtain the cracking time, constraint strain and constraint stress of specimens of different thicknesses under different constraint conditions.

[0066] With a material elastic modulus of 1.4 × 10⁻⁶ 5 MPa, using the Invar alloy constraint arm 4 with dimensions of 20mm*20mm*160mm as a reference, the constraint degree is controlled by adjusting the dimensions of the constraint arm 4, so that the constraint stress provided by the unit deformation energy is changed. The change coefficients are shown in Table 2 below.

[0067] Table 2 shows the influence coefficients of changing the size of constraint arm 4 on the constraint effect.

[0068]

[0069] As can be seen from Table 2, the larger the cross-sectional area of ​​constraint arm 4, the greater the elastic modulus of constraint arm 4.

Claims

1. A test method for a temperature and humidity driven cracking test device for constrained hardening cementitious materials, comprising the following steps: Step 1: Connect the clamp (3) and the constraint arm (4) with bolts to form a stable cracking test frame (1). Step 2: Select a tray (2) of the target height and place it inside the crack test frame (1). The space enclosed by the tray (2) and the groove (302) of the clamp (3) serves as the forming mold. Step 3: Pour the mixed cement-based material to be tested into the molding mold, vibrate and smooth it, and then cover the upper surface of the cement-based material to be tested with a plastic film. Step 4: After the cement-based material to be poured reaches the initial setting time point, pull the tray (2) out from the bottom of the space of the crack test frame (1), and release the bolts connecting the clamp (3) and the constraint arm (4), and remove the constraint arm (4) so ​​that the cement-based material to be tested is in a state without axial strong constraint. Step 5: 24 hours after the pouring is completed, place the hardened cement-based material to be tested and the clamp (3) connected to its end under standard curing conditions for 28 days or longer. During the curing period, the shrinkage of the material will not be affected by the constraint and the damage caused by the constraint. Step 6: Remove the cement-based material to be tested from the standard curing conditions and reconnect it through the bolt holes (301) of the bolted clamp (3) and the grooves (401) of the constraint arm (4) to form a stable cracking test frame (1). Step 7: Place the crack test frame (1) obtained in Step 6 into an environmental test chamber with set temperature cycle and constant humidity, so that the test sample can be subjected to sufficient temperature and humidity transfer in the environment; Step 8: Read the strain of strain gauge (7) as the temperature and humidity driven deformation of the cement-based material to be tested. Calculate the degree of constraint and the constraint stress based on the temperature and humidity driven deformation. Compare the constraint stress with the tensile strength of the measured material to obtain the cracking risk. The temperature and humidity driven cracking test device for hardened cementitious materials under constraint includes a cracking test frame (1) and a tray (2). The cracking test frame (1) includes a clamp (3) and a constraint arm (4). The two ends of the tray (2) are fixed by the clamp (3), and the clamps (3) are connected to each other by the constraint arm (4). The tray (2) includes a base (5) and a support (6). The base (5) is provided with base baffles (501) on both sides that are not connected to the clamp (3). The space enclosed by the base (5), the clamp (3), and the base baffles (501) is used to pour the cementitious material to be tested. The side of the base (5) where the cementitious material to be tested is not poured is connected to the support (6). The constraint arm (4) is provided with strain gauges (7) for measuring its deformation. The clamp (3) and the constraint arm (4) are connected by bolts; The clamp (3) is provided with a bolt hole (301) and a groove one (302), and the constraint arm (4) is provided with a groove two (401). The bolt passes through the bolt hole (301) and the groove two (401).

2. The test method of the temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: The width of the second groove (401) is the same as the diameter of the bolt hole (301).

3. The test method for a temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: The groove 1 (302) is trapezoidal in shape, and the clamp (3) fits tightly against the tray (2).

4. The test method for a temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: Both the clamp (3) and the restraint arm (4) are made of Invar alloy.

5. The test method of the temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: The tray (2) is made of PVC plastic.

6. The test method of the temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: The support (6) is cross-shaped.

7. The test method for a temperature and humidity driven cracking test device for constrained hardening cementitious materials according to claim 1, characterized in that: A plastic film is applied to the surface of the cement-based material to be tested.

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