Test apparatus and method for axial restraint cementitious material dry shrinkage cracking

Abstract

The application discloses a kind of axial restraint cement-based material dry shrinkage cracking test device, including axial cracking test frame, baffle and bottom plate, axial cracking test frame, baffle are all set on bottom plate;Axial cracking test frame includes chuck, restraint arm, chuck is symmetrically set, and the space enclosed by it and baffle is used to pour the concrete to be measured, recess is set on chuck, the cross-sectional area of recess gradually increases along chuck inside, the both ends of restraint arm, baffle are respectively connected with chuck;Strain gauge is set on restraint arm to measure its deformation;By replacing the size and material of restraint arm, the degree of restraint can be adjusted.The application also discloses a kind of axial restraint cement-based material dry shrinkage cracking test method.The test device of the application is assembled as a whole by screw, the material and cross-sectional area of restraint arm can be replaced flexibly, the degree of restraint can be controlled, and the possibility of changing the degree of restraint to adapt to different test requirements is provided.

Description

TECHNICAL FIELD

[0001] The present application relates to a cement cracking test device and method, in particular to a test device and method for axial constraint of drying shrinkage cracking of cement-based materials. BACKGROUND

[0002] After the cement-based material is poured, the temperature and humidity field continues to change under the action of internal hydration reaction and external temperature and humidity exchange, causing its own volume deformation. Drying shrinkage is the volume reduction of cement-based materials after setting and hardening due to the combined action of self-drying and drying. In structural members, such volume reduction is often constrained by surrounding members, thereby generating constraint stress, and when the constraint stress exceeds the tensile strength, the cement-based material will crack. Concrete member cracking caused by shrinkage, especially drying shrinkage, accounts for a considerable part of structural non-load cracks, and structural non-load cracks account for more than 70% of all structural cracks, which makes it particularly important to evaluate the deformation characteristics of cement-based materials under the action of drying shrinkage, especially the deformation, constraint stress, constraint degree and cracking risk of key parameters under the constraint state. The calculation of these key parameters depends on a stable constraint device and an accurate and reliable test system.

[0003] Traditional constraint test equipment is roughly divided into two types, one is a ring device for ring constraint, and the other is a traditional cracking frame device for axial constraint and its derived temperature stress test equipment. The defects of the ring device are that the ring constraint is not the real constraint form of the structural member, one face cannot be used as a drying surface during the test process because it needs to be in contact with the ring, and it is difficult to measure the test piece. The defects of the traditional cracking frame device are that the pouring is too cumbersome, generally needs to be sealed for testing, which leads to its only use for measuring cracking behavior caused by self-shrinkage under the constraint state, and the equipment is expensive. One of the common shortcomings of existing test methods is that the volume of the test piece is too large, which leads to the fact that only the crack width and cracking time can be extracted, and the parameters related to the crack morphology and crack volume, which can only be obtained by related supporting tests such as fault scanning of the test piece, cannot be extracted. Although the temperature stress test machine (TSTM) can control the constraint degree, it still has most of the shortcomings of the traditional cracking frame device, and additionally has a series of shortcomings such as further increasing the cost of the equipment, making the pouring and demolding process more cumbersome, and making it difficult to carry out multiple tests simultaneously, resulting in poor repeatability. SUMMARY

[0004] The purpose of the application is to overcome the deficiencies in the prior art, and provide a test device for axial restraint cement-based material drying shrinkage cracking, which has adjustable constraint degree, small size, high measurement accuracy, low manufacturing cost, and good adaptability to other matching tests.

[0005] Technical scheme: The test device for axial restraint cement-based material drying shrinkage cracking comprises an axial cracking test frame, a baffle and a bottom plate, and the axial cracking test frame and the baffle are arranged on the bottom plate; the axial cracking test frame comprises a chuck and a restraint arm, the chuck is symmetrically arranged, and a space enclosed by the chuck, the baffle and the bottom plate is used for pouring the measured concrete; a groove is arranged on the chuck, the cross-sectional area of the groove gradually increases along the inner side of the chuck, and the two ends of the restraint arm and the baffle are connected with the chuck; a strain gauge is arranged on the restraint arm to measure the deformation of the restraint arm; and the constraint degree is adjusted by replacing the size and material of the restraint arm.

[0006] Further, the groove is a dovetail groove or a circular truncated cone groove. The chuck and the restraint arm are fixedly connected through screws. The bottom plate is made of aluminum alloy.

[0007] Further, the chuck is made of invar alloy to eliminate the interference of the heat generated in the hydration reaction process of the cement-based material on the test system. Through holes for reducing weight and placing screws are arranged on the chuck.

[0008] Further, the restraint arm is made of metal with stable mechanical properties, preferably carbon steel, stainless steel, aluminum alloy, titanium alloy or invar alloy. The restraint arm is the constraint source of the axial cracking test frame, and the size and material of the restraint arm can be replaced to adjust the constraint degree to meet different experimental requirements.

[0009] Further, the baffle is an L-shaped plate comprising a horizontal segment and a vertical segment, the horizontal segment is fixedly connected with the bottom plate through screws, and the vertical segment is in close contact with the measured concrete. The material of the baffle is one of aluminum alloy, carbon steel, polyethylene or wood.

[0010] Further, the strain gauges are arranged uniformly along the circumference of the restraint arm.

[0011] The above-mentioned test method for axial restraint cement-based material drying shrinkage cracking comprises the following steps:

[0012] Step one, tightly connect the chuck and the restraint arm through screws to form an axial cracking test frame;

[0013] Step two, fix the chuck and the baffle on the bottom plate with screws;

[0014] Step three, brush lubricant on the enclosed inner wall formed by the chuck and the baffle;

[0015] Step four, pour the stirred cement-based material into the space surrounded by the chuck, baffle and bottom plate, and cover the top opening with plastic film to isolate the external moisture exchange;

[0016] Step five, after the poured cement-based material reaches the initial setting time, remove the baffle by unscrewing the screws fixed on the baffle and bottom plate;

[0017] Step six, remove the screws on the chuck, baffle and bottom plate, place the axial cracking test frame and the concrete to be tested clamped by it on the bottom plate, so that the four surfaces are in moisture exchange with the outside, and fix the bottom of one of the chucks to the bottom plate with screws;

[0018] Step seven, place the product obtained in step six in the test environment required, so that it deforms with the hydration reaction and temperature and humidity exchange;

[0019] Step eight, read the strain of the strain gauge as the restrained drying shrinkage deformation of the concrete to be tested, calculate the restraint degree and restraint stress according to the drying shrinkage deformation, and obtain the cracking risk according to the comparison between the restraint stress and the measured material tensile strength.

[0020] When only the constraint test of autogenous shrinkage is needed, the surface of the test piece can be wrapped with aluminum foil to isolate the influence of environmental drying.

[0021] Working principle: the calculation formula of restraint stress, restraint degree and cracking risk is as follows:

[0022]

[0023]

[0024]

[0025] In the formula, is the restraint stress at time t; is the restraint degree at time t; is the cracking risk at time t; is the strain value measured by the strain gauge on the constraint arm at time t, and the average value of the strain values of two constraint arms is taken to reduce the error; is the elastic modulus of the material used for the constraint arm; is the total cross-sectional area of the two constraint arms; is the cross-sectional area of the poured test piece, i.e. the cross-sectional area of the area surrounded by the two baffles and the bottom plate; is the free drying shrinkage deformation of the test piece at time t, which is measured by a matching test; is the tensile strength of the material at time t, which is measured by a matching test.

[0026] Beneficial effects: compared with the prior art, the present application has the following remarkable features:

[0027] 1. The overall test device is assembled by screws, the material and cross-sectional area of the constraint arm can be flexibly replaced, the constraint degree can be controlled, and the possibility of changing the constraint degree to adapt to different test requirements is provided;

[0028] 2. The overall test device is small, which saves space, is convenient to pour, has low cost, effectively reduces the influence of large temperature difference caused by the large volume of the test piece on the measurement of drying shrinkage cracking, and makes the smaller cracked test piece after disassembly available for further development of supporting tests to explore the morphological parameters of the cracks;

[0029] 3. The overall test system is small, after pouring, the test area is placed vertically to make each surface in contact with the environment, so that the drying process is more in line with the real state, and when only the constraint test of autogenous shrinkage is needed, the surface of the test piece can be wrapped with aluminum foil to isolate the influence of environmental drying. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a top view of the present application;

[0031] Figure 2 is a sectional view of the present application along A-A;

[0032] Figure 3 is a front view of the baffle 2 of the present application;

[0033] Figure 4 is a top view of the chuck 4 of the present application;

[0034] Figure 5 is a side view of the chuck 4 of the present application;

[0035] Figure 6 is a front view of the constraint arm 5 of the present application;

[0036] Figure 7 is a schematic diagram of the measurement process of the present application;

[0037] Figure 8 is a constraint shrinkage strain and shrinkage stress change curve of the present application. DETAILED DESCRIPTION

[0038] As Figures 1-2The test device for constraining the drying shrinkage cracking of cement-based materials includes an axial cracking test frame 1, a baffle 2 and a base plate 3. The axial cracking test frame 1 is a metal frame formed by two symmetrically arranged chucks 4 and two symmetrically arranged constraint arms 5 tightly connected by 3mm screws. The length of the constraint arm 5 is 200mm, which is also the distance between the two chucks 4, that is, the length of the cracking test measurement area. The chuck 4 and baffle 2 of the axial cracking test frame 1 are fixedly connected to the base plate 3 by 3mm screws. The connected axial cracking test frame 1 and baffle 2 are in the middle position of the base plate 3. The space enclosed by the chuck 4, baffle 2 and base plate 3 is used to cast the concrete to be tested. A layer of vaseline lubricant is brushed on the wall of the upper open area of ​​the enclosed space so that the cement-based material can be removed after the test. The degree of constraint is adjusted by changing the size and material of the constraint arm 5. The base plate 3 is made of aluminum alloy, and the constraint arm 5 is made of 304 stainless steel.

[0039] like Figure 3 Baffle 2 is made of aluminum alloy and is L-shaped, consisting of a horizontal section and a vertical section. The horizontal section is fixed to the base plate 3 via screws, while the vertical section is in close contact with the concrete being tested. The length of baffle 2 is equal to the length of restraining arm 5, which is 200 mm. The horizontal section of baffle 2 is provided with two 3 mm screw holes 201 symmetrically spaced 115 mm apart for connecting it to the base plate 3.

[0040] like Figures 4-5 The chuck 4 is made of Invar alloy and has dimensions of 60mm*50mm*190mm. A groove 6 is provided on one side to clamp the cement-based material. The cross-sectional area of ​​the groove 6 gradually increases as it moves inward from the chuck 4. The groove 6 in this embodiment is preferably a dovetail groove, but can also be replaced by a truncated cone groove. The chuck 4 is connected to the restraining arm 5 via a second 3mm screw hole 403, and a through hole 401 with a diameter of 14mm is provided near the second screw hole 403 to reduce weight, reduce material consumption, and accommodate screws. Two 3mm bolt holes 402 are provided on the chuck 4 to secure it to the base plate 3 during pouring and measurement, respectively.

[0041] like Figure 6 The material of the restraining arm 5 is 304 stainless steel, and the size is 20mm*20mm*200mm. The center position of the four surfaces of the restraining arm 5 is affixed with strain gauges 7 to measure deformation.

[0042] like Figure 7 After the cement-based material solidifies and hardens, during the test, the baffle 2 is removed and the test device is placed upright to achieve the purpose of drying.

[0043] Table 1 Mortar formula used in the examples

[0044]

[0045] The method for using the above-mentioned testing device for restraining drying shrinkage cracking of cement-based materials comprises the following steps:

[0046] (a) Tightly connect two chucks 4 and two restraining arms 5 with 3 mm screws to form an axial cracking test frame 1;

[0047] (b) Fix the two chucks 4 and the two baffles 2 to the base plate 3 using 3mm screws.

[0048] (c) Apply a layer of vaseline lubricant to the inner wall of the enclosure formed by the chuck 4 and the two baffles 2;

[0049] (d) Pour the mixed cement mortar with the formula shown in Table 1 above into the enclosure formed by the chuck 4, the two baffles 2, and the bottom plate 3, and cover the top opening with a layer of 0.02 mm transparent plastic film to isolate it from external moisture exchange;

[0050] (e) After the cement-based material poured inside reaches the initial setting time, the screws fixing the baffle 2 to the base plate 3 are removed and the baffle 2 is removed;

[0051] (f) Remove the screws securing the chuck 4, baffle 2, and base plate 3. Place the axial cracking test frame 1 and the cement-based material it holds on the base plate 3 so that all four sides of the test area of ​​the test piece maintain moisture exchange with the outside world. Connect the bottom of the left chuck 4 to the base plate 3 with a 3 mm screw to achieve the effect of limiting displacement.

[0052] (g) placing the axially restrained cementitious material drying shrinkage cracking behavior testing apparatus in a room at a temperature of 20±2°C and a relative humidity of (50±5)%, allowing the apparatus to deform due to hydration reaction and temperature and humidity exchange;

[0053] (h) Read the strain of the strain gauge 7 on the restraining arm 5 and use the average strain value as the restrained drying shrinkage deformation of the cement-based material. Calculate the restraint degree and restraint stress based on the drying shrinkage deformation. Compare the restraint stress with the measured tensile strength of the material to determine the cracking risk.

[0054] The calculation formulas for constraint stress, constraint degree, and cracking risk are as follows:

[0055]

[0056]

[0057]

[0058] Where, is the constraint stress at time t; is the degree of constraint at time t; is the cracking risk at time t; is the average strain value of the two constraint arms 5 to reduce errors; is the elastic modulus of the material used in the constraint arm 5; is the total cross-sectional area of the two constraint arms 5; is the cross-sectional area of the cast test piece, i.e. the cross-sectional area of the area enclosed by the two baffles 2 and the bottom plate 3; is the free drying shrinkage deformation of the test piece at time t, which is measured by a matching test; is the tensile strength of the material at time t, which is measured by a matching test.

[0059] As Figure 8 , the measured constraint shrinkage strain and constraint stress of the mortar in this embodiment are shown. As Figure 8 can be seen: the test started from about 9h, with the combined action of hydration and environmental moisture exchange, the test piece shrunk and was constrained by the constraint arm. The constraint strain developed to-24μm / m at about 27h, at which time the constraint stress developed to about 1.6MPa, and the test piece cracked. The curve records the deformation and stress development of the test piece from shrinkage to cracking.

[0060] The mortar test piece is taken out of the mold, and the 3mm screw between the two clamps 4 and the two constraint arms 5 is disengaged, and the material is replaced with aluminum alloy, which has an elastic modulus of 0.7×10 5 MPa, and the size of the constraint arm 5 is 10mm*10mm*200mm to reduce the constraint, and the steps of (a)-(h) are repeated to obtain the cracking time, constraint shrinkage strain and shrinkage stress of the test piece under different constraints.

[0061] The material is stainless steel with an elastic modulus of 2.0×10 5 MPa, and the size of the constraint arm 5 is 10mm*10mm*200mm as a reference, the material and size of the constraint arm 5 are adjusted to control the constraint degree, so that the constraint stress provided by the unit deformation changes, and the change coefficient is shown in Table 2:

[0062] Table 2 Influence coefficient of changing the material or size of the constraint arm 5 on the constraint effect

[0063]

Claims

1. A method for testing axially restrained drying shrinkage cracking of cement-based materials, characterized in that: The following steps are involved: Step 1: tightly connect the chuck (4) and the restraining arm (5) by screws to form an axial cracking test frame (1); Step 2: Fix the chuck (4) and the baffle (2) to the base plate (3) with screws; Step 3: Apply lubricant to the inner wall of the enclosure formed by the chuck (4) and the baffle (2); Step 4: Pour the mixed cement-based material into the space enclosed by the chuck (4), the baffle (2) and the bottom plate (3), and cover the open top with a plastic film; Step 5: After the poured cement-based material reaches the initial setting time point, the screws fixing the baffle (2) and the bottom plate (3) are released, and the baffle (2) is removed; Step 6: Unscrew the screws on the chuck (4), the baffle (2) and the bottom plate (3), and place the axial cracking test frame (1) and the concrete to be tested clamped therein on the bottom plate (3) so that all four surfaces thereof maintain wet exchange with the outside world, and fix the bottom of one of the chucks (4) to the bottom plate (3) with screws; Step 7: placing the product obtained in step 6 in an environment required for testing, so that it deforms due to hydration reaction and temperature and humidity exchange; Step eight, reading the strain of the strain gauge (7) as the constrained drying shrinkage deformation of the concrete to be tested, calculating the constraint degree and the constraint stress based on the drying shrinkage deformation, and obtaining the cracking risk based on the comparison between the constraint stress and the measured material tensile strength; The testing device for axially constrained drying shrinkage cracking of cement-based materials comprises an axial cracking test frame (1), a baffle (2) and a base plate (3), wherein the axial cracking test frame (1) and the baffle (2) are both arranged on the base plate (3); the axial cracking test frame (1) comprises a chuck (4) and a restraining arm (5), wherein the chuck (4) is symmetrically arranged, and the space enclosed by the chuck, the baffle (2) and the base plate (3) is used for pouring the concrete to be tested; a groove (6) is provided on the chuck (4), and the cross-sectional area of ​​the groove (6) gradually increases along the chuck (4) toward the inner side; the two ends of the restraining arm (5) and the baffle (2) are respectively connected to the chuck (4); a strain gauge (7) for measuring its deformation is provided on the restraining arm (5); and the restraining degree is adjusted by changing the size and material of the restraining arm (5).

2. The method for testing drying shrinkage cracking of axially restrained cementitious materials according to claim 1, characterized in that: The groove (6) is a dovetail groove or a frustum groove.

3. The method for testing drying shrinkage cracking of axially restrained cementitious materials according to claim 1, characterized in that: The clamp (4) and the restraining arm (5) are fixedly connected by screws.

4. The method for testing drying shrinkage cracking of axially restrained cementitious materials according to claim 1, characterized in that: The bottom plate (3) is made of aluminum alloy.

5. The method for testing drying shrinkage cracking of axially restrained cement-based materials according to claim 1, characterized in that: The chuck (4) is made of Invar alloy.

6. The method for testing drying shrinkage cracking of axially restrained cementitious materials according to claim 1, characterized in that: The chuck (4) is provided with a through hole (401) for reducing weight and placing screws.

7. The method for testing drying shrinkage cracking of axially restrained cementitious materials according to claim 1, characterized in that: The restraining arm (5) is made of metal with stable mechanical properties.

8. The method for testing drying shrinkage cracking of axially restrained cement-based materials according to claim 1, characterized in that: The baffle (2) is an L-shaped plate, comprising a horizontal section and a vertical section, the horizontal section being fixedly connected to the bottom plate (3) by screws, and the vertical section being in close contact with the concrete to be tested.

9. The method for testing drying shrinkage cracking of axially restrained cement-based materials according to claim 1, characterized in that: The strain gauges (7) are evenly spaced along the circumference of the constraint arm (5).

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