A method and device for testing non-uniform restraint shrinkage

Abstract

The present invention belongs to the technical field of concrete testing, and discloses a method and device for non-uniform restraint shrinkage testing, which includes grinding the part of the restraint where the strain gauge needs to be pasted; cleaning the part where the strain gauge needs to be pasted; pasting the strain gauge and fixing it with epoxy resin; pasting buffer foam on both sides of the restraint and covering it with a layer of polyethylene film; installing the inner plate and moving the processed restraint into the middle of one side of the mold in the length direction; pouring the stirred concrete into the mold, vibrating and compacting it, moving the specimen into the test environment after forming, and connecting the strain gauge to the strain gauge, recording the time and detecting the strain value; removing the inner plate after the initial setting of the concrete, and the time of the sudden change of the strain value is the cracking time of the specimen. The present invention uses a strain gauge to measure the strain of the internal restraint, and judges the cracking resistance of the material according to the cracking time and the strain development status. Guided by the restraint mode, a new type of concrete non-uniform restraint shrinkage testing method is proposed.

Description

Technical Field

[0001] The invention belongs to the technical field of concrete testing, and in particular relates to a method and a device for testing non-uniform restrained shrinkage. Background Art

[0002] Concrete shrinkage is very significant in high-strength concrete, self-compacting concrete and mass concrete. In severe cases, it can lead to shrinkage cracks, thereby reducing the bearing capacity and durability of the structure. Concrete shrinkage can be divided into free shrinkage and constrained shrinkage according to external conditions. Constrained shrinkage refers to the slight volume shrinkage of concrete over time after mixing and pouring. If there is no constraint (free shrinkage), the component will be displaced; if there is an external constraint (rebar, studs, etc.), the displacement of the concrete will be limited.

[0003] The test methods for evaluating the shrinkage and crack resistance of concrete also include free shrinkage test and restrained shrinkage test. According to the different ways of providing restraint, restrained shrinkage test is mainly divided into strip restraint test, plate restraint test and ring restraint test. Plate restraint test is complicated to operate, with low repeatability and no relatively mature theory to support it. Strip restraint test is a test method for testing uniform restraint shrinkage on the surface. However, in wet joints, in order to better transfer loads between prefabricated components, a large number of anchor steel bars and studs are usually required. At the same time, a certain thickness of protective layer must be ensured in the design, which makes the concrete on one side of the protective layer of the wet joint less restrained, while the constraint on the other side is greater. This uneven constraint will cause uneven stress inside the concrete; due to the presence of anchor steel bars and studs and the rigid provisions of the protective layer thickness, the constraint on the concrete is generally non-uniform. Ring shrinkage test is the most widely used test method for restrained shrinkage of concrete, which has a relatively mature study on the constitutive relationship between circular ring strain and internal stress of concrete. However, it has the following disadvantages: (1) The constraint caused by the annular constraint on concrete shrinkage is in the radial direction, and thus generates annular tensile stress. It is not applicable to the shrinkage problem of wet joint concrete to be solved by the present invention. (2) In the circular ring test, the cracking of the concrete ring is not only caused by the constraint of the steel ring, but the concrete ring itself will generate internal stress due to the self-constraint caused by the shrinkage difference between the inner ring and the outer ring. The weight of the self-constraint and external constraint of the concrete is unclear.

[0004] Chinese invention patent application CN201910109798.6 discloses a new type of concrete constrained shrinkage test device and its use method. The invention monitors concrete shrinkage by placing strain gauges inside the concrete. At the same time, the constraint of the device is generated by the wedge-shaped structure of the four corners, and the constraint is uniform.

[0005] Through the above analysis, the problems and defects of the existing technology are: the existing restrained shrinkage test does not have a test method for the restrained state of wet joint concrete between prefabricated components. Summary of the invention

[0006] In view of the problems existing in the prior art, the present invention provides a method and device for non-uniform constrained shrinkage testing.

[0007] The present invention is implemented in that a method for non-uniform restraint shrinkage testing comprises:

[0008] Step 1: Grind the part of the constraint where the strain gauge needs to be attached;

[0009] Step 2: Clean the area where the strain gauge needs to be pasted. Steps 1 and 2 are to ensure the pasting effect between the strain gauge and the constraint.

[0010] Step 3, pasting the strain gauge and fixing it with epoxy resin so that the strain gauge is consistent with the strain occurring at the part;

[0011] Step 4: Paste buffer foam on both sides of the constraint and cover it with a layer of polyethylene film; the buffer foam and polyethylene film are used to prevent the concrete from affecting the strain gauge;

[0012] Step 5: Install the inner plate and move the processed constraint into the middle of one side of the mold in the length direction;

[0013] Step 6: Pour the mixed concrete into the mold, vibrate and compact it, move the specimen into the test environment after forming, connect the strain gauge to the strain meter, record the time and detect the strain value;

[0014] Step seven, after the initial setting of the concrete, remove the inner plate, and the time of sudden change of strain value is the cracking time of the specimen.

[0015] Furthermore, the grinding operation in step one uses sandpaper.

[0016] Furthermore, the cleaning operation in step 2 uses anhydrous ethanol.

[0017] Furthermore, when cutting the buffer foam in step 4, a position for sticking the strain gauge and a channel for the wire need to be reserved;

[0018] Furthermore, in step six, the specimen is moved into the test environment within 20 minutes after the concrete is formed, and the strain gauge is connected to the strain meter within two minutes.

[0019] Another object of the present invention is to provide a device for non-uniform restraint shrinkage test, the device for non-uniform restraint shrinkage test comprising:

[0020] External mold;

[0021] The outer mold is a rectangular box structure with an open top and sealed sides, and an inner plate is provided on the inner side of the inner wall of the outer mold;

[0022] A restraint is placed in the middle of one side in the length direction of the outer mold, and strain gauges are respectively adhered and fixed on both sides of the restraint.

[0023] Furthermore, a layer of buffer foam is adhered to both sides of the restraint, and the outer side of the buffer foam layer is covered with a layer of polyethylene film.

[0024] Furthermore, the buffer foam layer is provided with a slot corresponding to the strain gauge, and the strain gauge is located inside the slot.

[0025] Furthermore, the strain gauge is an L-shaped structure, with the strain gauges on both sides symmetrically arranged. The strain gauge itself is a strip, but the strain gauge needs to be connected to a wire. The L-type is a strain gauge plus a wire connected to an external slot.

[0026] Furthermore, a concrete layer is filled between the outer side of the restraint and the inner plate.

[0027] In combination with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solutions to be protected by the present invention are as follows:

[0028] The present invention casts after pasting strain gauges, removes the mold after initial setting, measures the strain of internal constraints by strain gauges, and judges the crack resistance of the material according to the cracking time and strain development. The present invention is guided by the constraint mode, proposes a new concrete non-uniform constraint shrinkage test method, and gives the constitutive relationship between constraint strain and concrete stress.

[0029] In order to more realistically simulate the restraint state of wet joint concrete between prefabricated components and analyze the cracking performance of concrete specimens under the restraint state, the present invention provides a non-uniform concrete restrained shrinkage test device, which has the characteristics of simple structure, low cost and strong operability, and solves the difficult problem of concrete non-uniform restraint shrinkage test.

[0030] The present invention applies a constraint on one side so that non-uniform stress is generated on the concrete, and the strain of the constraint is monitored by sticking a strain gauge on the constraint, and the cracking time of the concrete is judged according to the strain of the constraint; it is closer to the actual working condition, has a simple structure and strong operability.

[0031] The technical solution of the present invention fills the technical gap in the industry at home and abroad: the arrangement of steel bars and other components inside the concrete structure will constrain the shrinkage of the concrete, and a concrete protective layer is arranged outside. The different internal and external constraints lead to uneven residual stress on the concrete, but the current invention does not have this kind of pure non-uniform constraint shrinkage test device.

[0032] The technical solution of the present invention solves a technical problem that people have been eager to solve but have never been successful: testing the shrinkage performance of concrete under a non-uniform constraint state caused by a constraint alone. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is a schematic structural diagram of a device for non-uniform constrained shrinkage testing provided by an embodiment of the present invention;

[0034] Figure 2 is a top view of a device for non-uniform constrained shrinkage testing provided by an embodiment of the present invention;

[0035] Figure 3 is a detailed diagram of the attachment position of the strain gauge provided in an embodiment of the present invention;

[0036] Figure 4 is a flow chart of a method for non-uniform constrained shrinkage testing provided by an embodiment of the present invention;

[0037] Figure 5 is a schematic diagram of the results of the finite element analysis provided by an embodiment of the present invention;

[0038] Figure 6 It is a schematic diagram of cracking of concrete under internal non-uniform stress provided by an embodiment of the present invention;

[0039] In the figure: 1. Outer mold; 2. Inner plate; 3. Buffer foam layer; 4. Strain gauge; 5. Constraint. DETAILED DESCRIPTION

[0040] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] In order to enable those skilled in the art to fully understand how to implement the present invention in detail, this section is an explanatory embodiment that expands and describes the technical solution of the claims.

[0042] like Figures 1 to 3 As shown, the outer mold 1 of the device for non-uniform constrained shrinkage testing provided by an embodiment of the present invention is a rectangular box-shaped structure with an open upper end and sealed on the other sides, and an inner plate 2 is provided on the inner side of the inner wall of the outer mold 1; a constraint 5 is placed in the middle position on one side of the length direction of the outer mold 1, and strain gauges 4 are respectively adhered and fixed on both sides of the constraint 5.

[0043] A layer of buffer foam layer 3 is respectively adhered to both sides of the restraining object 5 in the embodiment of the present invention, and the outer side of the buffer foam layer 3 is covered with a layer of polyethylene film.

[0044] The buffer foam layer 3 in the embodiment of the present invention is provided with a slot corresponding to the strain gauge 4 , and the strain gauge is located inside the slot.

[0045] The strain gauge 4 in the embodiment of the present invention is an L-shaped structure, and the strain gauges on both sides are symmetrically arranged.

[0046] In the embodiment of the present invention, a concrete layer is filled between the outer side of the restraint 5 and the inner plate.

[0047] like Figure 4 As shown, the method for non-uniform constrained shrinkage testing provided by an embodiment of the present invention includes:

[0048] S101: Use sandpaper to polish the part of the constraint where the strain gauge needs to be attached;

[0049] S102: Use anhydrous ethanol to clean the area where the strain gauge needs to be attached;

[0050] S103: Paste the strain gauge and fix it with epoxy resin;

[0051] S104: Paste cushioning foam on both sides of the restraint and cover it with a layer of polyethylene film;

[0052] S105: Install the inner plate and move the processed constraint into the middle of one side of the mold in the length direction;

[0053] S106: Pour the mixed concrete into the mold and vibrate it to make it dense. Move the specimen into the test environment within 20 minutes after forming. Connect the strain gauge to the strain meter within two minutes, record the time and detect the strain value.

[0054] S107: After the initial setting of the concrete, the inner plate is removed and the time of sudden change of the strain value is the cracking time of the specimen.

[0055] The embodiments of the present invention have achieved some positive effects during the development or use process, and indeed have great advantages over the prior art. The following content is described in conjunction with data, charts, etc. of the test process.

[0056] like Figure 5 As shown, it shows that the concrete produces non-uniform shrinkage and non-uniform stress inside.

[0057] like Figure 6 As shown, it is shown that concrete cracks under internal non-uniform stress.

[0058] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any modifications, equivalent substitutions and improvements made by any technician familiar with the technical field within the technical scope disclosed by the present invention and within the spirit and principle of the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for non-uniform restrained shrinkage testing, characterized in that: The method of the non-uniform restrained shrinkage test comprises: Step 1: Grind the part of the constraint where the strain gauge needs to be attached; Step 2: Clean the area where the strain gauge needs to be pasted; Step 3: Paste the strain gauge and fix it with epoxy resin; Step 4: Paste cushioning foam on both sides of the restraint and cover with a layer of polyethylene film; Step 5: Install the inner plate and move the processed constraint into the middle of one side of the mold in the length direction; Step 6: Pour the mixed concrete into the mold, vibrate and compact it, move the specimen into the test environment after forming, connect the strain gauge to the strain meter, record the time and detect the strain value; Step 7: After the concrete has initially set, the inner plate is removed, and the time of sudden change in strain value is the cracking time of the specimen; The grinding operation in step 1 uses sandpaper; The cleaning operation in step 2 uses anhydrous ethanol; The cushioning foam in step 4 needs to be reserved during cutting.

2. The method for non-uniform restrained shrinkage testing according to claim 1, characterized in that: In step six, the specimen is moved into the test environment within 20 minutes after the concrete is formed, and the strain gauge is connected to the strain meter within two minutes.

3. A device for performing a non-uniform restrained shrinkage test according to any one of claims 1 to 2, characterized in that: The apparatus for the non-uniform restrained shrinkage test comprises: External mold; The outer mold is a rectangular box structure with an open top and sealed sides, and an inner plate is provided on the inner side of the inner wall of the outer mold; A constraint is placed in the middle of one side of the outer mold in the length direction, and strain gauges are respectively attached and fixed on both sides of the constraint; A layer of buffer foam is attached to both sides of the restraint, and a layer of polyethylene film is covered on the outside of the buffer foam layer; The buffer foam layer is provided with a slot corresponding to the strain gauge, and the strain gauge is located inside the slot; The strain gauge is an L-shaped structure, and the strain gauges on both sides are symmetrically arranged; A concrete layer is filled between the outer side of the restraint and the inner plate.

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