Nondestructive testing method for testing internal steel fiber clotting of UHPC (Ultra High Performance Concrete) test piece

Abstract

The invention discloses a nondestructive testing method for testing steel fiber agglomeration in a UHPC (Ultra High Performance Concrete) test piece, which comprises the following steps: attaching a coiled material to the surface of the UHPC test piece to be tested, and arranging an electrode plate array in one surface of the coiled material; gridding area division is carried out on the surface of the test piece, the resistivity value of each block is tested, and a resistivity distribution cloud picture is drawn; a square formed by four electrode plates is used as a unit, an effective resistance value in the square unit is measured, and a caking rate coefficient is defined and used for representing whether a steel fiber caking phenomenon occurs in a detected area or not. Based on the influence of different distribution of the steel fibers on the resistivity of the test piece, the distribution condition of the steel fibers can be detected, the caking phenomenon can also be detected, and the influence caused by different resistance values of different test pieces can be eliminated to a certain extent. The problem that steel fiber distribution and caking in the UHPC cannot be accurately judged in the existing engineering application is solved, so that whether the UHPC test piece reaches the strength and mechanical properties required by engineering or not can be further judged.

Description

Technical Field

[0001] The invention belongs to the field of steel fiber reinforced concrete detection, and in particular relates to a nondestructive detection method for testing steel fiber agglomeration inside a UHPC specimen. Background Art

[0002] As a new type of building material, UHPC (ultra-high performance concrete) has been gradually studied and applied by many academic and engineering institutions. Compared with traditional concrete, it has higher compressive strength and flexural strength, and can withstand greater loads and stresses, making the structure more stable and strong. In addition, UHPC has good toughness and ductility, and can remain stable when subjected to large loads and deformations, reducing the risk of structural damage, and has significant advantages under load conditions such as earthquake resistance and wind resistance.

[0003] UHPC usually uses steel fibers as its reinforcement material. During the preparation process, the distribution characteristics of steel fibers have a direct impact on the mechanical properties and durability of UHPC materials. For example, the agglomeration of steel fibers will reduce the mechanical properties of UHPC, resulting in a decrease in the load-bearing capacity, seismic resistance and durability of structural components. Therefore, the detection of steel fiber agglomeration can verify whether the quality and performance of UHPC specimens meet the design requirements.

[0004] The methods currently used for UHPC testing can be divided into two categories: destructive testing and non-destructive testing. The most common method of destructive testing is to crush the sample, manually count or weigh the fibers in it to determine the fiber content; then evaluate the direction of fiber distribution by counting the number of fibers on the fracture surface along different directions. This method also destroys the tested piece during testing, and is difficult to apply to on-site product quality control. Non-destructive testing methods also have some problems. For example, X-ray methods are difficult to apply on-site due to their high price and impact on human health. At present, there are few non-destructive testing instruments on the market that can accurately determine (qualitatively or quantitatively) the agglomeration of steel fibers in UHPC, and cannot meet the requirements of measuring the distribution of steel fibers inside UHPC and inferring its mechanical properties. Summary of the invention

[0005] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a non-destructive testing method for detecting the distribution of steel fibers inside a UHPC specimen. The device is simple, convenient and safe to operate. By comparing the size of the agglomeration rate coefficient measured in each area of ​​the specimen, it is possible to judge whether the steel fibers in a certain area inside the UHPC are agglomerated.

[0006] The technical solution adopted by the present invention to solve the technical problem is: A nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen, comprising: (1) Select a coil according to the size of the UHPC specimen to be tested, and attach the coil to the surface of the specimen, wherein one side of the coil has an electrode sheet array built in, and the other side has a weight of the same size as the electrode sheet built in; The electrodes are made of porous foam copper material, and the spacing between each electrode sheet is twice the size of the electrode.

[0007] The coiled material and the weight are both non-conductive insulating materials.

[0008] (2) Divide the surface of the test piece into grid areas. One square unit contains four electrode sheets. There is a block between two adjacent electrode sheets. One square unit contains six blocks, which are in the horizontal, vertical and diagonal directions. When testing the resistance value, the electrode sheets are connected to the LCR detector. (3) Keep the surface of the test piece moist, connect the red and black test leads of the LCR tester to the adjacent electrode sheets respectively, and measure the resistance value Ri of the block between the two electrode sheets. The resistivity calculation formula is: ; Where: ρ is the resistivity, Ri is the resistance value measured in each block, i=1~6; S is the equivalent resistance cross-sectional area, which is equal to the side length of the electrode multiplied by the thickness of the UHPC specimen to be tested; L is the distance between the two electrode sheets; According to the resistivity values ​​of different blocks, a resistivity distribution cloud map is drawn to intuitively display the steel fiber distribution of the test piece; the larger the resistivity value, the lighter the color of the area, indicating that the steel fiber content is less; the smaller the resistivity value, the darker the color of the area, indicating that the steel fiber content is more; (4) When judging whether the steel fiber is clumping, a square composed of four electrode sheets is regarded as a unit, and the effective resistance value in the square unit is measured as: ; The m*n electrode sheet array on the coil forms several square units. For different square units, the effective resistance values ​​are calculated respectively and compared with the average value, and the agglomeration rate coefficient β is defined as: ; Where N represents the number of square units. It represents the effective resistance value of the i-th square unit; the larger the agglomeration rate coefficient is, the less the steel fiber content is; the smaller the agglomeration rate coefficient is, the more the steel fiber content is; if the agglomeration rate coefficient in an area is much less than 1, it proves that fiber agglomeration occurs in the area.

[0009] Beneficial effects of the present invention: The present invention applies the principle of resistivity method to the nondestructive identification of regional clustering of steel fibers in UHPC specimens, which can well detect the quality of UHPC components and has the characteristics of safety, convenience, speed and nondestructiveness, overcoming the economic waste and safety problems caused by traditional detection methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 A schematic diagram of the gridded area division of the test piece surface and resistivity testing in the present invention; Figure 2 This is a schematic diagram of theoretical calculation of effective resistance value of the present invention; Figure 3 It is a schematic diagram of the test of the agglomeration rate coefficient of the present invention. DETAILED DESCRIPTION

[0011] The present invention will be described in detail below with reference to the accompanying drawings and in combination with embodiments. It should be noted that, unless otherwise specified, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the art to which the present application belongs. Example 1

[0012] A nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen, comprising: (1) A coil is selected according to the size of the UHPC specimen to be tested, and the coil is attached to the surface of the specimen. One side of the coil has an electrode sheet array built in, and the other side has a weight of the same size as the electrode sheet built in. When measuring, the weight presses the electrode sheet on the specimen, so that the electrode sheet and the uneven surface of the specimen are in better contact, thereby achieving the purpose of reducing contact resistance.

[0013] The coiled material and the weight are both non-conductive insulating materials. The coiled material can be made of non-woven fabric, and the weight can be made of stone or wood.

[0014] The coil is attached to the surface of the specimen according to the shape of the specimen. For plate specimens, the flat laying method is selected, and for columnar specimens, the wrapping method is selected.

[0015] (2) The surface of the specimen is divided into grid areas. A square unit contains four electrode sheets. There is a block between every two adjacent electrode sheets. A square unit contains six blocks, which are in the horizontal, vertical and diagonal directions. The area of ​​each block is equal to the area of ​​the coil, such as Figure 2 As shown in the figure, the spacing between each electrode piece is twice the size of the electrode, and the electrode piece is connected to the LCR detector through a wire.

[0016] The present embodiment uses a Tonghui TH2811D LCR detector, which can measure the resistance of the area between two electrodes based on the principle of an alternating current two-electrode method.

[0017] The electrode of the present invention is a porous foam copper material, with a length and width of 10 mm and a thickness of 1.5 mm. Metal copper has good electrical conductivity, and its price is not as expensive as rare metals such as silver, and its cost is low; the porous foam structure makes the electrode fit closely and have good contact with the test piece, which can effectively reduce the phenomenon of excessive contact resistance caused by the uneven surface of the test piece.

[0018] (3) Wipe the surface of the test piece with a wet wipe to make it moist but not too wet. Use the red and black test leads of the LCR tester to connect the adjacent electrodes respectively, and wait for a while to eliminate the influence of the polarization reaction. Then, the resistance value Ri of the block between the two electrodes can be measured. The resistivity calculation formula is: ; Where: ρ is the resistivity, Ri is Figure 2 The resistance value measured in each block (i=1~6), S is the equivalent resistance cross-sectional area (equal to the side length of the electrode multiplied by the thickness of the UHPC specimen to be tested), and L is the distance between the two electrode sheets.

[0019] The above method can be used to obtain the resistivity value between each pair of electrode sheets. According to the resistivity values ​​of different blocks, a resistivity distribution cloud map can be drawn to intuitively display the steel fiber distribution of the test piece. Among them, the larger the resistivity value, the lighter the color of the area, indicating that the steel fiber content is less; the smaller the resistivity value, the darker the color of the area, indicating that the steel fiber content is more.

[0020] (4) When judging whether the steel fiber has agglomeration, a square composed of four electrode sheets is regarded as a unit. Assuming that the electrode size is a unit length (1*1), the distance between the two electrodes is 2. Since the test depth of the six blocks (two horizontal, two vertical and two diagonal blocks) is the same, which is the thickness of the specimen or the effective thickness that can be measured by the electrode sheet, the influence of the depth can be eliminated, and the relationship between the resistance values ​​of each area can be judged from the size of the two-dimensional area.

[0021] like Figure 2 As shown, the effective resistance value of this square unit can be obtained by area fitting method. R valid , which is approximately equal to the sum of the resistance values ​​of the six blocks minus the repeated calculation area. The sum of the areas of the six blocks can just cover the entire square unit, and the repeated calculation part (shaded area) is exactly equal to 1 / 2 of the square unit area. Note that all block areas refer to the area between the two electrodes, excluding the electrode area. Therefore, the effective resistance value in the square unit can be approximately considered by the area fitting method as: ; The m*n electrode array on the coil can form several square units. For different square units, the effective resistance values are calculated separately and compared with the average value. The agglomeration rate coefficient β is defined to eliminate the influence of the resistance value of the specimen to be tested itself, facilitating the horizontal comparison of different specimens.

[0022] ; Among them, N represents the number of square units, represents the effective resistance value of the i-th square unit. As Figure 3 shown, the larger the agglomeration rate coefficient, the less the steel fiber content; the smaller the agglomeration rate coefficient, the more the steel fiber content. If the agglomeration rate coefficient in an area is much less than 1, it proves that there is fiber agglomeration in this area. Example 2

[0023] In an embodiment of the present invention, the size of the component to be tested is a cuboid of 150*150*30 mm, containing steel fibers with a diameter of 0.6 mm and a length of 13 mm. Table 1 below shows the resistivity value data measured by the detection method of Example 1 of the present invention when the steel fiber content in the specimen is different.

[0024] Table 1

[0025] It can be found from the data that the less the steel fiber content in the specimen, the larger the measured resistivity value, which is in line with the expectation. Example 3

[0026] In an embodiment of the present invention, the size of the component to be tested is a cylinder with a diameter of 160 mm and a height of 30 mm, containing steel fibers with a diameter of 0.6 mm and a length of 13 mm. The specimen is divided into four regions in a "field" shape. A large number (about 300) of randomly distributed steel fibers are placed in the lower right corner region, and 100 radially distributed steel fibers are placed in each of the other regions. The agglomeration rate coefficients of the four regions are 0.98, 1.47, 1.39, and 0.15 respectively, which are tested and calculated by the detection method of Example 1 of the present invention.

[0027] It can be found from the data that the agglomeration rate coefficient of the agglomeration region (i.e., the region where steel fibers are densely aggregated) in the specimen is 0.15, which is much less than 1 compared with the coefficients of other regions, in line with the expectation.

[0028] In addition, the steel fiber distribution and agglomeration region measured by the detection method of the present invention are basically consistent with the internal situation of the component photographed by CT. Therefore, the results of the detection method of the present invention are correct and suitable for popularization and application.

[0029] The above description is only a preferred embodiment of the present invention and is 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 nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen, characterized in that: include: (1) Select a coil according to the size of the UHPC specimen to be tested, and attach the coil to the surface of the specimen, wherein one side of the coil has an electrode sheet array built in, and the other side has a weight of the same size as the electrode sheet built in; (2) Divide the surface of the specimen into grid areas. One square unit contains four electrode sheets. There is a block between every two adjacent electrode sheets. One square unit contains six blocks, namely, in the horizontal, vertical and diagonal directions. The electrode sheets are connected to the LCR detector. (3) Keep the surface of the test piece moist, connect the red and black test leads of the LCR tester to two adjacent electrode sheets respectively, and measure the resistance value Ri of the block between the two electrode sheets. The resistivity calculation formula is: ； Where: ρ is the resistivity, Ri is the resistance value measured in each block, i=1~6; S is the equivalent resistance cross-sectional area, which is equal to the side length of the electrode multiplied by the thickness of the UHPC specimen to be tested; L is the distance between the two electrode sheets; According to the resistivity values ​​of different blocks, a resistivity distribution cloud map can be drawn to intuitively display the steel fiber distribution of the test piece; the larger the resistivity value, the lighter the color of the area, indicating that the steel fiber content is less; the smaller the resistivity value, the darker the color of the area, indicating that the steel fiber content is more; (4) When judging whether the steel fiber is clumping, a square composed of four electrode sheets is regarded as a unit, and the effective resistance value in the square unit is measured as: ； The m*n electrode sheet array on the coil forms several square units. For different square units, the effective resistance values ​​are calculated respectively and compared with the average value, and the agglomeration rate coefficient β is defined as: ； Where N represents the number of square units. It represents the effective resistance value of the i-th square unit; the larger the agglomeration rate coefficient is, the less the steel fiber content is; the smaller the agglomeration rate coefficient is, the more the steel fiber content is; if the agglomeration rate coefficient in an area is much less than 1, it proves that fiber agglomeration occurs in the area.

2. A nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen according to claim 1, characterized in that: The distance between every two electrode sheets is twice the size of the electrode sheets.

3. A nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen according to claim 1, characterized in that: The electrode is made of porous foam copper material.

4. A nondestructive testing method for testing steel fiber agglomeration inside a UHPC specimen according to claim 1, characterized in that: The coiled material and the weight are both non-conductive insulating materials.

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