Method for testing moisture diffusion coefficient of concrete under surface negative pressure

Abstract

The present application relates to a kind of concrete moisture diffusion coefficient testing methods under the action of surface negative pressure, including negative pressure under the action of concrete humidity field testing system and the moisture diffusion coefficient calculation method based on humidity field.Therein, negative pressure under the action of concrete humidity field testing system is by negative pressure cavity, pressure gauge, controller, negative pressure pump, humidity sensor and data collector;Concrete moisture diffusion coefficient under the action of negative pressure is based on concrete humidity field test data back, and calculation process includes two parts of concrete dry surface diffusion and internal diffusion.The present application provides a kind of concrete humidity field testing method under the action of surface negative pressure and the moisture diffusion coefficient calculation method under the action of negative pressure, by the method of the present application, the internal humidity field variation law of concrete at different surface negative pressure level can be tested and corresponding moisture diffusion coefficient is calculated.The method will provide important reference for railway tunnel lining concrete moisture diffusion and shrinkage deformation research under train operation condition.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of building materials, in particular to a method for testing the moisture diffusion coefficient of concrete under the action of surface negative pressure. BACKGROUND

[0002] A large number of studies have shown that the internal humidity level and moisture diffusion properties of concrete are essential factors affecting its shrinkage deformation. Accurate determination of the internal moisture diffusion coefficient of concrete under dry conditions is the basis and premise for predicting the shrinkage and cracking behavior of concrete. Therefore, to study the cracking mechanism of railway tunnel lining concrete and then propose a shrinkage and cracking prediction method, the key problem to be solved is to accurately determine the internal moisture diffusion coefficient of concrete under negative pressure.

[0003] The internal moisture diffusion coefficient of concrete is a physical quantity used to describe the rate of water diffusion between adjacent capillary pores in concrete, usually represented by D. This parameter is a key factor affecting the internal humidity level and dry shrinkage development of concrete under dry conditions. Surface negative pressure will accelerate the migration rate of internal moisture to the surrounding environment, thereby accelerating the development of dry shrinkage deformation. However, existing patents and research have paid little attention to this issue. Only a few patents have proposed new methods for testing the humidity of concrete under normal conditions. Chinese patent CN202010524168.8 discloses a device for measuring the internal humidity of concrete and a method for arranging the same, which mainly proposes a humidity sensor protective shell. Chinese patent CN202122625789.8 discloses a test device for determining the relative humidity of concrete pores, which essentially proposes a concrete internal humidity testing device that can record data in real time. The above-mentioned patents all propose humidity testing methods for specific points in the internal concrete under normal conditions, but there is little research on the testing of the internal humidity field of concrete under surface negative pressure. More importantly, calculating the moisture diffusion coefficient of concrete based on the test humidity data is extremely important for evaluating the moisture diffusion properties of concrete, but existing patents have not addressed the testing of the moisture diffusion coefficient of concrete. Therefore, it is of great significance to propose a method for testing the internal humidity field and moisture diffusion coefficient of concrete under surface negative pressure to evaluate the shrinkage and cracking risk of tunnel lining concrete under train operation conditions. SUMMARY

[0004] To solve the above problems and accurately test the internal humidity field and moisture diffusion coefficient of concrete under surface negative pressure, the present application aims to provide a system for testing the internal humidity field of concrete under surface negative pressure and a method for calculating the internal moisture diffusion coefficient of concrete based on the humidity field. This method can test and calculate the internal humidity variation and moisture diffusion coefficient of concrete under different surface negative pressure levels, providing an important basis for analyzing the shrinkage and deformation mechanism of railway tunnel lining concrete under train operation conditions.

[0005] The present application is implemented as follows:

[0006] The present application provides a method for testing the moisture diffusion coefficient of concrete under surface negative pressure, comprising the following steps:

[0007] The moisture field testing system for concrete under negative pressure is used to test the relationship between the moisture at a typical position inside the concrete and the curing age, starting from a specified age after the concrete is poured.

[0008] The test results of the moisture of the concrete are substituted into the calculation method to inversely calculate the moisture diffusion coefficient of the concrete under negative pressure.

[0009] In some embodiments, the moisture field testing system for concrete under negative pressure is composed of a negative pressure cavity, a pressure gauge, a controller, a negative pressure pump, a humidity sensor, and a data collector, as shown in Figure 1 The negative pressure cavity is fixed to the dry surface of the concrete specimen to form a surface negative pressure environment; the pressure gauge is fixed above the negative pressure cavity to monitor the pressure value in the negative pressure cavity; the controller is connected to the pressure gauge to control the opening and closing of the negative pressure pump; the negative pressure pipe of the negative pressure pump is connected to the negative pressure cavity, and when the pressure in the negative pressure cavity is lower than the set value, the controller will issue an instruction to start the negative pressure pump until the pressure reaches the set value; the humidity sensor is fixed to a specified position inside the concrete through a PVC pipe, and the humidity data obtained by the test are transmitted and stored in the data collector in real time.

[0010] The edges of the negative pressure cavity and the edges of the dry surface of the concrete are sealed with a high-molecular sealant to ensure the sealing of the negative pressure cavity.

[0011] The pressure gauge can set a target negative pressure value, and when the negative pressure in the cavity is higher than the set value, the pressure gauge triggers the controller to issue an instruction to start the negative pressure pump until the negative pressure value in the cavity reaches the set value.

[0012] The controller can also issue an active pressure relief instruction to realize the disassembly operation of the negative pressure cavity.

[0013] The humidity sensor includes but is not limited to a capacitive humidity sensor, and the humidity sensor is poured into a specified position inside the concrete through a PVC pipe with a bottom opening, and is isolated from the external environment through a sealing ring and a sealant. The arrangement of the humidity sensor and the PVC pipe is shown in Figure 2 .

[0014] The data collector has data collection and storage functions.

[0015] In some embodiments, the method for calculating the moisture diffusion coefficient of concrete under negative pressure is based on the moisture diffusion theory of concrete under negative pressure. The moisture diffusion coefficient of concrete under negative pressure can be calculated by substituting the test results of humidity at a specific position into the method. The calculation of moisture diffusion of concrete under negative pressure is divided into surface diffusion and internal diffusion.

[0016] The surface diffusion of concrete under negative pressure can be described by the following formula. Figure 3 As shown in the formula, the moisture diffusion between the dry surface of concrete and the environment can be described by the following formula.

[0017] J x =a m (H1-H’ e )(1)

[0018] In the formula, J x is the moisture flux of the dry surface of concrete diffusing to the environment (kg / m 2 s); a m is the surface coefficient of concrete, which represents the migration rate of moisture on the surface of concrete to the surrounding environment. Under normal circumstances, this parameter can be taken as a constant 3.72×10 -5 kg / m 2 s; H1 is the humidity value of the dry surface of concrete; H' e is the humidity of the internal environment of the negative pressure cavity, which can be calculated by the following formula.

[0019] (2)

[0020] In the formula, P atm is the atmospheric pressure, taken as 101.325 kPa; Δp is the negative pressure value of the internal environment of the negative pressure cavity; H e is the humidity value of the test environment. Substituting formula (2) into formula (1) can obtain the diffusion equation of the dry surface of concrete.

[0021] The internal diffusion process of concrete under negative pressure can be described by formula (3).

[0022] (3)

[0023] In the formula, H d is the humidity value of the internal environment of the test; x is the coordinate position of the humidity test point from the dry surface; t is the drying time; D is the moisture diffusion coefficient of the internal environment of concrete, kg / m.s. In formula (3), the moisture diffusion coefficient D is a function of relative humidity H d and position x, so the equation is a nonlinear partial differential equation. The above equation is solved by introducing an intermediate variable S:

[0024]

[0025] In the formula, H m is an artificially selected humidity value, which does not affect the final calculation result. Formula (5) can be obtained by simply mathematically transforming formula (4) and substituting it into formula (3).

[0026] (5)

[0027] The present application solves the above equation by using the finite difference method, and the finite difference nodes used are as shown in Figure 4 The finite difference nodes are divided into internal nodes, external surface nodes and internal surface nodes. For the internal nodes, the finite difference equation thereof is shown in formula (6).

[0028] (6)

[0029] For the external surface nodes, the influence of the surface negative pressure on the moisture diffusion still needs to be considered, as shown in Figure 5 After considering the influence of the surface negative pressure, the finite difference equation of the external surface nodes of the concrete is shown in formula (7).

[0030] (7)

[0031] In the formula, h is the interval of the finite difference nodes; H 2 ,t is the humidity value of the second node. For the internal surface nodes, the finite difference equation thereof is shown in formula (8).

[0032] (8)

[0033] The present application uses the back-stepping method to inversely calculate the moisture diffusion coefficient of the concrete based on the humidity test data, and a multi-segment linear function is used to describe the moisture diffusion coefficient in the process.

[0034] D j =k j H d +d j j=1 to N (9)

[0035] In the present application, the moisture diffusion coefficient inverse process is realized as follows: since the moisture diffusion coefficients of the concrete are all unknown before being solved, a multi-segment linear function is used to describe the moisture diffusion coefficient, as shown in formula (9), and the moisture diffusion coefficient is solved from the high-humidity area (H d =1.0) to the low-humidity area. Specifically, formula (9) is used to segmentally and successively assume the moisture diffusion coefficient from the high-humidity area to the low-humidity area, as shown in Figure 6. Each assumed water diffusion coefficient is calculated to obtain the humidity change process of the specific position of the concrete at the corresponding humidity level, and the values of k j and d j in formula (9) are continuously optimized to make the difference between the calculated humidity change process and the test value meet the error requirement. Based on the above method, the water diffusion coefficient of the specific position inside the concrete can be solved based on the test value of the humidity inside the concrete under negative pressure.

[0036] The beneficial effects of the present application relative to the prior art are: the present application provides a method for testing the water diffusion coefficient of concrete under surface negative pressure, a concrete humidity field test system under surface negative pressure is constructed, and a method for calculating the water diffusion coefficient inside the concrete is proposed based on the water diffusion theory. The method can be used to calculate the water diffusion coefficient inside the concrete under different surface negative pressure conditions, and provides an important basis for analyzing the water migration law and shrinkage cracking mechanism of railway tunnel lining concrete under train operation conditions. BRIEF DESCRIPTION OF DRAWINGS

[0037] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed in the following embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only exemplary, and those skilled in the art can also obtain other implementation drawings based on the provided drawings without creating any creative labor.

[0038] The structures, proportions, sizes, etc. shown in the present specification are only used to cooperate with the content disclosed in the specification, to be understood and read by those skilled in the art, and do not define the limiting conditions for the implementation of the present application, so they do not have technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the present application, should still fall within the scope of the technical content disclosed by the present application.

[0039] Figure 1 The negative pressure concrete humidity field test system of a preferred embodiment of the present application is exemplarily shown, wherein (a) is a negative pressure cavity; (b) is a pressure gauge; (c) is a controller; (d) is a negative pressure pump; (e) is a humidity sensor; (f) is a data collector;

[0040] Figure 2 The schematic diagram of the setting mode of the humidity sensor of a preferred embodiment of the present application is exemplarily shown, wherein (a) is a device diagram; (b) is a PVC pipe opening mode;

[0041] Figure 3 The schematic diagram of the water diffusion principle of concrete under surface negative pressure in the present application is exemplarily shown;

[0042] Figure 4 A schematic diagram of a concrete moisture finite difference node illustrating a preferred embodiment of the present application;

[0043] Figure 5 A schematic diagram illustrating the principle of moisture diffusion at the surface of concrete under surface negative pressure in the present application;

[0044] Figure 6 A schematic diagram of concrete moisture diffusion coefficient back-calculation illustrating a preferred embodiment of the present application;

[0045] Figure 7 A schematic diagram of the relationship between the moisture at a typical position inside concrete and the curing age illustrating a preferred embodiment of the present application;

[0046] Figure 8 A schematic diagram of the results of concrete moisture diffusion coefficient back-calculation illustrating a preferred embodiment of the present application, wherein (a) is the back-calculated moisture diffusion coefficient, and (b) is the comparison between the calculated and experimental values of moisture. DETAILED DESCRIPTION

[0047] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the present application will be further described in detail below with reference to the embodiments and drawings. Herein, the illustrative embodiments of the present application and their descriptions are used to explain the present application, but are not intended to limit the present application.

[0048] In the description of the present application, the terms “comprise / comprising” and “consist of” or any other variant thereof are intended to cover non-exclusive inclusion, so that a product, device, process or method comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or further includes elements inherent to such product, device, process or method. Without more limitations, the elements defined by the statement “comprise / comprising” and “consist of” do not exclude the presence of other identical elements in the product, device, process or method comprising the elements.

[0049] It should be understood that, unless otherwise specified and limited, the terms “arrange”, “mount”, “connect”, “connect”, “fix” and the like should be understood in a broad sense, for example, it can be any suitable arrangement, it can be fixed connection or detachable connection, or it can be integrated; it can be mechanical connection or electrical connection; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0050] It should also be understood that the terms "upper", "lower", "front", "rear", "left", "right", "top", "bottom", "inner", "outer", "center", and the like refer to the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices, components or structures referred to must have a particular orientation, be constructed or operated in a particular orientation, and cannot be understood as a limitation of the present application.

[0051] The implementation of the present application is described in detail below in combination with the preferred embodiments.

[0052] The C40 concrete commonly used in railway tunnels is selected as the research object in this embodiment, and the concrete mix proportion is shown in Table 1.

[0053] Table 1: Tunnel lining C40 concrete mix proportion

[0054]

[0055] After the concrete is prepared using the above mix proportion, a 400mmx100mmx100mm cubic test block is cast and formed, and three humidity sensors are arranged at a distance of 2cm, 7cm and 12cm from the dry surface, and the mold is removed after 1d of curing. A plastic film is used to cover the surface of the concrete test piece, and only the end section (100mmx100mm) is left as the dry surface. The negative pressure cavity proposed in the present application is placed on the dry surface and sealed with sealing glue, and the air pressure in the negative pressure cavity is maintained at -50 kPa, as shown in Figure 1 The concrete humidity test piece is tested from the time of mold removal to 60d, and the obtained typical concrete internal humidity test results are shown in Figure 7 The water diffusion coefficient of the concrete under negative pressure obtained by the back-calculation method proposed in the present application is shown in Figure 8 .

Claims

1. A method for testing the water diffusion coefficient of concrete under surface negative pressure, characterized in that... This includes a concrete humidity field testing system under negative pressure and a method for calculating the moisture diffusion coefficient. The concrete humidity field testing system under negative pressure consists of a negative pressure chamber, a pressure gauge, a controller, a negative pressure pump, a humidity sensor, and a data acquisition unit. The negative pressure chamber is fixed to the dry surface of the concrete specimen to create a surface negative pressure environment. The pressure gauge is fixed above the negative pressure chamber to monitor the pressure value inside the chamber. The controller is connected to the pressure gauge to control the opening and closing of the negative pressure pump. The negative pressure pipe of the negative pressure pump is connected to the negative pressure chamber. When the pressure inside the negative pressure chamber is lower than the set value, the controller will issue a command to start the negative pressure pump until the pressure reaches the set value and then stops working. The humidity sensor is fixed inside the concrete at a designated position away from the negative pressure dry surface via a PVC pipe. The humidity data obtained from the test is transmitted in real time and stored in the data acquisition unit. The concrete moisture diffusion coefficient under negative pressure is derived from concrete humidity field test data, and the calculation process includes two parts: diffusion on the dry concrete surface and diffusion within the concrete. Specifically, The moisture diffusion between the dry concrete surface and the environment is described by the following formula: J x =a m (H1-H' e (1) ; In the formula, J x Moisture flux from a dry concrete surface to the environment (kg / m²) 2 s); a m H1 is the surface coefficient of the concrete; H2 is the moisture content of the dry concrete surface; H' e The humidity inside the negative pressure chamber is calculated using the following formula: (2) ； In the formula, P atm The ambient atmospheric pressure is represented by Δp, which is the negative pressure value inside the negative pressure chamber. e The humidity value represents the ambient humidity during the test. Substituting formula (2) into formula (1) yields the diffusion equation for the dry concrete surface. The diffusion process inside concrete under negative pressure is described by formula (3): (3) ； In the formula, H d t represents the internal humidity value of the concrete obtained from the test; x represents the coordinate position of the humidity test point from the drying surface; t represents the drying time; and D represents the internal moisture diffusion coefficient of the concrete. The above equation can be solved by introducing an intermediate variable S: ； In the formula, H m A humidity value selected manually; By performing a simple mathematical transformation on formula (4) and substituting it into formula (3), we can obtain formula (5): (5) ； Formula (5) is solved using the finite difference method. The finite difference nodes used are divided into internal nodes, external surface nodes and internal surface nodes. For internal nodes, the finite difference equation is given by equation (6): (6) ； For external surface nodes, the effect of surface negative pressure on moisture diffusion still needs to be considered. After considering the surface negative pressure, the finite difference equation for the concrete external surface node is shown in formula (7): (7) ； In the formula, h is the finite difference node spacing; H 2 ,t The humidity value at the second node; For the inner surface nodes, the finite difference equation is given by formula (8): (8) ； The concrete moisture diffusion coefficient was calculated by reverse calculation based on humidity test data using a backward method. Multiple linear functions were used to describe the moisture diffusion coefficient during the process. D j =k j H d +d j j=1 to N (9) 。 2. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The edge of the negative pressure cavity is sealed with polymer sealant to ensure the airtightness of the negative pressure cavity.

3. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The pressure gauge can be set to a target negative pressure value. When the negative pressure inside the cavity is higher than the set value, the pressure gauge triggers the controller to issue a command to start the negative pressure pump until the negative pressure inside the cavity reaches the set value.

4. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The controller can also issue an active pressure relief command to enable the disassembly of the negative pressure chamber.

5. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The humidity sensor mentioned includes, but is not limited to, a capacitive humidity sensor. The humidity sensor is cast into a designated location inside the concrete through a PVC pipe with a bottom opening, and is isolated from the external environment by a sealing ring and sealant.

6. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The data acquisition device described has data acquisition and storage functions.

7. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The concrete moisture diffusion coefficient under negative pressure is obtained by reverse deduction based on the concrete humidity test value under negative pressure. The concrete moisture diffusion coefficient under negative pressure can be calculated by substituting the humidity test results of a specific location obtained from the test into this method.

8. The method for testing the water diffusion coefficient of concrete under surface negative pressure according to claim 1, characterized in that... The concrete moisture diffusion coefficient test under negative pressure includes the following steps: Starting from a specified age after concrete pouring, the relationship between humidity at typical locations inside the concrete under surface negative pressure and curing age was tested. The concrete humidity test results were substituted into the calculation method to calculate the water diffusion coefficient of concrete under negative pressure.

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