A test method for determining isothermal adsorption curve of concrete under sustained load

Abstract

The application discloses a kind of test methods for determining concrete isothermal adsorption curve under sustained load, belong to civil engineering technical field, this method utilizes bolt combination to exert axial compressive stress to concrete test piece, and strain gauge is pasted in the middle of bolt, force sensor is arranged at the contact place of nut and steel plate, the load exertion of concrete test piece is realized continuously, monitorable and controllable, and the mass change of concrete in hygroscopic or dehumidification process is determined using real-time mass measuring equipment, the load is dynamically checked and adjusted by real-time monitoring bolt strain and contact force change, so that the target load level of concrete test piece is kept long-term in thermal-humid coupling environment;And the isothermal adsorption or desorption curve of concrete under sustained load is obtained by calculation.The method can also carry out concrete heat transport and moisture transport test under controlled sustained load condition, to provide reliable test means for studying the transport performance of concrete under thermal-humid-force multi-field coupling condition.

Description

Technical Field

[0001] This invention belongs to the field of civil engineering technology, specifically relating to a test method for determining the isothermal adsorption curve of concrete under continuous load. Background Technology

[0002] As a typical porous material, the internal moisture state of concrete has a significant impact on its mechanical properties, durability, and heat and moisture transport behavior. Isothermal adsorption and desorption curves can quantitatively reflect the moisture content of concrete under different relative humidity conditions, and are important fundamental parameters for studying the moisture transport mechanism, drying shrinkage, creep, and durability degradation of concrete.

[0003] In existing technologies, the determination of isothermal adsorption or desorption curves in concrete typically employs static testing under no-external-force conditions. This involves placing the specimen in environments with different relative humidity levels and periodically weighing the specimen to obtain data on the moisture absorption or desorption process. However, in actual engineering structures, concrete is usually under continuous load or service stress for extended periods. Continuous load can cause pore structure evolution, microcrack development, and changes in the interface transition zone, thus significantly affecting moisture adsorption, desorption, and transport behavior. Furthermore, achieving complete relative humidity equilibrium within the concrete is virtually impossible.

[0004] Existing experimental studies on isothermal adsorption or desorption curves of concrete have the following main problems: (1) it is difficult to maintain a constant load during long-term tests; (2) there is no dynamic verification and adjustment of the load; (3) it is impossible to obtain the relative humidity distribution inside the concrete; and (4) there is a lack of experimental studies on isothermal adsorption or desorption curves of concrete under sustained load.

[0005] Therefore, when conducting tests to determine the isothermal adsorption or desorption curves of concrete under continuous load, how to stably apply and maintain the target load level for a long period of time, obtain the relative humidity distribution inside the concrete, and accurately obtain the quality changes of the concrete during the test remains a key challenge in the existing technology. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a test method for determining the isothermal adsorption curve of concrete under continuous load, which can apply and maintain a stable continuous load on concrete specimens for a long time, while realizing real-time monitoring and correction of the load, obtaining the distribution of relative humidity inside the concrete, and accurately determining the mass change law of concrete during moisture absorption or desorption, so as to obtain the isothermal adsorption or desorption curve of concrete that is closer to the actual engineering service state.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention discloses a test method for determining the isothermal adsorption curve of concrete under continuous load, comprising the following steps: S1. Determine the basic parameters of the concrete specimen and the bolts used for loading, and calculate the compressive strength load of the concrete specimen based on the basic parameters. Preload of a single bolt Bolt torque value Strain of a single bolt The concrete specimens were subjected to pressure loading using a testing device until the sustained load on the concrete specimens reached the preset value. ; S2. The loaded concrete specimens are placed in a test chamber under a heat and humidity coupling environment for isothermal maintenance, and their humidity and mass changes are monitored; wherein, the interior of the concrete specimens is equipped with side-by-side embedded structures along the height direction. A humidity sensor; a mass sensor is installed at the bottom of the concrete specimen to monitor changes in the quality of the concrete specimen. S3. Real-time monitoring of concrete specimens during moisture absorption or desorption within the test chamber, at any given time. The quality of concrete Moisture content and readings from various humidity sensors inside the concrete. ; S4. Fit the isothermal adsorption or desorption curves of concrete using the GAB model. : In the formula, The equilibrium adsorption amount using the GAB model; This refers to the amount of adsorption on a single layer. The adsorption energy constant of the first layer of water vapor molecules; It is the multilayer adsorption energy constant; This refers to relative humidity.

[0008] Furthermore, in step S3, by measuring the first The water content is calculated from the mass at a given time. Calculated using the following formula: In the formula, The solid mass of concrete when it is absolutely dry. This refers to the quality of concrete in its hydrated state.

[0009] Furthermore, the isothermal adsorption or desorption curve fitting process of concrete is as follows: A1. Based on the location of the measuring point of the internal humidity sensor of the concrete from the center of the cross section. Divide the concrete section into Given a set of concentric circles, calculate the area of ​​each circle. as well as The total area of ​​the concentric circles ,in: In the formula, The radius of the cylindrical concrete specimen is given.

[0010] A2. Select the concrete isothermal adsorption or desorption model to be fitted. Calculate the time of each concentric circle. Moisture content : A3. Construct the fitting expression: ; A4. Use the least squares method to fit the fitting expression in step A3 and solve for the correlation coefficient. .

[0011] Furthermore, in step S1, the compressive strength load of the concrete specimen... Preload of a single bolt Bolt torque value Strain of a single bolt Calculated using the following formula: In the formula, Let be the radius of the cylindrical concrete specimen. This refers to the compressive strength of concrete. Pi The preset value for the continuous load on the concrete specimen. The number of bolts. This is the torque coefficient of the bolt. The nominal diameter of the bolts used for the loading device This represents the diameter of the bolt at the non-threaded section. This is the elastic modulus of the bolt.

[0012] Furthermore, the test apparatus includes steel plates fixedly installed on the upper and lower sides of the concrete specimen. The two steel plates are fixed by four bolts arranged circumferentially around the axis of the concrete specimen. Each bolt includes a screw and a nut threadedly connected to the screw. Rotating the nuts on both sides of the screw causes the two steel plates to clamp the concrete specimen. Applying torque to the nuts can adjust the pressure of the steel plates on the concrete specimen. A strain gauge for measuring the axial strain of the screw is attached to the middle of the screw. A force sensor for measuring the contact force applied by the nut to the steel plate is installed between the nut and the steel plate.

[0013] Furthermore, the method of loading concrete specimens using bolts includes: The concrete specimen was placed at the center between the two steel plates, and a continuous load was applied according to the pre-set parameters for the concrete specimen. Torque is applied to the nuts, and the application is carried out in a symmetrical, step-by-step manner from the inside out: first, torque is applied to the bolts located close to the center of the specimen. Then apply force to the bolts at their centrally symmetrical positions. Repeat this process until all bolts are applied. After determining the torque, apply the torque values ​​for each bolt in the same order. Repeat this loading process until all bolts have reached the design torque value. .

[0014] Furthermore, during the experiment, bolt strain and force sensor data were continuously monitored. When the load was detected to deviate from the target value, the load was corrected by readjusting the tightness of the nuts in order to maintain the stability of the load on the concrete specimen.

[0015] Furthermore, the surfaces of the concrete specimens that come into contact with the steel plate are coated with epoxy resin.

[0016] Furthermore, the humidity sensor is wrapped with a hydrophobic, dustproof, and heat-dissipating film on its outer side.

[0017] The beneficial effects of this invention are as follows: 1. This invention can apply and maintain a stable continuous load on concrete specimens, while simultaneously achieving real-time monitoring and correction of the load, obtaining the distribution of relative humidity inside the concrete, and accurately determining the mass change law of concrete during moisture absorption or desorption, thus obtaining isothermal adsorption or desorption curves of concrete that are closer to the actual engineering service conditions. 2. This invention can stably apply continuous loads during long-term tests; it can monitor and correct the load in real time through the combination of strain gauges and force sensors; it can obtain the real-time relative humidity distribution inside the concrete; it can obtain the isothermal adsorption or desorption curves of the concrete under load; the test method has a simple structure, strong repeatability, and is easy to promote and apply. Attached Figure Description

[0018] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following figures are provided for illustration: Figure 1 This is a schematic diagram of the experimental device structure according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the dimensions of the loading steel plate according to an embodiment of the present invention; Figure 3 This is a schematic diagram of the cross-sectional division of a concrete specimen according to an embodiment of the present invention; Figure 4 This is a schematic diagram of the arrangement of humidity sensors inside the concrete specimen according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the isothermal adsorption and desorption curves of concrete in an embodiment of the present invention.

[0019] The following are labeled in the attached diagram: 1. Concrete specimen; 2. Steel plate; 3. Screw; 4. Nut; 5. Strain gauge; 6. Force sensor; 7. Epoxy resin. Detailed Implementation

[0020] like Figures 1-5 As shown, this invention discloses a test method for determining the isothermal adsorption curve of concrete under continuous load, comprising the following steps: S1. Determine the basic parameters of the concrete specimen and the bolt group used: The basic parameters mentioned include the radius of the concrete specimen. The height of the specimen The compressive strength of concrete Number of humidity sensors inside the concrete Drying quality of concrete specimens The nominal diameter of the bolts used in the loading and loading devices Diameter of bolts at non-threaded locations The elastic modulus of bolts Number of bolts Bolt torque coefficient The level of the applied load ; S2. Application, real-time monitoring, dynamic verification and adjustment of continuous load: Calculate the compressive strength load of concrete based on the aforementioned basic parameters. Preload of a single bolt Bolt torque value Strain of a single bolt : S3. Based on the above calculation parameters, a loading test is conducted on the concrete specimen 1 using a testing device. The testing device includes steel plates 2. The concrete specimen 1 is fixedly installed between two steel plates 2. Through holes are symmetrically arranged at the four corners of the two steel plates 2. Bolts for connecting the two steel plates 2 are installed in two through holes on the same axis. The axial direction of the bolts is parallel to the compression direction of the concrete specimen 1. The bolts include a screw 3 that passes through the two through holes on the same axis and nuts 4 that are threaded to both ends of the screw 3. The nuts 4 are connected to the screw 3 so that the two steel plates 2 clamp the concrete specimen 1. Rotating the nuts 4 can adjust the pressure of the upper steel plate 3 on the concrete specimen 1. A strain gauge 5 is attached to the middle of the screw 3 for measuring the axial strain of the screw 3. A force sensor 6 is installed between the nuts 4 and the steel plates 2 for measuring the contact force applied by the nuts 4 to the steel plates 2. The method for loading concrete specimen 1 with bolts is as follows: Concrete specimen 1 is positioned at the center between two upper and lower steel plates 2; torque is applied to the bolts according to different load levels, and the torque is applied in a symmetrical, step-by-step manner from the inside out: The bolts are loaded sequentially... Figure 2 Apply 0.2T to bolts ①, ②, ③, and ④, and then apply the torque value to each bolt in the same order. Repeat this loading process, using the method described above, until all bolts have reached the design torque value. After the torque value is applied, the strain values ​​of each bolt and the force sensor readings are recorded to calibrate the applied load level. This method ensures that the specimen is not subjected to excessive eccentric loads during bolt loading. The strain values ​​of each bolt and the force sensor readings are recorded after the torque value is applied to calibrate the applied load level, ensuring that the expected target load is achieved.

[0021] During the experiment, the bolt strain and force sensor data were continuously monitored. When the load deviated from the target value, the load was corrected by readjusting the tightening of nut 4 to maintain the stability of the load on the concrete specimen. S4. Obtaining the relative humidity distribution inside the concrete: Select a humidity sensor with a suitable measurement range and accuracy. During the pouring of concrete specimen 1, humidity sensors were evenly distributed along the height of the specimen and from the center outwards along the radial direction. To prevent moisture from seeping into the humidity sensors and damaging them, a hydrophobic, dustproof, and heat-dissipating film was wrapped around the outside of each sensor during the pouring process. During the experiment, a layer of epoxy resin 7 was evenly applied to both the upper and lower surfaces of the concrete specimen 1. The influence of different heights was ignored, and the concrete was assumed to transmit moisture in one dimension radially. Therefore, by reading the humidity sensor readings during the experiment, the relative humidity at different locations inside the concrete specimen could be obtained in real time, thus revealing the relative humidity distribution inside the concrete specimen. S5. Determination of concrete quality: The entire loaded concrete specimen 1 was placed above a high-precision mass sensor and together in an environmental chamber to monitor changes in concrete quality in real time throughout the test.

[0022] S6. Calculation of isothermal adsorption or desorption curves: A1. Based on the location of the measuring point of the internal humidity sensor of the concrete from the center of the cross section. The concrete section was divided into 5 concentric circles, and the area of ​​the 5 circles was calculated. : A2. The experimental data during the moisture absorption and release process are shown in the table below. The data includes the mass of the concrete. Corresponding moisture content Readings of various humidity sensors inside the concrete at the same time .

[0023] Moisture absorption process: Dehumidification process: Isothermal adsorption or desorption curves of concrete fitted using the GAB model In the formula, To balance the adsorption amount ; Monolayer adsorption capacity ; The adsorption energy constant of the first layer of water vapor molecules; It is the multilayer adsorption energy constant; This refers to relative humidity.

[0024] Using the above experimental process, the sustained load level was calculated by employing least squares fitting. The following table shows the fitting results of the isothermal adsorption and desorption curves of concrete using the GAB model: The isothermal adsorption and desorption curves of concrete are plotted as follows: Figure 5 As shown.

[0025] This method can be used not only with the GAB model, but also with the BET model or other adsorption or desorption curve models that characterize the relationship between equilibrium adsorption and relative humidity. The physical quantities required for fitting are the same, namely water content and relative humidity.

[0026] Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that various changes can be made to it in form and detail without departing from the scope defined by the claims of the present invention.

Claims

1. A test method for determining the isothermal adsorption curve of concrete under continuous load, characterized in that, Includes the following steps: S1. Determine the basic parameters of the concrete specimen and the bolts used for loading, and calculate the compressive strength load of the concrete specimen based on the basic parameters. Preload of a single bolt Bolt torque value Strain of a single bolt The concrete specimens were subjected to pressure loading using a testing device until the sustained load on the concrete specimens reached the preset value. ; S2. The loaded concrete specimens are placed in a test chamber under a heat and humidity coupling environment for isothermal maintenance, and their humidity and mass changes are monitored; wherein, the interior of the concrete specimens is equipped with side-by-side embedded structures along the height direction. A humidity sensor; a mass sensor is installed at the bottom of the concrete specimen to monitor changes in the quality of the concrete specimen. S3. Real-time monitoring of concrete specimens during moisture absorption or desorption within the test chamber, at any given time. The quality of concrete Moisture content and readings from various humidity sensors inside the concrete. ; S4. Fit the isothermal adsorption or desorption curves of concrete using the GAB model. : In the formula, The equilibrium adsorption amount using the GAB model; This refers to the amount of adsorption on a single layer. The adsorption energy constant of the first layer of water vapor molecules; It is the multilayer adsorption energy constant; Relative humidity; The process of fitting the isothermal adsorption or desorption curve of concrete is as follows: A1. Based on the location of the measuring point of the internal humidity sensor of the concrete from the center of the cross section. Divide the concrete section into Given a set of concentric circles, calculate the area of ​​each circle. as well as The total area of ​​the concentric circles ,in: In the formula, The radius of the cylindrical concrete specimen; A2. Select the concrete isothermal adsorption or desorption model to be fitted. Calculate the time of each concentric circle. Moisture content : A3. Construct the fitting expression: ; A4. Use the least squares method to fit the fitting expression in step A3 and solve for the correlation coefficient. .

2. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: In step S3, by measuring the first The water content is calculated from the mass at a given time. Calculated using the following formula: In the formula, The solid mass of concrete when it is absolutely dry. This refers to the quality of concrete in its hydrated state.

3. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: In step S1, the compressive strength load of the concrete specimen is... Preload of a single bolt Bolt torque value Strain of a single bolt Calculated using the following formula: In the formula, Let be the radius of the cylindrical concrete specimen. This refers to the compressive strength of concrete. Pi The preset value for the continuous load on the concrete specimen. The number of bolts. This is the torque coefficient of the bolt. The nominal diameter of the bolts used for the loading device This represents the diameter of the bolt at the non-threaded section. This is the elastic modulus of the bolt.

4. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: The test apparatus includes steel plates fixedly installed on the upper and lower sides of a concrete specimen. The two steel plates are fixed by four bolts arranged circumferentially around the axis of the concrete specimen. Each bolt includes a threaded rod and a nut threadedly connected to the threaded rod. Rotating the nuts on both sides of the threaded rod causes the two steel plates to clamp the concrete specimen. Applying torque to the nuts can adjust the pressure of the steel plates on the concrete specimen. A strain gauge for measuring the axial strain of the threaded rod is attached to the middle of the threaded rod. A force sensor for measuring the contact force applied by the nut to the steel plate is installed between the nut and the steel plate.

5. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: Methods of loading concrete specimens using bolts include: The concrete specimen was placed at the center between the two steel plates, and a continuous load was applied according to the pre-set parameters for the concrete specimen. Torque is applied to the nuts, and the application is carried out in a symmetrical, step-by-step manner from the inside out: first, torque is applied to the bolts located close to the center of the specimen. Then apply force to the bolts at their centrally symmetrical positions. Repeat this process until all bolts are applied. After determining the torque, apply the torque values ​​for each bolt in the same order. Repeat this loading process until all bolts have reached the design torque value. .

6. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: During the experiment, bolt strain and force sensor data were continuously monitored. When the load deviated from the target value, the load was corrected by readjusting the tightness of the nuts to maintain the stability of the load on the concrete specimen.

7. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: The surfaces of the concrete specimens that come into contact with the steel plates are all coated with epoxy resin.

8. The test method for determining the isothermal adsorption curve of concrete under continuous load according to claim 1, characterized in that: The humidity sensor is wrapped with a hydrophobic, dustproof, and heat-dissipating film.

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