Hybrid fiber reinforced geopolymer concrete creep test method and system

Abstract

The application discloses a kind of hybrid fiber reinforced geopolymer concrete creep test method and system, it is related to hybrid fiber reinforced geopolymer concrete technical field, preparation different fiber species creep test piece;Different loading conditions are set, and the compression creep test is carried out to the creep test piece;In the compression creep test process, the creep data of each creep test piece is collected, and the creep data is handled;Based on the creep data after processing, the creep performance of hybrid fiber reinforced geopolymer concrete is evaluated.The application focuses on the creep behavior of GPC and HFGPC under long-term compression load.By analyzing the influence of fiber type, axial stress ratio and confining pressure level on creep characteristics, the deformation law of fiber reinforced composite materials under long-term service condition is revealed, which provides a theoretical basis for the engineering application of HFGPC and a reference for the prediction of its creep coefficient.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of concrete performance testing, and more particularly to a hybrid fiber reinforced geopolymer concrete creep testing method and system. BACKGROUND

[0002] As a promising structural material, the mechanical properties of HFGPC directly affect the safety and durability of engineering structures. Therefore, it is equally important to explore the deformation characteristics of HFGPC under short-term and long-term compression loads to ensure the service stability of materials and structures. The study of short-term compression load cannot fully reflect the performance of GPC and its HFGPC under long-term load in actual engineering. For the study of structural materials, not only the complete process of experiencing elastic, elastic-plastic deformation and finally failure under short-term load should be discussed, but also the deformation characteristics accumulated over time under constant load, i.e. creep, should be focused on. Creep has a crucial impact on the long-term performance of structural materials, especially in engineering fields such as long-span structures, bridges, building foundations, etc. that bear long-term loads. Creep may cause excessive deformation, shorten the service life, or even failure of the structure.

[0003] Therefore, how to provide a hybrid fiber reinforced geopolymer concrete creep testing method and system, focusing on the creep behavior of GPC and HFGPC under long-term compression load, and revealing the deformation law of fiber reinforced composite materials under long-term service state by analyzing the influence of fiber type, axial compression stress ratio and confining pressure level on creep characteristics is a problem that needs to be solved by those skilled in the art. SUMMARY

[0004] Therefore, the present application provides a hybrid fiber reinforced geopolymer concrete creep testing method and system, focusing on the creep behavior of GPC and HFGPC under long-term compression load. By analyzing the influence of fiber type, axial compression stress ratio and confining pressure level on creep characteristics, the deformation law of fiber reinforced composite materials under long-term service state is revealed, providing a theoretical basis for the engineering application of HFGPC and a reference for the prediction of its creep coefficient.

[0005] To achieve the above-mentioned purpose, the present application adopts the following technical solution: a hybrid fiber reinforced geopolymer concrete creep testing method, comprising:

[0006] Preparation of creep test pieces of different fiber types;

[0007] Setting different loading conditions, and performing compression creep test on the creep test pieces;

[0008] During the compression creep test, the creep data of each creep test piece are collected, and the creep data are processed;

[0009] Based on the processed creep data, the creep performance of the hybrid fiber reinforced geopolymer concrete is evaluated.

[0010] Preferably, the compression creep test adopts a pseudo-triaxial compression creep test.

[0011] Preferably, the step of the pseudo-triaxial compression creep test comprises:

[0012] (1) Assemble the pressure chamber components and the creep test piece from bottom to top in sequence, install on the creep tester, move the position of the pressure chamber to the top of the oil cylinder, center with the pressure sensor on the creep tester through the positioning bolt, and tighten the oil supply pipe;

[0013] (2) Connect the sensor leads of the prepared creep test piece and the two shrinkage compensation test pieces to the data acquisition instrument;

[0014] (3) Turn on the data acquisition system, turn on the pseudo-triaxial creep test machine control system, perform preloading, repeat the preloading process twice, and check whether the data acquisition system and the sensor are working normally;

[0015] (4) After confirming that the data acquisition system and the sensor are working normally, set the loading parameters on the pseudo-triaxial creep test machine control system; record the initial strain value, create a new data acquisition file, and start loading;

[0016] (5) Record the strain value after loading is completed, subtract the strain value before loading to obtain the initial creep strain, and record the strain value of the shrinkage compensation test piece as the reference value for subsequent calculation of natural shrinkage; Preferably, the preloading comprises first setting a first loading, the axial compression value is 20% of the preset axial compression, and the confining pressure value is 50% of the preset confining pressure, and after completing the first loading, unloading.

[0017] Preferably, the creep data comprises instantaneous elastic strain, free shrinkage strain, creep, and total creep during loading.

[0018] ε c = ε t - ε0- ε s ;

[0019] In the formula, ε c is the creep, ε t is the total creep at a certain loading age, ε0 is the instantaneous elastic strain at loading, and ε s is the shrinkage at the age.

[0020] Preferably, the creep degree and the creep coefficient are used to evaluate the creep performance of the concrete.

[0021] Preferably, the creep coefficient is the ratio of the creep deformation to the elastic deformation, and is expressed as:

[0022]

[0023] In the formula, is the creep coefficient at the loading time t (d) ;

[0024] The creep degree is the creep strain under the action of unit stress, which is expressed as:

[0025]

[0026] In the formula, C t is the creep degree at the loading time t (d) ; σ c is the creep stress;

[0027] The relationship between the creep degree and the creep coefficient is as follows, which is expressed as:

[0028]

[0029] In the formula, E t is the elastic modulus;

[0030] The creep function is the sum of the instantaneous elastic strain and the creep strain generated by unit stress, which is expressed as:

[0031]

[0032] Both the creep function and the creep degree can be expressed by the creep coefficient.

[0033] Preferably, the creep data is processed, including: intercepting data of several hours before and after the loading time in the data collected every day; then aggregating the intercepted data and taking the average value as the representative value of the strain of the day.

[0034] Preferably, a hybrid fiber reinforced polymer concrete creep test system comprises:

[0035] A creep test piece preparation module is used to prepare creep test pieces of different fiber types;

[0036] A test module is used to set different loading conditions and perform compression creep tests on the creep test pieces;

[0037] A data acquisition and processing module is used to collect creep data of each creep test piece during the compression creep test and process the creep data;

[0038] A creep performance evaluation module is used to evaluate the creep performance of the hybrid fiber reinforced polymer concrete based on the processed creep data.

[0039] Compared with the prior art, the application provides a hybrid fiber reinforced polymer concrete creep test method, which comprises the following steps: preparing creep test pieces of different fiber types; setting different loading conditions to compress the creep test pieces; collecting creep data of the creep test pieces during the compression creep test, and processing the creep data; and evaluating the creep performance of the hybrid fiber reinforced polymer concrete based on the processed creep data. The application focuses on the creep behavior of GPC and HFGPC under long-term compression load. By analyzing the influence of fiber type, axial compression stress ratio and confining pressure level on the creep characteristics, the deformation law of the fiber reinforced composite material under long-term service state is revealed, a theoretical basis for the engineering application of HFGPC is provided, and a reference for the creep coefficient prediction of HFGPC is provided. BRIEF DESCRIPTION OF DRAWINGS

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings in the following description only constitute the embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of the provided drawings.

[0041] Figure 1 A hybrid fiber reinforced polymer concrete creep test method flowchart is provided for the embodiments of the present application.

[0042] Figure 2 A structure schematic diagram of the pressure chamber and each component in the pseudo triaxial creep test is provided for the embodiments of the present application.

[0043] Figure 3 A comparison between the original creep data and the processed data (taking P group uniaxial longitudinal strain as an example) is provided for the embodiments of the present application.

[0044] Figure 4 A typical compression stress and total strain synchronous process line schematic diagram is provided for the embodiments of the present application.

[0045] Figure 5 A total strain-time full curve schematic diagram of each group of test pieces under uniaxial compression is provided for the embodiments of the present application.

[0046] Figure 6 A total strain-time full curve schematic diagram of each group of test pieces under pseudo triaxial compression is provided for the embodiments of the present application.

[0047] Figure 7 A relationship schematic diagram of volume creep and volume stress is provided for the embodiments of the present application.

[0048] Figure 8A schematic diagram of the relationship between partial creep and partial stress provided for the embodiment of the present application.

[0049] Figure 9 A schematic diagram of the pseudo triaxial compression creep curve provided for the embodiment of the present application.

[0050] Figure 10 A schematic diagram of the influence of confining pressure / axial pressure on HFGPC creep provided for the embodiment of the present application.

[0051] Figure 11 A schematic diagram of the influence of the same ratio increase of axial pressure and confining pressure on HFGPC creep provided for the embodiment of the present application.

[0052] Figure 12 A schematic diagram of the pseudo triaxial creep Poisson's ratio curve provided for the embodiment of the present application.

[0053] Figure 13 A schematic diagram of the creep coefficient under different axial pressure stress ratios provided for the embodiment of the present application.

[0054] Figure 14 A schematic diagram of the pseudo triaxial creep hysteresis recovery curve provided for the embodiment of the present application.

[0055] Figure 15 A schematic diagram of the pseudo triaxial creep recovery coefficient and recovery ratio provided for the embodiment of the present application. DETAILED DESCRIPTION

[0056] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0057] The embodiment of the present application discloses a hybrid fiber reinforced geopolymer concrete creep test method, as shown in Figure 1 including: preparing creep test pieces of different fiber types;

[0058] Specifically, the test adopts a cylindrical creep test piece with a size of φ150mm*450mm, uses an internal sensor to test the deformation of the creep test piece, and pours the creep test piece using a customized steel mold. During the pouring process, it is divided into 5 steps. First, pour half and vibrate it. Then, put wooden sticks along the axial direction and fix them with iron wire to prevent them from drifting in the slurry. Then, cover the film and move it into the constant temperature and humidity curing box for 10 minutes. After the slurry has initial reaction and has supporting force, remove the wooden sticks and place them into the 100mm sensor. Then, pour a small amount of mixture over the axial sensor, vibrate it, and then put the wooden sticks along the cross-sectional diameter. Cover the film and cure for 5 minutes, then remove the wooden sticks and place them into the 60mm sensor. Perform the last step of pouring and vibrating, and finally, fold the sensor wires and fix them at the center position. After pouring is completed, cover the film and move it into the constant temperature and humidity curing box for curing. After 4 days, remove it and place it in a room temperature environment for 28 days for testing.

[0059] Different loading conditions are set to perform compression creep tests on the creep test pieces.

[0060] During the compression creep test, the creep data of each creep test piece is collected, and the creep data is processed.

[0061] Based on the processed creep data, the creep performance of the hybrid fiber reinforced geopolymer concrete is evaluated.

[0062] Specifically, the compression creep test adopts a pseudo-triaxial compression creep test.

[0063] Currently, there are few studies on concrete compression creep considering the confining pressure factor. The basic principle of the test is to fill the pressure chamber with oil through an oil pump, apply confining pressure first and then axial pressure, and stop adding pressure when the target pressure value is reached and keep it. In this test, the instrument is a TR-XB50J type pseudo-triaxial creep test machine produced by the company, which includes a hydraulic control pump station, a compression creep frame, a pseudo-triaxial pressure chamber, a program control center and other parts. After test verification, the maximum axial pressure of this instrument can reach 17MPa, and the maximum confining pressure can reach 8MPa. To ensure the uniformity of the measurement method, the same strain gauge as the uniaxial compression creep test is used for the pseudo-triaxial compression creep test. The strain gauge with a range of 100mm is used to measure the longitudinal strain, and the strain gauge with a range of 60mm is used to measure the transverse strain. The DH-3818Y type static data acquisition system and the built-in sensor are used to obtain the strain data, and the axial pressure and confining pressure data are obtained through the pressure sensing system of the test machine.

[0064] The installation process of the creep test piece in this test is relatively complex. The important step in the whole process is to install the creep test piece in the pressure chamber. Install the components of the pressure chamber in turn, seal the creep test piece with a rubber cylinder, and install it in the pressure chamber. Move the pressure chamber to the creep frame, install the positioning bolt, connect the oil supply pipe, and wait for the test to start. The structure diagram of the pressure chamber and the description of each component are shown in Figure 2There are several key points to be noted during the installation of the creep test specimen: first, each component of the pressure chamber and the eight rubber rings need to be cleaned with machine oil to avoid the adverse effects of dust particles on the sealing performance of the pressure chamber; second, although the top surface of the creep test specimen has been smoothed as much as possible during pouring, in order to ensure that the upper pressure head is in close contact with the surface of the creep test specimen, the top surface of the creep test specimen needs to be ground flat before installation; third, the creep test specimen used in this test contains steel fiber filaments, and there are individual air holes on the side surface of the creep test specimen, which need to be plugged with plaster or fast-hardening cement to avoid the exposed steel fiber filament ends in the air holes from puncturing the rubber cylinder under the action of oil pressure, resulting in oil leakage.

[0065] Due to the large laboratory space and long test period of the pseudo-triaxial creep testing machine, it is difficult to maintain constant temperature and humidity. Therefore, this test is carried out in a room temperature environment, and the influence of environmental factors on each group of creep test specimens is considered to be the same. It can be considered that the comparison between groups of test data is effective and can reflect the influence of fiber type on creep performance.

[0066] Specifically, the compression creep test can also use uniaxial compression creep test, which is as follows:

[0067] (1) Test specimen size and quantity

[0068] The uniaxial compression creep test is carried out on a creep tester, and two creep test blocks are installed on each instrument, with three shrinkage control test blocks of the same ratio. According to the requirements of the Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete (GB / T 50082-2009, hereinafter referred to as the Standard), the size of the test specimen is 100mm x 100mm x 400mm, and the test specimen is removed after being cured at a temperature of 75℃ and a humidity of 85% for 4d, and is placed in a room temperature environment for 28d for testing.

[0069] (2) Test device and loading method

[0070] The CABR-XB50 uniaxial compression creep tester produced by the company is used for testing, with a maximum test force of 500kN and a pressure sensor error of ±0.5%. The loading process is completed by a hydraulic jack, and the jack is centered before loading to ensure loading along the central axis of the test block. The load data is checked every 30d after 7d, 28d and 60d of loading. If the load changes by more than 2%, the jack needs to be supplemented and the nut needs to be tightened. Real-time pressure data is read during the test through the pressure monitoring system of the creep tester.

[0071] (3) Data acquisition

[0072] According to the provisions of the Standard on the measurement device of creep, the internal strain gauge is selected to measure the deformation. The HNY type internal sensor is connected with the DH3816N static acquisition instrument to obtain the strain data. The internal sensor is placed in the middle layer of the specimen during the pouring of the specimen, the longitudinal sensor is arranged along the axis of the specimen, and the transverse strain sensor is perpendicular to the axis of the specimen.

[0073] In order to ensure that the sensor does not shift during pouring, the lower layer is poured first, and the sensor is positioned with a wooden stick after vibration. The sensor is placed after the slurry has a certain supporting force (about 10 minutes in a constant temperature and humidity curing box), and then the upper layer is poured and vibrated. The 100mm sensor is placed along the axis, and the 60mm sensor is placed transversely. Each strain sensor is attached with a factory-calibrated sensitivity coefficient, the data acquisition software sampling frequency is set to 1Hz, the measurement mode is set to bridge sensor (full bridge), the sensitivity coefficient of the strain gauge is input, and the measured result is the actual strain. The test values of two creep specimens in each group are averaged as the creep strain of the group, and the test values of three comparative shrinkage specimens are averaged as the free shrinkage of the group.

[0074] (4) Environmental conditions

[0075] According to the requirements of the specification, the creep test is carried out under the conditions of temperature (20±2)℃ and humidity (60±5)%. The selected creep tester does not have a matching constant temperature and humidity environment box, and the uniaxial compression creep test needs to last for more than 180 days, which has a long time span. Seasonal and diurnal changes will have a great impact on the temperature and humidity of the laboratory. In order to reduce this impact and ensure the test under the specified temperature and humidity conditions as much as possible, the constant temperature is realized through air conditioning during the test period, a closed plastic shed is built outside the creep tester, a humidifier is placed inside to keep the humidity constant, and a hygrometer is used for real-time monitoring and timely correction.

[0076] Specifically, the loading scheme and test steps

[0077] The influence factor settings of the axial compression strength and creep test of the test group are shown in Table 1. The test designed in the embodiment of the application focuses on the influence of the type of fiber and the combination mode of the fiber on the deformation performance of the HFGPC under long-term constant compression load, and the relationship between the pseudo-triaxial creep and the uniaxial creep. Therefore, the embodiment of the application focuses on the creep performance of the geopolymer concrete reinforced by MS and PVA fibers, four characteristic groups are selected, the four groups of test pieces are compared and analyzed between groups to investigate the influence of single fiber and mixed fiber on the creep; different loading conditions are set for one group of test pieces to investigate the influence of stress change on the creep. Three test pieces of the same size are cast in the same batch to obtain the average value of the axial compression strength. When the axial compression stress ratio is not greater than 0.4, the self-shrinkage of the creep loading test piece and the compensation shrinkage test piece can be considered to be equal, therefore the maximum uniaxial creep load in the test is set to 40% of the prism strength. The maximum axial compression stress ratio of the pseudo-triaxial creep test is 0.34, and the ratio of the confining pressure to the axial compression is set to two levels.

[0078] Table 1 Axial compression strength and creep load of each group of test pieces

[0079]

[0080] The operation steps of the uniaxial compression creep test are as follows:

[0081] (1) The center loading point is determined by measurement, that is, the position of the jack, the upper and lower compression plates are corrected by using a level, the levelness of the upper compression plate is controlled by adjusting the nut, and the levelness of the lower compression plate is controlled by adjusting the ball hinge support;

[0082] (2) The center positions of the upper and lower compression plates are determined, the two creep test pieces are overlapped and placed in the center, the axial line of the test piece is kept vertical, then the nut is loosened, and the upper compression plate is slowly lowered to contact the pressure bearing surface of the test piece;

[0083] (3) The sensor leads of the two creep test pieces and the three compensation shrinkage test pieces are connected to the data acquisition instrument;

[0084] (4) The jack is placed on the upper loading point of the creep frame, the data acquisition system is turned on, and the load is preloaded to 20% of the set axial compression, then unloaded, the preloading process is repeated twice, and in this process, it is checked whether the data acquisition system and the sensor are working normally;

[0085] (5) After confirming that the test system is working normally, the strain value before loading is recorded, a new data acquisition file is created, and the formal loading is started. In this stage, the oil feeding speed of the jack is carefully controlled, when the jack approaches the top plate of the creep frame, the oil feeding speed is slowed down, the pressure monitoring system is observed in real time, the pressure value is slowly increased, until the target value is reached, the oil feeding is stopped, the three nuts are tightened in turn, then the jack is removed, and the loading process is completed;

[0086] (6) Record the strain value after loading, and subtract the strain value before loading to obtain the initial creep strain. Record the strain value of the control shrinkage specimen as the reference value for subsequent calculation of natural shrinkage.

[0087] (7) After all the test groups are loaded, build a plastic shed and place a temperature and humidity meter inside. Start the test.

[0088] (8) Record the strain value 2h, 6h, and 24h after loading. Record the strain value every 24h thereafter. Calculate the creep of each day based on the initial creep. Monitor each group of specimens for 180d or 360d.

[0089] Specifically, the steps of the pseudo-triaxial compression creep test include:

[0090] (1) Assemble the pressure chamber components and the creep specimen from bottom to top, install on the creep meter, move the position of the pressure chamber to the top of the oil cylinder, center with the pressure sensor on the creep meter through the positioning bolt, and tighten the oil supply pipe.

[0091] (2) Connect the sensor leads of the prepared creep specimen and the two compensation shrinkage specimens to the data acquisition instrument.

[0092] (3) Turn on the data acquisition system and the pseudo-triaxial creep test machine control system, perform preloading, repeat the preloading process twice, and check whether the data acquisition system and the sensor are working normally.

[0093] (4) After confirming that the data acquisition system and the sensor are working normally, set the loading parameters on the pseudo-triaxial creep test machine control system. There are five levels of loading, each of which is 20% of the target axial pressure and confining pressure. Fine-tune the motor speed, load quickly at the first level to shorten the oil feeding time, and load slowly at the later levels to achieve stable deformation. Record the initial strain value, create a new data acquisition file, and start loading.

[0094] (5) Record the strain value after loading, and subtract the strain value before loading to obtain the initial creep strain. Record the strain value of the control shrinkage specimen as the reference value for subsequent calculation of natural shrinkage.

[0095] (6) Record the strain value 2h, 6h, and 24h after loading. Record the strain value every 24h thereafter. Calculate the creep strain of each day based on the initial creep strain. Monitor each group of creep specimens for 180 days.

[0096] Specifically, the preloading includes first setting the first level of loading, with an axial pressure value of 20% of the preset axial pressure and a confining pressure value of 50% of the preset confining pressure, and unloading after completing the first level of loading.

[0097] Specifically, the creep data includes: instantaneous elastic strain, free contraction strain, creep, and total creep at loading;

[0098] ε c =ε t -ε0-ε s (1)

[0099] In the formula, ε c For creep, ε t Let ε0 be the total creep at a certain loading age, and ε0 be the instantaneous elastic strain at loading. s This represents the shrinkage strain at this age.

[0100] Specifically, since the strength and loading stress of the creep specimens in each group are different, it is not appropriate to directly use creep as a measurement parameter to compare the creep performance of each group. Creep degree and creep coefficient are usually used to evaluate the creep performance of concrete.

[0101] Specifically, the creep coefficient is the ratio of creep to elastic strain, expressed as:

[0102]

[0103] In the formula, Let be the creep coefficient at the holding time t(d);

[0104] Creep is the creep under unit stress, expressed as:

[0105]

[0106] In the formula, C t σ is the creep degree at the holding time t(d), in units of 1 / MPa; c This refers to creep stress;

[0107] The relationship between creep degree and creep coefficient is as follows, expressed as:

[0108]

[0109] In the formula, E t The elastic modulus is taken as the elastic modulus at the initial loading time in the ACI-209 and B3 models, while it is taken as the 28-day elastic modulus of concrete in the CEB-FIP and GL2000 models.

[0110] The creep function is the sum of the instantaneous elastic strain and creep produced by a unit stress, expressed as:

[0111]

[0112] Both the creep function and the creep degree can be represented by the creep coefficient.

[0113] This invention studies the compression creep of geopolymer concrete from two perspectives: the influence of fiber type on the creep performance of HFGPC and the influence of stress conditions on the creep of specimens with the same mix proportion. The creep coefficient is used to measure the difference in creep performance among the specimens.

[0114] Since the data acquisition systems used in the experiments of this invention all employ creep data acquisition systems and involve long-term continuous monitoring, the data volume is extremely large. Furthermore, fluctuations, i.e., signal noise, are unavoidable during signal acquisition. Causes of noise include electromagnetic interference, electronic noise from the sensor itself, environmental factors (temperature fluctuations and mechanical vibrations), and signal transmission losses during data acquisition. These factors can all lead to errors between the measured data and the actual strain values, affecting the accuracy and reliability of the data.

[0115] Given the large volume and signal noise characteristics of the data, the acquired raw creep data needs to be processed appropriately to reduce the impact of noise and decrease the data volume. First, data from the two hours before and after the loading time of each day's collection (referred to as the "loading time") is extracted. Then, the extracted data segments are aggregated, and the average value is taken as the representative strain value for that day. Using data from the two hours before and after the loading time as the representative value effectively avoids data fluctuations caused by changes in ambient temperature and removes redundant data to avoid interference with the results analysis. Taking the longitudinal deformation measurement data of a typical group 60-64 days after loading as an example, the raw data and processed data are as follows: Figure 3 As shown. It can be seen that when calculating creep strain according to formula (1), due to the uniformity of the acquisition system and environment, the calculated true value of creep strain has removed a large amount of signal noise and periodic fluctuations caused by environmental influences. The aggregated data before and after the loading moment yields a representative creep value that can reflect the changing trend of the true value, and this processing method is considered effective. Figure 3 The creep in the figure has already been reduced by the instantaneous elastic strain under loading, which is hereby noted.

[0116] Specifically, the creep data is processed by: extracting data from several hours before and after the loading time of each day's collected data (referred to as the "loading time"); then aggregating the extracted data and taking the average value as the representative strain value for that day. This embodiment of the invention extracts data from two hours before and after the loading time; using data from these two hours as the representative value effectively avoids data fluctuations caused by changes in ambient temperature and removes redundant data to prevent interference with the results analysis.

[0117] Specifically, plot the total strain-time curves for uniaxial and pseudo-triaxial compression.

[0118] The measured data obtained from the beginning of the application of the compressive stress until the end of the final unloading is subtracted from the corresponding shrinkage strain, and the total strain-time curve of each group of test pieces is obtained, and a typical concrete stress and total strain process curve is shown in Figure 4 .

[0119] The instantaneous elastic strain ε0 is generated in the loading process (very short time, regarded as 0 time), and the creep ε is generated under the action of the constant compressive creep stress σ for a duration t; the creep recovery is generated after unloading, including two parts, one is the instantaneous elastic recovery ε generated in the process of unloading (t time), and the other is the hysteresis creep recovery t which increases with the extension of time since t time, and the test ends at t time. d d r c c

[0120] The total strain-time curves of each group of test pieces under uniaxial compression (a represents axial creep, and v represents transverse strain) are shown in Figure 5 , and the total strain-time curves of each group of test pieces under pseudo-triaxial compression (a represents axial creep, and v represents radial creep) are shown in Figure 6 . It can be clearly seen from Figure 5 and Figure 6 that under the uniaxial and pseudo-triaxial stress states, the creep development of GPC conforms to the general law of the creep development of concrete, and the total deformation from the loading time to the end of the test experiences four stages of instantaneous elastic strain, nonlinear creep development, instantaneous elastic recovery, and hysteresis elastic recovery. In the case that the test piece is subjected to uniaxial stress, compressive strain is generated in the axial direction, and tensile strain is generated in the transverse direction. When the test piece is subjected to the combined action of axial compression and surrounding pressure, compressive strain is generated in the axial and radial directions. In the following, the uniaxial creep and pseudo-triaxial creep processes will be analyzed in detail from the characteristics of the four stages. It is worth noting that due to the size effect of the compressive deformation caused by the inconsistent sizes of the uniaxial creep and pseudo-triaxial creep test pieces, the strain gauges with the same gauge length are used in the two tests, so the uniaxial creep strain and the pseudo-triaxial creep strain directly measured by the test cannot be directly compared. In the following, the creep coefficient will be used as a standard to further analyze the influence of the addition of fibers on the creep and to reveal the relationship between the uniaxial creep and the pseudo-triaxial creep.

[0121] In one specific embodiment of the present application, a hybrid fiber reinforced geopolymer concrete creep test system comprises:

[0122] A creep test piece preparation module for preparing creep test pieces of different fiber types;

[0123] A test module for setting different loading conditions and performing a compressive creep test on the creep test pieces;

[0124] ​​​​​a data acquisition and processing module, configured to acquire creep data of each creep test piece during the compression creep test, and process the creep data;

[0125] a creep performance evaluation module, configured to evaluate the HFGPC creep performance based on the processed creep data.

[0126] In the embodiments of the present application, the creep characteristics of HFGPC under multi-axial stress are mainly focused on, and the influence of the change of stress level on the HFGPC creep under the combined action of axial stress and confining pressure is emphatically analyzed. The influence of the change of axial stress and confining pressure on the HFGPC creep is analyzed in two series. Series I: under the condition that the axial stress is the same, the influence of the change of confining pressure level on the HFGPC creep is compared; Series II: under the condition that the ratio of axial stress and confining pressure is unchanged, the HFGPC creep performance is analyzed by simultaneously increasing the axial stress and confining pressure, so as to study the comprehensive influence of confining pressure and axial stress on the HFGPC creep performance. The analysis of the creep performance under multi-axial stress in the embodiments of the present application mainly starts from two aspects of volume creep and deviatoric creep.

[0127] According to the elastic-plastic theory, the stress tensor of a point of a material under multi-axial stress state can be decomposed into a spherical stress tensor (i.e. a volume stress tensor, or a hydrostatic stress tensor) and a deviatoric stress tensor, which correspond to volume strain and deviatoric strain. Thus, the creep under multi-axial stress can be correspondingly decomposed into two parts, the volume creep ε v produced by the hydrostatic pressure p and the deviatoric creep ε d produced by the deviatoric stress σ d , which can be respectively represented by the following formulae:

[0128]

[0129]

[0130]

[0131]

[0132] In the formulae, σ1, σ2 and σ3 are three principal stresses, and ε c1 , ε c2 and ε c3 are the creep produced in the direction of the principal stresses, respectively. Based on the test equipment adopted in the embodiments of the present application, σ1 is the axial stress, σ2 and σ3 are both confining pressures, ε c1 is the axial strain, and ε c2 and ε c3 are both radial strains. The relationship between the volume creep and the volume stress of the MP group at different loading ages is shown in Figure 7 , and the relationship between the deviatoric creep and the deviatoric stress is shown in Figure 8As shown in the figure, it can be seen that in the pseudo-triaxial test results, volumetric creep and volumetric stress are approximately linearly related, and deviatoric creep and deviatoric stress are also approximately linearly related; however, under the condition of no confining pressure (i.e., MP-A0.3 group), volumetric creep is approximately linearly related to the pseudo-triaxial test results, while deviatoric strain is not linearly related.

[0133] 1. Elastic deformation during the loading stage

[0134] The deformation during the pseudo-triaxial creep loading stage of HFGPC is listed in Table 2, combined with... Figure 4 The creep-strain curves reveal that under the combined action of axial compression and confining pressure, both axial and radial elastic deformations are greater than those under axial compression alone. This is because the specimen size used in the pseudo-triaxial creep test is larger than that in the uniaxial creep test. When the applied axial compression is comparable within the elastic deformation range, the larger specimen will undergo greater elastic deformation. Since the strain gauge lengths used in both tests are the same, the calculated pseudo-triaxial creep is greater than the axial compression creep. In Series I, with the same axial compression stress ratio, the instantaneous elastic strain of the pseudo-triaxial creep is greater than that of the axial compression creep. Furthermore, as the confining pressure increases, the axial instantaneous creep of the pseudo-triaxial creep decreases, while the radial instantaneous creep increases. In Series II, with the ratio of axial compression to confining pressure remaining constant, both the axial and radial instantaneous creep of the pseudo-triaxial creep increase when both axial and confining pressure increase simultaneously.

[0135] Table 2. Parameters related to pseudotriaxial compression creep.

[0136]

[0137] Note: For example, a and v represent the longitudinal and radial directions, respectively.

[0138] 2. Creep deformation during the load-bearing stage

[0139] Creep of HFGPC under triaxial compressive load Figure 9 As shown in the figure, when the axial compressive stress ratio and confining pressure / axial pressure are the same, the axial creep and radial creep of group MP are both smaller than those of group C. When the axial compressive stress ratio is the same and the confining pressure / axial pressure increases from 25% to 40%, the axial creep of group MP does not change significantly, but the radial creep increases by nearly 100%. When the confining pressure / axial pressure is fixed at 25%, the axial creep and radial creep of group MP increase uniformly with the proportional increase of confining pressure and axial pressure.

[0140] The effects of axial compression and confining pressure changes on volumetric creep and deviatoric creep are as follows: Figure 10 and Figure 11As shown, when the axial compression is constant, volumetric creep increases almost linearly with the increase of confining pressure / axial compression, while partial creep gradually decreases, and the linear characteristic is not obvious. When the confining pressure / axial compression is constant, and the axial compressive stress ratio increases, both confining pressure and axial compression increase proportionally. At this time, both volumetric creep and partial creep increase approximately linearly, and the longer the loading age, the more significant the effect of the axial compressive stress ratio on creep.

[0141] 3. Poisson effect during load holding process

[0142] Poisson's ratio under uniaxial compression can be calculated according to its definition, which is radial strain divided by longitudinal strain. However, under confining pressure, the specimen is in a complex stress state, and Poisson's ratio can no longer be simply calculated as the ratio of radial to longitudinal deformation. Under complex stress conditions, creep deformation caused by volumetric stress and deviatoric stress should be considered separately. The volumetric creep ε... v And partial creep ε d For volumetric stress p and deviatoric stress σ d After normalization, the volumetric creep coefficient C is obtained. v And creep variation C d According to the formula recommended by Jordaan and Illston (Formula (10), hereinafter referred to as the JI formula), when considering the triaxial creep Poisson's ratio, if the confining pressure and axial pressure are not equal, C v and C d The relationship can be expressed using the instantaneous Poisson's ratio:

[0143]

[0144] In the formula, p is the volume component of the stress, i.e., hydrostatic pressure; ε v It is the volume component of creep strain; σ d It is the Von Miles equivalent deviatoric stress, expressed as the shear stress in the octahedral plane; ε d is the partial component of creep strain, expressed as shear strain in the octahedral plane; K represents the bulk modulus, G the rigidity modulus; v is the instantaneous Poisson's ratio under complex stress. Calculating deformation under complex stress conditions in this way better reflects the physical mechanism of creep and is also of great significance in viscoelastic analysis. The pseudo-triaxial creep Poisson's ratio (calculated separately using the JI formula and the definition) is as follows: Figure 12 As shown.

[0145] Depend on Figure 12It can be seen that the value calculated by the Poisson ratio definition is greatly affected by the change of confining pressure, which overestimates the creep Poisson ratio of HFGPC under multi-axial stress. The result calculated by J-I formula considers the effect of volume stress and deviatoric stress, and the calculated Poisson ratio value is closer under different stress states of MP group, which can better reflect the properties of the material. From the calculation result of J-I formula, the creep Poisson ratio of C group under multi-axial stress can be approximately taken as 0.1, and the creep Poisson ratio of MP group can be approximately taken as 0.16, which are both smaller than the creep Poisson ratio under uniaxial stress.

[0146] 4. Creep coefficient in the sustained load stage

[0147] Due to the different sizes of the test specimens in the uniaxial creep test and the pseudo-triaxial creep test of the embodiments of the present application, the size effect exists in the creep deformation, which is not convenient for direct comparison. Therefore, the creep coefficient is selected as an index to evaluate the pseudo-triaxial creep performance of HFGPC in the embodiments of the present application, and is compared with the uniaxial compression creep coefficient. When the confining pressure increases from 0, the axial creep coefficient changes obviously under the conditions that the axial stress ratio is 0.25, 0.3 and 0.35, as shown in (a)-(c) of FIG. 6. Figure 13

[0148] Obviously, the effect of confining pressure makes the axial creep coefficient of HFGPC decrease greatly, which is close to 1 / 2 of the creep coefficient in the unconfined compression. Zhang Tianxu found that the creep coefficient of triaxial compression creep is close to 2 times of the uniaxial compression creep coefficient in the research on the true triaxial creep of concrete, and the possible reason is that the deformation caused by the wear plate and lubricating oil is included in the creep deformation of concrete. In the present test, the internal sensor is uniformly used for strain collection, which can collect the real deformation of the internal concrete, and avoids the interference of external factors on the collection of creep data, so that the test has high reliability. In addition, in the analysis of the creep strain of concrete, the triaxial compression creep is smaller than the uniaxial compression creep, and the difference between the triaxial compression creep and the uniaxial compression creep gradually decreases with the increase of the stress ratio. When the stress ratio is 0.3, the triaxial compression creep coefficient and the uniaxial compression creep coefficient are shown in (d) of FIG. 7. It can be seen from the figure that the triaxial compression creep coefficient is about 2 / 3 of the uniaxial compression creep coefficient, which is close to the test result. Therefore, it can be reasonably inferred that the axial creep coefficient of HFGPC under the action of pseudo-triaxial stress should be smaller than the uniaxial creep coefficient, and close to 1 / 2 of the uniaxial creep coefficient. Figure 13

[0149] 5. Creep recovery

[0150] The related parameters of pseudo-triaxial creep recovery are listed in Table 2. The hysteresis recovery curves of each group in the test are shown in FIG. 8, and the instantaneous creep recovery coefficient, the hysteresis creep recovery coefficient and the creep recovery ratio are shown in FIG. 9. Figure 14 Figure 15 ​​​As shown. From Figure 15 As can be clearly seen in (a) above, at the same stress level, the instantaneous creep recovery coefficient of the MP group is smaller than that of the C group, but the hysteretic creep recovery coefficient of the MP group is larger than that of the C group. This is because, under compression, the energy of HFGPC exists in the geopolymer matrix, fibers, matrix voids, and the interface between fibers and the matrix, while the energy of pure GPC is stored in the matrix and voids. Therefore, the energy release process during unloading differs between the MP group and the C group. Due to its higher stiffness, pure GPC releases its elastic energy more quickly during unloading, while for HFGPC, due to the presence of the fiber network structure and the interface transition zone, the energy is released gradually during unloading. Therefore, the instantaneous creep recovery coefficient of the MP group is smaller than that of the C group, while the hysteretic creep recovery coefficient is larger than that of the C group. Figure 14 The hysteresis recovery curves show that the deformation of group C quickly stabilizes, while the deformation of group MP recovers gradually.

[0151] This invention investigated the creep performance of HFGPC through uniaxial creep tests and pseudo-triaxial creep tests. Analysis of the experimental data yielded the following conclusions:

[0152] (1) The results of the uniaxial creep test show that the creep coefficient of HFGPC increases with age, and the creep coefficient of HFGPC is significantly higher than that of GPC. This is because the addition of fibers makes the pore structure inside the concrete more complex, increases the internal friction resistance of the concrete, and leads to an increase in the creep coefficient of the concrete.

[0153] (2) The results of the pseudo-triaxial creep test show that the creep coefficient of HFGPC increases with the increase of the confining pressure / axial pressure ratio, and the confining pressure / axial pressure ratio has a significant impact on the creep coefficient of HFGPC. When the confining pressure / axial pressure ratio is 40%, the creep coefficient of HFGPC is significantly higher than that when the confining pressure / axial pressure ratio is 25%.

[0154] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0155] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method of testing the creep of a hybrid fiber reinforced geopolymer concrete, characterized in that, The method comprises the following steps: preparing creep test pieces of different fiber types; setting different loading conditions to perform compression creep tests on the creep test pieces; collecting creep data of each creep test piece during the compression creep tests and processing the creep data; evaluating the creep performance of the hybrid fiber reinforced polymer concrete based on the processed creep data; the creep data comprises instantaneous elastic strain, free shrinkage strain, creep, and total creep strain during loading; ； wherein is the creep, is the creep at a certain loading age, is the instantaneous elastic strain at loading, is the shrinkage strain at that age; the creep performance of the concrete is evaluated by using creep degree and creep coefficient; the creep coefficient is the ratio of the creep deformation to the elastic deformation, and is expressed as: ； In the formula, is the creep coefficient at the loading time t (d); the creep degree is the creep strain under unit stress, and is expressed as: ； In the formula, is the creep degree at the loading time t (d), is the creep stress; there is a relationship between the creep degree and the creep coefficient, and is expressed as: ； In the formula, E is the modulus of elasticity; the creep function is the sum of the instantaneous elastic strain and the creep strain under unit stress, and is expressed as: ； both the creep function and the creep degree can be expressed by the creep coefficient; processing the creep data comprises the following steps: intercepting data of several hours before and after the loading time from the data collected every day; and then aggregating the intercepted data and taking the average value as the representative value of the strain of the day.

2. The method of claim 1, wherein the method is characterized by, The compression creep test adopts a pseudo-triaxial compression creep test.

3. The method of claim 2, wherein the method is characterized by, The steps of the pseudo-triaxial compression creep test comprise: (1) assembling the pressure chamber components and the creep test pieces from bottom to top in sequence, installing on the creep tester, moving the position of the pressure chamber to the top of the oil cylinder, centering by the positioning bolt and the pressure sensor on the creep tester, and tightening the oil supply pipe; (2) connecting the sensor leads of the prepared creep test pieces and the two compensation shrinkage test pieces to the data acquisition instrument; (3) turning on the data acquisition system, turning on the pseudo-triaxial creep test machine control system, preloading, repeating the preloading process twice, and checking whether the data acquisition system and the sensor are working normally; (4) after confirming that the data acquisition system and the sensor are working normally, setting the loading parameters on the pseudo-triaxial creep test machine control system; recording the initial strain value, creating a new data acquisition file, and starting loading; (5) recording the strain value after loading is completed, subtracting the strain value before loading to obtain the initial creep strain, and recording the strain value of the compensation shrinkage test piece as the reference value for subsequent calculation of the natural shrinkage.

4. The method of claim 3, wherein the method is characterized by, The preloading comprises first setting a first-level loading, the axial compression value is 20% of the preset axial compression, and the confining pressure value is 50% of the preset confining pressure; and after the first-level loading is completed, unloading.

5. A hybrid fiber reinforced geopolymer concrete creep testing system characterized by, The method comprises the following steps: a creep test piece preparation module for preparing creep test pieces of different fiber types; a test module for setting different loading conditions to perform compression creep tests on the creep test pieces; a data acquisition and processing module for collecting creep data of each creep test piece during the compression creep tests and processing the creep data; a creep performance evaluation module for evaluating the creep performance of the hybrid fiber reinforced polymer concrete based on the processed creep data; the creep data comprises instantaneous elastic strain, free shrinkage strain, creep, and total creep strain during loading; ； wherein is the creep, is the creep at a certain loading age, is the instantaneous elastic strain at loading, is the shrinkage strain at that age; the creep performance of the concrete is evaluated by using creep degree and creep coefficient; the creep coefficient is the ratio of the creep deformation to the elastic deformation, and is expressed as: ； In the formula, is the creep coefficient at the loading time t (d); The degree of creep is the creep strain under the action of unit stress, and is expressed as: ； In the formula, is the creep degree at the loading time t (d), is the creep stress; The degree of creep and the creep coefficient have the following relationship, which is expressed as: ； In the formula, E is the modulus of elasticity; The creep function is the sum of the instantaneous elastic strain and the creep strain generated by unit stress, and is expressed as: ； Both the creep function and the degree of creep can be expressed by the creep coefficient; The creep data are processed, including: intercepting data of several hours before and after the loading time in the data collected every day; then, the intercepted data are aggregated and processed, and the average value is taken as the strain representative value of the day.

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