Hybrid fiber reinforced geopolymer concrete creep test method and system

Abstract

The invention discloses a hybrid fiber reinforced geopolymer concrete creep test method and system, and relates to the technical field of hybrid fiber reinforced geopolymer concrete.The hybrid fiber reinforced geopolymer concrete creep test method comprises the steps that creep test pieces of different fiber types are prepared; setting different loading conditions, and performing a compression creep test on the creep test piece; in the compression creep test process, creep data of all creep test pieces are collected, and the creep data are processed; and evaluating the creep performance of the hybrid fiber reinforced geopolymer concrete based on the processed creep data. The method focuses on creep behaviors of GPC and HFGPC under the action of long-term compression load. By analyzing the influence of the fiber type, the axial compression stress ratio and the confining pressure level on the creep characteristic, the deformation rule of the fiber reinforced composite material in a long-term service state is disclosed, a theoretical basis is provided for engineering application of the HFGPC, and a reference is provided for prediction of the creep coefficient of the HFGPC.

Description

Technical Field

[0001] The invention 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 Art

[0002] As a structural material with great potential, 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 compressive loads to ensure the service stability of materials and structures. The study of short-term compressive loads cannot fully reflect the performance of GPC and its HFGPC under long-term loads in actual engineering. For the study of structural materials, it is necessary not only to explore the complete process of elastic and elastoplastic deformation until final destruction under short-term loads, but also to focus on the deformation characteristics of materials that gradually accumulate over time under constant loads, that is, creep. Creep has a vital impact on the long-term performance of structural materials, especially in engineering fields such as large-span structures, bridges, and building foundations that are subjected to long-term loads. Creep may cause excessive deformation of the structure, shorten its service life, and even cause failure.

[0003] Therefore, how to provide a hybrid fiber reinforced geopolymer concrete creep test method and system, focus on the creep behavior of GPC and HFGPC under long-term compressive load, analyze the influence of fiber type, axial compressive stress ratio and confining pressure level on creep characteristics, and reveal the deformation law of fiber reinforced composite materials under long-term service state is a problem that technical personnel in this field need to solve urgently. Summary of the invention

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

[0005] In order to achieve the above object, the present invention adopts the following technical scheme: a hybrid fiber reinforced geopolymer concrete creep test method, comprising:

[0006] Prepare creep specimens with different fiber types;

[0007] Setting different loading conditions, and performing compression creep tests on the creep specimens;

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

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

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

[0011] Preferably, the steps of the pseudo triaxial compression creep test include:

[0012] (1) Assemble the pressure chamber components and creep specimen from bottom to top, install them on the creep instrument, move the pressure chamber to the top of the oil cylinder, align it with the pressure sensor on the creep instrument through the positioning bolts, and tighten the oil pipe;

[0013] (2) Connecting the sensor wires of the prepared creep specimen and two compensating shrinkage specimens to a 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 sensors are working properly;

[0015] (4) After confirming that the data acquisition system and sensors are working properly, 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 it from the strain value before loading to obtain the initial creep strain, and record the strain value of the comparison compensation shrinkage specimen as a reference value for subsequent calculation of natural shrinkage; preferably, the preloading includes first setting a first-level loading, the axial pressure value is 20% of the preset axial pressure, the confining pressure value is 50% of the preset confining pressure, and unloading after completing the first-level loading.

[0017] Preferably, the creep data include: instantaneous elastic strain, free shrinkage strain, creep and total creep during loading;

[0018] ε c =ε t -ε 0 -ε s ;

[0019] In the formula, ε c is creep, ε t is the total creep at a certain loading age, ε 0 is the instantaneous elastic strain during loading, ε s is the shrinkage strain at that age.

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

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

[0022]

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

[0024] Creep is the creep strain under unit stress, expressed as:

[0025]

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

[0027] There is the following relationship between creep degree and creep coefficient, 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 creep strain produced by unit stress, expressed as:

[0031]

[0032] Among them, creep function and creep degree can both be expressed by creep coefficient.

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

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

[0035] Creep specimen preparation module, used to prepare creep specimens of different fiber types;

[0036] A test module, used to set different loading conditions and perform a compression creep test on the creep specimen;

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

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

[0039] It can be known from the above technical solutions that, compared with the prior art, the present invention discloses a creep test method for hybrid fiber reinforced geopolymer concrete, including: preparing creep specimens of different fiber types; setting different loading conditions, and performing compression creep tests on the creep specimens; during the compression creep test, collecting creep data of each creep specimen, and processing the creep data; based on the processed creep data, evaluating the creep performance of hybrid fiber reinforced geopolymer concrete. The present invention focuses on the creep behavior of GPC and HFGPC under long-term compressive loads. By analyzing the effects of fiber type, axial compressive stress ratio and confining pressure level on creep properties, the deformation law of fiber reinforced composite materials under long-term service is revealed, providing a theoretical basis for the engineering application of HFGPC, and providing a reference for the prediction of its creep coefficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0041] Figure 1 A schematic flow chart of a hybrid fiber reinforced geopolymer concrete creep test method provided in an embodiment of the present invention.

[0042] Figure 2 A schematic structural diagram of a pressure chamber and its components in a pseudo triaxial creep test provided in an embodiment of the present invention.

[0043] Figure 3 A schematic diagram of the comparison between the original creep data and the processed data provided in an embodiment of the present invention (taking the uniaxial longitudinal strain of group P as an example).

[0044] Figure 4 A schematic diagram of a typical synchronous process line of compressive stress and total strain provided in an embodiment of the present invention.

[0045] Figure 5 A schematic diagram of the total strain-time curve of each group of test specimens under uniaxial compression provided in an embodiment of the present invention.

[0046] Figure 6 Schematic diagram of the total strain-time curve of each group of test specimens under pseudo triaxial compression provided in an embodiment of the present invention.

[0047] Figure 7 A schematic diagram of the relationship between volume creep and volume stress provided in an embodiment of the present invention.

[0048] Figure 8A schematic diagram of the relationship between deviatoric creep and deviatoric stress provided in an embodiment of the present invention.

[0049] Fig. 9 A schematic diagram of a pseudo triaxial compression creep curve provided in an embodiment of the present invention.

[0050] Fig.10 Schematic diagram of the effect of confining pressure / axial pressure on HFGPC creep provided in an embodiment of the present invention.

[0051] Fig.11 Schematic diagram of the effect of year-on-year increase in axial pressure and confining pressure on HFGPC creep provided in an embodiment of the present invention.

[0052] Fig.12 A schematic diagram of a pseudo triaxial creep Poisson's ratio curve provided in an embodiment of the present invention.

[0053] Fig.13 A schematic diagram of creep coefficient under different axial compressive stress ratios provided in an embodiment of the present invention.

[0054] Fig.14 A schematic diagram of a pseudo triaxial creep hysteresis recovery curve provided in an embodiment of the present invention.

[0055] Fig.15 A schematic diagram of the pseudo triaxial creep recovery coefficient and recovery ratio provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0056] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0057] The embodiment of the present invention discloses a creep test method for hybrid fiber reinforced geopolymer concrete. Figure 1 As shown, it includes: preparing creep specimens of different fiber types;

[0058] Specifically, the test uses a cylindrical creep specimen with a size of φ150mm×450mm. The deformation of the creep specimen is tested with an embedded sensor. The creep specimen is cast with a customized steel mold. The casting process is carried out in 5 steps. First, half of it is cast and vibrated. Then a wooden stick is placed along the axis and fixed with wire to prevent it from drifting in the slurry. Then, it is covered with a film and moved into a constant temperature and humidity curing box for curing for 10 minutes. After the slurry has a preliminary reaction and has a supporting force, the wooden stick is taken out and a 100mm sensor is placed. Then a small amount of mixture is poured to cover the axial sensor. After vibrating, a wooden stick is placed along the cross-sectional diameter. After curing for 5 minutes with a film, the wooden stick is taken out and a 60mm sensor is placed. The last step is casting and vibrating. Finally, the sensor wire is gathered and fixed in the center. After the casting is completed, the film is covered and moved into a constant temperature and humidity curing box for curing. After 4 days, it is taken out and placed in a room temperature environment until 28 days for testing.

[0059] Setting different loading conditions, and performing compression creep tests on the creep specimens;

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

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

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

[0063] At present, there are few studies on concrete compression creep considering confining pressure. The basic principle of the test is to pump oil into the pressure chamber through an oil pump, apply confining pressure first and then axial pressure, and stop pressurizing and maintain when the target pressure value is reached. The instrument used in this test is the TR-XB50J pseudo triaxial creep tester produced by Tongrui Company, which includes a hydraulic control pump station, a compression creep frame, a pseudo triaxial pressure chamber, a program control center, etc. After test verification, the maximum axial pressure of the instrument can reach 17MPa and the maximum confining pressure can reach 8MPa. In order to ensure the uniformity of the measurement method, the pseudo triaxial compression creep test uses a strain gauge of the same specification as the uniaxial 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 static data acquisition system and built-in sensors are used to obtain strain data, and the axial pressure and confining pressure data are obtained through the pressure sensing system of the test machine.

[0064] The creep specimen installation process in this test is relatively complicated. Installing the creep specimen into the pressure chamber is an important step in the whole process. Install the various components of the pressure chamber in sequence, seal the creep specimen with a rubber tube, and install it into the pressure chamber. Move the pressure chamber to the creep frame, install the positioning bolts, connect the oil pipeline, and wait for the test to start. The schematic diagram of the pressure chamber structure and the description of each component are shown in Figure 2There are several key points to note during the installation of the creep specimen: First, the various components of the pressure chamber and the 8 rubber rings need to be cleaned with engine oil to prevent dust particles from adversely affecting the sealing of the pressure chamber; Second, although the top surface of the creep specimen has been smoothed as much as possible during pouring, in order to ensure that the upper pressure head fits the surface of the creep specimen, the top surface of the creep specimen needs to be smoothed with an angle grinder before installation; Third, the creep specimen used in this test contains steel fiber microfilaments, and there are individual pores on the side of the creep specimen. The pores need to be sealed with gypsum or quick-hardening cement to prevent the exposed steel fiber microfilament ends in the pores from puncturing the rubber tube under the action of oil pressure, causing oil leakage in the instrument.

[0065] Since the pseudo-triaxial creep test machine is located in a large laboratory space and the test cycle is long, it is difficult to ensure that the temperature and humidity remain constant. Therefore, this test is carried out at room temperature, and the impact of environmental factors on each group of creep specimens is considered to be the same. It can be considered that the inter-group comparison of each group of test data is effective and can reflect the impact of fiber type on creep performance.

[0066] Specifically, the compression creep test may also be a uniaxial compression creep test, as follows:

[0067] (1) Specimen size and quantity

[0068] The uniaxial compression creep test was carried out on a creep instrument. Two creep test blocks were installed on each instrument, together with three shrinkage control test blocks with the same ratio. According to the requirements of the Standard for Test Methods for Long-term Properties and Durability of Ordinary Concrete (GB / T50082-2009, hereinafter referred to as the Standard), the test pieces were all 100mm×100mm×400mm in size. After curing for 4 days at a temperature of 75°C and a humidity of 85%, they were taken out and placed at room temperature for 28 days for testing.

[0069] (2) Test equipment and loading method

[0070] The test was conducted using the CABR-XB50 uniaxial compression creep instrument produced by Tongrui Company, with a maximum test force of 500kN and a pressure sensor error of ±0.5%. The loading process was completed using a hydraulic jack, and the jack was aligned before loading to ensure loading along the center axis of the test block. The load data was checked every 30 days after 7d, 28d, and 60d after loading. If the load change is greater than 2%, it is necessary to make up the load with a jack and tighten the nut. During the test, real-time pressure data was read through the pressure monitoring system supporting the creep instrument.

[0071] (3) Data collection

[0072] According to the provisions of the "Standard" on creep measurement devices, this test selected embedded strain gauges to measure deformation. HNY type built-in sensors were connected to DH3816N static collectors to obtain strain data. The built-in sensors were placed in the middle layer of the specimen height during casting. The longitudinal sensors were arranged along the axis of the specimen, and the transverse strain sensors were arranged perpendicular to the axis of the specimen.

[0073] To ensure that the sensor does not shift during the pouring process, the lower layer is poured first, and then positioned with a wooden stick after vibration. After the slurry has a certain bearing force after initial reaction (about 10 minutes in a constant temperature and humidity curing box), the sensor is placed, and then the upper layer is quickly poured and vibrated. A 100mm sensor is placed axially and a 60mm sensor is placed horizontally. Each strain sensor is equipped with a factory-calibrated sensitivity coefficient. The sampling frequency of the data acquisition software is set to 1Hz, and the measurement mode is set to a 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 each group of two creep specimens are averaged as the creep strain of the group, and the test values ​​of the three contrast shrinkage specimens are averaged as the free shrinkage of the group.

[0074] (4) Environmental conditions

[0075] According to the specification requirements, the creep test is carried out under the conditions of (20±2)℃ and (60±5)%. The creep instrument selected for the test does not have a matching constant temperature and humidity environment chamber, and the uniaxial compression creep test needs to last for more than 180 days, which is a long time span. Seasonal changes and day and night changes will have a great impact on the temperature and humidity of the laboratory. In order to reduce this impact, the test is carried out under the specified temperature and humidity conditions as much as possible, and the constant temperature during the test cycle is achieved through air conditioning. A closed plastic shed is built outside the creep instrument, and a built-in humidifier is used to maintain constant humidity. The thermometer and hygrometer are combined for real-time monitoring and timely correction.

[0076] Specific loading scheme and test steps

[0077] The influencing factors of the axial compressive strength and creep tests of the test group are shown in Table 1. The experiment designed for the implementation of the present invention focuses on the influence of fiber types and fiber combinations on the deformation performance of HFGPC under long-term constant compression loads, as well as the relationship between pseudo-triaxial creep and uniaxial creep. Therefore, the implementation of the present invention focuses on the creep performance of geopolymer concrete reinforced with MS and PVA fibers, selects 4 characteristic groups, and conducts inter-group comparative analysis on the 4 groups of specimens to investigate the influence of single-mixed fibers and mixed fibers on creep; different loading conditions are set for 1 group of specimens to investigate the influence of stress changes on creep. Three specimens of equal size are cast in the same batch to obtain the average value of the axial compressive strength. When the axial compressive stress ratio is not greater than 0.4, it can be considered that the self-shrinkage of the creep-loaded specimen and the compensating shrinkage specimen is equal, so the maximum uniaxial creep load is set to 40% of the prism strength in this test. The maximum axial compressive stress ratio of the pseudo-triaxial creep test is 0.34, and the ratio of confining pressure to axial pressure is set at 2 levels.

[0078] Table 1 Axial compressive strength and creep load of each group of specimens

[0079]

[0080] Among them, the operating steps of the uniaxial compression creep test are as follows:

[0081] (1) Determine the central loading point, i.e., the position of the jack, by measuring, and use a level ruler to calibrate the upper and lower pressure plates. Control the levelness of the upper pressure plate by adjusting the nut, and control the levelness of the lower pressure plate by adjusting the ball joint support;

[0082] (2) Determine the center position of the upper and lower pressure plates, align and stack the two creep specimens, keep the specimen axis vertical, then loosen the nut and slowly lower the upper pressure plate until it contacts the pressure-bearing surface of the specimen;

[0083] (3) Connect the sensor wires of the two creep specimens and the three compensating shrinkage specimens to the data acquisition instrument;

[0084] (4) Place the jack at the upper loading point of the creep frame, turn on the data acquisition system, preload to 20% of the set axial pressure and then unload, repeat the preloading process twice, and check whether the data acquisition system and sensors are working properly during this process;

[0085] (5) After confirming that the test system is working properly, record the strain value before loading, create a new data acquisition file, and start formal loading. At this stage, the oil feeding speed of the jack must be carefully controlled. When the jack approaches the top plate of the creep frame, slow down the oil feeding speed, observe the pressure monitoring system in real time, and increase the pressure value slowly until it reaches the target value. Stop the oil feeding, tighten the three nuts in sequence, and then remove the jack to complete the loading process;

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

[0087] (7) After all test groups have been loaded, a plastic shed is built, a thermometer and hygrometer are placed inside, and the test begins;

[0088] (8) Record the strain values ​​2h, 6h, and 24h after loading, and record the strain values ​​every 24h thereafter. Calculate the daily creep based on the initial creep. The monitoring process for each group of specimens lasts for 180d or 360d.

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

[0090] (1) Assemble the pressure chamber components and creep specimen from bottom to top, install them on the creep instrument, move the pressure chamber to the top of the oil cylinder, align it with the pressure sensor on the creep instrument through the positioning bolts, and tighten the oil pipe;

[0091] (2) Connecting the sensor wires of the prepared creep specimen and two compensating shrinkage specimens to a data acquisition instrument;

[0092] (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 sensors are working properly;

[0093] (4) After confirming that the data acquisition system and sensors are working properly, set the loading parameters on the pseudo triaxial creep test machine control system; there are five levels of loading, each level is 20% of the target axial pressure and confining pressure, fine-tune the motor speed, load quickly in the first level to shorten the oil inlet time, and then load slowly to achieve smooth deformation. Record the initial strain value, create a new data acquisition file, and start loading;

[0094] (5) Record the strain value after loading is completed, subtract it from the strain value before loading to obtain the initial creep strain, and record the strain value of the comparison compensation shrinkage specimen as the reference value for subsequent calculation of natural shrinkage;

[0095] (6) Record the strain values ​​2h, 6h, and 24h after loading, and then record the strain values ​​every 24 hours. Calculate the daily creep strain based on the initial creep strain. The monitoring process for each group of creep specimens lasts for 180 days.

[0096] Specifically, the preloading includes first setting a first-level loading, the axial pressure value is 20% of the preset axial pressure, the confining pressure value is 50% of the preset confining pressure, and unloading is performed after the first-level loading is completed.

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

[0098] ε c =ε t -ε 0 -ε s ; (1)

[0099] In the formula, ε c is creep, ε t is the total creep at a certain loading age, ε 0 is the instantaneous elastic strain during loading, ε s is the shrinkage strain at that age.

[0100] Specifically, since the strength and loading stress of each group of creep specimens are different, it is not appropriate to directly use creep as a measurement parameter to compare the creep performance of each group. Usually, creep degree and creep coefficient are 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, is the creep coefficient at the load holding time t(d);

[0104] The creep degree is the creep under unit stress and is expressed as:

[0105]

[0106] In the formula, C t is the creep degree at the load holding time t(d), unit 1 / MPa; σ c is the creep stress;

[0107] There is the following relationship between creep degree and creep coefficient, which is expressed as:

[0108]

[0109] In the formula, E t It is the elastic modulus. In ACI-209 model and B3 model, it is taken as the elastic modulus at the initial loading moment, and in CEB-FIP model and GL2000 model, it is taken as the 28d elastic modulus of concrete.

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

[0111]

[0112] Among them, creep function and creep degree can both be expressed by creep coefficient.

[0113] The embodiment of the present invention studies the compression creep of geopolymer concrete from two levels, considering 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 ratio, and uses the creep coefficient to measure the difference in creep performance of each group of specimens.

[0114] Since the data acquisition system used in the experiments of the embodiments of the present invention acquires creep data and monitors it continuously for a long time, the amount of data is very large. In addition, fluctuations, i.e., signal noise, will inevitably occur during signal acquisition. The causes of noise include electromagnetic interference, electronic noise of the sensor itself, environmental factors (temperature fluctuations and mechanical vibrations), and signal transmission loss during data acquisition. These factors may cause errors between the measured data and the actual strain value, affecting the accuracy and reliability of the data.

[0115] Based on the above-mentioned characteristics of large data volume and signal noise, it is necessary to reasonably process the acquired original creep data to reduce the impact of noise and reduce the data volume. First, the data collected every day two hours before and after the loading moment (the time of initial loading is called the "loading moment"); then the intercepted data segments are aggregated and the average value is taken as the representative strain value of the day. Taking the data two hours before and after the loading moment as the representative value can effectively avoid data fluctuations caused by changes in ambient temperature, and remove redundant data to avoid interference with the result analysis. Taking the measured data of longitudinal deformation 60d-64d after loading of a typical group as an example, the original data and the 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 unity of the acquisition system and the environment, the calculated true value of creep strain has removed a large amount of signal noise and periodic fluctuations caused by environmental influences. Aggregating the data before and after the loading moment, the creep representative value obtained can reflect the change trend of the true value, and this processing method is considered effective. Figure 3 It is hereby noted that the creep in the figure has been subtracted from the instantaneous elastic strain at loading.

[0116] Specifically, the creep data is processed, including: intercepting data several hours before and after the loading moment from the data collected every day (the time of initial loading is called "loading moment"); then aggregating the intercepted data and taking the average value as the representative value of strain for the day. The embodiment of the present invention takes data two hours before and after the loading moment; taking the data two hours before and after the loading moment as the representative value can effectively avoid data fluctuations caused by changes in ambient temperature, and remove redundant data to avoid interference with result analysis.

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

[0118] Recording starts from the time when compressive stress is applied and stops at the final unloading. The total strain-time curve of each group of specimens can be obtained by subtracting the corresponding shrinkage strain from the measured data. The typical concrete stress and total strain process line is shown in Figure 4 shown.

[0119] The instantaneous elastic strain ε generated during the loading process (which takes a very short time, regarded as time 0) 0 , at a constant compressive creep stress σ c The creep ε is generated under the action of c ; Creep recovery occurs after unloading, which includes two parts. One is the instantaneous elastic recovery ε during the unloading process (at time t) r , one is the delayed creep recovery t which increases with time from time t d , t d End the test at this time.

[0120] The total strain-time curve of each group of specimens under uniaxial compression (a represents axial creep, v represents lateral strain) is as follows Figure 5 As shown in the figure, the total strain-time curve of each group of specimens under pseudo triaxial compression (a represents axial creep, v represents radial creep) is as follows Figure 6 As shown. Figure 5 and Figure 6 It can be clearly seen that under uniaxial and pseudo-triaxial stress states, the creep development of GPC conforms to the general law of concrete creep development. From the moment of loading to the end of the test, the total deformation experiences four stages: instantaneous elastic strain, nonlinear creep development, instantaneous elastic recovery, and delayed elastic recovery. When the specimen is subjected to uniaxial stress, compressive strain is generated in the axial direction and tensile strain is generated in the lateral direction. When the specimen is subjected to axial compression and confining pressure, compressive strain is generated in both the axial and radial directions. The following text will analyze the uniaxial creep and pseudo-triaxial creep processes in detail from the characteristics of the four stages. It is worth noting that due to the inconsistent specimen sizes of uniaxial creep and pseudo-triaxial creep, there is a certain size effect in the compressive deformation, but the strain gauge length used in the two tests is the same. Therefore, the uniaxial creep strain directly measured by the test and the pseudo-triaxial creep strain are not suitable for direct comparison. The creep coefficient will be used as a measurement standard in the following text to further analyze the effect of fiber addition on creep and reveal the relationship between uniaxial creep and pseudo-triaxial creep.

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

[0122] Creep specimen preparation module, used to prepare creep specimens of different fiber types;

[0123] A test module, used to set different loading conditions and perform a compression creep test on the creep specimen;

[0124] A data acquisition and processing module is used to collect creep data of each creep specimen during the compression creep test and process the creep data;

[0125] The creep performance evaluation module is used to evaluate the creep performance of HFGPC based on the processed creep data.

[0126] The embodiments of the present invention mainly focus on the creep characteristics of HFGPC under multi-axial stress, and focus on analyzing the influence of changes in stress levels on the creep of HFGPC under the combined action of axial pressure and confining pressure. The influence of changes in axial pressure and confining pressure on the creep of HFGPC is divided into two series for analysis. Series I: Under the condition of the same axial pressure stress, compare the influence of changes in confining pressure levels on the creep of HFGPC; Series II: Under the condition of the same ratio of axial pressure and confining pressure, increase the axial pressure and confining pressure at the same time to analyze the creep performance of HFGPC, so as to study the comprehensive influence of confining pressure and axial pressure on the creep performance of HFGPC. The creep performance analysis under multi-axial stress in the embodiments of the present invention mainly starts from two aspects: volume creep and partial creep.

[0127] According to the elastic-plastic theory, when a material is under multiaxial stress, the stress tensor at a certain point can be decomposed into the spherical stress tensor (i.e., volume stress tensor, or hydrostatic stress tensor) and the deviatoric stress tensor, which correspond to volume strain and deviatoric strain. Extending to the deformation under long-term load, the creep under multiaxial pressure can be decomposed into two parts: the volume creep ε generated by the hydrostatic pressure p v and by the deviatoric stress σ d The resulting partial creep ε d , which can be expressed by the following formulas:

[0128]

[0129]

[0130]

[0131]

[0132] In the formula, σ 1 , σ 2 , σ 3 are the three principal stresses, ε c1 , ε c2 , ε c3 are the creeps corresponding to the principal stress directions. Based on the test equipment used in the embodiment of the present invention, σ 1 is the axial pressure, σ 2 , σ3 Same as confining pressure, ε c1 is the axial strain, ε c2 , ε c3 The same is radial strain. The relationship between volume creep and volume stress of MP group at different load holding ages is as follows: Figure 7 As shown in the figure, the relationship between deviatoric creep and deviatoric stress is as follows Figure 8 As shown in the figure, it can be seen that in the test results of pseudo-triaxial test, the volume creep and volume stress are approximately linearly related, and the deviatoric creep and deviatoric stress are also approximately linearly related; but in the case of no confining pressure (i.e., MP-A0.3 group), the volume creep is approximately in line with the pseudo-triaxial test results, and the deviatoric strain does not conform to the linear relationship.

[0133] 1. Elastic deformation during loading

[0134] The deformation of HFGPC in pseudo triaxial creep loading stage is listed in Table 2. Figure 4 From the creep-strain curve, it can be found that under the combined action of axial pressure and confining pressure, the axial and radial elastic deformations are larger than those under the action of axial pressure alone. This is because the specimen size used in the pseudo-triaxial creep test is larger than that of the uniaxial creep test. When the axial pressure applied within the elastic deformation range is the same, the specimen with a larger size will undergo a larger elastic deformation. The strain gauge gauge length used in the two tests is the same, so the pseudo-triaxial creep calculated is larger than the axial compression creep. In series I, when the axial compression stress ratio is the same, the instantaneous elastic strain of the pseudo-triaxial creep is larger than that of the axial compression creep, and when the confining pressure increases, the axial instantaneous creep of the pseudo-triaxial creep decreases, while the radial instantaneous creep increases; in series II, the ratio of axial pressure to confining pressure remains unchanged. When the axial pressure and confining pressure increase simultaneously, the axial instantaneous creep and radial instantaneous creep of the pseudo-triaxial creep both increase.

[0135] Table 2 Pseudo-triaxial compression creep related parameters

[0136]

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

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

[0139] The creep of HFGPC under triaxial compression load is as follows: Fig. 9 As shown in the figure, when the axial compressive stress ratio and the confining pressure / axial pressure are the same, the axial creep and radial creep of the MP group are smaller than those of the C group; when the axial compressive stress ratio is the same and the confining pressure / axial pressure increases from 25% to 40%, the axial creep of the MP group does not change significantly, but the radial creep increases by nearly one-fold; when the confining pressure / axial pressure is fixed at 25%, with the year-on-year increase of the confining pressure and axial pressure, the axial creep and radial creep of the MP group increase evenly.

[0140] The effects of axial and confining pressure changes on volume creep and partial creep are as follows: Fig.10 and Fig.11 As shown in the figure, it can be seen that when the axial pressure is constant, as the confining pressure / axial pressure increases, the volume creep increases almost linearly, while the partial creep gradually decreases, and the linear characteristics are not obvious. When the confining pressure / axial pressure is constant and the axial pressure stress ratio increases, the confining pressure and axial pressure increase year-on-year. At this time, both the volume creep and partial creep increase approximately linearly, and the longer the load holding age, the more obvious the effect of the axial pressure stress ratio on creep.

[0141] 3. Poisson effect during load holding

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

[0143]

[0144] Where p is the volume component of stress, i.e., hydrostatic pressure; ε v is the volume component of creep strain; σ d is the VonMiles equivalent deviator 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 is the bulk modulus, G is the rigidity modulus; v is the instantaneous Poisson's ratio under complex stress. Calculating the deformation under complex stress in this way is more in line with the physical mechanism of creep generation, and is also of great significance in viscoelastic analysis. Pseudo-triaxial creep Poisson's ratio (JI formula and definition formula are calculated separately) is as follows Fig.12 shown.

[0145] Depend on Fig.12It can be seen that the value calculated by the Poisson's ratio definition is greatly affected by the change of confining pressure, which will overestimate the creep Poisson's ratio of HFGPC under multiaxial stress. The results calculated using the JI formula take into account the effects of volume stress and deviatoric stress. Under different stress states of the MP group, the calculated Poisson's ratio values ​​are closer and can better reflect the properties of the material itself. From the calculation results of the JI formula, the creep Poisson's ratio of the C group under multiaxial stress can be approximately taken as 0.1, and the creep Poisson's ratio of the MP group can be approximately taken as 0.16, both of which are smaller than the creep Poisson's ratio under uniaxial stress.

[0146] 4. Creep coefficient in the load-holding stage

[0147] Since the specimen sizes of the uniaxial creep test and the pseudo triaxial creep test in the embodiment of the present invention are different, there is a size effect in creep deformation, which is not convenient for direct comparison. Therefore, the embodiment of the present invention uses the creep coefficient as an indicator to evaluate the pseudo triaxial creep performance of HFGPC and compares it with the uniaxial compression creep coefficient. When the axial-compression stress ratio is 0.25, 0.3, and 0.35 respectively, when the confining pressure increases from 0, the axial creep coefficient changes more obviously, such as Fig.13 As shown in (a)-(c) in .

[0148] Obviously, the effect of confining pressure greatly reduces the axial creep coefficient of HFGPC, which is close to 1 / 2 of the creep coefficient under unconfined compression. Zhang Tianxu's research on the true triaxial creep of concrete found that the creep coefficient of triaxial compression creep is close to twice that of uniaxial compression creep. Through analysis, he gave a possible reason that the deformation caused by the wear-reducing plate and lubricating oil was included in the creep deformation of concrete. This test uniformly uses built-in sensors for strain acquisition, which can collect the real deformation inside the concrete and avoid the interference of external factors on creep data acquisition. Therefore, the test has high reliability. In addition, in the analysis of concrete creep strain, triaxial compression creep is smaller than uniaxial compression creep, and with the increase of stress ratio, the difference between triaxial compression creep and uniaxial compression creep gradually decreases. When the stress ratio is 0.3, the triaxial compression creep coefficient is similar to the uniaxial compression creep coefficient as shown in Figure 2. Fig.13 As shown in (d) in the figure, it can be clearly seen that the triaxial compression creep coefficient is about 2 / 3 of the uniaxial compression creep coefficient, which is close to the test results. Therefore, it can be reasonably inferred that the axial creep coefficient of HFGPC under pseudo-triaxial stress should be smaller than the uniaxial creep coefficient and close to 1 / 2 of the uniaxial creep coefficient.

[0149] 5. Creep recovery

[0150] The parameters related to pseudo triaxial creep recovery are listed in Table 2. The hysteresis recovery curves of each test group are shown in Fig.14 As shown, the instantaneous creep recovery coefficient, delayed creep recovery coefficient, creep recovery ratio, etc. Fig.15As shown. Fig.15 It can be clearly seen from (a) that at the same stress level, the instantaneous creep recovery coefficient of the MP group is smaller than that of the C group, but the hysteresis creep recovery coefficient of the MP group is larger than that of the C group. This is because when HFGPC is under pressure, the energy exists in the geopolymer matrix, fibers, matrix voids, and the interface between the fibers and the matrix, while the energy of plain GPC is stored in the matrix and voids. Therefore, compared with the C group, the energy release process during unloading is different in the MP group. Due to its greater stiffness, plain GPC releases elastic energy faster during unloading, while for HFGPC, due to the presence of the fiber mesh structure and the interface transition zone, the energy during unloading is gradually released. Therefore, the instantaneous creep recovery coefficient of the MP group is smaller than that of the C group, while the hysteresis creep recovery is greater than that of the C group. From Fig.14 It can be seen from the hysteresis recovery curve that the deformation of group C quickly stabilized, while the deformation of group MP gradually recovered.

[0151] The present invention studies the creep performance of HFGPC through uniaxial creep test and pseudo triaxial creep test. The following conclusions are drawn through analysis of the test data:

[0152] (1) The results of uniaxial creep tests 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, which 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 pseudo-triaxial creep tests 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 greater influence 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] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the device disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0155] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one 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 present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A hybrid fiber reinforced geopolymer concrete creep test method, characterized in that: include: Prepare creep specimens with different fiber types; Setting different loading conditions, and performing compression creep tests on the creep specimens; During the compression creep test, creep data of each creep specimen is collected and the creep data is processed; Based on the processed creep data, the creep performance of hybrid fiber reinforced geopolymer concrete was evaluated.

2. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 1, characterized in that: The compression creep test adopts a pseudo triaxial compression creep test.

3. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 2, characterized in that: The steps of the pseudo triaxial compression creep test include: (1) Assemble the pressure chamber components and creep specimen from bottom to top, install them on the creep instrument, move the pressure chamber to the top of the oil cylinder, align it with the pressure sensor on the creep instrument through the positioning bolts, and tighten the oil pipe; (2) Connecting the sensor wires of the prepared creep specimen and two compensating shrinkage specimens to a data acquisition instrument; (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 sensors are working properly; (4) After confirming that the data acquisition system and sensors are working properly, 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; (5) Record the strain value after loading is completed, subtract it from the strain value before loading to obtain the initial creep strain, and record the strain value of the comparison compensation shrinkage specimen as the reference value for subsequent calculation of natural shrinkage.

4. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 3, characterized in that: The preloading includes first setting a first-level loading, the axial pressure value is 20% of the preset axial pressure, the confining pressure value is 50% of the preset confining pressure, and unloading is performed after the first-level loading is completed.

5. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 1, characterized in that: The creep data include: instantaneous elastic strain during loading, free shrinkage strain, creep and total creep strain during loading; e c =e t -e0-e s ; In the formula, ε c is creep, ε t is the creep at a certain loading age, ε0 is the instantaneous elastic strain during loading, and ε s is the shrinkage strain at that age.

6. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 5, characterized in that: Creep degree and creep coefficient are used to evaluate the creep performance of concrete.

7. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 6, characterized in that: The creep coefficient is the ratio of creep deformation to elastic deformation, expressed as: In the formula, is the creep coefficient at the load holding time t(d); Creep is the creep strain under unit stress, expressed as: In the formula, C t is the creep degree at the load holding time t(d), σ c is the creep stress; There is the following relationship between creep degree and creep coefficient, which is expressed as: In the formula, E t is the elastic modulus; The creep function is the sum of the instantaneous elastic strain and creep strain produced by unit stress, expressed as: Among them, creep function and creep degree can both be expressed by creep coefficient.

8. A hybrid fiber reinforced geopolymer concrete creep test method according to claim 1, characterized in that: The creep data is processed, including: intercepting data several hours before and after the loading time from the data collected every day; then aggregating the intercepted data and taking the average value as the representative strain value of the day.

9. A hybrid fiber reinforced geopolymer concrete creep test system, characterized in that: include: Creep specimen preparation module, used to prepare creep specimens of different fiber types; A test module, used to set different loading conditions and perform a compression creep test on the creep specimen; A data acquisition and processing module is used to collect creep data of each creep specimen during the compression creep test and process the creep data; The creep performance evaluation module is used to evaluate the creep performance of hybrid fiber reinforced geopolymer concrete based on the processed creep data.

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