CN114323992A - Device and method for testing concrete creep characteristic under positive and negative temperature alternating action - Google Patents
Device and method for testing concrete creep characteristic under positive and negative temperature alternating action Download PDFInfo
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Abstract
The invention discloses a device and a method for testing concrete creep characteristics under the action of positive and negative temperature alternation, wherein the testing device comprises a loading cavity, a testing cavity and a testing cavity, wherein the loading cavity is used for matching and loading a concrete test piece; the loading system is used for applying axial load to the concrete test piece in the loading cavity; the heating/refrigerating system is used for heating or refrigerating the concrete sample; and a strain sensor is pre-embedded in the concrete test piece and is connected with a data acquisition system through a data line. The invention can realize variable temperature regulation by utilizing the heating/refrigerating system, can effectively simulate the actual large temperature difference environment of the concrete, is similar to the environment condition of the concrete structure serving in a large temperature difference area, can better master the creep performance of the concrete under the environment condition, overcomes the defects of the traditional test method only considering the creep characteristic of the concrete under the constant temperature condition, and has obvious superiority.
Description
Technical Field
The invention belongs to the technical field of concrete performance measurement, and particularly relates to a device and a method for testing concrete creep characteristics under the action of positive and negative temperature alternation.
Background
Concrete is the most widely used and most used building material worldwide. With the continuous development of concrete material technology, concrete structures are also continuously extended to areas with worse weather environments, and how to effectively test and evaluate the performance and durability of concrete in actual service environments is one of important tasks which need to be broken through urgently.
As the duration of the load increases, the material gradually develops deformation over time, collectively referred to as creep or creep deformation. Creep causes redistribution of internal force, relaxation of stress, increase in deformation, etc. of the concrete structure, which affects safety, durability, etc. of the concrete structure. However, in the western engineering construction of China, the concrete structure also faces the problem of a large temperature difference service environment, and the concrete creep at constant temperature is greatly different from the concrete creep in the actual service process, so that a creep test device and an evaluation method capable of simulating the natural temperature change are urgently needed.
Extensive practice has shown that the deformation of the material under the action of external loads and the surrounding environment often leads to concrete cracking [1 ]. In recent years, some researchers have conducted theoretical and experimental studies on the correlation of creep and temperature-time change. The temperature is vital to the concrete structure design in the actual environment, the concrete creep is greatly influenced by temperature and humidity, the basic creep is considered to be changed when being influenced by the temperature based on Arrhenius thermal analysis, the structure is damaged when the temperature is increased (no matter whether the structure is stressed), and the creep characteristic is influenced by the temperature gradient [2 ]. Temperature is an important external factor affecting the shrinkage creep of concrete. Generally, the creep rate increases with increasing temperature, decreasing the elastic modulus, and increasing temperature accelerates the drying of the concrete surface, so that the drying creep also increases [3 ]. Yangyongqing and the like [4] combine the special environmental conditions of China, indicate that the temperature changing along with time under the natural environment has larger influence on the creep rule according to the test data of nearly 3 years, provide a creep combination model considering the temperature influence, and can accurately analyze the shrinkage and creep effects of the concrete. Based on the three-dimensional characteristic of shrinkage and creep, the shrinkage and creep effect of the concrete in the natural temperature alternating environment is analyzed, and a mechanical model of the three-dimensional shrinkage and creep effect of the concrete in the temperature varying environment is established [5 ].
The main objective of the concrete creep test is to measure the time-dependent deformation of concrete samples under constant pressure or tension load (generally 30% -40% of the breaking load) under constant temperature and humidity environment [6], so that the influence of temperature on the concrete creep is less considered. Wangyong rock and other people invented a pressure type heating and refrigerating high-low temperature environment test box device (with the publication number of CN 106423311A), which heats and refrigerates a test piece through a temperature control module in a main shell of the environment box, so that the temperature control of the existing triaxial testing machine when the test piece is pressed by the test machine is realized, but the main object of the device is a solid-liquid transformation test of a soft rock similar material, and the device cannot be applied to the test of concrete creep; zhanling et al invented a CCZ type concrete member creep positive and negative temperature change tester [ publication number CN103217341A ], the device is mainly an environment box capable of realizing positive and negative temperature change, the coupling test of temperature and load is mainly implemented by putting a spring compression creep instrument with preset load, only automatic monitoring of temperature can be realized, real-time monitoring feedback of load cannot be adjusted, and the put spring creep instrument is easily affected by the temperature change of the environment box, and the practicability is low; jiangwei et al invented a concrete creep under pressure test device [ publication No. CN104483204A ] under high temperature environment, which was composed of a traditional creep test device and an oven heating structure, and was used for testing the concrete creep under pressure under high temperature environment, but could not satisfy the requirement of positive and negative temperature change, and the oven heating device also had a great influence on the humidity of the concrete, and the sealing performance was poor, and the feasibility was not high under the high temperature condition. In summary, reports of a test system of the concrete creep characteristic under the positive and negative temperature alternating action and an evaluation method thereof are not seen so far, and in order to meet the requirements of large-scale concrete engineering construction in China, particularly the requirements of durability design and material selection of a large engineering structure under the western large temperature difference service environment, the invention aims to develop a test device and a method capable of effectively testing and evaluating the concrete creep characteristic under the positive and negative temperature alternating action, and has important theoretical research significance and engineering application value for accurately evaluating the creep characteristic of the concrete under the large temperature difference environment and improving the safety and durability of the structure.
Reference documents:
[1] kuma-meita, paul j.m. montuterol, concrete microstructure, properties and materials [ M ] chinese power press, 2008.
[2] Zhanling, Ha Megfeng, Qiqiang, Wang Qi Shi, Marina, Huang Gua concrete creep test research and prediction model considering temperature change [ J ] materials guide report, 2021,35(20): 20028-.
[3] The test research on the shrinkage creep and temperature time-varying effect of the prestressed box girder of the high-speed railway [ D ]. university of China and south, 2014.
[4] Natural environment concrete creep test and prediction model studies [ J ]. proceedings of southwest university of transportation, 2015, 50 (6): 977-983
[5] Yangyongqing, Lishiwei, Lidao and the balance of the three-dimensional shrinkage and creep effect of concrete under the environment of variable temperature [ J ]. the school of transportation university in southwest, 2019,54(05):931 and 936.
[6] Marango. several problems with modern concrete creep are discussed [ D ]. university of river and sea, 2006.
Disclosure of Invention
The invention aims to provide a device for effectively and accurately testing the creep characteristic of concrete under the action of positive and negative temperature alternation.
The invention aims to provide a concrete creep characteristic testing method under the positive and negative temperature alternating action, which is effective and accurate.
The invention provides a concrete creep characteristic testing device under the condition of positive and negative temperature alternation, which comprises:
the loading cavity is used for matching and loading the concrete test piece;
the loading system is used for applying axial load to the concrete test piece in the loading cavity;
the heating/refrigerating system is used for heating or refrigerating the concrete sample;
and a strain sensor is pre-embedded in the concrete test piece and is connected with a data acquisition system through a data line.
Specifically, the heating/cooling system comprises a heating/cooling pipe, a refrigerant source and a heat medium source, the heating/cooling pipe is selected to be communicated with the refrigerant source and the heat medium source, the heating/cooling pipe is embedded in the inner wall of the loading cavity, and a temperature and humidity sensor is further arranged in the inner wall of the loading cavity.
Specifically, the heating/refrigerating system further comprises a medium storage, a circulating pump and a return pipe, wherein one end of the return pipe is communicated with the outlet end of the heating/cooling pipe, and the other end of the return pipe is selectively communicated with a heating pipeline and a cooling pipeline;
the heating pipeline and the cooling pipeline are communicated with the inlet end of the heating/cooling pipe, the medium storage device and the circulating pump are arranged on the return pipe, the heating pipeline is provided with a heater, the cooling pipeline is provided with a refrigerator, and the temperature and humidity sensor, the heater and the refrigerator are connected with a temperature control system.
Specifically, the heating/cooling pipe is a spiral pipe body surrounding the concrete sample.
Specifically, the periphery of the loading cavity is provided with a heat insulation layer.
Specifically, the loading system includes reaction frame and loading cylinder, the reaction frame comprises bottom plate, roof and four stands of connection between bottom plate and roof, the loading cylinder sets up on the roof, it arranges in to load the chamber on the bottom plate, and opens at both ends and set up, the loading cylinder is right through activity bearing spare the concrete test piece of loading intracavity carries out axial loading, the loading cylinder with be equipped with pressure sensor between the activity bearing spare.
Specifically, the bottom plate is connected with a bearing plate through a spherical hinge, and the bearing plate is provided with a bearing seat inserted into the loading cavity and contacted with the concrete test piece.
Specifically, the stand is located the below of bottom plate still is equipped with the base plate, the bottom plate with be equipped with the load spring between the base plate, the roof is fixed on the stand by lock nut, every it has a pair of adjusting nut still to go back threaded connection on the stand, bottom plate and base plate wear on the stand to be located between a pair of adjusting nut.
A method for testing concrete creep characteristic under the condition of positive and negative temperature alternation comprises the steps of placing a plurality of superposed concrete test pieces in a loading cavity, ensuring that the concrete test pieces are positioned at the center of the loading cavity, presetting test temperature and load, starting a loading system to axially load the concrete test pieces, starting a heating/refrigerating system to heat or refrigerate the concrete test pieces, and obtaining the creep characteristic under the positive and negative temperature alternation of concrete through concrete strain data measured by a strain sensor.
Compared with the prior art, at least one embodiment of the invention has the following beneficial effects:
1. the heating/refrigerating system can realize variable temperature regulation, can effectively simulate the actual large temperature difference environment of concrete service, is similar to the concrete structure environment condition of a large temperature difference area, can better master the creep performance of the concrete under the environment condition, overcomes the defect of the traditional concrete creep characteristic test method only considering the constant temperature condition, and has obvious superiority.
2. The adverse effect of temperature change on the loading instrument can be reduced to the minimum only by regulating and controlling the ambient temperature of the concrete test piece.
3. The effective transmission of load is realized through the movable heat preservation cover and the base fixed in the heat preservation device, and meanwhile, the heat preservation device has effective sealing performance.
4. Adopt 100X 400's cuboid test piece to test, not only guaranteed that the test piece can get into predetermined temperature state fast to can load 2 test pieces simultaneously, be favorable to reducing the discreteness of result, ensure the accuracy of test result.
5. The concrete strain change under the test condition is tracked in real time through the built-in concrete strain sensor, so that the concrete creep characteristic can be mastered in real time, and the concrete creep characteristic can be evaluated more effectively and scientifically.
6. The device is simple to operate, can be automatically started, stopped and adjusted according to actual conditions after the temperature and the load are set, is convenient to process data, and can be used for performing a positive and negative temperature alternating creep test for 6 months to 36 months.
Drawings
In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.
FIG. 1 is a schematic structural diagram of a concrete creep property testing apparatus provided in an embodiment of the present invention;
FIG. 2 is a schematic front view of a loading chamber according to an embodiment of the present invention;
FIG. 3 is a schematic top view of a load chamber according to an embodiment of the present invention;
wherein: 1. a column; 2. a top plate; 3. a stress plate; 4. a pressure bearing plate; 5. a loading cylinder; 6. a pressure sensor; 7. a base plate; 8. a loading chamber; 9. spherical hinge; 10. a base plate; 11. a force bearing spring; 12. a substrate; 13. a data acquisition system; 14. a switching valve; 15. a return pipe; 16. a movable pressure-bearing member; 17. an inlet end; 18. a temperature and humidity sensor; 19. a heating/cooling tube; 20. an outlet end; 21. a seal ring; 22. a heat-insulating layer; 23. a concrete sample; 24. a strain sensor; 25. a force bearing seat; 26. a refrigerator; 27. a heater; 28. a circulation pump; 29. a media storage.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
In the description of the present invention, it is to be understood that the terms "central," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," "circumferential," and the like are used in the orientations and positional relationships indicated in the drawings for convenience in describing the invention and to simplify the description, and are not intended to indicate or imply that the referenced devices or elements must have a particular orientation, be constructed and operated in a particular orientation, and are therefore not to be considered limiting of the invention.
Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
Referring to fig. 1, the device for testing the creep characteristic of the concrete under the condition of the positive and negative temperature alternating action comprises a loading cavity 8, a loading system and a heating/refrigerating system, wherein the loading cavity 8 is used for matching with a loaded concrete test piece 23, the loading system is used for applying an axial load to the concrete test piece 23 in the loading cavity 8, the heating/refrigerating system is used for heating or refrigerating the concrete test piece 23, a strain sensor 24 is pre-embedded in the concrete test piece 23, and the strain sensor 24 is connected with a data acquisition system 13 through a data line.
When the testing device with the structure is used for testing the creep characteristic of the concrete, a plurality of superimposed concrete test pieces 23 are placed in the loading cavity 8, the concrete test pieces 23 are ensured to be positioned at the center of the loading cavity 8, the test temperature and load are preset, the loading system is started to axially load the concrete test pieces 23, the heating/refrigerating system is started to heat or refrigerate the concrete test pieces 23, and the creep characteristic under the positive and negative temperature alternating action of the concrete can be obtained through the concrete strain data measured by the strain sensor 24.
In the embodiment, the heating/refrigerating system is utilized to realize variable temperature regulation, a real large temperature difference environment in which concrete is in service can be effectively simulated, the creep performance of the concrete under the environment condition can be better mastered by being similar to the environment condition of a concrete structure in a large temperature difference area in service, the defect of the conventional test method for the creep characteristic of the concrete under the constant temperature condition is overcome, and the method has obvious advantages. Wherein the temperature regulation and control range of the heating/refrigerating system is-20 ℃ to +60 ℃, the temperature precision reaches 0.1 ℃, and the constant temperature setting of a certain temperature can be carried out at the same time.
Referring to fig. 1 and 2, in some embodiments, the heating/cooling system includes a heating/cooling pipe 19, a cooling medium source and a heating medium source, the heating/cooling pipe 19 is selectively communicated with the cooling medium source and the heating medium source, the heating/cooling pipe 19 is pre-embedded in the inner wall of the loading cavity 8, a temperature and humidity sensor 18 is further disposed in the inner wall of the loading cavity 8, and the cooling medium source and the heating medium source are introduced into the heating/cooling pipe 19 and then exchange heat with the concrete sample 23 through the inner wall of the loading cavity 8, so that heating or cooling of the concrete sample 23 is achieved.
Referring to fig. 1 and 2, specifically, the heating/cooling system further includes a medium reservoir 29, a circulation pump 28, and a return pipe 15, one end of the return pipe 15 is communicated with the outlet end 20 of the heating/cooling pipe 19, the other end of the return pipe is selectively communicated with a heating pipeline and a cooling pipeline, both the heating pipeline and the cooling pipeline are communicated with the inlet end 17 of the heating/cooling pipe 19, the medium reservoir 29 and the circulation pump 28 are disposed on the return pipe 15, the heating pipeline is provided with a heater 27, the cooling pipeline is provided with a refrigerator 26, and the temperature and humidity sensor 18, the heater 27, and the refrigerator 26 are all connected to the temperature control system.
The medium storage 29 stores temperature transfer media such as silicone oil, and the temperature control system regulates and controls the operation of the circulating pump 28 under the data real-time data feedback of the temperature and humidity sensor 18, so that the temperature transfer media in the medium storage 29 flow in the pipeline, the flow path of the temperature transfer media is determined by the switching valve 14, and the heater 27 or the refrigerator 26 is started to heat and cool the temperature transfer media.
Referring to fig. 2 and 3, the heating/cooling pipe 19 is a spiral pipe body surrounding the concrete sample 23, and an insulating layer 22 is provided on the periphery of the loading chamber 8. The refrigerator 26 adopts a compressor to work on a refrigeration system, the temperature of the temperature transfer medium is reduced through the refrigeration system, the refrigeration system is manufactured by utilizing the principle that a refrigerant lower than the boiling point is changed into steam in the evaporator to absorb heat, the heater 27 utilizes the resistance wire of the heating tube to generate heat, and then the heat is transferred to the temperature transfer medium to heat the concrete test piece 23.
Referring to fig. 1, in some embodiments, the loading system includes a reaction frame and a loading cylinder 5, the reaction frame is composed of a bottom plate 10, a top plate 2 and four upright posts 1 connected between the bottom plate 10 and the top plate 2, the loading cylinder 5 is disposed on the top plate 2, a loading cavity 8 is disposed on the bottom plate 10, two ends of the loading cavity are open, the loading cylinder 5 axially loads a concrete test piece 23 in the loading cavity 8 through a movable bearing piece 16, and a pressure sensor 6 is disposed between the loading cylinder 5 and the movable bearing piece 16.
Referring to fig. 1, specifically, a bearing plate 4 is connected to a bottom plate 10 through a spherical hinge 9, a bearing seat 25 inserted into a loading cavity 8 and contacting a concrete test piece 23 is arranged on the bearing plate 4, sealing pieces 21 are arranged between the bearing seat 25 and the loading cavity 8 and between a movable bearing piece 16 and the loading cavity 8, a base plate 12 is further arranged below the bottom plate 10 on each upright post 1, a bearing spring 11 is arranged between the bottom plate 10 and the bearing plate 4, a top plate 2 is fixed on the upright posts 1 through locking nuts, a pair of adjusting nuts are further connected to each upright post 1 through threads, and the bottom plate 10 and the base plate 12 penetrate through the upright posts 1 and are located between the pair of adjusting nuts.
Referring to fig. 1, specifically, the loading cylinder 5 and the movable pressure-bearing member 16 are further provided with a stress plate 3, the stress plate 3 penetrates through the upright post 1 and is subjected to position adjustment through a nut, the pressure sensor 6 is arranged between the stress plate 3 and the movable pressure-bearing member 16, a backing plate 7 is further arranged between the pressure sensor 6 and the movable pressure-bearing member 16, the loading cavity 8 is arranged between the stress plate 3 and the pressure-bearing plate 4, the stress plate 3, the loading cavity 8 and the pressure-bearing plate 4 are axially positioned and load is applied through the four upright posts 1, and stable application of load is realized through the movable pressure-bearing member 16 which can freely move up and down in the loading cavity 8.
Referring to fig. 1, a method for testing concrete creep characteristics under the action of positive and negative temperature alternation comprises the following steps:
the first step is as follows: sample preparation
Preparing the stirred concrete according to test raw materials and proportion, embedding the strain sensor 24 in the sample, enabling the measurement base line to be positioned in the middle of the sample and to be superposed with the longitudinal axis of the sample, and obtaining a sample of the sample after molding and maintenance; and the same set of samples was made for compression and shrinkage testing.
The second step is that: sample placement
Two test samples are superposed and placed into the loading cavity 8 and aligned with the groove of the bearing seat 25 at the bottom of the loading cavity 8, the groove of the movable pressure-bearing piece 16 is aligned with the sample, the concrete sample 23 is ensured to be positioned in the center of the loading cavity 8, and then the movable pressure-bearing piece 16 is covered, so that the sealing effect is achieved. The cuboid test piece of 100X 400 is adopted to the sample, has not only guaranteed that the test piece can get into predetermined temperature state fast to can load 2 test pieces simultaneously, be favorable to reducing the discreteness of result, ensure the accuracy of test result.
The third step: creep test
After the loading chamber 8 is entirely placed on the pressure bearing plate 4 of the creep test device, the axes of the loading chamber 8 and the loading system are overlapped, the lead of the strain sensor 24 is connected with the data acquisition system 13, the zero setting condition of the strain sensor 24 is checked, and the initial reading is recorded. After the test sample is placed, setting the test temperature and the load, starting the device in time, and taking creep stress to be 40% of the compressive strength of the test sample when no special requirement exists. The deformation values of the test specimens should be measured under the same conditions as the initial temperatures at 1d, 3d, 7d, 14d, 28d, 45d, 60d, 90d, 120d, 150d, 180d, 270d and 360d after the loading, and the shrinkage values of the shrinkage test pieces for reference should be measured at the same time.
The fourth step: creep characteristics of concrete
The creep strain, the creep degree, the creep coefficient and the like are calculated according to the standard requirements by using the measured data, the arithmetic mean value of the test results of the creep strain (the creep degree or the creep coefficient) of 3 test pieces is respectively used as the measured value of the creep strain (the creep degree or the creep coefficient) of the test pieces under the corresponding environment temperature of the group, and the data such as the creep characteristic curve and the creep rate of the concrete under the set temperature change can be output by a testing device. Creep test parameters are determined according to the actual service conditions of the target evaluation component and the GB/T50082-2009 standard.
Any embodiment disclosed herein above is meant to disclose, unless otherwise indicated, all numerical ranges disclosed as being preferred, and any person skilled in the art would understand that: the preferred ranges are merely those values which are obvious or representative of the technical effect which can be achieved. Since the numerical values are too numerous to be exhaustive, some of the numerical values are disclosed in the present invention to illustrate the technical solutions of the present invention, and the above-mentioned numerical values should not be construed as limiting the scope of the present invention.
Meanwhile, if the invention as described above discloses or relates to parts or structural members fixedly connected to each other, the fixedly connected parts can be understood as follows, unless otherwise stated: a detachable fixed connection (for example using bolts or screws) is also understood as: non-detachable fixed connections (e.g. riveting, welding), but of course, fixed connections to each other may also be replaced by one-piece structures (e.g. manufactured integrally using a casting process) (unless it is obviously impossible to use an integral forming process).
In addition, terms used in any technical solutions disclosed in the present invention to indicate positional relationships or shapes include approximate, similar or approximate states or shapes unless otherwise stated. Any part provided by the invention can be assembled by a plurality of independent components or can be manufactured by an integral forming process.
The above examples are merely illustrative for clearly illustrating the present invention and are not intended to limit the embodiments. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. Nor is it intended to be exhaustive of all embodiments. And obvious variations or modifications of the invention may be made without departing from the scope of the invention.
Claims (9)
1. Concrete creep characteristic testing arrangement under positive negative temperature alternation action condition which characterized in that includes:
the loading cavity (8) is used for matching and loading the concrete test piece (23);
the loading system is used for applying axial load to the concrete test piece (23) in the loading cavity (8);
the heating/refrigerating system is used for heating or refrigerating the concrete test piece (23);
and a strain sensor (24) is embedded in the concrete test piece (23), and the strain sensor (24) is connected with a data acquisition system (13) through a data line.
2. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 1, wherein: the heating/refrigerating system comprises a heating/cooling pipe (19), a refrigerant source and a heat medium source, wherein the heating/cooling pipe (19) is selected to be communicated with the refrigerant source and the heat medium source, the heating/cooling pipe (19) is embedded in the inner wall of the loading cavity (8), and a temperature and humidity sensor (18) is further arranged in the inner wall of the loading cavity (8).
3. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 2, wherein: the heating/refrigerating system also comprises a medium storage (29), a circulating pump (28) and a return pipe (15), wherein one end of the return pipe (15) is communicated with the outlet end (20) of the heating/cooling pipe (19), and the other end of the return pipe is communicated with a heating pipeline and a cooling pipeline alternatively;
heating line and cooling line all with entrance point (17) intercommunication of heating/cooling tube (19), media accumulator (29) and circulating pump (28) set up on back flow (15), be equipped with heater (27) on the heating line, be equipped with refrigerator (24) on the cooling line, temperature and humidity sensor (18), heater (27) and refrigerator (24) all are connected with temperature control system.
4. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 2, wherein: the heating/cooling pipe (19) is a spiral pipe body which surrounds the concrete test piece (23).
5. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 2, wherein: and a heat insulation layer (22) is arranged on the periphery of the loading cavity (8).
6. The apparatus for testing creep characteristics of concrete under alternating positive and negative temperature conditions according to any one of claims 1 to 5, wherein: the loading system comprises a reaction frame and a loading cylinder (5), wherein the reaction frame is composed of a bottom plate (10), a top plate (2) and four stand columns (1) connected between the bottom plate (10) and the top plate (2), the loading cylinder (5) is arranged on the top plate (2), a loading cavity (8) is arranged on the bottom plate (10), two ends of the loading cylinder are arranged in an open mode, the loading cylinder (5) is subjected to axial loading through a movable bearing part (16) on a concrete test piece (23) in the loading cavity (8), and a pressure sensor (6) is arranged between the loading cylinder (5) and the movable bearing part (16).
7. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 6, wherein: the concrete test piece loading device is characterized in that a bearing plate (4) is connected to the bottom plate (10) through a spherical hinge (9), and a bearing seat (25) which is inserted into the loading cavity (8) and is in contact with the concrete test piece (23) is arranged on the bearing plate (4).
8. The apparatus for testing creep characteristics of concrete under positive and negative temperature alternating action according to claim 6, wherein: the upright post is characterized in that a base plate (12) is further arranged below the bottom plate (10) on the upright post (1), a force bearing spring (11) is arranged between the bottom plate (10) and the base plate (12), the top plate (2) is fixed on the upright post (1) through a locking nut, each upright post (1) is further in threaded connection with a pair of adjusting nuts, and the bottom plate (10) and the base plate (12) are arranged on the upright post (1) and located between the pair of adjusting nuts.
9. A method for testing creep characteristics of concrete under positive and negative temperature alternating action conditions, using the test apparatus according to any one of claims 1 to 8, wherein: the method comprises the steps of placing a plurality of superimposed concrete test pieces (23) in a loading cavity (8), ensuring that the concrete test pieces (23) are located at the center of the loading cavity (8), presetting test temperature and load, starting a loading system to axially load the concrete test pieces (23), starting a heating/refrigerating system to heat or refrigerate the concrete test pieces (23), and obtaining creep characteristics under the positive and negative temperature alternating action of concrete according to concrete strain data measured by a strain sensor (24).
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