Uniaxial compression performance test analysis method of HFGPC under quasi-static and seismic strain rates

Abstract

The invention discloses a uniaxial compression performance test analysis method of HFGPC under quasi-static and seismic strain rates, relates to the technical field of concrete material performance testing, and is characterized in that test data is collected and analyzed while a test piece is manufactured to carry out an experiment. The uniaxial compression performance of the HFGPC under the quasi-static state and the seismic strain rate can be comprehensively and systematically researched, including the aspects of failure form, stress-strain curve, mechanical property parameters, deformation parameters and the like, and abundant data support is provided for deeply knowing the mechanical behavior of the HFGPC. The test results are analyzed in detail and deeply, including strain rate effect analysis and mechanism analysis, so that the performance change rules and internal reasons of the HFGPC under different strain rates can be disclosed, a scientific theoretical basis is provided for the application of the HFGPC in practical engineering, and the engineering design can be optimized and the safety and durability of the structure can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete material performance testing, and more particularly to a method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates. Background Art

[0002] At present, HFGPC, as a new type of concrete, has shown great application potential in the construction field due to its excellent mechanical properties and crack resistance. However, as a heterogeneous material, HFGPC is often accompanied by the generation of initial defects such as pores and cracks during the preparation process, and these defects often become the starting point of its destruction under external loads. The hybrid addition of steel fiber and PVA fiber can significantly improve the compressive strength of GPC, but its stress-strain relationship, crack development mechanism, deformation characteristics and the specific role of fiber in the compression process have not been systematically elucidated. However, the current research on the uniaxial compressive performance of hybrid fiber reinforced geopolymer concrete (HFGPC) under different strain rates is not systematic and comprehensive enough.

[0003] Therefore, how to accurately evaluate the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates is an urgent problem to be solved by technicians in this field. Summary of the invention

[0004] In view of this, the present invention provides a method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates to solve the problems existing in the background technology.

[0005] In order to achieve the above object, the present invention adopts the following technical solution:

[0006] A method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, comprising:

[0007] Based on the test conditions and test method standards, a test piece with a size of φ100mm×100mm×300mm was manufactured;

[0008] A 10000kN electro-hydraulic servo pressure testing machine is used for loading, and four disc springs with a maximum load capacity of 400kN are used together with the specimen to bear the force;

[0009] A YBY-800 spoke-type pressure sensor was used with two pads to collect load data. Strain gauges were attached to the sides of the specimens and horizontal and vertical displacement gauges were set up to test the lateral and longitudinal displacements, respectively. A DH5920 dynamic data collector was used to connect the strain gauges and sensors to record time, force, strain, and displacement data.

[0010] A computer-controlled high-speed camera is set up on one side of the testing machine, with the lens facing the side of the specimen, to record the entire process of compression failure;

[0011] The uniaxial compression failure morphology of HFGPC was analyzed, the morphology of the specimen after failure was observed, the influence of fiber and strain rate on the failure morphology was analyzed, and the characteristics of different failure modes were summarized;

[0012] The collected displacement data are averaged and plotted against the load data to form stress-strain curves under quasi-static and seismic strain rates;

[0013] Based on the stress-strain curves under quasi-static and seismic strain rates, the compressive mechanical properties parameters and quasi-static compression deformation parameters of HFGPC under quasi-static strain rates are analyzed.

[0014] Optionally, the loading also includes loading in a displacement control mode, with loading rates of 0.18, 1.8, and 18 mm / min, corresponding to 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 strain rate; before the test officially started, loading and unloading were carried out three times, with the preload set to 10 kN and the loading speed to 0.5 MPa / s. The specimen position was aligned according to the data collected by the displacement sensor during the preloading stage until the deformation difference on both sides was no more than 15%, so as to reduce the influence of eccentric compression on the test results.

[0015] Optionally, the high-speed camera uses a camera resolution of 1280×800 and a strain rate of 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 The recording is done at frame rates of 68fps, 120fps, and 500fps respectively.

[0016] Optionally, the process of uniaxial compression failure of HFGPC is as follows:

[0017] (1) When the stress rises to 44% of the peak stress, cracks begin to appear at the corners of the pressure-bearing surface, and the observation time is recorded as 0;

[0018] (2) When the stress rises to 81% of the peak stress, fine cracks develop and penetrate, and the entire surface of the specimen peels off, accompanied by a popping sound;

[0019] (3) The stress continues to increase to a peak value, and then drops sharply. At the same time, cracks appear on the surface of the specimen, and there are thin flakes that bulge and peel off. The cracks develop rapidly from the cracked part and penetrate the specimen axially;

[0020] (4) Then, when the stress drops to 33% of the peak value, it enters the platform section, the crack width increases, and the axial strain increases by 2000 με. After that, the stress drops again, and the specimen loses its bearing capacity and eventually fails.

[0021] Optionally, the stress-strain curves drawn under quasi-static and seismic strain rates are specifically:

[0022] The displacement data collected synchronously are averaged and plotted into a stress-strain curve corresponding to the load data; the test results are presented in the form of average stress-strain, that is, the average value of the three stress values ​​under the same strain is taken as the average stress value to draw the curve.

[0023] Optionally, the stress-strain curve analysis under quasi-static and seismic strain rates includes curve characteristic analysis and mechanical property parameter analysis;

[0024] Curve characteristic analysis: Analyze the characteristics of the rising section, peak point, and falling section of the stress-strain curve, including elastic proportional limit, peak stress, peak strain, and residual strength parameters, and study the influence of fiber and strain rate on the parameters;

[0025] Mechanical performance parameter analysis:

[0026] Axial compressive strength and residual strength ratio: The peak stress is obtained from the stress-strain curve as the representative value of the axial compressive strength. The stress when the axial strain reaches 6εp is the residual strength. The ratio of residual strength to axial compressive strength is calculated, and the variation of axial compressive strength and residual strength ratio with fiber type and dosage is analyzed.

[0027] Ratio of axial compressive strength to cube compressive strength: Calculate the ratio of axial compressive strength to cube compressive strength based on the test results, analyze its relationship with fiber content, and compare it with relevant standard values ​​of ordinary concrete.

[0028] Optionally, the analysis of stress-strain curves at quasi-static and seismic strain rates also includes analysis of quasi-static compressive deformation parameters;

[0029] Strain characteristic value: take the axial strain and transverse strain corresponding to the peak point of the stress-strain curve as the axial peak strain and transverse peak strain, and take the strain at 50% stress in the descending section of the stress-strain curve as the axial limit strain; analyze the influence of adding fiber on the strain characteristic value, and the relationship between the axial limit strain ratio and the steel fiber content;

[0030] Elastic modulus: The secant modulus from the origin to 40% peak stress is selected as the elastic modulus; the effect of fiber content on the elastic modulus is studied, and the relationship between its change trend and peak stress is analyzed;

[0031] Poisson's ratio: Calculate the ratio of the transverse strain to the longitudinal strain in the elastic deformation section of each group of specimens to obtain the Poisson's ratio; Analyze the influence of fiber content on the Poisson's ratio;

[0032] Compressive toughness index: The ratio of the area under the curve corresponding to the axial limit strain and the axial peak strain is used to evaluate the compressive toughness of the material, namely the compressive toughness index; the effect of fiber content on the compressive toughness index is analyzed, and its change pattern with strain rate is studied.

[0033] Optionally, the mechanism analysis of the strain rate effect of HFGPC is also included;

[0034] The reasons for the differences in the mechanical performance parameters of HFGPC under quasi-static and seismic strain rates are analyzed from two aspects: the lateral inertial constraint and the Stefan effect of the viscous liquid inside the matrix; the influence of the lateral inertial constraint on the stress state of the specimen under high strain rate, as well as the relationship between viscous stress and strain rate in the Stefan effect are explained, and the influence of free water, pores, and the transition zone between the fiber and the matrix interface on the Stefan effect is analyzed.

[0035] It can be seen from the above technical solutions that, compared with the prior art, the present invention discloses a method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, which has the following beneficial effects:

[0036] 1. The present invention can comprehensively and systematically study the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, including failure morphology, stress-strain curve, mechanical property parameters and deformation parameters, providing rich data support for a deep understanding of the mechanical behavior of HFGPC.

[0037] 2. By precisely controlling test conditions, such as strain rate, loading method, etc., and using a variety of data acquisition methods, the test results can be accurately obtained, improving the reliability and accuracy of the test.

[0038] 3. Detailed and in-depth analysis of the test results, including strain rate effect analysis and mechanism analysis, will help reveal the performance change law and its internal reasons of HFGPC at different strain rates, provide a scientific theoretical basis for its application in practical engineering, and be conducive to optimizing engineering design and improving the safety and durability of the structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] 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.

[0040] Figure 1 A flow chart of the method provided by the present invention;

[0041] Figure 2 A schematic diagram of the quasi-static compressive failure crack morphology provided by the present invention;

[0042] Figure 3 A schematic diagram of a typical failure mode of HFGPC provided by the present invention;

[0043] Figure 4 A typical concrete uniaxial compressive stress-strain curve diagram provided by the present invention;

[0044] Figure 5 A curve diagram of axial compressive strength and residual strength ratio provided by the present invention;

[0045] Figure 6 f of each group of test pieces provided by the present invention cp / f cu Graphs;

[0046] Figure 7 A curve diagram of axial peak strain and axial limit strain ratio provided by the present invention;

[0047] Figure 8 A graph of elastic modulus provided by the present invention;

[0048] Fig. 9 A Poisson's ratio curve provided by the present invention;

[0049] Fig.10 A compression toughness index curve graph provided by the present invention;

[0050] Fig.11 A schematic diagram of the lateral inertial restraint provided by the present invention;

[0051] Fig.12 This is a schematic diagram of the Tefan effect mechanical analysis model provided by the present invention. DETAILED DESCRIPTION

[0052] 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.

[0053] The embodiment of the present invention discloses a method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, such as Figure 1 As shown, including:

[0054] Based on the test conditions and in accordance with the recommendations in the "Fiber Concrete Test Method Standard", the axial compression test was carried out using specimens with a size of φ100mm×100mm×300mm. Three specimens were prepared for each mix ratio and each strain rate, for a total of 126 specimens.

[0055] A 10000kN electro-hydraulic servo pressure testing machine is used for loading, and four disc springs with a maximum load capacity of 400kN are used together with the specimen to bear the force;

[0056] A YBY-800 spoke-type pressure sensor was used with two pads to collect load data. Strain gauges were attached to the sides of the specimens and horizontal and vertical displacement gauges were set up to test the lateral and longitudinal displacements, respectively. A DH5920 dynamic data collector was used to connect the strain gauges and sensors to record time, force, strain, and displacement data.

[0057] A computer-controlled high-speed camera is set up on one side of the testing machine, with the lens facing the side of the specimen, to record the entire process of compression failure;

[0058] The uniaxial compression failure morphology of HFGPC was analyzed, the morphology of the specimen after failure was observed, the influence of fiber and strain rate on the failure morphology was analyzed, and the characteristics of different failure modes were summarized;

[0059] The collected displacement data are averaged and plotted against the load data to form stress-strain curves under quasi-static and seismic strain rates;

[0060] Based on the stress-strain curves under quasi-static and seismic strain rates, the compressive mechanical properties parameters and quasi-static compression deformation parameters of HFGPC under quasi-static strain rates are analyzed.

[0061] In a specific embodiment, in order to ensure that a stable stress-strain curve descending section can be obtained at a faster loading rate, the following two measures are adopted: (1) based on a 10000kN rigidity testing machine, four disc springs with a maximum load capacity of 400kN are added to bear the force together with the specimen; (2) the displacement control mode is adopted for loading, and the loading rates are 0.18, 1.8, and 18mm / min, corresponding to 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 strain rate; before the test officially started, loading and unloading were carried out three times, with the preload set to 10 kN and the loading speed to 0.5 MPa / s. The specimen position was aligned according to the data collected by the displacement sensor during the preloading stage until the deformation difference on both sides was no more than 15%, so as to reduce the influence of eccentric compression on the test results.

[0062] In a specific embodiment, the high-speed camera uses a camera resolution of 1280×800 and a strain rate of 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 The recording is done at frame rates of 68fps, 120fps, and 500fps respectively.

[0063] In a specific embodiment, the compression failure process of the specimen is captured by a high-speed camera, from which the influence of fiber and strain rate changes on the GPC failure process can be known. -5 s -1 When HFGPC is subjected to uniaxial compression, the process of failure is as follows:

[0064] (1) When the stress rises to about 44% of the peak stress, fine cracks begin to appear at the corners of the pressure-bearing surface. At this time, the observation time is recorded as 0.

[0065] (2) When the stress rises to about 81% of the peak stress (15 seconds), fine cracks develop and penetrate, and the surface of the specimen peels off in its entirety, accompanied by a popping sound.

[0066] (3) The stress continued to increase to a peak value, and then dropped sharply (3 minutes and 25 seconds). At the same time, cracks appeared on the surface of the specimen, and there were thin flaky bulges and peeling. At 3 minutes and 26 seconds, the cracks developed rapidly from the cracked part and penetrated the specimen axially (3 minutes and 27 seconds).

[0067] (4) Then the stress dropped rapidly. When the stress dropped to 33% of the peak value, it entered the platform section and increased slightly. This process lasted for about 5 minutes. The crack width increased and the axial strain increased by about 2000με. After that, the stress dropped again, and the specimen lost its bearing capacity and was finally destroyed.

[0068] By comparing the failure process, we can see that: (1) GPC shows obvious brittleness when under pressure, and the failure process is accompanied by matrix cracking, which is dangerous. There are fewer cracks on the observation surface, but after the main crack is formed, it quickly penetrates the specimen axially. (2) Adding fibers can significantly improve the matrix peeling or cracking of the specimen during the failure process. Adding PVA fibers can best improve the matrix cracking, but adding MS or HS fibers cannot avoid the matrix peeling on the specimen surface; and when adding the same amount of steel fibers, compared with HS fibers, MS fibers can better maintain the integrity of the specimen due to their smaller single fiber volume and more uniform distribution in the matrix, and the cracks on the observation surface appear later in the entire compressive failure process. (3) Adding HS or MS fibers can significantly improve the brittle failure morphology of GPC. There are no long cracks that penetrate the specimen, but multiple dense short cracks appear. This is because the steel fibers block the cracks, causing them to change direction and develop along multiple trajectories, while PVA fibers cannot block the development of long cracks. (4) Mixing steel fibers and PVA fibers can slow down the crack propagation rate and avoid large-scale matrix shedding and bursting, which can better maintain the integrity of the damaged specimens.

[0069] Effect of fiber on crack morphology of HFGPC under compression

[0070] GPC specimens often show axial splitting or oblique shear failure after axial compression failure. After the large cracks are formed, they quickly penetrate the specimen and break it into multiple parts, making it difficult to maintain the original overall morphology of the specimen. GPC without fiber addition will suffer obvious brittle failure after exceeding the stress peak. The cracks start at the corners of the compression surface and quickly expand from small cracks. The crack development trajectory basically extends along the axial direction. The main cracks expand along the axial direction and develop secondary cracks in the middle of the specimen. Then the crack width expands sharply and the length elongates along the axial direction until it penetrates the specimen. Finally, the specimen splits into several parts. The main reason for the cracking of the specimen is due to the effect of the end constraints. When the specimen is under pressure, compression deformation occurs in the axial direction and tensile deformation occurs in the transverse direction. The lateral deformation of the end face is very small due to the friction between the two ends of the specimen and the pad. With the continuous action of the compression load, the middle part of the specimen expands laterally. When the ultimate tensile strain of the matrix is ​​reached, cracks are generated, and then continue to expand and extend as the compression proceeds.

[0071] The failure of the fiber-doped GPC specimens after reaching the ultimate bearing capacity showed the characteristics of plastic failure. Its main cracks developed along the diagonal, accompanied by secondary cracks of varying sizes. The failure morphology of the single fiber-doped GPC and C group specimens is completely different. The specimens did not break into multiple parts. Although there were obvious cracks, they were still able to remain as a whole, which reflects the bridging effect of the fibers between cracks. The main cracks in Group P almost penetrated the specimen axially, and although there were large areas of overall cracking in the surface matrix, there was no obvious peeling or matrix bursting; Groups H and M mainly showed end failure morphology, and there was obvious peeling of the matrix at the edge of the crack. This difference between Group P and Groups H and M is mainly caused by the following two reasons: the strength of Groups H and M is higher than that of Group P. When the specimens are completely destroyed, the internal stress of the matrix of Groups H and M is greater than that of Group P.

[0072] Therefore, the damage is more serious, which is manifested by the dense appearance of cracks; PVA fibers are relatively thin and soft, and they are bent and staggered in the matrix. The PVA fibers across the cracks will be twisted, and the fiber direction is perpendicular to the cracks to achieve the anti-crack effect. The macroscopic manifestation is that the cracks are uniform and fine, and there is little matrix peeling. Steel fibers are relatively stiff and staggered in the matrix, which will block the cracks from propagating along the initial direction. When the cracks extend to the steel fibers, stress concentration is more likely to occur due to the original defects in the interface between the steel fibers and the matrix, causing the cracks to develop along the direction of the steel fibers, thereby changing the crack trend. The macroscopic manifestation is that the cracks are discontinuous and distributed in a network, and the surface matrix peels off seriously.

[0073] Compared with the brittle failure morphology of GPC, the cracks after HFGPC failure are irregularly serrated, such as Figure 2 As shown. Due to the interlaced barrier effect of steel fibers, the cracks are hindered in the process of expansion and cannot directly penetrate the matrix. The crack direction changes. In addition, due to the anti-cracking effect of PVA fibers, the final failure form is a main crack accompanied by fine secondary cracks. Compared with the single fiber-doped P, H and M group specimens, the crack distribution of each group of mixed fiber specimens is more uniform. Mixed fibers make GPC

[0074] The integrity is stronger, so the stress on each part of the specimen is more uniform, avoiding the uneven cracking similar to that of Group H and Group M (one end is severely damaged, and the other end has no obvious damage). At the same time, the surface matrix of the mixed fiber GPC after destruction is peeled off, but due to the pulling effect of the PVA fiber, the peeled part rarely falls off the main body of the specimen, and the integrity of the specimen after destruction is good. Comparing the HS+PVA series and the MS+PVA series, from the perspective of crack morphology alone, when the steel fiber content is not more than 1.0% and the PVA fiber content is not more than 0.55%, the combination of MS and PVA shows better crack resistance; and when the steel fiber content is greater than 1.0% and the PVA fiber content is greater than 0.55%, the combination of HS and PVA shows better crack resistance. When the total fiber content is large, MS is more likely to agglomerate than HS, which affects the uniformity of the matrix and weakens the reinforcement ability and crack resistance of MS.

[0075] Crack morphology and failure mode of HFGPC at different strain rates

[0076] From the failure mode, the C and P groups still show obvious brittle failure mode, and the failure process is faster with the increase of strain rate. -3 s -1 When the cracks were observed, two main cracks were formed instantly and penetrated the specimen axially. The destruction process took less than 1 second, and the destruction time was 10 -3 s -1 The strain rates of H and M groups mixed with steel fibers and H10P55 and M10P55 groups mixed with steel fibers were 1 / 20 and 1 / 6, respectively. It can be seen that although the addition of PVA fibers cannot significantly improve the brittle failure mode of GPC under seismic strain rates, it can effectively slow down the failure process of GPC. -4 , 10 -3 s -1 The failure mode under strain rate is similar to 10 -5 s -1 The failure morphology is similar to that of the specimen, which is end failure combined with splitting failure. The failure morphology does not change significantly with the strain rate. The failure process lasts for 25 seconds, and the cracks gradually develop until the specimen is destroyed. The brittleness of the specimen is significantly improved.

[0077] Typical failure mode of HFGPC

[0078] Taking group C, group P4H10, and group P55M10 as examples, the image analysis method was used to extract the crack morphology and obtain the typical failure mode of HFGPC. Figure 3As shown. It can be clearly seen that under the quasi-static uniaxial compression load, the geopolymer concrete prism specimen may have three typical failure modes: (1) Type I - axial cracking type, the crack is perpendicular to the compression surface, develops along the axial direction or obliquely, forming a through crack that causes the specimen to split and fail; (2) Type II - oblique cracking type, the crack develops from one corner of the compression surface along the diagonal direction, forming a continuous non-through oblique crack, accompanied by axial fine cracks around, the specimen undergoes shear failure, and the angle between the crack and the compression surface is about 60-70°; (3) Type III - intermittent cracking type, the crack develops from one corner of the compression surface along the diagonal direction, and together with other fine cracks, forms a non-continuous non-through oblique crack, and the specimen undergoes shear failure. The fibers are evenly distributed in the matrix and interlaced into a three-dimensional network, which greatly limits the lateral expansion of the middle part of the specimen and delays the cracking process of the specimen. The barrier effect of steel fiber on cracks and the bridging effect between cracks largely maintain the integrity of the structure. Its addition changes the failure mode of GPC under axial compression, making the specimen change from typical brittle failure characteristics to plastic failure characteristics; PVA fiber can improve the tensile properties of the matrix, consume more energy to delay the expansion of cracks macroscopically, and its addition can reduce the shedding of the cracked part of GPC, which greatly improves the integrity of the specimen after failure; From the above analysis, it can be concluded that for a very brittle matrix such as geopolymer, in order to achieve the improvement of its bearing capacity and failure mode, the effect of adding a single fiber cannot be very ideal. Adding two "complementary" fibers can achieve a synergistic improvement upgrade effect, and the combination of steel fiber microfilaments and PVA fibers can make the matrix cracks develop more densely, which can maintain the integrity of the matrix after failure to the greatest extent.

[0079] In a specific embodiment, the stress-strain curve analysis under quasi-static and seismic strain rates

[0080] The uniaxial compressive stress-strain curve comprehensively covers the mechanical properties of concrete under uniaxial pressure: the stress at the peak point of the curve is the compressive strength of the prism f c , the corresponding strain is the peak strain ε p The slope of the rising section of the curve is its elastic modulus E, the shape of the curve reflects its plastic deformation capacity, and the area under the curve reflects its compressive toughness. A typical concrete stress-strain curve is shown in Figure 4 At the beginning of loading, the strain of the specimen increases approximately linearly with the stress growth (0-A section); thereafter, the strain of the specimen increases nonlinearly with the stress (AB section), and the corresponding stress at point B is about 0.8f c ~0.9f c , the corresponding strain is about 0.65ε p ~0.85ε pAt this stage, many micro cracks have appeared inside the concrete, but there are no visible cracks on the surface of the specimen; then the stress reaches its peak value (point C), and the internal micro cracks accelerate their development; when the stress drops to about 0.9f c When the axial strain increases, the cracks gradually widen and form a damaged zone. When the strain reaches about 6ε, the cracks gradually widen and form a damaged zone. p When the corresponding stress is called residual strength, its value is about 0.1f c ~0.4f c .

[0081] The stress-strain curves drawn under quasi-static and seismic strain rates are specifically:

[0082] The displacement data collected synchronously are averaged and plotted into a stress-strain curve corresponding to the load data; the test results are presented in the form of average stress-strain, that is, the average value of the three stress values ​​under the same strain is taken as the average stress value to draw the curve.

[0083] In a specific embodiment, the stress-strain curve analysis under quasi-static and seismic strain rates includes curve characteristic analysis and mechanical property parameter analysis;

[0084] Curve characteristic analysis: Analyze the characteristics of the rising section, peak point, and falling section of the stress-strain curve, including elastic proportional limit, peak stress, peak strain, and residual strength parameters, and study the influence of fiber and strain rate on the parameters;

[0085] The elastic proportional limit is the ratio of the maximum stress of the elastic deformation section of concrete to the peak stress during compression. It is the dividing point between the elastic working stage and the elastoplastic working stage of concrete. The elastic proportional limit of ordinary concrete is generally 0.4-0.5. The elastic proportional limit of each test group can be obtained from the stress-strain curve. The elastic proportional limit of GPC without fiber (Group C) reaches 0.78, which is much higher than that of ordinary concrete. It can be seen from the failure mode and stress-strain curve that GPC without fiber exhibits elastic-brittle failure when under pressure. After adding fibers, whether PVA fiber or steel fiber is added alone, or PVA fiber and steel fiber are mixed, the elastic proportional limit of HFGPC is reduced, fluctuating in the range of 0.50-0.69, but it is still generally higher than ordinary concrete. After adding fibers, the peak stress of the specimens generally increases, and the reduction of the elastic proportional limit means the extension of the plastic yield section during compression. This also shows that the addition of fibers will reduce the brittleness of GPC, making the brittle geopolymer concrete into an elastic-plastic material, which can absorb more energy through deformation. When the strain rate increases, the elastic proportional limit of HFGPC increases slightly but not significantly. This is because the incorporation of fibers will introduce more pores. Under rapid loading, the pores inside the specimen are quickly compacted, and the brittle characteristics of the geopolymer material itself become more prominent, thus causing the elastic proportional limit to increase.

[0086] Mechanical performance parameter analysis: Analyze the compressive mechanical performance parameters of HFGPC under quasi-static strain rate, obtain the peak stress of the group from the stress-strain curve as the representative value of axial compressive strength, and the axial strain reaches 6ε p The stress at the time is the residual strength, and the ratio of the residual strength to the axial compressive strength is the residual strength ratio. The following mainly analyzes the changes of HFGPC pressure mechanical properties with fiber type and dosage from three parameters: axial compressive strength, residual strength ratio, and ratio of axial compressive strength to cube compressive strength.

[0087] Axial compressive strength and residual strength ratio: The peak stress is obtained from the stress-strain curve as the representative value of the axial compressive strength. The stress when the axial strain reaches 6εp is the residual strength. The ratio of residual strength to axial compressive strength is calculated, and the variation of axial compressive strength and residual strength ratio with fiber type and dosage is analyzed.

[0088] The axial compressive strength and residual strength of each group of specimens are as follows: Figure 5As shown. From the test data, the addition of PVA fiber alone will slightly reduce the axial compressive strength of the test block, and the strength of group P is 5.5% lower than that of group C. The addition of steel fiber alone can improve the axial compressive strength of the test block, and the addition of MS has a more obvious effect on the axial compressive strength of the test block than HS. The axial compressive strength of groups H and M is increased by 5.2% and 13.0% respectively compared with group C. In addition, the mixed addition of PVA fiber and steel fiber generally has a more obvious strengthening effect than the single addition of one fiber. When the PVA fiber content is fixed, the axial compressive strength is increased by 25.0%, 33.8%, and 21.8% respectively when the HS content is 0.5%, 1.0%, and 1.5% relative to group P; when HS is replaced by an equal amount of MS, the axial compressive strength is increased by 28.5%, 44.0%, and 41.7% relative to group P. When the steel fiber content is fixed, for the combination of PVA and HS, the axial compressive strength increases by 7.4%, 21.5%, and -2.9% respectively when the PVA content is 0.4%, 0.55%, and 0.7% compared with the H group; for the combination of PVA and MS, the axial compressive strength increases by 10.6%, 20.4%, and 0.6% respectively when the PVA content is 0.4%, 0.55%, and 0.7% compared with the M group. It can be concluded that the mixed addition of fibers can improve the strength of geopolymer concrete and improve its cracking characteristics. The highest strength of the combination of PVA and HS appears in the H10P55 group, which increases by 26.5% compared with the C group; in the combination of PVA and MS, the M10P55 group has the highest strength, which increases by 36.1% compared with the C group, and the strengthening effect is higher than that of the H10P55 group. Overall, the addition of MS to GPC has a more obvious strengthening effect than the addition of HS, and the strengthening effect of PVA+MS is better than that of PVA+HS.

[0089] Ratio of axial compressive strength to cube compressive strength: Calculate the ratio of axial compressive strength to cube compressive strength based on the test results, analyze its relationship with fiber content, and compare it with relevant standard values ​​of ordinary concrete.

[0090] The ratio of the axial compressive strength of ordinary concrete to the cubic compressive strength (f cp / f cu ) fluctuates within the range of 0.7 to 0.8. The amount of PVA fiber has an effect on f cp / f cu There is no obvious effect, but with the increase of steel fiber content, f cp / f cu Gradually decrease, such as Figure 6As shown. The reasons can be analyzed from two aspects. On the one hand, the fiber has different degrees of improvement on the two compressive strengths of the specimen. When the specimen with fiber is subjected to axial pressure, the lateral expansion will be restricted by the addition of fiber. At this time, the appearance characteristics of the specimen will affect the lateral constraint effect. The prism specimen has a larger height-width ratio, and the lateral constraint effect of the fiber is smaller than that of the cube specimen; and the prism specimen has a larger volume, and the fiber distribution is worse. It is more likely to produce shear failure caused by stress concentration than the cube specimen. The overall performance is that the fiber has a smaller improvement on the compressive strength of the prism specimen. On the other hand, the end constraints generated by the press plate on the specimens of the two sizes have different degrees of influence on the middle section of the specimen. The addition of fiber can improve the compressive toughness of the specimen, thereby increasing the lateral constraint between the upper and lower plates and the pressure surface. The prism specimen is higher than the cube specimen, so the constraint effect of the plate on the middle of the prism specimen is less than that on the middle of the cube specimen. Combining the above two factors, the improvement of the axial compressive strength by adding fiber is small, which leads to the fact that the fiber-added specimen f cp / f cu reduce.

[0091] The ratios of the axial compressive strength and cubic compressive strength of the two types of steel-PVA hybrid fiber reinforced geopolymer concrete prepared in this embodiment can be approximately taken as 0.79 and 0.81, respectively.

[0092] In a specific embodiment, the stress-strain curve analysis under quasi-static and seismic strain rates further includes analyzing quasi-static compressive deformation parameters;

[0093] Strain characteristic value: take the axial strain and transverse strain corresponding to the peak point of the stress-strain curve as the axial peak strain and transverse peak strain, and take the strain at 50% stress in the descending section of the stress-strain curve as the axial limit strain; analyze the influence of adding fiber on the strain characteristic value, and the relationship between the axial limit strain ratio and the steel fiber content;

[0094] The axial strain and transverse strain corresponding to the peak point of the stress-strain curve are taken as the axial peak strain (ε a ) and transverse peak strain (ε l ), the strain at 50% stress in the descending section of the stress-strain curve is taken as the axial limit strain (ε au ). Axial peak strain and ultimate strain ratio (ε au / ε a )like Figure 7As shown. It can be seen that when the fibers are added, the axial and transverse peak strains increase significantly. There is no obvious regularity in the change of the transverse peak strain, but the change of the axial peak strain is consistent with the change trend of the peak stress. When mixed fibers are added, the H10P55 group and the M10P55 group have the largest peak strains, which are 41.1% and 79% higher than the C group, respectively. Axial compressive strength (MPa) residual (MPa) 3 70.4% of the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, and when the PVA fiber and steel fiber content are the same, the peak strain of the PVA+MS combination is larger than that of the PVA+HS combination, and when the steel fiber content exceeds 1.0%, this trend is more obvious, and the PVA+MS group is generally 600-700με higher than the PVA+HS group. From Figure 7 It can also be seen that the addition of fibers can improve the axial ultimate strain ratio. The addition of PVA fibers has little effect on the axial ultimate strain ratio, but with the increase of steel fiber content, the axial ultimate strain ratio gradually increases, which shows that the improvement of geopolymer concrete ductility mainly depends on steel fibers. The ultimate strain ratio of HFGPC under quasi-static conditions ranges from 1.91 to 2.96.

[0095] Elastic modulus: The secant modulus from the origin to 40% peak stress is selected as the elastic modulus; the effect of fiber content on the elastic modulus is studied, and the relationship between its change trend and peak stress is analyzed;

[0096] The static elastic modulus of ordinary concrete is usually in the range of 14 to 42 GPa. The results obtained in this example are as follows: Figure 8 As shown. This is because the elastic modulus of steel fiber is large, and adding it to the matrix can improve the overall stiffness. In addition, HFGPC generally has a higher elastic modulus than single fiber-doped specimens. The change trend of elastic modulus with fiber content is consistent with the change trend of peak stress. As the fiber content increases, the elastic modulus will increase accordingly. The elastic modulus of the H10P55 group and the M10P55 group is the largest, which is 14.3% and 18.9% higher than that of the C group, respectively. However, when the fiber content continues to increase, the elastic modulus will decrease.

[0097] Poisson's ratio: Calculate the ratio of the transverse strain to the longitudinal strain in the elastic deformation section of each group of specimens to obtain the Poisson's ratio; Analyze the influence of fiber content on the Poisson's ratio;

[0098] According to the definition of Poisson's ratio in GB / T50082, the ratio of the transverse strain to the longitudinal strain of the elastic deformation section of each group of specimens is calculated, and the Poisson's ratio results of each group of specimens are shown as follows: Fig. 9As shown in the figure, the Poisson's ratio of groups C and P is close to 0.2, while the Poisson's ratio of groups H and M is slightly lower than that of group C. When mixed fibers are added, the Poisson's ratio of each group fluctuates, but there is no obvious pattern with the change of fiber content, and the Poisson's ratio fluctuates around 0.18. It is worth noting that the Poisson's ratio of each group with single fiber addition is reduced compared with group C, and the Poisson's ratio of each group with mixed fibers is also reduced compared with single fiber addition.

[0099] Compressive toughness index: The ratio of the area under the curve corresponding to the axial limit strain and the axial peak strain is used to evaluate the compressive toughness of the material, namely the compressive toughness index; the effect of fiber content on the compressive toughness index is analyzed, and its change pattern with strain rate is studied.

[0100] The compression energy absorbed by concrete during compression can reflect its compression toughness, which can be represented by the area under the stress-axial strain curve. The ratio of the area under the curve corresponding to the axial limit strain and the axial peak strain is used to evaluate the compression toughness of the material, that is, the compression toughness index, which is calculated as follows: Fig.10 As shown. It can be seen that the compression toughness index of group C is only 1.20, which shows that GPC has great brittleness, accumulates compression energy in the rising section of the stress-strain curve, and releases compression energy immediately after exceeding the stress peak, causing the specimen to be damaged. With the addition of fibers, the compression toughness index has increased significantly. The compression toughness index of groups P, H and M increased by 37%, 103% and 97% respectively compared with group C. When mixed with fibers, the compression toughness increases more significantly, and with the increase of fiber content, the compression toughness index gradually increases. The fiber toughening effect is significant, and the compression toughness index of each PVA+MS group is slightly higher than that of each PVA+HS group. When the steel fiber content remains unchanged and the PVA fiber content is increased, the compression toughness index increases very little. When the PVA fiber content is fixed and the steel fiber content is increased, the compression toughness index increases significantly. The compression toughness index is the largest when the steel fiber content is 1.5%. The H15P55 group and the M15P55 group increased by 188% and 217% respectively compared with the C group. The toughening effect of steel fiber at 1.0% is also considerable, and the compression toughness index is increased by 159% and 177% respectively compared with group C. This is because when GPC has a strong bonding effect, the PVA fibers inside the matrix slip and break during destruction, and the bending, sliding and pulling out of the steel fibers enable the specimen to absorb more compression energy during the stress process, increasing the toughness of the concrete and thus improving the compression toughness index.

[0101] In a specific embodiment, a mechanism analysis of the HFGPC strain rate effect is also included;

[0102] The reasons for the differences in the mechanical performance parameters of HFGPC under quasi-static and seismic strain rates are analyzed from two aspects: the lateral inertial constraint and the Stefan effect of the viscous liquid inside the matrix; the influence of the lateral inertial constraint on the stress state of the specimen under high strain rate, as well as the relationship between viscous stress and strain rate in the Stefan effect are explained, and the influence of free water, pores, and the transition zone between the fiber and the matrix interface on the Stefan effect is analyzed.

[0103] When subjected to quasi-static compression, the stress state of the specimen can be regarded as a one-dimensional stress state, while at a higher strain rate, the stress state of the specimen changes to a multi-dimensional stress state similar to that formed by the combined action of axial pressure and confining pressure. Fig.11 The specimen on the left side of the middle directly shows the confining pressure effect caused by the inertia of the middle specimen under high strain rate. A tiny unit A inside the specimen is taken as the research object, and its outer ring matrix is ​​recorded as B. As the axial compressive stress increases, A will expand laterally, while B has not yet deformed laterally or its deformation is limited due to inertia. At this time, it is equivalent to B applying an inward confining pressure to A. At the same time, B is also subjected to the outward reaction force of A. Similarly, there is the same interaction force between B and the outer ring matrix C. As the axial compressive stress continues to increase, A inevitably forms lateral expansion, and at the same time causes B to produce cracks parallel to the axial direction, which rapidly widens, lengthens and finally penetrates, leading to the destruction of the specimen. It is precisely because of the obstruction of C that the lateral expansion of B is delayed, which hinders the rapid expansion of the cracks, causing the specimen to bear greater compressive stress in the axial direction, which is manifested as an increase in the peak compressive stress. On the other hand, after the addition of fibers, such as Fig.11 In the right specimen shown in the figure, whether it is a combination of HS and PVA fibers or a combination of MS and PVA fibers, the randomly distributed fibers in the matrix are interwoven, which further delays the expansion of the cracks, causing the specimen to bear greater pressure in the axial direction and increase the peak pressure. However, when the strain rate increases, the intricate structure of the fibers inside the specimen will make the transmission of the cracks more complicated, and the crack expansion path will become more tortuous, so that the destruction of the specimen has a stronger time lag, which is manifested as a more obvious strain rate effect on strength. This is also the reason why the strength growth factor and compression toughness index growth factor of the specimen are larger when the strain rate is higher.

[0104] The Stefan Effect

[0105] Under the earthquake strain rate (10 -4 ~10 -1 s -1 ), the Stefan effect of viscous liquid will also lead to the difference between dynamic damage and static damage of the material. The viscous fluid between the two disks will produce viscous resistance when the disks separate from each other. This is the Stefan effect, and its mechanical analysis model is as follows Fig.12 shown.

[0106] When the disk moves at a relative speed dh / dt, the equilibrium equation for the viscous fluid is established and solved, and the expression for the hydrostatic pressure at any radius r of the fluid can be obtained:

[0107]

[0108] Among them, p0 is the initial value of fluid pressure; η is the fluid viscosity coefficient; then according to the balance of forces, the external force required for disc separation can be obtained:

[0109]

[0110] It can be seen from the above formula that when two discs are separated or brought closer at a certain speed, the higher the relative speed dh / dt, the greater the disc separation resistance. In the concrete system, when subjected to dynamic loads, concrete can be regarded as a series of micro-disc systems. When the pores are compressed, the viscous liquid (i.e. free water) inside will trigger the Stefan effect, generating resistance that hinders the expansion of micropores. The viscous stress caused by the Stefan effect can be calculated:

[0111]

[0112] Also because Substituting into the above formula, we can get the relationship between viscous stress and strain rate:

[0113]

[0114] It can be seen from the above formula that the smaller R / h is, the flatter the gap is, the greater the viscous stress σ v The larger the value, the larger the value. v About The coefficient is the product of the fluid viscosity coefficient η and the mesoscopic pore geometry parameter k.

[0115] The viscosity of free water hinders the development of cracks in concrete, which increases the macro equivalent fracture toughness and dynamic strength of concrete. After the concrete is dried, the free water content is very small, and the influence of the Stefan effect is negligible, so the strain rate effect of dry concrete is not obvious. For the fly ash-based polymer concrete studied in this embodiment, high-temperature steam curing is adopted in the curing stage, so it is believed that the extra water added during the preparation process is not lost in the curing stage. In addition, after the geopolymerization reaction is completed, some water will still remain in the spatial network structure of the geopolymer in the form of free water, bound water and structural water. In addition to the viscous liquid present in the pores, the influence of aggregate and fiber on the complex HFGPC cannot be ignored. There is an interface transition zone with high porosity between the steel fiber and the concrete matrix. It can also be seen from the scanning electron microscope observation results of the GPC specimen that the interface transition zone between the matrix and the aggregate also has a similar situation.

[0116] In addition, PVA fiber is a hydrophilic material, free water easily gathers around the fiber, and after hydration is completed, it is also easy to form a densely porous interface transition zone. There is a certain amount of free water in the pores and microcracks inside the HFGPC, as well as in the capillary channels and interface transition zones. Therefore, the Stefan effect in HFGPC appears in the interface transition zone, pores and microcracks, which results in an increase in the macroscopic strength of HFGPC under seismic strain rate.

[0117] 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.

[0118] 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 method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates, characterized in that: include: Based on the test conditions and test method standards, a test piece with a size of φ100mm×100mm×300mm was manufactured; A 10000kN electro-hydraulic servo pressure testing machine is used for loading, and four disc springs with a maximum load capacity of 400kN are used together with the specimen to bear the force; A YBY-800 spoke-type pressure sensor was used with two pads to collect load data. Strain gauges were attached to the sides of the specimens and horizontal and vertical displacement gauges were set up to test the lateral and longitudinal displacements, respectively. A DH5920 dynamic data collector was used to connect the strain gauges and sensors to record time, force, strain, and displacement data. A computer-controlled high-speed camera is set up on one side of the testing machine, with the lens facing the side of the specimen, to record the entire process of compression failure; The uniaxial compression failure morphology of HFGPC was analyzed, the morphology of the specimen after failure was observed, the influence of fiber and strain rate on the failure morphology was analyzed, and the characteristics of different failure modes were summarized; The collected displacement data are averaged and plotted against the load data to form stress-strain curves under quasi-static and seismic strain rates; Based on the stress-strain curves under quasi-static and seismic strain rates, the compressive mechanical properties parameters and quasi-static compression deformation parameters of HFGPC under quasi-static strain rates are analyzed.

2. The method for analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The loading also includes loading in a displacement control mode, with loading rates of 0.18, 1.8, and 18 mm / min, corresponding to 10 -5 s -1 , 10 -4 s -1 , 10 -3 s -1 strain rate; before the test officially started, loading and unloading were carried out three times, with the preload set to 10 kN and the loading speed to 0.5 MPa / s. The specimen position was aligned according to the data collected by the displacement sensor during the preloading stage until the deformation difference on both sides was no more than 15%, so as to reduce the influence of eccentric compression on the test results.

3. The method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The high-speed camera has a resolution of 1280×800 and a strain rate of 10 -5 s -1 , 10 - 4 s -1 , 10 -3 s -1 The recording is done at frame rates of 68fps, 120fps, and 500fps respectively.

4. The method for analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The process of HFGPC uniaxial compression failure is as follows: (1) When the stress rises to 44% of the peak stress, cracks begin to appear at the corners of the pressure-bearing surface, and the observation time is recorded as 0; (2) When the stress rises to 81% of the peak stress, fine cracks develop and penetrate, and the entire surface of the specimen peels off, accompanied by a popping sound; (3) The stress continues to increase to a peak value, and then drops sharply. At the same time, cracks appear on the surface of the specimen, and there are thin flakes that bulge and peel off. The cracks develop rapidly from the cracked part and penetrate the specimen axially; (4) Then, when the stress drops to 33% of the peak value, it enters the platform section, the crack width increases, and the axial strain increases by 2000 με. After that, the stress drops again, and the specimen loses its bearing capacity and eventually fails.

5. The method for analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The stress-strain curves drawn under quasi-static and seismic strain rates are specifically: The displacement data collected synchronously are averaged and plotted into a stress-strain curve corresponding to the load data; the test results are presented in the form of average stress-strain, that is, the average value of the three stress values ​​under the same strain is taken as the average stress value to draw the curve.

6. The method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The stress-strain curve analysis under quasi-static and seismic strain rates includes curve characteristic analysis and mechanical property parameter analysis; Curve characteristic analysis: Analyze the characteristics of the rising section, peak point, and falling section of the stress-strain curve, including elastic proportional limit, peak stress, peak strain, and residual strength parameters, and study the influence of fiber and strain rate on the parameters; Mechanical performance parameter analysis: Axial compressive strength and residual strength ratio: The peak stress is obtained from the stress-strain curve as the representative value of the axial compressive strength. The stress when the axial strain reaches 6εp is the residual strength. The ratio of residual strength to axial compressive strength is calculated, and the variation of axial compressive strength and residual strength ratio with fiber type and dosage is analyzed. Ratio of axial compressive strength to cube compressive strength: Calculate the ratio of axial compressive strength to cube compressive strength based on the test results, analyze its relationship with fiber content, and compare it with relevant standard values ​​of ordinary concrete.

7. The method for testing and analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: The analysis of stress-strain curves under quasi-static and seismic strain rates also includes the analysis of quasi-static compressive deformation parameters; Strain characteristic value: take the axial strain and transverse strain corresponding to the peak point of the stress-strain curve as the axial peak strain and transverse peak strain, and take the strain at 50% stress in the descending section of the stress-strain curve as the axial limit strain; analyze the influence of adding fiber on the strain characteristic value, and the relationship between the axial limit strain ratio and the steel fiber content; Elastic modulus: The secant modulus from the origin to 40% peak stress is selected as the elastic modulus; the effect of fiber content on the elastic modulus is studied, and the relationship between its change trend and peak stress is analyzed; Poisson's ratio: Calculate the ratio of the transverse strain to the longitudinal strain in the elastic deformation section of each group of specimens to obtain the Poisson's ratio; Analyze the influence of fiber content on the Poisson's ratio; Compressive toughness index: The ratio of the area under the curve corresponding to the axial limit strain and the axial peak strain is used to evaluate the compressive toughness of the material, namely the compressive toughness index; the effect of fiber content on the compressive toughness index is analyzed, and its change pattern with strain rate is studied.

8. The method for analyzing the uniaxial compressive performance of HFGPC under quasi-static and seismic strain rates according to claim 1, characterized in that: It also includes the mechanism analysis of the strain rate effect of HFGPC; The reasons for the differences in the mechanical performance parameters of HFGPC under quasi-static and seismic strain rates are analyzed from two aspects: the lateral inertial constraint and the Stefan effect of the viscous liquid inside the matrix; the influence of the lateral inertial constraint on the stress state of the specimen under high strain rate, as well as the relationship between viscous stress and strain rate in the Stefan effect are explained, and the influence of free water, pores, and the transition zone between the fiber and the matrix interface on the Stefan effect is analyzed.

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