Method for testing bonding strength of chopped fibers and ultra-high performance concrete matrix under action of bending stress

Abstract

The invention relates to the technical field of fiber concrete performance detection methods, in particular to a method for testing the bonding strength of chopped fibers and an ultra-high performance concrete matrix under the action of bending stress. The method comprises the following steps: firstly, respectively cutting notches in the midspan positions of two opposite side surfaces of a test piece; applying tensile forces in opposite directions to the two ends of the test piece until a through crack appears between the two notches; performing a three-point bending test on the test piece subjected to tensile failure; and finally, obtaining the maximum load according to a load-time curve, and calculating to obtain the bonding strength of the chopped fibers and the ultra-high performance concrete matrix under the action of bending stress. According to the measurement result obtained by the method disclosed by the invention, the bonding strength of the chopped fiber and the ultra-high performance concrete matrix under the action of bending stress can be intuitively and accurately detected, and the technical problem that a test method which cannot truly reflect the bonding performance of the chopped fiber and the ultra-high performance concrete matrix is lacked in the prior art can be solved; and the method has ideal popularization and application prospects.

Description

Technical Field

[0001] The invention relates to the technical field of fiber concrete performance testing methods, and in particular to a method for testing the bonding strength between chopped fibers and an ultra-high performance concrete matrix under bending force. Background Art

[0002] As we all know, ultra-high performance concrete is mainly composed of two parts: active powder (ultra-high performance concrete matrix) and chopped fibers. When ultra-high performance concrete components are subjected to stress, the final cause of their destruction is often not due to the breaking of the internal chopped fibers themselves, but due to the insufficient interface bonding strength between the chopped fibers and the ultra-high performance concrete matrix, which causes the chopped fibers to be peeled off from the ultra-high performance concrete matrix. Therefore, the bonding performance between the chopped fibers and the ultra-high performance concrete matrix is ​​the most basic mechanical behavior in ultra-high performance concrete components, and it is also the main factor affecting the stress performance and deformation capacity of ultra-high performance concrete components. However, due to the differences in the materials, properties, surface morphology, etc. of different chopped fibers, there are large differences in the bonding performance between the chopped fibers and the ultra-high performance concrete matrix. Therefore, the bonding performance between the chopped fibers and concrete has become a key issue of concern to researchers in this field.

[0003] During the bending process of UHPC components, the interaction between the chopped fibers and the UHPC matrix is ​​generated under the influence of a series of external load factors such as the self-weight and constraints of the entire structure, which will have a great impact on the bonding performance between the chopped fibers and the UHPC matrix. Therefore, the traditional vertical pull-out test between a single fiber and the concrete matrix often has certain differences from the actual bonding performance between the chopped fibers and the UHPC matrix.

[0004] Chinese patent CN110333184B discloses a steel chopped fiber concrete bonding strength tensile test piece and tensile test device; Chinese patent CN101819135B discloses a method for determining the bonding strength between high-strength glass chopped fiber bars and concrete; Chinese patent CN210427321U discloses a chopped fiber-concrete bonding test device based on an in-situ tensile table; Chinese patent CN207894799U discloses a test device for the bonding strength between chopped fibers and concrete interfaces. This series of patents, like the commonly used "8"-shaped mortar specimen chopped fiber pull-out test method, realizes the bond strength test between the chopped fibers and the concrete matrix in the pulling direction. However, in the process of bending of ultra-high performance concrete components, the interaction between the chopped fibers and the ultra-high performance concrete matrix is ​​generated under the influence of a series of external load factors such as the self-weight and constraints of the entire structure, which will have a greater impact on the bonding performance between the chopped fibers and the concrete. A single pull-out test cannot truly reflect the bonding performance between the chopped fibers and the ultra-high performance concrete matrix. In addition, these methods have limitations such as difficult clamping, large test errors, and high experimental costs, and test failures often occur. Summary of the invention

[0005] The present invention aims to provide a method for testing the bonding strength between chopped fibers and an ultra-high performance concrete matrix under bending force, so as to solve the technical problem in the prior art that there is a lack of a test method that cannot truly reflect the bonding performance between chopped fibers and an ultra-high performance concrete matrix.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme:

[0007] A method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force comprises the following steps performed in sequence:

[0008] S1: Cut notches at the mid-span positions on two opposite sides of the specimen;

[0009] S2: Apply tension in opposite directions to the two ends of the specimen until a through crack appears between the two cuts, and obtain a specimen after tensile failure;

[0010] S3: Perform a three-point bending test on the specimen after tensile failure; load the specimen until the specimen fails again, and draw a load-time curve showing the load changing with time;

[0011] S4: The maximum load is obtained according to the load-time curve, and the bonding strength between the chopped fiber and the ultra-high performance concrete matrix under bending force is calculated according to Formula I;

[0012]

[0013] P is the bond strength between the chopped fiber and the ultra-high performance concrete matrix under bending force, F is the maximum load obtained in the load-time curve, L is the span between the two lower supports, b is the width of the specimen, and h is the height of the specimen.

[0014] Furthermore, in S1, the specimen is a rectangular parallelepiped, and the specifications of the specimen are: (160±0.5) mm×(40±0.5) mm×(40±0.5) mm.

[0015] Furthermore, in S1, the specifications of the incision are: (40±0.5) mm×(3±0.5) mm×(5±0.5) mm.

[0016] Further, in S1, the raw materials of the specimen include cementitious material, fine aggregate, admixture, fiber, and water in parts by weight; the mass ratio of cementitious material, fine aggregate, admixture, and water is 600-1100:150-530:5-8:200-350; and the fiber accounts for 0.5-2% of the total volume of the raw material mixture.

[0017] Further, in S1, the fibers used are short-cut non-continuous fibers; the short-cut non-continuous fibers are one of polyoxymethylene fibers, steel fibers, polypropylene fibers, basalt fibers or carbon fibers;

[0018] The gel material is prepared by mixing cement, fly ash and silica fume in a mass ratio of (8-11): (1-3.5): (1-4.5);

[0019] The admixture is a mixture of a water reducing agent and an air entraining agent in a mass ratio of 4-5.3:0-0.24.

[0020] Further, in S1, the preparation method of the test piece is: pour the fine aggregate and the cementitious material into a stirring device and dry mix for 60-90s, then fully mix the admixture with water, put it into the mixture of fine aggregate and cementitious material and continue to stir for 2-3min until the material is in a self-leveling state, and then stir it for 3-5min before discharging the material; pour the material into a mold, demould it after molding, and then obtain the test piece after curing and incision treatment.

[0021] Further, in S2, the stretching displacement was 0.2 mm and the stretching speed was 0.1 mm / min.

[0022] Further, in S3, the loading rate of the three-point bending test was 40-45 N / s.

[0023] Further, in S3, the span is set to 80-100 mm.

[0024] Furthermore, in S3, the specimen is fixed on two fulcrums of the three-point bending test device, and the notch is located in the middle of the two fulcrums, with one side with the notch facing upward and the other side facing downward; a load is applied downward at the notch of one side until the specimen is damaged again, and a load-time curve is drawn.

[0025] In summary, the principle of this scheme is:

[0026] The purpose of the present invention is to provide a testing method, using a conventional tensile testing machine and an integrated flexural and compressive testing machine to perform tensile and three-point bending tests on ultra-high performance concrete test blocks, respectively, selecting the maximum load in the three-point bending load-time curve and calculating the bending strength, which is the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending stress. In this way, the actual stress conditions of structural components can be simulated, and the bonding performance test of the chopped fibers and the concrete matrix can be carried out under conditions close to the actual bending stress.

[0027] More specifically, the present invention relates to a method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending stress, and a three-point bending test is performed on ultra-high performance concrete specimens based on a traditional tensile testing machine and an integrated flexural and compressive testing machine. First, symmetrical cuts are made at the mid-span positions of the two sides of the ultra-high performance concrete specimen containing chopped fibers inside, and then a preset load is applied to the ultra-high performance concrete specimen with the help of a tensile testing machine until the specimen is damaged, that is, a through crack appears in the mid-span cut area of ​​the specimen (showing a cracked but unbroken effect); then, the test block after tensile damage is placed on a three-point bending test device for bending loading until the specimen is damaged again, and the bending load-time curve is recorded; finally, the maximum load in the bending load-time curve is selected and the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending stress is calculated. After tensile loading, through cracks have appeared in the matrix material (brittle concrete) of the test block, and the fracture of the test block is only connected by the adhesion between the chopped fibers and the ultra-high performance concrete matrix. Therefore, after three-point bending loading, the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending force can be obtained. The measurement results obtained by the method of the present invention can intuitively and accurately measure the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending force, thereby filling the gap in the test method for the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending force.

[0028] The beneficial effects of this technical solution are:

[0029] (1) By making symmetrical cuts at the mid-span positions of the two sides of the specimen, it is ensured that the through cracks are located at the mid-span position of the specimen after tensile failure. This avoids the influence of the prefabricated through cracks not being in the same area on the test results to the greatest extent.

[0030] (2) By conducting a tensile test on an ultra-high performance concrete specimen (containing chopped fibers) with a symmetrical cut at the mid-span, a through-crack was prefabricated, so that the fracture of the specimen was maintained only by the adhesion between the chopped fibers and the specimen matrix material.

[0031] (3) By placing the tensile-damaged specimen on a three-point bending test device for bending loading until the specimen is damaged again, the obtained measurement results can more accurately reflect the bond strength between the chopped fibers and the concrete matrix. This provides the necessary test equipment and methods for studying the changing law of the bond anchoring performance of chopped fibers and concrete under bending loads.

[0032] (4) The tensile test equipment used in the present invention is a tensile test machine, and the three-point bending test device is a compression and flexure integrated machine. These two devices are conventional equipment for concrete and mortar experiments. One machine can be used for multiple purposes, saving costs. It can meet the needs of various short-cut fiber cement-based materials or components to study the bonding performance of short-cut fibers and matrix materials under bending loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is a flow chart of the bonding performance testing method between the chopped fibers and the ultra-high performance concrete matrix of Example 1.

[0034] Figure 2 Schematic diagram of the ultra-high performance concrete specimen containing chopped fibers and the incision of Example 1.

[0035] Figure 3 This is a schematic diagram of a tensile loading in Example 1.

[0036] Figure 4 This is a schematic diagram of secondary loading of Example 1.

[0037] Figure 5 The load-time curve of the ultra-high performance concrete member of Example 1 subjected to bending loading.

[0038] Figure 6 This is a schematic diagram of the first loading of test method 2 of Example 3. DETAILED DESCRIPTION

[0039] The present invention is further described in detail below in conjunction with examples, but the embodiments of the present invention are not limited thereto. Unless otherwise specified, the technical means used in the following examples and experimental examples are conventional means well known to those skilled in the art, and the materials, reagents, etc. used can be obtained from commercial sources.

[0040] Example 1

[0041] For ultra-high performance concrete containing chopped fibers, the bonding performance between the chopped fibers and the ultra-high performance concrete matrix was tested. The operation process can be found in Figure 1 The structure of the corresponding specimen 2 can be found in Figure 2 , a schematic diagram of tensile loading can be found in Figure 3 , the secondary loading diagram can be found in Figure 4 The specific reference numerals are incision 1, test piece 2, tensile fixture 3, connecting bolt 4, through crack 5, load 6, and steel plate 7.

[0042] S1: Prepare incision 1 on specimen 2

[0043] The bonding performance between the chopped fibers and the ultra-high performance concrete matrix was tested using specimen 2 (UHPC containing chopped fibers). Cutouts 1 (at the mid-span of the two opposite sides of specimen 2) were made. Figure 2 The upper and lower surfaces of the specimen 2 are cut for subsequent tensile and loading treatments.

[0044] Preferably, the specification (length×width×height) of the test piece 2 is: (160±0.5) mm×(40±0.5) mm×(40±0.5) mm, such as Figure 1 As shown, the side (length × height) of the specimen 2 is displayed. The width of the incision 1 is (3±0.5) mm, the depth of the incision 1 is (5±0.5) mm, and the length of the incision 1 is consistent with the width of the specimen 2. That is, the incision 1 is provided on the upper and lower surfaces of the specimen 2, and the incision 1 is perpendicular to the long side (the side of (160±0.5) mm) of the specimen 2, and is located in the middle of the long side of the specimen 2 (the distance from the midpoint of the width of the incision 1 to one end of the long side of the specimen 2 is about 80 mm). That is, the specifications of the incision 1 (length × width × depth) are: (40±0.5) mm × (3±0.5) mm × (5±0.5) mm.

[0045] S2: Single tensile loading

[0046] Use conventional tensile testing machines to carry out tensile tests, such as Figure 3 As shown, the two ends of the specimen 2 are fixed respectively using a tensile clamp 3 and a link bolt 4, and the long side of the specimen 2 is set vertically. Then, the two tensile clamps 3 apply tensile forces in opposite directions to the two ends of the specimen 2 respectively, and the tensile load is applied to the specimen 2 at a loading rate of 0.1mm / min from 0 until the tensile displacement reaches 0.2mm and the loading is stopped. That is, the tensile displacement is 0.2mm and the tensile speed is 0.1mm / min. More specifically, the upper tensile clamp 3 moves away from the lower tensile clamp 3 at a speed of 0.1mm / min until the displacement of the upper tensile clamp 3 reaches 0.2mm. Through the above-mentioned stretching operation, the specimen 2 has a through crack 5 (such as Figure 4 As shown), however, specimen 2 did not break due to the chopped fibers contained therein, and the tensile treated specimen 2 was subjected to subsequent loading tests.

[0047] S3: Secondary loading

[0048] The specimen 2 after tensile failure is placed on a three-point bending test device for bending loading. The three-point bending test device is a conventional experimental device in the prior art, which places the specimen on two supporting points at a certain distance, and applies a downward force to the specimen at the midpoint of the two supporting points. When the three contact points of the specimen form two equal moments, three-point bending occurs, and the specimen breaks at the midpoint. In the present technical solution, when in use, the specimen 2 is placed on the two supporting points below, and the incision 1 is located in the middle of the two supporting points below. A load 6 is applied to the incision 1 above the specimen 2, and the loading rate is 40-45N / s (a loading rate of 45N / s is specifically used in subsequent experimental studies), and the bending load-time curve is recorded (such as Figure 5 As shown in the figure, until specimen 2 is damaged again. According to the load-time curve, when load 6 increases to a certain amount, specimen 2 is damaged again, and load 6 drops sharply on the bending load-time curve. At this time, load 6 is the maximum load.

[0049] S4: Calculation of bond strength

[0050] The maximum load in the load-time curve is selected, and the bonding strength P between the chopped fibers and the ultra-high performance concrete matrix under bending force is calculated according to Formula I.

[0051]

[0052] In formula I, F is the maximum load in the secondary load-time curve, P is the bond strength between the chopped fiber and the ultra-high performance concrete matrix under bending force, L, b, and h are the span (the distance between the two support points below the three-point bending test device), the width of specimen 2, and the height of specimen 2. The L value can be set between 80 and 120 mm. In subsequent experimental studies, the L value is specifically set to 100 mm.

[0053] In this technical solution, in order to characterize the bonding strength between the chopped fibers and the ultra-high performance concrete matrix under bending force, the ultra-high performance concrete member with a notch is stretched. On the one hand, it can make the matrix material (brittle concrete) of the test block have through cracks under uniform tension, so that the fracture of the test block is only connected by the chopped fibers. On the other hand, symmetrical notches are made in the middle area of ​​the test piece 2, which can promote the through cracks to appear in the notch 1 area in the middle of the test piece 2, unifying the position of the through cracks in the test piece and improving the accuracy of the experimental data. Then, through the secondary flexural test of the ultra-high performance concrete test block, the real bonding strength between the chopped fibers and the ultra-high performance concrete matrix can be evaluated.

[0054] Example 2

[0055] The preparation of the ultra-high performance concrete specimen of this scheme is prepared in accordance with the standard of GB17671. The ultra-high performance concrete (specimen) formula includes cementitious materials, admixtures, fine aggregates, fibers and water. The cementitious materials, fine aggregates, admixtures and water are mixed according to the following weight parts: 600-1100 weight parts of cementitious materials, 150-530 weight parts of fine aggregates, 5-8 weight parts of admixtures, and 200-350 weight parts of water. After mixing according to the above proportions, a concrete primary mixture is obtained, and then fibers are added to the concrete primary mixture. The fibers are mixed by volume, and the percentage of the volume of the fibers to the total volume is 0.5-2%. The fibers used are short-cut non-continuous fibers, and the short-cut non-continuous fibers are one of polyoxymethylene fibers, steel fibers, polypropylene fibers, basalt fibers or carbon fibers. Preferably, when steel fibers are used, the fiber inclusion amount is 1%, and when synthetic fibers are used, the fiber inclusion amount is 2%. Too low a fiber dosage will cause the specimen to completely break under tension, failing to present a "broken but continuous" effect; while too high a dosage will cause some fibers to be unevenly dispersed.

[0056] The gel material is made of cement, fly ash and silica fume in a mass ratio of (8-11): (1-3.5): (1-4.5); the cement is P.II52.5, the fly ash is of grade I or above, and the mass fraction of SiO2 in the silica fume is not less than 94%. The particle sizes of silica fume and fly ash are both 400-300 meshes. The fine aggregate is quartz sand.

[0057] The admixture is a mixture of a water reducing agent and an air entraining agent, wherein the mass ratio of the water reducing agent to the air entraining agent is 4-5.3:0-0.24. The water reducing agent is a polycarboxylate water reducing agent, and the air entraining agent is a tea saponin-based air entraining agent.

[0058] The ultra-high performance concrete is prepared by the following method: adding water to a mixing device and running it idle for 1-2 minutes to wet the inner wall of the mixing device and then draining it; then pouring fine aggregate and cementitious material weighed according to the mix ratio into the mixing device and dry mixing for 60-90 seconds, then fully mixing the admixture weighed according to the mass ratio with water, adding it into the mixture of fine aggregate and cementitious material and continuing to stir for 2-3 minutes to ensure that the mixture in the mixing device is in a self-leveling state, and finally adding the fiber prepared according to the volume ratio and stirring for 3-5 minutes before discharging.

[0059] In subsequent specific experiments, in order to ensure the parallelism of the experimental results, the following formula and preparation method are specifically adopted:

[0060] 900 parts by weight of cementitious material, 350 parts by weight of fine aggregate, 7 parts by weight of admixture, 300 parts by weight of water, and 2% by volume of fiber.

[0061] Among them, the fiber used is short-cut non-continuous fiber, specifically polyoxymethylene fiber, with a length of 12mm and a diameter of 0.2mm (the optional range is 8-18mm in length and 0.1-1mm in diameter). The tensile strength of polyoxymethylene fiber is 1000MPa (the optional range is 800-1100Mpa), the elastic modulus is 10GPa (the optional range is 9-12GPa), the elongation at break is 15.0% (the optional range is 13-18%), and the density is 1.41g / cm 3 .

[0062] The gel material is made of cement, fly ash and silica fume in a mass ratio of 9:2:3; the cement is P.II52.5, the fly ash is fly ash of grade I or above; the mass fraction of SiO2 in the silica fume is 94%. The fine aggregate is quartz sand with a particle size of 0.15-0.21mm. The admixture is made of a water reducer and an air entraining agent in a mass ratio of 5:0.2. The water reducer is a conventional polycarboxylate water reducer (PC200 (powder), water reduction rate (mass fraction)>25%) in the prior art, and the air entraining agent is a conventional tea saponin-based air entraining agent in the prior art (the effective component of tea saponin is 60%).

[0063] Add water to the stirring device and run it idle for 2 minutes to wet the inner wall of the stirring device and then drain it; then pour the fine aggregate and cementitious material weighed according to the mix ratio into the stirring device and dry mix for 90 seconds, then fully mix the admixture weighed by mass with water, add it into the mixture of fine aggregate and cementitious material and continue to stir for 3 minutes to ensure that the mixture in the stirring device is in a self-leveling state, and finally add the fiber prepared according to the volume ratio and stir for 5 minutes before discharging. Put the self-leveling material into the mold, demould it after leaving it at room temperature for 24 hours, and then put it in a standard curing room for curing for 28 days, and then cut a notch on the concrete block (according to the method of Example 1) to obtain the test piece. The test piece prepared according to the above method will be used for subsequent specific experimental research.

[0064] Example 3

[0065] (1) Test method 1: This test method is carried out according to the method of Example 1, and the test piece prepared in Example 2 is tested.

[0066] The analysis of the test results showed that after 5 repeated tests, the coefficient of variation (CV) was only 1.554%, which was much lower than other test methods, indicating that it had good repeatability.

[0067] (2) Test method 2: This test method is used to test the specimen prepared in Example 2. The "S3: secondary loading" and "S4: calculation of bond strength" of this test method are the same as those of Example 1, except that the "S2: primary tensile loading" method is not used to make the base material (brittle concrete) of the specimen undergo through cracks under uniform tensile force, but the following method is used:

[0068] like Figure 6 As shown (with reference numerals: specimen 2, load 6, steel plate 7), specimen 2 (the incision 2 is not shown in the figure, but the specimen 2 used is the same as the specimen 2 used in test method 1) is placed on the two fulcrums of the three-point bending test device, and a 5mm thick steel plate 7 is placed between the specimen 2 and the fulcrum, and the area of ​​the steel plate 7 is consistent with the area of ​​the lower surface of the specimen 2. The middle part of the specimen 2 is loaded for the first time at a loading rate of 15N / s (load 6 is applied), and after the ultra-high performance concrete specimen 2 is damaged at the incision and a through crack occurs (a through crack just occurs), the steel plate 7 is pulled out. Continue to perform "S3: secondary loading" and "S4: calculation of bonding strength" according to the method of Example 1. Analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) of the results is 19.41%, indicating that the results obtained using this test scheme have high uncertainty and unsatisfactory repeatability.

[0069] (3) Test method 3: This test method is basically the same as test method 1, except that the width of the incision on specimen 2 is 1.5±0.5 mm. The analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) of the results is 11.29%, which is much higher than the method in Example 1. This is mainly because the notch width is too small. After the tensile test, the crack may not necessarily penetrate along the notches on both sides, and its expansion direction may deviate, resulting in unsatisfactory repeatability of the test method.

[0070] (4) Test method 4: This test method is basically the same as test method 1, except that the width of the incision on specimen 2 is 4.5±0.5 mm. The analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) of the results is 16.97%, which is much higher than the method in Example 1. This is mainly because the notch width is too wide. After the tensile test, the crack may not be able to straighten along the notches on both sides to show a through crack, and its expansion direction will be offset, resulting in unsatisfactory repeatability of the test method.

[0071] (5) Test method 5: This test method is basically the same as test method 1, except that the incision depth on specimen 2 is 6.5±0.5 mm. The analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) is 23.83%, which is much higher than the method in Example 1. This is mainly because the notch width is too wide. After the tensile test, the crack may not be able to straighten along the notches on both sides to show a through crack, and its expansion direction will be offset, resulting in unsatisfactory repeatability of the test method.

[0072] (6) Test method 6: This test method is basically the same as test method 1, except that the tensile load is applied to the specimen until the tensile displacement is 0.1 mm. The analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) is 4.65%, which is higher than the method in Example 1. At the same time, due to the small tensile displacement, the specimen may not be completely broken, so the secondary bending test results are significantly higher than normal values.

[0073] (7) Test method 7: This test method is basically the same as test method 1, except that the tensile load is applied to the specimen until the tensile displacement is 0.3 mm. Analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) of the results is 8.76%, which is higher than the method in Example 1, indicating that excessive tensile displacement will lead to inaccurate experimental results. This is mainly because excessive tensile displacement will cause some fibers inside the test block to be pulled out or broken prematurely.

[0074] (8) Test method 8: This test method is basically the same as test method 1, except that the tensile load is applied to specimen 2 until the tensile displacement is 0.5 mm. Analysis of the test results shows that after 5 repeated tests, the coefficient of variation (CV) of the results is 9.04%, which is higher than that of the method in Example 1, indicating that excessive tensile displacement will lead to inaccurate experimental results, which is mainly because excessive tensile displacement will cause some fibers inside the specimen to be pulled out or broken prematurely.

[0075] Table 1: Test results of bonding strength P of test methods 1-8

[0076]

[0077] The above is only an embodiment of the present invention, and the common knowledge such as the known specific technical solutions and / or characteristics in the solution is not described in detail here. It should be pointed out that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the effect of the implementation of the present invention and the practicality of the patent. The scope of protection required by this application shall be based on the content of its claims, and the specific implementation methods and other records in the specification can be used to interpret the content of the claims.

Claims

1. A method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force, characterized in that: The method includes the following steps in sequence: S1: Cut notches at the mid-span positions on two opposite sides of the specimen; S2: Apply tension in opposite directions to the two ends of the specimen until a through crack appears between the two cuts, and obtain a specimen after tensile failure; S3: Perform a three-point bending test on the specimen after tensile failure; load the specimen until the specimen fails again, and draw a load-time curve showing the load changing with time; S4: The maximum load is obtained according to the load-time curve, and the bonding strength between the chopped fiber and the ultra-high performance concrete matrix under bending force is calculated according to Formula I; P is the bond strength between the chopped fiber and the ultra-high performance concrete matrix under bending force, F is the maximum load obtained in the load-time curve, L is the span between the two lower supports, b is the width of the specimen, and h is the height of the specimen.

2. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 1, characterized in that: In S1, the specimen is a rectangular parallelepiped, and the specifications of the specimen are: (160±0.5) mm×(40±0.5) mm×(40±0.5) mm.

3. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 2, characterized in that: In S1, the specifications of the incision are: (40±0.5) mm×(3±0.5) mm×(5±0.5) mm.

4. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 3, characterized in that: In S1, the raw materials of the specimen include cementitious material, fine aggregate, admixture, fiber, and water in parts by weight; the mass ratio of cementitious material, fine aggregate, admixture, and water is 600-1100:150-530:5-8:200-350; and the fiber accounts for 0.5-2% of the total volume of the raw material mixture.

5. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 4, characterized in that: In S1, the fiber used is short-cut non-continuous fiber; the short-cut non-continuous fiber is one of polyoxymethylene fiber, steel fiber, polypropylene fiber, basalt fiber or carbon fiber; The gel material is prepared by mixing cement, fly ash and silica fume in a mass ratio of (8-11): (1-3.5): (1-4.5); The admixture is a mixture of a water reducing agent and an air entraining agent in a mass ratio of 4-5.3:0-0.

24.

6. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 5, characterized in that: In S1, the preparation method of the specimen is as follows: fine aggregate and cementitious material are poured into a stirring device and dry-mixed for 60-90 seconds, and then the admixture is fully mixed with water, and then put into the mixture of fine aggregate and cementitious material and continue to stir for 2-3 minutes until the material is in a self-leveling state, and then the material is stirred for 3-5 minutes before being discharged; the material is poured into a mold, demolded after molding, and then subjected to curing and incision treatment to obtain the specimen.

7. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 1, characterized in that: In S2, the stretching displacement was 0.2 mm and the stretching speed was 0.1 mm / min.

8. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 1, characterized in that: In S3, the loading rate of the three-point bending test was 40-45 N / s.

9. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 8, characterized in that: In S3, the span is set to 80-100mm.

10. The method for testing the bonding strength between chopped fibers and ultra-high performance concrete matrix under bending force according to claim 9, characterized in that: In S3, the specimen is fixed on the two fulcrums of the three-point bending test device, and the notch is located in the middle of the two fulcrums. The side with the notch is placed with one side facing up and the other side facing down. A load is applied downward at the notch on one side until the specimen is damaged again, and a load-time curve is drawn.

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