Test piece for testing forward pull-out performance of single fiber-cement, preparation method of test piece and testing method

Abstract

The invention provides a test piece for testing the forward pull-out performance of single fiber-cement, a preparation method of the test piece and a testing method, and the preparation method comprises the following steps: assembling a mold, arranging a fixed block on each of the two sides of a material injection port of the mold, and forming an insertion groove in each fixed block along the height direction of the fixed block; injecting cement-based slurry into the mold; pasting one end of the to-be-detected fiber on the fixed substrate, and enabling the other end to exceed the bottom edge of the fixed substrate and to be perpendicular to the fixed substrate; the free end of the to-be-detected fiber is vertically downward, and the fixed substrate is inserted into the two slots until the bottom edge of the fixed substrate is in contact with the upper surface of the mold; vertically lifting and quickly putting down the mold, and repeating for several times; and after the cement-based slurry is solidified, mold removal, sampling and maintenance are conducted, and finally the fixed base plate is removed. The method not only can ensure that the single fiber is vertically embedded into the cement matrix, but also can flexibly control the embedding length, is beneficial to improving the accuracy of an interface bonding strength test result, and is simple, easy to operate and low in cost.

Description

Technical Field

[0001] The present invention relates to the technical field of cement-based composite materials, and in particular to a test piece for testing the positive pull-out performance of a single fiber-cement, a preparation method of the test piece, and a testing method. Background Art

[0002] The single fiber pull-out test is a classic method for evaluating the interfacial bond strength between fibers and cement matrices. Its core is to pull a single fiber out of the matrix using a tensile load and directly measure the maximum stress at interface failure. In the prior art, the conventional preparation method for the single fiber pull-out test is to first pour the matrix slurry into a mold, and then use a precision fixture or microscope to vertically insert the single fiber into the uncured matrix material, so that one end of the single fiber is completely embedded and the other end is exposed outside the matrix. The length exposed outside the matrix is ​​generally 1-5 mm. When inserting a single fiber, it is generally impossible to ensure the perpendicularity of the single fiber to the matrix surface without auxiliary means, resulting in a certain tilt angle deviation, which has a significant impact on the interfacial bond strength test results between the fiber and the cement matrix. Using a laser positioning system, although verticality can be guaranteed, the operation is cumbersome and inefficient. Moreover, in order to accurately control the exposed length, the insertion of a single fiber also requires the use of instruments such as precision fixtures and microscopes, further reducing the efficiency of sample preparation. How to embed a single fiber into the cement matrix vertically, with controllable length, simply and efficiently is crucial for single fiber pull-out testing. Summary of the Invention

[0003] In view of the above-mentioned shortcomings of the prior art, the present invention provides a specimen for testing the positive pull-out performance of a single fiber-cement, a preparation method of the specimen, and a testing method.

[0004] First, the present invention provides a method for preparing a single fiber-cement forward pullout performance test specimen, comprising the following steps:

[0005] S1. Assemble a mold for cement matrix molding, wherein the upper surface of the mold is provided with an injection port, and a fixing block is provided on each side of the injection port. The fixing block has a slot extending through the upper surface of the mold in a square shape along its height.

[0006] S2. Injecting a uniformly stirred cement-based slurry into a mold; attaching one end of the fiber to be tested to a fixed substrate, with the free end of the fiber to be tested extending beyond the bottom edge of the fixed substrate and perpendicular to the bottom edge. By controlling the length of the free end of the fiber to be tested extending beyond the bottom edge of the fixed substrate, the insertion length of the fiber to be tested into the matrix can be controlled.

[0007] S3, with the free end of the fiber to be tested pointing vertically downward, insert the fixed base plate into the two slots until the bottom edge of the fixed base plate contacts the upper surface of the mold;

[0008] S4. Lift the mold vertically and quickly lower it to hit the table surface, repeating several times to compact the cement-based slurry;

[0009] S5. After the cement-based slurry solidifies, the mold is removed, samples are taken, and curing is performed, and finally the fixed base plate is removed.

[0010] In the prior art, a uniformly stirred cement-based slurry is injected into a mold, and is generally first vibrated and smoothed to obtain a cement matrix. The free end of the fiber to be tested, which is attached to a carrier, is then vertically inserted into the cement matrix. Since the diameter of the fiber to be tested is small and its rigidity is poor, the surface of the smoothed cement matrix is ​​relatively dense. When the fiber to be tested is manually inserted into the cement matrix, the verticality of the free end cannot be guaranteed due to factors such as the insertion speed, the softness of the fiber to be tested itself, and the density of the cement matrix surface. A certain tilt angle is easily present, which will adversely affect the test results of the interfacial bonding strength.

[0011] In the present invention: (1) slots for guiding and limiting the movement of the fixed substrate are provided on both sides of the injection port. When the mold repeatedly hits the table, the fixed substrate can move downward slowly and steadily, thereby gradually inserting the fiber to be tested into the cement-based slurry; (2) after the cement-based slurry is injected into the mold, it is not vibrated or smoothed first. At this time, it has both a certain viscous resistance and a certain gap. The fiber to be tested is slowly inserted and its position can be initially fixed by relying on the viscous resistance of the cement-based slurry. After that, the mold is vertically lifted and quickly lowered to provide a vertical downward vibration force for the fiber to be tested. The self-leveling property of the cement-based slurry prompts the cement-based slurry to fill around the fiber to be tested and wrap the insertion end of the fiber to be tested. The vertical downward vibration force guides the insertion end. Under the synergistic effect of the vibration force and the self-leveling property of the cement-based slurry, the free end of the fiber to be tested is vertically inserted into the cement-based slurry according to a predetermined length.

[0012] The present invention effectively realizes the vertical, controllable length, simple and efficient embedding of a single fiber into the cement matrix by controlling the timing of inserting the fiber to be tested into the cement matrix, which has a significant positive effect on the single fiber pull-out test.

[0013] Furthermore, in step S1, the fixing block is made of elastic material, such as a rubber fixing block or a foam fixing block, preferably a foam fixing block, and the size of the slot is adapted to the size of the fixing base plate;

[0014] Preferably, the size of the slot is slightly larger than that of the fixed substrate. For example, the single-side gap between the fixed substrate and the slot is controlled at 0.05-0.1 mm. This will neither cause the fixed substrate to get stuck nor cause the fixed substrate to tilt.

[0015] In this embodiment, the fixed block is designed to be made of an elastic material. When the fixed substrate gradually moves closer to the upper surface of the mold due to the action of vibration force, the fixed block can not only limit the fixed substrate, but also compensate for slight position deviations through local deformation to avoid jamming caused by rigid collision. When the mold is repeatedly lifted and lowered vertically in the subsequent process, the fixed block can dynamically adjust the shape of the slot along with the insertion action, gradually guiding the fixed substrate to be precisely positioned and ensuring the stability of the clamping after the fixed substrate is in place.

[0016] Furthermore, in step S2, the free end of the fiber to be measured extends beyond the edge of the fixed substrate by 0.5-15 mm, and the diameter of the fiber to be measured is 10-200 μm; preferably, the free end of the fiber to be measured extends beyond the edge of the fixed substrate by 1-5 mm, and the diameter of the fiber to be measured is 20-60 μm; further preferably, the free end of the fiber to be measured extends beyond the edge of the fixed substrate by 1.5 mm, and the diameter of the fiber to be measured is 40 μm;

[0017] Further preferably, the water-cement ratio of the cement-based slurry is 0.35-0.45. The cement-based slurry that meets this water-cement ratio requirement has appropriate self-leveling properties. After the fiber to be tested is inserted, it can be promptly filled around the fiber to be tested and quickly and effectively wrap the fiber, so as to cooperate with the vertical downward vibration force to guide the insertion end of the fiber to be tested to be vertical, and finally make the insertion end vertically inserted in the cement-based slurry.

[0018] In this embodiment, the present invention explores the preparation of test specimens with various embedding lengths of the fiber to be tested. The present invention found that: if the embedding length of the fiber to be tested is too long, although the cement-based slurry can cover its insertion end, due to the limited vibration force, the guiding effect provided for the insertion end is limited, and it is impossible to ensure that the entire embedding length remains in a vertical state, and the embedded part is prone to bending or tilting locally; if the embedding length of the fiber to be tested is too short, although it can be inserted vertically, it cannot be effectively anchored in the cement-based slurry, and it is very easy to be displaced and then "pulled out" or tilted when the cement-based slurry is vibrated or contracted. Taking into account the vertical effect and stability of the fiber to be tested after embedding, the present invention found through several experiments that, for a cement-based slurry with a water-cement ratio of 0.35-0.45, the embedding length of the fiber to be tested is 0.5-15mm, preferably 1-5mm, and particularly preferably 1.5mm.

[0019] In this embodiment, the present invention uses a variety of different diameters of test fibers to prepare the test specimens. The present invention found that: when the diameter of the test fiber is too large, it has good rigidity and strong deformation resistance, but requires a cement-based slurry with higher fluidity to completely wrap it, otherwise interface gaps will be formed; when the diameter of the test fiber is too small, the softness is too large, and it is easily disturbed by the flow of the cement-based slurry and deviates, and cannot maintain the verticality after embedment. Taking into account the verticality of the test fiber after embedment, for a cement-based slurry with a water-cement ratio of 0.35-0.45, a fiber with a diameter of 10-200 μm is better as the test fiber in the present invention, preferably with a diameter of 20-60 μm, and particularly preferably 40 μm.

[0020] In summary, a cement-based slurry with a water-cement ratio of 0.35-0.45 has appropriate fluidity and density. It is easy to fill around the fiber to be tested that meets the above-mentioned embedding length and diameter requirements, and provides appropriate viscosity and support for the fiber to be tested, preventing the fiber to be tested from sinking or tilting due to its own weight, ultimately making the embedded part of the fiber to be tested vertically located in the cement matrix material.

[0021] Furthermore, in step S2, the fixed substrate is a hard, oxidation-resistant fixed substrate; the fiber to be tested is any one of polymer fiber, carbon fiber, steel fiber, and glass fiber; one end of the fiber to be tested is adhered to the fixed substrate by any one of glue, single-sided tape, and double-sided tape; preferably, the fixed substrate is rectangular and made of any one of copper plate, aluminum alloy plate, titanium alloy plate, stainless steel plate, cardboard, glass plate, plastic plate, and wooden plate, and is particularly preferably a fixed substrate of metal material with a smooth surface; further preferably, one end of the fiber to be tested is adhered to the fixed substrate by a transparent single-sided tape. When removing the fixed substrate in step S5, a hair dryer can be used to blow hot air to the single-sided tape, and then the single-sided tape can be gently torn off to achieve lossless separation of the fiber to be tested and the fixed substrate.

[0022] Furthermore, in step S4, the height of the vertical lift of the mold does not exceed 5 cm, and the number of times it hits the table is not less than 2 times. Preferably, through summarizing the experience value through several experiments, in step S4, the height of the vertical lift of the mold is 2-4 cm, and the number of times it hits the table is 3-5 times. At this time, the high frequency and small amplitude lifting / falling not only prompts the fiber to be tested to be slowly and vertically inserted into the cement-based slurry, but also compacts the cement-based slurry.

[0023] Furthermore, in step S5, the cement-based slurry is first cured in air at room temperature for 24 hours, and then the sample is removed from the mold and placed in a curing room for curing for 7-28 days. The curing room has a temperature of 21-25° C. and a humidity of 93%-97%.

[0024] In this embodiment, the cement-based slurry must first be cured in air at room temperature for 24 hours before being removed for curing. Under these curing conditions, the shrinkage stress of the cement matrix is ​​small and relatively evenly distributed, which does not produce asymmetric tension on the test fibers, thereby preventing the test fibers from tilting, further improving the quality of the test specimen.

[0025] Secondly, the present invention proposes a test piece for testing the positive pull-out performance of a single fiber-cement.

[0026] Thirdly, the present invention also proposes a testing method for testing the forward pull-out performance of single fiber-cement, and uses the DIC imaging method to test the above-mentioned specimen.

[0027] Specifically, the specific operation of the test is as follows: first, drop accelerating dry glue on the free end of the fiber to be tested, and the quick-drying glue quickly solidifies to form a hard end; then, the specimen is placed on a tensile testing machine, and the tensile rate is controlled to pull the fiber to be tested out of the cement matrix to obtain a load-displacement curve, and DIC is used to synchronously capture the micro-displacement and strain distribution of the interface between the fiber to be tested and the cement matrix; then, the formula G=F / (πdL) is corrected and the interfacial bonding strength between the fiber to be tested and the cement matrix is ​​calculated, where: G is the interfacial bonding strength between the fiber to be tested and the cement matrix, in MPa; F is the maximum load, in N; d is the diameter of a single fiber, in mm; and L is the embedded length of the fiber to be tested in the cement matrix, in mm.

[0028] In this embodiment, the specific method for correcting the formula G=F / (πdL) is based on the prior art, which generally includes the following steps:

[0029] (1) DIC data acquisition and preprocessing

[0030] A speckle pattern was sprayed on the specimen surface, and a high-speed camera was used to synchronously record the image sequence during the stretching process. The micro-displacement u and strain distribution ε were calculated using DIC software, with a focus on the axial displacement gradient near the fiber-cement matrix interface.

[0031] (2) Determine the effective embedment length

[0032] Although the specimen preparation method proposed in this invention can effectively control the embedding length of the tested fiber in the cement matrix, in practice, the tested fiber may partially debond or slip locally, resulting in the effective stress transmission area being smaller than the theoretical embedding length. The critical position where the fiber starts to slip, such as the displacement sudden increase point, is determined by micro-displacement u. According to the dynamic process of the slip zone expansion, the effective embedding length is corrected in real time. For example, if the slip extends ΔL from the free end to the inside, then L eff =L-ΔL;

[0033] (3) Correction of stress distribution unevenness

[0034] The formula G = F / (πdL) assumes that the interface shear stress is uniformly distributed. However, DIC shows that the shear stress is nonlinearly distributed along the embedded length. In this case, a shear stress distribution model is constructed:

[0035] a. Calculate local shear stress T(x) through DIC strain field

[0036]

[0037] Where: E f is the fiber elastic modulus; A f =πd 2 / 4, fiber cross-sectional area; P = πd, fiber circumference; dε f (x) / dx is the strain gradient, calculated by numerical difference of DIC data;

[0038] b. Integrate T(x) along the embedded length to obtain the modified expression of the total load F

[0039]

[0040] c. Substitute the corrected bond strength formula

[0041]

[0042] (4) End effect and damage area elimination DIC can identify the non-representative damage area caused by stress concentration at the end of the tested fiber, so in the calculation of L eff When excluding the damaged area, for example, if the length of the damaged area is δ, then L eff =L-δ;

[0043] (5) Dynamic slip process correction

[0044] If DIC shows that the interface slip occurs in stages, such as the remaining length continues to bear the load after partial debonding, the G value of each stage can be calculated segmentally and the weighted average can be obtained:

[0045]

[0046] Among them: G i and L i is the bonding strength and corresponding embedment length at each stage;

[0047] In addition, the strain energy release rate measured by DIC can be compared with the theoretically calculated G value to ensure the rationality of the correction. At the same time, the shear stress distribution function can be fitted through multiple experiments to improve the universality of the correction formula.

[0048] In summary: Through the above method, a more realistic L eff, dynamically reflecting the actual load-bearing area; this approach allows for the determination of non-uniform stress distribution, avoiding underestimation or overestimation due to the uniform assumption; and allows for damage identification, eliminating abnormal end data, and improving reliability. Clearly, DIC technology has upgraded bond strength testing from a single-parameter model to a refined assessment based on full-field data. Working in conjunction with a tensile testing machine, this approach helps obtain more accurate test results, making it particularly suitable for studying non-uniform interfaces or complex failure modes.

[0049] Furthermore, the stretching rate was 0.1 mm / min.

[0050] The present invention realizes the preparation of specimens for single fiber-cement forward pull-out performance testing with efficient, simple and easy operation. It can ensure that the single fiber is vertically embedded in the cement matrix, improve the accuracy of the interface bonding strength test results between the fiber and the cement matrix, and flexibly control the embedding length. Moreover, it has low requirements on the length and diameter of the single fiber. On this basis, the present invention adopts the DIC imaging method combined with a tensile testing machine for testing, which can obtain more accurate single fiber pull-out test results. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0052] Figure 1 This is an implementation method of the method for preparing a specimen for testing the positive pull-out performance of a single fiber-cement proposed in the present invention.

[0053] Figure 2 These are the load-displacement curves obtained for Example 1 and Comparative Examples to Comparative Examples 6 of the present invention.

[0054] In the figure: 1. Mold; 11. Injection port; 2. Fixing block; 3. Fixed substrate; 4. Adhesive tape; 5. Fiber to be tested. DETAILED DESCRIPTION

[0055] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only some of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0056] It should be noted that the technical solutions not described in detail below all adopt conventional technical means in this field. Various raw materials involved in the following examples and comparative examples were commercially obtained.

[0057] Example 1

[0058] PVA fiber with a diameter of 40 μm and a cement-based interface strength of 7 MPa was used as the fiber to be tested. A specimen was prepared using the specimen preparation method shown in the present invention, and its cement-based interface strength was tested using the test method shown in the present invention.

[0059] like Figure 1 As shown: A method for preparing a single fiber-cement forward pullout performance test specimen includes the following steps:

[0060] S1. Assemble a mold 1 for cement matrix molding. The upper surface of the mold 1 is provided with an injection port 11. A foam fixing block 2 is fixed on each side of the injection port 11. The fixing block 2 has a slot extending through the upper surface of the mold 1 in a square shape along its height.

[0061] S2. A uniformly stirred cement-based slurry with a water-cement ratio of 0.4 is injected into the mold 1. One end of the fiber to be tested 5 is affixed to the fixed substrate 3 using transparent tape. The fixed substrate 3 is specifically a rectangular copper plate. The free end of the fiber to be tested 5 extends 1.5 mm beyond the bottom edge of the fixed substrate 3 and is perpendicular to the bottom edge.

[0062] S3, with the free end of the fiber to be tested 5 pointing vertically downward, insert the fixed base plate 3 into the two slots until the bottom edge of the fixed base plate 3 contacts the upper surface of the mold 1;

[0063] S4. Lift the mold 1 vertically by 3 cm and quickly lower it to hit the table surface. Repeat this 4 times to compact the cement-based slurry.

[0064] S5. The cement-based slurry is first cured in air at room temperature for 24 hours, then the mold is removed and the sample is placed in a curing room for curing for 18 days. The curing room temperature is 24° C. and the humidity is 95%. Finally, the fixed substrate is removed.

[0065] The obtained specimen was tested by DIC imaging method. The specific operation was as follows: first, the quick-drying glue was dripped on the free end of the fiber to be tested, and the quick-drying glue was quickly solidified to form a hard end. Then, the specimen was placed on a tensile testing machine, and the tensile rate was controlled to be 0.1 mm / min to pull the fiber 5 to be tested out of the cement matrix, and the load-displacement curve was obtained (specifically, Figure 2 As shown in FIG5 , the micro-displacement and strain distribution at the interface between the fiber 5 to be tested and the cement matrix are simultaneously captured by DIC. The maximum load value F is then corrected, and the corrected F is substituted into the formula G=F / (πdL) to calculate the interfacial bonding strength between the fiber 5 to be tested and the cement matrix.

[0066] Example 2

[0067] Compared with Example 1, the water-cement ratio of the cement-based slurry in step S2 is 0.35; in step S4: the mold 1 is vertically lifted 2 cm and quickly lowered and then hits the table, and this is repeated 5 times to compact the cement-based slurry; in step S5: the mold is cured in a curing room for 7 days, and the temperature and humidity of the curing room are 25° C. and 93%; the rest are consistent with Example 1.

[0068] Example 3

[0069] Compared with Example 1, the water-cement ratio of the cement-based slurry in step S2 is 0.45; in step S4: the mold 1 is vertically lifted 4 cm and quickly lowered and then hits the table, and repeated three times to compact the cement-based slurry; in step S5: the cement-based slurry is first cured in air at room temperature for 24 hours, and then the mold is removed and the sample is placed in a curing room for curing for 28 days. The temperature of the curing room is 21°C and the humidity is 97%. The rest is consistent with Example 1.

[0070] Example 4

[0071] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 0.5 mm, and the rest remains the same as in Example 1.

[0072] Example 5

[0073] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 1 mm, and the rest remains the same as in Example 1.

[0074] Example 6

[0075] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 3 mm, and the rest remains the same as in Example 1.

[0076] Example 7

[0077] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 5 mm, and the rest remains the same as in Example 1.

[0078] Example 8

[0079] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 10 mm, and the rest remains the same as in Example 1.

[0080] Example 9

[0081] Compared with Example 1, in step S2, the free end of the fiber to be tested 5 extends beyond the bottom edge of the fixed substrate 3 by 15 mm, and the rest remains the same as in Example 1.

[0082] Example 10

[0083] Compared with Example 1, the fiber 5 to be tested was adjusted to PVA with a diameter of 10 μm and a cement-based interface strength of 6.0 MPa, and the rest remained the same as Example 1.

[0084] Example 11

[0085] Compared with Example 1, the fiber 5 to be tested is adjusted to PVA with a diameter of 20 μm and a cement-based interface strength of 6.3 MPa, and the rest are consistent with Example 1.

[0086] Example 12

[0087] Compared with Example 1, the fiber 5 to be tested is adjusted to PVA with a diameter of 60 μm and a cement-based interface strength of 7.2 MPa, and the rest are consistent with Example 1.

[0088] Example 13

[0089] Compared with Example 1, the fiber 5 to be tested is adjusted to PVA with a diameter of 200 μm and a cement-based interface strength of 8.0 MPa, and the rest are consistent with Example 1.

[0090] Comparative Example 1

[0091] Compared with Example 1, the preparation method of the test piece has been adjusted. The step S2 of "injecting the evenly stirred cement-based slurry into the mold 1" is changed to "injecting the evenly stirred cement-based slurry into the mold 1, vibrating it, and smoothing the surface". The rest are consistent with Example 1.

[0092] Comparative Example 2

[0093] Compared with Example 1, the preparation method of the specimen is the same, but the testing method is different: only a tensile testing machine is used, and DIC assistance is not used. That is, the specific testing method is: during the test, the specimen is placed on the tensile testing machine, and the tensile rate is controlled to be 0.1 mm / min to pull the fiber to be tested out of the cement matrix to obtain a load-displacement curve. The obtained maximum load F is directly substituted into the formula G=F / (πdL) to calculate the interface bonding strength between the fiber to be tested and the cement matrix.

[0094] Comparative Example 3

[0095] Compared with Example 1, the curing conditions in step S5 are adjusted, specifically: the curing room temperature is 20° C., and the humidity is 98%. The rest are consistent with Example 1.

[0096] Comparative Example 4

[0097] Compared with Example 1, the curing conditions in step S5 are adjusted, specifically: the curing room temperature is 28° C. and the humidity is 90%. The rest are consistent with Example 1.

[0098] Comparative Example 5

[0099] Compared with Example 1, the water-cement ratio of the cement-based slurry in step S2 is 0.3, and the rest are consistent with Example 1.

[0100] Comparative Example 6

[0101] Compared with Example 1, the water-cement ratio of the cement-based slurry in step S2 is 0.5, and the rest are consistent with Example 1.

[0102] The maximum load F and interfacial bonding strength G obtained for each embodiment and comparative example were statistically analyzed, and their deviations from the standard values ​​of interfacial bonding strength were compared. The specific results are shown in Table 1. In addition, for ease of listing, Example 1 is referred to as S1, Comparative Example 1 is referred to as D1, and so on.

[0103] Table 1

[0104]

[0105] As shown in Table 1, Figure 1 and Figure 2 Commonly shown:

[0106] (1) By comparing the test results of Example 1, Comparative Example 1, and Comparative Example 2, it can be seen that: if the cement-based slurry is first vibrated and the surface smoothed before the fiber to be tested is inserted to prepare the test piece, the test result deviation is -10.67% compared with the standard value; if only the maximum load is tested by a tensile testing machine without DIC correction, the test result deviation is -11.28%; the present invention adjusts the insertion timing of the fiber to be tested 5 when preparing the test piece and uses DIC to correct the test result, and the test result deviation is only -0.16%. Obviously, the preparation method of the single fiber-cement positive pull-out performance test piece proposed by the present invention helps to improve the accuracy of the test results of the interface bonding strength between the fiber and the cement-based matrix.

[0107] Furthermore, the comparison Figure 2 It can be seen from the load-displacement curves of Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, and Comparative Example 6 that the preparation conditions in the specimen preparation method proposed in the present invention, especially the curing conditions of the curing room and the water-cement ratio of the cement-based slurry, have a significant impact on the test results.

[0108] As shown in Comparative Example 3, excessively low curing room temperature and high humidity have at least the following effects on the interface between PVA fibers and the cement matrix: PVA fibers are highly hydrophilic, and high humidity causes them to absorb large amounts of water, causing the fiber diameter to expand. After the matrix solidifies, drying and shrinkage may leave micro-gaps at the interface, weakening the anchoring effect. PVA fibers have hydroxyl groups on their surface, and hydrogen bonding between hydroxyl groups and cement hydration products is the main source of their bonding. Low temperatures delay hydration and reduce the formation of hydration products, thereby causing poor bonding. Moreover, moisture accumulation at the interface hinders direct contact between the PVA fibers and the cement matrix, further cracking the bonding effect. High humidity may also induce partial hydrolysis of the PVA molecular chains, reducing the strength of the PVA fibers themselves. Therefore, the PVA fiber-cement matrix interface of the specimen obtained in Comparative Example 3 is not an ideal "debonding + friction" mixed mode, and the measured bond strength is significantly reduced.

[0109] As shown in Comparative Example 4, excessively high curing room temperature and low humidity have at least the following effects on the interface between the PVA fiber and the cementitious base: The PVA fiber softens and deforms when heated, causing the shape of the embedded fiber to change and reducing the effective bonding area; high temperature and low humidity can cause the cementitious base to lose water too quickly, and the rapid extraction of water from the PVA fiber surface can lead to localized debonding points between the PVA fiber and the base; high temperature can exacerbate the heat release from hydration, exacerbating the early expansion and later cooling contraction of the cementitious base. This can lead to microcracks at the interface, possibly due to a mismatch between the elastic modulus of the PVA fiber and the cementitious base. Therefore, the specimens obtained in Comparative Example 4 have a high risk of PVA fiber damage during tensile testing, and fiber breakage is likely to occur. The pull-out force shown in the test is likely a result of fiber "brittle fracture" rather than actual bond strength.

[0110] As shown in Comparative Example 5, when the water-cement ratio was 0.3, the test deviation increased significantly compared to a water-cement ratio of 0.4. This is likely due to the low water-cement ratio reducing the porosity and increasing the strength of the cementitious matrix, making it difficult to insert the tested fiber 5. Furthermore, the fiber 5 is susceptible to mechanical damage such as scratches and bending during insertion. Furthermore, the increased brittleness and autogenous shrinkage stress caused by the low water-cement ratio not only induce early microcracks in the matrix, but also cause local deformation of the PVA fibers, reducing the effective bonding area. This results in fiber breakage or matrix damage, making the test results inaccurately reflect the bonding properties of the PVA fibers to the cementitious matrix.

[0111] As shown in Comparative Example 6: When the water-cement ratio is 0.5, the test results are significantly lower than the standard value compared with the water-cement ratio of 0.4. The reason for this is probably that the excess water evaporates and leaves behind pores and bubbles, making the cement-based matrix loose, which not only reduces its mechanical strength, but also causes microcracks to form at the interface when the cement-based matrix shrinks, reducing the friction anchoring effect. In addition, the water is enriched around the PVA fibers, which not only causes the diameter of the PVA fibers to expand, but also hinders the direct contact between the PVA fibers and the cement-based matrix, resulting in a decrease in the bonding strength between the two. As a result, the interface failure mode during the test is mainly "slip", with a small friction component, and the measured bonding strength is significantly reduced, and the bridging ability of the PVA fibers is not fully demonstrated.

[0112] From the comparison of the test results of Example 1 and Examples 4 to 9, it can be seen that the preparation method of the single fiber-cement forward pull-out performance test specimen proposed in the present invention can flexibly control the embedded length of the fiber 5 to be tested, and the embedded length is preferably 0.5-15 mm, preferably 1-5 mm, and particularly preferably 1.5 mm.

[0113] From the comparison of the test results of Example 1 and Example 10-Example 13, it can be seen that the preparation method of the single fiber-cement forward pull-out performance test specimen proposed in the present invention has low requirements on the diameter of the single fiber. Fibers with a diameter of 10-200μm can be used as the test fiber of the present invention, preferably 20-60μm, and particularly preferably 40μm.

[0114] In summary: The present invention realizes the preparation of specimens for single fiber-cement forward pull-out performance testing with efficient, simple and easy operation. It can not only ensure that the single fiber is vertically embedded in the cement matrix, thereby improving the accuracy of the test results of the interface bonding strength between the fiber and the cement-based matrix, but also flexibly control the embedding length, and has low requirements on the length and diameter of the single fiber. On this basis, the present invention adopts the DIC imaging method combined with the tensile testing machine for testing, which can obtain more accurate single fiber pull-out test results.

[0115] Although embodiments of the present invention have been shown and described above, it should be understood that the above embodiments are illustrative and are not to be construed as limiting the present invention. Those skilled in the art may alter, modify, replace, and modify the above embodiments within the scope of the present invention. Furthermore, those skilled in the art may combine and incorporate the various embodiments or examples described in this specification, as well as features thereof, without conflicting considerations.

Claims

1. A method for preparing a single fiber-cement forward pull-out performance test specimen, characterized in that: The steps are as follows: S1. Assembling a mold (1) for cement matrix molding, wherein an injection port (11) is provided on the upper surface of the mold (1), and a fixing block (2) is provided on both sides of the injection port (11), and a slot is provided on the fixing block (2) along its height and is directly connected to the upper surface of the mold (1); S2. Injecting the evenly stirred cement-based slurry into the mold (1); pasting one end of the fiber to be tested (5) onto the fixed substrate (3), with the free end of the fiber to be tested (5) extending beyond the bottom edge of the fixed substrate (3) and perpendicular to the bottom edge; S3, with the free end of the fiber to be tested (5) pointing vertically downward, the fixed base plate (3) is inserted into the two slots until the bottom edge of the fixed base plate (3) contacts the upper surface of the mold (1); S4, vertically lifting the mold (1) and quickly lowering it to hit the table surface, repeating several times to compact the cement-based slurry; S5. After the cement-based slurry solidifies, the mold is removed, samples are taken, and curing is performed, and finally the fixed base plate is removed.

2. The preparation method according to claim 1, characterized in that In step S1, the fixing block (2) is made of elastic material, and the size of the slot is adapted to the size of the fixing base plate (3); Preferably, the fixing block (2) is a fixing block made of foam material.

3. The preparation method according to claim 1, characterized in that In step S2, the free end of the fiber to be tested (5) extends beyond the edge of the fixed substrate (3) by 0.5-15 mm, and the diameter of the fiber to be tested (5) is 10-200 μm; Preferably, the free end of the fiber to be tested (5) extends 1-5 mm beyond the edge of the fixed substrate (3), and the diameter of the fiber to be tested (5) is 20-60 μm; Further preferably, the free end of the fiber to be tested (5) extends 1.5 mm beyond the edge of the fixed substrate (3), and the diameter of the fiber to be tested (5) is 40 μm; Still further preferably, the water-cement ratio of the cement-based slurry is 0.35-0.

45.

4. The preparation method according to claim 1, characterized in that In step S2, the fixed substrate (3) is a hard, oxidation-resistant fixed substrate; the fiber to be tested (5) is any one of polymer fiber, carbon fiber, steel fiber, and glass fiber; one end of the fiber to be tested (5) is adhered to the fixed substrate (3) by using any one of glue, single-sided tape, and double-sided tape; Preferably, the fixed substrate (3) is rectangular and made of any one of a copper plate, an aluminum alloy plate, a titanium alloy plate, a stainless steel plate, a cardboard, a glass plate, a plastic plate, and a wooden plate; Further preferably, one end of the fiber to be tested (5) is adhered to the fixed substrate (3) by means of a transparent single-sided adhesive tape (4).

5. The preparation method according to claim 1, characterized in that In step S4, the mold (1) is vertically lifted to a height not exceeding 5 cm and hits the table surface at least twice; Preferably, in step S4, the mold (1) is vertically lifted to a height of 2-4 cm and hits the table surface 3-5 times.

6. The preparation method according to claim 1, characterized in that In step S5, the cement-based slurry is first cured in air at room temperature for 24 hours, and then the mold is removed and the sample is placed in a curing room for curing for 7-28 days. The curing room has a temperature of 21-25° C. and a humidity of 93%-97%.

7. A specimen for testing the positive pull-out performance of a single fiber-cement prepared according to the preparation method according to any one of claims 1 to 6.

8. A method for testing the forward pull-out performance of a single fiber-cement composite, characterized in that: The test piece according to claim 7 is tested using the DIC imaging method.

9. The testing method according to claim 8, characterized in that: The specific operation is as follows: first, drop quick-drying glue on the free end of the fiber to be tested, and the quick-drying glue quickly solidifies to form a hard end. Then, the specimen is placed on a tensile testing machine, and the tensile rate is controlled to pull the fiber to be tested (5) out of the cement matrix to obtain a load-displacement curve. The micro-displacement and strain distribution of the interface between the fiber to be tested (5) and the cement matrix are synchronously captured using DIC. Then, the formula G=F / (πdL) is corrected and the interfacial bonding strength between the fiber to be tested (5) and the cement matrix is ​​calculated, wherein: G is the interfacial bonding strength between the fiber to be tested (5) and the cement matrix, in MPa; F is the maximum load, in N; d is the diameter of a single fiber, in mm; L is the embedded length of the fiber to be tested (5) in the cement matrix, in mm.

10. The testing method according to claim 8, characterized in that: The stretching rate was 0.1 mm / min.

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