Method for testing microscopic bonding performance of fiber and cement-based material

Abstract

The invention discloses a method for testing the microscopic bonding performance of fibers and a cement-based material, and belongs to the technical field of building materials. According to the method, a single fiber is placed in a poured set cement test piece, the set cement test piece is cut into slices, the lengths of the fibers in the slices are kept equal, a fiber push-out test is carried out by utilizing a nanoindentor, and the microcosmic cohesiveness of the fibers is calculated by utilizing a formula. The problems that a traditional pull-out test piece is low in preparation efficiency, insufficient in reliability, high in measurement precision requirement and the like are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and in particular relates to a method for testing the microscopic bonding performance of fibers and cement-based materials. Background Art

[0002] Fiber concrete is a cement-based composite material composed of fibers and cement-based materials (cement stone, mortar or concrete). The addition of fibers can effectively inhibit the expansion of cracks inside concrete and improve the crack resistance and ductility of concrete. Therefore, fiber concrete is widely used in various engineering structures. It is well known that the excellent crack resistance of fiber concrete mainly depends on the micro-bonding performance between fibers and cement-based materials. It is the basis for the coordinated work of fibers and cement-based materials, and it is also the key to ensuring the high strength and ductility of fiber concrete. Therefore, how to accurately measure the micro-bonding performance between the two is of great significance for in-depth research on the toughening mechanism of fibers and further improving the mechanical properties of fiber concrete.

[0003] At present, the commonly used method to characterize the bonding performance of fibers and cement-based materials is the fiber pull-out test. However, there are still many shortcomings in the preparation and testing of fiber pull-out specimens, mainly the following two problems: First, the preparation efficiency of pull-out specimens is low and the process is complicated. Only a single fiber pull-out specimen can be prepared in a single pouring. However, due to the high heterogeneity of the microstructure of fiber concrete, the test results are highly discrete, and a large number of tests are often required to ensure the reliability of the results; in addition, due to the small force and displacement of a single fiber pull-out process, the instrument is required to have a high measurement accuracy, and the currently commonly used extensometers and small force sensors are difficult to meet the test accuracy requirements. Therefore, it is necessary to propose a new fiber concrete micro-bonding performance test method that can not only meet the high-throughput requirements of micro-tests, but also ensure high-precision testing to accurately characterize the micro-bonding performance. Summary of the invention

[0004] In response to the above technical problems, the present invention proposes a method for testing the microscopic bonding properties of fibers and cement-based materials to overcome the limitations of traditional fiber pull-out tests and to achieve efficient and accurate measurement of the microscopic bonding strength of fibers and cement-based materials.

[0005] To achieve the above object, the present invention provides the following technical solutions:

[0006] A method for testing the microscopic bonding performance of fiber and cement-based material comprises the following steps:

[0007] Step 1: Place a single fiber in a casting mold, mix cement and deionized water and pour them into the mold to ensure that the cement slurry covers the fiber, vibrate, seal and cure, and then demould. The obtained sample is soaked in isopropyl ketone, taken out and air-dried to obtain a cement stone specimen;

[0008] Step 2: cutting the cement stone specimen in a direction perpendicular to the fiber length to obtain cement flakes;

[0009] Step 3: Fix the cement sheet on a metal gasket with holes to ensure that the fibers in the cement sheet are aligned with the gasket holes, and then place the whole on a nanoindentation test bench, perform in-situ imaging and calibration system by a nanoindenter to ensure that the center of the probe, the center of the gasket hole and the center of the fiber are aligned, and then perform a fiber push-out test by applying a uniformly increasing vertical load until the fiber is completely pushed out of the cement sheet, then stop the test, and record and save the load and displacement change curve of the probe;

[0010] Step 4: Based on the recorded maximum load value P k , the fiber bonding strength is calculated according to formula (1) as the main parameter for judging the microscopic bonding performance of the fiber:

[0011]

[0012] Where: τ f —Fiber bonding strength, MPa;

[0013] P max —maximum push-out load, N;

[0014] t—thickness of the specimen slice, i.e. fiber length, mm;

[0015] d—fiber diameter, mm;

[0016] Repeat the above fiber push-out test, calculate the fiber bonding strength value, and take the statistical mean value and standard deviation as the final result.

[0017] Furthermore, the fiber is steel fiber, basalt fiber or polyvinyl alcohol fiber.

[0018] Furthermore, the fiber has a diameter of 0.3 mm and a length of 40 mm.

[0019] Furthermore, the cement and deionized water are mixed according to a water-cement ratio of 0.4.

[0020] Furthermore, the curing temperature is 26° C. and the relative humidity is 40%.

[0021] Furthermore, the cutting speed is 100 rpm; the thickness of the cement sheet is 0.5 mm, and the length of a single fiber in each cement sheet is the same.

[0022] Furthermore, the loading speed of the vertical load is 50 nm / s.

[0023] Compared with the prior art, the present invention has the following advantages and technical effects:

[0024] By preparing cement stone thin slice specimens containing fibers at high throughput and combining them with a nanoindentation test system to carry out fiber push-out tests, the microscopic bonding performance of fibers and cement-based materials was accurately characterized, solving the problems of low efficiency, insufficient reliability, and high measurement accuracy required for traditional pull-out specimen preparation. This technology provides an important experimental means for in-depth research on the toughening mechanism of fibers in concrete. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings constituting a part of the present application are used to provide a further understanding of the present application. The illustrative embodiments and descriptions of the present application are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0026] Figure 1 Schematic diagram of preparation of fiber-containing cement stone slice specimen; wherein, 1-fiber-containing cement stone slice; 2-fiber; 3-metal blade;

[0027] Figure 2 Schematic diagram of fiber micro-pushing test, where 1-nanoindentation circular flat loading probe; 2-fiber; 3-cement stone matrix; 4-metal gasket;

[0028] Figure 3 The results of the microscopic push-out test of a single steel fiber, where: 1-steel fiber; 2-hardened cement stone matrix; 3-hole formed after the steel fiber is pushed out. DETAILED DESCRIPTION

[0029] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0030] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0031] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0032] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0033] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0034] The present invention provides a method for testing the microscopic bonding performance of fibers and cement-based materials, comprising the following steps:

[0035] Step 1: Preparation of fiber-containing cement paste specimens

[0036] Prepare a straight fiber and put it into the casting mold. The inner wall of the mold is coated with a release agent for easy demoulding; P.O42.5 ordinary silicate cement and deionized water are mixed and poured into the mold to fill the entire mold to ensure that the cement slurry covers the fiber. At the same time, it is placed on a vibration table for 20 seconds to remove the air inside the cement slurry, and then the lid is covered. The mold box is wrapped with a plastic film and then placed in a curing box for a period of sealed curing (maintaining a certain curing box temperature and relative humidity, the temperature is 26°C, and the relative humidity is 40%). After the curing is completed, the hardened cement stone specimen is demoulded; the specimen is immersed in isopropyl tone for 72 hours by a solvent exchange method to stop the cement stone from further hydration, and then the specimen is taken out and air-dried under natural ventilation conditions to remove the isopropyl tone;

[0037] Step 2: Preparation of fiber-containing cement stone thin slice specimens

[0038] The specimens were fixed on the fixture of the Minitom low-speed precision cutting machine, and a certain cutting distance and cutting speed were set. Then, a metal blade was used to perform multiple slow cuts perpendicular to the fiber length direction to cut the specimens into slices of a certain thickness, each of which contained a single fiber of the same length.

[0039] Step 3: Fiber Micro-Push-Out Test

[0040] Use cyanoacrylate glue to fix the bottom surface of the cement stone thin slice specimen on a perforated metal gasket to ensure that the bottom of the fiber is aligned with the gasket hole, and then place it on the KLAG200 nanoindentation test bench, where the nanoindenter is equipped with a round flat loading probe with a diameter smaller than the fiber diameter. Through the nanoindenter in-situ imaging and calibration system, ensure that the probe center, loading center and fiber center are aligned, and then perform a fiber push-out test by applying a uniformly increasing vertical load until the fiber is completely pushed out of the cement stone matrix. The test stops, and the load and displacement change curves of the probe are recorded and saved;

[0041] Step 4: Bond Strength Calculation

[0042] According to the recorded maximum load value P k , the fiber bonding strength is calculated according to formula (1) as the main parameter for judging the microscopic bonding performance of the fiber:

[0043]

[0044] Where: τ f —Fiber bonding strength, MPa;

[0045] P max —maximum push-out load, N;

[0046] t—thickness of the specimen slice, i.e. fiber length, mm;

[0047] d—fiber diameter, mm;

[0048] Repeat the above fiber push-out test for more than 10 times, calculate the fiber bonding strength value, and take the statistical average value and standard deviation as the final result.

[0049] The fiber is a steel fiber, a basalt fiber or a polyvinyl alcohol fiber. In the following embodiments of the present invention, for the sake of specific description, the following embodiments take steel fibers as an example, and the diameter of the steel fibers is 0.3 mm and the length is 40 mm.

[0050] The cement and deionized water can be compounded in a certain ratio so as to successfully prepare a test piece with composite performance. In the following embodiments of the present invention, for the purpose of specific description, the cement and deionized water are compounded according to a water-cement ratio of 0.4 (other parameters are similar and will not be repeated).

[0051] In the following embodiments of the present invention, for the purpose of specific description, the cutting speed is 100 rpm, the thickness of the cement sheet is 0.5 mm, the length of the single fiber in each cement sheet is the same, and the loading speed of the vertical load is 50 nm / s.

[0052] The invention focuses on solving the defect problem in the fiber pull-out test of the existing method for testing the microscopic bonding performance of fibers and cement-based materials.

[0053] The raw materials used in the present invention are all purchased from the market.

[0054] The technical solution of the present invention is further illustrated by the following embodiments.

[0055] Example 1

[0056] A method for testing the microscopic bonding performance of fiber and cement-based material comprises the following steps:

[0057] Step 1: Preparation of fiber-containing cement paste specimens

[0058] Prepare a steel fiber (diameter 0.3mm, length 40mm), straighten it, and then put it into the casting mold. The inner wall of the mold is coated with a release agent for easy demoulding; P.O42.5 ordinary silicate cement and deionized water are mixed in a water-cement ratio of 0.4 and poured into the mold to fill the entire mold to ensure that the cement slurry covers the fiber. At the same time, it is placed on a vibration table for 20 seconds to remove the air inside the cement slurry, and then the lid is covered, the mold box is wrapped with a plastic film, and it is placed in a curing box for sealed curing (curing conditions are: temperature 26°C, relative humidity 40%). After the curing is completed, the hardened cement stone specimen is demoulded; the specimen is immersed in isopropyl tone for 72 hours by a solvent exchange method to stop the cement stone from further hydration, and then the specimen is taken out and air-dried under natural ventilation conditions to remove the isopropyl tone;

[0059] Step 2: Preparation of fiber-containing cement stone thin slice specimens

[0060] The fiber-containing cement stone specimen was fixed on the fixture of the Minitom low-speed precision cutting machine, and a certain cutting distance (0.5 mm) and cutting speed (100 rpm) were set. Then, a metal blade was used to perform multiple slow cuts perpendicular to the fiber length direction to cut the specimen into 0.5 mm thick slices, each of which contained a single fiber of the same length (see the specific operation for details). Figure 1 );

[0061] Step 3: Fiber Micro-Push-Out Test

[0062] The bottom surface of the cement stone thin slice specimen was fixed on the perforated metal gasket using cyanoacrylate glue to ensure that the bottom of the fiber was aligned with the gasket hole, and then placed on the KLAG200 nanoindentation test bench, where the nanoindenter was equipped with a round flat loading probe whose diameter was smaller than the fiber diameter (probe diameter was 0.15 mm). The nanoindenter in-situ imaging and calibration system was used to ensure that the probe center, loading center and fiber center were aligned, and then the fiber push-out test was performed by applying a uniformly increasing vertical load (loading speed was 50 nm / s) (see Figure 2 ) until the fibers are completely pushed out of the cement matrix and the test is stopped. The test photos before and after the push-out are compared. Figure 3 As shown, the load and displacement curves of the probe are recorded and saved (the test force measurement accuracy of the nanoindentation system is 0.00001mN, and the force range can reach 10N, which fully meets the measurement accuracy requirements of the fiber micro-pullout test);

[0063] Step 4: Bond Strength Calculation

[0064] According to the recorded maximum load value P k , the fiber bonding strength is calculated according to formula (1) as the main parameter for judging the microscopic bonding performance of the fiber:

[0065]

[0066] Where: τ f —Fiber bonding strength, MPa;

[0067] P max —maximum push-out load, N;

[0068] t—thickness of the specimen slice, i.e. fiber length, mm;

[0069] d—fiber diameter, mm;

[0070] The above fiber push-out test was repeated 10 times, and the fiber bonding strength value was calculated, and the statistical mean value and standard deviation were taken as the final results.

[0071] The statistical results of the microscopic bonding performance of the steel fibers of this embodiment are shown in Table 1.

[0072] Table 1 Microscopic bonding performance results based on fiber push-out test

[0073]

[0074] The data are close to those of traditional steel fiber pull-out tests (the bond strength of steel fibers is 5.95-17.02 MPa in Table 4 of the paper titled Comparative pull-out behavior of half-hooked and commercial steel fibers embedded in UHPC under static and impact loads published by Doo-Yeol Yoo and Soonho Kim in the journal Cement and Concrete Composite in 2019, and 5.73-6.08 MPa in Table 4 of the paper titled Pull-out behavior of single steel fiber from SIFCON matrix published by Tuyan Murat and Yazici Halit in the journal Construction and Building Materials in 2012), which proves the accuracy and effectiveness of the test method of this scheme, and solves the problems of low efficiency, insufficient reliability and high measurement accuracy requirements of traditional pull-out specimen preparation.

[0075] The above are only preferred specific implementations of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A method for testing the microscopic bonding performance of fiber and cement-based materials, characterized in that: The following steps are involved: Step 1: Place a single fiber in a casting mold, mix cement and deionized water and pour them into the mold to ensure that the cement slurry covers the fiber, vibrate, seal and cure, and then demould. The obtained sample is soaked in isopropyl ketone, taken out and air-dried to obtain a cement stone specimen; Step 2: cutting the cement stone specimen in a direction perpendicular to the fiber length to obtain cement flakes; Step 3: Fix the cement sheet on a metal gasket with holes to ensure that the fibers in the cement sheet are aligned with the gasket holes, and then place the whole on a nanoindentation test bench, perform in-situ imaging and calibration system by a nanoindenter to ensure that the center of the probe, the center of the gasket hole and the center of the fiber are aligned, and then perform a fiber push-out test by applying a uniformly increasing vertical load until the fiber is completely pushed out of the cement sheet, then stop the test, and record and save the load and displacement change curve of the probe; Step 4: Based on the recorded maximum load value P k , the fiber bonding strength is calculated according to formula (1) as the main parameter for judging the microscopic bonding performance of the fiber: Where: τ f —Fiber bonding strength, MPa; P max —maximum push-out load, N; t—thickness of the specimen slice, i.e. fiber length, mm; d—fiber diameter, mm; Repeat the above fiber push-out test, calculate the fiber bonding strength value, and take the statistical mean value and standard deviation as the final result.

2. The method for testing the microscopic bonding performance of fibers and cement-based materials according to claim 1, characterized in that: The fiber is steel fiber, basalt fiber or polyvinyl alcohol fiber.

3. The method for testing the microscopic bonding performance of fibers and cement-based materials according to claim 2, characterized in that: The fiber has a diameter of 0.3 mm and a length of 40 mm.

4. The method for testing the microscopic bonding performance of fibers and cement-based materials according to claim 1, characterized in that: The cement and deionized water are mixed according to a water-cement ratio of 0.

4.

5. The method for testing the microscopic bonding performance of fiber and cement-based materials according to claim 1, characterized in that: The curing temperature is 26° C. and the relative humidity is 40%.

6. The method for testing the microscopic bonding performance of fiber and cement-based materials according to claim 1, characterized in that: The cutting speed is 100 rpm; the thickness of the cement sheet is 0.5 mm, and the length of a single fiber in each cement sheet is the same.

7. The method for testing the microscopic bonding performance of fibers and cement-based materials according to claim 1, characterized in that: The loading speed of the vertical load is 50 nm / s.

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