Joint experiment method for improving success probability of ECC performance test

Abstract

The invention relates to a combined experiment method for improving ECC performance test success probability, and belongs to the technical field of civil engineering material test and design. Aiming at the problems of low trial and error efficiency, large discreteness of experimental results and the like in the research and development of the existing high-strength and ultrahigh-ductility engineering cementitious composites (ECC), the invention provides a closed-loop experimental system based on the combination of micromechanics matching screening and macroscopic accurate verification. The method mainly comprises two stages: in a screening stage (step 1), respectively through a pure matrix three-point bending experiment and a single fiber bundle drawing experiment, acquiring matrix fracture toughness and interface bonding strength parameters, verifying an energy matching relationship according to a micromechanics criterion, if the energy matching relationship is not matched, feeding back and guiding fiber modification or matrix adjustment, and if the energy matching relationship is not matched, feeding back and guiding the interface bonding strength parameters; reverse design and pre-screening of the mix proportion are realized; in the verification stage (step 2), a special dumbbell test piece designed with a specific long scale distance (80mm effective observation area) is adopted to perform a uniaxial tensile test on the all-component material meeting the matching requirement, and end damage interference is avoided. By applying the method, the research and development period can be shortened by 40% or above, the high-performance ECC with the compressive strength larger than 135 MPa, the tensile strength larger than 10.0 MPa and the tensile strain capacity of 10%-12% is successfully guided, prepared and verified, and the test success rate and the data authenticity are remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of new composite materials technology in civil engineering, and specifically to a combined experimental method for improving the success probability of ECC performance testing. Background Technology

[0002] Ordinary concrete and high-strength concrete exhibit moderate to high compressive strength but significant brittleness. Their tensile strain capacity is very low, typically around 0.02%, making them prone to cracking and failure. This means that reinforced concrete is highly susceptible to cracking under sustained loads, and working with cracks is the norm for reinforced concrete, severely impacting the durability of concrete structures. Traditional engineering cementitious composites (ECCs) possess the core advantage of high ductility, typically with a tensile strain capacity greater than 3.0%, which can improve and compensate for the adverse effects of the brittleness of ordinary concrete. Currently, ECC performance often exhibits either moderate strength but high strain capacity, or very high strength but insufficient strain capacity. For ECC, it is difficult to simultaneously possess both high compressive strength and high tensile ductility.

[0003] The research and testing of high-performance ECC (especially those with both high strength and ultra-high ductility) currently face the following severe challenges: Low R&D efficiency: Existing R&D models mostly rely on the "trial and error method," that is, directly preparing full-component specimens for tensile testing. Due to numerous influencing factors (matrix brittleness, fiber dispersion, interfacial adhesion, etc.), it is difficult to quickly locate the cause once the results are unsatisfactory, resulting in long R&D cycles and serious material waste.

[0004] The experimental results exhibit significant dispersion: the macroscopic tensile properties of ECC (especially strain hardening behavior) are extremely sensitive to the fabrication process and specimen defects. Existing experimental methods lack pre-quantitative assessment of the fiber / matrix interface properties, and standard dumbbell specimens often fail prematurely during tensile testing due to end stress concentration, making the test results unable to accurately reflect the intrinsic properties of the material.

[0005] Lack of systematic verification methods: Existing technologies are unable to effectively predict whether a material has the potential for "saturated propagation of multiple cracks" in the early stages of material preparation, resulting in a low probability of achieving high strength and ultra-high ductility performance (literature data shows that when the compressive strength is >130MPa, the strain capacity is often difficult to exceed 6%).

[0006] Therefore, there is an urgent need for a scientific joint experimental method that combines microscopic mechanism screening with macroscopic precision testing to improve the success rate of ECC performance testing. Summary of the Invention

[0007] The purpose of this invention is to provide a combined experimental method for improving the success rate of ECC performance testing. This method establishes a closed-loop system of reverse design of mix proportions and performance verification, shortening the material development cycle by more than 40%, and ensuring the stable preparation and testing of high-performance ECC with compressive strength >135MPa and tensile strain >10%.

[0008] The specific technical solution is as follows: A joint experimental method for improving the success rate of ECC performance testing. This method includes step one: matrix and fiber compatibility design, and step two: precise uniaxial tensile verification. Step one, by quantifying the matching relationship between fiber / matrix interfacial bond strength and matrix fracture toughness, pre-screens the constituent materials of ECC and reverse-engineers the mix proportions, including the following sub-steps: (a) Prepare an ECC matrix slurry without any fibers, conduct a three-point bending test on a notched beam, and determine the fracture toughness of the matrix. The formula for calculating the matrix fracture toughness Km is: (1) Among them, P S : Peak load in the three-point bending test; S: Span of the notched beam specimen; B: Width of the specimen cross-section; W: Height of the specimen cross-section; a: Length of the notch; (b) Prepare a matrix slurry with the same proportions as in sub-step (a), embed the fiber bundle to be tested in it, and ensure that the matrix slurry does not contain any other randomly distributed fibers. Perform a fiber bundle pull-out test and determine the equivalent interfacial bond strength of the fiber bundle. and the interfacial bond strength of single fiber / matrix ; Fiber bundle equivalent interfacial bond strength τ b The calculation formula is (2) P max Peak load in fiber bundle pull-out test; d b : Fiber bundle diameter; l b : Fiber bundle embedding length; Single fiber / matrix interfacial bond strength τ 0 and fiber bundle equivalent interfacial bond strength τ b The relationship is (3) In formula (3) k sThe bonding strength scale factor was obtained through fiber bundle tensile testing. (c) Based on the micromechanical design theory, verify whether the matching relationship between the fiber / matrix interfacial bond strength and the matrix fracture toughness is satisfied. The matching relationship is shown in formula (4): (4) In the formula k (opt) The optimization coefficient was obtained through experimental data; for PE fiber, the optimization coefficient was... k (opt) The empirical value range is 0.15-0.40; E f Fiber elastic modulus; E m : Matrix elastic modulus; d f : Fiber diameter; (d) If the matching relationship is not satisfied, adjust the proportion of matrix components or modify the fiber surface according to the direction of deviation, and repeat the above sub-steps (a) to (c) until the matching relationship is satisfied. The second step involves preparing an ECC mixture incorporating randomly distributed fibers, under the premise that the matching relationship is satisfied in the first step. A dumbbell specimen of a specific size is then made for a uniaxial tensile test to verify its high strength and ultra-high ductility performance.

[0009] The matrix material includes cement, fly ash, silica fume, slag powder, nano-silica, water, fine silica sand, and water-reducing agent. The fiber system is a mixture of polyethylene fiber and steel fiber, and the component ratios meet the following requirements: The mass ratio of cement:fly ash:silica fume:slag powder is 0.54:0.21:0.17:0.08; The amount of nano-silica added is 0.8%-1.6% of the total mass of the cementitious material; The water-to-binder ratio is 0.16; The sand-to-rubber ratio is 0.30; The volumetric content of PE fiber is 1.8%-2.0%, and the volumetric content of steel fiber is 0.5%-0.7%. ECC preparation method: (a) Dry mix cement, fly ash, silica fume, slag powder, and fine silica sand at 140±5 rpm for 2-3 minutes; (b) Mix nano-silica, water-reducing agent and water, and then disperse the mixture using ultrasonication to form a suspension; (c) Add the suspension to the dry mixture and stir at low speed for 3-5 minutes to form a uniform slurry; (d) Add PE fiber and steel fiber in batches and mix at low speed for 3 minutes; (e) Mix at high speed (285±5 rpm) for 3 minutes; (f) Pour the fresh mixed slurry into the mold and cast it into shape.

[0010] The three-point bending test: The fracture toughness Km of the matrix can be obtained by a three-point bending test of a notched beam in the matrix material. The material used for the test specimen is an ECC matrix mixture without added fibers. The mixing ratio of the slurry mixture material of the specimen is the same as the ECC matrix mixing ratio. The preparation method of the slurry mixture material of the specimen adopts the ECC preparation method, except that the fiber addition operation of the above preparation method is removed.

[0011] The fiber bundle pull-out test: The equivalent interfacial bond strength of fiber bundles can be obtained through fiber bundle pull-out tests. τ b The matrix material of the fixed fiber bundle in this experimental specimen does not contain fibers. The mixing ratio of the slurry mixture material of this specimen is the same as the matrix mixing ratio of ECC. The preparation method of the slurry mixture material of this specimen adopts the ECC preparation method, except that the fiber addition operation of the above preparation method is removed.

[0012] If the fiber / matrix interfacial bond strength and matrix fracture toughness satisfy formula (4), continue preparing the dumbbell specimen. If not, adjust the matrix mix ratio as follows: If the fiber / matrix interfacial bond strength... τ If the value of 0 is too large and exceeds the empirical range, the relative mass ratios of fly ash, slag powder, and nano-silica need to be adjusted using orthogonal experimental methods; if the fiber / matrix interfacial bonding strength... τ When the value of 0 is too small and below the empirical range, surface modification treatment can be performed on the PE fiber.

[0013] The surface modification treatment method for PE fibers is as follows: (a) Prepare a solution of KH550: the solvent is a solution of water and ethanol, the volume ratio of ethanol to water in the solution is 9:1, and the mass ratio of silane added to the ethanol and water mixture is 10%; (b) Stir the mixture thoroughly for 30 minutes. From the observations, the hydrolysis process of KH-550 initially produces opaque or flocculent substances. After thorough stirring, the solution gradually becomes colorless and transparent, indicating that the hydrolysis of KH-550 is complete. (c) Place the PE fiber into the KH-550 hydrolysis solution, with a mass ratio of PE fiber to KH-550 of 3:100. Stir and react for about 30 minutes, then remove the PE fiber.

[0014] (d) Wash the PE fiber with tap water 2-3 times, dry the PE fiber indoors for 4 hours, and then dry it in an oven at 50℃-55℃ for 48 hours to ensure that the coupling agent is firmly bonded to the fiber.

[0015] Step 2: A uniaxial tensile test is performed on the dumbbell specimen. The specimen material is an ECC mixture, and its preparation method is completely in accordance with the ECC preparation method described above. Homemade mold: The mold for the dumbbell specimen is made with a width of 30mm, a thickness of 13mm, and a length of 120mm in the middle stretching section. The middle 80mm is taken as the gauge length, and 20mm is left at each end. This can avoid stress concentration within the 20mm range at each end during stretching. Homemade fixtures: Two trapezoidal fixtures are used to fix the specimen and are connected to the testing machine. The sharp corners of the stress concentration parts of the fixtures are all ground into rounded corners to ensure that the fixtures and the specimens are in surface contact during tensile testing, avoiding stress concentration caused by line contact. Electronic extensometer: The test specimen is loaded using a microcomputer-controlled electronic universal testing machine. The testing machine is equipped with an electronic extensometer, which can be clamped on the specimen to measure the elongation of the gauge length. Before the experiment, the relative positions of the specimen clamp, the specimen, and the extensometer need to be adjusted to ensure that the dumbbell specimen is subjected to axial tensile loading.

[0016] Technical effects: Significantly improve the compliance rate: Through the pre-screening in the first stage, a large number of formulas that are bound to fail are eliminated, ensuring that the materials entering the second stage have theoretically possess ultra-high ductility potential.

[0017] Eliminating experimental errors: The design of "long specimen, short gauge length" in Phase 2 effectively avoids the interference of stress concentration at the clamping end on the test results, making the measured tensile strain data more realistic and stable.

[0018] Highly instructive: This method is not only a testing approach but also a tool for materials design, guiding the successful development of materials such as high-strength, ultra-high-ductility HP-ECC (strength > 135 MPa, strain > 10%). Attached Figure Description

[0019] Figure 1 : Schematic diagram of the particle size distribution of the matrix material.

[0020] Figure 2 Appearance of raw materials (cement, fly ash, silica fume, silica sand, slag powder, nano-SiO2, PE fiber, steel fiber).

[0021] Figure 3 : Schematic diagram of a three-point bending test of a notched beam (used to determine the fracture toughness of the matrix).

[0022] Figure 4 : Schematic diagram of fiber bundle pull-out test (used to determine interfacial bond strength).

[0023] Figure 5 Schematic diagram of uniaxial tensile testing apparatus, extensometer arrangement, and specimen dimensions.

[0024] Figure 6 Tensile stress-strain curves of ECC specimens in embodiments of the present invention.

[0025] Figure 7 Photographs of HP-ECC crack development mode and multi-saturation crack morphology.

[0026] Figure 8 Microscopic analysis (SEM) images of crack morphology and fiber failure in uniaxial tensile specimens. Detailed Implementation

[0027] The present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings. However, it should be noted that these embodiments are not intended to limit the present invention. Any equivalent changes or substitutions in function, method, or structure made by those skilled in the art based on these embodiments are within the scope of protection of the present invention.

[0028] (1) Matrix design The matrix material of this high-strength, ultra-high-ductility engineering cementitious composite material (HP-ECC) includes cement, water, fly ash, silica fume, slag powder, nano-silica, and a water-reducing agent. The cement used is 52.5R ordinary Portland cement, and the fly ash used is type F fly ash. The mass ratio of cement to each supplementary cementitious material is cement:fly ash:silica fume:slag powder = 0.54:0.21:0.17:0.08. The nano-silica (NS) content is 0.8%-1.6% (by mass of total cementitious materials), and the water-cement ratio is 0.16-0.18. The fine silica sand has a maximum particle size of less than 0.30 mm, an average particle size of 0.155 mm, and a sand-cement ratio of 0.30. The polycarboxylate high-performance water-reducing agent used is added at a dosage of 1.5%. The particle size distribution of each material and the raw materials are shown in the attached figure. Figure 1 Appendix Figure 2 As shown.

[0029] Silica fume has the finest particle size (except for nano-silica), belonging to ultrafine powder. The use of silica fume can make the matrix more dense, and at the same time, a lower water-cement ratio is beneficial for improving ECC strength. Nano-silica has a very small specific surface area, exhibiting excellent filling effect; fly ash has a particle size of less than 0.050 mm. Due to the lubricating effect of its spherical particles, fly ash can improve ECC workability. The combined effect of fly ash and nano-silica can partially replace cement, which is beneficial for solid waste resource utilization and ecological environmental protection; it helps improve the fracture toughness of the matrix, compensating for the inherent brittleness tendency of high-strength matrices; it helps promote uniform fiber dispersion and improve fiber / matrix interfacial bonding, laying a good foundation for the full play of fiber bridging effect during tensile testing.

[0030] The rationality of the matrix material ratio can be verified and judged through the matrix material three-point bending test and fiber bundle pull-out test (see Experimental Method - Step 1 for details). To ensure the successful preparation of ECC, three key links need to be grasped. This is the first key link, namely, key link one: ensuring the rationality of the matrix material ratio.

[0031] (2) Fiber system design Due to its high density, steel fiber facilitates the uniform dispersion of PE fiber during stirring. The PE fiber composition is 50% 12mm long and 50% 18mm long, ensuring uniform fiber dispersion and preventing clumping, while also increasing the fiber embedment length during cracking and improving fiber bridging ability. Adding an appropriate amount of nano-silica improves the frictional bonding strength between the fiber and the matrix, further enhancing fiber bridging ability. Optimizing the processability of ECC manufacturing further promotes uniform fiber dispersion.

[0032] Table 1 Physical properties of PE fiber and steel fiber

[0033] (3) Preparation method The standard procedure for ECC preparation is as follows: (a) In a planetary cement mortar mixer, mix the main dry powder materials such as cement, fly ash, silica fume, slag powder and silica sand at a speed of 140±5 rpm for 2-3 minutes. (b) Take another container, pour nano silica (NS) and high-efficiency water-reducing agent into water, stir thoroughly or vibrate ultrasonically to ensure that NS is evenly dispersed in the suspension; (c) Then, add the suspension to the fully mixed dry powder material and continue to mix and stir at low speed for 3-5 minutes until it is uniformly mixed and has a slurry-like consistency; (d) Under low-speed mixing conditions, PE fibers and steel fibers are added in batches and dispersed, and mixed for 3 minutes; (e) Then, mix at a high speed of 285±5 rpm for 3 minutes to ensure uniform fiber dispersion; (f) Finally, pour the freshly mixed slurry into the mold.

[0034] Key details: Detail 1: To promote better fiber dispersion and prevent clumping, the PE fibers in step (d) can be first added to the dry powder material mixture in step (a) and mixed together until uniform. Detail 2: In step (b), in order to promote better dispersion of NS, an ultrasonic instrument can be used to vibrate the suspension with a small amplitude for about half an hour. Detail 3: The total time from adding the suspension to mixing and pouring the slurry into the mold should be controlled within 15 minutes. Detail 4: In step (f), when pouring the fresh mixed slurry into the dumbbell mold, it should be directional, that is, guided from one end of the mold to the other end, and then placed on a standard cement mortar vibrating table for compaction.

[0035] Key step two: Ensure the uniform dispersion of PE fibers.

[0036] (4) Experimental method - Step 1 The experimental method consists of two steps. Step one involves conducting a three-point bending test on the matrix material and a fiber bundle pull-out test to quantify the matching relationship between the fiber / matrix interfacial bond strength and the matrix fracture toughness, and to design the mix proportion of the ultra-high ductility ECC matrix material. Satisfying this matching relationship provides a prerequisite for the excellent strain hardening behavior of ECC. This experimental method can improve the probability of achieving the high-strength and ultra-high ductility properties of ECC and shorten the material development cycle by more than 40%.

[0037] (a) Three-point bending test Three-point bending test of a notched beam in a matrix material (attached) Figure 3 The matrix fracture toughness Km can be obtained. The material used in this experiment is an ECC matrix mixture without added fibers (the mixing ratio of the slurry mixture material is the same as the ECC matrix mixing ratio). The preparation method of the slurry mixture material is detailed in Part (4) above, except that the fiber addition operation in Part (4) above is removed. The formula for calculating the matrix fracture toughness Km is: (1) P S Peak load in a three-point bending test; S : The span of the beam specimen with notch; B : Width of the specimen cross section; W : Height of the specimen cross section;a : Gap length.

[0038] (b) Fiber bundle pull-out test like Figure 4 (a) A single PE fiber bundle is a "ultra-high molecular weight polyethylene fiber multifilament" with a specification of 1600D / 360F. "1600D" indicates the fiber fineness, meaning that 9000 meters of fiber weigh 1600 grams. Here, "D" is the unit of fineness, representing the thickness of the fiber, also known as denier. "F" refers to the number of strands; nylon and filaments generally have strands. 360F indicates that this fiber bundle consists of 360 finer fibers. These "finer fibers," if cut into 12mm or 18mm lengths, are the PE fibers dispersed within the ECC matrix, such as... Figure 2 (g) diagram.

[0039] Fiber bundle equivalent interfacial bond strength τ b This refers to "a single PE fiber bundle" according to... Figure 4 The measured frictional bond strength at the fiber bundle / matrix interface; Single fiber / matrix interfacial bond strength τ 0: This is the calculated frictional bond strength at the fiber / matrix interface of a "finer fiber".

[0040] First, through fiber bundle pull-out experiments (see attached) Figure 4 The equivalent interfacial bond strength of the fiber bundle can be obtained. τ b The matrix material of the specimen that fixes the fiber bundle does not contain fibers (the mixing ratio of the slurry mixture material of the specimen is the same as the matrix mixing ratio of ECC). The preparation method of the slurry mixture material of the specimen is detailed in the preparation method of Part (4) above, except that the fiber addition operation of the preparation method of Part (4) above is removed. Fiber bundle equivalent interfacial bonding strength τ b The calculation formula is (2) P max Peak load in fiber bundle pull-out test; d b : Fiber bundle diameter; l b : Fiber bundle embedding length.

[0041] Then, the interfacial bond strength of the single fiber / matrix τ 0 and fiber bundle equivalent interfacial bond strength τ b The relationship is (3) In formula (3) k s The bonding strength metric is obtained through fiber bundle tensile testing. In this embodiment of the invention, k s The value is determined to be 0.76.

[0042] (c) Matching relationship Matrix cracking stress σ c The relationship between the matrix fracture toughness and the fiber / matrix bond strength is as follows:

[0043] V f : Fiber volume fraction; E f Fiber elastic modulus; τ : Fiber / matrix interfacial bond strength; δ c Critical opening displacement; d f : Fiber diameter; a 0: Initial defect size; E m : Matrix elastic modulus; G fm Matrix fracture energy; f tm : Tensile strength of the matrix (excluding fibers).

[0044] The matching relationship between the ECC fiber / matrix interfacial bond strength and the matrix fracture toughness is shown in formula (4): (4) In the formula k (opt) The optimization coefficients were obtained through experimental data. For PE fiber, the optimization coefficients were... k (opt) The empirical value range is 0.15-0.40; in this embodiment of the invention, the optimization coefficient... k (opt) The numerical range is 0.35-0.40.

[0045] Based on experimental step one, if the fiber / matrix interface bonding strength and matrix fracture toughness satisfy formula (4), then continue with experimental step two; if not, the matrix mix ratio needs to be adjusted, and the above preparation and experimental step one steps are repeated until formula (4) is satisfied. Through this experimental step one, the mix ratio design and control of ultra-high ductility ECC matrix material are realized.

[0046] If these conditions are not met, cement-based cementitious composite materials mostly belong to τ When the value of 0 is too large, for τ A value that is too small (e.g., 0) is rare. This is important for the fiber / matrix interfacial bond strength. τ When the value of 0 is too large and exceeds the empirical range, the corresponding method for adjusting the matrix mix ratio is as follows: the relative mass ratios of fly ash, silica fume, slag powder, and nano silica need to be adjusted, and an orthogonal experimental design method should be used for adjustment.

[0047] Orthogonal experimental design considers three factors: Factor A: Fly ash content (levels: 19%, 21%, 23%); Factor B: Silica content (levels: 15%, 17%, 19%); Factor C: Nano silica doping (levels: 0.8%, 1.2%, 1.6%).

[0048] Table * shows the orthogonal experimental design. In the cementitious materials, the cement content is fixed at 54%, the fly ash and silica fume contents are two factor variables, the slag powder content is calculated, and the nano silica content is also a factor variable.

[0049] Table * Orthogonal Experimental Design Table

[0050] After adjusting the matrix mix ratio according to Table*, repeat the above preparation and experimental steps for each mix ratio specimen until the fiber / matrix interface friction strength and matrix fracture toughness satisfy formula (4).

[0051] If the fiber / matrix interfacial bond strength τ When the value of 0 is too small, below the empirical range, surface modification treatment of PE fibers can be used for adjustment. In fact, adding an appropriate amount of nano-silica can improve the adhesion between the PE fiber surface and the matrix. In addition, silane coupling agent KH550 can be used to modify the surface of PE fibers.

[0052] Silane coupling agent KH550 is aminopropyltriethoxysilane. Surface modification of hydrophobic PE fibers using a KH550 solution results in improved strain hardening behavior (ECC) of the modified PE fibers. The mechanism by which modified PE fibers influence ECC strain hardening is that the functional groups of the silane coupling agent attached to the PE fiber surface establish a strong chemical bond with the matrix. The free hydroxyl groups (—OH) undergo condensation reactions with the hydration product CSH in the matrix, and the hydroxyl groups (—OH) coordinate with Ca2+ in Ca(OH)2. The PE fiber surface modification technology is described below.

[0053] (a) Prepare a solution of KH550: the solvent is a solution of water and ethanol, the volume ratio of ethanol to water in the solution is 9:1, and the mass ratio of silane added to the ethanol and water mixture is 10%; (b) Stir the mixture thoroughly for 30 minutes. From the observations, the hydrolysis process of KH-550 initially produces opaque or flocculent substances. After thorough stirring, the solution gradually becomes colorless and transparent, indicating that the hydrolysis of KH-550 is complete. (c) Place the PE fiber into the KH-550 hydrolysis solution, with a mass ratio of PE fiber to KH-550 of 3:100. Stir and react for about 30 minutes, then remove the PE fiber.

[0054] (d) Wash the PE fiber with tap water 2-3 times, dry the PE fiber indoors for 4 hours, and then dry it in an oven at 50℃-55℃ for 48 hours to ensure that the coupling agent is firmly bonded to the fiber.

[0055] (5) Experimental method - Step 2 Experimental Step Two: The dumbbell specimen (attached) Figure 5 A uniaxial tensile test was conducted on the specimen material, which was an ECC mixture (with added fibers), and its preparation method was carried out entirely according to the preparation method in Part (3). The key technology is to ensure the accuracy of the experiment and obtain various data on the tensile properties of ECC through the uniaxial tensile test.

[0056] Self-made mold: The dimensions of the middle tensile section of a typical dumbbell specimen mold are: 80mm long, 30mm wide, and 13mm thick, with the gauge length being the length of the tensile section (80mm). The self-made dumbbell specimen mold of this invention has the same width and thickness of the middle tensile section as a typical dumbbell specimen mold, except that the length of the tensile section is 120mm. The middle 80mm is designated as the gauge length, with 20mm remaining at each end. This allows for stress concentration within the 20mm range at each end during tensile testing. For example, if numerous tensile cracks appear within the 20mm range at both ends, while there are no tensile cracks or very few cracks in the middle gauge length, the specimen is considered unqualified. Only when a large number of cracks appear in the gauge length can the specimen be considered qualified. Thus, the self-made mold places more stringent requirements on experimental precision.

[0057] Homemade fixtures: Two trapezoidal fixtures are used to fix the specimen and are connected to the testing machine. The sharp corners of the stress concentration parts of the fixtures are all ground into rounded corners to ensure that the fixtures and the specimens are in surface contact during tensile testing, avoiding stress concentration caused by line contact.

[0058] Electronic extensometer: A microcomputer-controlled electronic universal testing machine is used to load the specimen. The testing machine is equipped with an electronic extensometer, which can be clamped onto the specimen to measure the elongation of the gauge length. Before the experiment, the relative positions of the specimen clamp, the specimen, and the extensometer need to be adjusted to ensure that the dumbbell specimen is subjected to axial tensile loading.

[0059] Experimental Methods – Step Two highlights the third key step, namely, key step three: ensuring the accuracy of the experimental scheme and process for the uniaxial tensile test.

[0060] (6) Performance characteristics After a standard curing period of 28 days, the key performance indicators of the high-strength, ultra-high-ductility engineering cementitious composite material of the present invention are as follows.

[0061] Compressive strength: >135MPa, higher than traditional ECC; Tensile strength: >10.0MPa, higher than traditional ECC; Tensile strain capacity: 10%-12%, significantly higher than traditional ECC, and twice the strain capacity of traditional ECC (3%-5%); Multiple saturation cracking characteristics: number of cracks > 90 within an 80mm gauge length, average crack width < 85μm under maximum stress.

[0062] As attached Figure 6-8 As shown, this high-strength, ultra-high-ductility ECC exhibits significant strain hardening behavior and multiple saturation crack characteristics under tensile loading.

[0063] Comparative experiment: (1) Example The main raw materials used in high-strength, ultra-high-ductility engineering cementitious composites (HP-ECC) are as follows, including their types, specifications, quantities, and brands.

[0064] White PE fiber, specifications 12mm-0.024mm, 18mm-0.024mm, 2.0kg, Hubei Jiateng Textile; Steel fiber, specifications 13mm-0.20mm, 5.0kg; Ordinary Portland cement, 52.5mm specification, 2 bags, Zhucheng Jiuchang Building Materials; Nano silica, specification 20 nanometer hydrophilic silica, 1.0 kg, Hebei Weiyao New Materials.

[0065] One bag of fly ash, two bags of silica fume, and one bag of mineral powder; Fine silica sand, 0.155mm, 2 bags, 40.0kg.

[0066] Table 2 shows the component proportions in the examples and series of comparative tests of high-strength, ultra-high-ductility engineering cementitious composite material (HP-ECC). The cement content in the examples is 54%, and the cement content in the comparative test examples is 64%.

[0067] Table 2 Component proportions of the embodiments of the present invention

[0068] Note: In the table, (*) indicates an embodiment of the present invention.

[0069] The process parameters for preparing high-strength, ultra-high-ductility engineering cementitious composite materials (HP-ECC) are as follows.

[0070] Water-to-binder ratio: The water-to-binder ratios were 0.16 and 0.18 respectively. Tests were conducted on each formulation in Table 1 under different water-to-binder ratios. The water-to-binder ratio in this embodiment of the invention is 0.16. Fiber content: The volume ratio of PE fiber is 1.8%-2.0%, of which 12mm fiber and 18mm fiber are used in equal proportions, and the volume ratio of steel fiber is 0.5%-0.7%. Sand-to-rubber ratio: 0.30 for both. Stirring time: The process flow is described in the "Preparation Method" of the invention. From the addition of the suspension to the pouring of the slurry into the mold, this operation process is controlled within 15 minutes. Curing conditions: Immediately after the specimen is molded and smoothed, its surface is covered with plastic film to maintain the surface humidity. After molding, it is left to stand in the laboratory for 24 hours (not exceeding 48 hours). After standing, it is numbered, marked, and demolded. Immediately after demolding, the specimen is placed in a standard curing room with a temperature of 20℃±2℃ and a relative humidity of 95% or higher for curing. The curing period is 28 days.

[0071] (2) Comparative Example Table 3 compares the components and properties of the embodiments and comparative examples of the present invention. Generally, as in references [1] and [2], the compressive strength of ECC with polyvinyl alcohol fiber (PVA) as the reinforcing fiber is between 40-70 MPa, and the tensile strain is in the range of 1.5%-5.0%. The ordinary PE-ECC represented by reference [3] has a certain performance difference from the embodiments of the present invention. The ECC component of reference [4] has excellent performance, only the strain capacity is slightly smaller than that of the embodiments of the present invention, but its water-cement ratio is very small, only 0.14. The low water-cement ratio may have an adverse effect on the working performance of ECC. Reference [5] incorporates nano silica (NS) into ECC. The combined action of NS and fly ash can partially replace cement, and its performance has a certain difference from the embodiments of the present invention. The components in reference [6] have excellent performance, but the compressive strength is slightly lower than that of the present invention. The amount of cement used is higher than that of the present invention. It uses coarse-grained river sand with a maximum particle size of 4.75 mm, while the thickness of the dumbbell specimen in the tensile test is 13 mm, which increases the randomness of the tensile properties and makes them difficult to control. References [7] and [8] both use a mixture of PE fiber and steel fiber to reinforce ECC, which achieves ultra-high compressive strength, but its tensile strain capacity is small, only 5.2% and 5.1%. Reference [9] is an invention patent. Compared with the present invention, it has a high amount of cement, low compressive strength, and small strain capacity. Reference

[10] is an invention patent. Compared with the present invention, its compressive strength is slightly lower, but its cement content is low, which can be considered equivalent to the present invention, but its strain capacity is small.

[0072] Table 3 Comparison of components and performance between the embodiments and comparative examples of the present invention

[0073] Note: In the table, cement content is the mass percentage of cement in the total cementitious binder; PVA is polyvinyl alcohol fiber; (PE) represents polyethylene PE fiber; (S) represents steel fiber.

[0074] Comparative literature: [1]YL Li, W. Thielemans, Q. Yuan, JB Li, PVA fiber reinforcedcement composites with calcined cutter soil mixing residue as a partialcement replacement, Construct. Build. Mater. 326 (2022) 126924. [2]J.P. Zhuang, S.Zh. Shen, Y. Yang, K. Xu, P.J. Ni, Mechanicalperformance of basalt and PVA fiber reinforced hybrid-fiber engineeredcementitious composite with superimposed basalt fiber content, Construct.Build. Mater. 353 (2022) 129183. [3]W.H. Mao, J.P. Liu, Y. Ding, High-modulus and low-shrinkagehybrid-fiber reinforced engineered cementitious composites (ECC), Mater.Struct. (2022) 55:87. [4]K.Q. Yu, J.T. Yu, J.G. Dai, Z.D. Lu, S.P. Shah, Development ofultra-high performance engineered cementitious composites using polyethylene(PE) fibers, Construct. Build. Mater. 158 (2018) 217-227. [5]Zh.G. Zhang, Zh.P. Li, J.L. He, X.M. Shi, High-strength engineeredcementitious composites with nanosilica incorporated: Mechanical performanceand autogenous self-healing behavior, Cement Concr. Compos. 135(2023),104837. [6]Y.Zh. Li, [7]BT Huang, KF Weng, JX Zhu, Y. Xiang, JG Dai, Victor C. Li.Engineered / strain-hardening cementitious composites (ECC / SHCC) with an ultra-high compressive strength over 210 MPa, Composites Communications. 26 (2021)100775. [8]TA Liu, YZ Yang, Zh.T. Chen, Y.Zh. Li, RX Bai, Optimization of fiber volume fraction to enhance reinforcing efficiency in hybrid fiberreinforced strain hardening cementitious composite, Cement Concr. Compos. 113(2020) 103704. [9] Patent, Publication No.: CN119638311A, Title: A high-strength and high-ductility cement-based composite material and its preparation method and application.

[10] Patent, Publication No.: CN118324473A, Title: A high-strength and high-ductility concrete incorporating PE fiber and its preparation method.

[0075] (3) Performance test data The tensile stress-strain curves of embodiments of the present invention are shown in the appendix. Figure 4 As shown, the test data and index calculation results for the performance characteristics of each embodiment are shown in Table 4.

[0076] Table 4 Compressive strength and tensile ductility properties of each embodiment

[0077] In the embodiments, both Embodiments 2 and 3 of the present invention achieved the performance characteristics of compressive strength > 135 MPa, tensile strength > 10.0 MPa, and strain capacity of 10%-12%, and the cement content in the mix proportion was at the lower limit (54%). Compared with Comparative Test 2, Embodiment 2 showed a 5.3% decrease in compressive strength and a 10.7% decrease in tensile strength, but a 17.5% increase in strain capacity and a 10% reduction in cement content; compared with Comparative Test 3, Embodiment 3 showed an 8.7% decrease in compressive strength and a 5.7% decrease in tensile strength, but a 9.4% increase in strain capacity and a 10% reduction in cement content. Comparing the compressive and tensile strengths of Embodiments 2 and 3 of the present invention with “Comparative Test 1” in Table 4, the values ​​are comparable, indicating that in the embodiments of the present invention, 0.8%-1.6% nano-silica and 10% fly ash increment (by mass) can replace 15.6% of cement. (64%-54%) / 64%=15.6%. Meanwhile, compared with the strain capacity of "Comparative Test 1", the strain capacity of Embodiments 2 and 3 of the present invention increased from 9.29% to 11.55%-10.94%, an increase of 17.8%-24.3%. A tensile strain of 9.29% is already very high; further increasing it to 11.55% is quite difficult. The data shows that the material of the embodiments of the present invention simultaneously achieves high strength and ultra-high ductility, with a low cement content, and its performance is significantly better than the comparative material. The HP-ECC prepared by the present invention achieves a simultaneous improvement in strength and strain capacity, possessing high compressive and tensile strength, excellent crack control ability, and ultra-high toughness energy dissipation capacity. It demonstrates beneficial effects and potential application advantages in improving the seismic and crack resistance of structures, extending service life, applying to complex structural components, and reducing maintenance costs.

[0078] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0079] In the above embodiments of the present invention, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0080] Although embodiments of the invention have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. A joint experimental method for improving the success probability of ECC performance testing, characterized in that: The method includes step one: matrix and fiber matching design, and step two: uniaxial tensile precision verification; Step one involves pre-screening and reverse-engineering the mix proportions of ECC components by quantifying the matching relationship between the fiber / matrix interfacial bond strength and the matrix fracture toughness. This includes the following sub-steps: (a) Prepare an ECC matrix slurry without any fibers, conduct a three-point bending test on a notched beam, and determine the fracture toughness of the matrix. The formula for calculating the matrix fracture toughness Km is: （1） Among them, P S : Peak load in the three-point bending test; S: Span of the notched beam specimen; B: Width of the specimen cross-section; W: Height of the specimen cross-section; a: Length of the notch; (b) Prepare a matrix slurry with the same proportions as in sub-step (a), embed the fiber bundle to be tested in it, and ensure that the matrix slurry does not contain any other randomly distributed fibers. Perform a fiber bundle pull-out test and determine the equivalent interfacial bond strength of the fiber bundle. and the interfacial bond strength of single fiber / matrix ; Fiber bundle equivalent interfacial bond strength τ b The calculation formula is （2） P max Peak load in fiber bundle pull-out test; d b : Fiber bundle diameter; l b : Fiber bundle embedding length; Single fiber / matrix interfacial bond strength τ 0 and fiber bundle equivalent interfacial bond strength τ b The relationship is （3） In formula (3) k s The bonding strength scale factor was obtained through fiber bundle tensile testing. (c) Based on the micromechanical design theory, verify whether the matching relationship between the fiber / matrix interfacial bond strength and the matrix fracture toughness is satisfied. The matching relationship is shown in formula (4): （4） In the formula k (opt) The optimization coefficient was obtained through experimental data; for PE fiber, the optimization coefficient was... k (opt) The empirical value range is 0.15-0.40; E f Fiber elastic modulus; E m : Matrix elastic modulus; d f : Fiber diameter; (d) If the matching relationship (4) is not satisfied, adjust the proportion of matrix components or modify the fiber surface according to the deviation direction, and repeat the above sub-steps (a) to (c) until the matching relationship is satisfied; The second step involves preparing an ECC mixture incorporating randomly distributed fibers, under the premise that the matching relationship is satisfied in the first step. A dumbbell specimen of a specific size is then made for a uniaxial tensile test to verify its high strength and ultra-high ductility performance.

2. The combined experimental method according to claim 1, characterized in that, The matrix material includes cement, fly ash, silica fume, slag powder, nano-silica, water, fine silica sand, and water-reducing agent. The fiber system is a mixture of polyethylene fiber and steel fiber, and the component ratios meet the following requirements: The mass ratio of cement:fly ash:silica fume:slag powder is 0.54:0.21:0.17:0.08; The amount of nano-silica added is 0.8%-1.6% of the total mass of the cementitious material; The water-to-binder ratio is 0.16; The sand-to-rubber ratio is 0.30; The volumetric content of PE fiber is 1.8%-2.0%, and the volumetric content of steel fiber is 0.5%-0.7%. ECC preparation method: (a) Dry mix cement, fly ash, silica fume, slag powder, and fine silica sand at 140±5 rpm for 2-3 minutes; (b) Mix nano-silica, water-reducing agent and water, and then disperse the mixture using ultrasonication to form a suspension; (c) Add the suspension to the dry mixture and stir at low speed for 3-5 minutes to form a uniform slurry; (d) Add PE fiber and steel fiber in batches and mix at low speed for 3 minutes; (e) Mix at high speed (285±5 rpm) for 3 minutes; (f) Pour the fresh mixed slurry into the mold and cast it into shape.

3. The combined experimental method according to claim 2, characterized in that, The three-point bending test: The fracture toughness Km of the matrix can be obtained by a three-point bending test of a notched beam in the matrix material. The material used for the test specimen is an ECC matrix mixture without added fibers. The mixing ratio of the slurry mixture material of the specimen is the same as the ECC matrix mixing ratio. The preparation method of the slurry mixture material of the specimen adopts the ECC preparation method, except that the fiber addition operation of the above preparation method is removed.

4. The combined experimental method according to claim 2, characterized in that, The fiber bundle pull-out test: The equivalent interfacial bond strength of fiber bundles can be obtained through fiber bundle pull-out tests. τ b The matrix material of the fixed fiber bundle in this experimental specimen does not contain fibers. The mixing ratio of the slurry mixture material of this specimen is the same as the matrix mixing ratio of ECC. The preparation method of the slurry mixture material of this specimen adopts the ECC preparation method, except that the fiber addition operation of the above preparation method is removed.

5. The combined experimental method according to claim 2, characterized in that, Step (d) specifically includes: If the fiber / matrix interfacial bond strength and matrix fracture toughness satisfy formula (4), continue preparing the dumbbell specimen. If not, adjust the matrix mix ratio as follows: If the fiber / matrix interfacial bond strength... τ If the value of 0 is too large and exceeds the empirical range, the relative mass ratios of fly ash, slag powder, and nano-silica need to be adjusted using orthogonal experimental methods; if the fiber / matrix interfacial bonding strength... τ When the value of 0 is too small and below the empirical range, surface modification treatment can be performed on the PE fiber.

6. The combined experimental method according to claim 5, characterized in that, The surface modification treatment method for PE fibers is as follows: (a) Prepare a solution of KH550: the solvent is a solution of water and ethanol, the volume ratio of ethanol to water in the solution is 9:1, and the mass ratio of silane added to the ethanol and water mixture is 10%; (b) Stir the mixture thoroughly for 30 minutes. From the observations, the hydrolysis process of KH-550 initially produces opaque or flocculent substances. After thorough stirring, the solution gradually becomes colorless and transparent, indicating that the hydrolysis of KH-550 is complete. (c) Place the PE fiber into the KH-550 hydrolysis solution, with a mass ratio of PE fiber to KH-550 of 3:

100. Stir and react for about 30 minutes, then remove the PE fiber. (d) Wash the PE fiber with tap water 2-3 times, dry the PE fiber indoors for 4 hours, and then dry it in an oven at 50℃-55℃ for 48 hours to ensure that the coupling agent is firmly bonded to the fiber.

7. The combined experimental method according to claim 2, characterized in that, Step 2: A uniaxial tensile test is performed on the dumbbell specimen. The specimen material is an ECC mixture, and its preparation method is completely in accordance with the ECC preparation method described above. Homemade mold: The mold for the dumbbell specimen is made with a width of 30mm, a thickness of 13mm, and a length of 120mm in the middle stretching section. The middle 80mm is taken as the gauge length, and 20mm is left at each end. This can avoid stress concentration within the 20mm range at each end during stretching. Homemade fixtures: Two trapezoidal fixtures are used to fix the specimen and are connected to the testing machine. The sharp corners of the stress concentration parts of the fixtures are all ground into rounded corners to ensure that the fixtures and the specimens are in surface contact during tensile testing, avoiding stress concentration caused by line contact. Electronic extensometer: The test specimen is loaded using a microcomputer-controlled electronic universal testing machine. The testing machine is equipped with an electronic extensometer, which can be clamped on the specimen to measure the elongation of the gauge length. Before the experiment, the relative positions of the specimen clamp, the specimen, and the extensometer need to be adjusted to ensure that the dumbbell specimen is subjected to axial tensile loading.

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