Preparation method of modified micron bamboo fiber reinforced concrete

Abstract

This invention relates to the field of building materials technology and discloses a method for preparing modified micron-sized bamboo fiber reinforced concrete. The method involves preparing ordinary silicate cement, coarse and fine aggregates, water, a water-reducing agent, an alkali-resistant stabilizer, a water-retaining thickener, and 10-500 μm modified micron-sized bamboo fibers according to a specified ratio. During preparation, the aggregates and cement are dry-mixed, the first batch of fibers is added and dry-mixed again, then water and various additives are added and wet-mixed, and finally the remaining fibers are mixed in and thoroughly. The modified micron-sized bamboo fibers are prepared by heating and impurity treatment with sodium hydroxide solution, washing until neutral, and then drying. This invention removes impurities and maintains strength through alkaline heat treatment, reduces water absorption to prevent interfacial debonding; the step-by-step feeding and alternating wet and dry mixing process overcomes the problem of agglomeration and clumping of flexible fine fibers; and the synergistic effect of the alkali-resistant stabilizer and water-retaining thickener resists strong alkali erosion of cement and prevents fiber segregation, thus helping to improve the early-stage resistance to plastic shrinkage cracking of concrete and enhance its later-stage toughness.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a method for preparing modified micron-sized bamboo fiber reinforced concrete. Background Technology

[0002] Concrete is currently the most widely used building material in engineering construction, but it inherently suffers from low tensile strength, poor toughness, and susceptibility to cracking. To improve the overall performance of concrete, fiber materials are usually incorporated into the mixing system. Compared to traditional steel fibers and synthetic polymer fibers, natural bamboo fiber has the advantages of wide availability, biodegradability, and high specific strength, making it a widely recognized environmentally friendly reinforcing material in the concrete industry.

[0003] Processing bamboo into micron-sized fibers significantly increases its specific surface area, theoretically enabling it to better fill the micropores within the cement matrix and limit the expansion of primary microcracks. However, in actual production, due to the large aspect ratio and flexibility of micron-sized bamboo fibers, they easily become entangled and intertwined when mixed with cement and aggregates. Traditional single-batch feeding and conventional mixing processes struggle to disperse these aggregated microfiber bundles, leading to flocculent defects in the concrete slurry where fibers easily encapsulate dry clinker. This uneven dispersion not only fails to form an effective network load-bearing system in three-dimensional space but also makes the agglomerated areas weak points within the structure, causing localized concentration overload and ultimately deteriorating the density and crack resistance of the concrete.

[0004] On the other hand, the surface of untreated, raw bamboo fibers is coated with a large number of porous components such as hemicellulose, lignin, and pectin. These impurities not only mask the highly active hydroxyl groups inside the cellulose, hindering the physical anchoring and chemical bonding between the fiber and the cement matrix, but more seriously, natural bamboo fibers have extremely high water absorption. When directly added to concrete, the fibers will swell due to excessive water absorption and shrink due to water loss within the matrix. This repeated volume change will create pore defects between the fiber and the matrix, leading to severe interfacial debonding. When the component is under stress, the fibers can easily be pulled directly out of the matrix, failing to perform their high-ductility bridging and energy-dissipating function.

[0005] Furthermore, the application of natural plant fibers in concrete engineering faces the dual challenges of long-term durability and workability. Cement hydration generates a highly alkaline liquid environment, and long-term alkaline erosion can lead to structural degradation of the mechanical skeleton within the natural fibers, severely weakening the later-stage strength of the composite material. Simultaneously, due to the light weight and small size of micron-sized bamboo fibers, in freshly mixed concrete lacking sufficient cohesion, they are prone to lightweight floating and segregation, making it difficult to maintain optimal fluid suspension in the paste, resulting in inconsistent product quality. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, which solves the problems of traditional micron-sized bamboo fibers easily agglomerating and clumping in concrete, interfacial debonding due to surface impurities and high water absorption, easy degradation of the mechanical skeleton in a strongly alkaline environment, and easy floating and segregation of lightweight microfibers in slurry.

[0007] To achieve the above objectives, the present invention provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, comprising the following steps: Prepare, by weight, 100 parts ordinary silicate cement, 200-300 parts coarse aggregate, 120-200 parts fine aggregate, 35-50 parts water, 0.5-2.0 parts water-reducing agent, 0.2-0.8 parts alkali-resistant stabilizer, 0.1-0.5 parts water-retaining thickener, and 0.5-5.0 parts modified micron bamboo fiber; The coarse aggregate, the fine aggregate, and the ordinary silicate cement are dry-mixed to obtain a preliminary mixture; Add half of the modified micronized bamboo fiber to the preliminary mixture and continue dry mixing; After the dry mixing, the water, the water-reducing agent, the alkali-resistant stabilizer, and the water-retaining thickener are added, and the mixture is wet-mixed to obtain a mixture. Add the remaining modified micron-sized bamboo fibers to the mixture being stirred, and continue stirring until all fibers are evenly distributed in the mixture to obtain the concrete mixture.

[0008] Through the above technical solution, this invention improves the dispersion of fibers in the mixture by adding fibers in stages and combining dry and wet mixing, thereby enhancing the crack resistance of concrete. Specifically, half of the total amount of fiber is added in the early dry mixing stage. The mechanical collision and shear friction generated by the aggregate during high-speed mixing disperse the fine bamboo fiber bundles, breaking the initial electrostatic adsorption and mechanical entanglement between the fibers. After water and various admixtures are added to form a cement paste with a certain viscosity, the remaining fiber is mixed in for wet mixing. At the same time, the alkali-resistant stabilizer introduced into the system can effectively protect the modified micronized bamboo fiber from erosion in the strongly alkaline cement hydration environment, maintaining the long-term mechanical stability of the fiber; the water-retaining thickener increases the cohesion of the paste, prevents water loss, and better suspends and encapsulates the fiber. The modified micronized bamboo fiber is divided into two equal parts and added in the dry and wet mixing stages respectively, reasonably balancing the load of solid aggregate friction dispersion and liquid paste shear dispersion. This stepwise addition mechanism avoids the problems of localized concentration overload and clumping caused by adding fibers all at once, ensuring that the fibers form a uniform overlapping network inside the concrete, playing a bridging role under tensile stress and blocking the propagation of microcracks.

[0009] Preferably, the dry mixing time of the coarse aggregate, the fine aggregate and the ordinary silicate cement is 1 to 2 minutes; the dry mixing time is 2 to 4 minutes; and the wet mixing time is 2 to 3 minutes.

[0010] The above technical solution achieves initial homogenization of dry powder materials and aggregates through dry mixing for 1-2 minutes. Subsequent dry mixing for 2-4 minutes provides ample time for mechanical friction to disperse the initially added bamboo fibers, while also preventing fiber breakage caused by prolonged dry grinding. The final wet mixing stage, controlled at 2-3 minutes, ensures sufficient wetting and hydration of cement particles and the effective plasticizing effect of the water-reducing agent, achieving a suitable fluidity for the slurry and providing a favorable fluid environment for the incorporation of remaining fibers.

[0011] Preferably, the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent; the coarse aggregate includes crushed stone with a particle size of 5-20 mm, the fine aggregate includes river sand with a fineness modulus of 2.5-2.8, and the mud content of the aggregate is ≤1%; the alkali-resistant stabilizer is selected from sodium benzoate, sodium nitrite, or a mixture of the two; the water-retaining thickener is selected from hydroxypropyl methylcellulose or polyacrylamide; and the water is deionized water.

[0012] Through the above technical solutions, polycarboxylate-based high-efficiency water-reducing agents can impart excellent workability to fresh concrete at low water-cement ratios; strictly limiting the particle size and mud content of coarse and fine aggregates effectively reduces the isolation interference of impurities such as mud on the bonding between cement and fiber, ensuring stable strength development; alkali-resistant stabilizers and water-retaining thickeners complement each other, highly matching the surface characteristics of modified micron-sized bamboo fibers; deionized water avoids interference from excess impurity ions in tap water, ensuring the purity and long-term stability of the internal chemical environment of the system.

[0013] Preferably, the method further includes: pouring the well-mixed concrete mixture into a mold, placing it on a vibrating table for vibration, stopping vibration after the surface of the concrete mixture shows slurry, scraping off the excess concrete mixture above the mold until the top surface is flat, covering the surface with a plastic film, allowing it to stand and form, and then removing the mold to obtain a concrete test block. The concrete test block is then placed in a standard curing room for curing.

[0014] The above technical solution eliminates residual air bubbles inside the concrete, improving the density of the matrix. To prevent excessive vibration from causing coarse aggregate to sink and lightweight fibers to float and segregate, vibration should be stopped when surface laitance appears. Furthermore, subsequent standard curing provides stable temperature and humidity conditions for continuous cement hydration and the densification of the microstructure in the fiber-cement interface transition zone.

[0015] Preferably, the modified micronized bamboo fiber is prepared by the following steps: crushing bamboo segments and pulverizing and sieving them in a high-speed mixer to collect micronized bamboo fiber; immersing the micronized bamboo fiber in a sodium hydroxide solution and subjecting it to heat treatment; cooling the heated micronized bamboo fiber and then washing it until the filtrate is neutral; drying the washed micronized bamboo fiber to obtain the modified micronized bamboo fiber.

[0016] The above technical solution utilizes sodium hydroxide solution to heat and impregnate micron-sized bamboo fibers, removing impurities from the fiber surface and enhancing their interfacial adhesion to the cement matrix.

[0017] Preferably, the particle size of the collected micron-sized bamboo fibers is 10–500 μm.

[0018] Through the above technical solution, the micron-sized particles endow bamboo fibers with a high specific surface area, increasing their contact area with cement paste. Fibers of this size can fill the pores between cement particles and fine aggregates, inhibiting the initiation of primary microcracks at the scale.

[0019] Preferably, the sodium hydroxide solution is a sodium hydroxide solution with a mass fraction of 3-8 wt%; the micron-sized bamboo fiber is impregnated with the sodium hydroxide solution at a solid-liquid ratio of 1:15 by mass.

[0020] The above technical solution provides a suitable alkaline hydrolysis environment with an alkali concentration of 3–8 wt% and a solid-liquid ratio of 1:15. If the alkali concentration is too low, lignin and hemicellulose cannot be fully removed; conversely, if the alkali concentration is too high or the liquid-liquid ratio is inappropriate, irreversible degradation of the cellulose matrix will occur, reducing the tensile strength of the fiber. Therefore, this condition achieves a balance between the impurity removal effect and the retention rate of the fiber's mechanical properties.

[0021] Preferably, the heating treatment is performed by water bath heating, the heating temperature is 70-90°C, and the treatment time is 6-10 hours.

[0022] The above technical solution provides a uniform and constant temperature field through water bath heating. The thermal activation conditions of 70-90℃ accelerate the reaction rate between sodium hydroxide and impurity components, and the treatment time of 6-10 hours allows sufficient time for lignin macromolecules to degrade and dissolve, ensuring uniform and consistent fiber modification depth.

[0023] Preferably, the cooling method is to allow the water to cool naturally at room temperature for 30 minutes; the washing method is to repeatedly filter and wash the water using deionized water.

[0024] The above technical solution avoids stress damage to the fiber structure caused by sudden temperature drops through natural cooling. The washing process uses deionized water for repeated filtration to remove residual sodium ions and degraded organic matter from the fiber surface until it reaches a neutral state, thus eliminating the adverse interference of residual alkali metal ions on the subsequent normal cement hydration process and concrete durability.

[0025] Preferably, the drying is carried out in an oven at a temperature of 95–110°C for 6–10 hours, and the degree of drying is controlled to have a moisture content of ≤8%.

[0026] The above technical solution dries and evaporates the free and bound water inside the micron-sized bamboo fibers at 95–110℃, restoring the fibers to a dry state and controlling the moisture content below 8%. This not only prevents mold and degradation of the wet fibers during storage but also ensures the accuracy of weight-based measurements in subsequent dry-mixing processes, preventing the fibers from introducing excessive free moisture and altering the design water-cement ratio of the fresh concrete.

[0027] This invention provides a method for preparing modified micron-sized bamboo fiber reinforced concrete. It has the following beneficial effects: 1. This invention achieves a precise balance between impurity removal and strength preservation through mild alkaline heat treatment (3-8 wt% sodium hydroxide), improving the interfacial adhesion and volume stability of the fibers. It not only removes the porous hemicellulose and lignin coating the fiber surface, exposing abundant active hydroxyl groups to enhance physical anchoring and chemical bonding with the cement matrix, but also fully preserves the high-strength α-cellulose microcrystalline framework within, avoiding structural degradation caused by excessive reaction. Simultaneously, the degumming treatment significantly reduces the dynamic saturated water absorption rate of the fibers, cutting off the physical pathway of interfacial debonding and pore defects caused by excessive water absorption and shrinkage within the matrix.

[0028] 2. This invention pioneered a two-dimensional dispersion mixing process that couples batch feeding with alternating dry and wet mixing. This improves upon the engineering challenge of easily agglomerating and clumping high aspect ratio flexible microfibers. By first using the high-speed mechanical shearing force of coarse and fine aggregates to disperse the first batch of fibers during the dry mixing stage, and then using a cement slurry with a certain viscosity as an isolation suspension medium to mix in the remaining fibers during the wet mixing stage, the load of solid friction and liquid shearing is balanced. This avoids local concentration overload and floc defects that encapsulate dry clinker caused by one-time feeding, reducing the coefficient of variation of fiber distribution in three-dimensional space to an extremely low level (e.g., about 4%), ensuring the high efficiency of the full-content fiber network.

[0029] 3. This invention limits the scale range to the micrometer level (10–500 μm), achieving a dual leap in early-stage shrinkage crack resistance and later-stage fracture toughness without deteriorating concrete density. Compared to traditional millimeter-sized long fibers, micrometer-sized bamboo fibers can better fill micro-cracks between cement and aggregates without creating voids. Under tensile or bending failure, the highly dispersed microcrystalline skeleton fiber network can dissipate external energy through extremely high interfacial friction pull-out work, transforming brittle fracture into highly ductile failure (significantly increasing fracture energy). In the early rapid moisture evaporation stage, the dense fiber micro-network can also effectively resist the capillary negative pressure shrinkage stress generated by the slurry, transforming macroscopic cracks into harmless micro-cracks and reducing the cracking rate per unit area.

[0030] 4. This invention introduces a synergistic system of alkali-resistant stabilizers and water-retaining thickeners, providing dual protection for the long-term service of micron-sized bamboo fibers in strongly alkaline cement environments. Alkali-resistant stabilizers (such as sodium benzoate / sodium nitrite) can form a protective barrier on the surface of modified bamboo fibers, resisting the erosion of the strongly alkaline liquid phase generated by cement hydration, ensuring that the long-term mechanical skeleton of natural cellulose is not eroded or degraded. On the other hand, water-retaining thickeners (such as hydroxypropyl methylcellulose) enhance the cohesion and water retention of the paste, not only preventing the lightweight floating and segregation of microfibers, but also providing the fresh concrete with an excellent fluid suspension environment required to encapsulate the fibers. The two complement each other, ensuring the stable development of the composite material's strength. Attached Figure Description

[0031] Figure 1 This is a bar chart comparing the mass fraction of chemical composition of each group of fiber samples in this invention; Figure 2 The graph shows the dynamic water absorption rate of each group of fiber samples in this invention as a function of immersion time. Figure 3 This is a fiber extraction mass distribution diagram of fresh concrete mixture from different spatial sampling points according to the present invention; Figure 4 The bar charts for the mechanical properties of concrete test blocks of each group after 28 days of standard curing are as follows: (a) is the distribution of compressive strength of the test examples; (b) is the distribution of flexural strength of the test examples. Figure 5 The load-deflection response curve of the three-point bending fracture test of the precast cracked beam specimen of the present invention is shown in the figure. Figure 6 The following is a comparison of the characteristic parameters of plastic shrinkage cracking in concrete flat plate specimens of the present invention: (a) is the distribution of the initial cracking time of the test example; (b) is the distribution of the total cracking area per unit area of ​​the test example. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the preparation examples, examples, comparative examples, and test examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Preparation Examples 1-4: Preparation Example 1: This preparation example provides a method for preparing modified micron-sized bamboo fibers, comprising the following steps: crushing bamboo segments and pulverizing and sieving them in a high-speed mixer to collect micron-sized bamboo fibers with a particle size of 10 to 500 μm. The micron-sized bamboo fibers are then completely immersed in a 5 wt% sodium hydroxide solution at a solid-liquid ratio of 1:15. The sodium hydroxide solution containing the micron-sized bamboo fibers is placed in a water bath at 80°C and heated continuously for 8 hours. After heating, the solution is removed and allowed to cool naturally at room temperature for 30 minutes. The treated micron-sized bamboo fibers are then repeatedly washed and filtered with deionized water until the filtrate is neutral. The neutralized micron-sized bamboo fibers are evenly spread and dried in an oven at 103°C for 8 hours, with the moisture content controlled to ≤8%, ultimately obtaining the modified micron-sized bamboo fibers.

[0034] Preparation Example 2: This preparation example provides a method for preparing modified micronized bamboo fiber, comprising the following steps: crushing bamboo segments and pulverizing and sieving them in a high-speed mixer to collect micronized bamboo fiber with a particle size of 10 to 500 μm. The micronized bamboo fiber is then completely immersed in a 3 wt% sodium hydroxide solution at a solid-liquid ratio of 1:15. The sodium hydroxide solution containing the micronized bamboo fiber is placed in a water bath at 70°C and heated continuously for 6 hours. After heating, the solution is removed and allowed to cool naturally at room temperature for 30 minutes. The treated micronized bamboo fiber is then repeatedly washed and filtered with deionized water until the filtrate is neutral. The neutralized micronized bamboo fiber is evenly spread and dried in an oven at 95°C for 6 hours, with the moisture content controlled to ≤8%, ultimately obtaining modified micronized bamboo fiber.

[0035] Preparation Example 3: This preparation example provides a method for preparing modified micron-sized bamboo fibers, comprising the following steps: crushing bamboo segments and pulverizing and sieving them in a high-speed mixer to collect micron-sized bamboo fibers with a particle size of 10 to 500 μm. The micron-sized bamboo fibers are then completely immersed in an 8 wt% sodium hydroxide solution at a solid-liquid ratio of 1:15. The sodium hydroxide solution containing the micron-sized bamboo fibers is placed in a water bath at 90°C and heated continuously for 10 hours. After heating, the solution is removed and allowed to cool naturally at room temperature for 30 minutes. The treated micron-sized bamboo fibers are then repeatedly washed and filtered with deionized water until the filtrate is neutral. The neutralized micron-sized bamboo fibers are evenly spread and dried in an oven at 110°C for 10 hours, with the moisture content controlled to ≤8%, ultimately obtaining the modified micron-sized bamboo fibers.

[0036] Preparation Example 4: This preparation example provides a method for preparing modified micronized bamboo fiber, comprising the following steps: crushing bamboo segments and pulverizing and sieving them in a high-speed mixer to collect micronized bamboo fiber with a particle size of 10 to 500 μm. The micronized bamboo fiber is then completely immersed in a 6 wt% sodium hydroxide solution at a solid-liquid ratio of 1:15. The sodium hydroxide solution containing the micronized bamboo fiber is placed in a water bath at 85°C and heated continuously for 7 hours. After heating, the solution is removed and allowed to cool naturally at room temperature for 30 minutes. The treated micronized bamboo fiber is then repeatedly washed with deionized water until the filtrate is neutral. The neutralized micronized bamboo fiber is evenly spread and dried in an oven at 100°C for 7 hours, with the moisture content controlled to ≤8%, ultimately obtaining modified micronized bamboo fiber.

[0037] Examples 1-4: Example 1: This embodiment provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, comprising the following steps: Accurately weigh 100 parts by weight of ordinary Portland cement, 250 parts by weight of coarse aggregate (crushed stone with a particle size of 5-20 mm and a mud content ≤1%), 160 parts by weight of fine aggregate (river sand with a fineness modulus of 2.5-2.8 and a mud content ≤1%), 42 parts by weight of water (deionized water), 1.2 parts by weight of water-reducing agent (polycarboxylate-based high-efficiency water-reducing agent), 0.5 parts by weight of alkali-resistant stabilizer (sodium benzoate), 0.3 parts by weight of water-retaining thickener (hydroxypropyl methylcellulose), and 2.5 parts by weight of modified micron-sized bamboo fiber obtained from Preparation Example 1. Place the coarse aggregate, fine aggregate, and ordinary Portland cement together in a mixer and dry mix for 1 minute to initially mix the materials. Add half the total amount of modified micron-sized bamboo fiber to the above mixture and continue dry mixing for 3 minutes. Slowly add water, water-reducing agent, alkali-resistant stabilizer, and water-retaining thickener, and continue wet mixing for 2 minutes. Evenly sprinkle the remaining half of the modified micronized bamboo fiber into the mixing mixture and continue mixing until all fibers are evenly distributed in the mixture. Pour the well-mixed concrete mixture into the mold and place it on a vibrating table for compaction. Stop vibrating when a layer of slurry appears on the surface of the concrete mixture. Use a trowel to scrape off any excess concrete mixture from the top of the mold to ensure a smooth top surface of the poured body. Immediately after pouring, cover the surface with plastic film. After standing for 24 hours at a temperature of 20±2℃ and a relative humidity of ≥95%, remove the mold to obtain concrete test blocks, and number them with a marker. Finally, place all test blocks in a standard curing room for standard curing for 28 days.

[0038] Example 2: This embodiment provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, comprising the following steps: Accurately weigh 100 parts by weight of ordinary Portland cement, 200 parts by weight of coarse aggregate (crushed stone with a particle size of 5-20 mm and a mud content ≤1%), 120 parts by weight of fine aggregate (river sand with a fineness modulus of 2.5-2.8 and a mud content ≤1%), 35 parts by weight of water (deionized water), 0.5 parts by weight of water-reducing agent (polycarboxylate-based high-efficiency water-reducing agent), 0.2 parts by weight of alkali-resistant stabilizer (sodium nitrite), 0.1 parts by weight of water-retaining thickener (polyacrylamide), and 0.5 parts by weight of modified micron-sized bamboo fiber obtained in Preparation Example 2. Place the coarse aggregate, fine aggregate, and ordinary Portland cement together in a mixer and dry mix for 1 minute to initially mix the materials. Add half the total amount of modified micron-sized bamboo fiber to the above mixture and continue dry mixing for 2 minutes. Slowly add water, water-reducing agent, alkali-resistant stabilizer, and water-retaining thickener, and continue wet mixing for 2 minutes. Evenly sprinkle the remaining half of the modified micronized bamboo fiber into the mixing mixture and continue mixing until all fibers are evenly distributed in the mixture. Pour the well-mixed concrete mixture into the mold and place it on a vibrating table for compaction. Stop vibrating when laitance appears on the surface of the specimen. Use a trowel to scrape off the excess concrete mixture above the mold to ensure the top surface of the specimen is flat. Immediately after pouring, cover the surface with plastic film. After standing for 24 hours at a temperature of 20±2℃ and a relative humidity of ≥95%, remove the mold and number the specimens with a marker. Finally, place all specimens in a standard curing room for standard curing for 28 days.

[0039] Example 3: This embodiment provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, comprising the following steps: Accurately weigh 100 parts by weight of ordinary Portland cement, 300 parts by weight of coarse aggregate (crushed stone with a particle size of 5-20 mm and a mud content ≤1%), 200 parts by weight of fine aggregate (river sand with a fineness modulus of 2.5-2.8 and a mud content ≤1%), 50 parts by weight of water (deionized water), 2.0 parts by weight of water-reducing agent (polycarboxylate-based high-efficiency water-reducing agent), 0.8 parts by weight of alkali-resistant stabilizer (a mixture of sodium benzoate and sodium nitrite), 0.5 parts by weight of water-retaining thickener (hydroxypropyl methylcellulose), and 5.0 parts by weight of modified micron-sized bamboo fiber obtained from Preparation Example 3. Place the coarse aggregate, fine aggregate, and ordinary Portland cement together in a mixer and dry mix for 2 minutes to initially mix the materials. Add half the total amount of modified micron-sized bamboo fiber to the above mixture and continue dry mixing for 4 minutes. Slowly add water, water-reducing agent, alkali-resistant stabilizer, and water-retaining thickener, and continue wet mixing for 3 minutes. Evenly sprinkle the remaining half of the modified micronized bamboo fiber into the mixture and continue mixing until all fibers are evenly distributed. Pour the well-mixed concrete into a mold and place it on a vibrating table for compaction. Stop vibrating when laitance appears on the surface of the specimen. Use a trowel to scrape off excess concrete from the top of the mold to ensure a smooth top surface. Immediately after pouring, cover the surface with plastic film. After standing for 24 hours at 20±2℃ and relative humidity ≥95%, remove the mold and number the specimens with a marker. Finally, place all specimens in a standard curing room for standard curing for 28 days.

[0040] Example 4: This embodiment provides a method for preparing modified micron-sized bamboo fiber reinforced concrete, comprising the following steps: Accurately weigh 100 parts by weight of ordinary Portland cement, 280 parts by weight of coarse aggregate (crushed stone with a particle size of 5-20 mm and a mud content ≤1%), 180 parts by weight of fine aggregate (river sand with a fineness modulus of 2.5-2.8 and a mud content ≤1%), 45 parts by weight of water (deionized water), 1.5 parts by weight of water-reducing agent (polycarboxylate-based high-efficiency water-reducing agent), 0.6 parts by weight of alkali-resistant stabilizer (sodium benzoate), 0.4 parts by weight of water-retaining thickener (polyacrylamide), and 3.5 parts by weight of modified micron-sized bamboo fiber obtained in Preparation Example 4. Place the coarse aggregate, fine aggregate, and ordinary Portland cement together in a mixer and dry mix for 1.5 minutes to initially mix the materials. Add half the total amount of modified micron-sized bamboo fiber to the above mixture and continue dry mixing for 3 minutes. Slowly add water, water-reducing agent, alkali-resistant stabilizer, and water-retaining thickener, and continue wet mixing for 2.5 minutes. Evenly sprinkle the remaining half of the modified micronized bamboo fiber into the mixture and continue mixing until all fibers are evenly distributed. Pour the well-mixed concrete into a mold and place it on a vibrating table for compaction. Stop vibrating when laitance appears on the surface of the specimen. Use a trowel to scrape off excess concrete from the top of the mold to ensure a smooth top surface. Immediately after pouring, cover the surface with plastic film. After standing for 24 hours at 20±2℃ and relative humidity ≥95%, remove the mold and number the specimens with a marker. Finally, place all specimens in a standard curing room for standard curing for 28 days.

[0041] Comparative Examples 1-5: Comparative Example 1: Compared with Example 1, the difference is that no modified micronized bamboo fiber was added to this comparative example, and it is ordinary plain concrete; otherwise, they are the same.

[0042] Comparative Example 2: The difference between this comparative example and Example 1 is that this comparative example uses untreated, unprocessed, natural micron-sized bamboo fiber; all other aspects are the same.

[0043] Comparative Example 3: Compared with Example 1, the difference is that this comparative example adopts the traditional one-time feeding process, that is, all the modified micronized bamboo fibers are added to the mixer at the beginning along with the coarse and fine aggregates and cement, without being added in stages, and the rest are the same.

[0044] Comparative Example 4: Compared with Example 1, the difference lies in the step of preparing micron-sized bamboo fibers, where long bamboo fibers with a particle size greater than two millimeters are collected and then modified and added. All other steps are the same.

[0045] Comparative Example 5: Compared with Example 1, the difference is that in the step of preparing modified bamboo fiber, a 15wt% sodium hydroxide solution is used to continuously heat the bamboo fiber in a water bath at 100°C for 12 hours, while the rest are the same.

[0046] Test Examples 1-6: Test Example 1: This test example is used to quantitatively analyze the changes in chemical composition of micron-sized bamboo fibers under different treatment conditions. The specific test objects include unprocessed micron-sized bamboo fibers, modified micron-sized bamboo fibers prepared in Examples 1 to 4, and the destructively treated fibers in Comparative Example 5. The test process includes the following steps: Sample preparation and pre-extraction. Accurately weigh approximately 2,000 g of each group of dried fiber samples and place them in a vacuum filter bag. Place the bag in a Soxhlet extractor and reflux extract using a mixed solvent of ethanol and toluene at a volume ratio of 1:2 at 90°C for 6 hours to remove lipids, pectin, waxes, and other extracts from the fiber surface. Then wash with deionized water and dry in an oven at 105°C to constant weight.

[0047] Lignin content determination. Take 1.000 g of the pre-extracted, constant-weight sample and place it in a beaker. Add 15 mL of 72 wt% sulfuric acid solution and allow it to stand at 20°C for 2 hours to depolymerize, stirring continuously with a glass rod. Then transfer the reaction solution to a large beaker, add deionized water to dilute to a sulfuric acid concentration of 3 wt%, and reflux gently on an electric furnace for 4 hours. After the acid-insoluble lignin has completely precipitated, filter using a pre-weighed glass frit funnel. Wash the filter residue with hot distilled water until neutral. Finally, dry the funnel and precipitate at 105°C and weigh them to calculate the lignin mass fraction.

[0048] Determination of holocellulose content. Take another 2.000 g of the pre-extracted, constant-weight sample and place it in an Erlenmeyer flask. Add 150 mL of deionized water, 1.5 g of sodium chlorite, and 0.5 mL of glacial acetic acid. Heat in a 75°C water bath for 1 hour. During this period, add the same amount of sodium chlorite and glacial acetic acid every hour, continuing the reaction for 4 hours until the fibers turn bright white. After the reaction, cool to room temperature, filter, wash alternately with deionized water and acetone, and dry at 105°C to constant weight. Calculate the total mass of holocellulose containing hemicellulose and cellulose.

[0049] α-Cellulose content determination. Weigh 1.000 g of the obtained holocellulose sample into a beaker, add 25 mL of 17.5 wt% sodium hydroxide solution, and stir at 20 °C for 45 min to fully dissolve the hemicellulose. Then add 25 mL of deionized water to dilute, immediately transfer to a sintered glass funnel for filtration, and wash the residue sequentially with 10 wt% acetic acid solution and hot deionized water until neutral. Dry the residue at 105 °C and weigh it; this residue mass is the α-cellulose mass. Subtract the α-cellulose mass from the previously calculated cellulose mass to obtain the hemicellulose mass. Finally, convert all data to a percentage of the total mass of the original oven-dry sample.

[0050] Table 1. Test data of chemical composition (mass fraction) of fiber samples in each group

[0051] Conclusion Analysis: Combined with Table 1 and Figure 1 The bar chart clearly shows the effects of different processing techniques on the stripping and reshaping of the chemical components of the fiber. Figure 1 The horizontal axis represents the test groups, covering unprocessed micron-sized bamboo fiber, four preparation examples, and Comparative Example 5. The vertical axis represents the mass fraction (%) of each component. The legend clearly indicates that dark gray bars represent α-cellulose, medium gray bars represent hemicellulose, and light gray bars represent lignin, with the corresponding test value precisely labeled above each bar. In the unprocessed micron-sized bamboo fiber group on the far left, the dark gray α-cellulose bar is at a relatively low level of 45.31%, while the adjacent medium gray hemicellulose bar and light gray lignin bar are as high as 24.83% and 26.51%, respectively. This high impurity content directly reflects the original state of the natural fiber surface being thickly wrapped with a large amount of amorphous cementitious material. Previous mixing experience has shown that this porous coating will act like a sponge, plundering the moisture of the slurry in the early stages, severely hindering the formation of dense adhesion between the high-strength cellulose and hydrated calcium silicate gel.

[0052] Following the alkaline extraction reaction, the active components in the chart exhibited substantial reconstruction. Observing Preparation Example 1, which uses the preferred parameters, the dark gray α-cellulose column increased significantly to 72.18%, while the corresponding medium gray and light gray columns shrank to 9.38% and 12.24%, respectively. After the outer gelatinous layer dissolved and detached, a large area of ​​the previously deeply embedded high-strength cellulose microfibrils was exposed. This physical layer-by-layer erosion created abundant rough pits on the fiber surface, enabling the fibers to output stronger mechanical bonding forces when subjected to the hardening shrinkage stress of the cement matrix. Due to the removal of inert coatings such as lignin, the large number of exposed active hydroxyl groups also provided ample sites for subsequent interfacial chemical bonding.

[0053] Reducing the reaction intensity leads to a semi-stagnant degumming process. In Preparation Example 2, the dark gray column only increased to 58.65%, while the total of medium gray and light gray columns remained high (16.27% and 19.12%, respectively). Excessive residual hydrophilic impurities inevitably cause porosity defects in the interfacial hydration network during the later stages of service. As the water bath temperature and alkali concentration increased, the dark gray α-cellulose columns in Preparation Examples 3 and 4 further surged to 81.42% and 76.89%, respectively. The impurity stripping rate continued to increase, and the medium gray and light gray columns were compressed (e.g., the medium gray column in Preparation Example 3 decreased to 3.75%, and the light gray column decreased to 8.16%; the medium gray column in Preparation Example 4 was 6.51%, and the light gray column was 10.43%). Blindly pursuing purification often leads to structural collapse. Turning our attention to Comparative Example 5 on the far right of the horizontal axis, after introducing an extreme high-energy thermal field and high-concentration alkaline solution, the dark gray α-cellulose pillars, representing the main structural backbone, experienced a decline, with the top value recording only 34.26%, even falling below the initial baseline of the original fiber. Although the medium gray and light gray pillars were removed to 1.15% and 3.42% respectively, the extreme environment destroyed the β-1,4-glycosidic bonds within the cellulose macromolecular chains. The originally dense crystalline structure with excellent tensile modulus underwent irreversible alkaline fracture and degradation. These inferior fibers, lacking their own strength support, are prone to internal brittle fracture when concrete is subjected to tensile cracking, making them completely incapable of undertaking the energy-consuming task of pull-out. This transforms the intended toughening mechanism into a source of internal defects. The aforementioned materials science evolution trajectory confirms that the appropriate modification window must be precisely positioned within the dynamic balance between exposing binding sites and maintaining the integrity of the crystalline framework.

[0054] Test Example 2: This test example was used to determine the dynamic saturated water absorption rate of micron-sized bamboo fibers in an aquatic environment, aiming to investigate the effect of alkali treatment on the hydrophilicity and volume stability of the fibers. Specific test subjects included unprocessed micron-sized bamboo fibers, modified micron-sized bamboo fibers prepared in Examples 1 to 4, and the destructively treated fiber in Comparative Example 5. The testing process included the following steps: Weigh approximately 5,000 g of each group of fiber samples that have been dried to constant weight in an oven at 105℃ and record their initial dry weight. Place the weighed dry fibers into specially made nylon mesh bags with a known mass and a pore size of 400 mesh, and tie the bag openings tightly to ensure that the fibers will not leak out during subsequent immersion and centrifugation.

[0055] The nylon mesh bag containing the fibers was completely immersed in a constant temperature water bath containing 20°C deionized water. The mesh bag was removed from the water bath after immersion time of 1 hour, 2 hours, 4 hours, 8 hours, 12 hours, and 24 hours.

[0056] The removed mesh bag was quickly placed into a low-speed centrifuge and centrifuged at 1500 r / min for 3 min to remove free water attached to the fiber surface and the mesh bag, while retaining bound water inside the fiber and between the cell walls.

[0057] Immediately after centrifugation, the mesh bag, along with its internal fibers, was weighed on an analytical balance with an accuracy of 0.001 g. After deducting the background water absorption mass of the wet nylon mesh bag itself, the actual water absorption amount at this immersion time was calculated, and the ratio of water absorption amount to initial dry weight was converted into a water absorption rate percentage. After each weighing, the mesh bag was quickly re-immersed in the water tank to continue absorbing water until the 24-hour test was completed.

[0058] Table 2. Dynamic water absorption rate test data of fiber samples in each group at different immersion times.

[0059] Conclusion Analysis: Combined with Table 2 Figure 2 It can be seen that, Figure 2 The dynamic process of water absorption rate of each group of fiber samples changing with immersion time (h) is visually illustrated using line graphs with different marking symbols. Observing the overall distribution of water absorption rate (%) on the vertical axis, the raw micron-sized bamboo fiber marked with a gray circle shows an extremely rapid water absorption rate in the initial stage of immersion, with a steep curve slope. By the 24-hour mark, the saturated water absorption rate climbs to 181.27%. The volume expansion characteristics of the fiber in the water environment have a direct physical mapping relationship with its chemical composition state. The high-level operation of this curve is highly consistent with the structural characteristics of raw bamboo fiber surface rich in hemicellulose and amorphous pectin. In actual concrete mixing scenarios, this fiber with strong water absorption properties will quickly deplete the mixing water around the cement matrix, leading to a local water-cement ratio imbalance. As the concrete system hardens and internal moisture evaporates, the water-absorbing and expanding fiber will shrink in volume, inevitably leaving pores and debonding gaps at the interface between the fiber and the cement matrix.

[0060] After degumming, the moisture absorption capacity of all prepared groups was significantly suppressed. Figure 2In the graph, the curves of Preparation Example 1 (marked with a solid white square), Preparation Example 3 (marked with a dotted black diamond), and Preparation Example 4 (marked with a dashed black asterisk) are all located in the lower range. Taking Preparation Example 1 as an example, its final water absorption rate after 24 hours dropped to 114.38%, and the water absorption kinetic curve gradually flattened after 8 hours, with the plateau appearing earlier. Although the exposed cellulose matrix still contains free hydroxyl groups, due to the tightly packed microcrystalline structure inside high-purity cellulose, water molecules cannot enter the crystalline region in large quantities as they can infiltrate amorphous hemicellulose. This cuts off the physical pathways for excessive fiber expansion and contraction, fundamentally ensuring the volume compatibility of the fiber-cement interface during its service life. Comparing the data trajectory of Preparation Example 2 (marked with a dashed white triangle), its curve position is clearly between the original micron-sized bamboo fiber group (marked with a solid gray circle) and Preparation Example 1 (marked with a solid white square). This situation indirectly confirms that relatively weak degumming parameters retain some strongly hydrophilic groups, weakening the overall effect of interfacial porosity control.

[0061] In contrast, the data in Comparative Example 5, marked with a black dotted hexagonal line, exhibits an abnormal surge in water absorption, defying conventional degumming and precipitation patterns. This curve remains at the top of the chart throughout, with its water absorption rate surging to 226.51% at the 24-hour mark, far exceeding that of untreated, untreated, micron-sized bamboo fibers. This anomaly typically occurs after high-concentration strong alkalis disrupt the internal hydrogen bond network of the fibers. The originally dense crystalline cellulose undergoes severe degradation and mercerization expansion, causing the microfiber structure to collapse and transform into a loose, porous medium-like state. While this excessive modification removes impurities, it exponentially amplifies the capillary water absorption effect. Fibers with these porous defects, when incorporated into concrete, not only fail to fulfill their stress transfer function but also become internal water reservoirs, easily inducing through-cracks under freeze-thaw cycles or external loads. The above test results demonstrate that defining the structural integrity through appropriate alkali treatment parameters is a prerequisite for ensuring the macroscopic stability of composite material systems.

[0062] Test Example 3: This test example evaluates the impact of mixing process on the three-dimensional spatial uniformity of micron-sized bamboo fibers in fresh concrete. The dispersion mechanism is explored by quantitatively measuring the coefficient of variation of fiber content at different spatial sampling points. The test subjects are the fresh concrete mixture prepared in Example 1 and the fresh concrete mixture prepared in Comparative Example 3. The test process includes the following steps: Sampling Procedure. After the concrete mixer completes its designated operation and unloads the material, immediately transfer the freshly mixed material to a flat-bottomed iron pan measuring 800mm × 800mm and spread it evenly. Divide the area into four sections using the quartering method. Weigh 1000g of the mixed material, accurate to 1g, from each of the following eight different spatial locations: the upper left side, upper right side, middle center, lower front edge, lower rear edge, middle left side, middle right side, and lower center. Place the samples into separate large beakers.

[0063] Slurry washing and aggregate separation. Add 3000 mL of deionized water to a beaker containing the sample, and slowly stir in one direction at 60 rpm for 5 minutes using a glass rod to fully dilute and suspend the cement slurry. Let it stand for 2 minutes, and taking advantage of the density differences between the components, slowly pour the upper layer of slurry, containing bamboo fiber and a small amount of fine cement particles, into another large-capacity container. Add clean water again to the coarse and fine aggregates that have settled at the bottom of the original beaker, repeating the washing process until the aggregate surface is clean and the wash water is clear. Combine all suspensions.

[0064] Fiber purification and drying. The collected suspension was vacuum filtered through a specially designed nylon filter membrane with a pore size of 5 μm. To remove unhydrated cement clinker particles trapped on the surface of the fiber clusters on the filter membrane, a short-term acid washing reaction was performed by adding 5 wt% dilute hydrochloric acid to the filter cake, followed immediately by repeated rinsing with a large amount of deionized water until the filtrate was neutral. The filter membrane with the purified fibers was then dried in a drying oven at 105°C to constant weight.

[0065] Data statistics and calculations. The total weight of the dried filter membrane and fibers was subtracted from the pre-weighed baseline mass of the dry filter membrane to obtain the actual micron-sized bamboo fiber mass extracted from each sampling point. Based on the mass data of the eight sampling points, the arithmetic mean and standard deviation of each point were calculated, and finally the coefficient of variation, which characterizes the degree of dispersion, was determined. The calculation results were recorded as a percentage.

[0066] Table 3. Distribution data of fiber extraction quality and coefficient of variation at different spatial sampling points in Example 1 and Comparative Example 3

[0067] Conclusion Analysis: Combined with Table 3 Figure 3 It is evident that the spatial distribution of fibers in a multiphase concrete system is profoundly influenced by the mixing and feeding sequence. Observation Figure 3The gray solid line and solid dot trajectory representing Example 1 show that the extraction mass of the eight spatial sampling points fluctuates within a small range around the mean baseline of 2.988g, with a calculated coefficient of variation of only 4.05%. This highly consistent spatial distribution is attributed to the phased physical dispersion mechanism constructed in this invention. In the dry mixing stage, half of the fibers are pre-added. The micron-scale flexible bamboo, lacking moisture and agglomeration tension, undergoes initial untangling thanks to the strong rigid shear force generated by the coarse and fine aggregates driven by the mixing paddle. In the subsequent water addition and slurry preparation stage, the newly generated cement slurry quickly adheres to the surface of the dispersed individual fibers, forming a lubricating coating layer. This cuts off the pathway for secondary van der Waals force adsorption between fibers, thus providing a suspension barrier environment with suitable viscosity for the addition of the remaining fibers, ensuring the uniform placement of the full-volume fibers in three-dimensional space.

[0068] Shift of gaze Figure 3 The data trajectory of Comparative Example 3 exhibits a dramatic jump, with its hollow square scatter points showing extreme polarization at different spatial locations. The fiber weight at sampling point 4 abnormally surges to 8.43g, while at sampling point 5 it drops to 0.58g, with a coefficient of variation as high as 87.26%, confirming a severe structural imbalance within the system. During the purification process of Comparative Example 3, when the sample was washed out, fiber clumps with diameters of several centimeters were often observed to be tightly wrapped around the unhydrated, dry cement clinker. When micron-sized bamboo fibers are added in full to a high-speed mixing pot using the traditional one-step method, a large number of flexible fibers with extremely high aspect ratios instantly overlap and are sealed by the rapidly formed cement slurry shell, forming agglomerated clumps that are difficult to tear apart by mechanical force. These aggregated fiber clumps cannot function as bridges across cracks and will evolve into loose, weak physical pores and stress concentration points within the hardened concrete. Plain concrete areas without uniformly wrapped fibers will face an extremely high risk of cracking when subjected to external loads. The collapse of the dispersion uniformity means that high fiber content not only fails to generate benefits, but also disrupts the continuous transmission path of the original cementitious matrix.

[0069] Test Example 4: This test example was used to determine the macroscopic mechanical properties of concrete specimens under standard curing conditions, aiming to verify the influence of the modification state and spatial distribution of micron-sized bamboo fibers on the overall compressive and flexural strength of the composite material. The test subjects covered all groups of mixtures from Examples 1 to 4 and Comparative Examples 1 to 5. The specific testing procedure included the following steps: Specimen molding and curing. The fresh concrete mixtures prepared in Examples 1 to 4 and Comparative Examples 1 to 5 were respectively filled into cubic cast iron molds with dimensions of 150mm × 150mm × 150mm and prism molds with dimensions of 100mm × 100mm × 400mm. The filled molds were placed on a standard vibrating table and continuously vibrated and compacted until the surface was covered with slurry and no large air bubbles were expelled. The surface was then smoothed with a trowel. After the specimens were left to stand at room temperature for 24 hours, they were demolded and immediately transferred to a standard curing room with a temperature controlled at 20±2℃ and a relative humidity maintained above 95% for continuous curing for 28 days.

[0070] Compressive strength test. After reaching the specified age, remove the cubic specimen and wipe off excess moisture. Place the bearing surface of the specimen flat at the center of the lower pressure plate of the microcomputer-controlled electro-hydraulic servo universal testing machine. Start the equipment and continuously and uniformly apply a vertical load to the specimen at a loading rate of 0.5 MPa / s until the specimen fails. Record the ultimate failure load value displayed by the testing machine, and calculate the corresponding compressive strength value based on the pressure area. Take the average value of three specimens in each group as the final result.

[0071] Flexural strength test. The cured prism specimen was removed and subjected to a four-point bending test on a material testing machine. The specimen was placed on two cylindrical supports with a span of 300 mm, and two loading heads were applied at the middle third of the specimen's span. A constant load was applied at a loading rate of 0.05 MPa / s, and the crack initiation at the bottom of the specimen was closely observed. The peak load at the moment of complete fracture was recorded. The flexural strength was calculated using standard mechanical formulas, and the arithmetic mean of the three specimens was statistically analyzed.

[0072] Table 4. Mechanical property test data of the examples and comparative examples after 28 days of standard curing.

[0073] Conclusion Analysis: Combined with Table 4 Figure 4 It can be seen that, Figure 4 The diagram comprehensively presents the mapping response of structural design to the macroscopic mechanical properties of composite materials through two independent, vertically arranged sub-graphs. A closer look at the diagram structure reveals... Figure 4 (a) As a distribution diagram of compressive strength of test blocks in each group, the horizontal axis clearly lists the specific group names of Examples 1 to 4 and Comparative Examples 1 to 5 in sequence, and the vertical axis represents the compressive strength in MPa. The dark gray bar chart representing the data of each group in the figure intuitively reflects the strength difference, and the specific compressive strength test value is clearly marked at the top of each dark gray bar. Figure 4(b) is a distribution diagram of flexural strength of test blocks in each group. The horizontal axis is completely consistent with the above figure, and the vertical axis represents the flexural strength in MPa. The gray bars in the figure and the numerical labels at the top completely record the evolution of the material's tensile toughness.

[0074] observe Figure 4 (b) The gray column used as the benchmark in Comparative Example 1 shows that the flexural strength of ordinary plain concrete without any added fibers is only 4.32 MPa, exhibiting the inherent characteristics of a brittle material with extremely weak tensile strength. After the initial microcracks are induced by external loads, the lack of stress transfer mechanism within the plain concrete leads to rapid unstable propagation of cracks and subsequent specimen fracture. When modified micron-sized bamboo fibers prepared with optimized parameters are introduced, Example 1 shows... Figure 4 (b) shows a heightened medium-gray column, with top data indicating a significant increase in its flexural strength to 7.85 MPa, while corresponding to… Figure 4 The dark gray column in (a) also climbed to a global peak of 48.62 MPa. This mechanical enhancement is due to the three-dimensional bridging network constructed by micron-sized short chopped fibers in the cement matrix. When the matrix is ​​subjected to stress and deformation, the fibers spanning both sides of the microcracks can share the stress at the tip, converting the concentrated release of destructive energy into the work required to overcome the frictional slippage between the fibers and the matrix.

[0075] The impact of interface bonding strength is visually demonstrated in the data of Comparative Example 2. (Looking direction) Figure 4 (b) The gray column in Comparative Example 2 only recorded a value of 5.06 MPa at its top, which is a very limited improvement compared to the plain concrete in Comparative Example 1. The fiber surface without hemicellulose and other impurities was covered by a hydrophilic gel layer. During the hardening and dehydration process of the cement paste, volume shrinkage occurred and interfacial gaps were generated, which prevented the fibers from forming a tight mechanical bond with the hydrated calcium silicate network. Under bending loads, these untreated fibers often easily slipped and were pulled out of the matrix.

[0076] The destructive effects of mixing process and dimensional parameters on the overall load-bearing cross section are clearly demonstrated in other comparative bar charts. Figure 4 (a) and Figure 4 In (b), the dark gray and medium gray columns corresponding to Comparative Example 3 both exhibited a precipitous drop, with the column top values ​​plummeting to 34.56 MPa and 3.89 MPa respectively, even far below the baseline without fiber. A large number of flocculent fiber clumps formed in the initial mixing stage rapidly evolved into internal stress concentration sources under load, accelerating the connection of macroscopic through-cracks. The addition of long bamboo fibers in Comparative Example 4 led to... Figure 4The column top values ​​in (a) and (b) remained at 38.74 MPa and 4.51 MPa, respectively. This dimensional inconsistency inevitably introduced macroscopic physical pores into the concrete, weakening the overall density of the matrix. In contrast, the fibers in Comparative Example 5, which underwent destructive alkali treatment, showed... Figure 4 (a) and Figure 4 The values ​​at the top in (b) are only 36.92 MPa and 4.15 MPa. The severe degradation of the cellulose crystal region caused it to lose its inherent mechanical skeleton support. The fibers, having lost their tensile strength, instead became weak inclusions within the cement paste. The precisely labeled test data above, from both positive and negative perspectives, jointly confirm that limiting the micron-scale range, applying appropriate degumming modification, and combining it with a stepwise physical dispersion process constitute the core technical logic for substantially improving the toughness of composite materials.

[0077] Test Example 5: This test example is used to evaluate the toughening and crack-resistant mechanism and energy dissipation characteristics of micron-sized bamboo fiber in composite systems. Three-point bending fracture tests were conducted on small beam specimens with pre-existing cracks, and the load-mid-span deflection curves were recorded throughout the process to calculate the fracture energy. The test subjects included molded specimens from Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 5. The specific testing process included the following steps: Specimen preparation and pretreatment. Concrete mixtures for each group were prepared according to the standard mix proportions, poured into 100mm×100mm×400mm steel molds, and vibrated to form the final shape. After 28 days of curing under standard environmental conditions (temperature 20±2℃, relative humidity ≥95%), the specimens were removed. Using a precision water-cooled cutting machine with a diamond saw blade, an initial induced crack with a width of 2.0mm and a depth of 30.0mm was cut at the mid-span of the bottom of each small beam specimen to precisely control the initiation location of the main crack in the tension zone. The specimen surface was cleaned, repaired, and dried before testing.

[0078] Test setup. The cut specimen is placed on a microelectro-hydraulic servo universal testing machine with a closed-loop servo control system, with its bottom resting on two cylindrical support rollers with a span set at 300 mm. A loading head is placed above the mid-span of the specimen, directly opposite the pre-fabricated crack. A high-precision linear variable differential transformer (LVDT) with a measuring range of 5.0 mm is placed close to the bottom of the mid-span of the specimen side to accurately collect mid-span deflection displacement data during the loading process in real time.

[0079] Loading and Data Acquisition. A displacement control mode was used to drive the loading system. To capture a stable post-peak softening curve, the compression rate was set to a constant 0.02 mm / min. The equipment was started to synchronously record the applied vertical load and the corresponding mid-span deflection. Loading was stopped and all mechanical data were saved when the load exceeded the peak value and gradually decreased until it reached approximately 10% of the peak load or when the specimen showed obvious physical fracture.

[0080] Fracture parameter calculation. The load and deflection data points exported from the testing machine are integrated to obtain the complete area of ​​the envelope below the load-deflection curve. This area represents the total energy absorbed during the specimen's fracture process (work done, in J). Then, this total energy is divided by the cross-sectional area of ​​the ligament above the specimen crack (i.e., specimen width multiplied by the remaining height) to calculate the material's fracture energy parameters (in N / m).

[0081] Table 5. Test data of characteristic parameters of three-point bending fracture of precast cracked beams in Example 1 and the comparison.

[0082] Conclusion Analysis: Combined with Table 5 Figure 5 It can be seen that, Figure 5 The dynamic mechanism of energy dissipation during tensile fracture of composite materials is comprehensively revealed through load-deflection (P-δ) response curves of different linear shapes. Observing the black dotted line close to the vertical axis on the left side of the graph, this curve corresponds to Comparative Example 1 without any fiber. Ordinary plain concrete exhibits an approximately linear elastic rise in the initial stage of stress, but after reaching an extremely low peak load of 2.12 kN, the stress at the crack tip instantly expands uncontrollably, and the curve drops sharply to zero, with almost no post-peak decline segment characterizing the material's ductility. The statistical data in Table 5 clearly confirms this phenomenon, with an ultimate failure deflection of only 0.112 mm and a corresponding fracture energy as low as 68.35 N / m. This typical brittle fracture characteristic indicates that once a single cement matrix develops macroscopic cracks, it immediately loses its structural bearing capacity and is prone to catastrophic failure under dynamic impact or settlement deformation in actual engineering environments.

[0083] After introducing the fiber prepared by the preferred process of this invention, the macroscopic breaking mode was fundamentally reversed. Figure 5 The solid black line representing Example 1 not only significantly increases the peak load to 3.86 kN in the longitudinal direction, but more importantly, it exhibits an extremely full softening curve extending to the right in the post-peak stage. As the crack gradually opens, the micron-sized bamboo fibers spanning the crack plane act like tens of thousands of miniature tie rods, inhibiting the free development of the crack width through the strong mechanical interlocking force generated between them and the matrix. The ultimate breaking deflection of Example 1 extends to 2.315 mm, and the fracture energy surges to 874.62 N / m. This huge envelope area directly quantifies the external work consumed by the fiber from interface debonding, sliding friction, to final pull-out or breakage. The rough morphology exposed after the fiber surface is stripped of lignin forces the matrix to overcome higher frictional resistance during the failure process, thereby converting the originally destructive concentrated stress into dissipative frictional heat energy.

[0084] The interfacial bonding strength and the fiber's tensile modulus constitute the two core pillars of the bridging energy dissipation mechanism, a conclusion that was verified in reverse by the curve trajectories of two other sets of comparative examples. Figure 5 Although the gray dashed line representing Comparative Example 2 exhibits a certain post-peak decline, its rate of decline is significantly faster than that of Example 1, with the fracture energy dropping back to 315.48 N / m. The untreated, unprocessed micron-sized bamboo fibers, with their smooth, hydrophilic hemicellulose coating, hinder the formation of a dense hydration network. Under stress, these fibers typically slip off completely at relatively low stress levels, failing to reach their high strength load-bearing limit before ceasing operation. Re-examining the dark gray dashed line representing Comparative Example 5, the crystalline framework within the fiber is severely degraded due to the extreme alkaline thermal field damage. In the bending tension zone, these inferior fibers, lacking mechanical strength, are unable to withstand the tensile stress load transmitted from the matrix. In the initial stage as the cracks begin to open, the degraded fibers undergo brittle fracture rather than high-energy frictional pull-out, causing the curve to rapidly shrink after exceeding the peak of 2.31 kN, with the fracture energy sharply reduced to 162.74 N / m. The significant differences in fracture energy among different material ratios, through a rigorous logical chain, confirm that moderate degumming modification is the only feasible path to activate the bridging and toughening potential of high-strength microcrystalline skeletons of plant fibers.

[0085] Test Example 6: This test example was used to evaluate the crack resistance of composite materials during the early plastic shrinkage stage. Large-surface-area concrete components exposed to a dry environment are prone to volume shrinkage due to rapid moisture evaporation. This test aimed to investigate the effectiveness of different fiber admixture conditions in inhibiting the initiation and propagation of early microcracks in the matrix. The test subjects included fresh concrete mixtures prepared in Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 4. The specific operating procedures are as follows: Mold preparation and pouring. A steel flat mold with an inner diameter of 600mm × 600mm × 63mm is used. A layer of polyethylene film is laid flat on the bottom and inner walls of the mold to eliminate the constraint of bottom friction on the free shrinkage of the concrete. Each batch of fresh concrete mixture is poured into the mold, compacted using a mini plate vibrator, and the surface is smoothed with a trowel. During the smoothing process, excessive slurry should be avoided on the surface.

[0086] Accelerate drying and observe initial cracking. Immediately after molding, transfer the mold containing the specimen into a dedicated environmental simulation chamber. Adjust the chamber parameters to stabilize the ambient temperature at 30±2℃ and the relative humidity at 30±3%. Turn on the axial flow fan 0.5m directly above the specimen surface, maintaining a constant airflow speed of 5.0m / s to accelerate surface moisture evaporation. Assign a dedicated person to closely observe the specimen surface using a high-magnification magnifying glass with a light source, and record the time from when the fan is turned on to when the first visible crack appears on the specimen surface; define this as the initial cracking time.

[0087] Crack morphology measurement and statistics. The test was stopped after the specimens were continuously exposed in an environmental chamber for 24 hours. Using a portable crack microscope with a graduation of 0.01 mm, the length and maximum crack width of all cracks on the plate surface were measured one by one. For branching cracks, the main trunk and branches were measured as independent cracks.

[0088] Cracking index calculation. The total number of cracks on the surface of each group of specimens is summarized. The product of the length of each crack and its corresponding maximum width is defined as the area of ​​a single crack. The sum of all crack areas is then divided by the surface area of ​​the plate (0.36m²). 2 The final core parameters characterizing crack resistance are calculated, including the total crack area per unit area, expressed in mm. 2 / m 2 Recorded by unit.

[0089] Table 6. Test data of characteristic parameters of plastic shrinkage cracking of flat plates in Example 1 and Comparative Example

[0090] Conclusion Analysis: Combined with Table 6 Figure 6 It can be seen that, Figure 6 By independently quantifying time points and area spatial characteristics, the volumetric stability evolution process of the mixture under harsh dehydration conditions was fully disclosed. The baseline specimen of Comparative Example 1, without any reinforcing components, was brittle under high temperature and high wind conditions. Figure 6 The broken line in (a) shows that its initial cracking time was only 37.2 min, and Figure 6 (b) shows that the value at the top of the corresponding medium gray column increases dramatically to 1142.35 mm. 2 / m 2 Actual observation confirmed that the evaporation rate of moisture inside the plain concrete far exceeded the rate of internal bleeding. The huge negative pressure generated in the capillaries caused the slurry to shrink rapidly. Due to the lack of internal skeleton constraint, the microcracks that emerged penetrated the cement stone matrix without hindrance and evolved into deep through-cracks with a width of up to 1.84 mm.

[0091] Constructing a three-dimensional anti-crack mesh is key to delaying plastic shrinkage. When modified micron-sized bamboo fibers were introduced, the anti-drying shrinkage performance of Example 1 showed a qualitative leap. Figure 6 In (a), the starting point of the broken line in Example 1 is significantly raised, and the initial cracking time is delayed to 148.5 min; corresponding to... Figure 6 In (b), the crack area shrinks sharply to 34.62 mm. 2 / m 2The low position. A large number of uniformly distributed micron-sized chopped fibers form a dense spatial support system inside the slurry, which can bear and disperse the shrinkage stress caused by water evaporation. Once tensile stress concentration occurs at a certain point, the fibers crossing that area limit the displacement of the crack opening by the bonding force between their rough surfaces and the matrix, forcing the crack to change its path or consume energy, thereby transforming the macroscopic large crack into a small number of harmless microcracks with narrow width.

[0092] The dispersion state and physical size of the fibers play a decisive role in the functioning of this crack-blocking mechanism. Turning our attention to the data nodes in Comparative Example 3, in... Figure 6 (a) and Figure 6 In (b), the initial cracking time was 64.8 min, and the cracked area was as high as 687.91 mm. 2 / m 2 The traditional full-feeding method resulted in fiber agglomeration, creating numerous uncovered areas within the matrix. When shrinkage stress accumulated in these areas of the pulp, the localized stress could not be dissipated, leading to continued severe cracking. Simultaneously, the dry cement particles embedded within the fiber agglomerates absorbed surrounding moisture, exacerbating localized plastic shrinkage. Comparative Example 4, using millimeter-long bamboo fibers, showed an initial cracking time of 82.3 minutes and a cracked area of ​​453.28 mm². 2 / m 2 However, the crack prevention effect is also unsatisfactory. Excessively long fibers are difficult to distribute uniformly and isotropically with the mortar flow during concrete pouring and vibration. Instead, they easily form voids between aggregates and introduce excess interfacial bubbles. As moisture is lost, significant stress concentration occurs around these voids, becoming weak channels that induce crack propagation. Experimental data confirms that a micron-scale, staged physical dispersion strategy can maximize the use of bamboo fiber's elastic modulus to block early-stage volume shrinkage defects during hydration.

Claims

1. A method for preparing modified micron-sized bamboo fiber reinforced concrete, characterized in that, Includes the following steps: Prepare, by weight, 100 parts ordinary silicate cement, 200-300 parts coarse aggregate, 120-200 parts fine aggregate, 35-50 parts water, 0.5-2.0 parts water-reducing agent, 0.2-0.8 parts alkali-resistant stabilizer, 0.1-0.5 parts water-retaining thickener, and 0.5-5.0 parts modified micron bamboo fiber; The coarse aggregate, the fine aggregate, and the ordinary silicate cement are dry-mixed to obtain a preliminary mixture; Add half of the modified micronized bamboo fiber to the preliminary mixture and continue dry mixing; After the dry mixing, the water, the water-reducing agent, the alkali-resistant stabilizer, and the water-retaining thickener are added, and the mixture is wet-mixed to obtain a mixture. Add the remaining modified micron-sized bamboo fibers to the mixture being stirred, and continue stirring until all fibers are evenly distributed in the mixture to obtain the concrete mixture.

2. The preparation method according to claim 1, characterized in that, The time for dry mixing of the coarse aggregate, the fine aggregate and the ordinary silicate cement is 1 to 2 minutes. The dry mixing time is 2 to 4 minutes. The wet mixing time is 2 to 3 minutes.

3. The preparation method according to claim 1, characterized in that, The water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent; The coarse aggregate includes crushed stone with a particle size of 5-20 mm, and the fine aggregate includes river sand with a fineness modulus of 2.5-2.8, and the mud content of the aggregate is ≤1%. The alkali-resistant stabilizer is selected from sodium benzoate, sodium nitrite, or a mixture of both; The water-retaining and thickening agent is selected from hydroxypropyl methylcellulose or polyacrylamide; The water is deionized water.

4. The preparation method according to claim 1, characterized in that, Also includes: Pour the well-mixed concrete mixture into a mold and place it on a vibrating table for compaction. Stop compaction when slurry appears on the surface of the concrete mixture. Scrape off the excess concrete mixture from the top of the mold until the top surface is flat. Cover the surface with plastic film, let it stand to form, and then remove the mold to obtain a concrete test block. Place the concrete test block in a standard curing room for curing.

5. The preparation method according to claim 1, characterized in that, The modified micron-sized bamboo fiber is prepared through the following steps: The bamboo segments are crushed and placed in a high-speed mixer for pulverization and sieving to collect micron-sized bamboo fibers; The micron-sized bamboo fibers were impregnated in a sodium hydroxide solution and then subjected to heat treatment. The heat-treated micron-sized bamboo fibers are cooled and then washed until the filtrate is neutral. The micron-sized bamboo fibers, washed to neutral, are dried to obtain the modified micron-sized bamboo fibers.

6. The preparation method according to claim 5, characterized in that, The collected micron-sized bamboo fibers have a particle size of 10–500 μm.

7. The preparation method according to claim 5, characterized in that, The sodium hydroxide solution is a sodium hydroxide solution with a mass fraction of 3-8 wt%. The micron-sized bamboo fiber is impregnated with the sodium hydroxide solution at a solid-liquid ratio of 1:15 by mass.

8. The preparation method according to claim 5, characterized in that, The heating treatment is performed by water bath heating, with a temperature of 70–90°C and a duration of 6–10 hours.

9. The preparation method according to claim 5, characterized in that, The cooling method is to allow the room temperature to cool naturally for 30 minutes. The washing method involves repeatedly filtering and washing with deionized water.

10. The preparation method according to claim 5, characterized in that, The drying process involves placing the product in an oven at a temperature of 95–110°C for 6–10 hours, with the moisture content controlled to be ≤8%.

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