Nanofiber and steel fiber synergistically toughened ultra-high performance concrete and preparation method thereof

Abstract

The invention relates to the technical field of concrete preparation, and particularly provides nanofiber and steel fiber synergistically toughened ultra-high performance concrete and a preparation method thereof, the nanofiber and steel fiber synergistically toughened ultra-high performance concrete comprises the following components: ordinary Portland cement, silica fume, ISO standard sand, a polycarboxylic acid superplasticizer, cellulose nanofiber CNFs, copper-plated steel fiber and PVA fiber, the mixing amount of the cellulose nanofibers CNFs is 0.05-0.15% of the mass of the cement, the volume mixing amount of the steel fibers is 2%, and the volume mixing amount of the PVA fibers is 0.5%. Through the synergistic effect of the nanofibers CNFs and the steel fibers, the abrasion resistance, the mechanical property and the durability of the ultra-high performance concrete are remarkably improved, the microstructure of the UHPC is optimized, the performance of an interface transition area is enhanced, meanwhile, the requirements for environment friendliness and resource conservation are met, and wide application prospects and remarkable economic and social benefits are achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of concrete preparation, and more specifically, to a nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete and a preparation method thereof. Background Art

[0002] At present, in order to protect key hydraulic structures such as pipelines, flow surfaces, and spillways, epoxy mortar and polyurea coatings are generally used for protection. Among them, although the polyurea coating shows good protection effects in the field of engineering protection, its construction process has a high dependence on environmental conditions, and it is necessary to strictly control the temperature, humidity, and the quality of substrate treatment, resulting in high construction costs and insufficient long-term weather resistance. It is prone to aging degradation and surface cracking under the action of environmental factors such as ultraviolet radiation and alternating temperature and humidity, making it difficult to achieve long-term protection effects and unable to fundamentally solve the erosion problem of hydraulic structures. Although epoxy mortar does not have the disadvantages of strict construction conditions, environmental requirements, and high costs, however, under the strong impact of large bed load particles, the epoxy mortar is extremely prone to spalling, and cracks will also be generated inside, showing obvious deficiencies in protection performance. In this case, the epoxy mortar cannot provide long-term and stable protection for the base concrete and is difficult to meet the protection requirements of hydraulic structures under complex working conditions. Therefore, the existing protective materials for hydraulic structures are difficult to prevent actual damage caused by erosion and abrasion of high-speed sand-carrying water flows, and there is an urgent need to develop a protective layer material for hydraulic structures with excellent erosion and abrasion resistance and durability to serve the key structures of water conservancy projects. Summary of the Invention

[0003] In view of this, the present invention proposes a nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete and a preparation method thereof to solve the problems existing in the above-mentioned prior art.

[0004] On the one hand, to achieve the above object, the present invention proposes a nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete, comprising the following components:

[0005] Cement, silica fume, fine aggregate, water reducer, cellulose nanofiber CNFs, steel fiber, PVA fiber;

[0006] Wherein, the dosage of the cellulose nanofiber CNFs is 0.05%-0.15% of the mass of the cement, the volume dosage of the steel fiber is 2%, and the volume dosage of the PVA fiber is 0.5%.

[0007] Furthermore, the water-binder ratio of the nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete is 0.17, and the binder-sand ratio is 1.0.

[0008] Further, the cement is P·O 42.5 ordinary Portland cement, the silica fume is microsilica, the fine aggregate is ISO standard sand, the water reducing agent is polycarboxylate superplasticizer, and the steel fiber is copper-plated steel fiber.

[0009] Further, the specific surface area of the cement is 350 m 2 / kg, and the density is 3.1 g / cm 3 ; the average particle size of the silica fume is 0.3 μm, the density is 3.0 g / cm 3 , the specific surface area is 19,800 m 2 / kg, and the SiO2 content is 95%; the fineness modulus of the fine aggregate is 2.48, the particle size range is 0.15 - 4.75 mm, and the density is 2.547 kg / m 3 ; the water reducing efficiency of the water reducing agent is 30 - 45%; the equivalent diameter of the steel fiber is 0.22 mm, the average length is 12 mm, the density is 7.8 g / cm 3 , and the tensile strength is 3689 MPa; the equivalent diameter of the PVA fiber is 0.015 mm, the average length is 12 mm, the density is 1.3 g / cm 3 , and the tensile strength is 1400 - 1600 MPa.

[0010] Further, the diameter of the cellulose nanofiber CNFs is 10 - 50 nm, the length is 0.5 - 3 μm, the solid content is 99.5%, and the pH value is 7 - 8.

[0011] In addition, the present invention provides a method for preparing the above-mentioned nanofiber and steel fiber synergistically toughened ultra-high performance concrete, comprising the following steps:

[0012] (1) Dilute the cellulose nanofiber CNFs in deionized water to form a suspension with a specified concentration, add polycarboxylate water reducing agent, stir magnetically for 10 minutes, and then ultrasonically disperse for 20 minutes to ensure good dispersion of CNFs;

[0013] (2) Stir the cement and silica fume at low speed for 2 minutes, add the fine aggregate at 1 minute, then add water and the pre-mixed suspension, mix at low speed for 2 minutes, and then mix at high speed for 2 minutes;

[0014] (3) Add copper-plated steel fiber or PVA fiber to the slurry after high-speed mixing, and stir at high speed for 2 minutes;

[0015] (4) After filling the stirred slurry into the mold, vibrate and form it;

[0016] After the formed sample is placed at room temperature for 24 hours, the mold is removed, and then it is cured in a steam curing box at 60 °C for 72 hours to obtain the nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete.

[0017] Furthermore, the dosage of the cellulose nano-fiber CNFs is 0.05%-0.15% of the mass of cement, the volume dosage of steel fibers is 2%, and the volume dosage of PVA fibers is 0.5%.

[0018] Furthermore, the curing process includes: the specimen is left standing in an environment at 25 °C for 10 minutes, heated to 60 °C at a rate of 1 °C per minute, and then naturally cooled after being kept at a constant temperature for 72 hours.

[0019] Furthermore, the present invention proposes the application of the above-mentioned nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete in the engineering fields of hydraulic structures, bridges, tunnels or other projects requiring high abrasion resistance. Through the hydroxyl and carboxyl functional groups on the surface of the cellulose nano-fiber CNFs, Ca 2 + in the cement pore solution is adsorbed, promoting the growth of C-S-H gel, and inducing the ordered rearrangement of the layered structure through the space confinement effect.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] Through the synergistic effect of nano-fibers (CNFs) and steel fibers, the present invention significantly improves the abrasion resistance, mechanical properties and durability of ultra-high performance concrete (UHPC). Moreover, CNFs adsorb Ca 2 + in the cement pore solution through the hydroxyl and carboxyl functional groups on its surface, promoting the growth of C-S-H gel, and inducing the ordered rearrangement of the layered structure through the space confinement effect, optimizing the microstructure of UHPC, enhancing the performance of the interfacial transition zone, meeting the requirements of environmental friendliness and resource conservation at the same time, and having broad application prospects and significant economic and social benefits. Description of the Drawings

[0022] By reading the following detailed description of the preferred embodiments, various other advantages and benefits will become clear to those of ordinary skill in the art. The drawings are only for the purpose of showing the preferred embodiments and are not considered to be a limitation of the present invention. In the drawings:

[0023] Figure 1 It is a schematic diagram of the particle size distribution of portland cement and silica fume in the embodiments of the present invention;

[0024] Figure 2Physical property diagram of cellulose nanofibers in the embodiments of the present invention, where (a) is the appearance diagram of cellulose nanofibers, (b) is the Fourier transform infrared spectrum of cellulose nanofibers, (c) is the scanning electron microscope image of undispersed cellulose nanofibers, and (d) is the transmission electron microscope image of dispersed cellulose nanofibers;

[0025] Figure 3 Flow chart for preparing ultra-high performance concrete (UHPC) with synergistic toughening of nanofibers and steel fibers according to the present invention;

[0026] Figure 4 Schematic diagram of the hydration process of cement paste, where (a) is the heat release diagram of the mixture in the early stage of hydration and (b) is the cumulative hydration heat diagram;

[0027] Figure 5 Influence result diagram of cellulose nanofibers on the fluidity of fresh paste;

[0028] Figure 6 Influence result diagram of cellulose nanofibers on the mechanical properties of samples;

[0029] Figure 7 Schematic diagram of the influence of steel fibers on the failure mode of samples, where (a) is the sample without steel fibers and (b) is the sample with steel fibers. Detailed implementation manners

[0030] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be fully conveyed to those skilled in the art. It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments can be combined with each other. The present invention will be described in detail below with reference to the drawings and in combination with the embodiments.

[0031] This embodiment proposes a kind of ultra-high performance concrete (UHPC) with synergistic toughening of nanofibers and steel fibers, and the raw materials used are as follows:

[0032] (1) Cement: P·O 42.5 ordinary Portland cement is used. The specific surface area and density of the cement are 350 m 2 / kg and 3.1 g / cm 3 , respectively, both meeting the requirements specified in GB / T 1346–2011 and GB / T 17671-1999. The specific chemical composition of the cement is shown in Table 1, and its particle size distribution is as Figure 1 shown.

[0033] (2) Silica fume: Micro silica fume is used. It is a high pozzolanic material in the form of gray powder, with an average particle size of 0.3 μm and a density of 3.0 g / cm 3 , and the specific surface area is 19800 m 2 / kg. The SiO2 content is 95%, and its main chemical composition is shown in Table 1, and the particle size distribution is as shown in Figure 1 .

[0034] Table 1

[0035]

[0036] (3) Fine aggregate: ISO standard sand is used, with a fineness modulus of 2.48. Its particle size range is 0.15 - 4.75 mm, and the density is 2.547 kg / m 3 .

[0037] (4) Water reducing agent: Polycarboxylate superplasticizer is used, with a water reducing efficiency of 30 - 45%.

[0038] (5) Water: Tap water is used as mixing water.

[0039] (6) Cellulose nanofibers: Cellulose nanofibers (CNFs) are characterized by being granular in the solid undispersed state, as shown in Figure 2 . When dispersed in water, they are in the form of fibrous filaments, and their physical properties are shown in Table 2.

[0040] Table 2

[0041]

[0042] CNFs show a smooth surface in water, and a hollow tubular structure can be clearly observed. CNFs show an intertwined network structure in water. Their diameter is in the range of 23 - 30 nm, and the length is between 500 nm and 3 μm. The L / D ratio of CNFs is in the range of 25 - 100, and their morphology is as shown in Figure 2 (c), (d). Figure 2 (b) is the Fourier transform infrared spectrum of CNFs powder. The hydroxylation peak is generated by the stretching vibration of the hydroxyl group on CNFs near 3400 cm -1 , the C-H stretching vibration peak on the CNFs ring at 2900 cm -1 , there is a sharp peak at 1628 cm -1 , obtained from the stretching vibration of the C=O double bond in the carboxylate (-(C=O)-O-) group, and there are bending vibration peaks of -CH2- and -CH- at 1420 cm -1 and 1331 cm -1 respectively. These are the characteristic absorption peaks of cellulose nanofibers.

[0043] (7) Steel fibers and PVA fibers: Copper-plated steel fibers and PVA fibers are used. The detailed mechanical property indexes of the two kinds of fibers are shown in Table 3.

[0044] Table 3

[0045]

[0046] Mix proportion design

[0047] In this example, six groups of ultra-high performance concrete (UHPC) with different CNFs contents are prepared, including samples with CNFs dosages (cement mass) of 0%, 0.05%, 0.1% and 0.15%, and samples with 0.1% mass fraction of CNFs hybridized with 2% volume fraction of copper-plated steel fibers and 0.5% volume fraction of PVA fibers. The water-binder ratio of all samples is 0.17, and the mortar-binder ratio is 1.0. The mix proportions of each sample are shown in Table 4.

[0048] Table 4

[0049]

[0050] Material preparation and test method

[0051] Preparation and curing

[0052] Adding supplementary cementitious materials and nanomaterials into cement-based materials simultaneously and using steel fibers or PVA fibers to prepare nano-reinforced ultra-high performance abrasion-resistant concrete. The methods and environmental conditions adopted in the preparation (feeding sequence, mixing method, mixing time, etc.) and hardening process (curing environment, curing time, etc.) have an impact on the various properties of UHPC. Therefore, the preparation and curing process of the specimens should be strictly controlled.

[0053] The process of preparing UHPC materials is as follows, and its flow is as Figure 3 shown:

[0054] (1) Dilute CNFs in deionized water to form a suspension with a specified concentration. Then add polycarboxylate water reducer into the suspension, stir magnetically for 10 min, and then disperse ultrasonically for 20 min to ensure good dispersion of CNFs;

[0055] (2) Stir cement and silica fume at low speed for 2 min, add sand at 1 min, then add water and the pre-mixed suspension, mix at low speed for 2 min, and then mix at high speed for 2 min;

[0056] (3) After high-speed mixing, the fluidity of the slurry is good. Add copper-plated steel fibers or PVA fibers and stir at high speed for 2 min;

[0057] (4) After filling the stirred slurry into the mold, vibrate it to form.

[0058] The mold is removed after the sample is placed at room temperature for 24 h, and the taken-out sample is placed in a steam curing box for curing at 60 °C for 72 h, followed by mechanical property testing. The specific process is as follows: After the specimen is left standing in an environment of 25 °C for 10 min, it is heated to 60 °C at a rate of 1 °C / min, kept warm for 72 h, and then cooled naturally for mechanical property testing.

[0059] Test method

[0060] (1) Workability test of UHPC

[0061] The workability (fluidity) of UHPC is tested in accordance with the "Determination Method for Fluidity of Cement Mortar" (GB / T 2419-2015).

[0062] (2) Mechanical property test of UHPC

[0063] The mechanical properties of UHPC are tested in accordance with the "Test Method for Strength of Cement Mortar" (GB / T 17671-2021).

[0064] (3) Abrasion and erosion resistance test of UHPC

[0065] This section mainly studies the erosion and abrasion resistance of UHPC in a high-speed sand-carrying water flow environment, mainly considering the combined action of the impact and wear of high-speed water flow and sand-carrying. Therefore, a comprehensive method is needed to evaluate the erosion and abrasion resistance of the UHPC material prepared in this example.

[0066] Currently, the commonly used methods for evaluating the abrasion and erosion resistance of concrete mainly include the wind sand gun method, underwater steel ball method in the field of hydraulic engineering, and rotary abrasion method in the field of traffic engineering. Among them, the underwater steel ball method evaluates the erosion and abrasion of large particle sizes on the surface of cement-based materials, and the rotary abrasion method evaluates the wear of hard steel flower wheel blades on the surface of cement-based materials. These two methods cannot simulate the impact of water flow on cement-based materials. Therefore, in this example, the high-speed sand-carrying water jet method is selected to evaluate the abrasion and erosion resistance of UHPC in a high-speed sand-carrying water flow environment.

[0067] (4) Microscopic test of UHPC

[0068] 1) An isothermal calorimeter is used to measure the hydration heat of the mixture cement, and a synchronous thermal analyzer (STA449F3) and an X-ray diffractometer are used to measure the phase composition;

[0069] 2) A fully automatic mercury intrusion porosimeter (Micromeritics AutoPore V9620), a Quantachrome Autosorb IQ3 specific surface area and micropore analyzer, and an X-CT (CT2000) are used to test the pore structure and damage morphology;

[0070] 3) Use a solid nuclear magnetic resonance spectrometer to perform 29 Si NMR spectrum test;

[0071] 4) Use an X-ray photoelectron spectrometer (Thermo Scientific K-Alpha) to perform material analysis;

[0072] 5) Use a SIGMA300 scanning electron microscope to observe the microstructure.

[0073] Basic properties

[0074] The hydration and hardening properties of ultra-high performance concrete (UHPC) are closely related to its abrasion resistance. Compressive strength is an important evaluation index for the abrasion resistance of ultra-high performance cement-based materials. At the same time, the workability of fresh ultra-high performance cement paste has a significant impact on its pumpability and construction efficiency. Therefore, before studying the erosion resistance of UHPC, it is very necessary to carry out relevant research on its hydration and hardening properties.

[0075] Hydration characteristics

[0076] The basic properties of cement-based materials are closely related to the hydration process. Understanding the hydration process is the basic way to reveal the structure formation of cement-based materials. An isothermal calorimeter was used in the experiment to measure the hydration process characteristics of cement with different contents of CNFs (0 wt.%, 0.05 wt.%, 0.1 wt.% and 0.15 wt.%). From Figure 4 (a) It can be observed that the hydration characteristics of cement are affected by the change of CNFs content. With the addition of CNFs, the hydration induction period of cement is prolonged, the appearance of the hydration heat release peak is delayed, and the maximum heat release peak continuously decreases. After adding 0.05 wt.%, 0.1 wt.% and 0.15 wt.% of CNFs, the appearance of the maximum heat release peak is delayed by 1.8 h, 1.4 h and 3.1 h respectively. This delay phenomenon is mainly due to the network-structured CNFs adhering to the surface of cement particles, hindering their contact with water. Some studies have found that the delay phenomenon is related to the adsorption of CNFs on the surface of cement or the chemical combination of surface groups of CNFs with cations. Some studies believe that during the hydration process, CNFs exhibit a steric hindrance effect similar to that of water-reducing agents. This effect promotes the uniform distribution of cement particles during the hydration process. The reasons for the delay phenomenon can be summarized as follows: CNFs are electrostatically adsorbed on the cement surface to form a network covering. At the same time, when most CNFs are dispersed in the cement pore solution, the cement particles in contact with water release Ca 2+ and OH - . During the induction period of the hydration process, Ca 2+ is adsorbed on the surface of CNFs. At the same time, a "silica-rich and calcium-deficient" layer is formed on the surface of cement particles, and Ca in the solution2+ The double electric layer formed by the surface adsorption prevents the continuous hydration of cement particles, slows down the generation of Ca(OH)2 in the solution, and delays the induction period of cement hydration. ] In addition, due to the excellent hydrophilicity of the surface functional groups of CNFs, free water is adsorbed on their surface, which reduces the water consumption per unit cement, leading to a decrease in the local water-cement ratio, resulting in a decrease in the hydration rate and the dissolution reaction of C3S.

[0077] To further clarify the effect of CNFs on the hydration process, the cumulative hydration heat of the mixture from 0 to 120 h was calculated. Figure 4 (b). During the delay stage of CNFs hydration within 0-20h, the cumulative hydration heat of the control group samples was slightly higher than that of the samples doped with CNFs. This indicates that during this period, the presence of CNFs inhibited the rapid progress of the hydration reaction to a certain extent. However, after 20h, the cumulative hydration heat of the samples doped with CNFs significantly exceeded that of the control group. This means that with the passage of time, the long-term promoting effect of CNFs on the hydration reaction gradually emerges, and in the accelerated period of hydration, CNFs act as a catalyst, thereby generating more cumulative heat. This phenomenon is similar to the results of previous studies, that is, with the addition of nanocellulose materials, the cumulative heat tends to first decrease and then increase with the extension of the hydration reaction time.

[79] According to the results of hydration exotherm, it can be inferred that -COO - It can continuously absorb Ca2+ slowly released from the surface of cement particles. 2+ . This leads to a slow increase in the concentration of Ca(OH)2 in the solution and prolongs the time to reach supersaturation. When the supersaturation reaches the requirement for Ca(OH)2 crystallization, the effect of the double layer weakens and disappears, and the cement particles can continue to dissolve. It explains why the cumulative hydration heat release of the sample with added CNFs after 20h is greater than that of the control sample. On the other hand, due to the hydrophilicity and adsorption of CNFs, circuit diffusion occurs in the later stage of hydration. It promotes the further hydration of anhydrous cement and releases more heat.

[0078] Working performance

[0079] The fluidity of cement-based materials is the main indicator of their working performance. Insufficient fluidity may lead to reduced material density and affect strength, while excessive fluidity may cause segregation, which also damages mechanical properties. Reasonable fluidity helps to form a uniform microstructure and reduce porosity, thereby improving the durability of cement-based materials.

[0080] (1) Test method

[0081] 1) While preparing the mortar, wipe the table surface of the vibrating table, the inner wall of the mold, the tamping rod, and all utensils in contact with the mortar with a damp cotton cloth. Place the mold at the center of the vibrating table surface and cover it with a damp cotton cloth;

[0082] 2) Quickly fill the mold with the evenly mixed mortar in two layers. The first layer is filled to about two-thirds of the height of the truncated cone mold. Use a spatula to draw 5 times in two mutually perpendicular directions, and then use the tamping rod to tamp evenly 15 times from the edge to the center;

[0083] 3) Fill the second layer of mortar until it is about 20 mm higher than the truncated cone mold. Similarly, use a spatula to draw 5 times in two mutually perpendicular directions, and then use the tamping rod to tamp evenly 10 times from the edge to the center. After tamping, the mortar should be slightly higher than the mold;

[0084] 4) After tamping, remove the mold sleeve, tilt the spatula, and scrape off the mortar higher than the truncated cone mold from the middle to the edge in two nearly horizontal angles, and remove the mortar on the table. Then, gently lift the truncated cone mold vertically upward, immediately start the vibrating table, and complete 25 vibrations within 25 ± 1 s at a frequency of once per second;

[0085] 5) The fluidity test should be completed within 6 min from the start of adding water to the mortar to the end of measuring the spread diameter.

[0086] (2) Result analysis

[0087] The influence of CNFs on the fluidity of the fresh mixture is as Figure 5 shown. Judging from the test results, with the addition of CNFs, the fluidity of the mortar paste in the mixture gradually decreases. This is because CNFs have a high specific surface area, and the hydroxyl functional groups on them adsorb a large amount of free water to form a bound water film, resulting in a decrease in the proportion of free water used to lubricate the particles. At the same time, CNFs form a network skeleton structure, and their fiber structures overlap with each other in the paste through van der Waals forces, ionic bonds, etc. to form a network structure, thickening the paste. CNFs form a steric hindrance between cement particles, increasing the original distance between particles, resulting in an increase in the sliding resistance between particles. The thickening effect of low dosages of CNFs on the paste is not obvious. Especially, the mortar paste containing 0.05 wt.% CNFs still has a fluidity of 200 mm.

[0088] When nano - fibers and steel fibers are mixed, the fluidity is significantly better than that with a CNFs dosage of 0.15 wt.%. The surface of the steel fiber is smooth, and the "ball - bearing" effect reduces the frictional force between particles. Moreover, the interface between the steel fiber and the cement matrix only has a mechanical bonding effect. When CNFs and PVA fibers are mixed, the fluidity of the fresh cement - based paste decreases significantly through a dual - action mechanism: the hydrophilic groups on their surfaces cause the free water in the system to be adsorbed on the fiber surfaces, increasing the water demand per unit, enhancing the viscosity of the paste, and reducing the fluidity of the freshly - mixed paste.

[0089] Mechanical properties

[0090] (1) Test method

[0091] According to the "Test Method for Strength of Cement Mortar" (GB / T 17671 - 2021), the compressive strength and flexural strength tests after curing were carried out using the Hualong WAW - 600 micro - electromechanical hydraulic servo testing machine. The specific process is as follows:

[0092] Place the prismatic specimens on the testing machine and uniformly apply the load vertically on the opposite sides of the prism at a rate of 50 N / s until it breaks. Take out the two half - specimens and conduct the compressive strength test on the pressure machine respectively, uniformly applying the load at a rate of 2400 N / s until failure. One set of tests measures three samples of 40×40×160 mm.

[0093] (2) Result analysis

[0094] The compressive and flexural strengths of UHPC samples with different CNFs contents are as Figure 6 shown. It can be seen that as the CNFs content increases, the compressive and flexural strengths of the samples after steam curing at 60 °C for 72 h show an upward trend. Steam curing can reduce the drying shrinkage of UHPC and reduce the risk of forming harmful micro - cracks. Among them, when the CNFs dosage is 0.15 wt.%, the maximum compressive strength of UHPC is 142.4 MPa and the flexural strength is 25.7 MPa. Compared with the control group (103.4 MPa), adding CNFs increases the compressive strength by 37.6%. At the same time, the flexural strength increases by 17.9%. In contrast, the increase in flexural strength by adding CNFs is not as significant as that of compressive strength, which may be due to the small size and uneven distribution of CNFs. When CNFs are combined with steel fibers, the compressive strength exceeds 180 MPa, indicating that the synergistic effect of nano - materials and steel fibers with different scales improves the mechanical properties of the cementitious material. However, adding PVA fibers contributes little to the mechanical properties, only increasing the compressive strength by about 2.48%. The reason for the small increase may be that the elastic modulus and fracture strength of PVA fibers are lower than those of steel fibers. Although the hydrophilicity of PVA fibers enables them to bind tightly with the cement paste.

[0095] The specimens without steel fibers showed typical brittle failure characteristics in the compressive test. A large amount of energy was suddenly released at the moment of failure, accompanied by sharp cracking sounds. The edge part of the specimen fell off first, and then the whole specimen quickly disintegrated and was completely crushed. The failure mode is as Figure 7 shown; the specimens with 2 vol% steel fibers showed a completely different failure mode during the test. When the first crack appeared in the specimen, there was almost no obvious sound. As the load increased, the steel fibers played a "bridging" role between the cracks due to their good tensile properties, effectively hindering the further expansion of the cracks. At this time, only a weak dull sound could be heard. After the test, the main structure of the specimen still remained relatively intact, and only a small part of the surface material fell off, as Figure 7 shown. However, when PVA fibers were incorporated into the cement-based material, the improvement effect on the brittleness of the material was not significant. This is mainly attributed to the relatively low strength of PVA fibers themselves. During the process of bearing external forces, they cannot provide as strong constraints and reinforcement to the matrix as steel fibers, and it is difficult to effectively inhibit the generation and development of cracks, resulting in an insignificant reduction in the brittleness of the material.

[0096] In this embodiment, the hydration heat release characteristics, workability, compressive strength, and flexural strength of UHPC were studied, and the main conclusions are as follows:

[0097] (1) The addition of cellulose nanofibers (CNFs) significantly affected the cement hydration process, mainly manifested as a delay in the hydration heat release peak and an increase in the cumulative heat release. From the analysis of the action mechanism: during the induction period of cement hydration, CNFs adsorbed Ca 2+ on their surface, hindering the formation of a supersaturated state of Ca 2+ and OH - in the solution, delaying the formation of Ca(OH)2, thus prolonging the induction period and resulting in a delay in the hydration heat release peak; while in the acceleration period of hydration, the Ca 2+ adsorbed on the surface of CNFs and its high specific surface area characteristics made it act as a nucleation site, promoting the enrichment and growth of C-S-H gel, and thus increasing the cumulative heat release of the hydration reaction.

[0098] (2) The addition of CNFs had a great influence on the fluidity of the freshly mixed cement paste. The hydroxyl functional groups on the surface of CNFs adsorbed free water, resulting in a reduction in free water in the system. The water available in the cement paste for lubricating particles and reducing the friction between particles decreased, increasing the friction between particles and the viscosity of the cement paste, and thus affecting its fluidity.

[0099] (3) The addition of CNFs significantly improves the mechanical properties of ultra-high performance concrete. The optimal dosage is 0.15 wt.%, which increases the compressive strength of UHPC by 37.7% and the flexural strength by 17.9%. This phenomenon is mainly attributed to the internal curing effect and nucleation effect of CNFs, which promote the generation of more hydration products and improve the microstructure of UHPC.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications or equivalent replacements can still be made to the specific embodiments of the present invention. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete, characterized in that, It includes the following components: Cement, silica fume, fine aggregate, water reducer, cellulose nanofibers CNFs, steel fibers, PVA fibers; Among them, the dosage of the cellulose nanofibers CNFs is 0.05%-0.15% of the mass of the cement, the volume dosage of the steel fibers is 2%, and the volume dosage of the PVA fibers is 0.5%.

2. The nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete according to claim 1, characterized in that, The water-binder ratio of the ultra-high performance concrete synergistically toughened by the nanofibers and steel fibers is 0.17, and the mortar ratio is 1.

0.

3. The nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete according to claim 1, characterized in that, The cement is P·O 42.5 ordinary Portland cement, the silica fume is microsilica, the fine aggregate is ISO standard sand, the water reducer is polycarboxylate superplasticizer, and the steel fibers are copper-plated steel fibers.

4. The nanofiber and steel fiber synergistically toughened ultra-high performance concrete according to claim 1, wherein The specific surface area of the cement is 350 m 2 / kg, and its density is 3.1 g / cm 3 ; the average particle size of the silica fume is 0.3 μm, its density is 3.0 g / cm 3 , the specific surface area is 19,800 m 2 / kg, and the SiO2 content is 95%; the fineness modulus of the fine aggregate is 2.48, the particle size range is 0.15 - 4.75 mm, and the density is 2.547 kg / m 3 ; the water reducing efficiency of the water reducer is 30 - 45%; the equivalent diameter of the steel fiber is 0.22 mm, the average length is 12 mm, the density is 7.8 g / cm 3 , and the tensile strength is 3689 MPa; the equivalent diameter of the PVA fiber is 0.015 mm, the average length is 12 mm, the density is 1.3 g / cm 3 , and the tensile strength is 1400 - 1600 MPa.

5. The nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete according to claim 1, characterized in that, The cellulose nanofibers CNFs have a diameter of 10-50 nm, a length of 0.5-3 μm, a solid content of 99.5%, and a pH value of 7-8.

6. A preparation method for a nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete, characterized in that, It includes the following steps: (1) Dilute the cellulose nanofibers CNFs in deionized water to form a suspension with a specified concentration, add polycarboxylate water reducer, stir magnetically for 10 minutes, and then disperse ultrasonically for 20 minutes to ensure good dispersion of the CNFs; (2) Stir the cement and silica fume at low speed for 2 minutes, add the fine aggregate at 1 minute, then add water and the pre-mixed suspension, mix at low speed for 2 minutes, and then mix at high speed for 2 minutes; (3) Add copper-plated steel fibers or PVA fibers to the slurry after high-speed mixing, and stir at high speed for 2 minutes; (4) After filling the stirred slurry into the mold, vibrate it into shape; (5) After the formed sample is placed at room temperature for 24 hours, remove the mold, and then cure it in a steam curing box at 60°C for 72 hours to obtain the ultra-high performance concrete synergistically toughened by the nanofibers and steel fibers.

7. The preparation method of the nanofiber and steel fiber synergistically toughened ultra-high performance concrete according to claim 6, characterized in that, The dosage of the cellulose nanofibers CNFs is 0.05%-0.15% of the mass of the cement, the volume dosage of the steel fibers is 2%, and the volume dosage of the PVA fibers is 0.5%.

8. The preparation method of the nano-fiber and steel-fiber synergistically toughened ultra-high performance concrete according to claim 6, characterized in that, The curing process includes: leaving the specimen to stand in an environment of 25°C for 10 minutes, heating it to 60°C at a rate of 1°C per minute, keeping it warm for 72 hours, and then cooling it naturally.

9. Application of the ultra-high performance concrete synergistically toughened by nanofibers and steel fibers according to any one of claims 1-5 in the engineering fields such as hydraulic structures, bridges, tunnels or other projects that require high abrasion resistance.

Images