Fiber-modified concrete segments suitable for dual control of micro and macro cracks and their preparation method

Abstract

This invention belongs to the field of concrete building materials technology, and discloses a fiber-reinforced concrete segment suitable for dual control of micro-cracks and macro-cracks, and its preparation method. The fiber-reinforced concrete segment comprises cement-based concrete and crack-resistant fibers and structural fibers compositely added to the cement-based concrete. The crack-resistant fibers are fine steel fibers with a diameter of 0.15-0.25 mm and a length of 10-15 mm; the structural fibers are coarse polypropylene fibers with a diameter of 0.8-1.2 mm and a length of 40-60 mm; the volume fraction of the crack-resistant fibers is 0.5%-0.8%; and the volume fraction of the structural fibers is 0.5%-0.9%. The fiber-reinforced concrete segment provided by this invention can achieve continuous crack control, synergistic effect, and better economic efficiency.

Description

Technical Field

[0001] This invention belongs to the field of concrete building materials technology, and relates to a high-performance fiber-reinforced concrete segment for shield tunnels, its preparation method and quality inspection standards, and in particular a mixed fiber-reinforced concrete segment suitable for dual control of micro-cracks and macro-cracks and its preparation method. Background Technology

[0002] Fiber-reinforced concrete has been widely used in high-performance concrete structures such as shield tunnel segments due to its excellent crack resistance, toughness, and durability. Currently, the mainstream dual-fiber reinforcement scheme usually adopts a combination mode of "monofilament polymer fiber + steel fiber".

[0003] This traditional combination has the following inherent drawbacks:

[0004] (1) Insufficient performance of monofilament polymer fibers: Commonly used monofilament polymer fibers such as polypropylene and polyethylene have elastic modulus (usually 3-10 GPa) and tensile strength that are much lower than those of concrete matrix. Their main mechanism of action is to suppress non-structural microcracks caused by drying shrinkage and temperature drop through physical overlap during the plastic stage and early hardening stage of concrete. Once the concrete is stressed to its elastic limit (usually when the width of the microcrack is less than 0.05 mm), these low-modulus fibers quickly reach the yield strain, and the bridging effect drops sharply, causing the microcracks to develop and connect rapidly before the steel fibers can be effectively intervened, forming harmful cracks.

[0005] (2) Delayed intervention of steel fibers: Although steel fibers (usually diameter ≥0.5mm, length ≥35mm) used as structural supports have high modulus (≥200 GPa) and high tensile strength (≥1500MPa), their dosage in concrete is limited (volume fraction is usually 0.5%), resulting in a small number of fibers and large spacing. According to fiber spacing theory, they can only play a significant bridging role and provide residual strength when the crack width develops to more than 0.1mm (i.e., the macro-crack stage). This means that there is a "performance gap" or "weak zone" in the critical transition zone from micro-cracks to macro-cracks (crack width is about 0.05mm-0.1mm).

[0006] (3) Economic and construction issues: Steel fibers have a high density (about 7.8 g / cm³), and to achieve an effective structural reinforcement dosage, a large mass dosage is required, which significantly increases material costs and structural self-weight. At the same time, steel fibers with a large aspect ratio are prone to clumping during mixing and pouring, affecting the workability and homogeneity of concrete.

[0007] Therefore, existing technologies for crack control in fiber-reinforced concrete exhibit a "two-stage disconnect": premature failure of monofilament polymer fibers and delayed intervention of steel fibers. This restricts further improvements in the performance of fiber-reinforced concrete, especially in tunnel lining segments where crack control is extremely critical.

[0008] Moreover, the application of fiber-reinforced concrete in precast tunnel segments faces two major technical bottlenecks in its preparation: First, fibers, especially fine fibers with a large specific surface area, adsorb a large amount of slurry. If the mix design is inappropriate, the workability of the concrete mixture will deteriorate sharply, making it difficult to pour and compact. In addition, there is a lack of systematic mix design and preparation process guidance regarding the interaction between fibers and aggregates, as well as the synergistic effect of multiple fiber mixtures. Second, during the production process, multiple fibers are not easily dispersed evenly, and fiber clumping or unreasonable distribution orientation can easily occur, seriously affecting the homogeneity and reliability of the component performance. Summary of the Invention

[0009] The present invention aims to overcome the above-mentioned defects of existing dual-fiber-blended concrete technology and provide a fiber-blended concrete segment suitable for dual control of micro-cracks and macro-cracks and its preparation method. The fiber-blended concrete segment can achieve continuous crack control, synergistic effect, and better economy.

[0010] This invention achieves seamless, coordinated control of the entire process from microcracks to macrocracks by reconstructing the doped fiber system. Furthermore, by employing an innovative two-stage mix design method, this invention accurately calculates the fiber's occupation of the slurry and its replacement of aggregates, ensuring good workability and fiber dispersibility in the concrete mixture. It also optimizes the preparation process and quality testing methods, ultimately achieving the goal of high crack resistance and stable production quality for the tunnel segments.

[0011] To achieve the above objectives, the present invention adopts the following technical solutions.

[0012] This invention provides a fiber-reinforced concrete segment suitable for dual control of micro-cracks and macro-cracks, comprising cement-based concrete and crack-resistant fibers and structural fibers compositely added to the cement-based concrete; the crack-resistant fibers are fine steel fibers with a diameter of 0.15-0.25 mm and a length of 10-15 mm; the structural fibers are coarse polypropylene fibers with a diameter of 0.8-1.2 mm and a length of 40-60 mm; the volume fraction of the crack-resistant fibers is 0.5%-0.8%; and the volume fraction of the structural fibers is 0.5%-0.9%.

[0013] In one possible implementation, the aspect ratio of the microfiber is 50-100; the number of microfibers incorporated into each cubic meter of cement-based concrete is 13-25 million. Furthermore, the tensile strength of the microfiber is not less than 1500 MPa, and the elastic modulus is not less than 200 GPa.

[0014] In one possible implementation, the polypropylene coarse fiber is a fiber made from polypropylene with an aspect ratio of 35-60; the number of fine steel fibers incorporated per cubic meter of cement-based concrete is 150,000-250,000. Further, the tensile strength of the polypropylene coarse fiber is not less than 1000 MPa, and the elastic modulus is not less than 8 GPa. The surface of the polypropylene coarse fiber is further roughened with indentations and wavy lines to enhance its mechanical anchoring force with the concrete.

[0015] In one possible implementation, the ratio of the elastic modulus of the fine steel fibers to the coarse polypropylene fibers is not less than 20; the ratio of the tensile strength of the fine steel fibers to the coarse polypropylene fibers is not less than 1; and the ratio of the number of fine steel fibers to coarse polypropylene fibers per cubic meter is not less than 50.

[0016] In one possible implementation, the cement-based concrete comprises cement, fly ash, sand, gravel, water, and a water-reducing agent; the water-cement mass ratio r1 of water to cementitious material is 0.30-0.33; the cement-mortar mass ratio r2 of cementitious material to sand is 0.75-0.80; the mass ratio r3 of fly ash to cement is 0.18-0.22; and the mass of the cementitious material is the sum of the masses of cement and fly ash.

[0017] Furthermore, by weight, the cement-based concrete comprises 430-460 parts cement, 85-95 parts fly ash, 685-735 parts sand, 965-1065 parts aggregate, 163-178 parts water, and 7.9-8.5 parts water-reducing agent. The water-reducing agent is an alkali-free, high-performance water-reducing agent, such as a polycarboxylate-based water-reducing agent or an aminocarboxylate-based water-reducing agent.

[0018] This invention also provides a two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, comprising the following steps:

[0019] (a) Determining the mix proportion of concrete segments after adding crack-resistant fibers. This step includes the following sub-steps:

[0020] Determine the initial mix proportions of cement-based concrete; the cement-based concrete includes cement, fly ash, sand, gravel, water, and water-reducing agent; the water-cement mass ratio r1 of water to cementitious materials is 0.30-0.33; the cement-mortar mass ratio r2 of cementitious materials to sand is 0.75-0.80; the mass ratio r3 of fly ash to cement is 0.18-0.22; the mass of cementitious materials is the sum of the masses of cement and fly ash.

[0021] Determine the amount of cement mortar compensation required to wrap the crack-resistant fibers;

[0022] Based on the water-cement mass ratio r1, the mortar-cement mass ratio r2, and the fly ash-cement mass ratio r3 in cement-based concrete, determine the cement compensation amount, fly ash compensation amount, water compensation amount, and sand compensation amount in the cement mortar compensation amount.

[0023] The mass of crack-resistant fibers and the compensation amounts of cement, fly ash, water, and sand are added to the initial mix proportion of cement-based concrete to obtain the mix proportion of concrete segments after the addition of crack-resistant fibers.

[0024] (ii) Determine the mix proportion of concrete segments after adding structural fibers: According to the principle of equal surface area, structural fibers are used to replace part of the aggregate.

[0025] In one feasible embodiment, the initial mix proportion of the cement-based concrete, by weight, is: 350-400 parts cement, 70-80 parts fly ash, 560-630 parts sand, 1150-1300 parts aggregate, 130-150 parts water, and 6.5-8.0 parts water-reducing agent. The water-reducing agent is one of polycarboxylate-based water-reducing agents, aminocarboxylate-based water-reducing agents, etc.

[0026] In one feasible approach, the step of determining the amount of cement mortar compensation required to wrap the crack-resistant fiber is as follows: calculate the cement mortar wrapping volume v required to wrap a single crack-resistant fiber. gj :

[0027] v gj =s xg ×t gj ;

[0028] Among them, s xg t represents the surface area of ​​the crack-resistant fiber. gj The thickness of the cement mortar coating for crack-resistant fibers is 1-1.5d. xg d xg The diameter of the crack-resistant fiber;

[0029] Calculate the total volume of cement mortar required to encapsulate all crack-resistant fibers, and convert it to mass, i.e., the cement mortar compensation amount (m). gj :

[0030] m gj =v gj ×ρ gj ×n;

[0031] ;

[0032] Where, ρ gj ρ is the density of cement mortar. 砂 ρ is the density of sand. 水泥 is the density of cement; n is the number of crack-resistant fibers per cubic meter of cement-based concrete.

[0033] In one feasible approach, based on the calculated cement mortar compensation amount and the water-cement mass ratio and mortar-mortar mass ratio in the initial mix design of the cement-based concrete, the required compensation mass m of the cementitious material for this portion of the cement mortar is calculated. 裹胶 Water quality m 裹水 Sand quality m 裹砂 ;

[0034] m gj = m 裹胶 + m 裹水 + m 裹砂 ;

[0035] ; ; .

[0036] Furthermore, based on the mass ratio r3 of fly ash to cement, the cement compensation amount m is calculated. 裹水泥 and fly ash compensation amount m 裹粉煤灰 :

[0037] ; .

[0038] In one feasible approach, the core of stage (two) is to treat structural fibers as a special kind of "coarse aggregate," replacing part of the coarse aggregate (here referring to gravel) based on the principle of equal surface area, in order to optimize fiber distribution and reduce the adverse effects on concrete density.

[0039] Calculate the total surface area based on the set structural fiber volume content. Then, calculate the mass of coarse aggregate (stone) that can be replaced, assuming equal total surface areas.

[0040] ;

[0041] Where, m 石1 The mass of aggregate per cubic meter of cement-based concrete before the incorporation of structural fibers; m 石2 The mass of aggregate per cubic meter of cement-based concrete after incorporating structural fibers; m h The mass of structural fibers incorporated per cubic meter of cement-based concrete; φ sh The replacement ratio is determined based on the surface area ratio of the same mass of structured fibers to that of gravel. Specifically, the total surface area of ​​the structured fibers to be incorporated is defined as S. h S h =m h *s h s h The total surface area of ​​structural fibers per unit mass, in cm². 2 / g,m hThe mass of structural fiber to be incorporated; the total surface area of ​​the aggregate to be replaced in each cubic meter of cement-based concrete is defined as S. 石 S 石 =m 石 *s 石 s 石 The total surface area of ​​a unit mass of gravel, in cm². 2 / g,m 石 Let φ be the mass of the stone being replaced; then φ sh = s h / s 石 According to S h = S 石 , to obtain m 石 =φ sh ×m h .

[0042] Calculate the change in mass Δm per cubic meter of cement-based concrete caused by replacing some of the aggregate with structural fibers.

[0043] ;

[0044] Where, m 砼1 The total mass of cement-based concrete per cubic meter before replacement; m 砼2 ρ represents the total mass of cement-based concrete per cubic meter after replacement. h The density is the structural fiber density; V represents the unit volume, taken as 1m³. 3 ;m 砼2 = m 砼1 -(φ sh -1)×m h .

[0045] The change in mass of cement-based concrete per cubic meter is distributed according to the proportion of each component of cement-based concrete in the mix proportion of the concrete segment after the addition of crack-resistant fibers, so as to keep the total volume of concrete constant.

[0046] The final mix proportion is obtained by adjusting the mass of each component of cement-based concrete per cubic meter in the fiber-reinforced concrete segment.

[0047] ;

[0048] Where: m i1 and m i2 These represent the mass of each component (excluding aggregate) per cubic meter of cement-based concrete before and after the adjustment; m 石 The composition and mass of the adjusted stones.

[0049] This invention also provides a method for preparing fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, which is carried out according to the mix proportions obtained by the aforementioned design method, following these steps:

[0050] S1, Preparation of fiber-reinforced concrete mixture, this step includes the following sub-steps:

[0051] S11, mix sand, gravel and crack-resistant steel fibers evenly;

[0052] S12, then add structural fibers and mix evenly;

[0053] S13, then add cement, fly ash, water and water-reducing agent, and continue to stir evenly to obtain the fiber-reinforced concrete mixture;

[0054] S2, the fiber-reinforced concrete mixture is poured into the mold and vibrated to ensure that the crack-resistant and structural fibers are evenly distributed in the cement-based concrete and to remove air bubbles.

[0055] S3 involves curing and demolding the fiber-reinforced concrete component in the mold to obtain the fiber-reinforced concrete segment.

[0056] In step S11 above, to ensure uniform distribution of the crack-resistant fibers in the concrete, they are first dispersed in the aggregate under dry conditions. Specifically, a vibrating feeder evenly spreads the crack-resistant fibers onto the conveyor belt transporting sand and gravel, which is then fed into a vortex premixing silo. The crack-resistant fibers are pre-dispersed in the sand and gravel under vortex mixing to obtain a pre-dispersed material. The obtained pre-dispersed material is then fed into the main mixer. The vortex mixing time is 40-60 seconds.

[0057] The vortex premixing silo includes a silo body and a rotary motor mounted on the top of the silo body via a bracket. The top of the silo body is the inlet and outlet. One or more crossbeams are mounted at the end of the rotating main shaft of the rotary motor. One or more flexible levers facing the bottom of the silo body are mounted on the crossbeams. Two or more flow guide baffles are evenly arranged circumferentially at the bottom of the silo body. The flow guide baffles have an arc-shaped structure.

[0058] In step S12 above, the structural fibers are added to the main mixer containing the pre-dispersed material and stirred until homogeneous. The stirring time is 30-60 seconds.

[0059] In step S13 above, cement, fly ash, water and water-reducing agent are added to sand and gravel in which crack-resistant fibers and structural fibers are evenly dispersed, and stirring is continued for 90-120 seconds until a uniform fiber-reinforced concrete mixture is obtained.

[0060] In step S2 above, the well-mixed fiber-reinforced concrete mixture is transported to the top of the segment steel mold and poured into the mold. When the concrete begins to overflow from the discharge port at the bottom or side of the mold, it indicates that the mold is about to be filled, and the vibrating table located at the bottom of the mold is started. The vibration time and frequency are controlled to ensure thorough compaction, ensuring that the fibers are eventually and evenly dispersed in the concrete matrix and that large air bubbles are eliminated. The vibration time is determined by the complete formation of slurry on the concrete surface and the absence of large air bubbles.

[0061] In step S3 above, after vibration is completed, the concrete components in the mold are cured. Steam curing is preferred. After the demolding strength is reached, the mold is removed to obtain the finished product of high crack resistance double-fiber reinforced concrete precast segments.

[0062] Compared with the prior art, the present invention has the following significant advantages:

[0063] (1) Achieved full-process coordination of crack control:

[0064] Microcrack stage (<0.05mm): Crack-resistant fibers (micro-fine steel fibers), with their extremely high modulus and strength, and a huge number of fibers (approximately tens of times that of traditional coarse steel fibers at the same volume dosage), can be densely and uniformly distributed in concrete, forming a dense three-dimensional network. In the early stages of stress, when microcracks appear in the concrete matrix, the high-modulus fibers can immediately provide effective elastic restraint, significantly increasing the cracking stress of the concrete and strictly limiting the width and development of microcracks.

[0065] Crack transition stage (0.05mm-0.2mm): When the crack width exceeds the elastic working range of the fiber, due to its high strength, it will not break immediately, but will enter the stage of plastic yielding or pull-out, continuing to provide rising bridging stress and smoothly bearing the load. At this time, the crack propagation rate is effectively suppressed.

[0066] Macroscopic crack stage (>0.2mm): The structural fibers (coarse polypropylene fibers) begin to play a significant role. Their larger length and diameter, along with their optimized surface shape, provide strong mechanical interlocking and bridging capabilities, controlling the further propagation of macroscopic cracks and imparting excellent post-cracking toughness, impact resistance, and fatigue resistance to the concrete. The fibers also provide some residual stress in the early stages of this stage, creating a double layer of protection.

[0067] (2) Achieving scientific and precise mix design:

[0068] The "two-stage design method" proposed in this invention is the first to systematically consider the "sizing effect" of crack-resistant fibers and the "aggregate replacement effect" of structural fibers. It theoretically solves the problem of workability deterioration and volume instability caused by fiber incorporation, and ensures a high degree of consistency between the designed mix ratio and the actual workability.

[0069] (3) The preparation process ensures fiber dispersion:

[0070] This invention, by adding a vortex-type premixing chamber, pre-disperses crack-resistant fibers in the aggregates (sand and gravel) in a "dry state," eliminating the risk of fiber clumping during subsequent mixing. Compared to the traditional method of directly adding all raw materials to the main mixer, this represents a qualitative leap in dispersion uniformity. Furthermore, the segmented feeding and dry mixing process of "aggregate-crack-resistant fiber-structural fiber-cementing material" utilizes the mechanical friction of the aggregates to pre-disperse the fibers, further helping to solve the fiber clumping problem.

[0071] (4) Excellent economic efficiency:

[0072] Using low-density polypropylene coarse fibers as the main structural toughening component, while achieving the same or better post-cracking properties (such as flexural toughness index and residual strength), its mass content is much lower than that of traditional steel fibers, which significantly reduces material costs and component self-weight. Attached Figure Description

[0073] Figure 1 This is a schematic diagram of a vortex-type premixing chamber structure; where 1-chamber body; 2-support frame; 3-rotary motor; 4-rotating main shaft; 5-crossbeam; 6-flexible lever; 7-flow guide baffle;

[0074] Figure 2 This is a load-opening width diagram of the three-point bending test of the notched beam in Example 1;

[0075] Figure 3 This is a load-opening width diagram of the three-point bending test of the notched beam in Example 2;

[0076] Figure 4 This is a load-opening width diagram of the three-point bending test of the notched beam in Example 3;

[0077] Figure 5 This is a load-opening width diagram of the three-point bending test of the notched beam in Comparative Example 1. Detailed Implementation

[0078] The technical solutions of various embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0079] Because fine steel fibers are used as crack-resistant fibers, dispersion is more difficult. This invention designs a vortex premixing chamber to generate a gentle and uniform vortex material flow field, solving the risk of fiber clumping. The structure of the vortex premixing chamber is as follows: Figure 1 .

[0080] The aforementioned vortex-type premixing silo includes: a silo body 1, with an inlet and outlet at the top; a rotary motor 3 is mounted on the top of the silo body 1 via a bracket 2; a crossbeam 5 is mounted at the end of the rotary motor's rotating main shaft 4, the length of the crossbeam 5 being slightly smaller than the inner diameter of the silo body 1; six flexible levers 6, symmetrically arranged along the crossbeam and facing the bottom of the silo body, are mounted on the crossbeam 5; and six circumferentially evenly arranged baffles 7 are mounted at the bottom of the silo body 1. The baffles 7 have an arc-shaped structure.

[0081] The mechanical properties of the fiber-reinforced concrete segments obtained from the mix proportions of the following examples were verified according to the following concrete quality testing standards:

[0082] (1) Prepare the fiber-reinforced concrete mixture according to the mix design method given in the example, and pour it into the middle of a 150mm×150mm×550mm prism mold, without filling it completely;

[0083] (2) Place the test mold containing the central accumulation material on the vibrating table and vibrate it, so that the concrete mixture flows and fills both ends of the test mold under the action of vibration. This process simulates the orientation of the fibers in the flow during the casting of the tunnel lining segments, so that the fibers tend to lie flat along the long axis of the component (the main direction of force);

[0084] (4) After compaction and smoothing, cure for 28 days according to standard. After curing, pre-cut notches at the bottom of the mid-span of the specimen to make notched beam specimens; the notch depth is 25mm, the length is 150mm, and the width is 20mm;

[0085] (5) Three-point bending test was performed on the notched beam specimen, and the load-crack opening displacement (CMOD) curve was recorded.

[0086] The following criteria must be met simultaneously:

[0087] (i) Residual strength ratio f R1 / f LOP ≥ 1.0. Where, f LOP f is the initial crack strength (the peak value of linear elasticity). R1 The residual strength when the crack opening displacement reaches 0.2 mm; ; ;F Lop represents the maximum strength within the elastic range of the open beam, which is the maximum external load within the section where the test curve shows a linear relationship; L represents the span between the two supports below the open beam in the three-point bending test; b represents the cross-sectional height of the open beam; h sp F represents the height of the crack from the top of the open beam to the top surface of the open beam after cracking.0.2 This indicates the external load corresponding to an opening displacement of 0.2 mm in the test curve. This condition requires that the concrete immediately after cracking possesses a load-bearing capacity no less than that before cracking, without brittle fall.

[0088] (ii) Intensity development ratio f R3 / f R1 ≥ 1.0. Where, f R3 The residual strength when the crack opening displacement reaches 2.5 mm; ;F 2.5 This indicates the external load corresponding to an opening displacement of 2.5 mm in the test curve. This condition requires that as the crack further expands, the fiber bridging effect remains effective or is enhanced, and the post-cracking performance remains stable or improves.

[0089] The notched beam three-point bending test is adopted as a quality control method, and the f value is clearly specified. R1 / f LOP ≥1.0 and f R3 / f R1 Two quantitative indicators, ≥1.0, serve as the qualification benchmark. This standard is not only simple and easy to implement, but more importantly, it directly relates to and ensures the crucial post-cracking load-bearing capacity and toughness of concrete components, requiring that the material's performance not degrade after cracking and that it can continuously and stably withstand deformation. This provides an objective and rigorous basis for product factory acceptance and performance consistency control, avoiding the shortcomings of traditional single strength indicators that cannot reflect the toughness characteristics of fiber-reinforced concrete.

[0090] Example 1

[0091] In the fiber-reinforced concrete segments provided in this embodiment, the crack-resistant fibers are fine steel fibers, and the structural fibers are coarse polypropylene fibers.

[0092] Microfibers: 0.2mm in diameter, 12mm in length, tensile strength 2000MPa, elastic modulus 200GPa, volumetric content 0.76% per cubic meter of concrete segment. The number of microfibers per cubic meter of concrete segment is 20,169,851.

[0093] Polypropylene coarse fiber: 1.0 mm in diameter, 50 mm in length, tensile strength 1000 MPa, elastic modulus 8 GPa, volumetric content 0.73%. The number of polypropylene coarse fibers per cubic meter of concrete pipe segment is 185,987.

[0094] To enhance the mechanical anchoring force with concrete, the surface of the polypropylene coarse fiber can be roughened with indentations and wavy patterns.

[0095] This embodiment determines the mix proportion of fiber-reinforced concrete segments according to the following steps:

[0096] (a) Determining the mix proportion of concrete segments after adding crack-resistant fibers. This step includes the following sub-steps:

[0097] (1) Determine the initial mix proportion of cement-based concrete: 374 kg of cement, 75 kg of fly ash, 596 kg of sand, 1210 kg of gravel, 144 kg of water, and 7.2 kg of polycarboxylate superplasticizer.

[0098] The mass of the cementitious material is the sum of the masses of cement and fly ash, which is 449 kg.

[0099] The water-cement mass ratio r1 is 0.32, the mortar mass ratio r2 is 0.76, and the fly ash to cement mass ratio r3 is 0.2.

[0100] (2) Determine the amount of cement mortar compensation required to wrap the crack-resistant fiber.

[0101] Calculate the cement mortar encapsulation volume v required to encapsulate a single crack-resistant fiber. gj :

[0102] v gj =s xg ×t gj =3.14×0.2×12×0.2=1.5072mm 3 .

[0103] Calculate the total volume of cement mortar required to encapsulate all crack-resistant fibers, and convert it to mass, i.e., the cement mortar compensation amount (m). gj :

[0104] ;

[0105] m gj =v gj ×ρ gj ×n=1.5072×10 -9 ×1.799×10 3 ×20169851=54.69kg.

[0106] (3) Based on the water-cement mass ratio r1, the mortar mass ratio r2 and the fly ash-cement mass ratio r3 in cement-based concrete, determine the cement compensation amount, fly ash compensation amount, water compensation amount and sand compensation amount in the cement mortar compensation amount.

[0107] Based on the calculated cement mortar compensation amount, and the water-cement mass ratio and mortar-cement mass ratio in the initial mix design of cement-based concrete, calculate the required compensation mass m of cementitious materials for this portion of cement mortar. 裹胶 Water quality m 裹水 Sand quality m 裹砂 ;

[0108] ;

[0109] ;

[0110] .

[0111] Furthermore, based on the mass ratio r3 of fly ash to cement, the cement compensation amount m is calculated. 裹水泥 and fly ash compensation amount m 裹粉煤灰 :

[0112] The mass of cementitious materials is decomposed into cement compensation amount (m). 裹水泥 =17.29kg and fly ash compensation amount m 裹粉煤灰 =3.46kg.

[0113] (4) Add the mass of crack-resistant fiber and the compensation amounts of cement, fly ash, water and sand to the initial mix proportion of cement-based concrete to obtain the mix proportion of concrete segments after the addition of crack-resistant fiber.

[0114] The compensation amounts of cement, fly ash, water, and sand, as well as the mass of fine steel fibers, are added to the initial mix proportion of cement-based concrete to obtain the mix proportion of concrete segments with added fine steel fibers: cement 391kg, fly ash 78.5kg, sand 623kg, gravel 1210kg, water 150kg, water-reducing agent 7.2kg, and fine steel fibers 60kg.

[0115] (ii) Determine the mix proportion of concrete segments after adding structural fibers: According to the principle of equal surface area, structural fibers are used to replace part of the aggregate.

[0116] This stage determines the final mix proportion after adding polypropylene coarse fibers as structural fibers. The core of this stage is to treat polypropylene coarse fibers as a special kind of "coarse aggregate," replacing part of the coarse aggregate based on the principle of equal surface area, in order to optimize fiber distribution and reduce adverse effects on concrete density.

[0117] Calculate the total surface area based on the set structural fiber volume content. Then, calculate the mass of coarse aggregate (stone) that can be replaced, assuming equal total surface areas.

[0118] .

[0119] Calculate the change in mass Δm per cubic meter of cement-based concrete caused by replacing some of the aggregate with structural fibers.

[0120] m 砼1 =m 水泥 + m 粉煤灰 + m 砂 + m 石子 + m 水+ m 减水剂 =2460kg;

[0121] m 砼2 = m 砼1 -(φ sh -1)×m h =2105.72kg;

[0122] .

[0123] The change in mass of cement-based concrete per cubic meter is distributed according to the proportion of each component of cement-based concrete in the mix proportion of the concrete segment after the addition of crack-resistant fibers, so as to keep the total volume of concrete constant.

[0124] The final mix proportion is obtained by adjusting the mass of each component of cement-based concrete per cubic meter in the fiber-reinforced concrete segment.

[0125]

[0126] The final mix proportions are: 446 kg cement, 90 kg fly ash, 711 kg sand, 1016 kg gravel, 171 kg water, 8.2 kg water-reducing agent, 60 kg fine steel fiber, and 10.42 kg polypropylene coarse fiber.

[0127] Four notched beam specimens (numbered A, B, C, and D) were prepared according to the concrete quality testing standards given above, and three-point bending tests were conducted to verify the mechanical properties of the fiber-reinforced concrete segments with the corresponding mix proportions. The test results are as follows: Figure 2 As shown.

[0128] For specimen A, f R1 / f LOP =1.00≥1.0; f R3 / f R1 =1.20≥1.0;

[0129] For specimen B, f R1 / f LOP =1.02≥1.0;f R3 / f R1 =1.22≥1.0;

[0130] For specimen C, f R1 / f LOP =1.04≥1.0; f R3 / f R1 =1.15 ≥1.0;

[0131] For specimen D, f R1 / f LOP =1.20≥1.0; fR3 / f R1 =1.08 ≥1.0.

[0132] When the mechanical properties of the fiber-reinforced concrete segments obtained according to the design mix proportions meet the requirements, the fiber-reinforced concrete segments are prepared according to the following steps:

[0133] S1, Preparation of fiber-reinforced concrete mixture, this step includes the following sub-steps:

[0134] S11, mix sand, gravel and crack-resistant steel fibers evenly.

[0135] Weigh sand and gravel according to the final construction mix proportion and place them on the feeding conveyor belt. Weigh fine steel fibers according to the mix proportion and evenly spread the crack-resistant fibers on the surface of the sand and gravel on the feeding conveyor belt using a vibrating feeder, then send them into the vortex premixing bin. Start the rotary motor in the vortex premixing bin, set the speed to 50 rpm, and the mixing time to 50 seconds. During this process, the fine steel fibers fully collide and knead with the moving aggregates (gravel and sand) in the vortex field. The edges and surfaces of the aggregates effectively break up the fiber clumps, achieving pre-dispersion of the fine fibers before entering the main mixer, resulting in a pre-dispersed material.

[0136] S12, then add structural fibers and mix evenly.

[0137] The prepared pre-dispersed material is conveyed and unloaded into the main mixer. Polypropylene coarse fibers are weighed according to the mixing ratio and added to the main mixer via a vibrating feeder. After adding the polypropylene coarse fibers, dry mixing continues for 30 seconds to further mix the two fibers with the aggregate evenly.

[0138] S13, then add cement, fly ash, water and water-reducing agent, and continue to stir evenly to obtain the fiber-reinforced concrete mixture.

[0139] Specifically, cement, fly ash, water, and water-reducing agent are added to the main mixer in sequence according to the mix proportion, and the mixture is stirred for 90 seconds until a uniform fiber-reinforced concrete mixture is obtained.

[0140] S2, the fiber-reinforced concrete mixture is poured into the mold and vibrated to ensure that the crack-resistant and structural fibers are evenly distributed in the cement-based concrete and to remove air bubbles.

[0141] In the specific implementation, the well-mixed fiber-reinforced concrete mixture is transported to the top of the segment steel mold and poured into the mold. When the concrete begins to overflow from the discharge port at the bottom or side of the mold, it indicates that the mold is about to be filled, and the vibrating table located at the bottom of the mold is activated;

[0142] Control the vibration frequency of the vibrating table to 80Hz and the vibration time to 50 seconds to fully vibrate and ensure that the fibers are eventually evenly dispersed in the concrete matrix and that large air bubbles are eliminated, and that the concrete surface is completely covered with slurry without any large air bubbles escaping.

[0143] S3 involves curing and demolding the fiber-reinforced concrete component in the mold to obtain the fiber-reinforced concrete segment.

[0144] After vibration, the concrete components in the mold are cured. Traditional steam curing is preferred (see the national standard "Code for Construction and Acceptance of Shield Tunneling" GB 50446). After the demolding strength is reached, the mold is removed to obtain the finished product of high crack resistance fiber-reinforced precast tunnel segments.

[0145] Example 2

[0146] The initial mix proportions of cement-based concrete, micro steel fibers, and coarse polypropylene fibers used in the fiber-reinforced concrete segments prepared in this embodiment are the same as those in Example 1.

[0147] The difference between this embodiment and Embodiment 1 is that the volumetric content of fine steel fibers in each cubic meter of concrete pipe segment is 0.76%, and the volumetric content of polypropylene coarse fibers in each cubic meter of concrete pipe segment is 0.58%.

[0148] The mix proportion of the fiber-reinforced concrete pipe segment designed according to the method given in Example 1 is as follows: 435 kg of cement, 87 kg of fly ash, 693 kg of sand, 1056 kg of gravel, 167 kg of water, 8.0 kg of water-reducing agent, 60 kg of fine steel fiber, and 8.29 kg of polypropylene coarse fiber.

[0149] Four notched beam specimens (numbered A, B, C, and D) were prepared according to the concrete quality testing standards given above, and three-point bending tests were conducted to verify the mechanical properties of the fiber-reinforced concrete segments with the corresponding mix proportions. The test results are as follows: Figure 3 As shown.

[0150] For specimen A, f R1 / f LOP =1.05≥1.0; f R3 / f R1 =1.00≥1.0;

[0151] For specimen B, f R1 / f LOP =1.07≥1.0;f R3 / f R1 =1.11≥1.0;

[0152] For specimen C, f R1 / f LOP =1.06≥1.0;fR3 / f R1 =1.11≥1.0;

[0153] For specimen D, f R1 / f LOP =1.04≥1.0; f R3 / f R1 =1.08≥1.0.

[0154] Example 3

[0155] The initial mix proportions of cement-based concrete, micro steel fibers, and coarse polypropylene fibers used in the fiber-reinforced concrete segments prepared in this embodiment are the same as those in Example 1.

[0156] The difference between this embodiment and Embodiment 1 is that the volumetric content of fine steel fibers in each cubic meter of concrete pipe segment is 0.76%, and the volumetric content of polypropylene coarse fibers in each cubic meter of concrete pipe segment is 0.88%.

[0157] The mix proportion of the fiber-reinforced concrete pipe segment designed according to the method given in Example 1 is as follows: 457 kg of cement, 92 kg of fly ash, 729 kg of sand, 976 kg of gravel, 175 kg of water, 8.4 kg of water-reducing agent, 60 kg of fine steel fiber, and 12.56 kg of polypropylene coarse fiber.

[0158] Four notched beam specimens (numbered A, B, C, and D) were prepared according to the concrete quality testing standards given above, and three-point bending tests were conducted to verify the mechanical properties of the fiber-reinforced concrete segments with the corresponding mix proportions. The test results are as follows: Figure 4 As shown.

[0159] For specimen A, f R1 / f LOP =1.04≥1.0; f R3 / f R1 =1.35≥1.0;

[0160] For specimen B, f R1 / f LOP =1.04≥1.0; f R3 / f R1 =1.35≥1.0;

[0161] For specimen C, f R1 / f LOP =1.11≥1.0; f R3 / f R1 =1.10 ≥1.0;

[0162] For specimen D, f R1 / fLOP =1.04≥1.0; f R3 / f R1 =1.46≥1.0.

[0163] Comparative Example 1

[0164] The concrete segment mix proportion provided in this comparative example is: 448 kg cement, 90 kg fly ash, 713 kg sand, 1021 kg gravel, 172 kg water, 8.2 kg water-reducing agent, 0.9 kg monofilament polypropylene fiber, and 40 kg end-hook steel fiber.

[0165] Monofilament polypropylene fiber: diameter 0.03mm, length 12mm, tensile strength 450MPa, elastic modulus 4.3GPa, volumetric content in each cubic meter of concrete pipe segment 0.76%.

[0166] Hook-shaped steel fibers: 0.75mm in diameter, 50mm in length, tensile strength 1500MPa, elastic modulus 200GPa, and volumetric content of 0.5% per cubic meter of concrete segment. The middle section of the steel fiber is straight, and its two ends are double-folded into a hook shape.

[0167] Four notched beam specimens (numbered A, B, C, and D) were prepared according to the concrete quality testing standards given above, and three-point bending tests were conducted to verify the mechanical properties of the fiber-reinforced concrete segments with the corresponding mix proportions. The test results are as follows: Figure 5 As shown.

[0168] For specimen A, f R1 / f LOP =0.92<1.0; f R3 / f R1 =1.27 ≥1.0;

[0169] For specimen B, f R1 / f LOP =0.92<1.0; f R3 / f R1 =1.14 ≥1.0;

[0170] For specimen C, f R1 / f LOP =0.93<1.0; f R3 / f R1 =0.92 <1.0;

[0171] For specimen D, f R1 / f LOP =0.96<1.0; f R3 / f R1 =1.21 ≥1.0.

[0172] Combining the test results of Examples 1-3 and Comparative Example 1, it can be seen that using the traditional fiber-reinforced concrete mix proportion, the mechanical properties show a significant trough in the stage before macroscopic cracks appear (crack width less than 0.2 mm), making it unsuitable for segment-lined tunnels with strict crack resistance requirements. However, the present invention uses polypropylene coarse fibers as structural fibers, and the fiber-reinforced concrete segment mix proportion obtained by combining the mix proportion design method provided by the present invention shows a significant increasing trend in mechanical properties even after the crack width reaches 0.2 mm. This indicates that the concrete segment has entered the stage of plastic yielding or pull-out, continuing to provide rising bridging stress and smoothly bearing the load; at this point, the crack propagation rate is effectively suppressed.

[0173] Those skilled in the art will recognize that the embodiments described herein are intended to help the reader understand the principles of the invention, and should be understood that the scope of protection of the invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific modifications and combinations based on the technical teachings disclosed in this invention without departing from the spirit of the invention, and these modifications and combinations are still within the scope of protection of this invention.

Claims

1. A two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, characterized in that, The fiber-reinforced concrete segment comprises cement-based concrete and crack-resistant and structural fibers compositely added to the cement-based concrete; the crack-resistant fibers are fine steel fibers with a diameter of 0.15-0.25 mm and a length of 10-15 mm; the structural fibers are coarse polypropylene fibers with a diameter of 0.8-1.2 mm and a length of 40-60 mm; the volume fraction of the crack-resistant fibers is 0.5%-0.8%; the volume fraction of the structural fibers is 0.5%-0.9%. The two-stage mix design method for the fiber-reinforced concrete segments includes the following steps: (a) Determining the mix proportion of concrete segments after adding crack-resistant fibers. This step includes the following sub-steps: Determine the initial mix proportions of cement-based concrete; the cement-based concrete includes cement, fly ash, sand, gravel, water, and water-reducing agent; the water-cement mass ratio r1 of water to cementitious materials is 0.30-0.33; the cement-mortar mass ratio r2 of cementitious materials to sand is 0.75-0.80; the mass ratio r3 of fly ash to cement is 0.18-0.22; the mass of the cementitious materials is the sum of the masses of cement and fly ash. The steps to determine the amount of cement mortar compensation required to wrap crack-resistant fibers are as follows: Calculate the cement mortar wrapping volume v required to wrap a single crack-resistant fiber. gj : v gj =s xg ×t gj ; Among them, s xg t represents the surface area of ​​the crack-resistant fiber. gj The thickness of the cement mortar coating for crack-resistant fibers is 1-1.5d. xg d xg The diameter of the crack-resistant fiber; Calculate the total volume of cement mortar required to encapsulate all crack-resistant fibers, and convert it to mass, i.e., the cement mortar compensation amount (m). gj : m gj =v gj ×ρ gj ×n; ； Where, ρ gj ρ is the density of cement mortar. 砂 ρ is the density of sand. 水泥 Where n is the density of cement; n is the number of crack-resistant fibers per cubic meter of cement-based concrete. Based on the water-cement mass ratio r1, the mortar-cement mass ratio r2, and the fly ash to cement mass ratio r3 in cement-based concrete, determine the cement compensation amount, fly ash compensation amount, water compensation amount, and sand compensation amount in the cement mortar compensation amount. The steps are as follows: Based on the calculated cement mortar compensation amount, and the water-cement mass ratio and mortar-cement mass ratio in the initial mix design of the cement-based concrete, calculate the required cementitious material mass m for this portion of the cement mortar compensation. 裹胶 Water quality m 裹水 Sand quality m 裹砂 ; m gj = m 裹胶 + m 裹水 + m 裹砂 ; ； ； ； Furthermore, based on the mass ratio r3 of fly ash to cement, the cement compensation amount m is calculated. 裹水泥 and fly ash compensation amount m 裹粉煤灰 : ； ； The mass of crack-resistant fibers and the compensation amounts of cement, fly ash, water, and sand are added to the initial mix proportion of cement-based concrete to obtain the mix proportion of concrete segments after the addition of crack-resistant fibers. (ii) Determine the mix proportion of concrete segments after adding structural fibers: According to the principle of equal surface area, use structural fibers to replace part of the aggregate.

2. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, as described in claim 1, is characterized in that... The aspect ratio of the fine steel fibers is 50-100; the number of fine steel fibers incorporated in each cubic meter of cement-based concrete is 13-25 million; the tensile strength of the fine steel fibers is not less than 1500 MPa, and the elastic modulus is not less than 200 GPa; the aspect ratio of the polypropylene coarse fibers is 35-60; the number of polypropylene coarse fibers incorporated in each cubic meter of cement-based concrete is 150,000-250,000; the tensile strength of the polypropylene coarse fibers is not less than 1000 MPa, and the elastic modulus is not less than 8 GPa.

3. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, as described in claim 1, is characterized in that... The ratio of the elastic modulus of the fine steel fiber to the coarse polypropylene fiber is not less than 20; the ratio of the tensile strength of the fine steel fiber to the coarse polypropylene fiber is not less than 1; and the ratio of the number of fine steel fibers to coarse polypropylene fibers per cubic meter is not less than 50.

4. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks as described in claim 1, characterized in that, By weight, the cement-based concrete comprises 430-460 parts cement, 85-95 parts fly ash, 685-735 parts sand, 965-1065 parts gravel, 163-178 parts water, and 7.9-8.5 parts water-reducing agent.

5. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks as described in claim 1, characterized in that, The fiber-reinforced concrete tubular segments were subjected to a three-point bending test on notched beam specimens and met the following criteria: (i) Residual strength ratio f R1 / f LOP ≥ 1.0; where f LOP f is the initial crack strength. R1 The residual strength when the crack opening displacement reaches 0.2 mm; (ii) Intensity development ratio f R3 / f R1 ≥ 1.0; where f R3 The residual strength is given when the crack opening displacement reaches 2.5 mm.

6. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, as described in claim 1, is characterized in that... The initial mix proportion of the cement-based concrete, by weight, is: 350-400 parts cement, 70-80 parts fly ash, 560-630 parts sand, 1150-1300 parts gravel, 130-150 parts water, and 6.5-8.0 parts water-reducing agent.

7. The two-stage mix design method for fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks as described in claim 1, characterized in that, In stage (two), the total surface area is calculated based on the set structural fiber volume content; Calculate the mass of the stones that can be replaced based on the principle of equal total surface area: ； Where, m 石1 The mass of aggregate per cubic meter of cement-based concrete before the incorporation of structural fibers; m 石2 The mass of aggregate per cubic meter of cement-based concrete after incorporating structural fibers; m h The mass of structural fibers incorporated per cubic meter of cement-based concrete; φ sh The replacement ratio is determined based on the surface area ratio of structural fibers to pebbles of the same mass. Calculate the change in mass Δm per cubic meter of cement-based concrete caused by replacing some of the aggregate with structural fibers: ； Where, m 砼1 The total mass of cement-based concrete per cubic meter before replacement; m 砼2 ρ represents the total mass of cement-based concrete per cubic meter after replacement. h The density is the structural fiber density; V represents the unit volume, taken as 1m³. 3 ;m 砼2 = m 砼1 -(φ sh -1)×m h ; The change in mass of cement-based concrete per cubic meter is distributed according to the proportion of each component of cement-based concrete in the mix proportion of the concrete segment after the addition of crack-resistant fibers, so as to keep the total volume of concrete constant.

8. A method for preparing fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks, characterized in that, The mix proportion obtained according to the design method of any one of claims 1-7 shall be carried out according to the following steps: S1, Preparation of fiber-reinforced concrete mixture, this step includes the following sub-steps: S11, mix sand, gravel and crack-resistant fibers evenly; S12, then add structural fibers and mix evenly; S13, then add cement, fly ash, water and water-reducing agent, and continue to stir evenly to obtain the fiber-reinforced concrete mixture; S2, the fiber-reinforced concrete mixture is poured into the mold and vibrated to ensure that the crack-resistant and structural fibers are evenly distributed in the cement-based concrete and to remove air bubbles. S3 involves curing and demolding the fiber-reinforced concrete component in the mold to obtain the fiber-reinforced concrete segment.

9. The method for preparing fiber-reinforced concrete segments suitable for dual control of micro-cracks and macro-cracks according to claim 8, characterized in that, In step S11, the crack-resistant fiber is evenly spread on the conveyor belt for transporting sand and gravel by a vibrating feeder and sent into the vortex premixing bin; the crack-resistant fiber is pre-dispersed in the sand and gravel under vortex stirring to obtain pre-dispersed material; The vortex premixing bin includes a bin body (1) and a rotary motor (3) mounted on the top of the bin body via a bracket (2). The top of the bin body (1) is the inlet and outlet. The rotary motor (3) has one or more crossbeams (5) mounted at the end of its rotating main shaft (4). One or more flexible levers (6) facing the bottom of the bin body (1) are mounted on the crossbeams (5). Two or more flow guides (7) are mounted on the bottom of the bin body (1) and are evenly arranged in the circumferential direction. The flow guides (7) have an arc-shaped structure.

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