Fiber concrete toughening design method

Abstract

The application discloses a kind of fiber concrete toughening design methods, belong to concrete technical field.The method of the present application changes the passive control characteristics of fiber to the crack propagation process in traditional fiber concrete, from the source of composite material design, first design the composite cementitious material matrix with super-high ductility, secondly, in the specific matrix and aggregate parameter combination, make the composite material develop mesoscopic multi-crack cracking and strain hardening behavior between aggregate particles during external loading process.The fiber concrete prepared by this method actively disperses the brittle damage of the material inside, and avoids the crack instability propagation caused by the weak interface bonding between aggregate and matrix.Compared with the traditional fiber concrete design method, the present application more accurately constructs the correlation between material composition, microstructure and macroscopic mechanical response, and more efficiently improves the toughness of the material.

Description

Technical Field

[0001] This invention relates to the field of concrete technology, and in particular to a design method for toughening fiber-reinforced concrete. Background Technology

[0002] Cement-based materials, such as concrete, are typical brittle solid materials that absorb energy primarily through brittle fracture during failure. These materials generally exhibit high compressive strength but low tensile strength, making them highly susceptible to cracking during service. To improve material ductility, short, uniformly distributed short fibers are incorporated into concrete to bridge cracks. After cracking, the concrete absorbs additional energy through debonding, pull-out, or fiber rupture at the matrix interface, thereby enhancing the material's deformation capacity (ductility) and the total absorbed energy (toughness) before failure. Guided by this principle, current research has developed the concept of "fiber-reinforced concrete," which has been widely applied in various engineering facilities.

[0003] Traditional fiber-reinforced concrete design methods mainly involve two stages. First, designing the ordinary concrete matrix involves constructing a high-density, high-compressive-strength concrete mix based on a reasonable particle size distribution, water-cement ratio, and the type and amount of cementitious materials. Second, selecting appropriate fiber types, dimensions, and dosages to improve the material's ductility and toughness while meeting the requirements for fluidity and economy. These two stages of material design are independent, but their basic idea is to use fibers to bridge cracks in ordinary concrete, thereby achieving the goal of toughening.

[0004] For example, Chinese invention patent CN111620608A discloses an ultra-high toughness cement-based composite material and its design method. First, the gradation distribution of each solid component in the ultra-high toughness cement-based composite material with different mix proportions is determined according to the closest packing model, forming multiple matrices with different compositions. The amount of slender fibers in each matrix is ​​changed, and the matrix with the lowest slender fiber content is selected as the preferred group under the condition of satisfying the compressive strength. The slender fibers in the preferred group are partially replaced by short fibers, and the flexural strength at different replacement rates is measured. The preferred amounts of slender fibers and short fibers are determined according to the slender fiber content corresponding to the preferred group and the replacement rate that satisfies the flexural strength. The ultra-high toughness cement-based composite material is prepared according to the determined mix proportion of the preferred group and the preferred amounts of slender fibers and short fibers.

[0005] From the perspective of fracture mechanics, the improvement in toughness of traditional fiber-reinforced concrete mainly comes from three parts: (1) brittle fracture of the cement matrix, which is determined by the composition and proportion of cementitious materials; (2) the bridging, deflection, and retention effects of aggregates on cracks, which are determined by aggregate particle size, morphology, and dosage; and (3) the bridging effect of fibers on cracks, which is determined by fiber parameters and dosage. These three parts are superimposed to obtain the composite toughness of traditional fiber-reinforced concrete.

[0006] For example, Chinese invention patent CN107271257A provides a method for designing ECC (Engineered Cementitious Composites) formulations based on micromechanical and fracture mechanics tests, including the following steps: (1) determining the performance of the high-toughness cement-based engineering composite formulation; (2) determining fiber parameters, such as fiber type, fiber length, and fiber diameter, and selecting matrix parameters such as pre-mixing of matrix components and determining the matrix fracture toughness K. m Elastic modulus E m (3) Determine the fiber-matrix interface parameters, such as the frictional bond strength τ and the chemical bond strength G. d 1. Slip-hardening coefficient β and fiber-interface buffer factor g; (4) Determine the fiber volume fraction based on the above parameters, and then conduct tests according to the material mix ratio to verify whether the material meets the predetermined performance. The ECC component designed by the above method does not contain coarse aggregate and is a fiber-reinforced ultra-high ductility mortar. Its production cost is very high, which limits its application in engineering. Its design and production are not yet widely applicable.

[0007] The aforementioned traditional fiber-reinforced concrete design method treats the fibers as reinforcement of the brittle concrete matrix, based on a passive approach to the formation and development of the dominant crack. This design philosophy results in the fiber bridging effect primarily occurring after the formation of the dominant crack during the failure process, and ultimately leading to complete failure after the steady-state propagation of a single crack to critical instability. This method has low toughening efficiency, relying mainly on the bridging effect of the fibers passing through the dominant crack to absorb external energy.

[0008] Therefore, proposing a new design concept for fiber-reinforced concrete, where the bridging effect of the fibers acts simultaneously before and after the formation of the main crack, and actively disperses the main crack into multiple micro-cracks, can significantly improve the overall utilization efficiency of the fibers. The fiber-reinforced concrete design method developed based on this concept can significantly improve the toughness of the composite material while maintaining low cost. This is of great significance for optimizing the toughening of fiber-reinforced concrete. Summary of the Invention

[0009] In view of the above-mentioned deficiencies of the prior art, in a first aspect of the present invention, a fiber-reinforced concrete toughening design method with low cost, high toughening efficiency, and wide adaptability is provided, comprising the following steps:

[0010] (1) Mortar matrix design:

[0011] Based on the material's compressive strength requirements, the ratio of cementitious materials, sand, and water is determined, and the sand particle size D is controlled. 50 The particle size ranges from 0.1 to 4.75 mm, and the particle roundness ranges from 0.3 to 0.9.

[0012] Determine the Mode I fracture toughness of the matrix; while meeting the compressive strength requirements, adjust the proportions of the components to control the Mode I fracture toughness to 0.01–1.0 MPa·m. 1 / 2 ;

[0013] (2) Design of ductile composite mortar with strain hardening characteristics:

[0014] Based on the obtained mortar matrix, a fiber-reinforced ductile composite mortar was designed. The fiber length was 6–18 mm and the diameter was 10–50 μm. The minimum standard for the frictional bond force (τ) between the fiber and the mortar matrix was 0.8 MPa, and the chemical bond force (G) was ≤1.5 J / m. 2 The total volumetric fiber content is 1.5% to 2.5%, and it is uniformly distributed in a three-dimensional random manner in the mortar matrix.

[0015] The obtained ductile composite mortar was subjected to a notched direct tensile test under standard test conditions. The bridging effect of the fiber on the cracks must meet the stress and energy criteria for multi-crack cracking. The parameters of the mortar matrix and fiber were adjusted. The minimum standard for the tensile ductility of the ductile composite mortar was 2%, and the maximum crack width before unloading was ≤50~300μm.

[0016] (3) Interaction design between ductile composite mortar and aggregate:

[0017] Select aggregates and control aggregate particle size D. 50 The average spacing of aggregates is 4.75 to 50 mm, and the minimum standard for average aggregate spacing is 6 mm (i.e., minimum fiber length). The amount of aggregate in single-crystal fiber concrete can be determined based on the average aggregate spacing.

[0018] Based on the actual average spacing of aggregates, adjust the design parameters of fiber length in ductile composite mortar to meet the design criterion that average aggregate spacing > fiber length.

[0019] The minimum standard for the consistency (μ) of the mortar matrix is ​​8.0 Pa·s, and the minimum standard for the uniformity distribution coefficient (α) of the aggregate in the composite mortar is 0.9.

[0020] (4) Bearing capacity testing and optimization:

[0021] Based on the optimized mix proportions obtained from the above steps, the fiber-reinforced concrete required for the design is prepared.

[0022] After the fiber-reinforced concrete is cured to the specified age according to standard, the uniaxial compressive strength, four-point bending ultimate load and toughness are measured. If the compressive strength does not meet the bearing capacity requirements, feedback is given to step (1) to adjust the composition and proportion of the mortar matrix. The above steps are repeated until the bearing capacity meets the design requirements.

[0023] Based on the above design method, the concept of this invention is to control the key design parameters in each step, and to develop multi-crack opening and fiber bridging effect sequentially at the micro to meso level, thereby maximizing energy absorption, delaying the formation and instability development of the main crack, and improving the toughness of the resulting fiber-reinforced concrete.

[0024] In step (1), if the above conditions regarding D are not met... 50 If the requirements for particle roundness and Mode I fracture toughness are not met, the prepared mortar matrix will generate excessive matrix fracture absorption energy, causing the prepared composite mortar to deviate from the conditions for strain hardening and reduce ductility. In step (2), if the above requirements for fiber length, diameter, frictional bond strength between fiber and mortar matrix, chemical bond strength, and total fiber volume are not met, the designed composite mortar will deviate from the conditions for strain hardening and multi-cracking, resulting in a significant decrease in ductility and fracture in a form close to brittle failure, degenerating to ordinary mortar and concrete. In step (3), if the design standards for tensile ductility and maximum crack width are not met, it means that the absorption capacity of the composite mortar matrix is ​​insufficient, which will lead to a significant decrease in the toughness of the prepared fiber concrete, degenerating to traditional fiber concrete.

[0025] The design criterion of "average aggregate spacing > fiber length" in this design method is closely related to the parameters involved. The minimum standard of 6mm for the average aggregate spacing is a specific manifestation of this criterion, which is a necessary but not sufficient condition to meet the above criterion. Therefore, in the design, to constitute the necessary and sufficient condition, it is also necessary to determine the final aggregate spacing based on the actual fiber length used, and then estimate the aggregate particle size. In addition, meeting this criterion is a necessary condition for this design method to toughen fiber-reinforced concrete by using strain-hardening mortar. If this criterion is not met, the fibers in the prepared composite mortar matrix will not be distributed in the ideal three-dimensional form, which will result in the mortar matrix not being able to exert the fiber bridging effect when subjected to tensile stress between the aggregates, leading to a decrease in ductility and a decrease in the energy absorption capacity of tensile stress, thus resulting in a decrease in the toughness of the fiber-reinforced concrete.

[0026] Preferably, in step (1), the cementitious material system includes at least one of ordinary silicate cement, silicate composite cement, alkali-activated cementitious material, carbonate cementitious material, and aluminate cementitious material.

[0027] Preferably, in step (1), mineral admixtures can be added to the cementitious material according to application requirements to improve workability and strength development.

[0028] More preferably, the mineral admixture includes at least one of fly ash, silica fume, slag, metakaolin, and limestone powder.

[0029] Preferably, in step (1), the type of sand includes at least one of natural river sand, artificial manufactured sand, and artificial synthetic sand.

[0030] Preferably, in step (1), the water used is pure water that meets the ASTM C1602 standard for water used in concrete mixes.

[0031] The determination of fracture toughness in Mode I offers good flexibility and can be conveniently obtained using a three-point bending test on a notched beam. Other testing methods in this field are also applicable, and operators can choose the appropriate method based on actual conditions.

[0032] Preferably, in step (2), the fiber material type includes at least one of polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA), and polyethylene terephthalate (PET).

[0033] In some cases, the frictional or chemical bonding strength between the obtained fiber and the mortar matrix does not meet the range specified in the steps. In this case, a coating treatment can be applied to the fiber surface to make it meet the design requirements.

[0034] Preferably, in step (2), the stress and energy criteria for multi-crack initiation are as follows: initial crack strength of the matrix (σ c < Fiber bridging force on cracks (σ0); fracture absorption energy of the matrix (J) tip ) < Residual energy of fiber bridging effect on cracks (J) b ').

[0035] A further preferred energy criterion for multi-crack initiation is the flat crack propagation mode, with the following formula:

[0036]

[0037] Where σ0 is the bridging force of the fiber on the crack; δ0 is the crack width corresponding to σ0; J b 'The remaining energy of the fiber's bridging effect on the crack; J tip It absorbs energy for the fracture of the matrix.

[0038] The residual energy of fiber bridging to cracks and the fracture absorption energy of the matrix (the toughness of the matrix material at the crack tip) can both be obtained from the σ(δ) curve, which can be measured by a single crack test.

[0039] Preferably, in step (3), the aggregate type includes at least one of basalt crushed stone, granite crushed stone, limestone crushed stone, pebbles, recycled aggregate, and artificial ceramsite.

[0040] More preferably, the aggregate is crushed granite with a particle size D. 50 The spacing is 5–12.5 mm, with an average spacing of 12–20 mm.

[0041] In this field, aggregates can be considered as ideally round aggregates. The average spacing of the aggregates can be calculated based on the aggregate particle size and the aggregate volume ratio. During the design phase, the above method can be used for estimation; during the quality control phase, optical image analysis can be used on the hardened concrete cross-section to statistically determine the actual spacing of the aggregate particles.

[0042] Preferably, in step (3), when mixing ductile composite mortar and aggregate, chemical admixtures or mineral admixtures may be added to adjust the viscosity and uniformity of the matrix.

[0043] More preferably, the chemical additives include at least one of water-reducing agents and thickeners; the mineral admixtures include at least one of silica fume and fine stone powder.

[0044] Unlike existing design methods, this invention proposes a novel fiber-reinforced concrete design concept that allows the bridging effect of fibers to act simultaneously before and after the formation of the main crack, actively dispersing the main crack into multiple micro-cracks to maximize the overall utilization efficiency of the fibers. The fiber-reinforced concrete design method developed based on this concept can significantly improve the toughness of composite materials while maintaining low cost.

[0045] Unlike the ECC formulation design approach, this invention uses ECC as mortar to replace the brittle mortar and fibers in traditional fiber-reinforced concrete, thereby designing a novel composite material with costs close to those of traditional fiber-reinforced concrete and significantly higher toughness. In other words, this invention achieves toughening design of fiber-reinforced concrete by combining ECC with coarse aggregate (rather than by combining plain concrete and fibers as in traditional fiber-reinforced concrete).

[0046] As can be seen from the specific design steps, this method differs fundamentally from the design philosophy of traditional fiber-reinforced concrete. Unlike traditional fiber-reinforced concrete, which treats fibers as a means of controlling cracks in the existing concrete matrix, this method separates the design of the matrix and aggregates. First, it designs a fiber-reinforced composite cementitious matrix with ultra-high ductility. Then, combined with aggregates, through synergistic parameter design, the composite material develops multi-crack propagation and fiber bridging effects at the micro to mesoscopic levels before the formation of the main crack, thereby maximizing energy absorption and delaying the formation and instability development of the main crack. This design method can significantly improve the flexural toughness of the composite material and improve its mechanical response in the softening segment of the flexural load-deflection curve. The concrete matrix is ​​decomposed into mortar and aggregates. The fibers and the mortar matrix (excluding aggregates) are designed as an integrated system. By separately optimizing the fiber-mortar system, a strain-hardening ultra-high ductility mortar matrix is ​​achieved, which is then combined with aggregates to form a composite material system. The fiber-reinforced concrete material designed based on this method can adapt to the equipment conditions of traditional concrete mixing and production, and can be industrialized without special equipment. It has the characteristics of low cost and high toughening efficiency.

[0047] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0048] This invention provides a toughening design method for fiber-reinforced concrete, which can more effectively improve the ductility and flexural toughness of the material. It has the advantages of low cost, high toughening efficiency, high accuracy and wide adaptability, which helps in the material design and selection of fiber-reinforced concrete in application scenarios such as earthquake resistance and impact load resistance. Attached Figure Description

[0049] Figure 1 The image shows a photograph of the PVA fibers used in the example.

[0050] Figure 2 Example of a fiber-reinforced concrete flexural strength and toughness testing device;

[0051] Figure 3 Example 1 for calculating various bending toughness parameters;

[0052] Figure 4 Example 2 for calculating various bending toughness parameters;

[0053] Figure 5 The typical bending load-deflection curves of the fiber-reinforced concrete (FRC) of the present invention and conventional fiber-reinforced concrete are compared. (a) is the comparison result between Example 1 and Comparative Example 2, and (b) is the comparison result between Example 3 and Comparative Example 3. Detailed Implementation

[0054] The present invention is further illustrated below by way of embodiments, but the invention is not limited to the scope of the embodiments described herein. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0055] In the following embodiments:

[0056] PVA fiber, model: RECS15, is a short fiber obtained by cutting industrially manufactured fiber spinning; the amount of surface oil coating is expressed as a percentage by weight.

[0057] River sand, a common material available in ordinary building materials markets, has a roundness of 0.3 to 0.9.

[0058] Cement, ordinary Portland cement with a grade of 42.5.

[0059] Example 1

[0060] This embodiment is based on the toughening design method of fiber-reinforced concrete and establishes the design parameters of fiber-reinforced concrete. In order to study the design criterion of "average aggregate spacing > fiber length", this embodiment adjusts the average aggregate spacing to be less than the fiber length, and makes it the minimum value among the six embodiments, and prepares the corresponding test specimens.

[0061] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0062] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 Mix the mortar matrix with water at a mass ratio of 1:2.2:1.16 (212μm), and then mix it with water at a mass ratio of 1:0.18 to obtain the mortar matrix.

[0063] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0064] S3. Add 30% basalt crushed stone aggregate by volume to the ductile composite mortar. The aggregate has a single gradation of 5mm particle size. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniform distribution coefficient of the aggregate in the composite mortar meet the design requirements. After casting and molding, cure for 14 days to obtain fiber concrete samples.

[0065] Example 2

[0066] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the average spacing of the aggregate is similar to the fiber length.

[0067] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0068] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 Mix the mortar with water at a mass ratio of 1:2.2:1.16 (212μm), and then mix it with water at a mass ratio of 1:0.18 to obtain the mortar matrix.

[0069] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0070] S3. Add 30% by volume of basalt crushed stone aggregate to the ductile composite mortar. The aggregate particle size is 5-10mm with continuous gradation. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniformity of the aggregate distribution coefficient in the composite mortar meet the design requirements. After casting and molding, cure for 28 days to obtain fiber concrete samples.

[0071] Example 3

[0072] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the average spacing of the aggregates is the maximum value of the six embodiments and is much greater than the fiber length.

[0073] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0074] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 Mix the mortar with water at a mass ratio of 1:2.2:1.16 (212μm), and then mix it with water at a mass ratio of 1:0.18 to obtain the mortar matrix.

[0075] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0076] S3. Add 30% basalt crushed stone aggregate by volume to the ductile composite mortar. The aggregate has a single gradation of 22mm particle size. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniformity of the aggregate distribution in the composite mortar meet the design requirements. After casting and molding, cure for 14 days to obtain fiber concrete samples.

[0077] Example 4

[0078] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the average spacing between aggregates is greater than the fiber length.

[0079] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0080] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 Mix the mortar with water at a mass ratio of 1:2.2:1.16 (212μm), and then mix it with water at a mass ratio of 1:0.18 to obtain the mortar matrix.

[0081] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0082] S3. Add 40% by volume of basalt crushed stone aggregate to the ductile composite mortar. The aggregate particle size is continuously graded within the range of 5-20mm. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniformity of the aggregate distribution in the composite mortar meet the design requirements. After casting and molding, cure for 3 days to obtain fiber concrete samples.

[0083] Example 5

[0084] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the average spacing of the aggregate is much greater than the fiber length.

[0085] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0086] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 Mix the mortar with water at a mass ratio of 1:2.2:1.16 (212μm), and then mix it with water at a mass ratio of 1:0.18 to obtain the mortar matrix.

[0087] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0088] S3. Add 40% basalt crushed stone aggregate by volume to the ductile composite mortar. The aggregate has a single gradation of 22mm particle size. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniformity of the aggregate distribution in the composite mortar meet the design requirements. After casting and molding, cure for 3 days to obtain fiber concrete samples.

[0089] Example 6

[0090] This embodiment is basically the same as Embodiment 1, except that in this embodiment, the aggregate spacing is greater than the fiber length.

[0091] The design parameters and preparation process of the fiber-reinforced concrete specimen in this embodiment are as follows:

[0092] S1, Design of ordinary Portland cement, fly ash, and river sand (D) 50 The mortar matrix is ​​obtained by mixing the mortar with water at a mass ratio of 1:5:0.5 (for 212μm particles) and then mixing it with water at a mass ratio of 1:0.18.

[0093] S2. PVA fibers with a total volume of 2% are added to the mortar matrix. The PVA fibers are 8mm long and 39μm in diameter, and the surface oil coating is 1.5%. This ensures that the frictional and chemical bonding forces between the fibers and the mortar matrix meet the design requirements. The fibers are mixed evenly to ensure that they are evenly distributed in the mortar matrix in a three-dimensional random manner, resulting in a ductile composite mortar.

[0094] S3. Add 40% by volume of basalt crushed stone aggregate to the ductile composite mortar. The aggregate particle size is continuously graded within the range of 5-20mm. Mix the ductile composite mortar and aggregate. The consistency of the mortar matrix and the uniformity of the aggregate distribution coefficient in the composite mortar meet the design requirements. After casting and molding, cure for 28 days to obtain fiber concrete samples.

[0095] Comparative Example 1

[0096] The method for preparing the fiber-reinforced concrete specimens in this comparative example is as follows:

[0097] S1. Mix ordinary silicate cement, fly ash, and river sand evenly in a mass ratio of 1.0:2.2:1.16, add basalt crushed stone aggregate with a volume ratio of 30% and a continuous gradation of particle size within 5-10mm, and mix evenly with water in a mass ratio of 1:0.18.

[0098] S2. Add 2% PVA fiber by volume, with a PVA fiber length of 30mm and a diameter of 660μm. Mix evenly, cast into shape, and cure for 28 days according to standard.

[0099] Comparative Example 2

[0100] The method for preparing the fiber-reinforced concrete specimens in this comparative example is as follows:

[0101] S1. Mix ordinary silicate cement, fly ash, and river sand evenly in a mass ratio of 1.0:2.2:1.16, add 30% by volume of basalt crushed stone with a single gradation of 5mm particle size, and mix evenly with water in a mass ratio of 1:0.18.

[0102] S2. Add 2% PVA fiber by volume, with a PVA fiber length of 30mm and a diameter of 660μm. Mix evenly, cast into shape, and cure for 14 days according to standard.

[0103] Comparative Example 3

[0104] The method for preparing the fiber-reinforced concrete specimens in this comparative example is as follows:

[0105] S1. Mix ordinary silicate cement, fly ash, and river sand evenly in a mass ratio of 1.0:2.2:1.16, add basalt crushed stone aggregate with a volume ratio of 30% and a single gradation of 22mm particle size, and mix evenly with water in a mass ratio of 1:0.18.

[0106] S2. Add 2% PVA fiber by volume, with a PVA fiber length of 30mm and a diameter of 660μm. Mix evenly, cast into shape, and cure for 14 days according to standard.

[0107] Adopting such Figure 2 The apparatus shown was used to conduct four-point bending tests in both the embodiment and the control example. The obtained load-deflection curves were processed, and parameters such as bending toughness were calculated. The calculation process is as follows:

[0108] 1. First, determine the initial peak load. The initial peak load P1 is the load value corresponding to the first point on the load-deflection curve with a slope of zero. See the attached diagram for the calculation methods of various bending performance parameters when the initial peak load equals the peak load. Figure 3 When the initial peak load is lower than the peak load, the calculation methods for each bending performance parameter are shown in the attached diagram. Figure 4 ;

[0109] 2. Calculate the initial peak stress f1 and the peak stress f p The calculation formula is as follows:

[0110]

[0111] Where f is the strength, MPa; P is the corresponding load value, kN; L is the span used in the four-point bending test, mm; b is the average width of the cross-section on both sides near the fracture surface, mm; d is the average height on both sides near the fracture surface.

[0112] 3. Determine the initial peak deflection δ1 and the peak deflection δ based on the load-deflection curve. p ;

[0113] 4. Determine the corresponding residual load P for deflections of L / 600 and L / 150. D 600 P D 150 And calculate the residual strength f according to the above formula. D 600 f D 150 ;

[0114] 5. Calculate the area T enclosed by the load-deflection curves when the deflection is L / 150. D 150 , which is the bending toughness, J;

[0115] 6. Calculate the equivalent bending strength f based on bending toughness. D e,150 The calculation formula is as follows:

[0116]

[0117] 7. Calculate the equivalent flexural strength ratio R based on the initial peak strength. D T,150 The calculation formula is as follows:

[0118]

[0119] Based on the above calculation process, the test results of Examples 1 to 6 and Comparative Examples 1 to 3 can be obtained, and the corresponding calculation results are shown in Table 1.

[0120] Table 1:

[0121]

[0122] As can be seen from the table, the data on bending toughness and equivalent bending strength in Examples 1-6 show that, under the same fiber content, the present invention can efficiently utilize the toughening effect of fibers. In Examples 2-6, which meet the design criterion of "fiber length < average aggregate spacing," the bending toughness and equivalent bending strength of Example 2 are at least 30% higher than the comparative example; the bending toughness and equivalent bending strength of Example 3 are at least 50% higher than the comparative example; the bending toughness and equivalent bending strength of Example 4 are at least 15% higher than the comparative example; the bending toughness and equivalent bending strength of Example 5 are at least 30% higher than the comparative example; and the bending toughness and equivalent bending strength of Example 6 are at least 40% higher than the comparative example. Furthermore, the increase in toughness of the present invention is related to the design criterion of "fiber length < average aggregate spacing," as confirmed by the results of Examples 3 and 1.

[0123] Appendix Figure 5 Divided into appendix Figure 5 (a) and appendix Figure 5 (b) illustrates the bending load-deflection curves of the present invention and conventional fiber-reinforced concrete under different design conditions. (See attached diagram.) Figure 5 (a) shows the load-deflection curve under the condition of "aggregate spacing < fiber length". The solid red line in the figure corresponds to Example 1 of the present invention, and the dashed black line corresponds to Comparative Example 2 of conventional fiber-reinforced concrete; Appendix Figure 5 (b) shows the load-deflection curves under the condition of "aggregate spacing > fiber length". The solid red line in the figure corresponds to Example 3 of the present invention, and the dashed black line corresponds to Comparative Example 3 of conventional fiber-reinforced concrete. As can be seen from the figure, the softening stage after peak load exhibits significant differences between the present invention and conventional fiber-reinforced concrete. Specifically, the conventional fiber-reinforced concrete, represented by Comparative Examples 2 and 3, shows a sharp decrease in load-bearing capacity after peak load; while Examples 1 and 3 of the present invention show a relatively slow decrease in load-bearing capacity after initial cracking, and exhibit deflection-hardening characteristics when the design criterion of "aggregate spacing > fiber length" is met. Compared with conventional fiber-reinforced concrete, the present invention significantly improves the flexural toughness of the material under the same fiber content (see Table 1 for specific data comparison). This result demonstrates that the constructed fiber-reinforced concrete design method can significantly improve the utilization rate of fibers and their toughening efficiency on concrete, and improve the accuracy of toughness design.

[0124] This method alters the passive crack control characteristic of traditional fiber-reinforced concrete. Under external loads, it actively disperses a single master crack into multiple micron-sized cracks at the mesoscopic level. This distributes the work done by the external load to the fracture energy absorbed by each micron-sized crack during steady-state cracking, as well as the energy absorbed by the fiber-matrix interface debonding and friction under fiber bridging after cracking. This significantly enhances the composite material's energy absorption capacity under external loads. The resulting multi-crack structure at the mesoscopic level more effectively improves the material's ductility compared to traditional fiber-reinforced concrete.

[0125] This method improves fiber utilization and the toughening efficiency of fibers in concrete. It transforms the traditional fiber-reinforced concrete method, which focuses on toughening the main cracks, into one that simultaneously toughens numerous micron-sized cracks. Therefore, with a constant fiber content, it can more effectively enhance the toughness of the composite material and increase the area covered by the soft segment of the stress-strain curve. As demonstrated by one or more embodiments of this invention, the flexural toughness of fiber-reinforced concrete can be improved by at least 15%, and under the design criterion of "average aggregate spacing > fiber length," the flexural toughness can be improved by at least 30%.

[0126] This method also improves the accuracy of toughness design for fiber-reinforced concrete by incorporating the mechanical design parameters of ductile composite mortar into the toughness design of fiber-reinforced concrete. Based on sufficient empirical data, it can more efficiently and accurately predict the bending toughness of composite materials and will help with the material design and selection of fiber-reinforced concrete in applications such as seismic resistance and impact load resistance.

[0127] In summary, compared with traditional fiber-reinforced concrete, the design method of this invention can more effectively improve material ductility, increase fiber utilization and fiber toughening efficiency in concrete, and improve the accuracy of fiber-reinforced concrete toughness design.

[0128] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A design method for toughening fiber-reinforced concrete, characterized in that, Includes the following steps: (1) Mortar matrix design: Based on the material's compressive strength requirements, the ratio of cementitious materials, sand, and water is determined, and the sand particle size D is controlled. 50 The particle size ranges from 0.1 to 4.75 mm, and the particle roundness ranges from 0.3 to 0.

9. Determine the Mode I fracture toughness of the matrix; while meeting the compressive strength requirements, adjust the proportions of the components to control the Mode I fracture toughness to 0.01~1.0 MPa·m. 1 / 2 ; (2) Design of ductile composite mortar with strain hardening characteristics: Based on the obtained mortar matrix, a fiber-reinforced ductile composite mortar was designed. The fiber length was 6–18 mm and the diameter was 10–50 µm. The minimum standard for the frictional bond strength between the fiber and the mortar matrix was 0.8 MPa, and the chemical bond strength was ≤1.5 J / m. 2 The total volumetric fiber content is 1.5%~2.5%, and it is uniformly distributed in a three-dimensional random manner in the mortar matrix. The resulting ductile composite mortar was subjected to a notched direct tensile test under standard test conditions. The bridging effect of the fiber on the cracks must meet the stress and energy criteria for multi-crack cracking. The parameters of the mortar matrix and fiber were adjusted. The minimum standard for the tensile ductility of the ductile composite mortar was 2%, and the maximum crack width before unloading was ≤50~300 µm. (3) Interaction design between ductile composite mortar and aggregate: Select aggregates and control aggregate particle size D. 50 The average spacing of aggregates is 4.75~50 mm, and the minimum standard for average aggregate spacing is 6 mm. The amount of aggregate in single-unit fiber-reinforced concrete is determined based on the average aggregate spacing. Based on the actual average spacing of aggregates, adjust the design parameters of fiber length in ductile composite mortar to meet the design criterion that average aggregate spacing > fiber length. The minimum standard for the viscosity of the mortar matrix is ​​8.0 Pa·s, and the minimum standard for the uniformity distribution coefficient of the aggregate in the composite mortar is 0.

9. (4) Bearing capacity testing and optimization: Based on the optimized mix proportions obtained from the above steps, the fiber-reinforced concrete required for the design is prepared. After the fiber-reinforced concrete is cured to the specified age according to standard, the uniaxial compressive strength, four-point bending ultimate load and toughness are measured. If the compressive strength does not meet the bearing capacity requirements, feedback is given to step (1) to adjust the composition and proportion of the mortar matrix. The above steps are repeated until the bearing capacity meets the design requirements.

2. The method according to claim 1, characterized in that: In step (1), the cementitious material system includes at least one of ordinary silicate cement, silicate composite cement, alkali-activated cementitious material, carbonate cementitious material, and aluminate cementitious material.

3. The method according to claim 1, characterized in that: In step (1), mineral admixtures are added to the cementitious material according to the application requirements to improve workability and strength development; the mineral admixtures include at least one of fly ash, silica fume, slag, metakaolin, and limestone powder.

4. The method according to claim 1, characterized in that: In step (1), the type of sand includes at least one of natural river sand and artificial synthetic sand; the water used is pure water that meets the standards for water used in concrete mixtures.

5. The method according to claim 1, characterized in that: In step (2), the fiber material type includes at least one of polypropylene, polyethylene, polyvinyl alcohol, and polyethylene terephthalate.

6. The method according to claim 1, characterized in that, In step (2), the stress and energy criteria for multi-crack cracking are as follows: the initial cracking strength of the matrix < the bridging force of the fiber on the crack; the fracture absorption energy of the matrix < the residual energy of the bridging effect of the fiber on the crack.

7. The method according to claim 6, characterized in that, The energy criterion for multi-crack initiation is based on the flat crack propagation mode, and its formula is as follows: Where σ0 is the bridging force of the fiber on the crack; δ0 is the crack width corresponding to σ0; J b ’ This is the remaining energy for the bridging effect of fibers on cracks; J tip It absorbs energy for the fracture of the matrix.

8. The method according to claim 1, characterized in that: In step (3), the aggregate type includes at least one of basalt crushed stone, granite crushed stone, limestone crushed stone, pebbles, recycled aggregate, and artificial ceramsite.

9. The method according to claim 1, characterized in that: In step (3), when mixing ductile composite mortar and aggregate, chemical admixtures or mineral admixtures are added to adjust the viscosity and uniformity of the matrix.

10. The method according to claim 9, characterized in that: The chemical admixtures include at least one of water-reducing agents and thickeners; the mineral admixtures include at least one of silica fume and fine stone powder.

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