Toughening design method for fiber-reinforced concrete

Abstract

The present invention relates to the technical field of concrete. Disclosed is a toughening design method for fiber-reinforced concrete. The method of the present invention changes the passive control characteristics of fibers on a concrete crack propagation process in conventional fiber-reinforced concrete. Starting from the origin of composite material design, firstly, an ultrahigh-ductility composite cementitious matrix is designed; and secondly, an aggregate is incorporated. On the basis of a specific combination of matrix and aggregate parameters, a composite material develops meso-scale multiple cracking and strain-hardening behaviors between aggregate particles during external loading. The fiber-reinforced concrete prepared by this method actively disperses brittle damage and failure within the material, and avoids unstable crack propagation caused by weak interfacial bonding between the aggregate and the matrix. Compared to conventional fiber-reinforced concrete design methods, the present invention more accurately establishes relationships between material compositions, microstructures and macroscopic mechanical response, and more efficiently enhances the toughness of the material.

Description

Design method for toughening fiber reinforced concrete Technical Field

[0001] The present invention relates to the technical field of concrete, in particular to a fiber concrete toughening design method. Background Art

[0002] Cement-based materials, such as concrete, are a typical type of brittle solid material that absorbs energy primarily through brittle fracture during destruction. This type of material generally has high compressive strength but low tensile strength, making it very susceptible to cracking during service. To improve the ductility of the material, short fibers with three-dimensional uniform and random distribution are added to the concrete to bridge the cracks. After cracking, the concrete absorbs additional energy through debonding, pull-out, or rupture of the fibers themselves at the interface with the matrix, thereby increasing the material's deformation capacity (i.e., ductility) and the total energy absorbed (i.e., toughness) before failure. Guided by this, current research has developed the concept of "fiber concrete," which has been widely used in various engineering facilities.

[0003] The design method of traditional fiber concrete mainly consists of two stages. First, the ordinary concrete matrix is ​​designed to construct a concrete mix with high density and high compressive strength based on reasonable particle grading, water-cement ratio, and type and amount of cementitious materials. Second, the appropriate fiber type, geometric size, and amount are selected to improve the ductility and toughness of the material while meeting the requirements of mixture fluidity and economy. The material design of the above two stages is independent of each other. The basic idea is to use fibers to bridge the cracks in ordinary concrete, thereby achieving the goal of toughening.

[0004] For example, the Chinese invention patent with publication number CN111620608A discloses an ultra-high toughness cement-based composite material and a design method thereof. First, the gradation distribution state of each solid component of the ultra-high toughness cement-based composite material at different mix ratios is determined according to the closest packing model to form multiple groups of matrices with different compositions; the content of slender fibers in each group of matrices is changed, and a group of matrices with the lowest content of slender fibers is selected as the preferred group while meeting the compressive strength condition; 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 content of slender fibers and short fibers is determined according to the corresponding content of slender fibers in the preferred group and the replacement rate that meets the flexural strength; and the ultra-high toughness cement-based composite material is prepared according to the determined mix ratio of the preferred group and the preferred content of slender fibers and short fibers.

[0005] From the perspective of fracture mechanics, the improvement in toughness of traditional fiber reinforced concrete comes mainly from three factors: (1) the brittle fracture of the cement matrix, which is determined by the composition and proportion of the cementitious material; (2) the bridging, deflection, and retention of the aggregate on the cracks, which is determined by the aggregate particle size, morphology, and dosage; and (3) the bridging effect of the fiber on the cracks, which is determined by the fiber parameters and dosage. The superposition of these three parts gives rise to the toughness of the traditional fiber reinforced concrete composite material.

[0006] For example, the Chinese invention patent with publication number CN107271257A provides a method for designing ECC (Engineered Cementitious Composites) formulations based on micromechanics and fracture mechanics tests, which includes the following steps: (1) predetermining the properties 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-matching of matrix components and determining the matrix fracture toughness K m , elastic modulus E m ; (3) Determine the fiber-matrix interface parameters, such as friction bond strength τ, chemical bond strength G d , the slip-hardening coefficient β, and the fiber-interface buffer factor g; (4) Determine the fiber volume fraction based on the above parameters, and then conduct tests based on the material mix ratio to verify whether the material meets the predetermined performance. The ECC component designed using 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 traditional fiber-reinforced concrete design approach considers the fibers as reinforcement for the brittle concrete matrix, based on a design philosophy that passively addresses the formation and development of primary cracks. This design philosophy results in the fiber's bridging effect occurring primarily after the formation of the dominant crack during material failure. Complete failure occurs after the steady-state expansion of a single crack reaches critical instability. This approach has low toughening efficiency, relying primarily on the fiber's bridging effect across the primary crack to absorb external energy.

[0008] Therefore, if a new fiber-reinforced concrete design concept is proposed, where the fiber's bridging effect acts both before and after the formation of the main crack, actively dispersing the main crack into multiple microcracks to fully maximize the overall fiber utilization efficiency, a fiber-reinforced concrete design method based on this concept could significantly improve the toughness of the composite material while maintaining a low cost. This has important implications for optimizing the toughening of fiber-reinforced concrete.

[0009] Summary of the Invention

[0010] In view of the above-mentioned defects of the prior art, in a first aspect of the present invention, a fiber concrete toughening design method with low cost, high toughening efficiency and wide adaptability is provided, comprising the following steps:

[0011] (1) Mortar matrix design:

[0012] Based on the compressive strength requirements of the material, the ratio of cementitious materials, sand, and water is formulated, and the particle size of the sand is controlled. 50 The particle size is 0.1~4.75mm, and the particle roundness is 0.3~0.9;

[0013] Determine the Mode I fracture toughness of the matrix; adjust the proportion of components to control the Mode I fracture toughness to 0.01-1.0 MPa·m while meeting the compressive strength requirements. 1 / 2 ;

[0014] (2) Design of ductile composite mortar with strain hardening characteristics:

[0015] Combined with the obtained mortar matrix, a fiber-reinforced ductile composite mortar was designed. The fiber length was 6-18 mm, the diameter was 10-50 μm, the minimum standard of frictional adhesion (τ) between the fiber and the mortar matrix was 0.8 MPa, and the chemical adhesion (G) was ≤1.5 J / m 2 The total volume dosage of the fiber is 1.5% to 2.5%, and it is evenly distributed in the mortar matrix in a three-dimensional random manner;

[0016] The resulting ductile composite mortar was subjected to notched direct tensile testing under standard testing conditions. The fiber-bridging effect on cracks was required to meet the stress and energy criteria for multi-crack cracking. By adjusting the parameters of the mortar matrix and fibers, the minimum tensile strength of the ductile composite mortar was 2%, and the maximum crack width before unloading was ≤50-300 μm.

[0017] (3) Interaction design between ductile composite mortar and aggregate:

[0018] Select aggregate and control aggregate particle size D 50 The average spacing of aggregates is 4.75 to 50 mm, and the minimum standard for the average spacing of aggregates is 6 mm (i.e., the minimum fiber length). The amount of aggregate in a single cubic meter fiber concrete can be determined based on the average spacing of aggregates.

[0019] According to the actual average spacing of aggregates, the design parameters of fiber length in ductile composite mortar are adjusted to meet the design principle of average spacing of aggregates > fiber length;

[0020] When mixing ductile composite mortar and aggregate, the minimum standard for the mortar matrix viscosity (μ) is 8.0 Pa□s, and the minimum standard for the uniform distribution coefficient (α) of aggregate in the composite mortar is 0.9;

[0021] (4) Bearing capacity inspection and optimization:

[0022] Based on the mix ratio optimized in the above steps, the fiber concrete required by the design is prepared;

[0023] The fiber concrete is cured to the specified age by standard, and 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), and the mortar matrix composition and proportion are adjusted, and the above steps are repeated until the bearing capacity meets the design requirements.

[0024] Based on the above-mentioned design method, the concept of the present invention is to control the key design parameters in each step, and sequentially develop multi-crack cracking and fiber bridging effects at the microscopic to mesoscopic levels, thereby maximizing energy absorption, delaying the formation of main cracks and the process of instability development, and thus improving the toughness of the resulting fiber concrete.

[0025] In step (1), if the above conditions about D are not satisfied, 50 , particle roundness, and Mode I fracture toughness requirements, the prepared mortar matrix will produce too high a matrix fracture absorption energy, causing the prepared composite mortar to deviate from the conditions for strain hardening and reduced ductility. In step (2), if the above-mentioned requirements for fiber length, diameter, frictional bonding between the fiber and the mortar matrix, chemical bonding, and total fiber volume are not met, the designed composite mortar will deviate from the conditions for strain hardening and multi-crack cracking, resulting in a significant decrease in ductility, fracture in a form close to brittle failure, and degenerate to ordinary mortar and concrete. In step (3), if the design standards for tensile strength and maximum crack width are not met, it means that the absorption capacity of the composite mortar matrix is ​​insufficient, which will cause the toughness of the prepared fiber concrete to decrease significantly and degenerate to traditional fiber concrete.

[0026] In this design method, the design criterion of "average aggregate spacing > fiber length" is closely related to the parameters involved. The minimum standard of average aggregate spacing of 6mm is a specific embodiment of the criterion and a necessary but not sufficient condition for meeting the above criterion. Therefore, in the design, in order to constitute a necessary and sufficient condition, it is also necessary to formulate 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 concrete by using strain-hardening mortar. If this criterion is not met, in the prepared composite mortar base, the fibers will not be distributed in an ideal three-dimensional form, resulting in the mortar base being unable to play a fiber bridging role when subjected to tensile stress between aggregates, resulting in a decrease in ductility and a decrease in the energy absorption capacity of tensile stress, thereby resulting in a decrease in the toughness of the fiber concrete.

[0027] Preferably, in step (1), the cementitious material system includes at least one of ordinary Portland cement, Portland composite cement, alkali-activated cementitious material, carbonate cementitious material, and aluminate cementitious material.

[0028] Preferably, in step (1), mineral admixtures may be added to the cementitious material according to application requirements to improve workability and strength development.

[0029] Further preferably, the mineral admixture includes at least one of fly ash, silica fume, slag, metakaolin, and limestone powder.

[0030] Preferably, in step (1), the type of sand includes at least one of natural river sand, artificial machine-made sand, and artificial synthetic sand.

[0031] Preferably, in step (1), the water used is purified water that complies with the standard for water used in concrete mixtures ASTM C1602.

[0032] Mode I fracture toughness is a flexible measurement method that can be conveniently obtained using a three-point bending test on a notched beam. Other test methods available in the field are also applicable, and the operator can select the appropriate method based on actual conditions.

[0033] Preferably, in step (2), the material type of the fiber includes at least one of polypropylene (PP), polyethylene (PE), polyvinyl alcohol (PVA), and polyethylene terephthalate (PET).

[0034] In some cases, the frictional bonding force or chemical bonding force obtained between the fiber and the mortar matrix does not meet the range defined in the step. In this case, the fiber surface can be coated to meet the design requirements.

[0035] Preferably, in step (2), the stress and energy criteria for multi-crack cracking are as follows: the initial cracking strength of the matrix (σ c )<Fiber bridging force on cracks (σ0); Fracture absorption energy of matrix (J tip )<Fiber bridging effect on cracks (J b ').

[0036] More preferably, the energy criterion for multi-crack cracking is the flat crack expansion mode of the cracks, and its formula is as follows:

[0037] Where σ0 is the bridging force of the fiber on the crack; δ0 is the crack width corresponding to σ0; J b ' is the residual energy of fiber bridging effect on cracks; J tip Absorb energy for the fracture of the matrix.

[0038] The residual energy of the fiber's bridging effect on the crack and the matrix's fracture absorption energy (the toughness of the matrix material at the crack tip) can both be obtained through the σ(δ) curve, which can be measured through a single crack test.

[0039] Preferably, in step (3), the type of aggregate 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 granite crushed stone with a particle size D 50 The thickness of the strips is 5 to 12.5 mm, and the average spacing is 12 to 20 mm.

[0041] In this field, aggregates can be considered ideally round. Based on aggregate size and volume fraction, the average aggregate spacing can be calculated. During the design phase, this can be estimated using the aforementioned method. During quality control, optical image analysis can be used on hardened concrete cross-sections to determine the actual spacing between aggregate particles.

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

[0043] Further preferably, the chemical admixture includes at least one of a water reducer and a thickener; and the mineral admixture includes at least one of silica fume and fine stone powder.

[0044] Differentiating from existing design methods, this paper proposes a novel fiber-reinforced concrete design concept. This approach leverages the fiber's bridging effect both before and after the formation of a primary crack, actively dispersing the primary crack into multiple microcracks to maximize overall fiber utilization. The resulting fiber-reinforced concrete design method significantly improves the toughness of composite materials while maintaining a low cost.

[0045] Unlike the ECC formulation design approach, this invention uses ECC as the mortar, replacing the brittle mortar and fibers in traditional fiber-reinforced concrete. This results in a new composite material with a cost comparable to traditional fiber-reinforced concrete, yet significantly higher toughness. In other words, this invention achieves fiber-reinforced concrete toughness by combining ECC with coarse aggregate (rather than combining plain concrete with fibers as in traditional fiber-reinforced concrete).

[0046] The specific design steps reveal a fundamental difference between this method and the design philosophy of traditional fiber-reinforced concrete. Unlike traditional fiber-reinforced concrete, which treats fibers as a means of crack control within the existing concrete matrix, this method separates the design of the matrix from the aggregate. First, a fiber-reinforced composite cementitious material matrix is ​​designed to possess ultra-high ductility. This is then combined with the aggregate and, through the coordinated design of related parameters, the composite material undergoes a multi-crack initiation and fiber bridging effect at the microscopic to mesoscopic levels before the formation of the primary crack. This maximizes energy absorption, delays the formation of the primary crack, and leads to instability. This design approach significantly enhances the flexural toughness of the composite material and improves its mechanical response in the softening region of the flexural load-deflection curve. The concrete matrix is ​​decomposed into mortar and aggregate, and the fibers are integrated with the mortar matrix (excluding the aggregate) for an integrated design. By separately optimizing the fiber-mortar system, a strain-hardened, ultra-high ductility mortar matrix is ​​achieved. This is then combined with the aggregate to form a composite material system. The fiber concrete material designed based on this method can adapt to the equipment conditions of traditional concrete mixing and production, and can be industrially produced 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] The present invention provides a fiber concrete toughening design method, which can more effectively improve the ductility and bending toughness of the material. It has the advantages of low cost, high toughening efficiency, high accuracy and wide adaptability, and is helpful for the material design and selection of fiber concrete in application scenarios such as earthquake resistance and impact load resistance.

[0049] The present invention provides a fiber concrete which can meet various compressive strength grade requirements of existing concrete by adjusting the composition, content and mechanical properties of the ductile composite mortar, and can improve the toughness of the material by at least 15 to 50% while keeping the fiber content constant. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIG1 is a physical photograph of the PVA fiber used in the embodiment;

[0051] Figure 2 is an example of a fiber concrete flexural strength and toughness testing device;

[0052] FIG3 is a calculation example 1 for determining various bending toughness parameters;

[0053] FIG4 is a calculation example 2 for determining various bending toughness parameters;

[0054] FIG5 is a comparison of typical bending load-deflection curves of the fiber reinforced concrete (FRC) of the present invention and conventional fiber reinforced concrete, (a) is the comparison result of Example 1 and Comparative Example 2, and (b) is the comparison result of Example 3 and Comparative Example 3;

[0055] FIG6 is a summary of the design process of fiber reinforced concrete (FRC) according to the present invention. DETAILED DESCRIPTION

[0056] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0057] In the following embodiments:

[0058] PVA fiber, model: RECS15, is short fiber obtained by spinning industrial fiber and cutting; the surface oil coating amount is expressed as a percentage by mass;

[0059] River sand is a general-purpose material available in ordinary building material markets, with a roundness of 0.3 to 0.9;

[0060] Cement, ordinary Portland cement with grade 42.5.

[0061] Example 1

[0062] This embodiment is based on the fiber concrete toughening design method and establishes the design parameters of fiber concrete. In order to study the design principle of "average aggregate spacing > fiber length", this embodiment adjusts the average aggregate spacing to be less than the fiber length and the minimum value among the six embodiments, and prepares corresponding specimens.

[0063] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0064] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed according to a mass ratio of 1:2.2:1.16, and then uniformly mixed with water according to a mass ratio of 1:0.18 to obtain a mortar matrix;

[0065] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0066] S3. Add 30% by volume of basalt crushed stone aggregate to the ductile composite mortar, with a single gradation of 5 mm aggregate particle size; mix the ductile composite mortar and aggregate, and ensure that the mortar matrix viscosity and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 14 days to obtain a fiber concrete specimen.

[0067] Example 2

[0068] This embodiment is basically the same as embodiment 1, except that, in the design of this embodiment, the average spacing between aggregates is close to the fiber length.

[0069] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0070] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed according to a mass ratio of 1:2.2:1.16, and then uniformly mixed with water according to a mass ratio of 1:0.18 to obtain a mortar matrix;

[0071] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0072] S3. Add 30% by volume of basalt crushed stone aggregate to the ductile composite mortar, with the aggregate particle size being continuously graded from 5 to 10 mm; mix the ductile composite mortar and the aggregate, so that the mortar matrix viscosity and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 28 days to obtain a fiber concrete specimen.

[0073] Example 3

[0074] This embodiment is basically the same as embodiment 1, except that, in the design of this embodiment, the average spacing of aggregates is the maximum value of the six embodiments and is much larger than the fiber length.

[0075] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0076] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed according to a mass ratio of 1:2.2:1.16, and then uniformly mixed with water according to a mass ratio of 1:0.18 to obtain a mortar matrix;

[0077] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0078] S3. Add 30% by volume of basalt crushed stone aggregate to the ductile composite mortar, with a single gradation of 22 mm aggregate particle size; mix the ductile composite mortar and aggregate, and ensure that the mortar matrix viscosity and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 14 days to obtain a fiber concrete specimen.

[0079] Example 4

[0080] This embodiment is basically the same as embodiment 1, except that, in the design of this embodiment, the average spacing between aggregates is greater than the fiber length.

[0081] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0082] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed according to a mass ratio of 1:2.2:1.16, and then uniformly mixed with water according to a mass ratio of 1:0.18 to obtain a mortar matrix;

[0083] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0084] S3. Add 40% by volume of basalt crushed stone aggregate to the ductile composite mortar, with the aggregate particle size being continuously graded within the range of 5 to 20 mm; mix the ductile composite mortar and the aggregate, so that the viscosity of the mortar matrix and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 3 days to obtain a fiber concrete specimen.

[0085] Example 5

[0086] This embodiment is basically the same as embodiment 1, except that, in the design of this embodiment, the average spacing between aggregates is much greater than the fiber length.

[0087] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0088] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed according to a mass ratio of 1:2.2:1.16, and then uniformly mixed with water according to a mass ratio of 1:0.18 to obtain a mortar matrix;

[0089] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0090] S3. Add 40% by volume of basalt crushed stone aggregate to the ductile composite mortar, with a single gradation of 22 mm aggregate particle size; mix the ductile composite mortar and aggregate, and ensure that the mortar matrix viscosity and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 3 days to obtain a fiber concrete specimen.

[0091] Example 6

[0092] This embodiment is basically the same as embodiment 1, except that, in the design of this embodiment, the aggregate spacing is greater than the fiber length.

[0093] The design parameters and preparation process of the fiber concrete sample of this embodiment are as follows:

[0094] S1、Design of ordinary Portland cement, fly ash, river sand (D 50 212 μm) were mixed in a mass ratio of 1:5:0.5, and then mixed with water in a mass ratio of 1:0.18 to obtain a mortar matrix;

[0095] S2. Add 2% of PVA fiber by total volume to the mortar matrix. The PVA fiber has a length of 8 mm, a diameter of 39 μm, and a surface oil coating of 1.5%. Ensure that the frictional and chemical bonding strengths between the fiber and the mortar matrix meet the design range requirements. Mix the fibers uniformly so that the PVA fibers are uniformly distributed in the mortar matrix in a three-dimensional random manner, thereby obtaining a ductile composite mortar.

[0096] S3. Add 40% by volume of basalt crushed stone aggregate to the ductile composite mortar, with the aggregate particle size being continuously graded within the range of 5 to 20 mm; mix the ductile composite mortar and the aggregate, so that the mortar matrix viscosity and the uniform distribution coefficient of the aggregate in the composite mortar meet the design range requirements; after pouring and forming, perform standard curing for 28 days to obtain a fiber concrete specimen.

[0097] Comparative Example 1

[0098] The fiber concrete sample preparation method of this comparative example is as follows:

[0099] S1. Mix ordinary Portland cement, fly ash, and river sand in a mass ratio of 1.0:2.2:1.16, add 30% by volume of basalt crushed stone aggregate with a particle size of 5 to 10 mm and continuously graded, and mix with water in a mass ratio of 1:0.18;

[0100] S2. Add 2% PVA fiber with a total volume ratio of 30 mm in length and 660 μm in diameter, mix evenly, cast into shape and perform standard curing for 28 days.

[0101] Comparative Example 2

[0102] The fiber concrete sample preparation method of this comparative example is as follows:

[0103] S1. Ordinary Portland cement, fly ash, and river sand were uniformly mixed in a mass ratio of 1.0:2.2:1.16, 30% by volume of basalt crushed stone with a particle size of 5 mm and a single grade was added, and the mixture was uniformly mixed with water in a mass ratio of 1:0.18;

[0104] S2. Add 2% PVA fiber with a total volume ratio of 30 mm in length and 660 μm in diameter, mix evenly, cast into shape and perform standard curing for 14 days.

[0105] Comparative Example 3

[0106] The fiber concrete sample preparation method of this comparative example is as follows:

[0107] S1. Ordinary Portland cement, fly ash, and river sand were uniformly mixed in a mass ratio of 1.0:2.2:1.16, 30% by volume of basalt crushed stone aggregate with a particle size of 22 mm and a single grade was added, and the mixture was uniformly mixed with water in a mass ratio of 1:0.18.

[0108] S2. Add 2% PVA fiber with a total volume ratio of 30 mm in length and 660 μm in diameter, mix evenly, cast into shape and perform standard curing for 14 days.

[0109] The four-point bending test of the embodiment and the control example was carried out using the apparatus shown in FIG2 , and the obtained load-deflection curve was processed to calculate parameters such as bending toughness. The calculation process is as follows:

[0110] 1. First, determine the initial peak load. The load value corresponding to the first point on the load-deflection curve with a slope of zero is the initial peak load P1. When the initial peak load is equal to the peak load, the calculation method of each bending performance parameter is shown in Figure 3; when the initial peak load is lower than the peak load, the calculation method of each bending performance parameter is shown in Figure 4.

[0111] 2. Calculate the initial peak stress f1 and peak stress f p , the calculation formula is as follows:

[0112] 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 section on both sides near the fracture surface, mm; d is the average height on both sides near the fracture surface;

[0113] 3. Determine the initial peak deflection δ1 and peak deflection δ according to the load-deflection curve p ;

[0114] 4. Determine the residual load P corresponding to 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 ;

[0115] 5. Calculate the area T enclosed by the load-deflection curve when the deflection is L / 150 D 150 , which is the bending toughness, J;

[0116] 6. Calculate the equivalent bending strength f according to the bending toughness D e,150 , the calculation formula is as follows:

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

[0118] According to 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.

[0119] Table 1:

[0120] As can be seen from the table, the flexural toughness, equivalent flexural strength, and other data in Examples 1-6 demonstrate that, given the same fiber dosage, the present invention is able to efficiently utilize the toughening effect of fibers. In Examples 2-6, which meet the design criterion of "fiber length < average aggregate spacing," the flexural toughness and equivalent flexural strength of Example 2 are at least 30% higher than those of the control example; the flexural toughness and equivalent flexural strength of Example 3 are at least 50% higher than those of the control example; the flexural toughness and equivalent flexural strength of Example 4 are at least 15% higher than those of the control example; the flexural toughness and equivalent flexural strength of Example 5 are at least 30% higher than those of the control example; and the flexural toughness and equivalent flexural strength of Example 6 are at least 40% higher than those of the control example. Furthermore, the magnitude of the toughness improvement 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.

[0121] Figure 5, divided into Figure 5(a) and Figure 5(b), shows the bending load-deflection relationship curves of the present invention and conventional fiber concrete under different design conditions. Figure 5(a) shows the load-deflection curve for the case of "aggregate spacing < fiber length," with the red solid line corresponding to Example 1 of the present invention and the black dashed line corresponding to Comparative Example 2 of conventional fiber concrete. Figure 5(b) shows the load-deflection curve for the case of "aggregate spacing > fiber length," with the red solid line corresponding to Example 3 of the present invention and the black dashed line corresponding to Comparative Example 3 of conventional fiber concrete. As can be seen from the figure, the present invention and conventional fiber concrete exhibit significant differences in the softening phase after peak load. The conventional fiber concrete represented by Comparative Examples 2 and 3 experiences a sharp drop in load-bearing capacity after peak load, while Examples 1 and 3 of the present invention experience a relatively slow drop in load-bearing capacity after initial cracking and exhibit deflection-hardening characteristics when the design criterion of "aggregate spacing > fiber length" is met. Compared to conventional fiber concrete, the present invention significantly improves the material's flexural toughness at the same fiber content (see Table 1 for specific data comparison). The results show that the fiber concrete design method constructed by them can significantly improve the utilization rate of fibers and their toughening efficiency for concrete, and improve the accuracy of toughness design.

[0122] This method changes the passive crack control characteristics of traditional fiber-reinforced concrete. Under the action of an external load, a single main crack is actively dispersed 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 fiber-matrix interface debonding and friction due to fiber bridging after cracking. This significantly improves the composite material's ability to absorb the work done by the external load. The resulting multi-crack cracking at the mesoscopic level can more effectively improve the material's ductility than traditional fiber-reinforced concrete.

[0123] This method improves fiber utilization and the efficiency of fiber toughening in concrete. It shifts from fiber toughening at the main cracks in traditional fiber concrete to simultaneous fiber toughening at numerous micron-sized cracks. Therefore, at a constant fiber dosage, 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. One or more embodiments of the present invention can improve the flexural toughness of fiber concrete by at least 15%. When the design criterion of "average aggregate spacing > fiber length" is met, the flexural toughness of fiber concrete can be improved by at least 30%.

[0124] This method also improves the accuracy of fiber concrete toughness design, introduces the mechanical design parameters of ductile composite mortar into the toughness design of fiber concrete, and on the basis of sufficient empirical data, can more efficiently and accurately predict the flexural toughness of composite materials, and will help in the material design and selection of fiber concrete in application scenarios such as earthquake resistance and impact load resistance.

[0125] In summary, compared with traditional fiber concrete, the design method of the present invention can more effectively improve the ductility of the material, increase the utilization rate of the fiber and the toughening efficiency of the fiber on the concrete, and at the same time improve the accuracy of the toughness design of the fiber concrete.

[0126] The above describes in detail the preferred embodiments of the present invention. It should be understood that those skilled in the art can make numerous modifications and variations based on the concepts of the present invention without inventive effort. Therefore, any technical solutions that can be derived by those skilled in the art through logical analysis, reasoning, or limited experimentation based on the concepts of the present invention and the prior art should be within the scope of protection defined by the claims.

Claims

1. A fiber concrete toughening design method, characterized in that: The steps include: (1) Mortar matrix design: Based on the compressive strength requirements of the material, the ratio of cementitious materials, sand, and water is formulated, and the particle size of the sand is controlled. 50 The particle size is 0.1~4.75mm, and the particle roundness is 0.3~0.9; Determine the Mode I fracture toughness of the matrix; adjust the proportion of components to control the Mode I fracture toughness to 0.01-1.0 MPa·m while meeting the compressive strength requirements. 1 / 2 ; (2) Design of ductile composite mortar with strain hardening characteristics: Combined with the obtained mortar matrix, a fiber-reinforced ductile composite mortar was designed. The fiber length was 6-18 mm, the diameter was 10-50 μm, the minimum friction bond between the fiber and the mortar matrix was 0.8 MPa, and the chemical bond was ≤1.5 J / m 2 The total volume dosage of the fiber is 1.5% to 2.5%, and it is evenly distributed in the mortar matrix in a three-dimensional random manner; The resulting ductile composite mortar was subjected to notched direct tensile testing under standard testing conditions. The fiber-bridging effect on cracks was required to meet the stress and energy criteria for multi-crack cracking. By adjusting the parameters of the mortar matrix and fibers, the minimum tensile strength 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 aggregate and control aggregate particle size D 50 The average spacing between aggregates is 4.75 to 50 mm, and the minimum standard for the average spacing between aggregates is 6 mm. The amount of aggregate in a cubic meter of fiber concrete can be determined based on the average spacing between aggregates. According to the actual average spacing of aggregates, the design parameters of fiber length in ductile composite mortar are adjusted to meet the design principle of average spacing of aggregates > fiber length; When mixing ductile composite mortar and aggregate, the minimum standard for the mortar matrix viscosity is 8.0 Pa·s, and the minimum standard for the uniform distribution coefficient of aggregate in the composite mortar is 0.9; (4) Bearing capacity inspection and optimization: Based on the mix ratio optimized in the above steps, the fiber concrete required by the design is prepared; The fiber concrete is cured to the specified age by standard, and 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), and the mortar matrix composition and proportion are adjusted, and the above steps are repeated until the bearing capacity meets the design requirements.

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

3. The method according to claim 1, wherein: In the step (1), mineral admixtures may be added to the cementitious material according to 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, wherein: In the step (1), the type of sand includes at least one of natural river sand, artificial machine-made sand, and artificial synthetic sand; and the water used is pure water that meets the water standards for concrete mixtures.

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

6. The method according to claim 1, characterized in that In the step (2), the stress and energy criteria for multi-crack cracking are as follows: the initial cracking strength of the matrix is ​​less than the bridging force of the fiber on the crack; the fracture absorption energy of the matrix is ​​less than 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 cracking is the flat crack propagation mode of the cracks, 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 ' is the residual energy of fiber bridging effect on cracks; J tip Absorb energy for the fracture of the matrix.

8. The method according to claim 1, wherein: In the step (3), the type of aggregate 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, wherein: In the step (3), when mixing the ductile composite mortar and the aggregate, chemical admixtures or mineral admixtures may be added to adjust the matrix viscosity and uniform distribution coefficient.

10. The method according to claim 9, characterized in that: The chemical admixture includes at least one of a water reducer and a thickener; and the mineral admixture includes at least one of silica fume and fine stone powder.

11. The method according to claim 1, wherein: In the step (3), the aggregate particle size D 50 The aggregate is one or more of 4.75-20mm, 4.75-40mm, 4.75-80mm, and 4.75-120mm, and the aggregate adopts one of continuous gradation or single gradation; According to the actual average spacing of aggregates, the design parameters of fiber length in the ductile composite mortar are adjusted. When the fiber length is less than 6 mm, the average spacing of aggregates can be 0-6 mm, meeting the design principle of average spacing of aggregates > fiber length.

12. A fiber concrete produced by the method according to claims 1 to 11, which improves the toughness of the material by at least 15-50% while meeting the various compressive strength grade requirements of existing concrete.

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