Evaluation method for flexural capacity and failure mode of three-dimensional fiber grid reinforced concrete slab

Abstract

The invention relates to a three-dimensional fiber grid reinforced concrete plate flexural capacity and failure mode evaluation method, relates to a concrete plate flexural capacity and failure mode evaluation method, and aims to solve the problems that three-dimensional fiber grid reinforced concrete plate flexural capacity and failure mode evaluation is high in cost, difficult to realize and high in reliability. The method comprises the following steps: measuring geometric parameters of the three-dimensional fiber grid reinforced concrete slab; measuring and calculating material parameters of the three-dimensional fiber grid reinforced concrete slab; calculating the flexural capacity of the normal section of the three-dimensional fiber grid reinforced concrete slab; calculating the shear bearing capacity of the oblique section of the three-dimensional fiber grid reinforced concrete slab; according to the normal section flexural capacity limit value Mu and the oblique section shear capacity limit value Vu, the failure mode of the three-dimensional fiber grid reinforced concrete slab is obtained according to Pm and Pv. The invention belongs to the field of concrete bearing capacity and failure mode evaluation.

Description

Technical Field

[0001] The present invention relates to a method for evaluating the flexural bearing capacity and failure mode of concrete slabs, and specifically to a method for evaluating the flexural bearing capacity and failure mode of three-dimensional fiber grid reinforced concrete slabs. The present invention belongs to the field of evaluation of concrete bearing capacity and failure mode. Background Art

[0002] The three-dimensional fiber grid reinforced concrete slab is a new type of slab prepared by using a single-layer or multi-layer three-dimensional fiber grid as the reinforcement and cement-based materials such as concrete as the matrix through processes such as casting and spraying. The bearing capacity and failure mode of the three-dimensional fiber grid reinforced concrete slab under bending load are important indicators that must be considered in the design and application stages of concrete slabs. Therefore, it is very necessary to establish a systematic and reliable method for evaluating the flexural bearing capacity and failure mode of three-dimensional fiber grid reinforced concrete slabs. At present, a complete system of coordinated use of three means, namely, experiment, theory and numerical simulation, has been established for the bearing capacity test schemes of traditional flexural members such as reinforced concrete beams and slabs. However, as a high-performance new type of building slab, the three-dimensional fiber grid reinforced concrete slab has not been sufficiently studied, and the evaluation schemes for its bearing capacity and failure mode under bending load are mostly carried out through pure experiments. For the three-dimensional fiber grid reinforced concrete slab, its bearing capacity refers to the magnitude of the external force that can be borne, and the failure mode can be divided into two categories: flexural failure of the normal section and shear failure of the inclined section. According to the size of the test specimens, the tests can be divided into full-scale tests and reduced-scale tests. For building slabs, the dimensions in the length and width directions can often reach several meters. A full-scale test means preparing a three-dimensional fiber grid reinforced concrete slab with the same or similar size as the actual engineering slab and conducting a bending test. A reduced-scale test, on the other hand, refers to preparing a small-size concrete slab and conducting a bending test with reference to relevant specifications. After the test is completed, the designer evaluates the bearing capacity and failure mode of the three-dimensional fiber grid reinforced concrete slab by analyzing the test results.

[0003] The full-scale test specimens of the bending performance of the three-dimensional fiber grid reinforced concrete slab have large sizes, high preparation costs and high load levels, which pose high requirements for the test devices and test personnel, and it is difficult for relevant manufacturers or testing institutions to meet the relevant requirements. For the reduced-scale test, on the one hand, the relevant specifications are not perfect and the guiding role for the test is not sufficient; on the other hand, affected by size effects and the like, the evaluation effect of the test results on the bearing capacity is limited. At the same time, the working conditions involved in the application stage of the three-dimensional fiber grid reinforced concrete slab are very complex. Both full-scale tests and reduced-scale tests can only conduct bearing capacity tests under simple working conditions and cannot fully reflect the conditions of the three-dimensional fiber grid reinforced concrete slab under complex working conditions. In summary, evaluating the flexural bearing capacity and failure mode of the three-dimensional fiber grid reinforced concrete slab through pure experiments is not only costly and difficult to implement, but also the reference significance of the evaluation results is limited. Summary of the Invention

[0004] The object of the present invention is to solve the problems that the evaluation of the flexural bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab is costly, difficult to achieve, and the reference significance of the evaluation results is limited, and further to provide a method for evaluating the flexural bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab.

[0005] The technical solution of the present invention is as follows:

[0006] A method for evaluating the flexural bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab, and the method is realized according to the following steps:

[0007] Step 1: Measure the geometric parameters of the three-dimensional fiber grid reinforced concrete slab:

[0008] Step 2: Measure and calculate the material parameters of the three-dimensional fiber grid reinforced concrete slab;

[0009] Step 3: Calculate the flexural bearing capacity of the normal section of the three-dimensional fiber grid reinforced concrete slab;

[0010] Step 4: Calculate the shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab;

[0011] Step 5: According to the ultimate value M u of the flexural bearing capacity of the normal section and the ultimate value V u of the shear bearing capacity of the inclined section obtained in Step 3 and Step 4, the ultimate values of the external load can be obtained respectively: the flexural load limit P m and the shear load limit P v . According to the magnitude relationship between P m and P v , the failure mode of the three-dimensional fiber grid reinforced concrete slab can be obtained.

[0012] Furthermore, the parameters measured in Step 1 for the three-dimensional fiber grid reinforced concrete slab include measuring the cross-sectional width b, cross-sectional height H0, the distance H i from each surface layer to the upper surface of the concrete slab, the total area A f of each row of longitudinal fiber bundles on the cross-section, the total area A f ′ of each row of core columns, the height h i of each layer of the three-dimensional fiber grid, and the spacing C between adjacent two weft yarns.

[0013] Furthermore, the material parameters measured and calculated in Step 2 for the three-dimensional fiber grid reinforced concrete slab include the prism compressive strength f c of the concrete, the uniaxial tensile strength f t of the concrete, the concrete compressive strain ε0 when the concrete compressive stress just reaches f c , the ultimate concrete compressive strain ε cu , and the ultimate tensile stress of the fiber bundles of the three-dimensional fiber grid is ffu 。

[0014] Furthermore, substitute ε0 and ε obtained in Step 2 cu into Equations (1) and (2) to obtain the simplified stress pattern coefficients α and β:

[0015]

[0016] The relationship between the distance H from each surface layer to the upper surface of the concrete slab i and the distance from the neutral axis to the upper surface of the concrete slab, which is the compression zone height x0, satisfies H i > x0. Then, based on Equations (3)-(5), solve for the ultimate value M of the flexural bearing capacity of the cross-section of the three-dimensional fiber grid-reinforced concrete slab u ;

[0017]

[0018] f f,1 = f fu (5)

[0019] where m is the number of surface layers that satisfy H i > x0.

[0020] Furthermore, before calculating the bearing capacity in Step 3, if the number of three-dimensional fiber grid surface layers that cannot be clearly located in the compression zone is unknown, first assume that all fiber bundles are located on the tensile side, and then solve Equation

[0021]

[0022] f f,1 = f fu (8)

[0023]

[0024] Take:

[0025]

[0026] to obtain x0; judge whether each surface layer satisfies H i > x0. After removing the surface layers that do not meet the requirements, continue to obtain x0 according to the above formula until all the remaining surface layers satisfy H i > x0.

[0027] Furthermore, substituting the relevant data from Step 1 and Step 2 into Equation (12) can obtain the following under the action of the bending load

[0028]

[0029] V uUltimate shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab.

[0030] Further, in step five, P m and P v are used to judge the failure mode of the three-dimensional fiber grid reinforced concrete slab:

[0031] If P m > P v , it indicates that the flexural bearing capacity of the normal section of the slab is greater than the shear bearing capacity of the inclined section, and the failure mode is controlled by the inclined section with weaker bearing capacity, and shear failure of the inclined section occurs;

[0032] If P m < P v , it indicates that the flexural bearing capacity of the normal section of the slab is less than the shear bearing capacity of the inclined section, and the failure mode is controlled by the normal section with weaker bearing capacity, and flexural failure of the normal section occurs;

[0033] If P m ≈P v , it indicates that the flexural bearing capacity of the normal section of the slab is similar to the shear bearing capacity of the inclined section, and both failure modes may occur.

[0034] The present invention has the following effects compared with the prior art:

[0035] 1. In this application, a three-dimensional fiber grid is used as the reinforcing material for the concrete matrix. The warp and weft yarns of the three-dimensional fiber grid are crisscrossed to form upper and lower surface layers, and the core columns connect the two surface layers to form an integral body.

[0036] 2. This application establishes a theoretical calculation method for the bearing capacity of the three-dimensional fiber grid reinforced concrete slab, forming a systematic evaluation method for the bearing capacity and failure mode. Since the bearing capacity calculation method is based on reasonable assumptions and the actual state of stress to establish equations for solution, it can be quickly completed through software, and the obtained results are reliable, which can fully reflect the bearing capacity of the slab, reduce the evaluation cost and improve the evaluation efficiency. At the same time, through the theoretical calculation method of bearing capacity, the bearing capacity and failure mode of the slab under complex working conditions can be evaluated, overcoming the limitation of the test in evaluating complex working conditions.

[0037] 3. This application overcomes the implementation difficulties brought by the complexity of the test through a simple and easy-to-implement calculation formula, and can effectively improve the efficiency of the evaluation scheme for the bearing capacity and failure mode of the three-dimensional fiber grid reinforced concrete slab.

[0038] 4. The calculation formula of this application is based on the stress characteristics of the three-dimensional fiber grid reinforced concrete slab, and the obtained results are reliable, which can correctly reflect the bearing capacity of the three-dimensional fiber grid reinforced concrete slab.

[0039] 5. Based on the calculation formula, the flexural bearing capacity of the plate in the normal section and the shear bearing capacity of the inclined section can be obtained in this application. By comparing the magnitude relationship between the two parts, the failure mode of the concrete slab can be effectively predicted.

[0040] 6. The calculation scheme and the test in this application complement each other to form a systematic evaluation scheme for the three-dimensional fiber grid reinforced concrete slab, which is conducive to the popularization and application of this type of slab in the construction field. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a schematic diagram of a multi-layer three-dimensional fiber grid reinforced concrete slab. Figure 1 (a) is the front view of the three-dimensional fiber grid reinforced concrete slab. Figure 1 (b) is the cross-sectional view of the three-dimensional fiber grid reinforced concrete slab;

[0042] Figure 2 It is a schematic diagram of the concrete prism test scheme;

[0043] Figure 3 It is a schematic diagram of the concrete splitting tensile strength test scheme;

[0044] Figure 4 It is a schematic diagram of the tensile strength test scheme for a single fiber bundle

[0045] Figure 5 It is the stress and strain diagram of the rectangular section in the pure bending section of the three-dimensional fiber grid reinforced concrete slab. Figure 5 (a) is the cross-sectional view of the specimen. Figure 5 (b) is the strain diagram. Figure 5 (c) is the stress diagram. Figure 5 (d) is the equivalent stress diagram;

[0046] Figure 6 It is the process diagram of calculating the flexural bearing capacity of the normal section of the three-dimensional fiber grid reinforced concrete slab;

[0047] Figure 7 It is the schematic diagram of the four-point bending test in the embodiment;

[0048] Figure 8 It is the failure mode diagram of the normal section flexural failure S1 in the embodiment;

[0049] Figure 9 It is the failure mode diagram of the inclined section shear failure S2 in the embodiment. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0050] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Components of the embodiments of the present invention usually described and illustrated in the drawings here can be arranged and designed in various different configurations.

[0051] Step 1: Measure the geometric parameters of the three-dimensional fiber grid-reinforced concrete slab:

[0052] As Figure 1 shown, the parameters measured in Step 1 for the three-dimensional fiber grid-reinforced concrete slab include measuring the cross-sectional width b, cross-sectional height H0, the distance H from each surface layer to the upper surface of the concrete slab, the total area A of each row of longitudinal fiber bundles on the cross-section i , the total area A f ′ of each row of core columns, the height h of each layer of three-dimensional fiber grid f , and the spacing C between adjacent two weft yarns; i

[0053] Step 2: Measure and calculate the material parameters of the three-dimensional fiber grid-reinforced concrete slab;

[0054] Measuring and calculating the material parameters of the three-dimensional fiber grid-reinforced concrete slab include the prism compressive strength f c of concrete, the uniaxial tensile strength f t of concrete, the concrete compressive strain ε0 when the concrete compressive stress just reaches f c , the ultimate compressive strain ε cu of concrete, and the ultimate tensile stress f fu of the fiber bundles of the three-dimensional fiber grid.

[0055] f c and f t are determined according to GB-T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". While preparing the three-dimensional fiber-reinforced concrete slab, prepare 3 concrete prism specimens with dimensions of 150 mm × 150 mm × 300 mm and 3 concrete cube specimens of 150 mm × 150 mm × 300 mm, and cure them for the same age under the same conditions as the concrete slab. As Figure 7 shown, after curing, conduct tests through a compressive testing machine. The calculation formula for f c is:

[0056]

[0057] F1 is the failure load of the prism compressive test; A1 is the bearing area of the specimen.

[0058] ThroughFigure 8 The following scheme can obtain f t The calculation formula is:

[0059]

[0060] F2 is the failure load of the splitting tensile test; A2 is the splitting surface area of the specimen.

[0061] ε0, ε cu According to the "Code for Design of Concrete Structures" GB / T 50010-2015 (2024 Edition), it is calculated through the following formula:

[0062]

[0063] The determination of relevant parameters of the three-dimensional fiber grid fiber bundle is carried out with reference to standards such as GB / T 1446-2005 "General Rules for Test Methods of Fiber Reinforced Plastics Properties", GB / T 3354-2014 "Test Method for Tensile Properties of Oriented Fiber Reinforced Polymer Matrix Composites", and GB20310-2006 "Manufacture and Determination of Tensile Strength of Sized Yarns of E-Glass Rovings". The specific scheme is as follows:

[0064] As Figure 9 shown, first cut a fiber bundle from the three-dimensional fiber grid, clamp both ends of the fiber bundle with the clamping plate of the testing machine, and then conduct a tensile test. The calculation formula of f fu is:

[0065]

[0066] F3 is the failure load of the fiber bundle tensile test; A3 is the cross-sectional area of a single fiber bundle.

[0067] Step 3: Calculate the flexural bearing capacity of the normal section of the three-dimensional fiber grid reinforced concrete slab;

[0068] As Figure 5 (a) shown, the normal section of the three-dimensional fiber grid reinforced concrete slab is divided into two parts by the neutral axis (the dotted line in the figure indicates the neutral axis). The part above the neutral axis is the compression zone, and the part below the neutral axis is the tension zone.

[0069] The three-dimensional fiber grid can withstand large tensile stresses but cannot withstand compressive stresses, while concrete can only bear compressive stresses and is difficult to bear tensile stresses. Therefore, as Figure 5 (c) shown, the stress in the compression zone above the neutral axis is borne by the concrete, and the stress in the tension zone is borne by the three-dimensional fiber grid. Among them, the distance from the neutral axis to the upper surface of the concrete slab is the height x0 of the compression zone. Therefore, for the three-dimensional fiber grid, when H i ≤x0, it is located in the compression zone and does not work; when H i >x0, it is located in the tension zone and bears the tensile stress.

[0070] As Figure 5 shown in (c), the stress state of concrete under compression is relatively complex. For the sake of simplified calculation, it is equivalent to the state shown in Figure 5 (d). The complex stress state in the compression zone of concrete is equivalent to a simple uniformly distributed load. Before equivalence, the maximum compressive stress is f c , and after equivalence, the compressive stress across the entire cross-section of the compression zone is αf c ; before equivalence, the height of the compression zone is x0, and after equivalence, the height of the compression zone is βx0. Therefore, α and β are coefficients for simplifying the stress diagram. The equivalence process needs to satisfy that the magnitude of the resultant force and the position of the action point remain unchanged before and after equivalence. Based on this, substituting ε0 and ε cu obtained in step 2 into equations (7) and (8), α and β are obtained:

[0071]

[0072] As Figure 6 shown, since the number of three-dimensional fiber grid surface layers located in the compression zone cannot be determined before calculating the bearing capacity, it is first assumed that all fiber bundles are located on the tension side. After substituting equations (11) and (12) into equation (9), a quadratic equation of one variable (13) with x0 as the unknown is obtained. This equation has two solutions, one is shown in equation (14); the other must be greater than H1 and is discarded. Determine whether each surface layer satisfies H i >x0. After removing the surface layers that do not meet the requirements, continue to solve equations (9), (11), and (12) to obtain x0 until all the non-removed surface layers satisfy H i >x0, and count the number m of surface layers that meet the requirements. Then, based on equations (10)-(12), solve for the ultimate value M u of the flexural bearing capacity of the cross-section of the three-dimensional fiber grid reinforced concrete slab.

[0073]

[0074] f f,1 =f fu (12)

[0075]

[0076] Take: (14)

[0077]

[0078] Step 4: Calculate the shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab;

[0079] Substituting the relevant data obtained in steps 1 and 2 into equation (16), the ultimate value V u of the shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab under the action of bending load can be obtained.

[0080]

[0081] Step Five: Failure Mode Evaluation

[0082] According to the load application scheme and the ultimate flexural capacity limit value M of the normal section obtained in Steps Three and Four u and the ultimate shear capacity limit value V of the inclined section u the ultimate external load values can be obtained respectively: the flexural load ultimate P m and the shear load ultimate P v . The flexural load ultimate P m is the maximum external load that the concrete slab can bear calculated based on the ultimate flexural capacity limit value M of the normal section u and the actual stress state; the shear load ultimate P v is the maximum external load that the concrete slab can bear calculated based on the ultimate shear capacity limit value V of the inclined section u and the actual stress state. P m and P v can be either concentrated load or distributed load. According to the magnitude relationship between P m and P v the failure mode of the three-dimensional fiber grid reinforced concrete slab can be obtained: P m >P v , indicating that the flexural capacity of the normal section of the slab is greater than the shear capacity of the inclined section, and the failure mode is controlled by the inclined section with weaker bearing capacity, and inclined section shear failure occurs; P m <P v , indicating that the flexural capacity of the normal section of the slab is less than the shear capacity of the inclined section, and the failure mode is controlled by the normal section with weaker bearing capacity, and normal section flexural failure occurs; P m ≈P v , indicating that the flexural capacity of the normal section of the slab is similar to the shear capacity of the inclined section, and both failure modes may occur.

[0083] Example: Take the three-dimensional fiber grid reinforced concrete slab with a two-layer fiber grid structure as an example.

[0084] Step One: Measure the cross-sectional width b, cross-sectional height H0 of the three-dimensional fiber grid reinforced concrete slab, the distance H from each surface layer to the upper surface of the concrete slab i , the total area A f of each row of longitudinal fiber bundles on the cross-section and the total area A f ′ of each row of core columns.

[0085]

[0086] Step Two: Measure the compressive strength f c of the concrete prism and the uniaxial tensile strength f t, the concrete compressive strain ε0 when the concrete compressive stress just reaches f c , the ultimate concrete compressive strain ε cu , the ultimate tensile stress of the three-dimensional fiber grid fiber bundle is f fu , the height h i of each layer of three-dimensional fiber grid and the spacing C between two adjacent weft yarns.

[0087]

[0088] Step 3: Calculate the flexural bearing capacity of the cross-section of the three-dimensional fiber grid reinforced concrete slab.

[0089]

[0090] Step 4: Calculate the shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab

[0091]

[0092] Step 5: Evaluate the failure mode

[0093]

[0094] P v,t = 2V u,t (14)

[0095] Where: L is the span of the specimen, that is, the distance between the supports.

[0096] The M u and V u obtained from Step 2 and Step 3 are available:

[0097]

[0098] As known from the table: The S1 group satisfies P m,t < P v,t , so the flexural failure of the cross-section occurs; The S2 group satisfies P m,t > P v,t , so the shear failure of the inclined section occurs.

[0099] Step 7: Compare and analyze the theoretical results with the experimental results. The following table shows the comparison between the theoretical values and experimental values of the bearing capacity.

[0100]

[0101] As known from the table: The calculated results are close to the experimental results and the failure modes are the same. The failure modes of the two embodiments are shown in Figure 8 、 Figure 9 . That is, the method for evaluating the flexural bearing capacity and failure mode of the three-dimensional fiber grid reinforced concrete slab proposed by the present invention has high accuracy and strong practicability.

[0102] Note that the above is only a preferred embodiment of the present invention and the technical principles applied. Those skilled in the art will understand that the present invention is not limited to the specific embodiments described herein. Various obvious changes, re-adjustments, and substitutions can be made by those skilled in the art without departing from the protection scope of the present invention. Therefore, although the present invention has been described in detail through the above embodiments, the present invention is not limited to the above embodiments. Without departing from the concept of the present invention, it can also include more other equivalent embodiments, and the scope of the present invention is determined by the scope of the appended claims.

Claims

1. A method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab, characterized by: The method is implemented according to the following steps: Step 1: Measure the geometric parameters of the three-dimensional fiber grid reinforced concrete slab; Step 2: Measure and calculate the material parameters of the three-dimensional fiber grid reinforced concrete slab; Step 3: Calculate the bending bearing capacity of the positive section of the three-dimensional fiber grid reinforced concrete slab; Step 4: Calculate the shear bearing capacity of the inclined section of the three-dimensional fiber grid reinforced concrete slab; Step 5: According to the bending bearing capacity limit value M of the normal section obtained in step 3 and step 4 u and the shear bearing limit value V of the inclined section u , the external load limit values ​​can be obtained respectively: bending load limit P m and the shear load limit P v , bending load limit P m It is based on the maximum bending bearing capacity of the normal section M u The maximum external load that the concrete slab can withstand calculated based on the actual stress state; the shear load limit P v According to the shear bearing limit value V of the inclined section u The maximum external load that the concrete slab can withstand is calculated based on the actual stress state. m and P v The failure mode of the three-dimensional fiber grid reinforced concrete slab can be obtained based on the relationship between the sizes of .

2. According to claim 1, a method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab is characterized by: The parameters of the three-dimensional fiber grid reinforced concrete slab measured in step 1 include measuring the cross-sectional width b, cross-sectional height H0, and the distance H from each surface layer to the upper surface of the concrete slab. i , the total area of ​​each row of longitudinal fiber bundles on the cross section A f 、Total area of ​​each row of core columns A f ′, the height of each layer of three-dimensional fiber grid h i And the spacing C between two adjacent weft yarns.

3. The method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab according to claim 1, characterized in that: In step 2, the material parameters of the three-dimensional fiber grid reinforced concrete slab are measured and calculated, including the compressive strength of the concrete prism f c , uniaxial tensile strength of concrete f t 、Concrete compressive stress just reaches f c The concrete compressive strain ε0 and the ultimate compressive strain ε cu , the ultimate tensile stress of the three-dimensional fiber grid fiber bundle is f fu .

4. The method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab according to claim 1, characterized in that: The ε0 and ε obtained in step 2 are cu Substituting into equation (1) and equation (2), we can obtain simplified stress graph coefficients α and β: The distance H from each surface layer to the upper surface of the concrete slab i The relationship between the distance from the neutral axis to the upper surface of the concrete slab and the height of the compression zone x0 satisfies H i >x0, and then according to equations (3)-(5), the moment bearing capacity limit value M of the positive section of the three-dimensional fiber grid reinforced concrete slab is solved u ; f f,1 =f fu (5) Where m is the value that satisfies H i >x0 number of surface layers.

5. According to claim 1, a method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab is characterized by: Before calculating the bearing capacity in step 3, the number of three-dimensional fiber grid surface layers in the compression zone cannot be determined. First, assume that all fiber bundles are located on the tension side, and then solve the equation f f,1 =f fu (8) Pick: Get x0; determine whether each surface layer satisfies H i >x0, after removing the surface layers that do not meet the requirements, continue to obtain x0 according to the above formula until all the surface layers that have not been removed meet H i >x0.

6. According to claim 1, a method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab is characterized by: Substituting the relevant data of step 1 and step 2 into formula (12), we can obtain the following equation under bending load: V u Limit value of shear bearing capacity of inclined section of three-dimensional fiber grid reinforced concrete slab.

7. The method for evaluating the bending bearing capacity and failure mode of a three-dimensional fiber grid reinforced concrete slab according to claim 1, characterized in that: Step 5 P m and P v The size of the three-dimensional fiber grid reinforced concrete slab determines the failure mode: If P m >P v , indicating that the bending bearing capacity of the normal section of the plate is greater than the shear bearing capacity of the inclined section, and the failure mode is controlled by the inclined section with weaker bearing capacity, and the shear failure of the inclined section occurs; If P m <P v , indicating that the bending bearing capacity of the normal section of the plate is less than the shear bearing capacity of the inclined section, and the failure mode is controlled by the normal section with weaker bearing capacity, and the normal section bending failure occurs; If P m ≈P v , indicating that the bending bearing capacity of the plate's normal section is similar to the shear bearing capacity of the inclined section, and both failure modes may occur.

Images