Fire resistance evaluation method for reinforced concrete beams with magnesium phosphate inorganic adhesive and carbon fiber cloth

Abstract

The present invention discloses a fire resistance assessment method for a concrete beam reinforced with carbon fiber cloth using magnesium phosphate inorganic adhesive, which relates to the technical field of fire resistance limit assessment. The method comprises the following steps: obtaining the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP cloth, and the concrete when the concrete beam reinforced with CFRP cloth using magnesium phosphate inorganic adhesive reaches a preset fire resistance limit; then, combining the degradation law of the tensile strength of the steel bars and CFRP cloth as a function of the fire temperature, and the degradation law of the compressive strength of the concrete, determining the effective area of ​​the concrete beam cross section, and then determining the height of the compression zone and the residual bending bearing capacity under fire. The actual fire resistance limit is determined based on the judgment result of the residual bending bearing capacity and the bending moment caused by the external load. The thermal insulation effect of the magnesium phosphate inorganic adhesive and the mechanical contribution of the CFRP cloth under fire are explicitly considered, thereby shortening the assessment time and reducing the assessment cost.

Description

Technical Field

[0001] The present invention relates to the technical field of fire resistance evaluation, in particular to a fire resistance evaluation method for a concrete beam reinforced with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive. Background Art

[0002] Carbon fiber-reinforced polymer (CFRP) cloth reinforcement has been widely used in the field of concrete structure reinforcement due to its advantages such as light weight and high strength, small additional load, little disturbance to the original structure and surrounding environment, good corrosion resistance, and convenient construction.

[0003] However, the fire safety of CFRP-reinforced concrete structures faces significant threats. This is because the adhesives used for CFRP reinforcement are typically epoxy-based organic adhesives, which have extremely low glass transition temperatures. Exposure to fire causes the epoxy-based adhesive to soften rapidly, resulting in a sharp decrease in the bond strength between the CFRP sheet and the concrete matrix. This can lead to reinforcement failure and, in severe cases, collapse of the reinforced structure. While fire-retardant coatings and panels can be used to protect CFRP-reinforced concrete structures from fire, this significantly increases the cost of the reinforcement. Furthermore, to meet the required fire resistance rating, the thickness of the coating or panels is typically quite high, necessitating mechanical anchoring measures such as wire mesh to prevent the fireproofing layer from detaching. These complex construction processes and long construction cycles make them unsuitable for large-scale, extensive concrete reinforcement projects. Furthermore, epoxy-based adhesives have poor durability. Concrete structures reinforced with CFRP sheets require regular inspections of their working condition, with inspection intervals no longer than 10 years. This not only increases maintenance costs, causes damage to building decoration, affects the normal use of the structure, but also poses challenges to the long-term safety of the structure.

[0004] Magnesium phosphate inorganic adhesive is a new type of inorganic cementitious material, primarily composed of a mixture of dead-burned magnesium oxide, acid phosphate (typically ammonium dihydrogen phosphate or potassium dihydrogen phosphate), a retarder, admixtures, and water in a specific proportion. It offers advantages such as rapid hardening and early strength, low shrinkage, excellent heat resistance and durability, and excellent bonding with the concrete matrix. Given its excellent heat resistance and durability, magnesium phosphate inorganic adhesive is being used as an alternative to traditional epoxy adhesives, offering an opportunity to improve the fire safety and long-term durability of CFRP-reinforced concrete structures.

[0005] As an important horizontal load-bearing member in a building structure, the fire resistance of beams is crucial to the fire safety of the building structure. Current research on the high-temperature resistance of magnesium phosphate inorganic adhesives is mostly limited to the material level. Few reports have been published on the fire resistance of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesives. It is unclear whether the fire resistance limit of such structural members meets the fire resistance requirements of relevant codes. Therefore, assessing the fire resistance limit of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesives can not only guide the fire resistance design of such structural members, but also have important guiding significance for emergency protection and rescue escape in fire situations.

[0006] Currently, fire testing is a common method for evaluating the fire resistance of structural components. Its advantages are directness and reliability. The basic procedure involves placing the structural component in a fire test furnace. Depending on actual needs, the specimen can be loaded or unloaded. The furnace is then heated according to a standard heating curve. During the test, parameters characterizing the structural component's fire resistance (temperature, deformation, fire resistance, etc.) are measured to assess the component's fire resistance. However, the disadvantages of using fire testing to determine the fire resistance of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesives are high cost, long testing cycles, and difficulty in examining the impact of all key parameters on the component's fire resistance.

[0007] Another method for evaluating the fire resistance of structural components is finite element analysis. Its basic procedure is to use a sequential thermal-mechanical coupling approach to establish a finite element model for the structural component's temperature field analysis and a finite element model for its mechanical properties. The model calculations then determine the structural component's temperature, deformation, fire resistance, and other parameters. However, determining the fire resistance of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive using finite element analysis presents the following issues:

[0008] (1) Due to the poor high-temperature resistance of epoxy organic adhesives, they quickly stop working after being exposed to fire, causing the CFRP sheets to also stop working. Therefore, when evaluating the fire resistance limit of concrete beams reinforced with CFRP sheets bonded with epoxy organic adhesives based on finite element analysis, the thermal insulation effect of epoxy organic adhesives is usually not considered in the temperature field analysis finite element model, and the mechanical contribution of CFRP sheets is usually ignored in the mechanical performance analysis finite element model. However, due to the excellent high-temperature resistance of magnesium phosphate inorganic adhesives, they will not stop working immediately after being exposed to fire, and they play a certain role in thermal insulation and protection for CFRP sheets. Therefore, if the thermal insulation effect of magnesium phosphate inorganic adhesives and the mechanical contribution of CFRP sheets are ignored during finite element analysis, the fire resistance of concrete beams reinforced with CFRP sheets based on this type of adhesive will be underestimated, which will in turn affect the assessment of the disaster situation and the formulation of rescue plans.

[0009] (2) The finite element analysis process of first performing temperature field analysis and then mechanical properties analysis is very time-consuming, especially for CFRP reinforced concrete beams with large cross-sectional dimensions, more reinforcement, and thicker magnesium phosphate inorganic adhesive layer. The workload of establishing a finite element model for mechanical properties analysis is greater and the calculation time is more consuming, which is not conducive to the subsequent expansion parameter analysis to study the influence of various factors on the fire resistance limit of such structural components.

[0010] Based on this, it is urgent to propose a fire resistance evaluation method for CFRP cloth reinforced concrete beams considering the thermal insulation effect of magnesium phosphate inorganic adhesive to solve the above technical problems. Summary of the Invention

[0011] In order to solve the above problems, the present invention proposes a fire resistance assessment method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive. The method explicitly considers the thermal insulation effect of magnesium phosphate inorganic adhesive and the mechanical contribution of CFRP cloth under fire, shortens the assessment time of the fire resistance limit of concrete beams reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive, reduces the assessment cost, provides scientific guidance for disaster assessment and rescue plan formulation of reinforced concrete structures under fire, and also provides a scientific basis for the fire resistance design of concrete structures reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive.

[0012] In order to achieve the above object, the present invention adopts the following technical solutions:

[0013] In a first aspect, the present invention provides a method for evaluating the fire resistance of a concrete beam reinforced with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive, comprising:

[0014] Step 1: Preset the fire resistance limit of concrete beams reinforced with CFRP sheets using magnesium phosphate inorganic adhesive;

[0015] Step 2: Obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with the CFRP sheet bonded with the magnesium phosphate inorganic adhesive reaches the preset fire resistance limit;

[0016] Step 3: Based on the degradation of the tensile strength of the steel bars and CFRP sheets as they change with the fire temperature, as well as the temperatures of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets, the residual strength of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets is obtained;

[0017] Step 4: Determine the effective area of ​​the concrete beam cross section based on the degradation law of concrete compressive strength as it changes with fire temperature and concrete temperature;

[0018] Step 5: Determine the height of the compression zone of the concrete beam reinforced with CFRP sheets using magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheets, as well as the effective area of ​​the concrete beam cross section;

[0019] Step 6: Determine the residual flexural bearing capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire based on the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0020] Step 7: Determine whether the residual bending bearing capacity and the bending moment caused by the external load meet the preset conditions; if so, use the preset fire resistance limit as the actual fire resistance limit of the concrete beam reinforced with CFRP sheet glued with magnesium phosphate inorganic adhesive; if not, adjust the preset fire resistance limit and repeat steps 2 to 7 until the preset conditions are met to obtain the actual fire resistance limit of the concrete beam reinforced with CFRP sheet glued with magnesium phosphate inorganic adhesive.

[0021] As an optional implementation method, step 2 specifically includes:

[0022] Construct a geometric model of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0023] Set thermal parameters for concrete and magnesium phosphate inorganic adhesive in the geometric model; set the run time for the geometric model; set the contact relationship between concrete and magnesium phosphate inorganic adhesive in the geometric model; set boundary conditions for the geometric model; set initial conditions for the geometric model; mesh the concrete and magnesium phosphate inorganic adhesive in the geometric model; set the element type for the concrete and magnesium phosphate inorganic adhesive in the geometric model;

[0024] Output the temperature of the longitudinal reinforcement in the tension zone, longitudinal reinforcement in the compression zone, CFRP cloth and concrete when the geometric model runs for the preset fire resistance limit.

[0025] As an optional implementation, in step 3: the degradation law of the tensile strength of the steel bar with the change of the over-fire temperature is:

[0026] ;

[0027] Where, T is the over-fire temperature of the steel bar, Overheating temperature T The tensile strength of steel bars, is the tensile strength of steel bars at room temperature.

[0028] As an optional implementation, in step 3: the degradation law of the tensile strength of the CFRP cloth with the change of the overfire temperature is:

[0029] ;

[0030] Where, T is the over-fire temperature of CFRP cloth, is the tensile strength of CFRP cloth at room temperature, Overheating temperature T The tensile strength of CFRP cloth;A 、 B 、 C and n Is related to the overheating temperature T Related parameters: When hour, A Take 1.00, B Take 22, C Take 200, n Take 0.9; when hour, A Take 0.59, B Take 150, C Take 490, n Take 0.7; when hour, A Take 0.48, B Take 420, C Take 76000, n Take 1.8.

[0031] As an optional implementation, in step 4: the degradation law of concrete compressive strength with the change of over-fire temperature is:

[0032] ;

[0033] Where, T is the overfire temperature of concrete, Overheating temperature T The compressive strength of concrete, is the compressive strength of concrete at room temperature;

[0034] The 500℃ isotherm of the concrete beam section is drawn according to the concrete temperature. According to the degradation law of concrete compressive strength, the area outside the 500℃ isotherm of the concrete beam section is eliminated, and the area within the 500℃ isotherm is the effective area of ​​the concrete beam section.

[0035] As an optional implementation method, in step 5: the height of the compression zone of the concrete beam reinforced with CFRP cloth by magnesium phosphate inorganic adhesive is:

[0036] ;

[0037] ;

[0038] Where, is the coefficient; b 、 h are the width and height of the effective area of ​​the concrete beam section, respectively; x is the height of the compression zone; is the axial compressive strength of concrete at room temperature; The residual compressive strength of the longitudinal reinforcement in the compression zone when the preset fire resistance limit is reached; The residual tensile strength of the longitudinal reinforcement in the tension zone when the preset fire resistance limit is reached; To determine the residual tensile strength of the CFRP cloth when the preset fire resistance limit is reached; is the cross-sectional area of ​​the tensile reinforcement; is the cross-sectional area of ​​the compressive reinforcement; is the effective cross-sectional area of ​​CFRP cloth; is the CFRP strength utilization coefficient; is the ultimate compressive strain of concrete; is the ultimate tensile strain of CFRP cloth; is the hysteresis strain of CFRP when considering the influence of secondary force.

[0039] As an optional implementation, in step 6: if , the residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire M for:

[0040] ;

[0041] Where, The effective height of the concrete beam section before reinforcement; c is the thickness of magnesium phosphate inorganic glue; It is the distance from the resultant point of longitudinal reinforcement in the compression zone to the compression edge of the section.

[0042] As an optional implementation, in step 6: if , the residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire M for:

[0043] ;

[0044] ;

[0045] Where, is the effective height of the concrete beam section before reinforcement; c is the thickness of magnesium phosphate inorganic glue; It is the distance from the resultant point of longitudinal reinforcement in the compression zone to the compression edge of the section.

[0046] As an optional implementation method, step seven specifically includes:

[0047] calculate ;

[0048] when When the current preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive;

[0049] when When the fire resistance limit is adjusted, the current preset fire resistance limit is adjusted, including:

[0050] when When the current preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;in, r When it is a positive number, take +; r When it is a negative number, take -; R is the preset fire resistance limit.

[0051] In a second aspect, the present invention provides a fire resistance evaluation system for reinforced concrete beams using magnesium phosphate inorganic adhesive bonded to carbon fiber cloth, comprising:

[0052] An initialization module is configured to preset the fire resistance limit of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0053] The finite element analysis module is configured to obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with the CFRP sheet bonded with the magnesium phosphate inorganic adhesive reaches a preset fire resistance limit;

[0054] a residual strength determination module configured to determine the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet based on the degradation law of the tensile strength of the steel bars and the CFRP sheet as a function of the fire temperature and the temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet;

[0055] an effective region determination module configured to determine an effective region of a concrete beam cross section based on a degradation law of concrete compressive strength that varies with fire temperature and concrete temperature;

[0056] A compression zone height determination module is configured as a module configured to determine the compression zone height of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheets, and the effective area of ​​the concrete beam cross section;

[0057] The residual bending capacity determination module is configured to determine the residual bending capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire according to the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0058] The comparison module is configured to determine whether the residual bending bearing capacity and the bending moment caused by the external load meet the preset conditions; if so, the preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive; if not, the preset fire resistance limit is adjusted, and steps 2 to 7 are repeated until the preset conditions are met, thereby obtaining the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive.

[0059] Compared with the prior art, the present invention has the following beneficial effects:

[0060] The present invention explicitly considers the excellent high-temperature resistance of magnesium phosphate inorganic adhesive, which will not stop working immediately after being exposed to fire, thereby playing a certain role in heat insulation and protection for CFRP cloth. A fire resistance evaluation method for CFRP cloth reinforced concrete beams using magnesium phosphate inorganic adhesive is proposed. This solves the problem that in the past, when evaluating CFRP cloth reinforced concrete beams, the heat insulation effect of the adhesive and the mechanical contribution of the CFRP cloth were not considered, resulting in the fire resistance performance of the CFRP cloth reinforced concrete beams being underestimated, which in turn affected the judgment of fire disaster conditions and the formulation of rescue plans.

[0061] The method of the present invention only performs finite element analysis of the temperature field of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive, thereby solving the problems of high cost, long test cycle and difficulty in examining the influence of all key parameters on the fire resistance limit of such structural components when determining the fire resistance limit of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive through fire tests.

[0062] The method of the present invention only performs a finite element analysis of the temperature field of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive, and does not involve a finite element analysis of mechanical properties. This solves the problem of large modeling workload and time-consuming calculations when first performing a finite element analysis of the temperature field and then a finite element analysis of the mechanical properties to determine the fire resistance limit. At the same time, the present invention converts the calculation of the fire resistance limit of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive into a static equilibrium iterative calculation under various fire moments. The fire resistance limit of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive can be determined in a relatively short period of time. The calculation is highly efficient and meets accuracy requirements, facilitating subsequent expanded parameter analysis to study the influence of various factors on the fire resistance of such structural components, and provides a scientific basis for the fire resistance design of concrete structures reinforced with CFRP sheets based on magnesium phosphate inorganic adhesives.

[0063] Advantages of additional aspects of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0064] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0065] Figure 1 Flowchart of the fire resistance evaluation method for reinforced concrete beams using carbon fiber cloth bonded with magnesium phosphate inorganic adhesive provided in Example 1 of the present invention;

[0066] Figure 2 This is a dimension diagram of a concrete beam in Example 1 of the present invention;

[0067] Figure 3 for Figure 2 Reinforcement diagram of middle section A;

[0068] Figure 4 for Figure 2 Reinforcement diagram of middle section B;

[0069] Figure 5 This is a schematic diagram of the arrangement of the CFRP sheet pasted on the bottom surface of the concrete beam in Example 1 of the present invention;

[0070] Figure 6 Schematic diagram of component types and mesh division of the finite element model for temperature field analysis of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive in Example 1 of the present invention;

[0071] Figure 7 This is a cross-sectional boundary condition diagram of a finite element model for temperature field analysis of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive in Example 1 of the present invention;

[0072] Figure 8 Graph showing the temperature-time curves of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet in Example 1 of the present invention;

[0073] Figure 9 The concrete temperature cloud diagram of the beam cross section during the identification process of the effective area of ​​the concrete beam cross section when exposed to fire for 100 minutes in Example 1 of the present invention;

[0074] Figure 10 Schematic diagram of various isothermal lines of a concrete beam cross section during identification of an effective area of ​​a concrete beam cross section when exposed to fire for 100 minutes in Example 1 of the present invention;

[0075] Figure 11 Schematic diagram of the effective area of ​​the concrete beam cross section during the identification process of the effective area of ​​the concrete beam cross section when exposed to fire for 100 minutes in Example 1 of the present invention;

[0076] Figure 12Schematic diagram of the force analysis of the cross section of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive in Example 1 of the present invention;

[0077] Figure 13 Schematic diagram of the loading equipment for the fire resistance test of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive in Example 1 of the present invention;

[0078] Figure 14 This is a comparison chart of the fire resistance test results of the concrete beam BL-1 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive using the conventional method in Example 1 of the present invention and the method of this embodiment;

[0079] Figure 15 This is a comparison chart of the fire resistance test results of the concrete beam BL-2 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive using the conventional method in Example 1 of the present invention and the method of this embodiment;

[0080] Among them, 1. Concrete beam, 2. Sliding hinge support, 3. Fixed hinge support, 4. CFRP cloth, 5. Magnesium phosphate inorganic adhesive, 6. Thermal convection, 7. Thermal radiation, 8. Reaction frame, 9. Hydraulic jack, 10. Pressure sensor, 11. Distribution beam. DETAILED DESCRIPTION

[0081] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0082] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0083] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that the terms "include" and "comprise" and any variations are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0084] In the absence of conflict, the embodiments of the present invention and the features thereof may be combined with each other.

[0085] Example 1

[0086] This embodiment provides a method for evaluating the fire resistance of concrete beams reinforced with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive. The fire resistance limit of a concrete beam BL-1 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive is calculated using the above method of this embodiment and verified according to the fire resistance test method for building components required by relevant standards. The dimensions of the concrete beam used are as follows: Figure 2 As shown, the reinforcement of section A is as follows Figure 3 As shown, the reinforcement of section B is as follows Figure 4 As shown. The span of concrete beam 1 is 3300mm, the calculated span is 3000mm, and the cross-sectional dimensions are 150mm×300mm (cross-sectional width×cross-sectional height). The longitudinal reinforcement at the bottom of the beam is 2 12, beam top reinforcement is 2 12. The stirrup diameter is 8mm, the stirrup spacing in the reinforced area is 100mm, the stirrup spacing in the non-reinforced area is 200mm, and the stirrup cover thickness is 25mm. All steel bars are HRB400 grade hot-rolled steel bars, and the concrete is C30 commercial concrete. All steel bars are HRB400 grade hot-rolled steel bars. The reinforcement plan for concrete beam 1 is as follows: a layer of CFRP cloth 4 (length × width = 3300mm × 100mm) is attached to the center of the bottom surface of concrete beam 1. Figure 5 The thickness of the CFRP cloth 4 used is 0.167 mm and the density is 300 g / m2. The load level of BL-1 is 0.4.

[0087] The raw materials for preparing magnesium phosphate inorganic glue 5 are as follows: dead-burned magnesium oxide, potassium dihydrogen phosphate, glass powder, a homemade composite modifier, borax, calcium chloride hexahydrate and water. The proportions of the components are shown in Table 1.

[0088] The preparation process of magnesium phosphate inorganic glue 5 is as follows: first, pour the weighed potassium dihydrogen phosphate, glass powder, homemade composite modifier and borax into a blender and dry mix for 1 minute; then, dissolve the weighed calcium chloride hexahydrate in 3 / 4 of the total amount of water and pour it into the blender and stir for 2 minutes; then, slowly add the weighed dead-burned magnesium oxide to the mixture and continue stirring for 2 minutes; finally, pour the remaining 1 / 4 of the total amount of water into the mixture and stir rapidly for at least 5 minutes until it becomes a uniform slurry.

[0089] The process of pasting CFRP cloth 4 on the bottom surface of concrete beam 1 is as follows: (1) roughen the bottom surface area to be reinforced and remove impurities in the area to be reinforced; (2) moisten the bottom surface area to be reinforced and evenly apply the prepared magnesium phosphate inorganic adhesive to the area to be reinforced with a thickness of 5 mm; (3) wet the cut CFRP cloth and paste it on the magnesium phosphate inorganic adhesive base glue, and use a scraper to repeatedly scrape along the fiber direction of the CFRP cloth to fully infiltrate the CFRP cloth; (4) evenly apply a layer of magnesium phosphate inorganic adhesive top glue on the surface of the CFRP cloth with a thickness of 5 mm, and smooth the top glue with a scraper. After pasting the CFRP cloth, the specimen was naturally cured for 14 days.

[0090] Table 1 Magnesium phosphate inorganic glue mix ratio;

[0091] .

[0092] In this embodiment, if Figure 1 As shown in FIG, the fire resistance evaluation method of reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive specifically includes the following steps:

[0093] S1: Preset fire resistance of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive.

[0094] In this embodiment, the fire resistance limit R of the concrete beam BL-1 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive is preset to be 100 min.

[0095] S2: Obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with CFRP sheet bonded with magnesium phosphate inorganic adhesive reaches the preset fire resistance limit.

[0096] Specifically, it includes: establishing a temperature field finite element model based on the ABAQUS finite element platform to obtain the fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive to achieve the preset fire resistance limit in step S1. R The temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP cloth and the concrete at that time.

[0097] In this embodiment, the modeling process includes the following steps:

[0098] S201: Establish the geometric model of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive.

[0099] S202: Set thermal parameters for concrete and magnesium phosphate inorganic adhesive in the geometric model.

[0100] In this embodiment, the thermal parameters include: density of concrete, specific heat capacity of concrete, thermal conductivity of concrete, density of magnesium phosphate inorganic glue, specific heat capacity of magnesium phosphate inorganic glue and thermal conductivity of magnesium phosphate inorganic glue.

[0101] Among them, the density of concrete is 2350kg / m 3 The thermal conductivity of concrete, the specific heat of concrete, the density of magnesium phosphate inorganic glue, the thermal conductivity of magnesium phosphate inorganic glue and the specific heat of magnesium phosphate inorganic glue are respectively related to the overfire temperature.

[0102] In this embodiment, the thermal conductivity of concrete is calculated according to formula (1):

[0103] (1);

[0104] Where, is the thermal conductivity of concrete (unit: W / (m·℃)), T is the fire temperature of concrete (unit: ℃).

[0105] The specific heat capacity of concrete is determined according to formula (2):

[0106] (2);

[0107] Where, is the specific heat capacity of concrete (unit: J / (kg·℃)), is the fire temperature of concrete (unit: ℃).

[0108] The density of magnesium phosphate inorganic glue is determined according to formula (3):

[0109] (3);

[0110] Where, is the density of magnesium phosphate inorganic glue (unit: g / cm 3 ), is the overheating temperature of magnesium phosphate inorganic glue (unit: ℃), , is the mass percentage of glass powder (unit: %).

[0111] The thermal conductivity of magnesium phosphate inorganic glue is calculated according to formula (4):

[0112] (4);

[0113] Where, is the thermal conductivity of magnesium phosphate inorganic glue (unit: W / (m·℃)), T is the overheating temperature of magnesium phosphate inorganic glue (unit: ℃), , is the mass percentage of glass powder (unit: %).

[0114] The specific heat capacity of magnesium phosphate inorganic glue is determined according to formula (5):

[0115] (5);

[0116] Where, is the specific heat capacity of magnesium phosphate inorganic glue (unit: J / (g·℃)), T is the overheating temperature of magnesium phosphate inorganic glue (unit: ℃), , is the mass percentage of glass powder (unit: %).

[0117] S203: Setting the running time for the geometric model.

[0118] In this embodiment, the geometric model running time is set to 100 minutes.

[0119] S204: Setting the contact relationship between the concrete and the magnesium phosphate inorganic adhesive in the geometric model.

[0120] In this embodiment, the contact relationship between the concrete and the magnesium phosphate inorganic adhesive in the geometric model is set to "Tie".

[0121] S205: Setting boundary conditions for the geometric model.

[0122] In this embodiment, if Figure 7 As shown in the figure, the bottom and two sides of the geometric model are heated according to the ISO834 standard fire, and are affected by heat convection 6 and heat radiation 7, while the top surface is not affected by the fire; the convection coefficients of the geometric model facing the fire and facing away from the fire are 23~27W / (m 2 ·℃) and 8~10W / (m 2 ·℃); the comprehensive radiation coefficient of the fire-facing surface of the geometric model is 0.65~0.75.

[0123] As an optional implementation, the flow coefficients of the geometric model facing the fire surface and the back-fire surface are 25W / (m2·℃) and 9W / (m2·℃) respectively; the comprehensive radiation coefficient of the geometric model facing the fire surface is 0.7.

[0124] S206: Setting initial conditions for the geometric model.

[0125] In this embodiment, the initial temperature of the geometric model is set to 25°C.

[0126] S207: Mesh the concrete and magnesium phosphate inorganic adhesive in the geometric model.

[0127] In this embodiment, if Figure 6 As shown, the grid size of the concrete beam 1 is set to 25 mm × 25 mm × 50 mm (height × width × length), and the grid size of the magnesium phosphate inorganic glue 5 is set to 5 mm × 25 mm × 50 mm (height × width × length).

[0128] S208: Set the element type for concrete and magnesium phosphate inorganic adhesive in the geometric model.

[0129] In this embodiment, if Figure 6 As shown, the concrete beam 1 is set as a three-dimensional eight-node solid thermal analysis unit (DC3D8), and the magnesium phosphate inorganic glue 5 is set as a three-dimensional eight-node solid thermal analysis unit (DC3D8).

[0130] S209: Geometric model calculation, output geometric model running time is R The temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP cloth and the concrete.

[0131] In this embodiment, the temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete is output when the geometric model runs for 100 minutes. The temperature of the longitudinal reinforcement in the tension zone and the compression zone is replaced by the temperature of the concrete at their location, and the temperature of the CFRP sheet is replaced by the temperature of the magnesium phosphate inorganic adhesive at their location. The temperature-time curves of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet are shown in Figure 1. Figure 8 As shown, the temperature cloud of the concrete in the beam section at 100 minutes is as follows: Figure 9 shown.

[0132] S3: Based on the degradation law of the tensile strength of the steel bars and CFRP sheets as they change with the fire temperature, as well as the temperatures of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets, the residual strength of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets is obtained.

[0133] Specifically:

[0134] The degradation law of the tensile strength of steel bars with the change of over-fire temperature is calculated according to formula (6):

[0135] (6);

[0136] Where, T is the fire temperature of the steel bar (unit: °C), Temperature T The tensile strength of the steel bar (unit: MPa), is the tensile strength of steel bars at room temperature (unit: MPa).

[0137] The degradation law of the tensile strength of CFRP cloth with the change of over-fire temperature is taken according to formula (7):

[0138] (7);

[0139] Where, T is the fire temperature of CFRP cloth (unit: °C), is the tensile strength of CFRP cloth at room temperature (unit: MPa), Temperature T Tensile strength of CFRP cloth at (unit: MPa); A 、 B 、 C and n is a temperature-dependent parameter: when hour, A Take 1.00, B Take 22, C Take 200, n Take 0.9; when hour, A Take 0.59, B Take 150, C Take 490, n Take 0.7; when hour, A Take 0.48, B Take 420, C Take 76000, n Take 1.8.

[0140] Depend on Figure 8 It can be seen that at 100 minutes, the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet are 558.3°C, 467.3°C, and 890.5°C, respectively. The tensile strength of the longitudinal reinforcement at room temperature is 360 MPa, and the tensile strength of the CFRP sheet at room temperature is 2300 MPa. According to Equations (6) and (7), the residual strengths of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet are 149.2 MPa, 202.9 MPa, and 0 MPa, respectively.

[0141] S4: Determine the effective area of ​​the concrete beam cross section based on the degradation law of concrete compressive strength that changes with fire temperature and concrete temperature.

[0142] Specifically:

[0143] The degradation law of concrete compressive strength with fire temperature is given by formula (8):

[0144] (8);

[0145] Where, T is the fire temperature of concrete (unit: °C), Temperature T The compressive strength of concrete (unit: MPa) is the compressive strength of concrete at room temperature (unit: MPa).

[0146] according to Figure 9The temperature cloud of the concrete beam section at 100 minutes is shown in the figure, and the 500℃ isotherm of the concrete beam section is drawn, as shown in the figure. Figure 10 As shown, according to formula (8), the area outside the 500℃ isotherm of the concrete beam section is eliminated, and the area within the 500℃ isotherm is identified as the effective area of ​​the section, thereby obtaining the width of the effective area of ​​the concrete beam section b =61.6mm, height h = 239.6mm, such as Figure 11 shown.

[0147] S5: Determine the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone and the CFRP sheets, as well as the effective area of ​​the concrete beam cross section.

[0148] Specifically:

[0149] According to the static equilibrium condition, the height x of the compression zone of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive is calculated. The stress analysis is as follows: Figure 12 As shown, Figure 12 (a) and (b) are schematic diagrams of force analysis of different sections;

[0150] (9);

[0151] (10);

[0152] Where, is a coefficient. When the concrete strength grade does not exceed C50, it is taken as 1.0. When the concrete strength grade is C80, it is taken as 0.94. The linear interpolation method is used in between. b 、 h Reaching the preset fire resistance limit R= The width and height of the effective area of ​​the concrete beam section at 100 minutes; x is the height of the compression zone; is the axial compressive strength of concrete at room temperature, ; To achieve the preset fire resistance limit R= At 100min, the residual compressive strength of the longitudinal reinforcement in the compression zone is ; To achieve the preset fire resistance limit R= At 100min, the residual tensile strength of the longitudinal reinforcement in the tension zone is ; To achieve the preset fire resistance limit R= At 100min, the residual tensile strength of CFRP cloth is ; is the cross-sectional area of ​​the tensile reinforcement, ; is the cross-sectional area of ​​the compressive reinforcement, ; is the effective cross-sectional area of ​​CFRP cloth, ; is the CFRP strength utilization coefficient, when When, take ; is the ultimate compressive strain of concrete, which is taken as 0.0033; is the ultimate tensile strain of CFRP cloth, which is taken as 0.01; To consider the hysteresis strain of CFRP when the secondary force is affected, if the secondary force is not considered, it is taken as 0.

[0153] S6: Determine the residual flexural bearing capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire based on the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive.

[0154] Specifically:

[0155] If we calculate according to formula (9) and formula (10) , then the residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire is calculated according to formula (11):

[0156] (11);

[0157] Where, M The residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire; The effective height of the concrete beam section before reinforcement; c is the thickness of magnesium phosphate inorganic glue; is the distance from the resultant point of longitudinal reinforcement in the compression zone to the compression edge of the section, .

[0158] If we calculate according to formula (9) and formula (10) When x=0mm , then the resistance provided by the longitudinal reinforcement in the compression zone is ignored, and the compression zone height x and the residual bending bearing capacity M under fire of the concrete beam BL-1 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive are calculated according to formula (12)-formula (13):

[0159] (12);

[0160] (13);

[0161] Where, is the effective height of the concrete beam section before reinforcement, ; c is the thickness of magnesium phosphate inorganic glue, c=10mm. Therefore, we can calculate: x=47.6mm, M=10.0 .

[0162] S7: Determine whether the residual bending bearing capacity and the bending moment caused by the external load meet the preset conditions; if so, use the preset fire resistance limit as the actual fire resistance limit of the concrete beam reinforced with CFRP sheet glued with magnesium phosphate inorganic adhesive; if not, adjust the preset fire resistance limit, and repeat steps S2 to S7 until the preset conditions are met, thereby obtaining the actual fire resistance limit of the concrete beam reinforced with CFRP sheet glued with magnesium phosphate inorganic adhesive.

[0163] Specifically:

[0164] According to formula (14), it is judged whether the residual bending bearing capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire and the bending moment caused by external load meet the preset conditions:

[0165] (14);

[0166] when When the current preset fire resistance limit R That is the actual fire resistance limit of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive.

[0167] when When the current preset fire resistance limit R Make adjustments as follows:

[0168] when When the current preset fire resistance limit R Adjust to ( r If it is a positive number, take +; r If it is a negative number, take -);

[0169] when When the preset fire resistance limit R Adjust to ( r If it is a positive number, take +; r If it is a negative number, take -);

[0170] when When the preset fire resistance limit R Adjust to ( r If it is a positive number, take +; r If it is a negative number, take -);

[0171] when When the preset fire resistance limit R Adjust to ( rIf it is a positive number, take +; r If it is a negative number, take -);

[0172] when When the preset fire resistance limit R Adjust to ( r If it is a positive number, take +; r If it is a negative number, take -);

[0173] when When the preset fire resistance limit R Adjust to ( r If it is a positive number, take +; r If it is a negative number, take -);

[0174] Adjusting the preset fire resistance limit R back ， Repeat steps S2 to S7 until , that is, the actual fire resistance limit of concrete beams reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive was obtained.

[0175] According to the relevant provisions of the "Standard for Experimental Methods of Concrete Structures", the test results of the bending bearing capacity of the concrete beam BL-1 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive at room temperature are 27.5 According to the load level of BL-1, the bending moment caused by the external load is 11 . Make the following judgment:

[0176] .

[0177] Then the preset initial fire resistance limit R is adjusted to Repeat steps S2 to S7, that is, the initial fire resistance limit R = 95min. When this initial fire resistance limit is reached, the temperature of the longitudinal reinforcement in the tension zone, the temperature of the longitudinal reinforcement in the compression zone and the temperature of the CFRP cloth are 538.2℃, 448.4℃ and 875.3℃ respectively; the design value of the tensile strength of the longitudinal reinforcement in the tension zone at the above temperature is The design value of the compressive strength of the longitudinal reinforcement in the compression zone is 161.1 MPa. The design value of CFRP cloth tensile strength is 214.1MPa =0MPa; the width b of the effective area of ​​the concrete beam section is 65.7mm, and the height h of the effective area of ​​the concrete beam section is 250.0mm, then the calculation results are: x=49.6mm, M=11.1 . Make the following judgment:

[0178] .

[0179] The current preset fire resistance limit R = 95min is the actual fire resistance limit of the concrete beam BL-1 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive.

[0180] Effect verification:

[0181] According to the provisions of the "Fire Resistance Test Method for Building Components", a fire resistance test was conducted on the concrete beam BL-1 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive in this embodiment. Figure 13 As shown, BL-1 was placed in a fire test furnace. The bottom ends of BL-1 were simply supported by sliding hinge supports 2 and fixed hinge supports 3, respectively. The upper surface of BL-1, at 1 / 3 of its span, was connected to distribution beam 11 via sliding hinge supports 2 and fixed hinge supports 3. A hydraulic jack 9, fixed to a reaction frame 8, pressed distribution beam 11 downward, applying two symmetrical concentrated loads to BL-1. A pressure sensor 10 was installed between the hydraulic jack 9 and distribution beam 11 to measure the applied load. BL-1 was heated using a constant load with a load level of 0.4. The fire test furnace was heated according to ISO834 standard fire conditions. The bottom and two sides of BL-1 were exposed to fire, while the top was not. The load applied to BL-1 remained constant during the heating process.

[0182] Comparison of the fire resistance test results of BL-1 concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive and the calculation results of this embodiment Figure 14 As shown. Based on the experimental measurement, the fire resistance limit of the concrete beam BL-1 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive is 104 minutes. Based on the calculation of this embodiment, the fire resistance limit of the concrete beam BL-1 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive is 95 minutes, with an error of 9%. Therefore, this embodiment explicitly considers the thermal insulation effect of the magnesium phosphate inorganic adhesive and the mechanical contribution of the CFRP sheet. Only the finite element analysis of the temperature field of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive is performed to determine its fire resistance limit. This solves the problem of high cost, long test cycle, and difficulty in examining the impact of all key parameters on the fire resistance limit of such structural components when determining the fire resistance limit of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive through fire tests.

[0183] In order to better verify the effectiveness of the method of this embodiment, the fire resistance limit of another concrete beam BL-2 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive was calculated using the method of this embodiment. The load level of BL-2 is 0.5, and other working conditions such as component size, reinforcement, and reinforcement scheme are the same as BL-1 in this embodiment. According to the "Standard for Experimental Methods of Concrete Structures", the flexural bearing capacity test result of BL-2 at room temperature is 27.5 , based on the given load level, the bending moment caused by the external load is 13.75 The calculation method of BL-2 fire resistance limit is as follows:

[0184] The preset fire resistance limit R=100min. When this initial fire resistance limit is reached, the temperature of the longitudinal reinforcement in the tension zone, the temperature of the longitudinal reinforcement in the compression zone and the temperature of the CFRP sheet are 558.4℃, 467.3℃ and 890.5℃ respectively; the design value of the tensile strength of the longitudinal reinforcement in the tension zone at the above temperature is The design value of the compressive strength of the longitudinal reinforcement in the compression zone is 149.2MPa. The design value of CFRP cloth tensile strength is 202.9MPa =0MPa; the width b of the effective area of ​​the concrete beam section is 61.6mm, and the height h of the effective area of ​​the concrete beam section is 239.6mm. Then the calculation results are: x=47.6mm, M=10.0 . Make the following judgment:

[0185] .

[0186] The preset fire resistance limit R is adjusted to Repeat steps S2 to S7, that is, the initial fire resistance limit R = 85min. When this initial fire resistance limit is reached, the temperature of the longitudinal reinforcement in the tension zone, the temperature of the longitudinal reinforcement in the compression zone, and the temperature of the CFRP cloth are 494.3℃, 407.9℃, and 842.1℃ respectively; the design value of the tensile strength of the longitudinal reinforcement in the tension zone at the above temperature is The design value of the compressive strength of the longitudinal reinforcement in the compression zone is 187.0 MPa. The design value of CFRP cloth tensile strength is 238.0MPa =0MPa; the width b of the effective area of ​​the concrete beam section is 79.7mm, and the height h of the effective area of ​​the concrete beam section is 260.2mm, then the calculation results are: x=51.6mm, M=14.1 . Make the following judgment:

[0187] .

[0188] The preset fire resistance limit R is adjusted to Repeat steps S2 to S7, that is, the initial fire resistance limit R = 86min. When this initial fire resistance limit is reached, the temperature of the longitudinal reinforcement in the tension zone, the temperature of the longitudinal reinforcement in the compression zone, and the temperature of the CFRP sheet are 499.0℃, 412.1℃, and 845.7℃ respectively; the design value of the tensile strength of the longitudinal reinforcement in the tension zone at the above temperature is The design value of the compressive strength of the longitudinal reinforcement in the compression zone is 184.2MPa. The design value of CFRP cloth tensile strength is 235.5MPa =0MPa; the width b of the effective area of ​​the concrete beam section is 78.6mm, and the height h of the effective area of ​​the concrete beam section is 258.7mm, then the calculation results are: x=51.4mm, M=13.9 . Make the following judgment:

[0189] .

[0190] The current preset fire resistance limit R=86min is the actual fire resistance limit of the concrete beam BL-2 reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive.

[0191] Effect verification:

[0192] Comparison of the fire resistance test results of BL-2 concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive and the calculation results of this embodiment Figure 15 The fire resistance limit of the concrete beam BL-2 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive was measured to be 90 minutes. The fire resistance limit of the concrete beam BL-2 reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive was calculated based on this embodiment to be 86 minutes, with an error of 5%. Therefore, the fire resistance limit analysis results of this embodiment meet the accuracy requirements.

[0193] In summary, the fire resistance evaluation method for concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive in this embodiment only performs a temperature field finite element analysis of the concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive, without involving a mechanical properties finite element analysis. This solves the problem of heavy modeling workload and computational time required to determine the fire resistance limit by first performing a temperature field finite element analysis followed by a mechanical properties finite element analysis. Furthermore, the fire resistance limit calculation of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive is converted into a static equilibrium iterative calculation under various fire conditions. This method allows the fire resistance limit of concrete beams reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive to be determined in a relatively short time. This method is computationally efficient and meets accuracy requirements (with an error of less than 10%), facilitating subsequent parameter expansion analysis to study the influence of various factors on the fire resistance performance of such structural components. This method provides a scientific basis for the fire resistance design of concrete structures reinforced with CFRP sheets bonded to magnesium phosphate inorganic adhesive.

[0194] Example 2

[0195] This embodiment provides a fire resistance evaluation system for reinforced concrete beams using magnesium phosphate inorganic adhesive and carbon fiber cloth, comprising:

[0196] An initialization module is configured to preset the fire resistance limit of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0197] The finite element analysis module is configured to obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with the CFRP sheet bonded with the magnesium phosphate inorganic adhesive reaches a preset fire resistance limit;

[0198] a residual strength determination module configured to determine the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet based on the degradation law of the tensile strength of the steel bars and the CFRP sheet as a function of the fire temperature and the temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet;

[0199] an effective region determination module configured to determine an effective region of a concrete beam cross section based on a degradation law of concrete compressive strength that varies with fire temperature and concrete temperature;

[0200] A compression zone height determination module is configured as a module configured to determine the compression zone height of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheets, and the effective area of ​​the concrete beam cross section;

[0201] The residual bending capacity determination module is configured to determine the residual bending capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire according to the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive;

[0202] The comparison module is configured to determine whether the residual bending bearing capacity and the bending moment caused by the external load meet the preset conditions; if so, the preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive; if not, the preset fire resistance limit is adjusted, and steps 2 to 7 are repeated until the preset conditions are met, thereby obtaining the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive.

[0203] It should be noted that the above modules correspond to the steps described in Example 1, and the examples and application scenarios implemented by the above modules and the corresponding steps are the same, but are not limited to the contents disclosed in the above Example 1. It should be noted that the above modules, as part of the system, can be executed in a computer system such as a set of computer-executable instructions.

[0204] Although the above describes the specific embodiments of the present invention in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art on the basis of the technical solution of the present invention without any creative work are still within the scope of protection of the present invention.

Claims

1. A fire resistance evaluation method for reinforced concrete beams using magnesium phosphate inorganic adhesive and carbon fiber cloth, characterized in that: include: Step 1: Preset the fire resistance limit of concrete beams reinforced with CFRP sheets using magnesium phosphate inorganic adhesive; Step 2: Obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with the CFRP sheet bonded with the magnesium phosphate inorganic adhesive reaches the preset fire resistance limit; Step 3: Based on the degradation of the tensile strength of the steel bars and CFRP sheets as they change with the fire temperature, as well as the temperatures of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets, the residual strength of the longitudinal bars in the tension zone, the longitudinal bars in the compression zone, and the CFRP sheets is obtained; Step 4: Determine the effective area of ​​the concrete beam cross section based on the degradation law of concrete compressive strength as it changes with fire temperature and concrete temperature; Step 5: Determine the height of the compression zone of the concrete beam reinforced with CFRP sheets using magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheets, as well as the effective area of ​​the concrete beam cross section; Step 6: Determine the residual flexural bearing capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire based on the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; Step 7: Determine whether the residual bending bearing capacity and the bending moment caused by the external load meet the preset conditions; if so, use the preset fire resistance limit as the actual fire resistance limit of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; if not, adjust the preset fire resistance limit and repeat steps 2 to 7 until the preset conditions are met, thereby obtaining the actual fire resistance limit of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; The step seven specifically includes: calculate ; when When the current preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive; when When the fire resistance limit is adjusted, the current preset fire resistance limit is adjusted, including: when When the current preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;in, r When it is a positive number, take +; r When it is a negative number, take -; R is the preset fire resistance limit.

2. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 1, characterized in that: The second step specifically includes: Construct a geometric model of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; Set thermal parameters for concrete and magnesium phosphate inorganic adhesive in the geometric model; set the run time for the geometric model; set the contact relationship between concrete and magnesium phosphate inorganic adhesive in the geometric model; set boundary conditions for the geometric model; set initial conditions for the geometric model; mesh the concrete and magnesium phosphate inorganic adhesive in the geometric model; set the element type for the concrete and magnesium phosphate inorganic adhesive in the geometric model; Output the temperature of the longitudinal reinforcement in the tension zone, longitudinal reinforcement in the compression zone, CFRP cloth and concrete when the geometric model runs for the preset fire resistance limit.

3. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 1, characterized in that: In step 3, the degradation law of the tensile strength of the steel bar with the change of the over-fire temperature is: ； Where, T is the over-fire temperature of the steel bar, Overheating temperature T The tensile strength of steel bars, is the tensile strength of steel bars at room temperature.

4. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 1, characterized in that: In step 3, the degradation law of the tensile strength of the CFRP cloth with the change of the over-fire temperature is: ； Where, T is the over-fire temperature of CFRP cloth, is the tensile strength of CFRP cloth at room temperature, Overheating temperature T The tensile strength of CFRP cloth; A 、 B 、 C and n Is related to the overheating temperature T Related parameters: When hour, A Take 1.00, B Take 22, C Take 200, n Take 0.9; when hour, A Take 0.59, B Take 150, C Take 490, n Take 0.7; when hour, A Take 0.48, B Take 420, C Take 76000, n Take 1.

8.

5. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 1, characterized in that: In the fourth step, the degradation law of the concrete compressive strength as the fire temperature changes is: ； Where, T is the overfire temperature of concrete, Overheating temperature T The compressive strength of concrete, is the compressive strength of concrete at room temperature; The 500℃ isotherm of the concrete beam section is drawn according to the concrete temperature. According to the degradation law of concrete compressive strength, the area outside the 500℃ isotherm of the concrete beam section is eliminated, and the area within the 500℃ isotherm is the effective area of ​​the concrete beam section.

6. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 1, characterized in that: In the step 5, the height of the compression zone of the concrete beam reinforced with CFRP cloth by magnesium phosphate inorganic adhesive is: ； ； Where, is the coefficient; b 、 h are the width and height of the effective area of ​​the concrete beam section, respectively; x is the height of the compression zone; is the axial compressive strength of concrete at room temperature; The residual compressive strength of the longitudinal reinforcement in the compression zone when the preset fire resistance limit is reached; The residual tensile strength of the longitudinal reinforcement in the tension zone when the preset fire resistance limit is reached; To determine the residual tensile strength of the CFRP cloth when the preset fire resistance limit is reached; is the cross-sectional area of ​​the tensile reinforcement; is the cross-sectional area of ​​the compressive reinforcement; is the effective cross-sectional area of ​​CFRP cloth; is the CFRP strength utilization coefficient; is the ultimate compressive strain of concrete; is the ultimate tensile strain of CFRP cloth; is the hysteresis strain of CFRP when considering the influence of secondary force.

7. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 6, characterized in that: In step six: if , the residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire M for: ； Where, The effective height of the concrete beam section before reinforcement; c is the thickness of magnesium phosphate inorganic glue; It is the distance from the resultant point of longitudinal reinforcement in the compression zone to the compression edge of the section.

8. The fire resistance evaluation method for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive according to claim 6, characterized in that: In step six: if , the residual flexural bearing capacity of concrete beams reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire M for: ； ； Where, is the effective height of the concrete beam section before reinforcement; c is the thickness of magnesium phosphate inorganic glue; It is the distance from the resultant point of longitudinal reinforcement in the compression zone to the compression edge of the section.

9. Fire resistance evaluation system for reinforced concrete beams with carbon fiber cloth bonded with magnesium phosphate inorganic adhesive, characterized in that: include: An initialization module is configured to preset the fire resistance limit of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; The finite element analysis module is configured to obtain the temperatures of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, the CFRP sheet, and the concrete when the concrete beam reinforced with the CFRP sheet bonded with the magnesium phosphate inorganic adhesive reaches a preset fire resistance limit; a residual strength determination module configured to determine the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet based on the degradation law of the tensile strength of the steel bars and the CFRP sheet as a function of the fire temperature and the temperature of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheet; an effective region determination module configured to determine an effective region of a concrete beam cross section based on a degradation law of concrete compressive strength that varies with fire temperature and concrete temperature; A compression zone height determination module is configured as a module configured to determine the compression zone height of a concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive based on the residual strength of the longitudinal reinforcement in the tension zone, the longitudinal reinforcement in the compression zone, and the CFRP sheets, and the effective area of ​​the concrete beam cross section; The residual bending capacity determination module is configured to determine the residual bending capacity of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive under fire according to the height of the compression zone of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; The comparison module is configured to determine whether the residual bending bearing capacity and the bending moment caused by the external load meet preset conditions; if so, the preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; if not, the preset fire resistance limit is adjusted, and the processing from the finite element analysis module to the comparison module is repeated until the preset conditions are met, thereby obtaining the actual fire resistance limit of the concrete beam reinforced with CFRP sheets bonded with magnesium phosphate inorganic adhesive; The comparison module specifically includes: calculate ; when When the current preset fire resistance limit is used as the actual fire resistance limit of the concrete beam reinforced with CFRP cloth bonded with magnesium phosphate inorganic adhesive; when When the fire resistance limit is adjusted, the current preset fire resistance limit is adjusted, including: when When the current preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;when When the preset fire resistance limit is adjusted to ;in, r When it is a positive number, take +; r When it is a negative number, take -; R is the preset fire resistance limit.

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