U-shaped recycled concrete composite beam, and design and construction method therefor

Abstract

A U-shaped recycled concrete composite beam, and a design and construction method therefor, the U-shaped recycled concrete composite beam comprising a prefabricated concrete casing (11) and cast-in-place compression-zone concrete (12), wherein the prefabricated concrete casing (11) is a recycled concrete casing, and has a U-shaped appearance; the cast-in-place compression-zone concrete (12) is high strength concrete, and the cast-in-place compression-zone concrete (12) is concrete with a strength grade of C50-C80; the thickness h1 of the bottom of the prefabricated concrete casing (11) is not less than 80 mm, and the widths b1 and b3 of lateral surfaces of the prefabricated concrete casing (11) are both not less than 45 mm; the width b2 of the cast-in-place compression-zone concrete (12) is not less than 100 mm; and a steel bar cage is provided in the U-shaped recycled concrete composite beam to serve as a framework. Compared with the prior art, the fire-resistant characteristic of recycled concrete and the excellent mechanical properties of high-strength concrete are fully exerted, the flexural bearing capacity and ductility of the recycled concrete beam can be effectively improved at room temperature, the bursting phenomenon of a fire-exposed surface is reduced at a high temperature, and the temperature field in the beam is significantly reduced.

Description

A U-shaped recycled concrete composite beam and its design and construction method Technical Field

[0001] The present invention relates to the field of building structure and construction, and in particular to a U-shaped recycled concrete composite beam and a design and construction method thereof. Background Art

[0002] To achieve the sustainability of civil engineering structures, disaster prevention and mitigation are crucial aspects of structural design. With the development of high-rise and long-span structures, the application of high-strength and high-performance concrete (HPC) has become increasingly widespread. However, the dense microstructure of HPC results in high internal stresses at high temperatures, making it susceptible to concrete cracking. This, in turn, exposes internal rebar, rapidly increases rebar temperature, and significantly reduces the structure's fire resistance. This restricts the application of HPC in building structures. The incorporation of fire-retardant coatings, for example, increases structural costs, increases the number of construction steps, and raises long-term maintenance challenges.

[0003] In contrast, recycled concrete, made from recycled aggregates generated from crushed waste concrete, has been shown to exhibit excellent fire resistance. Due to the abundant old mortar on the surface of the recycled aggregate, the recycled concrete has a loose and porous interior. Its thermal conductivity is lower than that of conventional concrete at the same water-cement ratio, and it is less susceptible to cracking at high temperatures. However, the use of recycled aggregates somewhat reduces the bearing capacity and stiffness of recycled concrete components at room temperature, which restricts the application of recycled concrete structures and limits the material's excellent thermal properties.

[0004] Patent CN208329371U discloses a T-shaped steel-reinforced recycled concrete composite beam, comprising a concrete composite beam box. A retaining plate is placed at the bottom of the concrete composite beam box, and the top of the retaining plate is provided with evenly spaced engaging grooves. Stirrups are placed evenly spaced within the concrete composite beam box, and the retaining plate is placed atop the stirrups. Reinforcement is provided at both the top and bottom ends of the stirrups. The combined use of steel and flange plates enhances the bearing capacity of the device, making better use of the ductility of the T-shaped steel, saving formwork and improving construction efficiency. It also facilitates the filling of the composite beam with waste concrete, protecting the environment and enabling waste recycling. However, this design significantly increases the amount of steel used in the concrete beam and complicates the construction, increasing construction costs. Furthermore, this design places steel, such as the retaining plate, on the surface of the concrete beam, causing rapid performance degradation after fire. Furthermore, the use of recycled concrete in the middle of the concrete beam prevents the full utilization of its excellent fire resistance.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a U-shaped recycled concrete composite beam and its design and construction method, which fully utilizes the fire resistance characteristics of recycled concrete and the excellent mechanical properties of high-strength concrete. At room temperature, the flexural bearing capacity and ductility of the recycled concrete beam can be effectively improved. At high temperature, the cracking phenomenon on the fire-exposed surface is reduced, and the temperature field inside the beam is significantly reduced, which fully utilizes the thermal insulation performance of the recycled concrete, protects the internal stressed steel bars, and avoids the cracking of high-strength concrete causing the weakening of the structural cross-section size. The construction method can effectively reduce the amount of on-site pouring work and speed up the project progress.

[0007] The purpose of the present invention can be achieved by the following technical solutions:

[0008] The present invention provides a U-shaped recycled concrete composite beam, comprising a precast concrete shell and cast-in-situ compression zone concrete;

[0009] The precast concrete shell is a recycled concrete shell with a U-shaped cross-section, good fire resistance, and a low cracking resistance. The recycled concrete shell is made by crushing and screening waste concrete to form recycled coarse aggregate, which partially or completely replaces natural aggregate. Preferably, the recycled concrete has a recycled coarse aggregate replacement rate greater than 50%, where the recycled coarse aggregate replacement rate refers to the ratio of recycled coarse aggregate to natural coarse aggregate. More preferably, the recycled concrete has a 100% recycled coarse aggregate replacement rate. The strength grade of the recycled concrete is no higher than C40. The precast concrete shell is prefabricated in a factory, and the casting quality of the precast concrete shell is improved by controlling the recycled coarse aggregate grade, optimizing the recycled coarse aggregate grading, and ensuring good curing conditions.

[0010] The cast-in-place compression zone concrete is high-strength concrete, which refers to concrete with a strength grade of C50-C80. This can be poured on-site. It is located within the U-shaped trough of the precast concrete shell.

[0011] Preferably, the maximum particle size of the recycled coarse aggregate used in the precast concrete shell is not greater than 1 / 2 of the minimum value of the bottom thickness and side width of the precast concrete shell.

[0012] Preferably, according to the actual bearing capacity requirements, ultra high performance concrete (UHPC) material can be used for the cast-in-place compression zone. Ultra high performance concrete refers to fiber reinforced concrete material with a strength grade of C80 or above.

[0013] The cross-sectional dimensions of the U-shaped recycled concrete composite beam are: width b, not less than 200mm, and height h, not less than 300mm. The bottom thickness h1 of the precast concrete shell is not less than 80mm, and the tensile reinforcement is located within the precast concrete shell. The side widths b1 and b3 of the precast concrete shell are not less than 45mm. The width b2 of the cast-in-place compression zone concrete is not less than 100mm.

[0014] Preferably, b1 and b3 are equal.

[0015] Preferably, under the premise of meeting the bearing capacity requirements, the cross-sectional area of ​​the precast concrete shell accounts for no less than 40% of the total cross-sectional area of ​​the U-shaped recycled concrete composite beam.

[0016] The bottom of the precast concrete shell is roughened by roughening after casting the formwork or by mechanical chiseling, or by pre-setting teeth and grooves in the formwork.

[0017] Preferably, interface reinforcement steel bars are provided at the bottom of the precast concrete shell. Preferably, the length of the interface reinforcement steel bars is not less than 100 mm, and the longitudinal spacing along the beam is not greater than 150 mm.

[0018] Furthermore, the sides of the precast concrete shell should also be roughened, preferably with teeth in the formwork.

[0019] The U-shaped recycled concrete composite beam is constructed with a steel cage as its skeleton. The cage, housed within the precast concrete shell, includes tension bars, support bars, and stirrups. The stirrups are arranged in a U-shape to reflect the member's shape. The top of the stirrups has a bend, with the upper surface of the stirrup bottom abutting the tension bars and the lower surface of the bend abutting the support bars. Furthermore, the cage includes additional steel and waist bars.

[0020] The construction method of the steel cage complies with the relevant requirements of "11G101-1 Drawing Rules and Structural Details of the Overall Representation Method of the Plane of Concrete Structure Construction Drawings (Cast-in-place Concrete Frame, Shear Wall, Beam, Slab)" and "GB50666-2011 Concrete Structure Engineering Construction Code".

[0021] When the upper part of the U-shaped recycled concrete composite beam is connected to the concrete slab, an optimized U-shaped combination form is adopted, that is, the heights on both sides of the precast concrete shell are set to h1+h2, and the upper height h3 is poured at the same time as the cast-in-place compression zone concrete, and the same concrete material as the cast-in-place compression zone concrete is used.

[0022] Furthermore, the precast portion of the precast concrete shell is not lower than the bottom of the connected concrete slab.

[0023] Furthermore, the upper h3 height is the same as the thickness of the concrete slab.

[0024] Furthermore, a supplementary stirrup is provided above the U-shaped stirrup to connect the U-shaped stirrups to form a ring stirrup.

[0025] Furthermore, the U-shaped stirrups and the supplementary stirrups above them can be connected by binding.

[0026] Furthermore, considering the torsion of the concrete beam, the U-shaped stirrups and the supplementary stirrups above them are connected by welding.

[0027] The present invention also provides a construction method for a U-shaped recycled concrete composite beam, which is cast on-site in sections and stages or in a semi-prefabricated and semi-cast-in-place manner. When the semi-prefabricated and semi-cast-in-place method is adopted, the casting step of the precast concrete shell can be completed in the factory, and only the cast-in-place concrete portion is poured on-site. Specifically, the following steps are included:

[0028] S1: Set up a reinforcement cage inside the U-shaped recycled concrete composite beam, and the reinforcement cage uses U-shaped stirrups;

[0029] S2: Set up the external formwork and the U-shaped formwork of the corresponding size inside the U-shaped recycled concrete composite beam, and fix the U-shaped formwork to the external formwork at the top and sides through wooden boards;

[0030] S3: Use recycled concrete to pour into the precast concrete shell, and vibrate it thoroughly with a vibrator;

[0031] S4: After S3 pouring is completed, after final setting and curing for at least one week, remove the internal U-shaped formwork;

[0032] S5: Roughening the bottom interface of the precast concrete shell;

[0033] S6: Use high-strength concrete to form cast-in-place compression zone concrete.

[0034] Furthermore, in S2, the allowable deviation of the U-shaped template is ±5 mm.

[0035] Furthermore, in S2, the net distance between the U-shaped template and the U-shaped stirrup is not less than 10 mm.

[0036] Furthermore, in S5 , the bottom interface of the precast concrete shell is roughened.

[0037] Furthermore, S5 is omitted, and in S2, a special template with pre-arranged teeth and grooves is used at the side and bottom interfaces of the precast concrete shell to obtain a rough interface.

[0038] Furthermore, the average roughness of the tooth grooves of the special template is not less than 3 mm, and the spacing between the tooth grooves is not less than 10 mm.

[0039] Furthermore, custom templates can be made using 3D printed plastic materials.

[0040] Furthermore, in S2, a slurry outlet hole is provided at the bottom of the U-shaped template.

[0041] Furthermore, in S3, the slump of the recycled concrete material used is not less than 120 mm.

[0042] Furthermore, in S3, the diameter of the vibrating rod should be smaller than the side width of the U-shaped concrete shell.

[0043] Furthermore, in S3, the vibrating rod should be extended into the outside of the U-shaped formwork for vibration.

[0044] Furthermore, in S3, the vibration compaction means that the recycled concrete slurry no longer sinks, the surface of the recycled concrete slurry appears as floating slurry, and cement slurry overflows from the slurry outlet.

[0045] Furthermore, in S3, part of the recycled concrete may be added first, and after the bottom recycled concrete is poured densely, the U-shaped side concrete may be poured.

[0046] Furthermore, steel bar holes are provided at the bottom of the U-shaped formwork to arrange interface reinforcement steel bars.

[0047] Furthermore, when the U-shaped recycled concrete composite beam is connected to the concrete slab above, an optimized U-shaped combination is used. In S3, the beam is poured to a height of h2, retaining the naturally cast rough surface. Before proceeding to S6, additional reinforcement is placed above the U-shaped stirrups and tied to the reinforcement cage.

[0048] The present invention also provides a method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance, the specific steps of which are as follows:

[0049] A1: Determine the load effect and fire resistance rating of U-shaped recycled concrete composite beams;

[0050] A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam;

[0051] A3: Analyze the normal temperature bearing capacity and normal service capacity of U-shaped recycled concrete composite beams, and design the reinforcement of U-shaped recycled concrete composite beams;

[0052] A4: Conduct fire resistance design of U-shaped recycled concrete composite beams, using either a simple or complex design method;

[0053] A5: Based on the analysis in A3, design the roughness and reinforcement of the combined interface, using either a simple or complex design method;

[0054] A6: Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.

[0055] The load effects in A1 should comply with the relevant provisions of the Code for Loads on Building Structures (GB50009-2012) and the Code for Loads on Building Structures (GB50009-2012). The fire resistance rating should comply with the relevant provisions of the Code for Fire Protection Design of Buildings (GB 50016-2014) and the General Code for Fire Protection of Buildings (GB 55037-2022). The cross-sectional dimensions of the U-shaped recycled concrete composite beam in A2 should comply with the component dimension requirements in the Code for Design of Concrete Structures (GB50010-2010). The dimensional parameters of the U-shaped recycled concrete composite beam should meet the engineering design requirements.

[0056] In A3, the reinforcement design of the U-shaped recycled concrete composite beam should meet the requirements of the "Code for Design of Concrete Structures" (GB50010-2010) for the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of the concrete beam.

[0057] The bending bearing capacity M of the normal section of the U-shaped recycled concrete composite beam cu Calculated by the following steps:

[0058] First, through α 1,H f c,H bx0=f y A s Calculate x0

[0059] If x0≤h3,x=x0,

[0060] If x0>h3,α 1,H f c,H bh3+α 1,H f c,H b2(x-h3)+α 1,R f c,R (b1+b3)(x1-h3)=f y A s ,

[0061] If h3<x0≤h3+h2,x=x1,

[0062] If x1>h3+h2,

[0063] α 1,H f c,H bh3+α 1,H f c,H b2(h2+h3)+α 1,R f c,R (b1+b3)h2+α 1,R f c,Rb(x-h2-h3)=f y A s .

[0064] Among them, M cu is the bending bearing capacity of the normal section of the U-shaped recycled concrete composite beam; b, b1, b2, b3 are the width of the composite beam, the left thickness, middle thickness and right thickness of the precast concrete shell respectively; h, h0, h1, h2, h3 are the cross-sectional height of the composite beam, the effective cross-sectional height of the composite beam, the bottom thickness of the precast concrete shell, the core height of the cast-in-place compression zone concrete and the top height of the cast-in-place compression zone concrete respectively; f c,R ,f c,H ,f y are respectively the calculated compressive strength value of recycled concrete, the designed compressive strength value of high-strength concrete, and the designed yield strength value of the stressed steel bars; α 1,H and α 1,R The coefficient of high-strength concrete and recycled concrete materials is calculated in accordance with Article 6.2.6 of the Code for Design of Concrete Structures (GB50010-2010).

[0065] Shear bearing capacity V of the oblique section of U-shaped composite recycled concrete beam cs It can be calculated by the following formula:

[0066] Among them, V cs is the shear bearing capacity of the oblique section of the U-shaped recycled concrete composite beam; f t,R ,f t,H, f yv are respectively the calculated value of tensile strength of recycled concrete, the design value of tensile strength of high-strength concrete, and the design value of yield strength of stirrups; α cv A is the shear bearing capacity coefficient of inclined section concrete, calculated according to Article 6.3.4 of the Code for Design of Concrete Structures (GB50010-2010); sv It is the total cross-sectional area of ​​each limb of the stirrups arranged in the same section; s is the stirrup spacing along the length of the component.

[0067] In A3, the bearing capacity of a U-shaped recycled concrete composite beam should be no less than that of a corresponding cast-in-place conventional concrete beam, where the conventional and recycled concrete materials have the same effective water-cement ratio. The serviceability analysis of the U-shaped recycled concrete composite beam described in A3 should comply with the relevant provisions of the Code for Design of Concrete Structures (GB50010-2010), including verification of maximum crack width, deflection, and other factors.

[0068] Furthermore, the simple design method in A4 refers to meeting the minimum values ​​of the beam width and the thickness of the longitudinal tensile reinforcement protective layer for simply supported beams or continuous beams in the "Technical Code for Fire Resistance Design of Building Concrete Structures" (DBJ / T15-81-2022). The impact of the reduction in the thermal conductivity of recycled concrete should not be considered and it should only be used as a safety reserve.

[0069] Furthermore, the complex design method in A4 involves conducting a full-process, nonlinear analysis under fire conditions. This involves using a large-scale, general-purpose finite element program to calculate the time-varying internal temperature field of concrete components and structures, taking into account the impact of the reduced thermal conductivity of recycled concrete. This is then followed by a high-temperature mechanical analysis of the components and structures to determine whether their fire resistance meets the engineering design requirements.

[0070] Furthermore, the effect of the reduction in thermal conductivity of recycled concrete is considered when calculating the time-varying internal temperature field.

[0071] Furthermore, the fire resistance limit should be determined by whether the component has lost its bearing capacity, integrity and thermal insulation. Among them, the judgment condition for losing bearing capacity is that the deflection reaches the limit bending deformation, or the growth rate of the deflection reaches the limit bending deformation rate, which should comply with the following formula:

[0072] Limit bending deformation: mm

[0073] Limiting bending deformation rate: mm / min

[0074] Where D is the mid-span deflection, L is the clear span of the specimen, mm; d is the distance between the compression point and the tension point on the specimen interface, mm.

[0075] The criterion for loss of integrity is that the specimen can continue to maintain fire resistance and fire isolation performance during the fire test.

[0076] The judgment condition for losing thermal insulation is that the average temperature rise of the unfired surface of the specimen exceeds the initial average temperature by 140℃ or the temperature rise of any point exceeds the initial temperature by 180℃.

[0077] Furthermore, the thermal conductivity of recycled concrete in the complex design method adopts the measured value.

[0078] Furthermore, in A5, the simple design method means that the design interface is a rough interface, and the roughness of the rough interface is not less than the roughness corresponding to the case where half of the coarse aggregate is exposed, or the natural casting surface that is not smoothed, and structural interface steel bars should be arranged, and the interface steel bar reinforcement ratio should not be less than the stirrup reinforcement ratio.

[0079] The interface shear stress under normal use should satisfy the following formula:

[0080] τ≤[τ],

[0081] Wherein, τ is the interface shear stress obtained from experiments or numerical simulations, and [τ] is the maximum shear stress that the combined interface can withstand, in MPa.

[0082] The maximum shear stress [τ] that the composite interface can withstand can be calculated by the following formula:

[0083] Among them, τ a The shear stress provided for bonding and aggregate bite; μ is the interface friction coefficient; κ1, κ2 are the action coefficients; f y ,f cc are the tensile strength of the interface steel bars and the uniaxial cylindrical compressive strength of the concrete, respectively; ρ is the reinforcement ratio of the interface steel bars; β c and ν are the effective coefficients of the interface reinforcement and concrete respectively; σ n is the interface normal stress.

[0084] The interface reinforcement ratio ρ is calculated by the following formula:

[0085] Where A s Interface reinforcement area; A c is the interface area.

[0086] Furthermore, in A5, the complex design method refers to the use of nonlinear full-process analysis under fire conditions to perform high-temperature safety analysis of the combined interface.

[0087] Furthermore, the high-temperature safety analysis of the composite interface should consider the strength and stiffness loss of the interface bearing capacity under high temperature, and the degradation law of the interface tensile bearing capacity and shear bearing capacity under high temperature should be tested through experiments.

[0088] Furthermore, the security analysis satisfies the following formula:

[0089] R T >S T ,

[0090] Among them, R T is the resistance of the combined interface at temperature T, S T is the load effect of the combined interface at temperature T. Safety requirements should be met within the fire resistance limit of the structure.

[0091] Furthermore, the safety of the combined interface in A5 also requires the calculation of the relative slip and maximum crack width under normal use, which should comply with the following formula:

[0092] Relative slip: s≤[s],

[0093] Where s is the relative slip obtained from the test or numerical simulation, and [s] is the slip limit specified in the specification, in mm.

[0094] The maximum crack width should conform to the following formula: w≤[w],

[0095] Where w is the maximum crack width obtained from the test or numerical simulation, and [w] is the maximum crack width limit specified in the specification, in mm.

[0096] Compared with the prior art, the present invention has the following advantages:

[0097] (1) The U-shaped recycled concrete composite beam provided by the present invention can effectively improve the bending bearing capacity and ductility of the recycled concrete beam at room temperature, and solve the problem of the weakening of the structural bearing capacity and ductility caused by the application of recycled concrete. At high temperatures, the cracking phenomenon of the fire-exposed surface is reduced, and the temperature field inside the beam is significantly reduced, giving full play to the thermal insulation performance of the recycled concrete, which not only protects the internal stress-bearing steel bars, but also avoids the weakening of the structural cross-sectional dimensions caused by the cracking of high-strength concrete. On the basis of meeting the structural stress requirements, a large amount of construction solid waste is consumed, which not only broadens the application scope of recycled concrete and promotes the sustainable and green development of the construction industry, but also reduces the project cost and reflects the cost advantage.

[0098] (2) The present invention provides a construction method for a U-shaped recycled concrete composite beam. The construction method of the composite recycled concrete composite beam is on-site casting or semi-prefabricated and semi-cast-in-place construction. The semi-prefabricated and semi-cast-in-place construction method can effectively reduce the on-site casting workload, speed up the project progress, and the construction quality of the prefabricated part of the recycled concrete can be well guaranteed, reducing the variability of the performance of the recycled concrete material.

[0099] (3) The present invention provides a design method for a U-shaped recycled concrete composite beam, which provides a simplified design method based on existing specifications and an advanced calculation method based on nonlinear high-temperature full-process analysis. While reflecting the advanced nature of the structural design, it also takes into account the difficulty of design and construction, which is conducive to the promotion of actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0100] Figure 1 is the cross-sectional design drawing of the U-shaped recycled concrete composite beam.

[0101] Figure 2 is a cross-sectional design diagram of a U-shaped recycled concrete composite beam considering that the composite beam is connected to the upper concrete slab.

[0102] Figure 3 is a flow chart of the structural design of a U-shaped recycled concrete composite beam with good mechanical and fire resistance properties.

[0103] Figure numerals: 11 - precast concrete shell; 12 - cast-in-situ compression zone concrete; 21 - tension steel bars; 22 - U-shaped stirrups; 23 - frame bars; 24 - supplementary stirrups; 25 - interface reinforcement steel bars. DETAILED DESCRIPTION

[0104] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Component models, material names, connection structures, control methods, algorithms, and other features not explicitly described in this technical solution are considered common technical features disclosed in the prior art.

[0105] Example 1

[0106] As shown in FIG3 , this embodiment provides a method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance, using a simple design method. The specific steps are as follows:

[0107] A1: Determine the load effect and fire resistance rating of the U-shaped recycled concrete composite beam. The load effect and fire resistance rating of the U-shaped recycled concrete composite beam are determined based on the structural requirements. The load effect is calculated in accordance with the relevant provisions of the Code for Loads on Building Structures (GB50009-2012) and the Code for Loads on Building Structures (GB50009-2012). The fire resistance rating is determined in accordance with the relevant provisions of the Code for Fire Protection Design of Buildings (GB50016-2014) and the General Code for Fire Protection of Buildings (GB55037-2022).

[0108] A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam. The cross-sectional dimensions of the U-shaped recycled concrete composite beam comply with the requirements for component dimensions in the Code for Design of Concrete Structures (GB50010-2010), and the dimensional parameters of the U-shaped recycled concrete composite beam can meet the engineering design requirements.

[0109] A3: Analyze the normal temperature bearing capacity and normal service capacity of U-shaped recycled concrete composite beams, and design the reinforcement of U-shaped recycled concrete composite beams. The reinforcement design of U-shaped recycled concrete composite beams should meet the requirements of the "Code for Design of Concrete Structures" (GB50010-2010) for the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of concrete beams. Among them, the bending bearing capacity M of the normal section of the U-shaped recycled concrete composite beam is cu Calculated by the following steps:

[0110] First, through α 1,H f c,H bx0=f y A s Calculate x0

[0111] If x0≤h3,x=x0,

[0112] If x0>h3,α1,H f c,H bh3+α 1,H f c,H b2(x-h3)+α 1,R f c,R (b1+b3)(x1-h3)=f y A s ,

[0113] If h3<x0≤h3+h2,x=x1,

[0114] If x1>h3+h2,

[0115] α 1,H f c,H bh3+α 1,H f c,H b2(h2+h3)+α 1,R f c,R (b1+b3)h2+α 1,R f c,R b(x-h2-h3)=f y A s .

[0116] Among them, M cu is the bending bearing capacity of the normal section of the U-shaped composite concrete beam; b, b1, b2, b3 are the width of the composite beam, the left thickness, middle thickness and right thickness of the precast concrete shell 11 respectively; h, h0, h1, h2, h3 are the cross-sectional height of the composite beam, the effective cross-sectional height of the composite beam, the bottom thickness of the precast concrete shell 11, the core height of the cast-in-place compression zone concrete 12 and the top height of the cast-in-place compression zone concrete 12 respectively; f c,R ,f c,H ,f y are respectively the calculated compressive strength value of recycled concrete, the designed compressive strength value of high-strength concrete, and the designed yield strength value of the stressed steel bars; α 1,H and α 1,R The coefficient of high-strength concrete and recycled concrete materials is calculated in accordance with Article 6.2.6 of the Code for Design of Concrete Structures (GB50010-2010).

[0117] Among them, the shear bearing capacity of the oblique section of the U-shaped recycled concrete composite beam is V cs Calculated by the following formula:

[0118] Among them, V cs is the shear bearing capacity of the oblique section of the U-shaped recycled concrete composite beam. t,R ,f t,H, f yvThey are respectively the calculated value of the tensile strength of recycled concrete, the design value of the tensile strength of high-strength concrete, and the design value of the yield strength of stirrups. cv is the shear bearing capacity coefficient of inclined section concrete, according to the Code for Design of Concrete Structures

[0119] (GB50010-2010) 6.3.4. sv is the total cross-sectional area of ​​each limb of the stirrups arranged in the same section, and s is the stirrup spacing along the length of the component.

[0120] Verify that the bearing capacity of the U-shaped recycled concrete composite beam should be no less than that of the corresponding ordinary concrete monolithic beam, where the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio.

[0121] A4: Conduct fire resistance design for U-shaped recycled concrete composite beams using a simple design method. Ensure that the cross-sectional dimensions of the U-shaped recycled concrete composite beam meet the minimum beam width and longitudinal tensile reinforcement 21mm cover thickness specified in the Technical Code for Fire Resistance Design of Building Concrete Structures (DBJ / T15-81-2022). Do not consider the impact of the reduced thermal conductivity of recycled concrete, which serves only as a safety reserve.

[0122] A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface using a simple design method. The design interface is a rough interface, and the roughness of the rough interface is not less than the roughness corresponding to the case where the coarse aggregate is half exposed, or the natural casting surface that is not smoothed. Structural interface reinforcement should be arranged, and the reinforcement ratio of the interface reinforcement should be not less than the reinforcement ratio of the stirrups.

[0123] The interface shear stress under normal use should satisfy the following formula:

[0124] τ≤[τ],

[0125] Wherein, τ is the interface shear stress obtained from experiments or numerical simulations, and [τ] is the maximum shear stress that the combined interface can withstand, in MPa.

[0126] The maximum shear stress [τ] that the composite interface can withstand can be calculated by the following formula:

[0127] Among them, τ a The shear stress provided for bonding and aggregate bite; μ is the interface friction coefficient; κ1, κ2 are the action coefficients; f y ,f cc are the tensile strength of the interface steel bars and the uniaxial cylindrical compressive strength of the concrete, respectively; ρ is the reinforcement ratio of the interface steel bars; β c and ν are the effective coefficients of the interface reinforcement and concrete respectively; σ nis the interface normal stress.

[0128] The interface reinforcement ratio ρ is calculated by the following formula:

[0129] Where A s Interface reinforcement area; A c is the interface area.

[0130] The safety of the combined interface also requires calculation of the relative slip and maximum crack width under normal use, which should comply with the following formula:

[0131] Relative slip: s≤[s],

[0132] Where s is the relative slip obtained from the test or numerical simulation, and [s] is the slip limit specified in the specification, in mm.

[0133] The maximum crack width should conform to the following formula: w≤[w],

[0134] Where w is the maximum crack width obtained from the test or numerical simulation, and [w] is the maximum crack width limit specified in the specification, in mm.

[0135] A6: Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.

[0136] Example 2

[0137] As shown in FIG3 , this embodiment provides a method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance, using a complex design method. The specific steps are as follows:

[0138] A1: Determine the load effect and fire resistance rating of the U-shaped recycled concrete composite beam. The load effect and fire resistance rating of the U-shaped recycled concrete composite beam are determined based on the structural requirements. The load effect is calculated in accordance with the relevant provisions of the Code for Loads on Building Structures (GB50009-2012) and the Code for Loads on Building Structures (GB50009-2012). The fire resistance rating is determined in accordance with the relevant provisions of the Code for Fire Protection Design of Buildings (GB50016-2014) and the General Code for Fire Protection of Buildings (GB55037-2022).

[0139] A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam. The cross-sectional dimensions of the U-shaped recycled concrete composite beam comply with the requirements for component dimensions in the Code for Design of Concrete Structures (GB50010-2010), and the dimensional parameters of the U-shaped recycled concrete composite beam can meet the engineering design requirements.

[0140] A3: Analyze the normal temperature bearing capacity and normal service capacity of U-shaped recycled concrete composite beams, and design the reinforcement of U-shaped recycled concrete composite beams. The reinforcement design of U-shaped recycled concrete composite beams should meet the requirements of the "Code for Design of Concrete Structures" (GB50010-2010) for the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of concrete beams. Among them, the bending bearing capacity M of the normal section of the U-shaped recycled concrete composite beam is cu Calculated by the following steps:

[0141] First, through α 1,H f c,H bx0=f y A s Calculate x0

[0142] If x0≤h3,x=x0,

[0143] If x0>h3,α 1,H f c,H bh3+α 1,H f c,H b2(x-h3)+α 1,R f c,R (b1+b3)(x1-h3)=f y A s ,

[0144] If h3<x0≤h3+h2,x=x1,

[0145] If x1>h3+h2,

[0146] α 1,H f c,H bh3+α 1,H f c,H b2(h2+h3)+α 1,R f c,R (b1+b3)h2+α 1,R f c,R b(x-h2-h3)=f y A s .

[0147] Among them, M cu is the bending bearing capacity of the normal section of the U-shaped composite concrete beam; b, b1, b2, b3 are the width of the composite beam, the left thickness, middle thickness and right thickness of the precast concrete shell 11 respectively; h, h0, h1, h2, h3 are the cross-sectional height of the composite beam, the effective cross-sectional height of the composite beam, the bottom thickness of the precast concrete shell 11, the core height of the cast-in-place compression zone concrete 12 and the top height of the cast-in-place compression zone concrete 12 respectively; f c,R ,f c,H ,f yare respectively the calculated compressive strength value of recycled concrete, the designed compressive strength value of high-strength concrete, and the designed yield strength value of the stressed steel bars; α 1,H and α 1,R The coefficient of high-strength concrete and recycled concrete materials is calculated in accordance with Article 6.2.6 of the Code for Design of Concrete Structures (GB50010-2010).

[0148] Among them, the shear bearing capacity of the oblique section of the U-shaped recycled concrete composite beam is V cs Calculated by the following formula:

[0149] Among them, V cs is the shear bearing capacity of the oblique section of the U-shaped recycled concrete composite beam. t,R ,f t,H, f yv They are respectively the calculated value of the tensile strength of recycled concrete, the design value of the tensile strength of high-strength concrete, and the design value of the yield strength of stirrups. cv is the shear bearing capacity coefficient of inclined section concrete, according to the Code for Design of Concrete Structures (GB

[0150] 50010-2010) 6.3.4. sv is the total cross-sectional area of ​​each limb of the stirrups arranged in the same section, and s is the stirrup spacing along the length of the component.

[0151] Verify that the bearing capacity of the U-shaped recycled concrete composite beam should be no less than that of the corresponding ordinary concrete monolithic beam, where the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio.

[0152] A4: Conduct fire resistance design for U-shaped recycled concrete composite beams using a complex design approach. Perform a nonlinear, full-process analysis under fire conditions. Use a large-scale, general-purpose finite element program to calculate the time-varying internal temperature field of concrete components and structures, taking into account the impact of the reduced thermal conductivity of recycled concrete. Based on this, conduct high-temperature mechanical analysis of the components and structures to determine whether the fire resistance design of the components or structures meets the fire resistance requirements of the Code for Fire Protection of Building Structures (GB 50016-2014). Consider the impact of the reduced thermal conductivity of recycled concrete when calculating the time-varying internal temperature field.

[0153] The fire resistance limit should be determined by whether the component has lost its bearing capacity, integrity and thermal insulation. Among them, the judgment condition for losing bearing capacity is that the deflection reaches the limit bending deformation, or the growth rate of the deflection reaches the limit bending deformation rate, which should comply with the following formula:

[0154] Limit bending deformation: mm

[0155] Limiting bending deformation rate: mm / min

[0156] Where D is the mid-span deflection, L is the clear span of the specimen, mm; d is the distance between the compression point and the tension point on the specimen interface, mm.

[0157] The criterion for loss of integrity is that the specimen can continue to maintain fire resistance and fire isolation performance during the fire test.

[0158] The judgment condition for losing thermal insulation is that the average temperature rise of the unfired surface of the specimen exceeds the initial average temperature by 140℃ or the temperature rise of any point exceeds the initial temperature by 180℃.

[0159] In the complex design method, the thermal conductivity of recycled concrete adopts the measured value.

[0160] A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface using a complex design method. Perform a high-temperature safety analysis of the composite interface using a nonlinear full-process analysis under fire conditions.

[0161] The high-temperature safety analysis of the composite interface should consider the strength and stiffness loss of the interface bearing capacity under high temperature, and the degradation law of the interface tensile bearing capacity and shear bearing capacity under high temperature should be tested through experiments.

[0162] The safety analysis satisfies the following formula:

[0163] R T >S T ,

[0164] Among them, R T is the resistance of the combined interface at temperature T, S T is the load effect of the combined interface at temperature T. Safety requirements should be met within the fire resistance limit of the structure.

[0165] The safety of the combined interface in A5 also requires calculation of the relative slip and maximum crack width under normal use, which should comply with the following formula:

[0166] Relative slip: s≤[s],

[0167] Where s is the relative slip obtained from the test or numerical simulation, and [s] is the slip limit specified in the specification, in mm.

[0168] The maximum crack width should conform to the following formula: w≤[w],

[0169] Where w is the maximum crack width obtained from the test or numerical simulation, and [w] is the maximum crack width limit specified in the specification, in mm.

[0170] A6: Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.

[0171] Comparative Example 1

[0172] The composite concrete beam designed in Example 2 was tested to compare the room temperature performance of the U-shaped recycled concrete composite beam and the single concrete material cast-in-one beam, and a U-shaped recycled concrete composite beam, a recycled concrete cast-in-one beam, and a common concrete cast-in-one beam were manufactured.

[0173] (1) Design and production

[0174] As shown in Figure 1, the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and cast-in-situ compression zone concrete 12. The cross-sectional dimensions of the U-shaped recycled concrete composite beam are 200mm×300mm, with specific dimensions of h1=80mm, h2=220mm, h3=0mm, b1=45mm, b2=110mm, and b3=45mm. The top reinforcement 23 is Tensile reinforcement 21 U-shaped stirrups 22 are 10@100, no interface reinforcement is provided. The U-shaped recycled concrete composite beam is 2700mm long, with a clear span of 2400mm and a bottom concrete cover thickness of 40mm.

[0175] The raw materials and equipment selected are as follows:

[0176] PO42.5 ordinary Portland cement, natural yellow sand with a maximum particle size of 0.9 mm, polycarboxylic acid high-efficiency water-reducing agent, natural coarse aggregate with a particle size range of 5-16 mm (provided by the Key Laboratory of Civil Engineering of the Ministry of Education, Tongji University); recycled coarse aggregate with a particle size range of 5-16 mm (provided by Shanghai Youhong Environmental Protection Technology Co., Ltd.

[0177] As shown in Table 1, recycled concrete with a recycled coarse aggregate replacement rate of 100% is used to cast the precast concrete shell 11, and high-strength concrete is used to cast the cast-in-situ compression zone. First, a reinforcement cage is set inside the U-shaped recycled concrete composite beam, and U-shaped stirrups 22 are used in the reinforcement cage; an external formwork and a U-shaped formwork of corresponding size are set inside the U-shaped beam, and the U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards, and a slurry outlet is set below the U-shaped formwork; recycled concrete with a slump of 120 mm is used to cast the precast concrete shell 11, and part of the recycled concrete is first added, and a vibrating rod is inserted into the outside of the U-shaped formwork for sufficient vibration until the bottom concrete is dense, the recycled concrete slurry no longer sinks, the surface of the recycled concrete slurry shows floating slurry, and cement slurry overflows from the slurry outlet; after the bottom recycled concrete is poured densely, the U-shaped side concrete is poured; vibration compaction means that the recycled concrete slurry no longer sinks, the surface of the recycled concrete slurry shows floating slurry, and cement slurry overflows from the slurry outlet; after the final setting is completed and the curing is carried out for one week, the internal U-shaped formwork is removed; the bottom interface is roughened; and high-strength concrete is used to cast the cast-in-place part of the concrete.

[0178] At the same time, recycled concrete monolithic beams and ordinary concrete monolithic beams with the same specifications and sizes as the composite beams were used. The mix ratios of recycled concrete, ordinary concrete and high-strength concrete used in the construction are shown in Table 1.

[0179] Table 1 Mix ratios of recycled concrete, ordinary concrete and high-strength concrete used in construction

[0180] (2) Results and Discussion

[0181] U-shaped recycled concrete composite beams, recycled concrete monolithic beams, and conventional concrete monolithic beams were subjected to room-temperature flexural bearing capacity tests. The specimens were simply supported beams with a mid-span loading point spacing of 500 mm. Loading was performed using a four-column hydraulic servo testing machine. The yield moment, ultimate bending moment, and corresponding deflection were measured. Specific data are shown in Table 2.

[0182] Table 2 shows that the yield moment and ultimate bending moment of the U-shaped recycled concrete composite beams are significantly higher than those of the recycled concrete monolithic beams, reaching and exceeding the levels of conventional concrete monolithic beams. The deflection corresponding to the yield moment is roughly the same. Therefore, composite recycled concrete beams can optimize the low bearing capacity of recycled concrete beams. In structural design, the proposed U-shaped recycled concrete composite beams can be used to replace conventional concrete monolithic beams.

[0183] Table 2 Comparison of normal temperature bearing capacity of composite beams, recycled concrete monolithic beams, and ordinary concrete monolithic beams

[0184] Comparative Example 2

[0185] The fire resistance and post-fire residual bearing capacity of U-shaped recycled concrete composite beams and single-material cast-in-place beams were compared, and U-shaped recycled concrete composite beams, recycled concrete cast-in-place beams, and high-strength concrete cast-in-place beams were manufactured.

[0186] (1) Design and production

[0187] As shown in Figure 2, the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and cast-in-place compression zone concrete 12. The cross-sectional dimensions of the U-shaped recycled concrete composite beam are 200mm×300mm, with specific dimensions of h1=80mm, h2=160mm, h3=60mm, b1=45mm, b2=110mm, and b3=45mm. The top reinforcement 23 is Tensile reinforcement 21 U-shaped stirrups 22 are 10@100, interface reinforcement steel 25 10@100, length is 100mm. U-shaped recycled concrete composite beam is 2700mm long, with a clear span of 2400mm and a bottom concrete cover thickness of 40mm.

[0188] The raw materials and equipment used are the same as those in Example 2.

[0189] As shown in Table 3, recycled concrete with a recycled coarse aggregate replacement rate of 100% is used to cast the precast concrete shell 11, and high-strength concrete is used to cast the cast-in-situ compression zone. First, a reinforcement cage is set inside the U-shaped recycled concrete composite beam, and U-shaped stirrups 22 are used in the reinforcement cage; an external formwork and a U-shaped formwork of corresponding size are set inside the U-shaped beam, and the U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards, and slurry holes and reinforcement holes are set below the U-shaped formwork; the precast concrete shell 11 is cast with recycled concrete with a slump of 120mm, and part of the recycled concrete is added first, and a vibrating rod is inserted into the outside of the U-shaped formwork for sufficient vibration until the bottom concrete is dense, the recycled concrete slurry no longer sinks, the surface of the recycled concrete slurry shows floating slurry, and cement slurry overflows from the slurry holes; after the bottom recycled concrete is poured densely, the U-shaped side concrete is poured; vibration compaction means that the recycled concrete slurry no longer sinks, the surface of the recycled concrete slurry shows floating slurry, and cement slurry overflows from the slurry holes; interface reinforcement is arranged through the reinforcement holes; after the final setting is completed and cured for one week, the internal U-shaped formwork is removed; the bottom interface is roughened; and high-strength concrete is used to cast the cast-in-place part of the concrete.

[0190] At the same time, recycled concrete monolithic beams and high-strength concrete monolithic beams with the same specifications and sizes as the U-shaped recycled concrete composite beams were used. The mix ratios of recycled concrete and high-strength concrete used in the construction are shown in Table 3.

[0191] Table 3 Mix ratios of recycled concrete, ordinary concrete and high-strength concrete used in construction

[0192] (2) Results and Discussion

[0193] Constant-load temperature-increasing tests were conducted on U-shaped recycled concrete composite beams, recycled concrete monolithic beams, and high-strength concrete monolithic beams. The furnace temperature curve followed the ISO 834 standard temperature-increasing curve. The specimens were exposed to fire from three sides, with a load ratio of 0.55. The load was applied to the midspan using a 50t jack and a distribution beam, and the specimens were exposed to fire for 103 minutes. The specimens were simply supported beams, with a 500mm spacing between the midspan loading points. During the tests, the midspan deflection of the beams was measured, the deflection growth rate was calculated, and surface cracking was observed after the tests. Specific data are shown in Table 4.

[0194] After the fire test, the U-shaped recycled concrete composite beams, recycled concrete monolithic beams, and high-strength concrete monolithic beams were allowed to cool naturally to room temperature in the furnace before testing their post-fire residual bearing capacity. The specimens were simply supported beams with loading points spaced 500 mm apart at mid-span. They were loaded using a four-column hydraulic servo testing machine. The yield moment, ultimate bending moment, and corresponding deflection were measured. Specific data are shown in Table 5.

[0195] Table 4 shows that the deflections and corresponding deflection growth rates of the U-shaped recycled concrete composite beams at 30, 60, 90, and 99 minutes after exposure to fire were lower than those of the recycled concrete monolithic beams and significantly lower than those of the high-strength concrete monolithic beams. The high-strength concrete monolithic beams reached their fire resistance limit at 99 minutes, while the U-shaped recycled concrete composite beams did not. Localized cracking occurred in the high-strength concrete monolithic beams, but not in the U-shaped recycled concrete composite beams. Therefore, the composite recycled concrete beams exhibit significant fire resistance advantages, exceeding those of both recycled concrete monolithic beams and high-strength concrete monolithic beams. This improves the fire resistance of composite concrete beams and addresses the problem of high-strength concrete being prone to cracking and premature failure under fire.

[0196] Table 5 shows that the post-fire residual bearing capacity of the U-shaped recycled concrete composite beam is significantly better than that of the recycled concrete monolithic beam, and the deflection corresponding to the yield moment is significantly lower than that of the recycled concrete monolithic beam. This shows that the recycled concrete monolithic beam experiences significant strength and stiffness degradation after fire, while the U-shaped recycled concrete composite beam can optimize this phenomenon, reaching or even exceeding the post-fire residual bearing capacity of the high-strength concrete monolithic beam 1. Therefore, the U-shaped recycled concrete composite beam has a superior post-fire residual bearing capacity.

[0197] Table 4 Fire resistance performance indicators of U-type recycled concrete composite beams, recycled concrete monolithic beams, and high-strength concrete monolithic beams

[0198] Table 5 Post-fire residual bearing capacity performance indicators of U-type recycled concrete composite beams, recycled concrete monolithic beams, and high-strength concrete monolithic beams

[0199] The above description of the embodiments is intended to facilitate understanding and use of the invention by those skilled in the art. It will be apparent that those skilled in the art can readily make various modifications to these embodiments and apply the general principles described herein to other embodiments without requiring inventive effort. Therefore, the present invention is not limited to the above-described embodiments. Improvements and modifications made by those skilled in the art based on the disclosure of the present invention, without departing from the scope of the present invention, should be within the scope of protection of the present invention.

Claims

1. A U-shaped recycled concrete composite beam, characterized in that: It comprises a prefabricated concrete shell (11) and cast-in-place compression zone concrete (12); The precast concrete shell (11) is a recycled concrete shell, the cross-section of the precast concrete shell (11) is U-shaped, and the strength grade of the precast concrete shell (11) is not higher than C40; The cast-in-place compression zone concrete (12) is high-strength concrete arranged in the U-shaped trough of the precast concrete shell (11), and the cast-in-place compression zone concrete (12) is concrete with a strength grade of C50-C80; A steel cage is provided as a skeleton in a U-shaped recycled concrete composite beam. The steel cage is placed inside a precast concrete shell (11) and comprises: horizontally arranged tension steel bars (21), frame bars (23) and vertically arranged stirrups. The stirrups are used to restrain the tension steel bars (21). The stirrups are U-shaped structures with a bent portion at the top. The upper surface of the bottom of the stirrups abuts against the tension steel bars (21), and the lower surface of the bent portion of the stirrups abuts against the frame bars (23).

2. The U-shaped recycled concrete composite beam according to claim 1, characterized in that: The bottom of the precast concrete shell (11) is provided with interface reinforcement steel bars (25); The cross-sectional dimensions of the U-shaped recycled concrete composite beam are as follows: the width b is not less than 200 mm, and the height h is not less than 300 mm; the bottom thickness h1 of the precast concrete shell (11) is not less than 80 mm, and the side widths b1 and b3 of the precast concrete shell (11) are not less than 45 mm; and the width b2 of the cast-in-place compression zone concrete (12) is not less than 100 mm.

3. The U-shaped recycled concrete composite beam according to claim 1, characterized in that: The cast-in-place compression zone concrete (12) is cast simultaneously with the concrete slab, the heights of both sides of the precast concrete shell (11) are set to h1+h2, and the height h3 above is cast simultaneously with the cast-in-place compression zone concrete (12), the height h3 is the thickness of the concrete slab, and the same concrete material as the cast-in-place compression zone concrete (12) is used.

4. A construction method of a U-shaped recycled concrete composite beam, used to realize the construction of the U-shaped recycled concrete composite beam according to claims 1 to 3, characterized in that: The following steps are involved: S1: Set a reinforcement cage inside the U-shaped recycled concrete composite beam, and the reinforcement cage uses U-shaped stirrups; S2: Setting the external formwork and the U-shaped formwork of the corresponding size inside the U-shaped recycled concrete composite beam, the U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards; S3: Use recycled concrete to cast a precast concrete shell (11), and vibrate it thoroughly with a vibrator. Reality; S4: After S3 pouring is completed, the internal U-shaped formwork is removed after final setting and curing for at least one week; S5: roughening the bottom interface of the precast concrete shell (11); S6: high-strength concrete is used to form cast-in-place compression zone concrete (12).

5. The construction method of a U-shaped recycled concrete composite beam according to claim 4, characterized in that: In S2, a slurry outlet hole is provided at the bottom of the U-shaped template.

6. The construction method of a U-shaped recycled concrete composite beam according to claim 1, characterized in that: In S3, the slump of the recycled concrete material used is not less than 120 mm.

7. A method for normal temperature and fire resistance design of a U-shaped recycled concrete composite beam, used to achieve the normal temperature and fire resistance design of the U-shaped recycled concrete composite beam according to claims 1 to 3, characterized in that: The following steps are involved: A1: Determine the load effect and fire resistance rating of U-shaped recycled concrete composite beams; A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam; A3: Analyze the normal temperature bearing capacity and normal use capacity of U-shaped recycled concrete composite beams, and design the reinforcement of U-shaped recycled concrete composite beams; A4: Carry out fire resistance design of U-shaped recycled concrete composite beams, using simple design method or complex design method; A5: Based on the analysis in A3, the roughness and reinforcement design of the combined interface is carried out, using a simple design method or a complex design method; A6: Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.

8. The method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance according to claim 7, characterized in that: The load effect in A1 shall comply with the relevant provisions of the Code for Loads on Building Structures (GB 50009-2012) and the Code for Loads on Building Structures (GB 50009-2012), and the fire resistance level shall comply with the relevant provisions of the Code for Fire Protection Design of Buildings (GB 50016-2014) and the General Code for Fire Protection of Buildings (GB 55037-2022); the cross-sectional dimension design of the U-shaped recycled concrete composite beam in A2 shall comply with the requirements for component dimensions in the Code for Design of Concrete Structures (GB50010-2010), and the dimensional parameters of the U-shaped recycled concrete composite beam shall meet the requirements of engineering design; in A3, the reinforcement design of the U-shaped recycled concrete composite beam shall meet the requirements for the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of the concrete beam in the Code for Design of Concrete Structures (GB50010-2010).

9. A method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance according to claim 7, characterized in that: The simple design method in A4 refers to: meeting the minimum values ​​of the beam width and the protective layer thickness of the longitudinal tensile reinforcement (21) for simply supported beams or continuous beams in the Technical Code for Fire Resistance Design of Building Concrete Structures (DBJ / T15-81-2022), and the effect of the reduction in the thermal conductivity of recycled concrete should not be considered, only as a safety reserve; The complex design method in A4 refers to the nonlinear full-process analysis under fire conditions: the time-varying internal temperature field of concrete components and structures is calculated using a general finite element program, the impact of the reduced thermal conductivity of recycled concrete is considered, and on this basis, high-temperature mechanical analysis of components and structures is carried out to determine whether the fire resistance limit of the components or structures meets the engineering design requirements.

10. The method for designing a U-shaped recycled concrete composite beam at room temperature and fire resistance according to claim 7, characterized in that: In A5, the simple design method means: the design interface is a rough interface, the roughness of the rough interface is not less than the corresponding roughness when the coarse aggregate is half exposed, or the natural casting surface that is not smoothed, and structural interface steel bars should be arranged, and the reinforcement ratio of the interface steel bars should not be less than the reinforcement ratio of the stirrups; In A5, the complex design method refers to: using nonlinear full-process analysis under fire conditions to conduct high-temperature safety analysis of the combined interface, the strength and stiffness loss of the interface bearing capacity under high temperature should be considered, and the degradation law of the interface tensile bearing capacity and shear bearing capacity under high temperature should be tested through experiments.

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