WO2025103288A1 - 一种u型再生混凝土组合梁及其设计、施工方法 - Google Patents

一种u型再生混凝土组合梁及其设计、施工方法 Download PDF

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Publication number
WO2025103288A1
WO2025103288A1 PCT/CN2024/131478 CN2024131478W WO2025103288A1 WO 2025103288 A1 WO2025103288 A1 WO 2025103288A1 CN 2024131478 W CN2024131478 W CN 2024131478W WO 2025103288 A1 WO2025103288 A1 WO 2025103288A1
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concrete
shaped
recycled concrete
composite beam
design
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French (fr)
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肖建庄
陈子璇
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Tongji University
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Tongji University
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    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
    • E04C3/00—Structural elongated elements designed for load-supporting
    • E04C3/02—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces
    • E04C3/20—Joists; Girders, trusses, or trusslike structures, e.g. prefabricated; Lintels; Transoms; Braces of concrete or other stone-like material, e.g. with reinforcements or tensioning members
    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04G—SCAFFOLDING; FORMS; SHUTTERING; BUILDING IMPLEMENTS OR AIDS, OR THEIR USE; HANDLING BUILDING MATERIALS ON THE SITE; REPAIRING, BREAKING-UP OR OTHER WORK ON EXISTING BUILDINGS
    • E04G21/00—Preparing, conveying, or working-up building materials or building elements in situ; Other devices or measures for constructional work
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
    • Y02W30/00—Technologies for solid waste management
    • Y02W30/50—Reuse, recycling or recovery technologies
    • Y02W30/91—Use of waste materials as fillers for mortars or concrete

Definitions

  • the 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.
  • recycled concrete made from recycled aggregates generated by crushing waste concrete has been shown to have good fire resistance. Due to the rich old mortar on the surface of the recycled aggregate, the recycled concrete has the characteristics of looseness and porosity inside.
  • the thermal conductivity of recycled concrete is lower than that of ordinary concrete under the same water-cement ratio, and it is not easy to burst at high temperatures.
  • the use of recycled aggregates has reduced the bearing capacity and stiffness of recycled concrete components at room temperature to a certain extent, which restricts the application of recycled concrete structures and also limits the excellent thermal performance of recycled concrete materials.
  • Patent CN208329371U discloses a T-shaped steel-frame recycled concrete composite beam, including a concrete composite beam box, a limit fixing plate is placed at the bottom of the concrete composite beam box, and the top of the limit fixing plate is provided with evenly distributed clamping grooves, and stirrups evenly distributed in the inner cavity of the concrete composite beam box are placed on the top of the limit fixing plate, and a limit fixing plate located in the inner cavity of the concrete composite beam box is placed on the top of the stirrups, and steel bars are arranged at the top and bottom of the stirrups.
  • the bearing capacity of the device is enhanced, which facilitates better utilization of the ductility of the T-shaped steel, saves the use of formwork, improves construction efficiency, and facilitates filling of waste concrete into the interior of the composite beam, which protects the environment and enables waste recycling.
  • this design greatly increases the amount of steel used in the concrete beam, and the structure is relatively complex, which increases the construction cost.
  • this design places steel materials such as limit fixing plates on the surface of concrete beams, which degrades quickly after being exposed to fire.
  • the recycled concrete is used in the middle of the concrete beam, which cannot fully utilize the excellent fire resistance of recycled concrete.
  • the purpose of the present invention is to provide a U-shaped recycled concrete composite beam and its design and construction method in order to overcome the defects of the above-mentioned prior art, give full play to the fire resistance characteristics of recycled concrete and the excellent mechanical properties of high-strength concrete, at room temperature, can effectively improve the bending bearing capacity and ductility of the recycled concrete beam, at high temperature, the bursting phenomenon of the fire-exposed surface is reduced, the temperature field inside the beam is significantly reduced, and the thermal insulation performance of the recycled concrete is fully utilized, which not only protects the internal stress-bearing steel bars, but also avoids the weakening of the structural cross-section size caused by the bursting of high-strength concrete.
  • the construction method can effectively reduce the amount of on-site pouring work and speed up the progress of the project.
  • the present invention provides a U-shaped recycled concrete composite beam, comprising a precast concrete shell and cast-in-place compression zone concrete;
  • the precast concrete shell is a recycled concrete shell.
  • the cross-section of the precast concrete shell is U-shaped, with good fire resistance and not easy to burst.
  • the recycled concrete shell is a recycled concrete prepared by crushing and screening waste concrete to form recycled coarse aggregate, which partially or completely replaces natural aggregate.
  • the recycled concrete is a recycled concrete with a recycled coarse aggregate replacement rate greater than 50%.
  • the recycled coarse aggregate replacement rate refers to the ratio of recycled coarse aggregate to natural coarse aggregate. More preferably, the recycled concrete is a recycled concrete with a 100% recycled coarse aggregate replacement rate.
  • the strength grade of the recycled concrete is not 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 grade of recycled coarse aggregate, optimizing the grading of recycled coarse aggregate, and ensuring good maintenance conditions.
  • the cast-in-place compression zone concrete is high-strength concrete, which refers to concrete with a strength grade of C50-C80.
  • the cast-in-place compression zone concrete can be cast on the construction site.
  • the cast-in-place compression zone concrete is high-strength concrete set in the U-shaped trough of the precast concrete shell.
  • 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.
  • ultra high performance concrete (Ultra High Performance Concrete) 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.
  • the cross-sectional dimensions of the U-shaped recycled concrete composite beam are as follows: width b is not less than 200mm, and height h is not less than 300mm.
  • the thickness of the bottom of the precast concrete shell h1 is not less than 80mm, and the tensile reinforcement is arranged inside the precast concrete shell.
  • the widths b1 and b3 of the side 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.
  • b1 and b3 are equal.
  • 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.
  • 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.
  • interface reinforcement steel bars are provided at the bottom of the precast concrete shell.
  • 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.
  • the sides of the precast concrete shell should also be roughened, preferably with teeth in the formwork.
  • a steel cage is provided as a skeleton in the U-shaped recycled concrete composite beam.
  • the steel cage is placed inside the precast concrete shell.
  • the steel cage includes tension steel bars, frame bars and stirrups.
  • the stirrups are arranged in a U shape according to the shape of the component.
  • a bending portion is provided at the top of the stirrups. The upper surface of the bottom of the stirrups abuts against the tension steel bars, and the lower surface of the bending portion of the stirrups abuts against the frame bars.
  • the steel cage also includes additional steel bars and waist bars.
  • 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 plan view of concrete structure construction drawings (cast-in-place concrete frames, shear walls, beams, slabs)" and "GB50666-2011 Concrete Structure Engineering Construction Code”.
  • an optimized U-shaped combination form is adopted, that is, the heights of both sides of the precast concrete shell are set to h 1 +h 2 , and the upper height h 3 is cast 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.
  • the precast portion of the precast concrete shell is not lower than the bottom of the connected concrete slab.
  • the upper height h 3 is the same as the thickness of the concrete slab.
  • a supplementary stirrup is provided above the U-shaped stirrup to connect the U-shaped stirrup to form a ring stirrup.
  • the U-shaped stirrups are connected to the supplementary stirrups above them by welding.
  • the present invention also provides a construction method for a U-shaped recycled concrete composite beam, which is to cast the beam in sections and in batches on site.
  • semi-prefabricated and semi-cast-in-place method when the semi-prefabricated and semi-cast-in-place method is adopted, the pouring steps of the prefabricated concrete shell can be completed in the factory, and only the pouring of the cast-in-place concrete part is carried out on site. Specifically, the following steps are included:
  • the allowable deviation of the U-shaped template is ⁇ 5 mm.
  • the net distance between the U-shaped template and the U-shaped stirrup is not less than 10 mm.
  • 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.
  • 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.
  • custom templates can be made using 3D printed plastic materials.
  • a slurry outlet hole is provided at the bottom of the U-shaped template.
  • the slump of the recycled concrete material used is not less than 120 mm.
  • the diameter of the vibrating rod should be smaller than the side width of the U-shaped concrete shell.
  • the vibrating rod should be extended into the outside of the U-shaped template for vibration.
  • the 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.
  • 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.
  • steel bar holes are provided at the bottom of the U-shaped formwork to arrange interface reinforcement steel bars.
  • the present invention also provides a normal temperature and fire resistance design method for a U-shaped recycled concrete composite beam, and the specific steps are as follows:
  • 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, design the roughness and reinforcement of the combined interface, 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.
  • the load effect 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), and the fire resistance level 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 dimension design of the U-shaped recycled concrete composite beam in A2 should comply with the requirements for component dimensions in the Code for Design of Concrete Structures (GB50010-2010).
  • the dimension parameters of the U-shaped recycled concrete composite beam can meet the engineering design requirements.
  • 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.
  • the bending bearing capacity Mcu of the positive section of the U-shaped recycled concrete composite beam is calculated by the following steps:
  • Mcu 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 section height of the composite beam, the effective section height of the composite beam, the bottom thickness of the precast concrete shell, the core height of the cast-in-place concrete in the compression zone and the top height of the cast-in-place concrete in the compression zone respectively
  • fc ,R , fc ,H , fy are the calculated compressive strength of recycled concrete, the design value of the compressive strength of high-strength concrete and the design value of the yield strength of the stressed steel bars respectively
  • ⁇ 1 ,H and ⁇ 1 ,R are the coefficients of high-strength concrete and recycled concrete materials, which are calculated in accordance with Article 6.2.6 of Code for Design of
  • the shear bearing capacity Vcs of the oblique section of the U-shaped composite recycled concrete beam can be calculated by the following formula:
  • Vcs is the shear bearing capacity of the inclined section of the U-shaped recycled concrete composite beam
  • ft ,R , ft ,H, fyv are the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups, respectively
  • ⁇ cv is the shear bearing capacity coefficient of the inclined section concrete, which is calculated in accordance with Article 6.3.4 of the Code for Design of Concrete Structures (GB50010-2010)
  • Asv 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 direction of the member.
  • the bearing capacity of the U-shaped recycled concrete composite beam should not be less than the bearing capacity of the corresponding ordinary concrete monolithic beam, where the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio.
  • the normal service capacity analysis of the U-shaped recycled concrete composite beam described in A3 should meet the relevant provisions of the "Concrete Structure Design Code" (GB50010-2010), and the maximum crack width, deflection, etc. should be verified.
  • the simple design method in A4 refers to 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).
  • 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.
  • 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 large-scale general finite element program, and the influence of the reduced thermal conductivity of recycled concrete is considered.
  • high-temperature mechanical analysis of components and structures is carried out to determine whether the fire resistance limit of components or structures meets the engineering design requirements.
  • the fire resistance limit should be determined by whether the component has lost its bearing capacity, integrity and thermal insulation.
  • 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 meet the following formula:
  • 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 tensile point on the specimen interface, mm.
  • the criterion for loss of integrity is that the specimen can continue to maintain fire resistance and fire isolation performance during the fire test.
  • the judgment condition for loss of thermal insulation is that the average temperature rise of the unfired surface of the specimen exceeds the initial average temperature by 140°C or the temperature rise at any point exceeds the initial temperature by 180°C.
  • thermo conductivity of recycled concrete in the complex design method adopts the measured value.
  • the simple design method refers to designing the interface as 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 interface steel bar reinforcement ratio should not be less than the stirrup reinforcement ratio.
  • ⁇ is the interface shear stress obtained from experiments or numerical simulations
  • [ ⁇ ] is the maximum shear stress that the combined interface can withstand, in MPa.
  • the maximum shear stress [ ⁇ ] that the composite interface can withstand can be calculated by the following formula:
  • ⁇ a is the shear stress provided by bonding and aggregate bite
  • ⁇ is the interface friction coefficient
  • ⁇ 1 and ⁇ 2 are action coefficients
  • fy and fcc are the tensile strength of the interface steel bars and the uniaxial cylindrical compressive strength of concrete, respectively
  • ⁇ is the interface steel bar reinforcement ratio
  • ⁇ c and ⁇ are the effective coefficients of the interface steel bars and concrete, respectively
  • ⁇ n is the interface normal stress.
  • the interface reinforcement ratio ⁇ is calculated by the following formula:
  • the complex design method refers to the use of nonlinear full-process analysis under fire conditions to High temperature safety analysis of composite interface.
  • 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.
  • RT 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 structural fire resistance limit.
  • the maximum crack width should meet the following formula: w ⁇ [w],
  • w is the maximum crack width obtained from the test or numerical simulation
  • [w] is the maximum crack width limit specified in the specification, mm.
  • the present invention has the following advantages:
  • 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 weakening the structural bearing capacity and ductility caused by the application of recycled concrete.
  • the bursting phenomenon of the fire-exposed surface is reduced, and the temperature field inside the beam is significantly reduced, which fully exerts the thermal insulation performance of the recycled concrete, protects the internal stress-bearing steel bars, and avoids the weakening of the structural cross-sectional dimensions caused by the bursting of high-strength concrete.
  • 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.
  • the present invention provides a construction method for a U-shaped recycled concrete composite beam, wherein 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 of the recycled concrete prefabricated part is The quality can be well guaranteed, reducing the variability of recycled concrete material properties.
  • 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 embodying 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.
  • Figure 1 is a cross-sectional design diagram of a U-shaped recycled concrete composite beam.
  • FIG2 is a cross-sectional design diagram of a U-shaped recycled concrete composite beam when the composite beam is connected to an upper concrete slab.
  • Figure 3 is a structural design flow chart of a U-shaped recycled concrete composite beam with good mechanical and fire resistance properties.
  • Figure numerals 11 - precast concrete shell; 12 - cast-in-place compression zone concrete; 21 - tension steel bars; 22 - U-shaped stirrups; 23 - frame bars; 24 - supplementary stirrups; 25 - interface reinforcement steel bars.
  • this embodiment provides a normal temperature and fire resistance design method for a U-shaped recycled concrete composite beam, using a simple design method, and the specific steps are as follows:
  • 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 by structural requirements, where 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), and 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).
  • 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 meet 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.
  • A3 Analyze the normal temperature bearing capacity and normal use capacity of the U-shaped recycled concrete composite beam, and design the reinforcement of the U-shaped recycled concrete composite beam.
  • the reinforcement design of the U-shaped recycled concrete composite beam should meet the requirements of the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of the concrete beam in the "Concrete Structure Design Code" (GB50010-2010). Among them, the bending bearing capacity M cu of the normal section of the U-shaped recycled concrete composite beam is calculated by the following steps:
  • Mcu 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, the middle thickness and the right thickness of the precast concrete shell 11 respectively
  • h, h0 , h1 , h2 , h3 are the section height of the composite beam, the effective section 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
  • fc ,R , fc ,H , fy are the calculated compressive strength of recycled concrete, the design value of compressive strength of high-strength concrete, and the design value of yield strength of the stressed steel bars respectively
  • ⁇ 1 ,H and ⁇ 1 ,R are the coefficients of high-strength concrete and recycled concrete materials, which are calculated in accordance with Article 6.2.6 of Code for Design of Concrete
  • the shear bearing capacity Vcs of the oblique section of the U-shaped recycled concrete composite beam is calculated by the following formula:
  • 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 the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups.
  • ⁇ cv is the shear bearing capacity coefficient of the oblique section concrete, according to the Code for Design of Concrete Structures
  • Asv 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 member.
  • A4 Conduct fire resistance design of U-shaped recycled concrete composite beams using a simple design method. Confirm that the cross-sectional dimensions of the U-shaped recycled concrete composite beams meet the minimum values of beam width and 21 protective layer thickness of longitudinal tensile reinforcement for simply supported beams or continuous beams in the Technical Code for Fire Resistance Design of Building Concrete Structures (DBJ/T15-81-2022), without considering the impact of reduced thermal conductivity of recycled concrete, only as a safety reserve.
  • A5 According to the analysis in A3, the roughness and reinforcement design of the combined interface is carried out, and a simple design method is adopted.
  • the design interface is a rough interface, and 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.
  • ⁇ is the interface shear stress obtained from experiments or numerical simulations
  • [ ⁇ ] is the maximum shear stress that the combined interface can withstand, in MPa.
  • ⁇ a is the shear stress provided by bonding and aggregate bite
  • ⁇ is the interface friction coefficient
  • ⁇ 1 and ⁇ 2 are action coefficients
  • fy and fcc are the tensile strength of the interface steel bars and the uniaxial cylindrical compressive strength of concrete, respectively
  • ⁇ is the interface steel bar reinforcement ratio
  • ⁇ c and ⁇ are the effective coefficients of the interface steel bars and concrete, respectively
  • ⁇ n is the interface normal stress.
  • the interface reinforcement ratio ⁇ is calculated by the following formula:
  • the maximum crack width should meet the following formula: w ⁇ [w],
  • w is the maximum crack width obtained from the test or numerical simulation
  • [w] is the maximum crack width limit specified in the specification, mm.
  • A6 Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.
  • this embodiment provides a normal temperature and fire resistance design method for a U-shaped recycled concrete composite beam, using a complex design method, and the specific steps are as follows:
  • 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 by structural requirements, where 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), and 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).
  • 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 meet 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.
  • A3 Analyze the normal temperature bearing capacity and normal use capacity of the U-shaped recycled concrete composite beam, and design the reinforcement of the U-shaped recycled concrete composite beam.
  • the reinforcement design of the U-shaped recycled concrete composite beam should meet the requirements of the bending bearing capacity of the normal section and the shear bearing capacity of the inclined section of the concrete beam in the "Concrete Structure Design Code" (GB50010-2010). Among them, the bending bearing capacity M cu of the normal section of the U-shaped recycled concrete composite beam is calculated by the following steps:
  • Mcu 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, the middle thickness and the right thickness of the precast concrete shell 11 respectively
  • h, h0 , h1 , h2 , h3 are the section height of the composite beam, the effective section 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
  • fc ,R , fc ,H , fy are the calculated compressive strength of recycled concrete, the design value of compressive strength of high-strength concrete, and the design value of yield strength of the stressed steel bars respectively
  • ⁇ 1 ,H and ⁇ 1 ,R are the coefficients of high-strength concrete and recycled concrete materials, which are calculated in accordance with Article 6.2.6 of Code for Design of Concrete
  • the shear bearing capacity Vcs of the oblique section of the U-shaped recycled concrete composite beam is calculated by the following formula:
  • 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 the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups.
  • ⁇ cv is the shear bearing capacity coefficient of the oblique section concrete, which is calculated according to the Code for Design of Concrete Structures (GB
  • Asv is the total cross-sectional area of each leg of the stirrups arranged in the same section, and s is the stirrup spacing along the length of the member.
  • A4 Carry out fire resistance design of U-shaped recycled concrete composite beams, using complex design methods. Perform nonlinear full-process analysis under fire conditions. Use a large-scale general finite element program to calculate the time-varying internal temperature field of concrete components and structures, consider the impact of the reduced thermal conductivity of recycled concrete, and on this basis carry out high-temperature mechanical analysis of components and structures to determine whether the fire resistance design of components or structures meets the fire resistance limit requirements of the Code for Fire Protection of Building Structures (GB 50016-2014). When calculating the time-varying internal temperature field, consider the impact of the reduced thermal conductivity of recycled concrete.
  • the fire resistance limit should be determined by whether the component loses its bearing capacity, integrity and thermal insulation.
  • the 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 meet the following formula:
  • 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 tensile point on the specimen interface, mm.
  • the criterion for loss of integrity is that the specimen can continue to maintain fire resistance and fire isolation performance during the fire test.
  • the judgment condition for loss of thermal insulation is that the average temperature rise of the unfired surface of the specimen exceeds the initial average temperature by 140°C or the temperature rise at any point exceeds the initial temperature by 180°C.
  • the thermal conductivity of recycled concrete adopts the measured value.
  • A5 Based on the analysis in A3, the roughness and reinforcement design of the combined interface is carried out, and a complex design method is adopted. The nonlinear full process analysis under fire conditions is used to conduct high temperature safety analysis of the combined interface.
  • 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.
  • RT 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 structural fire resistance limit.
  • the maximum crack width should meet the following formula: w ⁇ [w],
  • w is the maximum crack width obtained from the test or numerical simulation
  • [w] is the maximum crack width limit specified in the specification, mm.
  • A6 Based on the analysis results obtained in A2 to A5, select the optimized construction method for U-shaped recycled concrete composite beams.
  • 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.
  • the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and a cast-in-place compression zone concrete 12.
  • the top reinforcement 23 is
  • the tensile reinforcement 21 is
  • the U-shaped stirrup 22 is 10@100, no interface reinforcement is provided.
  • the length of the U-shaped recycled concrete composite beam is 2700mm, the clear span is 2400mm, and the thickness of the bottom concrete cover is 40mm.
  • the raw materials and equipment are as follows:
  • recycled concrete with a recycled coarse aggregate replacement rate equal to 100% is used to cast the precast concrete shell 11, and high-strength concrete is used to cast the cast-in-place compression zone.
  • a steel cage is set inside the U-shaped recycled concrete composite beam, and U-shaped stirrups 22 are used in the steel cage; an external formwork and a U-shaped formwork of corresponding size inside the U-shaped beam are set, the U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards, and a slurry outlet hole is set below the U-shaped formwork; recycled concrete with a slump of 120 mm is used to cast the precast concrete shell 11, first a part of the recycled concrete is 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 presents floating slurry, and cement slurry overflows from the slurry outlet hole; after the bottom recycled concrete
  • the bending bearing capacity test at room temperature was carried out on U-shaped recycled concrete composite beams, recycled concrete monolithic beams, and ordinary concrete monolithic beams.
  • the specimens were simply supported beams with a mid-span loading point spacing of 500 mm.
  • the specimens were loaded using a four-column hydraulic servo testing machine. The yield bending moment, ultimate bending moment, and their corresponding deflections were measured, respectively.
  • the specific data are shown in Table 2.
  • the yield moment and ultimate moment of the U-shaped recycled concrete composite beam are significantly higher than those of the recycled concrete monolithic beam, reaching and exceeding the level of the ordinary concrete monolithic beam, and the deflection corresponding to the yield moment is roughly the same. Therefore, the composite recycled concrete beam can optimize the problem of low bearing capacity of the recycled concrete beam.
  • the proposed U-shaped recycled concrete composite beam can be used to replace the ordinary concrete monolithic beam.
  • the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and cast-in-place compression zone concrete 12.
  • the top reinforcement 23 is The tensile reinforcement 21 is The U-shaped stirrup 22 is 10@100, interface reinforcement steel bar 25 10@100, length is 100mm.
  • the length of U-shaped recycled concrete composite beam is 2700mm, the clear span is 2400mm, and the thickness of the bottom concrete cover is 40mm.
  • recycled concrete with a recycled coarse aggregate replacement rate equal to 100% is used to cast the precast concrete shell 11, and high-strength concrete is used to cast the cast-in-place compression zone.
  • a steel cage is set inside the U-shaped recycled concrete composite beam, and U-shaped stirrups 22 are used in the steel cage; an external formwork and a U-shaped formwork of corresponding size inside the U-shaped beam are set, the U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards, and a slurry outlet hole and a steel bar hole are 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 presents floating slurry, and cement slurry overflows from the slurry outlet hole;
  • the U-shaped recycled concrete composite beam, recycled concrete monolithic beam, and high-strength concrete monolithic beam were subjected to constant load temperature rise tests.
  • the furnace temperature curve was the ISO 834 standard temperature rise curve.
  • the specimen was subjected to fire on three sides, the load ratio was 0.55, and the load was applied to the mid-span through a 50t jack and a distribution beam.
  • the specimen was subjected to fire for 103 minutes.
  • the specimen was a simply supported beam with a mid-span loading point spacing of 500mm. During the test, the mid-span deflection of the beam was measured, the deflection growth rate was calculated, and the surface cracking was observed after the test.
  • the specific data are shown in Table 4.
  • the deflection and corresponding deflection growth rate of the U-shaped recycled concrete composite beam at 30, 60, 90, and 99 minutes of fire exposure are lower than those of the recycled concrete monolithic beam, and significantly lower than those of the high-strength concrete monolithic beam.
  • the high-strength concrete monolithic beam reaches its fire resistance limit at 99 minutes of fire exposure, while the U-shaped recycled concrete composite beam does not. Local bursting occurs in the high-strength concrete monolithic beam, but not in the U-shaped recycled concrete composite beam. Therefore, the composite recycled concrete beam has a significant advantage in fire resistance, and its fire resistance exceeds that of the recycled concrete monolithic beam and the high-strength concrete monolithic beam. It can optimize the fire resistance of the composite concrete beam and solve the problem that high-strength concrete is prone to bursting and premature destruction under fire.
  • 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. It can be seen that the recycled concrete monolithic beam has significant strength and stiffness degradation after the fire, and the U-shaped recycled concrete composite beam can optimize this phenomenon and reach or even exceed the post-fire residual bearing capacity of the high-strength concrete monolithic beam 1. Therefore, the U-shaped recycled concrete composite beam has a more excellent post-fire residual bearing capacity.

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Abstract

一种U型再生混凝土组合梁及其设计、施工方法，U型再生混凝土组合梁包括预制混凝土外壳（11）、现浇受压区混凝土（12），预制混凝土外壳（11）为再生混凝土外壳，外形为U型；现浇受压区混凝土（12）为高强混凝土，现浇受压区混凝土（12）为C50-C80强度等级的混凝土；预制混凝土外壳（11）底部厚度h 1不小于80mm，预制混凝土外壳（11）侧面宽度b 1和b 3均不小于45mm；现浇受压区混凝土（12）的宽度b 2不小于100mm；U型再生混凝土组合梁内设有钢筋笼作为骨架。与现有技术相比，充分发挥再生混凝土的耐火特性和高强混凝土优异的力学性能，在常温下，可以有效提高再生混凝土梁的抗弯承载力和延性，在高温下，受火表面的爆裂现象减少，梁内温度场显著降低。

Description

一种U型再生混凝土组合梁及其设计、施工方法 技术领域
本发明涉及建筑结构和施工领域，尤其是涉及一种U型再生混凝土组合梁及其设计、施工方法。
背景技术
为实现土木工程结构的可持续性，防灾减灾是结构设计中相当重要的一个内容。随着高层结构、大跨结构的发展，高强混凝土以及高性能混凝土的应用愈加广泛。然而，高强混凝土致密的微观结构使得其在高温作用下有较高的内应力，易发生混凝土爆裂，进而造成内部钢筋裸露，钢筋温度快速升高，而结构的耐火性能显著下降，这制约了高强混凝土在建筑结构中的应用。采用掺入防火涂料等方法，提高了结构造价，增加了施工工序，并且带来了建筑结构长期使用的养护问题。
与之相反，采用废旧混凝土破碎生成的再生骨料制备的再生混凝土，已被证明具有良好的耐火性能。由于再生骨料表面具有丰富的老砂浆，再生混凝土具有内部松散、多孔的特性。再生混凝土的导热系数与同等水灰比条件下的普通混凝土更低，并且高温下不易发生爆裂。但是，再生骨料的应用一定程度上降低了再生混凝土构件在常温下的承载力和刚度，这制约了再生混凝土结构的应用，也限制了再生混凝土材料优异的热工性能的发挥。
专利CN208329371U公开了一种T形钢骨再生混凝土组合梁，包括混凝土组合梁箱，混凝土组合梁箱的底部放置有限位固定板，且限位固定板的顶部开设有均匀分布的卡接槽，限位固定板的顶部放置有位于混凝土组合梁箱内腔且均匀分布的箍筋，箍筋的顶部放置有位于混凝土组合梁箱内腔的限位固定板，箍筋内部的顶端和底端均设置有钢筋。通过钢骨和翼缘板的配合使用，使得该装置的承载能力更强，便于更好的利用T形钢的延展性能，节省了模板的使用，提高了施工的效率，同时便于将废旧混凝土填充在该组合梁的内部，既保护了环境又能够废物再利用。但是，这一设计大量增加了混凝土梁中钢材的使用量，并且构造较为复杂，提高了建 造成本。另外，这一设计将限位固定板等钢材布置于混凝土梁表面，受火后性能退化较快，而再生混凝土应用于混凝土梁中部，无法充分利用再生混凝土优异的耐火性能。
发明内容
本发明的目的就是为了克服上述现有技术存在的缺陷而提供一种U型再生混凝土组合梁及其设计、施工方法，充分发挥再生混凝土的耐火特性和高强混凝土优异的力学性能，在常温下，可以有效提高再生混凝土梁的抗弯承载力和延性，在高温下，受火表面的爆裂现象减少，梁内温度场显著降低，充分发挥再生混凝土的隔热性能，既保护内部受力钢筋，又避免高强混凝土爆裂导致结构截面尺寸的削弱，施工方式可以有效减少现场浇筑作业量，加快工程进度。
本发明的目的可以通过以下技术方案来实现：
本发明提供一种U型再生混凝土组合梁，包括预制混凝土外壳、现浇受压区混凝土；
预制混凝土外壳为再生混凝土外壳，预制混凝土外壳的截面为U型，耐火性能较好且不易爆裂，再生混凝土外壳为将将废旧混凝土经过破碎筛分后形成再生粗骨料，部分或全部替代天然骨料配制的再生混凝土。优选的，再生混凝土为再生粗骨料取代率大于50％的再生混凝土，再生粗骨料取代率指再生粗骨料代替天然粗骨料的比率，更优选的，再生混凝土为100％再生粗骨料取代率的再生混凝土。再生混凝土的强度等级不高于C40。预制混凝土外壳在工厂中预制而成，并通过控制再生粗骨料等级、优化再生粗骨料级配、保证良好养护条件，以提高预制混凝土外壳的浇筑质量。
现浇受压区混凝土为高强混凝土，高强混凝土指C50-C80强度等级的混凝土。现浇受压区混凝土可以在施工现场浇筑。现浇受压区混凝土为设于预制混凝土外壳的U型槽体中的高强混凝土。
优选的，预制混凝土外壳所用再生粗骨料的最大粒径不大于预制混凝土外壳底部厚度和侧面宽度最小值的1/2。
优选的，可按照实际承载能力需求，现浇受压区可使用超高性能混凝土(Ultra high performance concrete)材料，超高性能混凝土指C80强度等级以上的纤维增强混凝土材料。
U型再生混凝土组合梁的截面尺寸，宽度b不小于200mm，高度h不小于300mm。其中，预制混凝土外壳底部厚度h1不小于80mm，受拉钢筋设于预制混凝土外壳内部。预制混凝土外壳侧面宽度b1和b3均不小于45mm。现浇受压区混凝土的宽度b2不小于100mm。
优选的，b1和b3相等。
优选的，在满足承载能力要求的前提下，预制混凝土外壳的截面面积占U型再生混凝土组合梁的总截面面积不小于40％。
预制混凝土外壳底部进行粗糙化处理，可以使用模板浇筑后拉毛或机械凿毛，或在模板中预先设置齿槽等施工方法。
优选的，预制混凝土外壳底部设有界面增强钢筋，优选的，界面增强钢筋的长度不小于100mm，沿梁纵向间距不大于150mm。
进一步的，预制混凝土外壳侧面应同样进行粗糙处理，并优先选择在模板中设置齿槽的方法。
U型再生混凝土组合梁内设有钢筋笼作为骨架，钢筋笼置于预制混凝土外壳内部，钢筋笼包括受拉钢筋、架立筋和箍筋，箍筋根据构件形状设置为U形，箍筋顶部设有折弯部，箍筋底部的上表面和受拉钢筋抵接，箍筋的折弯部的下表面和架立筋抵接。进一步的，钢筋笼还包括附加钢筋和腰筋。
钢筋笼的施工方式符合《11G101-1混凝土结构施工图平面整体表示方法制图规则和构造详图(现浇混凝土框架、剪力墙、梁、板)》和《GB50666-2011混凝土结构工程施工规范》的相关要求。
当U型再生混凝土组合梁上方与混凝土板连接时，采用优化的U形组合形式，即预制混凝土外壳两侧高度设置为h1+h2，上方h3高度与现浇受压区混凝土同时浇筑，且采用与现浇受压区混凝土相同的混凝土材料。
进一步的，预制混凝土外壳的预制部分不低于相连混凝土板的底部。
进一步的，上方h3高度与混凝土板厚度相同。
进一步的，U形箍筋上方设有一根补充箍筋，将U形箍筋连接成为环形箍。
进一步的，U形箍筋与其上方补充箍筋可采用绑扎连接。
进一步的，考虑混凝土梁受扭作用的情况下，U形箍筋与其上方补充箍筋采用焊接连接。
本发明还提供一种U型再生混凝土组合梁的施工方法，为现场分段分次浇筑 或半预制半现浇方式，当采用半预制半现浇的制作方式时，预制混凝土外壳的浇筑步骤可以在工厂内完成，现场仅进行现浇部分混凝土的浇筑工作。具体包括以下步骤：
S1：设置U型再生混凝土组合梁内部的钢筋笼，钢筋笼采用U形箍筋；
S2：设置外部模板和U型再生混凝土组合梁内部相应尺寸的U形模板，U形模板通过木板在上方和侧面与外部模板固定；
S3：采用再生混凝土浇筑形成预制混凝土外壳，并用振捣棒充分振捣密实；
S4：在S3浇筑完毕后，在终凝结束并养护至少一周后，拆除内部U形模板；
S5：对预制混凝土外壳底部界面进行粗糙化处理；
S6：采用高强混凝土浇筑形成现浇受压区混凝土。
进一步的，在S2中，U形模板的允许偏差为±5mm。
进一步的，在S2中，所述U形模板与U形箍筋的净距不小于10mm。
进一步的，在S5中，对预制混凝土外壳底部界面进行凿毛处理。
进一步的，省去S5，在S2中，在预制混凝土外壳的侧面和底部界面处，采用预先布置齿槽的特制模板，以获得粗糙界面。
进一步的，特制模板的齿槽的平均粗糙度不小于3mm，齿槽之间的间距不小于10mm。
进一步的，特制模板可以使用3D打印的塑料材料进行制作。
进一步的，在S2中，在U形模板底部设置出浆孔。
进一步的，在S3中，所使用的再生混凝土材料坍落度不小于120mm。
进一步的，在S3中，所述振捣棒的直径应小于U形混凝土外壳侧面宽度。
进一步的，在S3中，所述振捣棒应伸入U形模板外侧进行振捣。
进一步的，S3中，所述振捣密实指再生混凝土浆体不再沉落，再生混凝土浆体表面呈现浮浆，出浆孔有水泥浆体溢出。
进一步的，S3中，可先加入部分再生混凝土，待底部再生混凝土浇筑密实后，再浇筑U形侧面混凝土。
进一步的，在U形模板底部设置钢筋孔洞，以布置界面增强钢筋。
进一步的，当U型再生混凝土组合梁上方与混凝土板连接时，采用优化的U形组合形式，S3中浇筑至h2高度，并保留自然浇筑粗糙表面。进行S6之前在U形箍筋上方布置补充钢筋，并与钢筋笼绑扎。
本发明还提供一种U型再生混凝土组合梁的常温和耐火设计方法，具体步骤如下：
A1：确定U型再生混凝土组合梁的荷载效应及耐火等级；
A2：进行U型再生混凝土组合梁的截面尺寸设计；
A3：进行U型再生混凝土组合梁常温承载能力和正常使用能力分析，进行U型再生混凝土组合梁的配筋设计；
A4：进行U型再生混凝土组合梁的耐火设计，采用简单设计方法或复杂设计方法；
A5：根据A3中的分析，进行组合界面的粗糙度和配筋设计，采用简单设计方法或复杂设计方法；
A6：根据A2至A5中获得的分析结果，选择U型再生混凝土组合梁的优化施工方法。
A1中荷载效应应符合《建筑结构荷载规范》(GB50009-2012)、《建筑结构荷载规范》(GB50009-2012)中的相关规定，耐火等级应符合《建筑设计防火规范》(GB 50016-2014)和《建筑防火通用规范》(GB 55037-2022)中的相关规定。A2中U型再生混凝土组合梁的截面尺寸设计，应符合《混凝土结构设计规范》(GB50010-2010)中对于构件尺寸的要求。构成U型再生混凝土组合梁的尺寸参数能够满足工程设计需求。
A3中，U型再生混凝土组合梁的配筋设计应满足《混凝土结构设计规范》(GB50010-2010)中对于混凝土梁的正截面受弯承载力、斜截面受剪承载力的要求。
U型再生混凝土组合梁的正截面受弯承载力Mcu由以下步骤计算：
首先，通过α1,Hfc,Hbx0＝fyAs计算x0
若x0≤h3,x＝x0，
若x0＞h3,α1,Hfc,Hbh3+α1,Hfc,Hb2(x-h3)+α1,Rfc,R(b1+b3)(x1-h3)＝fyAs，
若h3＜x0≤h3+h2,x＝x1，
若x1＞h3+h2,
α1,Hfc,Hbh3+α1,Hfc,Hb2(h2+h3)+α1,Rfc,R(b1+b3)h2+α1,Rfc,Rb(x-h2-h3)＝fyAs。
其中，Mcu为U型再生混凝土组合梁的正截面受弯承载力；b,b1,b2,b3分别为组合梁的宽度，预制混凝土外壳的左侧厚度、中部厚度和右侧厚度；h,h0,h1,h2,h3分别为组合梁的截面高度，组合梁的截面有效高度，预制混凝土外壳的底部厚度、现浇受压区混凝土核心高度和现浇受压区混凝土的顶部高度；fc,R,fc,H,fy分别为再生混凝土的受压强度计算值，高强混凝土的受压强度设计值，受力钢筋的屈服强度设计值；α1,H和α1,R为高强混凝土和再生混凝土材料的系数，按照《混凝土结构设计规范》(GB50010-2010)6.2.6条的规定计算。
U型组合再生混凝土梁的斜截面受剪承载力Vcs可由以下公式计算：
其中，Vcs为U型再生混凝土组合梁的斜截面受剪承载力；ft,R,ft,H,fyv分别为再生混凝土的受拉强度计算值，高强混凝土的受拉强度设计值，箍筋的屈服强度设计值；αcv为斜截面混凝土受剪承载力系数，按照《混凝土结构设计规范》(GB50010-2010)6.3.4条的规定计算；Asv为配置在同一截面内箍筋各肢的全部截面面积；s为沿构件长度方向的箍筋间距。
A3中，U型再生混凝土组合梁的承载力应不小于对应的普通混凝土整浇梁的承载力，其中普通混凝土材料与再生混凝土材料具有相同的有效水灰比。A3中所述U型再生混凝土组合梁的正常使用能力分析，应满足《混凝土结构设计规范》(GB50010-2010)中的相关规定，对最大裂缝宽度、挠度等进行验算。
进一步的，A4中简单设计方法指满足《建筑混凝土结构耐火设计技术规程》(DBJ/T15-81-2022)中对于简支梁或连续梁梁宽和纵向受拉钢筋保护层厚度的最小值，应不考虑再生混凝土导热系数的降低的影响，仅作为安全储备。
进一步的，A4中复杂设计方法指进行火灾条件下的非线性全过程分析。采用大型通用有限元程序对混凝土构件和结构的时变内部温度场进行计算，考虑再生混凝土导热系数降低的影响，在此基础上开展构件和结构的高温力学分析，进而判断构件或结构的耐火极限是否满足工程设计要求。
进一步的，在时变内部温度场计算时，考虑再生混凝土导热系数的降低的影响。
进一步的，耐火极限的判定应通过构件是否失去承载能力、完整性和隔热性进 行判定。其中，失去承载能力的判定条件为挠度达到极限弯曲变形量，或挠度的增长速率达到极限弯曲变形速率，应符合下述公式：
极限弯曲变形量：mm
极限弯曲变形速率：mm/min
式中，D为跨中挠度，L为试件净跨，mm；d为试件界面上抗压点与抗拉点之间的距离，mm。
失去完整性的判定条件为试件在耐火试验期间能够持续保持耐火隔火性能。
失去隔热性的判定条件为试件背火面的平均温度温升超过初始平均温度140℃或任一点位置的温度温升超过初始温度180℃。
进一步的，复杂设计方法中再生混凝土导热系数采用实测值。
进一步的，A5中，简单设计方法指设计界面为粗糙界面，粗糙界面的粗糙程度不低于粗骨料裸露出一半情况下对应的粗糙程度，或未抹平的自然浇筑面，并且应布置构造界面钢筋，界面钢筋配筋率应不小于箍筋配筋率。
正常使用情况下的界面剪应力应满足如下公式：
τ≤[τ]，
其中，τ为试验或数值模拟得出的界面剪应力，[τ]为组合界面的所能承受的最大剪应力，MPa。
组合界面的所能承受的最大剪应力[τ]可由以下公式计算：
其中，τa为黏结和骨料咬合作用提供的剪应力；μ为界面摩擦系数；κ1，κ2为作用系数；fy,fcc分别为界面钢筋的受拉强度和混凝土的单轴圆柱体受压强度；ρ为界面钢筋配筋率；βc和ν分别为界面钢筋和混凝土的有效系数；σn为界面正应力。
界面钢筋配筋率ρ由以下公式计算：
式中，As界面钢筋面积；Ac为界面面积。
进一步的，A5中，复杂设计方法指采用火灾条件下的非线性全过程分析进行 组合界面高温安全性分析。
进一步的，组合界面高温安全性分析，应考虑界面承载力在高温作用下的强度和刚度损失，界面受拉承载力及受剪承载力在高温下的退化规律应通过试验进行测试。
进一步的，安全性分析满足如下公式：
RT＞ST，
其中，RT为温度T下的组合界面的抗力，ST为温度T下的组合界面的荷载效应。应在结构耐火极限时长内，均满足安全性要求。
进一步的，A5中组合界面安全性，还要计算正常使用情况下的相对滑移量和最大裂缝宽度，应符合如下公式：
相对滑移量：s≤[s]，
其中，s为试验或数值模拟得出的相对滑移量，[s]为规范规定的滑移量限值，mm。
最大裂缝宽度应符合如下公式：w≤[w]，
其中，w为试验或数值模拟得出的最大裂缝宽度，[w]为规范规定的最大裂缝宽度限值，mm。
与现有技术相比，本发明具有以下优点：
(1)本发明提供的一种U型再生混凝土组合梁，在常温下，可以有效提高再生混凝土梁的抗弯承载力和延性，解决再生混凝土的应用对结构承载力和延性的削弱。在高温下，受火表面的爆裂现象减少，梁内温度场显著降低，充分发挥再生混凝土的隔热性能，既保护内部受力钢筋，又避免高强混凝土爆裂导致结构截面尺寸的削弱。在满足结构受力需求的基础上，大量消纳建筑固废，既拓宽了再生混凝土的应用范围，推动建筑产业可持续化和绿色化发展，又可以降低工程造价，体现成本优势。
(2)本发明提供的一种U型再生混凝土组合梁的施工方法，组合再生混凝土组合梁的施工方式为现场浇筑或半预制半现浇的施工方式，半预制半现浇的施工方式可以有效减少现场浇筑作业量，加快工程进度，并且再生混凝土预制部分的施工 质量可以得到良好保证，减少了再生混凝土材料性能的变异性。
(3)本发明提供的一种U型再生混凝土组合梁的设计方法，提供了基于现有规范的简化设计方法和基于非线性高温全过程分析的高级计算方法，在体现结构设计先进性的同时，兼顾设计和施工难度，有利于实际工程推广。
附图说明
图1为U型再生混凝土组合梁的截面设计图。
图2为考虑组合梁与上方混凝土板相连的情况下的U型再生混凝土组合梁的截面设计图。
图3为具有良好受力性能和耐火性能的U型再生混凝土组合梁的结构设计流程图。
附图标记：11-预制混凝土外壳；12现浇受压区混凝土；21-受拉钢筋；22-U形箍筋；23-架立筋；24-补充箍筋；25-界面增强钢筋。
具体实施方式
下面结合附图和具体实施例对本发明进行详细说明。本技术方案中如未明确说明的部件型号、材料名称、连接结构、控制方法、算法等特征，均视为现有技术中公开的常见技术特征。
实施例1
如图3所示，本实施例提供U型再生混凝土组合梁的常温和耐火设计方法，采用简单设计方法，具体步骤如下：
A1：确定U型再生混凝土组合梁的荷载效应及耐火等级。通过结构要求确定U型再生混凝土组合梁的荷载效应及耐火等级，其中，荷载效应按照《建筑结构荷载规范》(GB50009-2012)、《建筑结构荷载规范》(GB50009-2012)中的相关规定计算，耐火等级按照《建筑设计防火规范》(GB50016-2014)和《建筑防火通用规范》(GB55037-2022)中的相关规定确定。
A2：进行U型再生混凝土组合梁的截面尺寸设计。U型再生混凝土组合梁的截面尺寸符合《混凝土结构设计规范》(GB50010-2010)中对于构件尺寸的要求，并且构成U型再生混凝土组合梁的尺寸参数能够满足工程设计需求。
A3：进行U型再生混凝土组合梁常温承载能力和正常使用能力分析，进行U型再生混凝土组合梁的配筋设计。U型再生混凝土组合梁的配筋设计应满足《混凝土结构设计规范》(GB50010-2010)中对于混凝土梁的正截面受弯承载力、斜截面受剪承载力的要求。其中，U型再生混凝土组合梁的正截面受弯承载力Mcu由以下步骤计算：
首先，通过α1,Hfc,Hbx0＝fyAs计算x0
若x0≤h3,x＝x0，
若x0＞h3,α1,Hfc,Hbh3+α1,Hfc,Hb2(x-h3)+α1,Rfc,R(b1+b3)(x1-h3)＝fyAs，
若h3＜x0≤h3+h2,x＝x1，
若x1＞h3+h2,
α1,Hfc,Hbh3+α1,Hfc,Hb2(h2+h3)+α1,Rfc,R(b1+b3)h2+α1,Rfc,Rb(x-h2-h3)＝fyAs。
其中，Mcu为U型组合混凝土梁的正截面受弯承载力；b,b1,b2,b3分别为组合梁的宽度，预制混凝土外壳11的左侧厚度、中部厚度和右侧厚度；h,h0,h1,h2,h3分别为组合梁的截面高度，组合梁的截面有效高度，预制混凝土外壳11的底部厚度、现浇受压区混凝土12核心高度和现浇受压区混凝土12的顶部高度；fc,R,fc,H,fy分别为再生混凝土的受压强度计算值，高强混凝土的受压强度设计值，受力钢筋的屈服强度设计值；α1,H和α1,R为高强混凝土和再生混凝土材料的系数，按照《混凝土结构设计规范》(GB50010-2010)6.2.6条的规定计算。
其中，U型再生混凝土组合梁的斜截面受剪承载力Vcs由以下公式计算：
其中，Vcs为U型再生混凝土组合梁的斜截面受剪承载力。ft,R,ft,H,fyv分别为再生混凝土的受拉强度计算值，高强混凝土的受拉强度设计值，箍筋的屈服强度设计值。αcv为斜截面混凝土受剪承载力系数，按照《混凝土结构设计规范》
(GB50010-2010)6.3.4条的规定计算。Asv为配置在同一截面内箍筋各肢的全部截面面积，s为沿构件长度方向的箍筋间距。
验证U型再生混凝土组合梁的承载力应不小于对应的普通混凝土整浇梁的承载力，其中普通混凝土材料与再生混凝土材料具有相同的有效水灰比。
A4：进行U型再生混凝土组合梁的耐火设计，采用简单设计方法。确认U型再生混凝土组合梁的截面尺寸满足《建筑混凝土结构耐火设计技术规程》(DBJ/T15-81-2022)中对于简支梁或连续梁梁宽和纵向受拉钢筋21保护层厚度的最小值，不考虑再生混凝土导热系数的降低的影响，仅作为安全储备。
A5：根据A3中的分析，进行组合界面的粗糙度和配筋设计，采用简单设计方法。设计界面为粗糙界面，粗糙界面的粗糙程度不低于粗骨料裸露出一半情况下对应的粗糙程度，或未抹平的自然浇筑面，并且应布置构造界面钢筋，界面钢筋配筋率应不小于箍筋配筋率。
正常使用情况下的界面剪应力应满足如下公式：
τ≤[τ]，
其中，τ为试验或数值模拟得出的界面剪应力，[τ]为组合界面的所能承受的最大剪应力，MPa。
组合界面的所能承受的最大剪应力[τ]可由以下公式计算：
其中，τa为黏结和骨料咬合作用提供的剪应力；μ为界面摩擦系数；κ1，κ2为作用系数；fy,fcc分别为界面钢筋的受拉强度和混凝土的单轴圆柱体受压强度；ρ为界面钢筋配筋率；βc和ν分别为界面钢筋和混凝土的有效系数；σn为界面正应力。
界面钢筋配筋率ρ由以下公式计算：
式中，As界面钢筋面积；Ac为界面面积。
组合界面安全性，还要计算正常使用情况下的相对滑移量和最大裂缝宽度，应符合如下公式：
相对滑移量：s≤[s]，
其中，s为试验或数值模拟得出的相对滑移量，[s]为规范规定的滑移量限值， mm。
最大裂缝宽度应符合如下公式：w≤[w]，
其中，w为试验或数值模拟得出的最大裂缝宽度，[w]为规范规定的最大裂缝宽度限值，mm。
A6：根据A2至A5中获得的分析结果，选择U型再生混凝土组合梁的优化施工方法。
实施例2
如图3所示，本实施例提供U型再生混凝土组合梁的常温和耐火设计方法，采用复杂设计方法，具体步骤如下：
A1：确定U型再生混凝土组合梁的荷载效应及耐火等级。通过结构要求确定U型再生混凝土组合梁的荷载效应及耐火等级，其中，荷载效应按照《建筑结构荷载规范》(GB50009-2012)、《建筑结构荷载规范》(GB50009-2012)中的相关规定计算，耐火等级按照《建筑设计防火规范》(GB50016-2014)和《建筑防火通用规范》(GB55037-2022)中的相关规定确定。
A2：进行U型再生混凝土组合梁的截面尺寸设计。U型再生混凝土组合梁的截面尺寸符合《混凝土结构设计规范》(GB50010-2010)中对于构件尺寸的要求，并且构成U型再生混凝土组合梁的尺寸参数能够满足工程设计需求。
A3：进行U型再生混凝土组合梁常温承载能力和正常使用能力分析，进行U型再生混凝土组合梁的配筋设计。U型再生混凝土组合梁的配筋设计应满足《混凝土结构设计规范》(GB50010-2010)中对于混凝土梁的正截面受弯承载力、斜截面受剪承载力的要求。其中，U型再生混凝土组合梁的正截面受弯承载力Mcu由以下步骤计算：
首先，通过α1,Hfc,Hbx0＝fyAs计算x0
若x0≤h3,x＝x0，
若x0＞h3,α1,Hfc,Hbh3+α1,Hfc,Hb2(x-h3)+α1,Rfc,R(b1+b3)(x1-h3)＝fyAs，
若h3＜x0≤h3+h2,x＝x1，
若x1＞h3+h2,
α1,Hfc,Hbh3+α1,Hfc,Hb2(h2+h3)+α1,Rfc,R(b1+b3)h2+α1,Rfc,Rb(x-h2-h3)＝fyAs。
其中，Mcu为U型组合混凝土梁的正截面受弯承载力；b,b1,b2,b3分别为组合梁的宽度，预制混凝土外壳11的左侧厚度、中部厚度和右侧厚度；h,h0,h1,h2,h3分别为组合梁的截面高度，组合梁的截面有效高度，预制混凝土外壳11的底部厚度、现浇受压区混凝土12核心高度和现浇受压区混凝土12的顶部高度；fc,R,fc,H,fy分别为再生混凝土的受压强度计算值，高强混凝土的受压强度设计值，受力钢筋的屈服强度设计值；α1,H和α1,R为高强混凝土和再生混凝土材料的系数，按照《混凝土结构设计规范》(GB50010-2010)6.2.6条的规定计算。
其中，U型再生混凝土组合梁的斜截面受剪承载力Vcs由以下公式计算：
其中，Vcs为U型再生混凝土组合梁的斜截面受剪承载力。ft,R,ft,H,fyv分别为再生混凝土的受拉强度计算值，高强混凝土的受拉强度设计值，箍筋的屈服强度设计值。αcv为斜截面混凝土受剪承载力系数，按照《混凝土结构设计规范》(GB 
50010-2010)6.3.4条的规定计算。Asv为配置在同一截面内箍筋各肢的全部截面面积，s为沿构件长度方向的箍筋间距。
验证U型再生混凝土组合梁的承载力应不小于对应的普通混凝土整浇梁的承载力，其中普通混凝土材料与再生混凝土材料具有相同的有效水灰比。
A4：进行U型再生混凝土组合梁的耐火设计，采用复杂设计方法。进行火灾条件下的非线性全过程分析。采用大型通用有限元程序对混凝土构件和结构的时变内部温度场进行计算，考虑再生混凝土导热系数降低的影响，在此基础上开展构件和结构的高温力学分析，进而判断构件或结构的耐火设计是否满足《建筑结构防火规范》(GB 50016-2014)中耐火极限的要求。在时变内部温度场计算时，考虑再生混凝土导热系数的降低的影响。
耐火极限的判定应通过构件是否失去承载能力、完整性和隔热性进行判定。其 中，失去承载能力的判定条件为挠度达到极限弯曲变形量，或挠度的增长速率达到极限弯曲变形速率，应符合下述公式：
极限弯曲变形量：mm
极限弯曲变形速率：mm/min
式中，D为跨中挠度，L为试件净跨，mm；d为试件界面上抗压点与抗拉点之间的距离，mm。
失去完整性的判定条件为试件在耐火试验期间能够持续保持耐火隔火性能。
失去隔热性的判定条件为试件背火面的平均温度温升超过初始平均温度140℃或任一点位置的温度温升超过初始温度180℃。
复杂设计方法中再生混凝土导热系数采用实测值。
A5：根据A3中的分析，进行组合界面的粗糙度和配筋设计，采用复杂设计方法。采用火灾条件下的非线性全过程分析进行组合界面高温安全性分析。
组合界面高温安全性分析，应考虑界面承载力在高温作用下的强度和刚度损失，界面受拉承载力及受剪承载力在高温下的退化规律应通过试验进行测试。
安全性分析满足如下公式：
RT＞ST，
其中，RT为温度T下的组合界面的抗力，ST为温度T下的组合界面的荷载效应。应在结构耐火极限时长内，均满足安全性要求。
A5中组合界面安全性，还要计算正常使用情况下的相对滑移量和最大裂缝宽度，应符合如下公式：
相对滑移量：s≤[s]，
其中，s为试验或数值模拟得出的相对滑移量，[s]为规范规定的滑移量限值，mm。
最大裂缝宽度应符合如下公式：w≤[w]，
其中，w为试验或数值模拟得出的最大裂缝宽度，[w]为规范规定的最大裂缝宽度限值，mm。
A6：根据A2至A5中获得的分析结果，选择U型再生混凝土组合梁的优化施工方法。
对比例1
将实施例2中设计的组合混凝土梁进行测试，用以比较U型再生混凝土组合梁与单一混凝土材料整浇梁的常温性能，制作U型再生混凝土组合梁、再生混凝土整浇梁、普通混凝土整浇梁。
(1)设计与制作
如图1所示，U型再生混凝土组合梁，包括预制混凝土外壳11、现浇受压区混凝土12；U型再生混凝土组合梁截面尺寸为200mm×300mm，具体尺寸为h1＝80mm,h2＝220mm,h3＝0mm,b1＝45mm,b2＝110mm,b3＝45mm。顶部架立筋23为受拉钢筋21为U形箍筋22为10@100，未设置界面钢筋。U型再生混凝土组合梁长2700mm，净跨2400mm，底部混凝土保护层厚度40mm。
选用原材料和设备如下所示：
PO42.5普通硅酸盐水泥、最大粒径为0.9mm的天然黄砂、聚羧酸高效减水剂、粒径范围5-16mm的天然粗骨料(同济大学土木工程教育部重点试验室统一提供)；粒径范围5-16mm的再生粗骨料(上海又宏环保科技有限公司)。
如表1所示，采用再生粗骨料取代率等于100％的再生混凝土浇筑预制混凝土外壳11，采用高强混凝土浇筑现浇受压区。首先设置U型再生混凝土组合梁内部的钢筋笼，钢筋笼中采用U形箍筋22；设置外部模板和U型梁内部相应尺寸的U形模板，U形模板通过木板在上方和侧面与外部模板固定，U形模板下方设置出浆孔；采用坍落度为120mm的再生混凝土浇筑预制混凝土外壳11，先加入部分再生混凝土，并用振捣棒伸入U形模板外侧进行充分振捣，至底部混凝土密实，再生混凝土浆体不再沉落，再生混凝土浆体表面呈现浮浆，出浆孔有水泥浆体溢出；待底部再生混凝土浇筑密实后，再浇筑U形侧面混凝土；振捣密实指再生混凝土浆体不再沉落，再生混凝土浆体表面呈现浮浆，出浆孔有水泥浆体溢出；终凝结束并养护一周后，拆除内部U形模板；对底部界面进行凿毛处理；采用高强混凝土浇筑现浇部分混凝土。
同时，采用与组合梁相同规格和尺寸的再生混凝土整浇梁、普通混凝土整浇梁，施工中使用的再生混凝土与普通混凝土和高强混凝土配合比如表1所示。
表1施工中使用的再生混凝土、普通混凝土和高强混凝土配合比
(2)结果与讨论
通过对U型再生混凝土组合梁、再生混凝土整浇梁、普通混凝土整浇梁进行常温下的受弯承载力试验，试件为简支梁，跨中加载点间距500mm，通过四立柱液压伺服试验机加载。分别测定屈服弯矩、极限弯矩及其对应挠度，具体数据见表2。
由表2可知，U型再生混凝土组合梁的屈服弯矩和极限弯矩均显著高于再生混凝土整浇梁，达到并且超过普通混凝土整浇梁的水平，屈服弯矩对应的挠度大致相同。因此，组合再生混凝土梁可以达到优化再生混凝土梁承载能力低的问题，在结构设计中，所提出的U型再生混凝土组合梁可以用来替代普通混凝土整浇梁。
表2组合梁与再生混凝土整浇梁、普通混凝土整浇梁的常温承载力指标比较

对比例2
比较U型再生混凝土组合梁和单一材料整浇梁的耐火性能和火灾后残余承载能力，制作U型再生混凝土组合梁、再生混凝土整浇梁、高强混凝土整浇梁。
(1)设计与制作
如图2所示，U型再生混凝土组合梁，包括预制混凝土外壳11、现浇受压区混凝土12；U型再生混凝土组合梁截面尺寸为200mm×300mm，具体尺寸为h1＝80mm,h2＝160mm,h3＝60mm,b1＝45mm,b2＝110mm,b3＝45mm。顶部架立筋23为受拉钢筋21为U形箍筋22为10@100，界面增强钢筋25为10@100，长度为100mm。U型再生混凝土组合梁长2700mm，净跨2400mm，底部混凝土保护层厚度40mm。
选用原材料和设备与实施例2相同。
如表3所示，采用再生粗骨料取代率等于100％的再生混凝土浇筑预制混凝土外壳11，采用高强混凝土浇筑现浇受压区。首先设置U型再生混凝土组合梁内部的钢筋笼，钢筋笼中采用U形箍筋22；设置外部模板和U型梁内部相应尺寸的U形模板，U形模板通过木板在上方和侧面与外部模板固定，U形模板下方设置出浆孔和钢筋孔洞；采用坍落度为120mm的再生混凝土浇筑预制混凝土外壳11，先加入部分再生混凝土，并用振捣棒伸入U形模板外侧进行充分振捣，至底部混凝土密实，再生混凝土浆体不再沉落，再生混凝土浆体表面呈现浮浆，出浆孔有水泥浆体溢出；待底部再生混凝土浇筑密实后，再浇筑U形侧面混凝土；振捣密实指再生混凝土浆体不再沉落，再生混凝土浆体表面呈现浮浆，出浆孔有水泥浆体溢出；通过钢筋孔洞布置界面钢筋；终凝结束并养护一周后，拆除内部U形模板；对底 部界面进行凿毛处理；采用高强混凝土浇筑现浇部分混凝土。
同时，采用与U型再生混凝土组合梁相同规格和尺寸的再生混凝土整浇梁、高强混凝土整浇梁，施工中使用的再生混凝土和高强混凝土配合比如表3所示。
表3施工中使用的再生混凝土、普通混凝土和高强混凝土配合比
(2)结果与讨论
通过对U型再生混凝土组合梁、再生混凝土整浇梁、高强混凝土整浇梁进行恒载升温试验，炉温曲线为ISO 834标准升温曲线，试件为三面受火，荷载比为0.55，通过50t千斤顶和分配梁对跨中施加荷载，试件受火时间为103mins。试件为简支梁，跨中加载点间距500mm。试验过程中测定梁的跨中挠度，计算挠度增长速率，并在试验结束后观察表面爆裂情况，具体数据见表4。
火灾试验结束后，待U型再生混凝土组合梁、再生混凝土整浇梁、高强混凝土整浇梁在炉内自然降温至室温后，测试火灾后的残余承载能力，试件为简支梁，跨中加载点间距500mm，通过四立柱液压伺服试验机加载。分别测定屈服弯矩、极限弯矩及其对应挠度，具体数据见表5。
由表4可知，U型再生混凝土组合梁在受火30、60、90、99mins时的挠度及对应的挠度增长速率均低于再生混凝土整浇梁，并显著低于高强混凝土整浇梁。高强混凝土整浇梁在受火99分钟时达到耐火极限，此时U型再生混凝土组合梁未达到耐火极限。高强混凝土整浇梁出现了局部爆裂现象，U型再生混凝土组合梁未出现。因此，组合再生混凝土梁具有显著的耐火性能优势，耐火性能超过了再生混凝土整浇梁和高强混凝土整浇梁，可以起到优化组合混凝土梁耐火性能的作用，并且解决高强混凝土在火灾下易出现爆裂导致提前破坏的问题。
由表5可知，U型再生混凝土组合梁的火后残余承载力性能显著优于再生混凝土整浇梁，并且屈服弯矩对应挠度显著低于再生混凝土整浇梁，可见再生混凝土整浇梁在受火后出现显著的强度及刚度退化，而U型再生混凝土组合梁可以优化这一现象，达到甚至超过高强混凝土整浇梁1的火后残余承载能力。因此，U型再生混凝土组合梁具有更为优异的火后残余承载能力。
表4U型再生混凝土组合梁与再生混凝土整浇梁、高强混凝土整浇梁的耐火性能指标
表5U型再生混凝土组合梁与再生混凝土整浇梁、高强混凝土整浇梁的火后残 余承载力性能指标
上述的对实施例的描述是为便于该技术领域的普通技术人员能理解和使用发明。熟悉本领域技术的人员显然可以容易地对这些实施例做出各种修改，并把在此说明的一般原理应用到其他实施例中而不必经过创造性的劳动。因此，本发明不限于上述实施例，本领域技术人员根据本发明的揭示，不脱离本发明范畴所做出的改进和修改都应该在本发明的保护范围之内。

Claims (10)

  1. 一种U型再生混凝土组合梁，其特征在于，包括预制混凝土外壳(11)、现浇受压区混凝土(12)；
    所述预制混凝土外壳(11)为再生混凝土外壳，所述预制混凝土外壳(11)的截面为U型，所述预制混凝土外壳(11)的强度等级不高于C40；
    所述现浇受压区混凝土(12)为设于所述预制混凝土外壳(11)的U型槽体中的高强混凝土，所述现浇受压区混凝土(12)为C50-C80强度等级的混凝土；
    U型再生混凝土组合梁内设有钢筋笼作为骨架，所述钢筋笼置于预制混凝土外壳(11)内部，包括：水平设置的受拉钢筋(21)、架立筋(23)和竖直设置的箍筋，所述箍筋用于约束所述受拉钢筋(21)，所述箍筋为U形结构，顶部设有折弯部，所述箍筋底部的上表面和受拉钢筋(21)抵接，所述箍筋的折弯部的下表面和架立筋(23)抵接。
  2. 根据权利要求1所述的一种U型再生混凝土组合梁，其特征在于，所述预制混凝土外壳(11)底部设有界面增强钢筋(25)；
    所述U型再生混凝土组合梁的截面尺寸，宽度b不小于200mm，高度h不小于300mm；所述预制混凝土外壳(11)底部厚度h1不小于80mm，所述预制混凝土外壳(11)侧面宽度b1和b3均不小于45mm；所述现浇受压区混凝土(12)的宽度b2不小于100mm。
  3. 根据权利要求1所述的一种U型再生混凝土组合梁，其特征在于，所述现浇受压区混凝土(12)与混凝土板同时浇筑，所述预制混凝土外壳(11)两侧高度设置为h1+h2，上方h3高度与所述现浇受压区混凝土(12)同时浇筑，h3高度即为混凝土板厚度，且采用与所述现浇受压区混凝土(12)相同的混凝土材料。
  4. 一种U型再生混凝土组合梁的施工方法，用于实现权利要求1至3所述的的U型再生混凝土组合梁的施工，其特征在于，包括以下步骤：
    S1：设置U型再生混凝土组合梁内部的钢筋笼，钢筋笼采用U形箍筋；
    S2：设置外部模板和U型再生混凝土组合梁内部相应尺寸的U形模板，U形模板通过木板在上方和侧面与外部模板固定；
    S3：采用再生混凝土浇筑形成预制混凝土外壳(11)，并用振捣棒充分振捣密 实；
    S4：在S3浇筑完毕后，在终凝结束并养护至少一周后，拆除内部U形模板；
    S5：对预制混凝土外壳(11)底部界面进行粗糙化处理；
    S6：采用高强混凝土浇筑形成现浇受压区混凝土(12)。
  5. 根据权利要求4所述的一种U型再生混凝土组合梁的施工方法，其特征在于，在S2中，在所述U形模板底部设置出浆孔。
  6. 根据权利要求1所述的一种U型再生混凝土组合梁的施工方法，其特征在于，在S3中，所使用的再生混凝土材料坍落度不小于120mm。
  7. 一种U型再生混凝土组合梁的常温和耐火设计方法，用于实现权利要求1至3所述的U型再生混凝土组合梁的常温和耐火设计，特征在于，包括以下步骤：
    A1：确定U型再生混凝土组合梁的荷载效应及耐火等级；
    A2：进行U型再生混凝土组合梁的截面尺寸设计；
    A3：进行U型再生混凝土组合梁常温承载能力和正常使用能力分析，进行U型再生混凝土组合梁的配筋设计；
    A4：进行U型再生混凝土组合梁的耐火设计，采用简单设计方法或复杂设计方法；
    A5：根据A3中的分析，进行组合界面的粗糙度和配筋设计，采用简单设计方法或复杂设计方法；
    A6：根据A2至A5中获得的分析结果，选择U型再生混凝土组合梁的优化施工方法。
  8. 根据权利要求7所述的一种U型再生混凝土组合梁的常温和耐火设计方法，其特征在于，A1中荷载效应应符合《建筑结构荷载规范》(GB 50009-2012)、《建筑结构荷载规范》(GB 50009-2012)中的相关规定，耐火等级应符合《建筑设计防火规范》(GB 50016-2014)和《建筑防火通用规范》(GB 55037-2022)中的相关规定；A2中U型再生混凝土组合梁的截面尺寸设计，应符合《混凝土结构设计规范》(GB50010-2010)中对于构件尺寸的要求，构成U型再生混凝土组合梁的尺寸参数能够满足工程设计需求；A3中，U型再生混凝土组合梁的配筋设计应满足《混凝土结构设计规范》(GB50010-2010)中对于混凝土梁的正截面受弯承载力、斜截面受剪承载力的要求。
  9. 根据权利要求7所述的一种U型再生混凝土组合梁的常温和耐火设计方法， 其特征在于，A4中简单设计方法指：满足《建筑混凝土结构耐火设计技术规程》(DBJ/T15-81-2022)中对于简支梁或连续梁梁宽和纵向受拉钢筋(21)保护层厚度的最小值，应不考虑再生混凝土导热系数的降低的影响，仅作为安全储备；
    A4中复杂设计方法指进行火灾条件下的非线性全过程分析：采用通用有限元程序对混凝土构件和结构的时变内部温度场进行计算，考虑再生混凝土导热系数降低的影响，在此基础上开展构件和结构的高温力学分析，进而判断构件或结构的耐火极限是否满足工程设计要求。
  10. 根据权利要求7所述的一种U型再生混凝土组合梁的常温和耐火设计方法，其特征在于，A5中，简单设计方法指：设计界面为粗糙界面，粗糙界面的粗糙程度不低于粗骨料裸露出一半情况下对应的粗糙程度，或未抹平的自然浇筑面，并且应布置构造界面钢筋，界面钢筋配筋率应不小于箍筋配筋率；
    A5中，复杂设计方法指：采用火灾条件下的非线性全过程分析进行组合界面高温安全性分析，应考虑界面承载力在高温作用下的强度和刚度损失，界面受拉承载力及受剪承载力在高温下的退化规律应通过试验进行测试。
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