Normal temperature and fire resistant design method of u-shaped recycled concrete composite beam

Abstract

The present application relates to a kind of U-shaped recycled concrete composite beam normal temperature and fire resistance design method, comprising the following steps: A1: determining the load effect and fire resistance grade of U-shaped recycled concrete composite beam;A2: section size design;A3: carry out U-shaped recycled concrete composite beam normal temperature bearing capacity and normal use capacity analysis, reinforcement design;A4: carry out the fire resistance design of U-shaped recycled concrete composite beam, using simple design method or complex design method;A5: according to the analysis in A3, carry out the roughness of combination interface and reinforcement design, using simple design method or complex design method;A6: select the optimization construction method of U-shaped recycled concrete composite beam.Compared with prior art, the fire resistance characteristics of recycled concrete and the excellent mechanical properties of high-strength concrete are fully utilized, the flexural bearing capacity and ductility of recycled concrete beam can be effectively improved at normal temperature, the spalling phenomenon of the surface under high temperature is reduced, and the temperature field in the beam is significantly reduced.

Description

Technical Field

[0001] This invention relates to the field of building structures and construction, and in particular to a method for designing U-shaped recycled concrete composite beams for ambient temperature and fire resistance. Background Technology

[0002] To achieve sustainability in civil engineering structures, disaster prevention and mitigation are crucial aspects of structural design. With the development of high-rise and large-span structures, the application of high-strength and high-performance concrete is becoming increasingly widespread. However, the dense microstructure of high-strength concrete results in high internal stress under high temperatures, making it prone to concrete bursting. This leads to exposed reinforcing steel, rapid temperature rise in the steel, and a significant decrease in the structure's fire resistance, thus limiting its application in building structures. Methods such as incorporating fire-retardant coatings increase structural costs, complicate construction procedures, and introduce long-term maintenance challenges for building structures.

[0003] Conversely, recycled concrete made from recycled aggregates derived from crushed waste concrete has proven to have excellent fire resistance. Due to the abundant old mortar on the surface of recycled aggregates, recycled concrete possesses a loose and porous internal structure. The thermal conductivity of recycled concrete is lower than that of ordinary concrete under the same water-cement ratio, and it is less prone to spalling at high temperatures. However, the use of recycled aggregates reduces the load-bearing capacity and stiffness of recycled concrete components at room temperature to some extent, which limits the application of recycled concrete structures and restricts the full realization of the excellent thermal properties of recycled concrete.

[0004] Patent CN208329371U discloses a T-shaped steel-reinforced recycled concrete composite beam, including a concrete composite beam box. A limiting and fixing plate is placed at the bottom of the concrete composite beam box, and the top of the limiting and fixing plate has evenly distributed snap-fit ​​grooves. Stirrups, evenly distributed within the cavity of the concrete composite beam box, are placed on top of the limiting and fixing plate. The top and bottom ends of the stirrups are reinforced with steel bars. The combined use of the steel frame and flange plates enhances the load-bearing capacity of the device, facilitates better utilization of the ductility of the T-shaped steel, saves on formwork, improves construction efficiency, and allows for the filling of waste concrete into the composite beam, protecting the environment and enabling waste recycling. However, this design significantly increases the amount of steel used in the concrete beam and has a more complex structure, increasing construction costs. Furthermore, this design places the limiting and fixing plate and other steel materials on the surface of the concrete beam, resulting in rapid performance degradation after fire exposure. Since the recycled concrete is applied to the middle of the concrete beam, its excellent fire resistance cannot be fully utilized. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology by providing a method for designing U-shaped recycled concrete composite beams at both room temperature and fire resistance. This method fully utilizes the fire resistance characteristics of recycled concrete and the excellent mechanical properties of high-strength concrete. At room temperature, it can effectively improve the flexural strength and ductility of the recycled concrete beam. At high temperatures, it reduces the cracking phenomenon on the fire-exposed surface and significantly lowers the temperature field inside the beam, thus fully utilizing the thermal insulation properties of recycled concrete. This method protects the internal reinforcing steel bars and avoids the weakening of the structural cross-sectional dimensions caused by the cracking of high-strength concrete. The construction method can effectively reduce the amount of on-site pouring work and accelerate the project progress.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] This invention provides a U-shaped recycled concrete composite beam, comprising a precast concrete shell and a cast-in-place compression zone concrete;

[0008] The precast concrete shell is made of recycled concrete with a U-shaped cross-section, exhibiting good fire resistance and resistance to cracking. The recycled concrete shell is prepared 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 is improved by controlling the grade of recycled coarse aggregate, optimizing its gradation, and ensuring good curing conditions.

[0009] The cast-in-place compression zone concrete is high-strength concrete, referring to concrete with strength grades C50-C80. This concrete can be poured on-site. The cast-in-place compression zone concrete is high-strength concrete housed in a U-shaped trough within a precast concrete shell.

[0010] Preferably, the maximum particle size of the recycled coarse aggregate used in the precast concrete shell is no greater than 1 / 2 of the minimum value of the bottom thickness and side width of the precast concrete shell.

[0011] Preferably, according to the actual load-bearing capacity requirements, ultra-high performance concrete (UHVPC) can be used in the cast-in-place compression zone. UHVPC refers to fiber-reinforced concrete with a strength grade of C80 or higher.

[0012] The cross-sectional dimensions of the U-shaped recycled concrete composite beam are as follows: width b not less than 200 mm, height h not less than 300 mm. The bottom thickness h1 of the precast concrete shell is not less than 80 mm, and the tensile reinforcement is located inside the precast concrete shell. The side widths b1 and b3 of the precast concrete shell are both not less than 45 mm. The width b2 of the cast-in-place compression zone concrete is not less than 100 mm.

[0013] Preferably, b1 and b3 are equal.

[0014] Preferably, provided that the load-bearing capacity requirements are met, 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.

[0015] The bottom of the precast concrete shell can be roughened by methods such as roughening after casting with formwork, mechanical chiseling, or pre-setting grooves in the formwork.

[0016] Preferably, the bottom of the precast concrete shell is provided with interface reinforcing steel bars. Preferably, the length of the interface reinforcing steel bars is not less than 100mm and the spacing along the longitudinal direction of the beam is not greater than 150mm.

[0017] Furthermore, the sides of the precast concrete shell should also be roughened, and the method of setting grooves in the template should be preferred.

[0018] The U-shaped recycled concrete composite beam has a steel cage as its framework, which is placed inside a precast concrete shell. The steel cage includes tensile bars, stirrups, and reinforcing bars. The stirrups are U-shaped according to the shape of the component, with a bend at the top. The upper surface of the bottom of the stirrup abuts against the tensile bars, and the lower surface of the bend abuts against the stirrups. Furthermore, the steel cage also includes additional reinforcing bars and web reinforcement.

[0019] The construction method of the reinforcing cage complies with the relevant requirements of "11G101-1 Rules for Drawing Overall Plan Representation of Concrete Structure Construction Drawings and Construction Details (Cast-in-place Concrete Frames, Shear Walls, Beams, Slabs)" and "GB50666-2011 Code for Construction of Concrete Structures".

[0020] When the U-shaped recycled concrete composite beam is connected to the concrete slab, an optimized U-shaped combination is adopted, that is, the height of the two sides of the precast concrete shell is set as h1 + h2, and the height of the upper 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.

[0021] Furthermore, the precast portion of the precast concrete shell is not lower than the bottom of the connected concrete slab.

[0022] Furthermore, the height h3 above is the same as the thickness of the concrete slab.

[0023] Furthermore, an additional stirrup is provided above the U-shaped stirrup to connect the U-shaped stirrups into a ring stirrup.

[0024] Furthermore, the U-shaped stirrups and the additional stirrups above them can be connected by tying.

[0025] Furthermore, considering the torsional load on the concrete beam, the U-shaped stirrups and the supplementary stirrups above them are connected by welding.

[0026] This invention also provides a construction method for a U-shaped recycled concrete composite beam, which can be either on-site segmented and phased casting or a semi-precast and semi-cast-in-place method. When using the semi-precast and semi-cast-in-place method, the casting of the precast concrete shell can be completed in the factory, and only the on-site casting of the concrete portion is performed. Specifically, it includes the following steps:

[0027] S1: Set up a steel cage inside the U-shaped recycled concrete composite beam, and use U-shaped stirrups for the steel cage;

[0028] S2: Set up external formwork and U-shaped formwork of corresponding size inside the U-shaped recycled concrete composite beam. The U-shaped formwork is fixed to the external formwork by wooden boards on the top and sides.

[0029] S3: Use recycled concrete to pour a precast concrete shell and then fully compact it with a vibrator.

[0030] S4: After S3 is poured, and after final setting and curing for at least one week, remove the internal U-shaped formwork;

[0031] S5: Roughen the bottom interface of the precast concrete shell;

[0032] S6: High-strength concrete is used to form the cast-in-place compression zone concrete.

[0033] Furthermore, in S2, the allowable deviation of the U-shaped template is ±5 mm.

[0034] Furthermore, in S2, the net distance between the U-shaped template and the U-shaped stirrup is not less than 10 mm.

[0035] Furthermore, in S5, the bottom interface of the precast concrete shell is roughened.

[0036] Furthermore, S5 is omitted, and in S2, a special template with pre-arranged grooves is used at the side and bottom interfaces of the precast concrete shell to obtain a rough interface.

[0037] Furthermore, the average roughness of the grooves in the specially made template is not less than 3 mm, and the spacing between the grooves is not less than 10 mm.

[0038] Furthermore, custom templates can be made using 3D-printed plastic materials.

[0039] Furthermore, in S2, slurry outlet holes are provided at the bottom of the U-shaped template.

[0040] Furthermore, in S3, the slump of the recycled concrete material used is not less than 120 mm.

[0041] Furthermore, in S3, the diameter of the vibrator should be smaller than the width of the side of the U-shaped concrete shell.

[0042] Furthermore, in S3, the vibrating rod should be inserted into the outside of the U-shaped template for vibration.

[0043] Furthermore, in S3, the vibration compaction refers to the recycled concrete slurry no longer settling, the surface of the recycled concrete slurry showing a floating slurry, and cement slurry overflowing from the slurry outlet.

[0044] Furthermore, in S3, some recycled concrete can be added first, and after the bottom recycled concrete is poured and compacted, the U-shaped side concrete can be poured.

[0045] Furthermore, reinforcing bar holes are set at the bottom of the U-shaped formwork to accommodate interface reinforcement bars.

[0046] Furthermore, when the U-shaped recycled concrete composite beam is connected to the concrete slab above, an optimized U-shaped composite form is adopted. In S3, it is poured to a height of h2, and the naturally poured rough surface is retained. Before proceeding to S6, supplementary reinforcement is arranged above the U-shaped stirrups and tied to the reinforcement cage.

[0047] This invention also provides a method for the room temperature and fire resistance design of U-shaped recycled concrete composite beams, the specific steps of which are as follows:

[0048] A1: Determine the load effects and fire resistance rating of the U-shaped recycled concrete composite beam;

[0049] A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam;

[0050] A3: Conduct an analysis of the ambient temperature bearing capacity and normal serviceability of the U-shaped recycled concrete composite beam, and design the reinforcement of the U-shaped recycled concrete composite beam.

[0051] A4: Conduct fire-resistant design of U-shaped recycled concrete composite beams, using either a simple or complex design method;

[0052] A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface, using either a simple or complex design method.

[0053] A6: Based on the analysis results obtained from A2 to A5, select the optimized construction method for the U-shaped recycled concrete composite beam.

[0054] The load effects in A1 should comply with the relevant provisions of the "Load Code for Design of Building Structures" (GB50009-2012), and 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 Design of Buildings" (GB 55037-2022). The cross-sectional dimensions of the U-shaped recycled concrete composite beams in A2 should comply with the requirements for component dimensions in the "Code for Design of Concrete Structures" (GB50010-2010). The dimensional parameters constituting the U-shaped recycled concrete composite beams should meet the engineering design requirements.

[0055] In A3, 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 flexural bearing capacity of the normal section and the shear bearing capacity of the inclined section of concrete beams.

[0056] The flexural bearing capacity M of the U-shaped recycled concrete composite beam in its normal section cu Calculated using the following steps:

[0057] First, through Calculate x0

[0058] like , ,

[0059] like , ,

[0060] like , ,

[0061] like ,

[0062] .

[0063] .

[0064] Among them, M cu denoted as σb, where σb is the flexural capacity of the U-shaped recycled concrete composite beam; b, b1, b2, b3 represent the width of the composite beam, the left side thickness, the middle thickness, and the right side thickness of the precast concrete shell, respectively; h, h0, h1, h2, h3 represent 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 compression zone concrete, and the top height of the cast-in-place compression zone concrete, respectively; f0 c,R , f c,H , f yThese represent the calculated compressive strength of recycled concrete, the design compressive strength of high-strength concrete, and the design yield strength of reinforcing steel, respectively; α 1,H and α 1,R The coefficient for 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] The shear capacity V of the inclined section of the U-shaped composite recycled concrete beam cs It can be calculated using the following formula:

[0066] .

[0067] Among them, V cs The shear capacity of the inclined section of the U-shaped recycled concrete composite beam; f t,R , f t,H, f yv These represent the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups, respectively; α cv The shear capacity coefficient of the inclined section concrete is calculated according to Article 6.3.4 of the "Code for Design of Concrete Structures" (GB50010-2010); A sv s represents the total cross-sectional area of ​​each leg of the stirrups within the same cross-section; s represents the stirrup spacing along the length of the member.

[0068] In section A3, the bearing capacity of the U-shaped recycled concrete composite beam should not be less than that of the corresponding ordinary concrete monolithic beam, wherein the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio. The normal serviceability analysis of the U-shaped recycled concrete composite beam described in section A3 should meet the relevant provisions in the "Code for Design of Concrete Structures" (GB50010-2010), and the maximum crack width, deflection, etc. should be verified.

[0069] Furthermore, the simplified design method in A4 refers to meeting the minimum values ​​of beam width and longitudinal tensile reinforcement cover thickness for simply supported beams or continuous beams as specified in the "Technical Specification for Fire-Resistant Design of Concrete Structures" (DBJ / T15-81-2022). The impact of the reduced thermal conductivity of recycled concrete should not be considered, and it should only be used as a safety reserve.

[0070] Furthermore, the complex design method in A4 refers to the nonlinear full-process analysis under fire conditions. A large-scale general-purpose finite element program is used to calculate the time-varying internal temperature field of concrete components and structures, considering the impact of reduced thermal conductivity of recycled concrete. Based on this, high-temperature mechanical analysis of the components and structures is carried out to determine whether the fire resistance limit of the components or structures meets the engineering design requirements.

[0071] Furthermore, the effect of the reduced thermal conductivity of recycled concrete is considered when calculating the time-varying internal temperature field.

[0072] Furthermore, the determination of fire resistance limit should be based on whether the component has lost its load-bearing capacity, integrity, and thermal insulation properties. The criterion for losing load-bearing capacity is that the deflection reaches the ultimate bending deformation, or the rate of deflection increase reaches the ultimate bending deformation rate, and should conform to the following formula:

[0073] Ultimate bending deformation: , mm

[0074] Ultimate bending deformation rate: , mm / min

[0075] In the formula, D is the mid-span deflection, L is the net span of the specimen (mm), and d is the distance between the compressive and tensile points on the specimen interface (mm).

[0076] The criterion for loss of integrity is that the specimen can maintain its fire resistance and fire-proof performance during the fire resistance test.

[0077] The criteria for determining loss of thermal insulation are that the average temperature rise on the unexposed side 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.

[0078] Furthermore, the thermal conductivity of recycled concrete in complex design methods is based on measured values.

[0079] Furthermore, in A5, the simple design method refers to a design interface that is rough, with a roughness not less than that corresponding to the condition where half of the coarse aggregate is exposed, or the roughness of an unsmoothed natural pouring surface, and structural interface reinforcement should be arranged, with the interface reinforcement ratio not less than the stirrup reinforcement ratio.

[0080] The interfacial shear stress under normal use conditions should satisfy the following formula:

[0081] τ≤[τ],

[0082] Where τ is the interfacial shear stress obtained from experiments or numerical simulations, and [τ] is the maximum shear stress that the combined interface can withstand, in MPa.

[0083] The maximum shear stress [τ] that the composite interface can withstand can be calculated by the following formula:

[0084] ,

[0085] Where, τ a Shear stress provided for bonding and aggregate interlocking; μ is the interfacial friction coefficient; κ1 and κ2 are the interaction coefficients; f y , f ccThese represent the tensile strength of the interfacial reinforcement and the uniaxial cylindrical compressive strength of the concrete, respectively; ρ is the reinforcement ratio of the interfacial reinforcement; β c σ and ν are the effective coefficients of the interface reinforcement and concrete, respectively; n This represents the interfacial normal stress.

[0086] The reinforcement ratio ρ at the interface is calculated using the following formula:

[0087] ,

[0088] In the formula, A s Area of ​​interfacial reinforcement; A c Let be the interface area.

[0089] Furthermore, in A5, the complex design method refers to the use of nonlinear full-process analysis under fire conditions to conduct high-temperature safety analysis of the combined interface.

[0090] 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 by experiments.

[0091] Furthermore, the security analysis satisfies the following formula:

[0092] R T >S T ,

[0093] Among them, R T S represents the resistance of the composite interface at temperature T. T This represents the load effect at the combined interface at temperature T. Safety requirements must be met throughout the structure's fire resistance limit duration.

[0094] Furthermore, for the safety of the composite interface in A5, the relative slip and maximum crack width under normal use conditions must also be calculated, and should conform to the following formula:

[0095] Relative slip: s≤[s],

[0096] Where s is the relative slip obtained from experiments or numerical simulations, and [s] is the slip limit specified in the standard, in mm.

[0097] The maximum crack width should conform to the following formula: w≤[w],

[0098] Where w is the maximum crack width obtained from experiments or numerical simulations, and [w] is the maximum crack width limit specified in the standard, in mm.

[0099] Compared with the prior art, the present invention has the following advantages:

[0100] (1) The U-shaped recycled concrete composite beam provided by this invention can effectively improve the flexural bearing capacity and ductility of recycled concrete beams at room temperature, solving the problem of the weakening of structural bearing capacity and ductility caused by the application of recycled concrete. At high temperatures, the cracking phenomenon on the fire-exposed surface is reduced, and the temperature field inside the beam is significantly reduced, giving full play to the heat insulation performance of recycled concrete, protecting the internal reinforcing steel bars, and avoiding 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 disposed of, 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.

[0101] (2) The present invention provides a construction method for a U-shaped recycled concrete composite beam. The construction method of the composite recycled concrete beam is on-site casting or semi-precast and semi-cast construction. The semi-precast and semi-cast construction method can effectively reduce the amount of on-site casting work, speed up the project progress, and ensure the construction quality of the recycled concrete precast part, thereby reducing the variability of recycled concrete material properties.

[0102] (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 structural design, it also takes into account the design and construction difficulties, which is conducive to the promotion of practical engineering. Attached Figure Description

[0103] Figure 1 This is a cross-sectional design drawing of a U-shaped recycled concrete composite beam.

[0104] Figure 2 This is a cross-sectional design drawing of a U-shaped recycled concrete composite beam, considering the connection between the composite beam and the upper concrete slab.

[0105] Figure 3 This is a flowchart illustrating the structural design of a U-shaped recycled concrete composite beam with good load-bearing and fire-resistant properties.

[0106] Figure reference numerals: 11-Precast concrete shell; 12-Cast-in-place compression zone concrete; 21-Tension reinforcement; 22-U-shaped stirrup; 23-Stirrup reinforcement; 24-Supplementary stirrup; 25-Interface reinforcement. Detailed Implementation

[0107] The present invention will now be described in detail 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.

[0108] Example 1

[0109] like Figure 3 As shown, this embodiment provides a method for the design of U-shaped recycled concrete composite beams at room temperature and with fire resistance. A simple design method is adopted, and the specific steps are as follows:

[0110] A1: Determine the load effects and fire resistance rating of the U-shaped recycled concrete composite beam. The load effects and fire resistance rating of the U-shaped recycled concrete composite beam are determined based on structural requirements. The load effects are calculated according to the relevant provisions of the "Code for Design of Building Structures" (GB50009-2012), and the fire resistance rating is determined according to the relevant provisions of the "Code for Fire Protection Design of Buildings" (GB50016-2014) and the "General Code for Fire Protection Design of Buildings" (GB55037-2022).

[0111] 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 constituting the U-shaped recycled concrete composite beam can meet the engineering design requirements.

[0112] A3: Conduct a normal temperature bearing capacity and serviceability analysis 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 "Code for Design of Concrete Structures" (GB50010-2010) for the flexural bearing capacity and shear bearing capacity of the inclined section of concrete beams. Specifically, the flexural bearing capacity M of the U-shaped recycled concrete composite beam... cu Calculated using the following steps:

[0113] First, through Calculate x0

[0114] like , ,

[0115] like , ,

[0116] like , ,

[0117] like ,

[0118] .

[0119] .

[0120] Among them, M cudenoted as , where is the flexural capacity of the U-shaped composite concrete beam; b, b1, b2, b3 represent the width of the composite beam, and the thicknesses of the left, middle, and right sides of the precast concrete shell 11, respectively; h, h0, h1, h2, h3 represent 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; f c,R , f c,H , f y These represent the calculated compressive strength of recycled concrete, the design compressive strength of high-strength concrete, and the design yield strength of reinforcing steel, respectively; α 1,H and α 1,R The coefficient for 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).

[0121] Among them, the shear bearing capacity V of the inclined section of the U-shaped recycled concrete composite beam cs Calculated using the following formula:

[0122]

[0123] Among them, V cs The shear capacity of the inclined section of the U-shaped recycled concrete composite beam. t,R , f t,H, f yv These represent the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups, respectively. α cv The shear capacity coefficient of the inclined section concrete is calculated according to Article 6.3.4 of the "Code for Design of Concrete Structures" (GB50010-2010). A sv The total cross-sectional area of ​​each leg of the stirrups within the same cross-section is given by s, where s is the stirrup spacing along the length of the member.

[0124] The load-bearing capacity of the U-shaped recycled concrete composite beam should be no less than that of the corresponding ordinary concrete monolithic beam, wherein the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio.

[0125] A4: Conduct fire resistance design for U-shaped recycled concrete composite beams using a simplified design method. Confirm that the cross-sectional dimensions of the U-shaped recycled concrete composite beams meet the minimum values ​​for beam width and longitudinal tensile reinforcement cover thickness for simply supported beams or continuous beams as specified in the "Technical Specification for Fire Resistance Design of Concrete Structures" (DBJ / T15-81-2022). The impact of reduced thermal conductivity of recycled concrete is not considered, and is only used as a safety margin.

[0126] A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface using a simple design method. The interface should be a rough interface, with a roughness not lower than that of half of the coarse aggregate exposed, or the roughness of an unsmoothed natural cast surface. Structural interface reinforcement should be provided, and the reinforcement ratio of the interface reinforcement should not be less than the stirrup reinforcement ratio.

[0127] The interfacial shear stress under normal use conditions should satisfy the following formula:

[0128] τ≤[τ],

[0129] Where τ is the interfacial shear stress obtained from experiments or numerical simulations, and [τ] is the maximum shear stress that the combined interface can withstand, in MPa.

[0130] The maximum shear stress [τ] that the composite interface can withstand can be calculated by the following formula:

[0131] ,

[0132] Where, τ a Shear stress provided for bonding and aggregate interlocking; μ is the interfacial friction coefficient; κ1 and κ2 are the interaction coefficients; f y , f cc These represent the tensile strength of the interfacial reinforcement and the uniaxial cylindrical compressive strength of the concrete, respectively; ρ is the reinforcement ratio of the interfacial reinforcement; β c σ and ν are the effective coefficients of the interface reinforcement and concrete, respectively; n This represents the interfacial normal stress.

[0133] The reinforcement ratio ρ at the interface is calculated using the following formula:

[0134] ,

[0135] In the formula, A s Area of ​​interfacial reinforcement; A c Let be the interface area.

[0136] For the safety of the composite interface, the relative slippage and maximum crack width under normal use conditions must also be calculated, and should conform to the following formula:

[0137] Relative slip: s≤[s],

[0138] Where s is the relative slip obtained from experiments or numerical simulations, and [s] is the slip limit specified in the standard, in mm.

[0139] The maximum crack width should conform to the following formula: w≤[w],

[0140] Where w is the maximum crack width obtained from experiments or numerical simulations, and [w] is the maximum crack width limit specified in the standard, in mm.

[0141] A6: Based on the analysis results obtained from A2 to A5, select the optimized construction method for the U-shaped recycled concrete composite beam.

[0142] Example 2

[0143] like Figure 3 As shown, this embodiment provides a method for the ambient temperature and fire resistance design of U-shaped recycled concrete composite beams, employing a complex design approach. The specific steps are as follows:

[0144] A1: Determine the load effects and fire resistance rating of the U-shaped recycled concrete composite beam. The load effects and fire resistance rating of the U-shaped recycled concrete composite beam are determined based on structural requirements. The load effects are calculated according to the relevant provisions of the "Code for Design of Building Structures" (GB50009-2012), and the fire resistance rating is determined according to the relevant provisions of the "Code for Fire Protection Design of Buildings" (GB50016-2014) and the "General Code for Fire Protection Design of Buildings" (GB55037-2022).

[0145] 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 constituting the U-shaped recycled concrete composite beam can meet the engineering design requirements.

[0146] A3: Conduct a normal temperature bearing capacity and serviceability analysis 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 "Code for Design of Concrete Structures" (GB50010-2010) for the flexural bearing capacity and shear bearing capacity of the inclined section of concrete beams. Specifically, the flexural bearing capacity M of the U-shaped recycled concrete composite beam... cu Calculated using the following steps:

[0147] First, through Calculate x0

[0148] like , ,

[0149] like , ,

[0150] like , ,

[0151] like ,

[0152] .

[0153] .

[0154] Among them, M cu denoted as , where is the flexural capacity of the U-shaped composite concrete beam; b, b1, b2, b3 represent the width of the composite beam, and the thicknesses of the left, middle, and right sides of the precast concrete shell 11, respectively; h, h0, h1, h2, h3 represent 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; f c,R , f c,H , f y These represent the calculated compressive strength of recycled concrete, the design compressive strength of high-strength concrete, and the design yield strength of reinforcing steel, respectively; α 1,H and α 1,R The coefficient for 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).

[0155] Among them, the shear bearing capacity V of the inclined section of the U-shaped recycled concrete composite beam cs Calculated using the following formula:

[0156]

[0157] Among them, V cs The shear capacity of the inclined section of the U-shaped recycled concrete composite beam. t,R , f t,H, f yv These represent the calculated tensile strength of recycled concrete, the design tensile strength of high-strength concrete, and the design yield strength of stirrups, respectively. α cv The shear capacity coefficient of the inclined section concrete is calculated according to Article 6.3.4 of the "Code for Design of Concrete Structures" (GB 50010-2010). A sv The total cross-sectional area of ​​each leg of the stirrups within the same cross-section is given by s, where s is the stirrup spacing along the length of the member.

[0158] The load-bearing capacity of the U-shaped recycled concrete composite beam should be no less than that of the corresponding ordinary concrete monolithic beam, wherein the ordinary concrete material and the recycled concrete material have the same effective water-cement ratio.

[0159] A4: Fire resistance design of U-shaped recycled concrete composite beams using a complex design methodology. Nonlinear full-process analysis under fire conditions is conducted. A large-scale general-purpose finite element program is used to calculate the time-varying internal temperature field of the concrete components and structure, considering the impact of the reduced thermal conductivity of recycled concrete. Based on this, high-temperature mechanical analysis of the components and structure is performed to determine whether the fire resistance design of the components or structure meets the fire resistance limit requirements of the "Code for Fire Protection Design of Building Structures" (GB 50016-2014). The impact of the reduced thermal conductivity of recycled concrete is considered when calculating the time-varying internal temperature field.

[0160] The fire resistance rating should be determined by whether the component has lost its load-bearing capacity, integrity, and thermal insulation properties. The criterion for losing load-bearing capacity is that the deflection reaches the ultimate bending deformation, or the rate of deflection increase reaches the ultimate bending deformation rate, and should conform to the following formula:

[0161] Ultimate bending deformation: , mm

[0162] Ultimate bending deformation rate: , mm / min

[0163] In the formula, D is the mid-span deflection, L is the net span of the specimen (mm), and d is the distance between the compressive and tensile points on the specimen interface (mm).

[0164] The criterion for loss of integrity is that the specimen can maintain its fire resistance and fire-proof performance during the fire resistance test.

[0165] The criteria for determining loss of thermal insulation are that the average temperature rise on the unexposed side 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.

[0166] In complex design methods, the thermal conductivity of recycled concrete is based on measured values.

[0167] A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface using a complex design method. Perform high-temperature safety analysis of the composite interface using nonlinear full-process analysis under fire conditions.

[0168] The high-temperature safety analysis of the composite interface should consider the loss of strength and stiffness of the interface bearing capacity under high temperature. The degradation law of the interface tensile bearing capacity and shear bearing capacity under high temperature should be tested by experiments.

[0169] Security analysis satisfies the following formula:

[0170] R T >S T ,

[0171] Among them, RT S represents the resistance of the composite interface at temperature T. T This represents the load effect at the combined interface at temperature T. Safety requirements must be met throughout the structure's fire resistance limit duration.

[0172] For the safety of the combined interface in A5, the relative slip and maximum crack width under normal use conditions must also be calculated, and should conform to the following formula:

[0173] Relative slip: s≤[s],

[0174] Where s is the relative slip obtained from experiments or numerical simulations, and [s] is the slip limit specified in the standard, in mm.

[0175] The maximum crack width should conform to the following formula: w≤[w],

[0176] Where w is the maximum crack width obtained from experiments or numerical simulations, and [w] is the maximum crack width limit specified in the standard, in mm.

[0177] A6: Based on the analysis results obtained from A2 to A5, select the optimized construction method for the U-shaped recycled concrete composite beam.

[0178] Comparative Example 1

[0179] 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 monolithic beam made of a single concrete material. U-shaped recycled concrete composite beam, monolithic recycled concrete beam, and monolithic ordinary concrete beam were then manufactured.

[0180] (1) Design and production

[0181] like Figure 1 As shown, the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and a cast-in-place compression zone concrete 12. The cross-sectional dimensions of the U-shaped recycled concrete composite beam are 200 mm × 300 mm, specifically h1 = 80 mm, h2 = 220 mm, h3 = 0 mm, b1 = 45 mm, b2 = 110 mm, and b3 = 45 mm. The top support reinforcement 23 has 2... 10, tensile reinforcement 21 is 2 22, U-shaped stirrup 22 is 10@100, no interface reinforcement is provided. The U-shaped recycled concrete composite beam is 2700 mm long, with a clear span of 2400 mm and a bottom concrete cover thickness of 40 mm.

[0182] The selected raw materials and equipment are as follows:

[0183] PO42.5 ordinary Portland cement, natural yellow sand with a maximum particle size of 0.9mm, polycarboxylate superplasticizer, and natural coarse aggregate with a particle size range of 5-16mm (provided by the Key Laboratory of Civil Engineering of the Ministry of Education, Tongji University); recycled coarse aggregate with a particle size range of 5-16mm (Shanghai Youhong Environmental Protection Technology Co., Ltd.).

[0184] As shown in Table 1, the precast concrete shell 11 is poured with recycled concrete with a 100% replacement rate of recycled coarse aggregate, and the cast-in-place compression zone is poured with high-strength concrete. First, a reinforcing cage is installed inside the U-shaped recycled concrete composite beam, with U-shaped stirrups 22 inside. External formwork and U-shaped formwork of corresponding dimensions are installed inside the U-shaped beam. The U-shaped formwork is fixed to the external formwork at the top and sides with wooden boards, and grout outlets are provided at the bottom of the U-shaped formwork. A precast concrete shell 11 is poured using recycled concrete with a slump of 120mm. A portion of the recycled concrete is added first, and a vibrator is inserted into the outside of the U-shaped formwork for thorough vibration until the bottom concrete is compacted, the recycled concrete slurry no longer settles, a surface layer of slurry appears, and cement slurry overflows from the grout outlets. After the bottom recycled concrete is compacted, the U-shaped side concrete is poured. Compacted concrete means the recycled concrete slurry no longer settles, a surface layer of slurry appears, and cement slurry overflows from the grout outlets. After final setting and curing for one week, the internal U-shaped formwork is removed. The bottom interface is roughened. High-strength concrete is then used to pour the cast-in-place concrete portion.

[0185] Meanwhile, recycled concrete monolithic beams and ordinary concrete monolithic beams with the same specifications and dimensions as the composite beams were used. The ratio of recycled concrete to ordinary concrete and high-strength concrete used in construction is shown in Table 1.

[0186] Table 1. Mix proportions of recycled concrete, ordinary concrete, and high-strength concrete used in construction.

[0187]

[0188] (2) Results and Discussion

[0189] Bending capacity tests were conducted at room temperature on U-shaped recycled concrete composite beams, monolithic recycled concrete beams, and monolithic ordinary concrete beams. The specimens were simply supported beams with a mid-span loading point spacing of 500 mm, and were loaded using a four-column hydraulic servo testing machine. The yield moment, ultimate moment, and corresponding deflection were measured respectively, and the specific data are shown in Table 2.

[0190] As 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 monolithic recycled concrete beam, reaching and exceeding the levels of the monolithic concrete beam. The deflection corresponding to the yield moment is roughly the same. Therefore, the composite recycled concrete beam can effectively address the low load-bearing capacity of recycled concrete beams. In structural design, the proposed U-shaped recycled concrete composite beam can be used to replace the monolithic concrete beam.

[0191] Table 2 Comparison of ambient temperature bearing capacity indices of composite beams, recycled concrete monolithic beams, and ordinary concrete monolithic beams

[0192]

[0193] Comparative Example 2

[0194] The fire resistance and residual load-bearing capacity after a fire were compared between U-shaped recycled concrete composite beams and monolithic beams made of single material. U-shaped recycled concrete composite beams, monolithic recycled concrete beams, and monolithic high-strength concrete beams were then fabricated.

[0195] (1) Design and production

[0196] like Figure 2 As shown, the U-shaped recycled concrete composite beam includes a precast concrete shell 11 and a cast-in-place compression zone concrete 12. The cross-sectional dimensions of the U-shaped recycled concrete composite beam are 200 mm × 300 mm, specifically h1 = 80 mm, h2 = 160 mm, h3 = 60 mm, b1 = 45 mm, b2 = 110 mm, and b3 = 45 mm. The top support reinforcement 23 has 2... 10, tensile reinforcement 21 is 2 22, U-shaped stirrup 22 is 10@100, interface reinforcement steel 25 10@100, with a length of 100 mm. The U-shaped recycled concrete composite beam is 2700 mm long, with a clear span of 2400 mm and a bottom concrete cover thickness of 40 mm.

[0197] The raw materials and equipment used are the same as in Example 2.

[0198] As shown in Table 3, the precast concrete shell 11 is poured with recycled concrete with a 100% replacement rate of recycled coarse aggregate, and the cast-in-place pressure zone is poured with high-strength concrete. First, a reinforcing cage is installed inside the U-shaped recycled concrete composite beam, with U-shaped stirrups 22 inside. External formwork and U-shaped formwork of corresponding dimensions are installed inside the U-shaped beam. The U-shaped formwork is fixed to the external formwork at the top and sides by wooden boards. Grout outlets and rebar holes are provided at the bottom of the U-shaped formwork. A precast concrete shell 11 is poured using recycled concrete with a slump of 120mm. A portion of the recycled concrete is added first, and a vibrator is inserted into the outside of the U-shaped formwork for thorough vibration until the bottom concrete is compacted, the recycled concrete slurry no longer settles, a surface layer of slurry appears, and cement slurry overflows from the grout outlets. After the bottom recycled concrete is compacted, the U-shaped side concrete is poured. Compacted concrete means the recycled concrete slurry no longer settles, a surface layer of slurry appears, and cement slurry overflows from the grout outlets. Interface reinforcement is arranged through the rebar holes. After final setting and curing for one week, the internal U-shaped formwork is removed. The bottom interface is roughened. High-strength concrete is used to pour the cast-in-place concrete portion.

[0199] Meanwhile, recycled concrete monolithic beams and high-strength concrete monolithic beams with the same specifications and dimensions as the U-shaped recycled concrete composite beams were adopted. The mix proportions of recycled concrete and high-strength concrete used in the construction are shown in Table 3.

[0200] Table 3. Mix proportions of recycled concrete, ordinary concrete, and high-strength concrete used in construction.

[0201]

[0202] (2) Results and Discussion

[0203] Constant load temperature rise tests were conducted on U-shaped recycled concrete composite beams, monolithic recycled concrete beams, and monolithic high-strength concrete beams. The furnace temperature curves followed the ISO 834 standard temperature rise curves. The specimens were exposed to fire on three sides, with a load ratio of 0.55. A 50-ton jack and a distribution beam were used to apply the load at mid-span, and the specimens were exposed to fire for 103 minutes. The specimens were simply supported beams with a mid-span loading point spacing of 500 mm. During the tests, the mid-span 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.

[0204] After the fire test, the U-shaped recycled concrete composite beam, the recycled concrete monolithic beam, and the high-strength concrete monolithic beam were allowed to cool naturally to room temperature in the furnace before the residual load-bearing capacity after the fire was tested. The specimens were simply supported beams with a mid-span loading point spacing of 500 mm, and were loaded using a four-column hydraulic servo testing machine. The yield moment, ultimate moment, and corresponding deflection were measured respectively, and the specific data are shown in Table 5.

[0205] Table 4 shows that the deflection and corresponding deflection growth rate of the U-shaped recycled concrete composite beam after 30, 60, 90, and 99 minutes of fire exposure are lower than those of the monolithic recycled concrete beam and significantly lower than those of the monolithic high-strength concrete beam. The monolithic high-strength concrete beam reaches its fire resistance limit after 99 minutes of fire exposure, while the U-shaped recycled concrete composite beam does not. The monolithic high-strength concrete beam exhibits localized bursting, which is not observed in the U-shaped recycled concrete composite beam. Therefore, the composite recycled concrete beam has a significant advantage in fire resistance, exceeding that of both monolithic recycled concrete and monolithic high-strength concrete beams. It can optimize the fire resistance of composite concrete beams and address the problem of premature failure caused by bursting of high-strength concrete under fire conditions.

[0206] As shown in Table 5, the U-shaped recycled concrete composite beam exhibits significantly better post-fire residual bearing capacity than the monolithic recycled concrete beam, and its yield moment-to-deflection ratio is significantly lower. This indicates that the monolithic recycled concrete beam experiences significant strength and stiffness degradation after fire exposure, while the U-shaped recycled concrete composite beam can mitigate this phenomenon, achieving or even exceeding the post-fire residual bearing capacity of the high-strength monolithic concrete beam 1. Therefore, the U-shaped recycled concrete composite beam possesses superior post-fire residual bearing capacity.

[0207] Table 4 Fire resistance performance indicators of U-shaped recycled concrete composite beams, monolithic recycled concrete beams, and monolithic high-strength concrete beams

[0208]

[0209] Table 5 Post-fire residual bearing capacity performance indicators of U-shaped recycled concrete composite beams, recycled concrete monolithic beams, and high-strength concrete monolithic beams.

[0210]

[0211] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any 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 invention should be within the protection scope of the present invention.

Claims

1. A method for designing U-shaped recycled concrete composite beams at ambient temperature and with fire resistance, characterized in that, Includes the following steps: A1: Determine the load effects and fire resistance rating of the U-shaped recycled concrete composite beam; A2: Design the cross-sectional dimensions of the U-shaped recycled concrete composite beam; A3: Conduct an analysis of the ambient temperature bearing capacity and normal serviceability of the U-shaped recycled concrete composite beam, and design the reinforcement of the U-shaped recycled concrete composite beam. A4: Conduct fire-resistant design of U-shaped recycled concrete composite beams, using either a simple or complex design method; A5: Based on the analysis in A3, design the roughness and reinforcement of the composite interface, using either a simple or complex design method. A6: Based on the analysis results obtained from A2 to A5, select the optimized construction method for the U-shaped recycled concrete composite beam; Specifically, in A1, the load effects should comply with the relevant provisions of the "Load Code for Design of Building Structures" (GB 50009-2012), and 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); in A2, the cross-sectional dimensions of the U-shaped recycled concrete composite beam should comply with the requirements for component dimensions in the "Code for Design of Concrete Structures" (GB50010-2010), and the dimensional parameters constituting the U-shaped recycled concrete composite beam should meet the engineering design requirements; in A3, the reinforcement design of the U-shaped recycled concrete composite beam should meet the requirements for the flexural bearing capacity and shear bearing capacity of the concrete beam in the "Code for Design of Concrete Structures" (GB50010-2010). The simple design method in A4 refers to: meeting the minimum values ​​of beam width and longitudinal tensile reinforcement (21) protective layer thickness for simply supported beams or continuous beams in the "Technical Specification for Fire-resistant Design of Concrete Structures" (DBJ / T15-81-2022), without considering the impact of the reduction in thermal conductivity of recycled concrete, and only as a safety reserve; The complex design method in A4 refers to the nonlinear full-process analysis under fire conditions: using a general finite element program to calculate the time-varying internal temperature field of concrete components and structures, considering the influence of the reduced thermal conductivity of recycled concrete, and on this basis, carrying out high-temperature mechanical analysis of components and structures, and then judging whether the fire resistance limit of components or structures meets the engineering design requirements. In A5, the simple design method refers to: the design interface is a rough interface, 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 unsmoothed natural pouring surface, and structural interface reinforcement should be arranged, and the reinforcement ratio of the interface reinforcement 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 by experiments; The designed U-shaped recycled concrete composite beam includes a precast concrete shell (11) and a cast-in-place compression zone concrete (12). The precast concrete shell (11) is a recycled concrete shell, the precast concrete shell (11) has a U-shaped cross section, and the strength grade of the precast concrete shell (11) is not higher than C40; the bottom of the precast concrete shell (11) is provided with interface reinforcing steel bars (25). The cast-in-place compression zone concrete (12) is high-strength concrete located 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. The U-shaped recycled concrete composite beam is equipped with a steel cage as a skeleton. The steel cage is placed inside the precast concrete shell (11) and includes: horizontally arranged tensile steel bars (21), stirrups (23) and vertically arranged stirrups. The stirrups are used to restrain the tensile steel bars (21). The stirrups are U-shaped structures with a bend at the top. The upper surface of the bottom of the stirrups abuts against the tensile steel bars (21), and the lower surface of the bend of the stirrups abuts against the stirrups (23). The cross-sectional dimensions of the U-shaped recycled concrete composite beam are as follows: width b is not less than 200 mm and height h is not less than 300 mm; the bottom thickness h1 of the precast concrete shell (11) is not less than 80 mm; the side widths b1 and b3 of the precast concrete shell (11) are both not less than 45 mm; and the width b2 of the cast-in-place compression zone concrete (12) is not less than 100 mm. The cast-in-place pressure zone concrete (12) is poured simultaneously with the concrete slab. The height of the two sides of the precast concrete shell (11) is set as h1 + h2, and the height of the top h3 is poured simultaneously with the cast-in-place pressure zone concrete (12). The height of h3 uses the same concrete material as the cast-in-place pressure zone concrete (12). h1, h2, and h3 are the bottom thickness of the precast concrete shell, the core height of the cast-in-place pressure zone concrete, and the top height of the cast-in-place pressure zone concrete, respectively.

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