Composite beam capable of enhancing ultimate strength of ultimate concrete structure

Abstract

The invention discloses a composite beam capable of enhancing the ultimate strength of an ultimate concrete structure. The span of the beam is measured, a bending moment graph of the beam is drawn, according to analysis of the bending moment graph, UHPC is selected for a pure bending section (the middle of the span) above a neutral axis of the beam, the upper half part of the beam is covered, and common concrete is used for the rest parts; half of the beam height is selected to use UHPC on the upper portion, and the structural ultimate strength of the composite beam can be improved; the FRP rib is used on the lower portion of the beam, the tensile property and ductility of the lower portion can be improved, a sleeve grouting anchorage device (the inner wall of a metal sleeve is provided with threads) and a wedge-shaped anchorage device (the clamping section is wrapped with a copper sleeve) are adopted, the anchoring reliability is ensured through the double effects of chemical bonding and mechanical meshing, and due to the fact that the FRP rib can be subjected to brittle failure and is preset to be in a super rib failure mode, the tensile strength and ductility of the lower portion can be improved. The problem of brittle failure of the FRP rib is solved. The beam is simple in design, convenient to construct, reasonable in stress, high in structural ultimate strength, high in reliability, wide in application range and material-saving.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building construction, and particularly relates to a composite beam for enhancing the ultimate structural strength of concrete. BACKGROUND

[0002] Reinforced concrete is a widely used structural material in the fields of construction and engineering. The steel reinforcement in reinforced concrete is prone to rusting in humid or chloride-containing environments, leading to expansion and cracking of the concrete cover, which in turn affects the durability and safety of the structure. At high temperatures, the steel reinforcement loses its strength and elasticity, leading to structural failure. Therefore, reinforced concrete structures have poor fire resistance in the event of a fire. The self-weight of reinforced concrete structures is relatively large, which limits their use in certain applications, such as long-span bridges or high-rise buildings, which may require more complex foundation and support structures. The construction of reinforced concrete requires formwork support and concrete pouring, which usually takes a long time to complete, especially in large structures. Due to factors such as shrinkage, temperature changes, and drying of concrete, reinforced concrete structures may develop cracks, which need to be controlled through design and construction.

[0003] Ultra-High Performance Concrete (UHPC) is a new type of building material, with a compressive strength of over 200 MPa, far exceeding that of traditional concrete. Its flexural strength and tensile strength are also very high, providing excellent structural performance. Due to the presence of a certain amount of fibers in the UHPC matrix, it can effectively prevent the cracking of concrete and inhibit the development of cracks. UHPC has a very low water-cement ratio and a reasonable particle size distribution, resulting in a material with very small porosity and a dense microstructure, which has outstanding advantages in terms of resistance to carbonation, chloride ion penetration, sulfate attack, chemical solution attack, and wear resistance. UHPC has high toughness and ductility, which can resist severe impact and vibration. UHPC materials have good flowability and can be applied to various construction processes. Due to the high strength and durability of UHPC, the maintenance frequency and cost of buildings or structures are greatly reduced, resulting in a lower life cycle cost in long-term operating buildings and infrastructure.

[0004] FRP, Fiber-Reinforced Polymer, is a new type of high-performance composite material, which is composed of reinforcing fibers and resin matrix. FRP has a low density, usually between 1.5 and 2.0, only 1 / 4 to 1 / 5 of carbon steel, but its tensile strength is close to or even exceeds that of carbon steel, and its specific strength can be compared with high-grade alloy steel. FRP has excellent corrosion resistance and can be used in acid, alkali, chlorine salt and humid environment for a long time, improving the service life of the structure. The tensile strength of FRP is significantly higher than that of steel bar, generally 2 to 10 times that of steel bar. Although the fire resistance of FRP is not as good as that of concrete, its fire resistance can be improved by adding fire retardant to the resin and surface fireproofing treatment.

[0005] UHPC has a wide application prospect in the field of construction and engineering due to its excellent mechanical properties, outstanding durability, good self-graded performance and construction performance, and better environmental protection performance. Combined with FRP, the ultimate strength of the beam can be better improved, the durability of the beam can be improved, and the excellent mechanical properties can be improved.

[0006] Compared with the prior art, the ultimate bearing capacity of the traditional reinforced concrete beam is limited due to the problems of steel bar corrosion and excessive self-weight; and the cost of the pure UHPC beam is high. The present application balances performance and economy through material zoning optimization. SUMMARY

[0007] The present application is based on the phenomenon of insufficient flexural bearing capacity of ordinary reinforced concrete beam, and is combined with traditional construction process for beam with rigid bearing capacity. The high strength advantage of UHPC can effectively improve the flexural bearing capacity of the beam, and the high ductility and high strength of FRP can effectively improve the tensile performance and strain of the lower part of the beam, thereby effectively solving the problem of beam damage caused by insufficient bearing capacity.

[0008] The technical scheme adopted by the present application to solve the above technical problems is: a composite beam capable of enhancing the ultimate strength of a concrete structure, wherein the upper pure bending section of the composite beam is UHPC super high performance concrete, and the rest is ordinary concrete, and the interior of the ordinary concrete is FRP bar.

[0009] Further, the FRP bar is positioned in the lower part of the beam by using a concave mold, and the ordinary concrete is poured and cured to initial setting.

[0010] Further, the upper half of the beam height is selected to use UHPC to improve the structural ultimate strength of the composite beam; and FRP bar is used in the lower part of the beam to improve the tensile performance and ductility of the lower part.

[0011] Further, UHPC pouring: UHPC is poured again in the concave area to cover the pure bending section; curing control: the environmental humidity is kept at ≥95%, the temperature is kept at 20±5℃, and the steam curing is kept for 48 hours.

[0012] Further, the UHPC proportioning parameter is: water-binder ratio (mass ratio) 0.18-0.22, the volume fraction of steel fiber content is 2%-3%, and the cement is replaced by 10%-15% of silica ash; the dense structure of UHPC is immune to carbonization, the unhydrated cement particles continue to react in the crack, the microcrack is closed, and the durability is improved.

[0013] Further, the upper part adopts UHPC super high performance concrete to balance the stress of the lower FRP tendon.

[0014] Further, the FRP tendon anchoring system comprises a sleeve grouting anchor and a wedge-shaped anchor, and the anchoring efficiency coefficient is greater than or equal to 0.9.

[0015] Further, the adopted anchoring mode is mechanical anchoring, the sleeve grouting anchor is first used, the FRP tendon is inserted into the metal sleeve, and then epoxy resin or cement-based grouting material is poured, and the FRP tendon is anchored through the double action of chemical bonding and mechanical engagement.

[0016] Further, the wedge-shaped anchor mechanically locks the FRP tendon through the conical clamping piece, and the local stress is dispersed by wrapping the clamping section with soft metal or fiber cloth to avoid crushing the FRP.

[0017] Further, the FRP tendon will be brittle when it is broken, and the preset beam using the FRP tendon is super-reinforced.

[0018] The present application has the following advantages:

[0019] (1) The present application utilizes the high strength performance of UHPC. When the span of the ordinary concrete is calculated, the ordinary concrete is first poured using a fixed formwork. After the concrete is formed, UHPC is used in the compression zone in the middle upper part of the reinforced concrete beam according to the calculated span, and then UHPC is poured to better connect it, thereby maintaining its integrity.

[0020] (2) The present application uses FRP tendon in the lower part, effectively solving the problem of easy damage of the lower tensile zone of ordinary reinforced concrete beam, thereby improving the tensile performance of the beam.

[0021] (3) The present application uses UHPC super high performance concrete only in the middle upper part of the beam, and uses ordinary concrete in the remaining part, thereby saving materials, facilitating construction, and at the same time making the beam have better performance.

[0022] (4) According to the four-point bending test verification, the ultimate bearing capacity of the composite beam of the present application is improved by 45%-60% compared with that of the ordinary concrete beam, and the crack width is controlled within 0.1mm. BRIEF DESCRIPTION OF DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 This is a schematic diagram of the main structure of the shaped beam of the present invention;

[0025] Figure 2 This is a schematic diagram of the main structure of the formed beam of the present invention;

[0026] Figure 3 is a schematic cross-sectional view of a formed beam of the present invention;

[0027] In the figure: 1-UHPC ultra-high performance concrete; 2-conventional concrete; 3-FRP reinforcement. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described below with reference to the accompanying drawings and examples. It should be understood that the specific examples described herein are only used to explain the present invention and are not intended to limit the present invention.

[0029] like Figure 1 、 Figure 2 and Figure 3 As shown, the present invention provides a composite beam that can enhance the ultimate strength of ultimate concrete structures. This beam utilizes a zoned casting process (conventional concrete and UHPC are constructed in stages), simplifying the formwork erection process and improving construction efficiency. This process provides a rational force distribution, high structural ultimate strength, high reliability, a wide range of applications, and material savings. The beam comprises UHPC ultra-high performance concrete 1, conventional concrete 2, and FRP bars 3. The concrete is divided into two sections: UHPC ultra-high performance concrete 1 and conventional concrete 2. The UHPC ultra-high performance concrete is primarily used in the upper, purely bending section of the beam, while the remaining section is conventional concrete 2. The FRP bars 3 are internally constructed, providing excellent tensile strength in the tension zone below the central axis. Formwork pre-setting: A concave mold is used to position the FRP bars 3 at the lower portion of the beam, and conventional concrete 2 is poured and cured to initial set. UHPC casting: A secondary UHPC cast is placed in the concave area to cover the purely bending section. Curing control: The ambient humidity is maintained at ≥95% and the temperature at 20±5°C, with steam curing for 48 hours.

[0030] This invention utilizes two different types of concrete, each designed to overcome different forces: UHPC (ultra-high performance concrete) 1 and conventional concrete 2. UHPC surpasses the limits of conventional materials, boasting ultra-high compressive strength and excellent tensile properties. Its crack resistance is over ten times better than conventional concrete, with a porosity of less than 2% and virtually no interconnectedness. Its chloride ion permeability is 100 times greater than that of conventional concrete. Its dense structure is immune to carbonation, and unhydrated cement particles continue to react within cracks, sealing microcracks and improving durability. Its high fluidity supports the casting of complex shapes. Under the same load, component thickness can be reduced by 50%, and deadweight by over 30%. Under cyclic loading, its fatigue life is 10 times that of conventional concrete. Because FRP rebar 3 is used in the lower portion, the tensile stresses in the lower portion are significant. Therefore, UHPC (ultra-high performance concrete) 1 is used in the upper portion to maintain force balance. UHPC mix parameters include a water-cement ratio of 0.18-0.22, a steel fiber content of 2%-3% (volume fraction), and silica fume as a cement replacement of 10%-15%.

[0031] FRP bars 3 are used inside the composite beam. FRP bars 3 are light, high-strength, low-density, and light in weight. The density is 1.5-1.9g / cm 3, about 1 / 4 of ordinary steel, greatly reducing the structure self weight, excellent tensile strength, tensile strength is higher than 20% of the same specification steel, close to high strength steel wire, meet the high strength demand, FRP bar anchoring system includes sleeve grouting anchor and wedge anchor (wedge anchor is a kind of anchoring device widely used in prestressed concrete structure, the structure mainly includes anchor ring, wedge, steel strand and anti-loosening device, sealing ring or anchor plate and other auxiliary components), anchoring efficiency coefficient is greater than or equal to 0.9. Strong corrosion resistance, can resist acid, alkali, chloride and low pH value solution erosion, especially suitable for marine, chemical and other corrosive environment. Good material bonding force, close to the thermal expansion coefficient of concrete, cooperative deformation when temperature changes, reduce internal stress, surface thread design enhances the gripping force with concrete, improves the overall structure performance. Strong designability, FRP bar elastic modulus is stable, dimensionally stable under thermal stress, easy to thermoforming, and good safety performance. Convenient construction, factory prefabrication, on-site binding with non-metallic tension belt, simple operation. Strong durability, environmental protection and many other advantages. When using FRP bar, the preset beam is over-reinforced failure, because FRP bar will occur brittle fracture when failure, so the preset beam is over-reinforced failure. The adopted anchoring mode is mechanical anchoring, and its solution is: first, sleeve grouting anchor, FRP bar is inserted into the metal sleeve, and then high-strength epoxy resin or cement-based grouting material is grouted, and the FRP bar is anchored through the double action of chemical bonding and mechanical engagement, so that the dependence on concrete bonding can be avoided, the anchoring reliability is high, and the key is to note that the inner wall of the sleeve needs to be provided with threads or rough surface to enhance the engagement, and then the wedge anchor is used to mechanically lock the FRP bar through the conical clamping piece, but the clamping pressure needs to be optimized (to avoid crushing the FRP), which can be improved by wrapping the clamping section with soft metal (such as copper sleeve) or fiber cloth to disperse local stress. Water-cooled diamond saw blade is used for cutting, and the cutting rate is less than or equal to 5mm / s to avoid fiber delamination.

[0032] The present application fully considers that when the upper compression zone of the concrete is subjected to heavy pressure, micro cracks are generated, and under the action of sustained load, the micro cracks in the compression zone expand into through cracks along the length direction of the beam. The lower half uses FRP bars, so that the tensile strength of the lower tensile zone is improved. Construction preparation: material adaptability verification: the bonding strength of the FRP bar and the UHPC is determined by a pull-out test (greater than or equal to 8 MPa), and the anchoring length is increased by 20% according to the calculated value of the specification. Formwork design: the ribbed steel form (stiffness greater than or equal to 15 kN / mm) is used in the pure bending section, and the wooden form (surface film treatment) is used in the non-pure bending section. Anchoring process: sleeve grouting anchor: the FRP bar is inserted into the 304 stainless steel sleeve (inner diameter = bar diameter + 2 mm) with threads on the inner wall, and the non-shrinkage epoxy grouting material (compressive strength greater than or equal to 80 MPa) is grouted, and the curing time is 72 hours. Wedge anchor: the clamping piece is made of high-strength aluminum alloy (hardness HRB 80), and the clamping pressure is controlled at 20-25 MPa to avoid local crushing of the FRP.

[0033] The specific implementation method is as follows:

[0034] I. The first part is the construction preparation stage, which carries out technical composite;

[0035] 1.1) Confirm the structural design documents: clearly define the scope of the pure bending section (usually located on the upper side of the beam neutral axis compression zone), mark the UHPC and ordinary concrete interface position;

[0036] 1.2) Material adaptability verification: test the bonding strength of FRP bars and UHPC / ordinary concrete (it is recommended to increase the anchorage length by 20%).

[0037] 1.3) Material preparation:

[0038] FRP bars: precast according to the design length, use water-cooled diamond saw blades for cutting, cutting speed ≤5mm / s, avoid fiber delamination, and make mechanical anchor heads at the ends.

[0039] UHPC: Choose premixed dry powder, store in humidity ≤60%, avoid vibration during transportation, embed φ8@150mm shear bolts at the UHPC and ordinary concrete interface to enhance the interface shear strength.

[0040] 1.4) Ordinary concrete: prepare according to C30-C40 proportioning, control the slump to 180±20mm.

[0041] II. The second part is the steel engineering (focus on controlling FRP characteristics) processing and installation, using a three-dimensional laser scanner to position the steel mesh;

[0042] 2.1) Cross node processing: use epoxy sleeve to connect between FRP bars, wrap carbon fiber cloth on the contact surface of ordinary steel bars to enhance adhesion, before grouting the sleeve, use ultrasonic detection to detect the roughness of the inner wall of the sleeve (R a ≥6.3μm), the grouting material flowability needs to be ≥300mm.

[0043] 2.2) Erection sequence: first lay the lower ordinary steel mesh → reserve FRP bars in the UHPC section → install the upper ordinary steel.

[0044] 2.3) Cover control:

[0045] Pure bending section: FRP bar protection layer thickness ≥30mm (considering the brittleness of the material characteristics);

[0046] Non-pure bending section: the protection layer of ordinary steel bars is executed according to the specification.

[0047] Thirdly, the third part is template engineering (double-section special treatment), interface isolation, UHPC and ordinary concrete junction set Z-type water stop steel plate (height 300mm), template inside brushing release agent, then, in UHPC section, polypropylene fiber isolation layer is added. The lower template of the pure bending section adopts dense rib steel mold (deformation control ≤2mm), and the overall stability calculation needs to consider the temperature stress caused by the UHPC ultra-early strength characteristics.

[0048] Fourthly, the fourth part is concrete construction, which needs to be poured in time and in sections;

[0049] 4.1) Firstly, UHPC pouring, special mixer truck is adopted, rotation speed ≥60rpm, mixing time is extended to 8 minutes, pouring temperature is controlled at 15-25℃, pumping pressure ≤25MPa, layer thickness ≤300mm, high-frequency vibrating rod (12000 times / minute) is used;

[0050] 4.2) Secondly, ordinary concrete pouring, adjacent area pouring is completed before UHPC initial setting (about 2 hours), interface inserts φ16 threaded steel anchor (spacing 200mm) to enhance the combination, threaded anchor head + double nut is installed at the tensioning end, and the tightening torque is controlled at 120% of the design value, the fixed end adopts glue pouring anchor, after injecting epoxy resin, vacuum pumping is used to remove bubbles, and hierarchical tensioning is used to control stress (σ con =0.75f pk ), after holding for 2 minutes, the anchor is locked;

[0051] 4.3) Finally, some special processes are needed, UHPC section steam curing: temperature rising rate ≤15℃ / h, constant temperature for 48 hours (temperature 90℃±5℃), ordinary concrete water spraying curing ≥7 days.

[0052] The above examples are used to explain and illustrate the present application, but not to limit the present application, any modification and change made to the present application within the spirit and protection scope of the claims, falls into the protection scope of the present application.

Claims

1. A composite beam capable of enhancing the ultimate strength of an ultimate concrete structure, characterized in that: The upper pure bending section of the composite beam is made of UHPC ultra-high performance concrete (1), and the remaining section is made of ordinary concrete (2), with FRP bars (3) inside the ordinary concrete.

2. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: A concave mold is used to position the FRP bars at the bottom of the beam, and ordinary concrete is poured and cured to initial setting.

3. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: UHPC is selected to be used in the upper part of the beam height to improve the structural ultimate strength of the composite beam; FRP bars are used in the lower part of the beam to improve the tensile performance and ductility of the lower part.

4. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: UHPC pouring: Secondary pouring of UHPC in the concave area to cover the pure curved section; Curing control: Maintain ambient humidity ≥ 95%, temperature 20±5℃, and steam curing for 48 hours.

5. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: UHPC mix parameters: water-cement ratio (mass ratio) 0.18-0.22, steel fiber content volume fraction 2%-3%, silica fume replacement of cement mass 10%-15%; UHPC's dense structure is immune to carbonization, and unhydrated cement particles continue to react in cracks, sealing micro-cracks and improving durability.

6. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: The upper part is made of UHPC ultra-high performance concrete (1) to balance the tensile stress of the lower FRP bars (3).

7. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: The FRP reinforcement anchoring system includes sleeve grouting anchors and wedge anchors, with an anchoring efficiency coefficient ≥ 0.

9.

8. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 7, characterized in that: The anchoring method adopted is mechanical anchoring. First, the sleeve is grouted with anchors. After the FRP bars are inserted into the metal sleeve, epoxy resin or cement-based grouting material is poured in. The anchoring is achieved through the dual action of chemical bonding + mechanical bite.

9. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 7, characterized in that: The wedge anchor mechanically locks the FRP bar through the conical clip, and by wrapping the clamping section with soft metal or fiber cloth, it disperses local stress to avoid crushing the FRP.

10. The composite beam capable of enhancing the ultimate strength of an ultimate concrete structure according to claim 1, characterized in that: FRP bars will undergo brittle fracture when they fail, and when using FRP bars, it is assumed that the beam will fail due to over-reinforcement.

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