Test apparatus and method for creep of two-way prestressed concrete

Abstract

This invention provides a bidirectional prestressed concrete creep testing device and method, relating to the technical fields of concrete testing technology and long-term structural deformation monitoring. The device includes: a concrete slab specimen; multiple prestressing units, disposed on the concrete slab specimen and bonded integrally with it, orthogonally distributed in the horizontal plane and multi-layered in the thickness direction, for applying bidirectional prestress to the concrete slab specimen; a testing unit for monitoring changes in prestress, temperature, and humidity of the concrete slab specimen; a boundary protection unit disposed on the sidewall of the concrete slab specimen to block lateral heat and moisture exchange, ensuring that temperature and humidity within the concrete slab specimen are transmitted only along the thickness direction; and a support frame on which the concrete slab specimen is mounted for suspended support, allowing ambient air to flow freely across the upper and lower surfaces of the concrete slab specimen.

Description

Technical Field

[0001] This invention relates to the technical field of concrete testing technology and long-term monitoring of structural deformation, specifically to a two-way prestressed concrete creep test device and method. Background Technology

[0002] Creep in concrete refers to the physical phenomenon where hardened concrete undergoes continuous deformation over time under sustained loads. Accurate assessment of concrete creep is crucial for ensuring the long-term mechanical performance of engineering structures. In practical large-scale and complex prestressed engineering projects (such as nuclear power plant containment structures and two-dimensional load-bearing slabs), concrete members are typically not under simple uniaxial compression, but rather under complex biaxial or even multiaxial stress. Biaxial creep is not a simple linear superposition of uniaxial creep; influenced by the Poisson effect under biaxial stress and the multidimensional evolution of internal microcracks, its creep characteristics differ significantly from those of the uniaxial state. Therefore, revealing the mechanism of biaxial creep is fundamental to accurately establishing a multidimensional creep constitutive model.

[0003] However, existing concrete creep testing methods and equipment have significant limitations, making it difficult to meet the testing needs of complex prestressed structures. First, traditional creep testing is mainly limited to uniaxial constant load tests. In real biaxial prestressed structures, over time, due to the shrinkage and creep of the concrete itself and the relaxation of the prestressing tendons, the effective stress inside the structure is in a dynamic decay process, i.e., a state of "variable stress." Existing conventional creep apparatuses (such as spring compression systems) can often only maintain a constant load, failing to realistically and accurately simulate the complex process of long-term variable stress in actual structures. Second, the few existing biaxial creep loading devices are usually complex in construction, and during long-term loading, the two vertical force transmission systems are prone to coupling interference. Similar to the material and interface problems of the force transmission blocks commonly found in existing conventional testing equipment, existing biaxial loading easily creates strong lateral constraints and friction at the specimen ends, leading to uneven stress distribution within the specimen and even causing end tensile cracking failure, severely affecting the accuracy and repeatability of biaxial creep data. Therefore, there is an urgent need for devices and corresponding testing methods that can accurately reproduce the stress history of a structure during its service life, so as to provide reliable experimental support for the long-term deformation prediction of complex engineering projects. Summary of the Invention

[0004] This invention was made to solve the above-mentioned problems, and its purpose is to provide a two-way prestressed concrete creep test device.

[0005] This invention provides a bidirectional prestressed concrete creep testing device, characterized by comprising: a concrete slab specimen; multiple prestressing units disposed on the concrete slab specimen, bonded integrally with the concrete slab specimen, and orthogonally distributed in the horizontal plane and distributed in multiple layers in the thickness direction, for applying bidirectional prestress to the concrete slab specimen; a testing unit for monitoring changes in prestress, temperature, and humidity of the concrete slab specimen; a boundary protection unit disposed on the side wall of the concrete slab specimen for blocking lateral heat and moisture exchange, ensuring that the temperature and humidity inside the concrete slab specimen are transmitted only along the thickness direction; and a support frame on which the concrete slab specimen is disposed for suspended support, allowing ambient air to flow freely across the upper and lower surfaces of the concrete slab specimen.

[0006] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following features: wherein the prestressing unit includes: a corrugated pipe disposed inside the concrete slab specimen; prestressing tendons passing through the corrugated pipe with both ends extending out of the concrete slab specimen, and the prestressing tendons being tensioned to apply a preset prestress; bearing plates disposed at both ends of the corrugated pipe, with one side of the bearing plate connected to the corrugated pipe; and anchors disposed on both sides of the concrete slab specimen, with one side of the anchor abutting against the other side of the bearing plate.

[0007] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following features: the multi-layer distribution is a three-layer distribution, the tensioning sequence of the prestressing tendons is to first tension the prestressing tendons of the middle layer, and then tension the prestressing tendons of the upper and lower layers, and the tensioning is symmetrical at both ends.

[0008] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following feature: the interior of the concrete slab specimen is provided with spiral ribs, which are respectively sleeved on both ends of the corrugated pipe, and the side of the spiral ribs near the bearing pad abuts against the bearing pad.

[0009] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following features: wherein the test unit includes: at least one force gauge, disposed on the part of the prestressed tendon extending out of the concrete slab specimen, for monitoring changes in prestress; at least one thermocouple, disposed on the concrete slab specimen, for monitoring changes in temperature; and at least one humidity sensor, disposed on the concrete slab specimen, for monitoring changes in humidity.

[0010] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following feature: the force gauge, thermocouple and humidity sensor are connected to a cloud server for real-time uploading of monitoring data.

[0011] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following features: a measuring hole is provided on the side wall of the concrete slab specimen, and a thermocouple and a humidity sensor are embedded in the measuring hole. The measuring hole with the thermocouple embedded is sealed with cement grout, and the measuring hole with the humidity sensor embedded is sealed with silicone.

[0012] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following features: wherein the boundary protection unit comprises, from the inside to the outside, the following: an epoxy resin sealing layer with a thickness greater than 2 mm; a heat insulation layer with a thickness greater than 20 mm, made of polyurethane insulation material; and an aluminum foil reflective layer.

[0013] The bidirectional prestressed concrete creep test device provided by the present invention may also have the following feature: wherein the height of the support frame is ≥600mm.

[0014] This invention also provides a method for testing the creep of bidirectional prestressed concrete, characterized by the following steps: setting up multiple prestressing units, which are orthogonally distributed in the horizontal plane and multi-layered in the thickness direction; pouring concrete to bond the prestressing units to the concrete slab specimen; setting boundary protection units on the sidewalls of the concrete slab specimen; setting up a test unit on the concrete slab specimen to monitor changes in prestress, temperature, and humidity; setting the concrete slab specimen on a support frame for suspended support; activating the test unit to conduct the creep test and collect data.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] The biaxial prestressed concrete creep testing device and method of this invention ingeniously integrates prestressing loading with concrete slab specimens, constructing a biaxial variable stress testing system capable of realistically reproducing the dynamic stress evolution process of actual structures during their service life. Through the synergistic design of lateral heat and moisture isolation and suspended support, a one-dimensional heat and moisture transfer environment within a large-volume concrete structure is simulated on a small-sized concrete slab specimen. The use of multi-layer orthogonal arrangement of prestressing tendons along the thickness direction and a "middle-first, then two-sided, symmetrical tensioning at both ends" process fundamentally eliminates the lateral constraint and tensile cracking risks caused by end friction in traditional tests. This invention integrates simultaneous monitoring of prestress, temperature, and humidity across multiple physical fields, and achieves real-time data transmission and analysis through a cloud platform, providing reliable experimental support for the long-term creep prediction of complex prestressed concrete structures. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the bidirectional prestressed concrete creep test device in an embodiment of the present invention.

[0018] Figure 2 This is a partial structural schematic diagram of the bidirectional prestressed concrete creep test device in an embodiment of the present invention.

[0019] Figure 3 This is a cross-sectional view of the prestressed unit in an embodiment of the present invention.

[0020] Figure 4 This is a diagram showing the sequence of applying prestress during tensioning in an embodiment of the present invention.

[0021] Figure 5 This is a schematic diagram of the thermocouple arrangement in an embodiment of the present invention.

[0022] Figure 6 This is a schematic diagram of the humidity sensor setup in an embodiment of the present invention.

[0023] Figure 7 This is a schematic diagram of the structure of the epoxy coating auxiliary device on the side of the concrete slab in an embodiment of the present invention.

[0024] Figure 8 This is a schematic diagram of the structure of the epoxy resin coating auxiliary device for the corner of the concrete specimen in an embodiment of the present invention.

[0025] Figure 9 This is a schematic diagram of the boundary protection unit in an embodiment of the present invention.

[0026] Explanation of symbols for main components:

[0027] In the diagram: 100, Bidirectional prestressed concrete creep test device; 30, Prestressed unit; 40, Test unit; 50, Boundary protection unit; 60, Epoxy coating auxiliary device for the side of concrete slab; 70, Epoxy resin coating auxiliary device for the corner of concrete specimen; 1, Concrete slab specimen; 2, Anchorage; 3, Steel strand; 4, Force gauge; 6, Tensioner; 8, Bearing pad; 10, Corrugated pipe; 11, Spiral reinforcement; 12, Measuring hole; 13, Thermocouple; 14, Silicone; 15, Humidity sensor; 16, First front baffle; 17, Side baffle; 18, Back plate; 19, First bottom plate; 20, Sealing plate; 21, Side sealing plate; 22, Second bottom plate; 23, Side plate; 24, Second front baffle; 26, Epoxy resin sealing layer; 27, Thermal insulation layer; 28, Aluminum foil reflective layer; 29, Support frame. Detailed Implementation

[0028] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication between two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0029] To make the technical means, creative features, objectives and effects of the present invention easy to understand, the following embodiments, in conjunction with the accompanying drawings, provide a detailed description of the bidirectional prestressed concrete creep test device and method of the present invention.

[0030] Figure 1 This is a schematic diagram of the structure of the bidirectional prestressed concrete creep test device in an embodiment of the present invention. Figure 2 This is a partial structural schematic diagram of the bidirectional prestressed concrete creep test device in an embodiment of the present invention.

[0031] like Figures 1-2 As shown, the bidirectional prestressed concrete creep test device 100 in this embodiment includes a concrete slab specimen 1, multiple prestressed units 30, a test unit 40, a boundary protection unit 50, and a support frame 29.

[0032] Concrete slab specimen 1 was made of C60 concrete. To ensure the strength of concrete slab specimen 1 during the creep process, the specific composition was: 0.33 parts cement, 0.16 parts water, 1 part coarse aggregate, 0.77 parts fine aggregate, 0.07 parts silica fume, 0.08 parts mineral powder, and 0.007 parts water-reducing agent. In this embodiment, a square concrete slab specimen 1 with dimensions of 1230mm*1230mm*410mm was used.

[0033] Figure 3 This is a cross-sectional view of the prestressed unit in an embodiment of the present invention.

[0034] like Figure 3 As shown, multiple prestressed units 30 are disposed on the concrete slab specimen 1 and bonded to the concrete slab specimen 1 by grouting. They are orthogonally distributed in the horizontal plane and distributed in three layers in the thickness direction. They are used to apply bidirectional prestress to the concrete slab specimen 1 and include corrugated pipe 10, prestressing tendons, bearing pad 8 and anchor 2.

[0035] The corrugated pipes 10 are installed inside the concrete slab specimen 1, and are orthogonally distributed in the horizontal plane and distributed in three layers in the thickness direction.

[0036] In this embodiment, steel strand 3 is used as the prestressing tendon. The steel strand 3 is a 1860MPa low-relaxation steel strand. The steel strand 3 is threaded inside the corrugated pipe 10, with both ends extending out of the concrete slab specimen 1. The steel strand 3 is subjected to a preset prestress through the tensioner 6. In this embodiment, the tensioner 6 is a hydraulic jack.

[0037] Figure 4 This is a diagram showing the sequence of applying prestress during tensioning in an embodiment of the present invention.

[0038] The tensioning sequence of the steel strands 3 is as follows: first tension the middle layer of steel strands 3, then tension the upper and lower layers of steel strands 3, and tensioning is carried out symmetrically at both ends. In this embodiment, the specific tensioning sequence is as follows: Figure 4 As shown, the positions of each corrugated pipe 10 are numbered sequentially from 1 to 6. During tensioning, the steel strands 3 running through each corrugated pipe 10 are tensioned one by one in the order of number 1→2→3→4→5→6.

[0039] Pressure bearing plates 8 are respectively installed at both ends of the corrugated pipe 10, and one side of the pressure bearing plate 8 is connected to the corrugated pipe 10. Grouting holes are opened on the pressure bearing plate 8 and are connected to the corrugated pipe 10.

[0040] The concrete slab specimen 1 is also provided with spiral ribs 11, which are respectively sleeved on both ends of the corrugated pipe 10. The side of the spiral ribs 11 closest to the bearing pad 8 abuts against the bearing pad 8.

[0041] Anchor 2 is made of 20CrMnTi alloy structural steel, which has low shrinkage and high elasticity. It is set on both sides of the concrete slab specimen 1, with one side of anchor 2 abutting against the other side of the bearing pad 8.

[0042] In this embodiment, each steel strand 3 is equipped with three anchors 2. Two of them are working anchors, which are respectively set on both sides of the concrete slab specimen 1, close to the outer side of the bearing pad 8, for permanently locking the steel strand 3. The third is a tool anchor, which is set at a preset appropriate position for temporary locking during the tensioning process. After tensioning is completed, the tool anchor is removed, leaving only the two working anchors on the specimen.

[0043] Specifically, the length of the portion of the steel strand 3 extending beyond the concrete slab specimen 1 needs to accommodate the tensioner 6, the tool anchor, and the force gauge 4 of the test unit 40. The tensioner 6 and the force gauge 4 are positioned at the portion of the steel strand 3 extending beyond the concrete slab specimen 1, and the tool anchor is positioned between the tensioner 6 and the force gauge 4. Before tensioning, the working anchor and the tool anchor are installed and fixed in the aforementioned preset positions. During tensioning, the tensioner 6 tightly grips the steel strand 3 through its internal locking structure, applying tension to the steel strand 3 through hydraulic pressure. After tensioning is completed, cement grout is injected under high pressure through the grouting holes on the pressure bearing plate 8, filling the gap between the corrugated pipe 10 and the steel strand 3. After the cement grout hardens, the steel strand 3 is bonded to the corrugated pipe 10 and the concrete slab specimen 1 as a whole. At the same time, two working anchors lock the steel strand 3 and abut against the side of the bearing pad 8 away from the corrugated pipe 10, converting the tension of the steel strand 3 into pressure on the bearing pad 8. This pressure is transmitted to the side wall of the concrete slab specimen 1 through the bearing pad 8, thereby applying bidirectional prestress to the concrete slab specimen 1.

[0044] The testing unit 40 is used to monitor the changes in prestress, temperature and humidity of the concrete slab specimen 1. It includes multiple force gauges 4, multiple thermocouples 13 and multiple humidity sensors 15, all of which are connected to the cloud server and can upload monitoring data in real time.

[0045] The force gauge 4 is a high-precision vibrating wire force gauge, which is installed on the part of the steel strand 3 that extends out of the concrete slab specimen 1, and is used to monitor the prestress fluctuation data of the concrete slab specimen 1 in real time.

[0046] Multiple measuring holes 12 are provided on the side wall of the concrete slab specimen 1, and multiple thermocouples 13 and multiple humidity sensors 15 are respectively embedded in the measuring holes 12.

[0047] Figure 5 This is a schematic diagram of the thermocouple arrangement in an embodiment of the present invention.

[0048] Specifically, such as Figure 5 As shown, thermocouple 13 is a type K thermocouple. The measuring hole 12 for embedding thermocouple 13 is opened on the side wall where the double-layer steel strand 3 is located, and the drilling depth is uniformly 200mm. Multiple thermocouples 13 are arranged in a straight line along the thickness direction of the concrete slab specimen 1, forming a row of thermocouples 13, used to monitor the internal temperature field gradient data of the concrete slab specimen 1. The multiple thermocouples 13 are arranged with variable spacing in the thickness direction: in the area near the upper and lower surfaces of the concrete slab specimen 1, the temperature and humidity gradient is larger, so the thermocouples 13 are arranged more densely (smaller spacing); near the central area, the temperature and humidity field tends to be stable, so the thermocouples 13 are arranged more sparsely (larger spacing). This optimized arrangement can accurately capture the nonlinear gradient changes of the boundary layer, effectively characterize the distribution law of the overall temperature and humidity field, and reduce the number of thermocouples 13, thus reducing the test cost.

[0049] In this embodiment, the horizontal distance between the thermocouples 13 and the nearest side edge of the concrete slab specimen 1 is 150mm. A total of ten thermocouples 13 are arranged in this embodiment, distributed from top to bottom along the thickness direction. The uppermost thermocouple 13 is 10mm from the upper edge of the concrete slab specimen 1, and the lowermost thermocouple 13 is 10mm from the lower edge. From top to bottom, the spacing between adjacent thermocouples 13 is: 30mm, 40mm, 50mm, 50mm, 50mm, 50mm, 50mm, 40mm, 30mm.

[0050] After the thermocouple 13 is installed, the measuring hole 12 is initially sealed with cement grout.

[0051] Figure 6 This is a schematic diagram of the humidity sensor setup in an embodiment of the present invention.

[0052] like Figure 6 As shown, the specific number and depth of the measuring holes 12 for embedding the humidity sensor 15 are determined according to the experimental requirements, and are used to monitor the humidity field gradient data inside the concrete slab specimen 1. When the humidity sensor 15 is embedded in the measuring hole 12, the top of the humidity sensor 15 (the end facing the outside of the measuring hole 12) is 5 mm away from the bottom of the measuring hole 12. After the humidity sensor 15 is embedded, the measuring hole 12 is sealed with high-density silicone 14.

[0053] The boundary protection unit 50 is set on the side wall of the concrete slab specimen 1 to block lateral heat and moisture exchange, so that the temperature and humidity inside the concrete slab specimen 1 are transmitted only along the thickness direction. From the inside to the outside, it includes an epoxy resin sealing layer 26, a heat insulation layer 27 and an aluminum foil reflective layer 28.

[0054] The epoxy resin sealing layer 26 has a thickness greater than 2 mm. To ensure coating quality and uniform thickness, an auxiliary device is used during epoxy resin application. Before using the auxiliary device, a polytetrafluoroethylene (PTFE) film must be placed over the interface between the auxiliary device and the epoxy resin to prevent adhesion between the epoxy resin and the metal auxiliary device, which would affect the sealing effect. The auxiliary devices include an epoxy resin coating auxiliary device 60 for the side of the concrete slab and an epoxy resin coating auxiliary device 70 for the corner of the concrete specimen.

[0055] Figure 7 This is a schematic diagram of the structure of the epoxy coating auxiliary device on the side of the concrete slab in an embodiment of the present invention.

[0056] like Figure 7 As shown, the epoxy coating auxiliary device 60 for the side of the concrete slab includes a first front baffle 16, two side baffles 17, a back plate 18, a first bottom plate 19, and two sealing plates 20.

[0057] The length of the back plate 18 matches the length of the concrete slab specimen 1. The length of the first front baffle 16 is the same as the length of the back plate 18. The first front baffle 16 and the back plate 18 are connected by two sealing plates 20 respectively disposed on both sides of the back plate 18, forming a uniform gap with a width of 2-3 mm between the first front baffle 16 and the back plate 18. The height of the top edge of the first front baffle 16 is the same as the height of the top edge of the back plate 18, and the heights of both the first front baffle 16 and the back plate 18 are greater than the thickness of the concrete slab specimen 1, to ensure that the epoxy resin can completely cover the entire side of the concrete slab specimen 1. The first bottom plate 19 is disposed at the bottom of the back plate 18. Two side baffles 17 are respectively disposed on both sides of the back plate 18, located between the first front baffle 16 and the first bottom plate 19.

[0058] When applying epoxy resin, the concrete slab specimen 1 is placed on the epoxy coating auxiliary device 60 on the side of the concrete slab, with the surface to be coated facing the back plate 18 and flush with the first front baffle 16. The first base plate 19 is pressed tightly against the bottom of the concrete slab specimen 1 to prevent epoxy resin from flowing out from the bottom. Two side baffles 17 are respectively pressed tightly against the two sides adjacent to the surface to be coated to prevent epoxy resin from flowing out from the sides. Two sealing plates 20 also prevent epoxy resin from flowing out from the sides. Epoxy resin is poured into the gap between the first front baffle 16 and the back plate 18. Due to the flow characteristics of epoxy resin, it flows downwards under gravity, evenly filling the space between the surface to be coated of the concrete slab specimen 1 and the back plate 18, forming a uniform and smooth sealing layer.

[0059] Figure 8 This is a schematic diagram of the structure of the epoxy resin coating auxiliary device for the corner of the concrete specimen in an embodiment of the present invention.

[0060] like Figure 8 As shown, the epoxy resin coating auxiliary device 70 for the corner of the concrete specimen includes two side sealing plates 21, a second bottom plate 22, a side plate 23, and a second front baffle 24.

[0061] The second base plate 22 is square, and the side plate 23 is L-shaped, its dimensions matching those of the second base plate 22, and is positioned on two adjacent sides of the second base plate 22. Two side sealing plates 21 are respectively positioned on both sides of the side plate 23 (i.e., the ends of the two sides of the L-shape), each side sealing plate 21 having a width of 2-3 mm, and its top edge height matching that of the top edge of the side plate 23. The shape of the second front baffle 24 matches that of the side plate 23, and it is positioned on the side of the side sealing plate 21 away from the side plate 23, forming a uniform gap of 2-3 mm between the second front baffle 24 and the side plate 23. The heights of both the second front baffle 24 and the side plate 23 are greater than the thickness of the concrete slab specimen 1 to ensure that the epoxy resin can completely cover the entire corner of the concrete slab specimen 1.

[0062] When applying epoxy resin, the concrete slab specimen 1 is placed on the epoxy resin application aid 70 at the corner of the concrete specimen, with the corner to be coated facing the right-angle bend of the side plate 23, and the two adjacent surfaces of the corner to be coated flush with the second front baffle 24. The second bottom plate 22 is pressed tightly against the bottom of the concrete slab specimen 1 to prevent epoxy resin from flowing out from the bottom; the two side sealing plates 21 are respectively pressed tightly against the two adjacent surfaces of the corner to be coated of the concrete slab specimen 1, forming a closed injection space together with the second front baffle 24 to prevent epoxy resin from flowing out from the sides. Then, epoxy resin is poured into the gap between the side plate 23 and the second front baffle 24, flowing downwards under gravity and evenly filling the space between the corner of the concrete slab specimen 1 and the side plate 23, forming a uniform and smooth sealing layer.

[0063] Figure 9 This is a schematic diagram of the boundary protection unit in an embodiment of the present invention.

[0064] like Figure 9 As shown, the outer side of the epoxy resin sealing layer 26 is a 20mm thick thermal insulation layer 27 made of polyurethane insulation material. The outer side of the thermal insulation layer 27 is an aluminum foil reflective layer 28. In this embodiment, components exposed outside the concrete slab specimen 1 (such as the force gauge 4) are also provided with thermal insulation and rust prevention measures.

[0065] The concrete slab specimen 1 is placed on the support frame 29 (an iron frame is used in this embodiment). The support frame 29 is 600mm high and provides suspended support for the concrete slab specimen 1, so that ambient air can flow freely over the upper and lower surfaces of the concrete slab specimen 1.

[0066] This embodiment also provides a method for testing the creep of two-dimensional prestressed concrete, based on a two-dimensional prestressed concrete creep testing device 100, including the following steps:

[0067] S1: Orthogonally embed corrugated pipes 10 in the mold according to a three-layer arrangement, and tie spiral reinforcements 11 at the ends of the ducts of the corrugated pipes 10 and install pressure bearing plates 8. Pour concrete and perform standard curing to form concrete slab specimen 1.

[0068] S2: Grind the four sides of the concrete slab specimen 1 to remove surface laitance and impurities, ensuring a smooth base surface. Fix the epoxy coating auxiliary device 60 to the ground sides, prepare epoxy resin with appropriate fluidity, and slowly pour it into the device. After standing for 12 hours, remove the epoxy coating auxiliary device 60. Perform local repairs and smoothing on the epoxy resin sealing layer 26, and check the thickness of the epoxy resin sealing layer 26 to ensure it is greater than 2 mm. When applying epoxy resin to the corners of the concrete slab specimen 1, use the concrete specimen corner epoxy resin coating auxiliary device 70, following the same procedure as above.

[0069] S3: Thread the steel strand 3 into the pre-installed corrugated pipe 10, following the tensioning principle of "first the middle layer, then the upper and lower layers, and symmetrical tensioning at both ends," and perform symmetrical tensioning at both ends in the X and Y directions (e.g., Figure 4 (As shown). After tensioning to the correct position, anchor 2 (working anchor) is used to lock the steel strand 3 to the concrete slab specimen 1, and high-pressure grouting is immediately performed on the corrugated pipe 10 to bond the steel strand 3 to the concrete slab specimen 1 to form a whole.

[0070] S4: On the side of the tensioned concrete slab specimen 1, according to the test requirements, drill holes at the preset positions using a drilling machine, embed thermocouple 13 and humidity sensor 15 into the holes, and seal the holes. The holes of thermocouple 13 are sealed with cement grout, and the holes of humidity sensor 15 are sealed with silicone 14.

[0071] S5: A 20mm thick heat insulation layer 27 is tightly bonded to the outside of the epoxy resin sealing layer 26, and then aluminum foil is tightly wrapped on the outermost layer as an aluminum foil reflective layer 28, thus forming a heat-insulating and moisture-proof boundary. The same heat insulation treatment method is also used for the exposed force gauge 4 and anchor 2, and lubricating oil is applied to prevent rust.

[0072] S6: Place the grouting-completed concrete slab specimen 1 stably on the support frame 29 approximately 600mm above the ground. Turn on the force gauge 4 to transmit the dynamic data of prestress generated by environmental fluctuations and concrete creep to the cloud platform in real time for summary analysis.

[0073] The bidirectional prestressed concrete creep test apparatus 100 and method according to the present invention have the following beneficial effects:

[0074] (1) It breaks through the limitations of traditional uniaxial constant pressure and realizes biaxial real variable stress capture: Traditional concrete creep tests mostly rely on external pressure equipment (such as spring creep meter) to apply and maintain a uniaxial constant load. This invention cleverly integrates prestressing loading with concrete slab specimen 1. Through the combination of "pre-embedded corrugated pipe 10 + high-pressure grouting + force gauge 4 real-time cloud monitoring", it not only realizes the complex biaxial stress state, but also captures the dynamic attenuation and nonlinear oscillation of prestress caused by the alternating fluctuations of temperature and humidity in the natural environment and the long-term shrinkage and creep of the concrete itself at high frequency and with high precision. It truly restores the "variable stress" evolution process of the prestressed structure during its service life.

[0075] (2) Constructing a one-dimensional heat and moisture transfer boundary to eliminate the experimental size effect: Traditional small-sized creep specimens are severely affected by the external environment, and multi-directional heat and mass transfer leads to severe coupling between basic creep and drying creep, making accurate separation difficult. This invention innovatively adopts a process of "side-drilled sensor implantation + three-layer composite boundary protection (epoxy resin moisture insulation, 20mm heat insulation layer 27, aluminum foil reflective layer 28 radiation resistance)" in conjunction with a 600mm suspended support frame 29, which completely cuts off the lateral water and heat exchange of the concrete slab specimen 1. This forces the temperature and moisture transfer inside the concrete slab specimen 1 to only proceed in one dimension along the thickness direction (Z direction), and the side drilling perfectly protects the physical integrity of the upper and lower mass transfer surfaces. This structure successfully simulates the pure physical environment of the deep layers of large-volume concrete structures such as nuclear power plant containment vessels on small-sized specimens, greatly reducing the error caused by the specimen size effect.

[0076] (3) Eliminating the hidden dangers of end-constraint tensile cracking and eccentric compression in traditional tests: In response to the common problem in traditional creep tests (especially the loading of force transmission blocks or the stacking of two blocks), which is prone to strong lateral constraints due to end friction, leading to local tensile cracking failure of the specimen, this invention adopts a "three-layer distribution of prestressed tendons" in the internal structure of the concrete slab specimen 1, and strictly follows the tensioning sequence of "first the middle and then the two sides, and symmetrical tensioning at both ends" in the process. This symmetrical loading mechanism based on the self-balancing of the concrete slab specimen 1 ensures the absolute uniform transmission of bidirectional prestress, eliminates end stress concentration and eccentric compression from the root, and ensures the safety and data reliability of long-cycle creep tests.

[0077] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A bidirectional prestressed concrete creep testing device, characterized in that, include: Concrete slab specimens; Multiple prestressing units are disposed on the concrete slab specimen, bonded to the concrete slab specimen as a whole, and orthogonally distributed in the horizontal plane and distributed in multiple layers in the thickness direction, for applying bidirectional prestress to the concrete slab specimen; The testing unit is used to monitor the changes in prestress, temperature, and humidity of the concrete slab specimen. A boundary protection unit is installed on the side wall of the concrete slab specimen to block lateral heat and moisture exchange, so that the temperature and humidity inside the concrete slab specimen are transmitted only along the thickness direction. A support frame is provided on which the concrete slab specimen is placed to provide suspended support for the concrete slab specimen, allowing ambient air to flow freely over the upper and lower surfaces of the concrete slab specimen.

2. The bidirectional prestressed concrete creep test device according to claim 1, Its features are: The prestressed unit includes: A corrugated pipe is installed inside the concrete slab specimen; The prestressing tendons are inserted inside the corrugated pipe, with both ends extending out of the concrete slab specimen, and the prestressing tendons are subjected to a preset prestress through tensioning. Pressure bearing plates are respectively disposed at both ends of the corrugated pipe, and one side of the pressure bearing plate is connected to the corrugated pipe; Anchors are respectively installed on both sides of the concrete slab specimen, with one side of the anchor abutting against the other side of the bearing pad.

3. The bidirectional prestressed concrete creep test device according to claim 2, characterized in that: in, The multi-layer distribution is a three-layer distribution. The tensioning sequence of the prestressing tendons is to first tension the prestressing tendons of the middle layer, and then tension the prestressing tendons of the upper and lower layers, and to use symmetrical tensioning at both ends.

4. The bidirectional prestressed concrete creep test device according to claim 2, characterized in that: in, The concrete slab specimen is also provided with spiral ribs inside, which are respectively sleeved on both ends of the corrugated pipe, and the side of the spiral ribs near the pressure plate abuts against the pressure plate.

5. The bidirectional prestressed concrete creep test device according to claim 2, Its features are: The test unit includes: At least one force gauge is installed at the portion of the prestressing tendon that extends out of the concrete slab specimen to monitor changes in prestress; At least one thermocouple is installed on the concrete slab specimen to monitor the temperature change; At least one humidity sensor is installed on the concrete slab specimen to monitor humidity changes.

6. The bidirectional prestressed concrete creep test device according to claim 5, characterized in that: in, The force gauge, the thermocouple, and the humidity sensor are connected to a cloud server for real-time uploading of monitoring data.

7. The bidirectional prestressed concrete creep test device according to claim 5, characterized in that: in, A measuring hole is provided on the side wall of the concrete slab specimen. The thermocouple and the humidity sensor are embedded in the measuring hole. The measuring hole where the thermocouple is embedded is sealed with cement grout, and the measuring hole where the humidity sensor is embedded is sealed with silicone.

8. The bidirectional prestressed concrete creep test device according to claim 1, Its features are: The boundary protection unit comprises, from the inside out, the following components: Epoxy resin sealing layer, with a thickness greater than 2mm; Thermal insulation layer, with a thickness greater than 20mm, is made of polyurethane insulation material; Aluminum foil reflective layer.

9. The bidirectional prestressed concrete creep test device according to claim 1, characterized in that: in, The height of the support frame is ≥600mm.

10. A method for testing the creep of biaxial prestressed concrete, based on the biaxial prestressed concrete creep testing apparatus according to any one of claims 1-9, characterized in that, Includes the following steps: Multiple prestressed units are set up so that they are orthogonally distributed in the horizontal plane and multi-layered in the thickness direction. Concrete is poured to bond the prestressed units to the concrete slab specimen as a whole. The boundary protection unit is installed on the side wall of the concrete slab specimen; The test unit is set on the concrete slab specimen to monitor the changes in prestress, temperature and humidity of the concrete slab specimen. The concrete slab specimen is placed on the support frame to provide suspended support for the concrete slab specimen; The test unit is activated to conduct a creep test and collect data.

Images