System and method for testing the flexural performance of wet joints based on uhpc

Abstract

The present application relates to the field of civil engineering structure test and detection technology, especially to a wet joint bending resistance performance test system and method based on UHPC. The spliced test piece is obtained, the initial axial tension target is determined and the axial tension is applied, the constant tension servo follow-up of the horizontal traction device is executed according to the tension feedback, the sinking of the one side support base is controlled, the cross mid anti-bending load is applied, the real-time axial tension, the support base displacement, the loading force and the cross mid deflection are synchronously collected, when the real-time axial tension meets the initial cracking criterion, the tension servo follow-up of the horizontal traction device is stopped and the traction end position thereof is locked, the support base position is kept, the load increment is stopped and the displacement loading is switched. The present application can solve or at least alleviate the problems of axial tension drift caused by fixed end constraint in wet joint bending resistance test and inconvenient continuous collection of residual bending resistance performance data after initial cracking, and provides a wet joint bending resistance performance test system and method based on UHPC.

Description

Technical Field

[0001] This invention relates to the field of civil engineering structural testing and inspection technology, and in particular to a wet joint bending performance testing system and method based on UHPC. Background Technology

[0002] In prefabricated bridge structures, wet joints are typically incorporated during the splicing of bridge decks, widening of existing bridges, connection of new and old bridges, and on-site connection of precast components. These wet joints connect adjacent ordinary concrete components to form a continuous load-bearing structure. Ultra-high performance concrete (UHVPC) possesses high compressive strength, tensile cracking strength, and fiber-reinforced properties. In engineering projects, UHVPC has been used for wet joint pouring to improve the cracking problems of ordinary concrete wet joints under bending, shear, and repeated loading.

[0003] Existing mechanical property testing of wet joints typically employs simply supported beam specimens, plate splice specimens, or partially connected specimens. These specimens are subjected to mid-span loading using a standard testing machine, reaction frame, and vertical actuators, and data such as loading force, mid-span deflection, crack width, or reinforcement strain are collected. Some testing methods also collect vibration data near the wet joint at the bridge widening construction site and obtain the construction vibration response through frequency domain filtering, threshold screening, or Kalman filtering. These testing methods are used for construction process monitoring or structural vibration assessment.

[0004] However, existing flexural strength testing devices typically employ fixed or symmetrical supports at the specimen boundaries. These fixed or symmetrical supports are insufficient to simultaneously represent both the early shrinkage constraint state and the construction misalignment state of ultra-high performance concrete wet joints. If only vertical mid-span loading is used, the stress state of the specimen is mainly controlled by the vertical bending moment, making it difficult to reflect the cracking process of the wet joint under the combined action of axial shrinkage tension, support height difference, and mid-span flexural load.

[0005] Meanwhile, in the existing testing process, if the specimen is fixedly clamped at both ends to simulate early shrinkage constraint, the axial elongation caused by mid-span loading will cause the constraint force at both ends to continuously change with the increase of mid-span deflection. This change in constraint force will affect the stability of the axial constraint tensile force before the initial crack. If the axial constraint tensile force is still used as the criterion for judging the initial crack, it is easy to see a mixture of tensile force drift caused by bending elongation and tensile force drop caused by cracking of wet joints. This mixture will reduce the consistency of the initial crack judgment.

[0006] Furthermore, when using load control for mid-span loading in existing testing machines, the stiffness of the specimen decreases after initial cracking. The vertical loading module may still continue to execute load increment commands, leading to rapid crack propagation and shortened data acquisition time for post-peak load and deflection. For ultra-high performance concrete wet joints containing steel fibers, the load-deflection curve and residual tensile force change curve after initial cracking are used to characterize crack propagation, fiber bridging, and pull-out processes. If a single load increment method is maintained after initial cracking, it is not conducive to continuously obtaining residual flexural performance data.

[0007] Therefore, there is a need to provide a wet joint bending performance testing system and method based on UHPC, which can form a combined loading state of initial axial tensile force, misaligned shear deformation and mid-span bending load during the test, maintain constant tension servo following of the horizontal traction device when the axial length of the specimen changes due to bending at mid-span, and maintain the position of the support base, stop the load increase and switch to displacement loading when the real-time axial tensile force meets the initial cracking criterion, so as to obtain continuous bending performance data of the wet joint before and after the initial cracking. Summary of the Invention

[0008] To achieve the above-mentioned objectives, this invention provides a wet joint bending performance testing system and method based on UHPC, aiming to solve or at least alleviate the problems in existing wet joint bending performance testing, such as single boundary state, axial tensile force drift caused by fixed end constraints, and difficulty in continuously collecting residual bending performance data after initial cracking load control.

[0009] To achieve the above objectives, the present invention provides the following technical solution: a method for testing the flexural strength of wet joints based on UHPC, comprising:

[0010] The spliced ​​specimen containing the wet joint is placed on a left support base and a right support base that are separated from each other; an initial axial tensile force is applied to the spliced ​​specimen by means of horizontal traction devices set at both ends of the spliced ​​specimen;

[0011] Under the condition that the horizontal traction device performs constant tension servo following, the support base on one side is controlled to sink, so that the wet joint forms a misaligned shear deformation.

[0012] While maintaining the misaligned shear deformation, a mid-span bending load is applied to the spliced ​​specimen through a vertical loading module;

[0013] The system synchronously collects real-time axial tensile force, support base displacement, loading force, and mid-span deflection. When the real-time axial tensile force meets the initial crack criterion, it controls the horizontal traction device to stop constant tension servo following and lock its current traction position, maintain the support base position, stop the load increase, and switch to displacement loading to obtain the bending performance data of the wet joint.

[0014] To further realize the present invention, the following technical solutions may be preferred:

[0015] Preferably, the spliced ​​specimen includes ordinary concrete components and ultra-high performance concrete wet joints, and the horizontal traction device is connected to the spliced ​​specimen through a hinged component;

[0016] The step of applying an initial axial tensile force to the spliced ​​specimen through horizontal traction devices located at both ends of the spliced ​​specimen includes: determining the initial axial tensile force target based on the shrinkage strain parameters of the ultra-high performance concrete and the equivalent axial stiffness of the spliced ​​specimen, and controlling the horizontal traction device to load to the initial axial tensile force target.

[0017] Preferably, the real-time tension value of the horizontal traction device is collected by a tension sensor, and the traction end of the horizontal traction device is controlled to perform displacement compensation based on the deviation between the real-time tension value and the initial axial tension target, so that the additional axial tension caused by bending at mid-span is compensated.

[0018] Preferably, under the condition that the horizontal traction device maintains constant tension servo following, the left support base or the right support base is controlled to move vertically, and the height difference between the left support base and the right support base is maintained after the movement is completed.

[0019] Preferably, the vertical loading module is controlled to apply a mid-span force to the spliced ​​specimen in a load-controlled manner, and the mid-span deflection of the spliced ​​specimen is collected by a displacement sensor.

[0020] Preferably, the real-time tension value of the horizontal traction device is taken as the real-time axial tension, the vertical position of the left support base and the right support base is taken as the support base displacement, the output force of the vertical loading module is taken as the loading force, and the output value of the displacement sensor is taken as the mid-span deflection.

[0021] Preferably, the decrease in the real-time axial tension within the preset sampling window reaches a preset decrease threshold, or the decrease rate of the real-time axial tension within the preset sampling window reaches a preset decrease rate threshold.

[0022] Preferably, the control module issues a position locking command to the horizontal traction device to maintain the current horizontal displacement of its traction end, keeps the vertical position of the left support base and the right support base when the real-time axial tensile force meets the initial crack criterion, stops the load increment command of the vertical loading module, and continues displacement loading with the mid-span deflection at the switching moment as the relative displacement starting point.

[0023] A UHPC-based wet joint bending performance testing system is provided for performing the above-mentioned method. The testing system includes a differential support module, a horizontal traction module, a vertical loading module, a sensing module, and a control module. The differential support module includes a left support base and a right support base that are separate from each other and can be raised and lowered independently. The horizontal traction module is disposed at both ends of the spliced ​​specimen, and its traction end is provided with a hinge component between itself and the spliced ​​specimen to release rotational freedom. The vertical loading module is disposed above the spliced ​​specimen. The sensing module is used to collect real-time axial tensile force, support base displacement, loading force, and mid-span deflection. The control module is used to control the differential support module, the horizontal traction module, and the vertical loading module.

[0024] Preferably, the control module includes a tension servo control unit and an interlocking switching control unit. The tension servo control unit is used to control the horizontal traction module to perform constant tension servo following based on the real-time axial tension collected by the sensing module. The interlocking switching control unit is used to control the horizontal traction module to stop servo following and lock the current traction position when the real-time axial tension meets the initial crack criterion, control the differential support module to maintain its position, and control the vertical loading module to stop load increment and switch to displacement loading.

[0025] The beneficial effects of this invention are:

[0026] This invention enables spliced ​​specimens containing UHPC wet joints to be tested under the combined action of initial axial tension, misaligned shear deformation, and mid-span bending load through the cooperation of a horizontal traction module, a differential support module, and a vertical loading module. During the test, the horizontal traction module performs constant tension servo following, which compensates for the additional axial tension caused by mid-span bending and allows the real-time axial tension to be used as input data for the initial crack criterion.

[0027] Meanwhile, when the real-time axial tensile force meets the initial crack criterion, the present invention controls the left and right support bases to maintain their vertical positions, controls the vertical loading module to stop the load increase and switches to displacement loading, so that the spliced ​​specimen continues to undergo residual loading after the initial crack while maintaining the misaligned shear deformation, and obtains the initial crack load, peak load, residual bearing capacity, load-deflection curve and residual tensile force change curve. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the test system structure of the present invention.

[0029] Figure 2 This is a flowchart of the testing method of the present invention.

[0030] Figure 3 This is a schematic diagram of the constant tension servo following control of the present invention.

[0031] Figure 4 This is a schematic diagram of the initial crack criterion and interlocking switching timing of the present invention.

[0032] Figure 5 This is a schematic diagram of the spliced ​​specimen and differential support working conditions of the present invention.

[0033] Reference numerals: 100, spliced ​​specimen; 110, ordinary concrete component; 120, UHPC wet joint; 200, differential support module; 210, left support base; 220, right support base; 300, horizontal traction module; 310, servo traction cylinder; 320, traction clamp; 330, tension sensor; 400, vertical loading module; 410, vertical actuator; 420, loading head; 430, loading force sensor; 440, displacement sensor; 500, control module. Detailed Implementation

[0034] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set," "install," "connect," and "link" 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 or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1

[0037] like Figure 1As shown, this embodiment provides a wet joint bending performance testing system based on UHPC. The testing system includes a reaction frame, a differential support module 200, a horizontal traction module 300, a vertical loading module 400, a sensing module, and a control module 500. The reaction frame is used to install the vertical loading module 400 and bear the reaction force during the test. The differential support module 200 is located below the reaction frame. The horizontal traction modules 300 are respectively located at both ends of the spliced ​​specimen 100. The vertical loading module 400 is located above the spliced ​​specimen 100. The sensing modules are respectively located on the differential support module 200, the horizontal traction module 300, and the vertical loading module 400. The control module 500 is connected to the differential support module 200, the horizontal traction module 300, the vertical loading module 400, and the sensing modules.

[0038] The spliced ​​specimen 100 includes ordinary concrete components 110 and ultra-high performance concrete wet joints. The ordinary concrete components 110 are respectively disposed on both sides of the ultra-high performance concrete wet joints. The ultra-high performance concrete wet joints are formed by casting UHPC material. The ordinary concrete components 110 and the ultra-high performance concrete wet joints can be connected by roughening the interface, reserving reinforcement, shear keys, or pre-embedded connectors. The spliced ​​specimen 100 can be a transverse spliced ​​specimen 100 of bridge deck or a longitudinal spliced ​​specimen 100 of bridge deck.

[0039] Specifically, the differential support module 200 includes a left support base 210 and a right support base 220, which are separately arranged and have independent lifting degrees of freedom. The left support base 210 and the right support base 220 are respectively mounted on the bottom bearing seat through vertical guide members. The vertical guide members can be heavy-duty linear guides, hydraulic guide columns, or dovetail guides. The left support base 210 and the right support base 220 are respectively connected to independent lifting drive members, which are used to drive the corresponding support base to move vertically.

[0040] Furthermore, support rollers or support pads are respectively provided on the left support base 210 and the right support base 220. The support rollers or support pads are used to support the lower surface of the spliced ​​specimen 100. The axis of the support roller is set along the width direction of the spliced ​​specimen 100. The upper surface of the support pad can be provided with a wear-resistant steel plate or a replaceable shim. No rigid synchronous beam is provided between the left support base 210 and the right support base 220 so that the left support base 210 and the right support base 220 can form a relative height difference.

[0041] In one specific embodiment, the differential support module 200 further includes a position sensor and a position holding component. The position sensor is used to acquire the vertical position of the left support base 210 and the right support base 220. The position sensor can be a grating ruler, a magnetic grating ruler, or a servo driver with a built-in encoder. The position holding component is used to maintain the current position of the left support base 210 and the right support base 220 when the control module 500 outputs a position holding command. The position holding component can be an electromagnetic brake, a hydraulic locking valve, a mechanical wedge mechanism, or a servo holding brake.

[0042] like Figure 1 As shown, the horizontal traction module 300 includes horizontal traction devices respectively disposed at both ends of the spliced ​​specimen 100. Each horizontal traction device includes a servo traction cylinder 310, a traction clamp 320, and a tension sensor 330. The servo traction cylinder 310 is fixed to the reaction frame or independent traction seat. The traction clamp 320 is disposed at the traction end of the servo traction cylinder 310, and a hinge component such as a pin or universal ball joint is provided between the traction clamp 320 and the traction end of the servo traction cylinder 310 or the end of the spliced ​​specimen 100. The hinge component allows the end of the spliced ​​specimen 100 to undergo a corresponding angular deflection when a relative height difference is formed between the left support base 210 and the right support base 220, thereby avoiding unexpected rigid shear damage or breakage of the specimen end due to specimen tilting to the servo traction cylinder 310. The traction clamp 320 is used to connect with the pre-embedded steel bar, end steel plate, anchor seat or clamping block at the end of the spliced ​​specimen 100, and the tension sensor 330 is connected in series between the servo traction cylinder 310 and the traction clamp 320.

[0043] The horizontal traction device is used to apply axial tension to the spliced ​​specimen 100, and to perform constant tension servo following when the spliced ​​specimen 100 experiences support misalignment or mid-span bending. The constant tension servo following means that the control module 500 controls the traction end of the servo traction cylinder 310 to perform retraction compensation or extension compensation based on the deviation between the real-time tension value collected by the tension sensor 330 and the initial axial tension target, so that the tension feedback value of the horizontal traction device is kept within the allowable range corresponding to the initial axial tension target.

[0044] like Figure 3As shown, the constant tension servo following includes tension feedback input, target tension input, deviation calculation, traction end displacement compensation, and tension feedback update. The control module 500 receives the real-time tension value output by the tension sensor 330, compares the real-time tension value with the initial axial tension target, and outputs a displacement compensation command to the servo traction cylinder 310 based on the comparison result. When the real-time tension value is higher than the allowable range corresponding to the initial axial tension target, the traction end of the servo traction cylinder 310 performs outward extension force relief compensation. When the real-time tension value is lower than the allowable range corresponding to the initial axial tension target, the traction end of the servo traction cylinder 310 performs retraction tension compensation.

[0045] The vertical loading module 400 includes a vertical actuator 410, a loading head 420, a loading force sensor 430, and a displacement sensor 440. The vertical actuator 410 is installed on the top crossbeam of the reaction frame. The loading head 420 is located at the lower end of the vertical actuator 410. The loading force sensor 430 is located between the vertical actuator 410 and the loading head 420. The displacement sensor 440 is located on the loading head 420, at the mid-span position of the spliced ​​specimen 100, or on an independent measuring bracket. The displacement sensor 440 is used to collect the mid-span deflection of the spliced ​​specimen 100.

[0046] Accordingly, the loading head 420 can be a single-point loading head 420 or a double-point loading head 420 formed by the distribution beam and two loading rollers; when the loading head 420 is a single-point loading head 420, the test system forms a three-point bending loading mode; when the loading head 420 is a double-point loading head 420 formed by the distribution beam and two loading rollers, the test system forms a four-point bending loading mode. The vertical actuator 410 has a load control mode and a displacement control mode. The load control mode is used for mid-span bending loading before the real-time axial tensile force meets the initial crack criterion, and the displacement control mode is used for residual loading after the real-time axial tensile force meets the initial crack criterion.

[0047] The sensing module includes a tension sensor 330, a support base position sensor, a loading force sensor 430, and a displacement sensor 440. The tension sensor 330 outputs the real-time tension value of the horizontal traction device. The support base position sensor outputs the vertical positions of the left support base 210 and the right support base 220. The loading force sensor 430 outputs the loading force of the vertical loading module 400. The displacement sensor 440 outputs the mid-span deflection of the spliced ​​specimen 100. The sensing module may also include a crack gauge, strain gauge, acoustic emission sensor, or digital image acquisition device, which are used as auxiliary recording components.

[0048] The control module 500 includes an industrial control computer, a real-time controller, a data acquisition unit, a tension servo control unit, and an interlocking switching control unit. The industrial control computer is used to set test parameters, display acquired data, and store test records. The real-time controller is used to execute differential support control, horizontal traction control, and vertical loading control. The data acquisition unit is used to acquire data output by the sensing module. The tension servo control unit is used to control the horizontal traction device to perform constant tension servo following. The interlocking switching control unit is used to control the horizontal traction device to stop constant tension servo following and lock its current position at the traction end when the real-time axial tensile force meets the initial crack criterion, control the differential support module 200 to maintain its position, and control the vertical loading module 400 to stop load increment and switch loading modes.

[0049] Example 2

[0050] like Figure 2 As shown, this embodiment provides a method for testing the bending resistance of wet joints based on UHPC. The testing method includes specimen placement, initial axial tensile force target determination, axial tensile force application, constant tension servo following, differential support sinking, mid-span bending loading, initial crack determination, support base position maintenance, load increment stopping, and displacement loading switching.

[0051] A spliced ​​specimen 100 comprising a conventional concrete component 110 and a wet joint of ultra-high performance concrete is obtained. The spliced ​​specimen 100 is placed on the left support base 210 and the right support base 220, which are separated from each other at the bottom of the testing system. Before placing the spliced ​​specimen 100, the support positions on the lower surface of the spliced ​​specimen 100 are marked, the traction connection at the end of the conventional concrete component 110 is inspected, and the longitudinal centerline of the spliced ​​specimen 100 is aligned with the loading centerline of the vertical loading module 400.

[0052] Specifically, after the spliced ​​specimen 100 is placed on the left support base 210 and the right support base 220, the left support base 210 and the right support base 220 are initially at the same vertical height. The traction clamps 320 of the horizontal traction device are respectively connected to both ends of the spliced ​​specimen 100. The loading head 420 of the vertical loading module 400 is located above the mid-span of the spliced ​​specimen 100. The probe of the displacement sensor 440 is in contact with or aligned with the mid-span measuring point of the spliced ​​specimen 100.

[0053] The initial axial tensile force target is determined based on the shrinkage strain parameters of the ultra-high performance concrete (UHPC), and an axial tensile force is applied to the spliced ​​specimen 100 by a horizontal traction device connected by a hinged component and located at both ends of the specimen. The shrinkage strain parameters can be obtained from shrinkage tests of the same batch of UHPC materials, or from material mix design documents, curing condition records, or pre-test calibration data. The initial axial tensile force target is determined according to the shrinkage strain parameters and the equivalent axial stiffness of the spliced ​​specimen 100.

[0054] In one specific embodiment, the initial axial tensile force target is determined according to the following formula: ,in, As the initial axial tensile force target, The equivalent elastic modulus of the spliced ​​specimen 100 in the axial tensile direction is given by [the relevant parameter]. This refers to the equivalent tensile area of ​​the spliced ​​specimen 100. The shrinkage strain parameter of the ultra-high performance concrete is given.

[0055] Furthermore, before the horizontal traction device applies axial tension to the spliced ​​specimen 100, the tension sensor 330 performs zero-point calibration, the servo traction cylinder 310 performs no-stroke check, and after the traction clamp 320 is connected to the end of the spliced ​​specimen 100, the control module 500 controls the horizontal traction devices at both ends of the spliced ​​specimen 100 to load synchronously until the feedback value of the tension sensor 330 enters the allowable range corresponding to the initial axial tension target.

[0056] Under the condition that the horizontal traction device performs constant tension servo following based on the tension feedback, the support base on one side is controlled to sink relative to the support base on the other side, so that misaligned shear deformation is formed at the wet joint. After the horizontal traction device enters constant tension servo following, the control module 500 does not fix or lock the traction end of the horizontal traction device, but controls the traction end to perform micro-displacement compensation based on the feedback value of the tension sensor 330.

[0057] Specifically, the displacement compensation at the traction end can be performed according to the following formula: ,in, This refers to the compensation displacement of the traction end of the horizontal traction device. For force difference compensation coefficient, The real-time tension value collected by the tension sensor 330 The initial axial tensile force target is defined as follows.

[0058] like Figure 5 As shown, the control module 500 controls the left support base 210 or the right support base 220 to sink vertically, creating a relative height difference between the two supports. This relative height difference causes misaligned shear deformation at the wet joint of the spliced ​​specimen 100. During the sinking of one support base, the position sensor continuously collects the vertical positions of the left support base 210 and the right support base 220, and the horizontal traction device continues to perform constant tension servo following based on the feedback value from the tension sensor 330.

[0059] While maintaining the misaligned shear deformation, a mid-span bending load is applied to the spliced ​​specimen 100 via the vertical loading module 400. After one side support base sinks to a preset position, the control module 500 maintains the relative height difference between the left support base 210 and the right support base 220. The loading head 420 of the vertical loading module 400 moves down to contact the upper surface of the spliced ​​specimen 100, and the loading force sensor 430 and the displacement sensor 440 perform zero-point processing before loading.

[0060] Correspondingly, the vertical loading module 400 applies a mid-span force to the spliced ​​specimen 100 in a load control manner. The load control manner can be a continuous load increase or a graded load increase. During the application of the mid-span force by the vertical loading module 400, the control module 500 simultaneously collects the real-time axial tensile force, the displacement of the support base, the loading force, and the mid-span deflection.

[0061] In this embodiment, real-time axial tensile force, support base displacement, loading force, and mid-span deflection are simultaneously acquired. When the real-time axial tensile force meets the initial crack criterion, the horizontal traction device is controlled to stop constant tension servo following and lock its current traction end position, maintain the support base position, stop load increase, and switch to displacement loading to obtain real-time axial tensile force and bending performance data after the initial crack of the wet joint. The real-time axial tensile force is the real-time tensile force value of the horizontal traction device, the support base displacement is the vertical position of the left support base 210 and the right support base 220, the loading force is the output force of the vertical loading module 400, and the mid-span deflection is the output value of the displacement sensor 440.

[0062] like Figure 4 As shown, the real-time axial tensile force is within the allowable range corresponding to the initial axial tensile force target before the initial crack of the spliced ​​specimen 100. It should be noted that when microcracks initiate and the initial crack response occurs at the wet joint, the rapid expansion of the crack at the moment of cracking causes a sudden change in the local axial elongation of the lower edge of the specimen. This sudden elongation rate instantaneously exceeds the physical compensation response limit of the servo system in the horizontal traction device—that is, the inherent mechanical response hysteresis characteristic of the equipment. This causes the traction device to be unable to perform displacement compensation in time, thus inevitably resulting in a momentary real-time axial tensile force drop under macroscopic constant tension control. This invention utilizes the unavoidable physical response hysteresis of the testing machine, transforming it into a highly sensitive microcrack probe. At this time, the real-time axial tensile force shows a change in the amount or rate of decrease within the preset sampling window. The control module 500 determines whether the real-time axial tensile force meets the initial crack criterion based on the amount or rate of decrease.

[0063] In one specific embodiment, the rate of decrease of the real-time axial tensile force is determined according to the following formula: Where RF is the real-time axial tensile force descent rate. This represents the real-time axial tension at the current sampling moment. The real-time axial tension at the start of the preset sampling window. The time length corresponding to the preset sampling window.

[0064] Specifically, when the decrease in real-time axial tensile force within a preset sampling window reaches a preset decrease threshold, or when the decrease rate of real-time axial tensile force within the preset sampling window reaches a preset decrease rate threshold, the control module 500 determines that the real-time axial tensile force meets the initial crack criterion. The preset sampling window, the preset decrease threshold, and the preset decrease rate threshold can be set according to the range of the tensile sensor 330, the size of the spliced ​​specimen 100, the type of wet joint material, and the loading method of the vertical loading module 400.

[0065] Furthermore, when the real-time axial tensile force meets the initial crack criterion, the control module 500 outputs a servo stop and position lock command to the horizontal traction device, causing it to stop constant tension servo following and fix the current traction end position; simultaneously, the control module 500 outputs a position hold command to the differential support module 200, causing the left support base 210 and the right support base 220 to remain in the vertical position when the real-time axial tensile force meets the initial crack criterion; the control module 500 outputs a load increase stop command to the vertical loading module 400, causing the vertical loading module 400 to stop the load increase under the load control mode; the control module 500 switches the vertical loading module 400 from the load control mode to the displacement control mode, and continues displacement loading with the mid-span deflection at the switching moment as the relative displacement starting point.

[0066] During the displacement loading process, the vertical loading module 400 continues to apply vertical displacement to the spliced ​​specimen 100 according to a preset displacement step size or a preset displacement rate. The control module 500 continues to collect the loading force, the mid-span deflection, and the residual tensile force until the spliced ​​specimen 100 reaches a preset termination condition. The preset termination condition may be that the loading force decreases to a preset proportion of the peak load, the mid-span deflection reaches the test limit, the crack width reaches the test limit, or the spliced ​​specimen 100 fails entirely.

[0067] The bending performance data includes initial crack load, initial crack mid-span deflection, peak load, peak mid-span deflection, residual bearing capacity, load-deflection curve, and residual tensile force variation curve. The load-deflection curve is formed by the loading force and the mid-span deflection. The residual tensile force variation curve is formed by the residual tensile force and the sampling time. The initial crack load and the initial crack mid-span deflection are determined by the loading force and mid-span deflection corresponding to when the real-time axial tensile force satisfies the initial crack criterion.

[0068] Example 3

[0069] This embodiment verifies the test system described in Embodiment 1 and the test method described in Embodiment 2. The prototype consists of a reaction frame, a differential support module 200, a horizontal traction module 300, a vertical loading module 400, a sensing module, and a control module 500. The differential support module 200 uses two independent lifting support platforms. The horizontal traction module 300 uses two servo traction cylinders 310 and a spoke-type tension sensor 330. The vertical loading module 400 uses an electro-hydraulic servo actuator, a loading force sensor 430, and a mid-span displacement gauge. The control module 500 uses an industrial control computer and a real-time controller.

[0070] The specimens consisted of ordinary concrete end members and a central UHPC wet joint 120. The interfaces of the ordinary concrete end members were roughened, and the UHPC wet joint 120 was made of steel fiber reinforced material. After curing, the specimens were numbered, dimensionally measured, and end clamps were installed. The test log recorded the initial axial tensile target, the height difference of the support base, the vertical loading rate, the sampling frequency, the initial crack detection time, and the displacement loading termination status.

[0071] Under the baseline condition of no support misalignment and no initial axial tensile force applied, the spliced ​​specimen 100 developed a single main crack during mid-span bending loading. The real-time axial tensile force channel was not used as the basis for initial crack determination. The test results were used to obtain the load-deflection curve under ordinary bending loading conditions.

[0072] Under the condition of applying initial axial tensile force and not sinking the support base, the horizontal traction module 300 maintains constant tension servo following, and the vertical loading module 400 performs mid-span bending load. It is detected that the real-time axial tensile force is in a small fluctuation state before cracking, and it is determined that the constant tension servo following can compensate for the additional axial tensile force caused by mid-span bending.

[0073] Under the condition of applying initial axial tensile force and sinking of the unilateral support base, misaligned shear deformation is formed at the wet joint. Before the initial crack is loaded by the vertical loading module 400, the real-time axial tensile force shows a decrease in both the amount and rate of decrease exceeding a preset threshold within the sampling window. The control module 500 outputs a support base position holding command and a vertical loading module 400 switching command.

[0074] Under the residual loading condition after the displacement loading switch, the vertical loading module 400 continues to load with the mid-span deflection at the switching moment as the relative displacement starting point. The loading force decreases in a stepwise manner as the mid-span deflection increases, and the residual tensile force decreases in segments in sync. It is determined that the specimen has entered the crack propagation and fiber pull-out stage.

[0075] In a set of scaled-down specimens, the initial axial tensile force target was set at 38.5 kN, and the tensile force fluctuation range during the stabilization phase before loading was 37.9 kN to 39.2 kN. During the mid-span load increase phase, the real-time axial tensile force was maintained between 38.0 kN and 39.6 kN before the initial crack, and the mid-span deflection increased continuously with the increase of the loading force.

[0076] Under the condition that the single-sided support base sinks by 5.0 mm, the stable height difference recorded by the support base position sensor fluctuates between 4.96 mm and 5.04 mm; when the mid-span loading force reaches around 76.8 kN, the real-time axial tensile force decreases from 38.7 kN to 34.9 kN within a 0.08 s sampling window, and the descent rate reaches the preset descent rate threshold. The control module 500 records the initial crack trigger time.

[0077] After switching the displacement loading, the peak loading force appeared between 78.1 kN and 79.4 kN. The residual bearing capacity fluctuated and decreased with the increase of mid-span deflection. At the final termination, the recorded mid-span deflection was 18.6 mm, and the loading force decreased to approximately 42% to 46% of the peak load. In the same batch of three specimens, after the initial crack was triggered, the mid-span deflection increased further by approximately 1.5 mm to 3.2 mm before reaching the peak load as the displacement loading continued. This crack propagation and fiber pull-out process lasted approximately 15 to 45 seconds. The test data show that the system successfully avoided instantaneous brittle fracture after the specimen cracked, effectively obtained continuous post-cracking flexural mechanical characteristics, and the initial crack determination data eliminated fluctuations caused by sensor noise and micro-slippage of the clamps.

[0078] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for testing the flexural strength of wet joints based on UHPC, characterized in that, include: The spliced ​​specimen (100) containing the wet joint was placed on the mutually separated left support base (210) and right support base (220); An initial axial tensile force is applied to the spliced ​​specimen (100) by means of horizontal traction devices located at both ends of the spliced ​​specimen (100); Under the condition that the horizontal traction device performs constant tension servo following, the support base on one side is controlled to sink, so that the wet joint forms a misaligned shear deformation. While maintaining the misaligned shear deformation, a mid-span bending load is applied to the spliced ​​specimen (100) through the vertical loading module (400); The system synchronously collects real-time axial tensile force, support base displacement, loading force, and mid-span deflection. When the real-time axial tensile force meets the initial crack criterion, it controls the horizontal traction device to stop constant tension servo following and lock its current traction position, maintain the support base position, stop the load increase, and switch to displacement loading to obtain the bending performance data of the wet joint.

2. The method according to claim 1, characterized in that, The spliced ​​specimen (100) includes a common concrete component (110) and a wet joint of ultra-high performance concrete, and the horizontal traction device is connected to the spliced ​​specimen (100) through a hinged component. The initial axial tensile force target is determined based on the shrinkage strain parameters of the ultra-high performance concrete and the equivalent axial stiffness of the spliced ​​specimen (100), and the horizontal traction device is controlled to be loaded to the initial axial tensile force target.

3. The method according to claim 1, characterized in that, The real-time tension value of the horizontal traction device is collected by the tension sensor (330). The displacement compensation of the traction end of the horizontal traction device is controlled according to the deviation between the real-time tension value and the initial axial tension target, so that the axial additional tension caused by bending at the mid-span is compensated.

4. The method according to claim 1, characterized in that, Under the condition that the horizontal traction device maintains constant tension servo following, the left support base (210) or the right support base (220) is controlled to move vertically, and the height difference between the left support base (210) and the right support base (220) is maintained after the movement is completed.

5. The method according to claim 1, characterized in that, The vertical loading module (400) is controlled to apply a mid-span force to the spliced ​​specimen (100) in a load-controlled manner, and the mid-span deflection of the spliced ​​specimen (100) is collected by the displacement sensor (440).

6. The method according to claim 1, characterized in that, The real-time tension value of the horizontal traction device is taken as the real-time axial tension, the vertical position of the left support base (210) and the right support base (220) is taken as the support base displacement, the output force of the vertical loading module (400) is taken as the loading force, and the output value of the displacement sensor (440) is taken as the mid-span deflection.

7. The method according to claim 1, characterized in that, The decrease in the real-time axial tension within the preset sampling window reaches a preset decrease threshold, or the decrease rate of the real-time axial tension within the preset sampling window reaches a preset decrease rate threshold.

8. The method according to claim 1, characterized in that, The control module (500) issues a position locking command to the horizontal traction device to maintain the current horizontal displacement of its traction end, keep the left support base (210) and the right support base (220) in the vertical position when the real-time axial tensile force meets the initial crack criterion, stop the load increment command of the vertical loading module (400), and continue displacement loading with the mid-span deflection at the switching moment as the relative displacement starting point.

9. A wet joint bending performance testing system based on UHPC, characterized in that, The testing system for performing the method according to any one of claims 1 to 8 includes a differential support module (200), a horizontal traction module (300), a vertical loading module (400), a sensing module, and a control module (500). The differential support module (200) includes a left support base (210) and a right support base (220) that are separated from each other and can be raised and lowered independently. The horizontal traction module (300) is disposed at both ends of the spliced ​​specimen (100), and its traction end is provided with a hinge component for releasing the rotational degree of freedom between it and the spliced ​​specimen (100). The vertical loading module (400) is disposed above the spliced ​​specimen (100). The sensing module is used to collect real-time axial tension, support base displacement, loading force, and mid-span deflection. The control module (500) is used to control the differential support module (200), the horizontal traction module (300), and the vertical loading module (400).

10. The system according to claim 9, characterized in that, The control module (500) includes a tension servo control unit and an interlocking switching control unit. The tension servo control unit is used to control the horizontal traction module (300) to perform constant tension servo following based on the real-time axial tension collected by the sensing module. The interlocking switching control unit is used to control the horizontal traction module (300) to stop servo following and lock the current traction position when the real-time axial tension meets the initial crack criterion, control the differential support module (200) to maintain the position, and control the vertical loading module (400) to stop the load increase and switch to displacement loading.

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