Comprehensive evaluation method and device for shrinkage cracking performance of 3D printing UHPC (Ultra High Performance Concrete)

Abstract

The invention provides a comprehensive evaluation method and device for the shrinkage cracking performance of 3D printing UHPC, and the method comprises the following steps: S1, preparing a UHPC material, and testing the final setting time T1; s2, molding shrinkage test: taking T1 as a zero point to obtain first shrinkage data for evaluating the shrinkage performance of the UHPC material; s3, 3D printing shrinkage test: taking the initial reading time T2 of the displacement measurement device as a zero point to obtain second shrinkage data for evaluating the shrinkage performance of the UHPC material of the 3D printing process; s4, constraint cracking testing: after the UHPC is printed and formed according to a preset path, constraint is applied in the printing direction, and cracking data are obtained; and S5, comprehensive evaluation: comprehensively evaluating the shrinkage cracking risk of the 3D printing UHPC material based on the first shrinkage data, the second shrinkage data and the cracking data. The technical problem that the shrinkage and cracking performance of the 3D printing ultra-high performance concrete (UHPC) cannot be accurately and comprehensively evaluated by an existing concrete shrinkage and cracking test method is solved.

Description

Technical Field

[0001] This invention relates to the technical field of concrete testing, and in particular to a comprehensive evaluation method and apparatus for the shrinkage cracking performance of 3D-printed UHPC. Background Technology

[0002] Ultra-high performance concrete (UHPC) has broad application prospects in the field of 3D printing construction due to its high strength, high toughness and excellent durability. However, due to the characteristics of UHPC, such as low water-cement ratio, large amount of cementitious materials and no coarse aggregate, its early self-shrinkage phenomenon is significant. Coupled with the coupling effect of chemical shrinkage and drying shrinkage, the risk of cracking is greatly increased, which directly affects the safety and durability of the structure.

[0003] Currently, methods and devices for evaluating the shrinkage and crack resistance of concrete (including chemical shrinkage tests and constrained shrinkage tests) are mainly aimed at traditional casting processes. However, 3D printed UHPC is formed by extrusion and layer-by-layer stacking. Its process principle, material structure and internal interface are fundamentally different from traditional casting. Therefore, existing testing methods based on casting processes cannot accurately evaluate the shrinkage behavior and cracking performance of 3D printed UHPC.

[0004] In summary, there is an urgent need to develop a multi-dimensional comprehensive evaluation method and device that integrates molding shrinkage testing, 3D printing shrinkage testing, and constraint cracking testing, in order to accurately assess the shrinkage deformation and cracking risk of UHPC under 3D printing process, and to provide a reliable basis for material selection and process optimization. Summary of the Invention

[0005] The purpose of this invention is to provide a comprehensive evaluation method and apparatus for the shrinkage and cracking performance of 3D-printed UHPC, which solves the technical problem that existing concrete shrinkage and cracking test methods cannot accurately and comprehensively evaluate the shrinkage and cracking performance of 3D-printed ultra-high performance concrete (UHPC).

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: This invention provides a comprehensive evaluation method for the shrinkage and cracking performance of 3D-printed UHPC, comprising the following steps: S1. Material preparation: preparing UHPC material and testing the final setting time T1 of the UHPC material for conducting molding shrinkage tests, 3D printing shrinkage tests, and constraint cracking tests, respectively; S2. Molding shrinkage test: casting the first part of the UHPC material into a mold with a built-in strain monitoring component, using the final setting time T1 as the data recording zero point, and obtaining the first shrinkage data for evaluating the shrinkage performance of the UHPC material itself; S3. 3D printing shrinkage test: molding the second part of the UHPC material into a solid specimen using a 3D printing device, curing and hardening it, and cutting it into standard specimens. Monitoring the standard specimens using an externally clamped displacement measuring device, using the first shrinkage time T1 as the data recording zero point, and obtaining the first shrinkage data for evaluating the shrinkage performance of the UHPC material itself; The initial reading time T2 of the displacement measuring device is the zero point for data recording. The second shrinkage data is obtained to evaluate the shrinkage performance of the 3D printed UHPC. S4. Constraint cracking test: The UHPC material described in the third part is formed into a cracking test specimen by a 3D printing device using a preset path. The cracking test specimen is constrained along the printing direction, and the surface of the 3D printed cracking test specimen is continuously observed to obtain the cracking data. S5. Comprehensive evaluation: Based on the first shrinkage data, the second shrinkage data, and the cracking data, the shrinkage cracking risk of the 3D printed UHPC is comprehensively evaluated. Among them, the first shrinkage data is used to compensate for the lack of shrinkage data of the second shrinkage data during T1 to T2, so as to judge the ultra-early shrinkage performance of the 3D printed UHPC.

[0007] Furthermore, in S2, the mold is a cuboid mold, and the long side of the cuboid mold is provided with a through hole for fixing the strain monitoring component. The strain monitoring component is positioned at the center of the mold through the through hole. The inner wall of the mold is provided with a release agent. The UHPC material is demolded 24 hours after molding.

[0008] Furthermore, in S4, the preset path is a loop-shaped printing path, and the 3D printed crack test specimen has a loop-shaped structure extending along the length direction, and adjacent printing strips are in contact or slightly overlapping.

[0009] Furthermore, S4 also includes an environmental simulation step: simulating a dry environment to accelerate the drying and shrinkage of 3D printed crack test specimens.

[0010] Furthermore, the S4 observation period is 1 to 3 months, and the observation content includes the number, length, width and location information of cracks.

[0011] The present invention also provides a comprehensive evaluation device for the shrinkage and cracking performance of 3D printed UHPC, comprising: a molding shrinkage test module, including a mold with a built-in strain monitoring component and a data acquisition system connected to the strain monitoring component; a printing shrinkage test module, including a displacement measuring device; and a constraint cracking test module, including a base, a counterweight block disposed on the side of the base, and an anchoring bracket detachably mounted on the base by fasteners.

[0012] Furthermore, the strain monitoring component in the molded shrinkage test module is an embedded vibrating wire strain gauge.

[0013] Furthermore, the displacement measuring device includes a dial indicator and a clamp for fixing the dial indicator; The fixture includes at least two pairs of L-shaped metal components, which are connected by connectors to circumferentially clamp the standard test piece. The L-shaped metal components are provided with mounting parts for fixing the dial indicator dial and the dial indicator extension rod.

[0014] Furthermore, the anchoring bracket is an insert-type anchoring bracket used to provide end constraints for the 3D printed crack test specimen; the insert-type anchoring bracket is provided with multiple exposed anchor bars at intervals along the height direction, and the anchor bars are threaded steel bars; the counterweight is a counterweight with a handle.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: This invention addresses the process characteristics of 3D printing extrusion molding and strip layer-by-layer stacking, constructing a multi-dimensional, comprehensive evaluation system. Through a three-dimensional system—"molding shrinkage testing to capture material shrinkage, 3D printing testing to reflect process influences, and constraint cracking testing to simulate actual working conditions"—it comprehensively covers key factors influencing shrinkage and cracking. This system places material properties, process effects, and structural constraint behavior within a unified framework for correlation analysis, achieving a root cause analysis and comprehensive evaluation of shrinkage and cracking risks in 3D-printed UHPCs. This provides a direct and comprehensive basis for material selection, process optimization, and crack-resistant design.

[0016] This invention innovatively solves the problem of difficult monitoring of early shrinkage in 3D-printed solid specimens, enabling monitoring of the complete shrinkage process from final setting. Addressing the deficiency that 3D-printed specimens must be hardened and cut before measuring instruments can be installed, resulting in a data gap between final setting and measurable shrinkage (T1 to T2), this invention utilizes continuously collected molding shrinkage data starting from final setting time T1 to compensate for the data deficiency in this critical early stage. This data complementarity mechanism makes it possible to evaluate the complete shrinkage development process of 3D-printed UHPC components from final setting, particularly enabling quantitative assessment of their ultra-early shrinkage performance.

[0017] The testing device provided by this invention adopts a modular and detachable design, and the various testing modules can be flexibly combined, combining specialization and economy. In the core constraint cracking testing module, except for the anchor bracket and anchor bar, the remaining components can be disassembled and reused, reducing the cost of testing equipment; in addition, the blower and heater can simulate complex environments such as drying and wind blowing, and the test results are more in line with engineering practice. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of the self-shrinkage test method for molded UHPC specimens provided in this embodiment; Figure 2 This is a schematic diagram of the 3D printed UHPC cut specimen shrinkage testing device provided in this embodiment; Figure 3 This is a schematic diagram of the 3D printed UHPC fixing fixture provided in this embodiment; Figure 4 This is a schematic diagram of the specimen used for constrained cracking testing of a 3D-printed UHPC provided in this embodiment. Figure 5 This is a schematic diagram of the overall structure of the 3D-printed UHPC constrained cracking test device provided in this embodiment. Figure 6 This is a schematic diagram of the anchor frame structure of the 3D printed UHPC constraint cracking test device provided in this embodiment; Figure 7 Shrinkage curves of two UHPC specimens molded in this embodiment; Figure 8 Shrinkage curves of two UHPC specimens 3D printed in this embodiment; Figure 9 This is a schematic diagram of the surface crack condition of two UHPC specimens 3D printed in this embodiment under constraint cracking test.

[0020] Figure label: 1-Mold; 2-Strain monitoring component; 3-Data acquisition system; 4-Standard specimen; 5-Fixture; 6-Dial indicator; 7-L-shaped metal component; 8-Dial indicator extension rod; 9-Connector; 10-Connector; 11-Connector; 12-3D printed crack test specimen; 13-Constrained crack test device; 14-Stainless steel base; 15-Counterweight with handle; 16-Insertion anchor bracket; 17-Anchor bar on bracket; 18-Bracket fixing nut; 19-Blower; 20-Heater; 21-Crack. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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, 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.

[0022] Some embodiments of the present invention will be described in detail below. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0023] This embodiment provides a comprehensive evaluation method for the shrinkage and cracking performance of 3D-printed UHPC, comprising the following five steps: First, UHPC material is prepared, and the final setting time T1 is determined as a unified time reference. The material is divided into three parts: the first part is used for molding shrinkage testing, the second part for printing shrinkage testing, and the third part for constraint cracking testing. Molding shrinkage testing uses a mold 1 with a built-in strain monitoring component 2 to collect first shrinkage data reflecting the material's own shrinkage characteristics, with T1 as the zero point. 3D printing shrinkage testing involves 3D printing a solid specimen, curing and hardening it, and then cutting it into standard specimens 4. Second shrinkage data reflecting the influence of the printing process is collected, with the initial reading time T2 of the displacement measuring device as the zero point. Constraint cracking testing involves forming a specimen using a 3D printing device along a preset path, applying constraint along the printing direction using a constraint device, and continuously observing and acquiring cracking data. Finally, based on the correlation analysis of the three sets of data, a shrinkage and cracking risk assessment is completed. The shrinkage curve of the first shrinkage data is used to obtain the amount of shrinkage change from the final setting time T1 to the initial reading time T2 of the displacement measuring device, which is used to determine the ultra-early shrinkage performance of the 3D-printed UHPC. Those skilled in the art can export the continuous shrinkage curve of the molding shrinkage test through the data acquisition system 3, define the time interval from T1 to T2, calculate the difference in shrinkage values ​​within the interval, that is, the shrinkage amount of the molding test specimen in the time interval, and use it as an estimate or compensation value of the shrinkage amount of the 3D printed specimen in the same time period for comprehensive evaluation, so as to complete the quantitative judgment of the ultra-early shrinkage performance.

[0024] Specifically, the first part of the mixed UHPC material is poured into mold 1, and the strain monitoring component 2, which is fixed in the center of the mold, is gradually covered. The UHPC material is poured onto the upper surface of mold 1 and smoothed. After curing, the lead end of the strain monitoring component 2 is connected to the data acquisition system 3 and data acquisition begins. The acquisition frequency is set to 2-5 minutes, and the final setting time T1 of UHPC is set as zero. The molding shrinkage data is obtained, and the shrinkage curve of the molded UHPC material is plotted. The second part of the mixed UHPC material is placed into the hopper of the 3D printer to form a solid 3D printed UHPC specimen. After printing, it is cured with a film. After the solid 3D printed specimen has hardened (24 hours after molding), it is cut with a cutting machine to cut at least 2-3 standard 3D printed specimens 4. The size is the same as the molded cuboid specimen. The direction of the long side of the specimen can be set to the direction of the nozzle printing or two other directions in the 3D printed three-dimensional system according to the design requirements. The zero point of the shrinkage test curve is set to the initial reading time T2 of the displacement measuring device.

[0025] This solution constructs a complete system of "three-dimensional testing + comprehensive evaluation" for the shrinkage and cracking performance of 3D-printed UHPCs. It breaks through the limitations of traditional testing methods that only target cast-formed specimens, and is specifically adapted to the special process of 3D printing with layer-by-layer stacking. Through full-chain coverage of "material properties + process influence + actual constraint state", it solves the core problem of the disconnect between traditional testing and the actual application scenarios of 3D-printed UHPCs. Through three interrelated but distinct testing links, it can systematically separate and quantify the material's own shrinkage, the additional influence of the printing process, and the actual cracking tendency under constraint state, thus providing a complete method for solving the problem of shrinkage and cracking risk assessment of 3D-printed UHPCs. A creative data correlation was established between molding shrinkage testing and 3D printing shrinkage testing, filling the technical gap in 3D printing shrinkage testing, which requires curing (after 24 hours) before specimens can be cut and monitoring can be initiated, and cannot capture ultra-early shrinkage during T1 to T2. Ultra-early shrinkage is one of the key causes of UHPC cracking. This supplement makes the evaluation system's assessment of the shrinkage development process of 3D printed UHPC components throughout their entire lifespan continuous and complete, improving the accuracy of the evaluation and the ability to provide early warning.

[0026] In this embodiment, the mold 1 used in the molding shrinkage test is specified as a cuboid mold with a through hole on the long side for fixing the strain monitoring component 2. The strain monitoring component 2 is positioned at the center of the mold 1 through the through hole in the side wall of the mold 1. A release agent is provided on the inner wall of the mold 1. The demolding time for UHPC material molding is specified as 24 hours after molding is completed.

[0027] Specifically, mold 1 is a standard concrete mold of 100×100×300mm or 100×100×400mm. Two holes are made on the long side of mold 1 at one-third of the distance from both ends, and strain monitoring components 2 are fixed by steel wire through the holes, ensuring that the strain monitoring components 2 are at the exact center of mold 1. A release agent is applied to the inner wall of mold 1. Demolding is performed 24 hours after the specimen is formed by first cutting the exposed steel wire outside mold 1, and then demolding. The data recording period for the molding shrinkage test is as follows: starting from the final setting time T1, the rapidly changing shrinkage values ​​of the UHPC material within 1-2 days are accurately captured to evaluate the self-shrinkage performance of the selected UHPC material.

[0028] This scheme ensures the stability of the strain monitoring component 2 installation (avoiding displacement during testing) and the integrity of the molded specimen (reducing damage during demolding) through standardized mold 1 structure, perforation design, and demolding process. The use of release agent and the limitation of 24-hour molding cycle conform to the solidification and hardening law of UHPC, avoiding test errors caused by improper mold 1 design or unreasonable demolding timing, thus ensuring the reliability and repeatability of the first shrinkage data.

[0029] In this embodiment, the core elements of the constraint crack test are defined: the preset path of the 3D printed crack test specimen 12 adopts a loop printing path, and adjacent printing strips need to contact or slightly overlap.

[0030] The loop-shaped printing path of this scheme simulates the actual forming structure and interface characteristics of 3D printed components, solving the core problem that traditional test specimens (such as cuboids and rings) cannot reproduce the layer-by-layer stacking and strip splicing interface of 3D printing. The contact or slight overlap of adjacent strips perfectly matches the actual process parameters of 3D printing, making the structural shape and internal interface of the 3D printed crack test specimen 12 consistent with the actual engineering component. This ensures that the cracking data can truly reflect the cracking of 3D printed UHPC in actual application scenarios, and enhances the engineering guidance value of constrained cracking test.

[0031] In this embodiment, an environmental simulation and observation step is added: simulating a dry environment to accelerate the drying and shrinkage of the 3D printed crack test specimen 12.

[0032] Specifically, to simulate the drying environment conditions of a 3D printing site, a blower 19 and a heater 20 can be set up next to the specimen. The blower 19 has a wind speed of 5 to 10 m / s, and the heater 20 has a power of 800 to 1200 W, so as to achieve a certain period of heat and air coupling effect, thereby accelerating or increasing the occurrence of drying shrinkage.

[0033] The supplementary environmental simulation steps in this scheme can reproduce the complex environment of the 3D printing site (such as dryness and strong winds), solving the problem that traditional tests can only be carried out in standard laboratory environments and are out of touch with actual application scenarios. The clear observation period and observation content ensure the comprehensiveness and systematic nature of cracking data, avoiding evaluation bias caused by incomplete observation or insufficient period, making cracking risk assessment more in line with engineering practice, and improving the practicality and guidance value of the evaluation results.

[0034] In this embodiment, the observation period for the constraint cracking test is limited to 1 to 3 months, and the observation content includes the number, length, width and location information of cracks 21.

[0035] Specifically, the observation steps include using a magnifying camera to observe the occurrence of cracks 21 around the outer surface of the 3D printed crack test specimen 12, and recording the number, length, width and location of cracks in a timely manner; the data recording observation period is 1 to 3 months: in the first week, observation is conducted once every day; in the later period, the observation frequency is reduced to 2 to 3 times per week.

[0036] Figure 5 A schematic diagram of the overall structure of the 3D-printed UHPC constraint cracking test device 13 is shown. The constraint cracking test device 13 includes two stainless steel bases 14, two counterweights 15 with handles placed on the sides of the bases, and two insertable anchor brackets 16 that provide constraint. Each bracket contains three anchor bars, and each base requires two bracket fixing nuts 18. The counterweights 15 with handles weigh 6-12 kg. The anchor bars 17 on the anchor brackets are made of 150 mm long M6-M12 threaded steel bars. Three constraint anchor bars are set at heights of 1 cm, 4.5 cm, and 9 cm on the brackets. The height of the bracket column is 100 mm. The device is a detachable structure. After the test is completed, the anchor brackets and their anchor bars are not reusable because they are bonded to the 3D-printed UHPC. Other components can be separated by removing the bracket fixing nuts 18, so they can all be reused. A blower 19 and / or a heater 20 were placed next to the 3D printed crack test specimen 12. The wind speed of the blower 19 was 5-10 m / s, and the power of the heater 20 was 800-1200 W. The distance between the two and the specimen surface was ≤200 mm. The blower 19 and the heater 20 were turned on simultaneously and kept on for 48 hours. Then the blower 19 and the heater 20 were turned off, and the specimen was continuously observed until 90 days of age.

[0037] This invention also provides a comprehensive evaluation device for the shrinkage and cracking performance of 3D-printed UHPC, integrating three functionally independent modules: a molding shrinkage test module, including a mold 1 with a built-in strain monitoring component 2 and a data acquisition system 3 connected to the strain monitoring component 2; a printing shrinkage test module, including a displacement measuring device; and a constraint cracking test module, which includes a base, a counterweight block disposed on the side of the base, and an anchoring bracket detachably mounted on the base by fasteners. The modules are connected through standardized interfaces, allowing those skilled in the art to flexibly combine and disassemble the modules according to testing requirements, prepare module components using conventional machining processes, and complete assembly.

[0038] Specifically, it adopts a modular and detachable design, which can be quickly assembled and adjusted according to different test requirements to meet the testing needs of different sized specimens, environmental conditions and printing processes. It is easy to operate and has a high reusability, which greatly reduces the testing cost.

[0039] This solution integrates modular design into a coordinated hardware system, overcoming the limitations of existing testing devices with fixed structures and poor versatility. It allows users to flexibly select or combine modules according to evaluation needs, supporting both comprehensive evaluation processes and individual tests, thus improving the equipment's practicality and adaptability. The detachable anchor bracket design reflects the modularity of the device and the design concept of some consumable parts (the bracket is usually replaced after bonding to the specimen, while other components are detachable and reusable), facilitating adjustment and maintenance and improving the device's usability. It provides a dedicated and efficient hardware platform for the implementation of the method. Through comprehensive analysis of crack observation and shrinkage data, it provides scientific support for optimizing UHPC material ratios, printing parameters, and curing strategies, improving structural safety and durability. It is suitable for laboratory and field environments and can be easily promoted to engineering practice.

[0040] In this embodiment, the strain monitoring component 2 of the molded shrinkage test module is specified as an embedded vibrating wire strain gauge. Specifically, the vibrating wire strain gauge has the characteristics of high measurement accuracy and strong anti-interference ability, which is suitable for monitoring the rapid changes in early shrinkage of UHPC.

[0041] In this embodiment, the displacement measuring device for measuring the shrinkage of the 3D printed standard specimen 4 is defined as follows: it includes a dial indicator 6 and a clamp 5 for fixing the dial indicator 6; the clamp 5 includes at least two pairs of L-shaped metal components 7, which are connected to the standard specimen 4 in a circumferential manner by connectors 9, 10, and 11, and the L-shaped metal components 7 are provided with mounting parts for fixing the dial of the dial indicator 6 and the dial indicator extension rod 8.

[0042] For example, the dial indicator 6 has a measuring range of 0~1mm, and the inner diameter of the dial indicator 6 fixing clamp 5 is 100×100mm. The clamp 5 includes two sets of identical L-shaped metal components 7, which are used to fix the dial of the dial indicator 6 and the dial indicator extension rod 8, respectively. The ends of the L-shaped metal components 7 are provided with threaded holes, and bolts are inserted and tightened so that the pair of L-shaped components can circumferentially clamp the 3D printed standard specimen 4. There are two threaded holes on each side of the L-shaped components, and bolts are inserted to finely adjust their positions. The dial indicator extension rod 8 has a diameter of 5mm and a length of 200mm to meet the testing requirements of the standard specimen 4. The zero point for plotting the shrinkage data curve is set to the initial reading time T2 of the dial indicator 6, and the observation data recording period is 1~3 months: in the first week, readings are taken 2~3 times a day; in the later period, readings are taken 1~2 times a week to form the shrinkage data of the 3D printed specimen.

[0043] The L-shaped clamp 5 in this design features a circumferential clamping design that provides uniform clamping force, achieving a stable connection with the 3D printed standard specimen 4 and avoiding interference from localized compression on the specimen's free shrinkage. The mounting section is equipped with a dial indicator 6 and its extension rod 8, which can accurately and stably align and contact the measurement points. This device is a key tool for accurately acquiring the second shrinkage data (affected by the printing process). The adaptability design of each component enhances the overall stability and testing reliability of the device, providing hardware support for the accurate acquisition of multi-dimensional data.

[0044] In this embodiment, the constraint cracking test module is further refined: the anchoring bracket is an insert-type anchoring bracket 16, which is used to provide end constraints for the 3D printed cracking test specimen 12; multiple exposed anchoring bars are arranged at intervals along the height direction on the anchoring bracket, and the anchoring bars are threaded steel bars; the counterweight is a counterweight 15 with a handle.

[0045] Specifically, the crack test specimen size for the 3D printed UHPC crack risk test is 500×60×150mm. After the crack test specimen is formed, it is moved to a flat table or ground and fixed using a constraint crack test device 13. The constraint crack test device 13 includes a stainless steel base 14, a counterweight with a handle 15, an insertable anchor bracket 16 that provides constraint, an anchor bar on the bracket 17, and a bracket fixing nut 18. The depth of the anchor bar inserted into the specimen is controlled to be 1 / 6 to 1 / 5 of the specimen length.

[0046] This solution describes a method for accurately simulating the process characteristics and structural morphology of layer-by-layer stacking in 3D printing using a spiral printing path. The anchoring ribs provide clear end constraint strength and control over the insertion depth range, preventing premature cracking of the specimen due to excessive constraint and avoiding insufficient constraint to simulate actual working conditions. The combined design of the insertable anchor bracket 16 and the threaded steel bar anchoring ribs provides stable and uniform end constraint for the crack test specimens. The design of the counterweight block 15 with a handle improves the ease of operation and solves the problems of unstable constraint strength and poor compatibility with 3D printed specimens in traditional constraint devices, ensuring the authenticity and reliability of constraint crack testing.

[0047] The present invention also provides the following specific embodiments: Example 1: Two UHPC materials were selected, one was ordinary UHPC material (UHPC-ordinary), and the other was UHPC material with shrinkage reduction by incorporating internal curing medium (UHPC-low shrinkage). The internal curing medium used was porous sintered bauxite aggregate (hereinafter referred to as CB aggregate).

[0048] UHPC mix ratio: Table 1. Mixing proportions of UHPC (kg / m³) 3 )

[0049] UHPC materials were prepared by stirring according to the mixing ratio in Table 1; like Figure 1-9 As shown, a 100×100×300mm concrete mold 1 was drilled and a embedded intelligent vibrating wire strain gauge was fixed with a steel wire rope. The embedded intelligent vibrating wire strain gauge was model JMZX-215HAT(V2.2). The release agent was evenly applied to the inner surface of the mold 1. A portion of the mixed UHPC material was poured into the mold 1, and the pouring was carried out layer by layer, slowly covering the vibrating wire strain gauge. The UHPC material was poured to the upper surface of the mold 1 and smoothed. The mold was then covered and cured. Subsequently, the embedded intelligent vibrating wire strain gauge was connected to the strain acquisition box for data acquisition. The acquisition box was model JMZX-20SH(V1.07), and the acquisition frequency was set to 2-5 minutes to form the first set of shrinkage data (A) of the UHPC material for mold shrinkage test. The zero point of the shrinkage data curve was set to the final setting time of UHPC T1=6 hours.

[0050] Freshly mixed UHPC was placed into the hopper of a 3D printer and extruded into a large test block measuring 700×200×200mm to measure drying shrinkage. After printing, the block was covered with a film for curing. After 24 hours of hardening, it was cut into smaller test blocks of 100×100×300mm. The cut 3D printed test pieces were immediately fixed with a dial indicator 6 using a special clamp 5, and readings were taken. The age of the test piece at the time of the first reading was recorded. The reading frequency was 2-3 times per day (first week) and 1-2 times per week (later period), forming the shrinkage data of the second set of 3D printed test pieces. The zero point for plotting the shrinkage data curve was set to the initial reading time of dial indicator 6, T2 = 25 hours.

[0051] Freshly mixed UHPC from the same batch was placed into the hopper of a 3D printer. A loop-shaped printing path was used to form specimens for testing shrinkage cracking risk. The printing spacing should be set to ensure contact or slight overlap between the left and right strips of the long loop shape. The formed 3D-printed crack test specimens were moved to a flat table or ground, and anchor brackets were immediately inserted. The insertion depth was controlled to 1 / 6 of the specimen length. Blower 19 and heater 20 were simultaneously started and continued for 48 hours, after which blower 19 and heater 20 were turned off. Continuous observation was then conducted until 90 days of age. Cracks were observed in the 3D-printed crack test specimen 12. Based on the shrinkage test results and cracks 21 (including the number, length, and location of cracks), the shrinkage cracking performance of the 3D-printed UHPC was evaluated.

[0052] Two types of UHPC materials (ordinary UHPC and low-shrinkage UHPC with CB aggregate) were selected for 3D printing. Their shrinkage and cracking properties were tested and analyzed. The results showed that there were significant differences in early shrinkage between the two materials (Data A). Ordinary UHPC exhibited higher shrinkage, while CB aggregate UHPC showed relatively lower shrinkage. In the 3D printing process, the shrinkage of the 3D printed ordinary UHPC specimen was still much greater than that of the 3D printed low-shrinkage UHPC (Data B). The aggravated shrinkage deformation of UHPC material leads to further accumulation of internal shrinkage stress under constrained conditions. Consequently, the cracking assessment showed (Data C) that 3D printed ordinary UHPC was more prone to more macroscopic cracks, affecting structural integrity and durability. In contrast, the low-shrinkage UHPC using shrinkage-reducing materials did not show cracking in the constrained shrinkage test due to its smaller shrinkage value. Based on these results, experimental apparatus and methods are provided for the selection of UHPC materials and crack risk assessment in 3D printed structure design. In 3D printed structure design, low-shrinkage UHPC materials should be given priority to significantly reduce the risk of shrinkage cracking and ensure the durability and structural safety of printed components.

[0053] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. The invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A comprehensive evaluation method for the shrinkage and cracking performance of 3D-printed UHPC, characterized in that, Includes the following steps: S1. Material preparation: Prepare UHPC material and test the final setting time T1 of the UHPC material for mold shrinkage test, 3D printing shrinkage test and constraint cracking test respectively; S2. Molding shrinkage test: The UHPC material described in the first part is poured into the mold (1) with the built-in strain monitoring component (2), and the first shrinkage data is obtained with the final setting time T1 as the zero point of data recording, which is used to evaluate the shrinkage performance of the UHPC material itself. S3.3D Printing Shrinkage Test: The UHPC material described in Part 2 is formed into a solid test block by a 3D printing equipment. After curing and hardening, it is cut into a standard test piece (4). The standard test piece (4) is monitored by an externally clamped displacement measuring device. The initial reading time T2 of the displacement measuring device is taken as the zero point of data recording. The second shrinkage data is obtained to evaluate the shrinkage performance of 3D printed UHPC. S4. Constraint cracking test: The UHPC material described in Part III is formed into a cracking test specimen by a 3D printing device using a preset path. By applying constraints to the 3D printed cracking test specimen (12) along the printing direction and continuously observing the surface of the 3D printed cracking test specimen (12), cracking data is obtained. S5. Comprehensive evaluation: Based on the first shrinkage data, the second shrinkage data, and the cracking data, the shrinkage and cracking risk of the 3D printed UHPC is comprehensively evaluated; wherein, the first shrinkage data is used to compensate for the missing shrinkage data of the second shrinkage data during T1 to T2, in order to determine the ultra-early shrinkage performance of the 3D printed UHPC.

2. The comprehensive evaluation method for shrinkage and cracking performance of 3D printed UHPC according to claim 1, characterized in that, In S2, the mold (1) is a cuboid mold, and the long side of the cuboid mold is provided with a through hole for fixing the strain monitoring component (2). The strain monitoring component (2) is positioned at the center of the mold (1) through the through hole. The mold (1) has a release agent on its inner wall. The UHPC material is demolded 24 hours after molding.

3. The comprehensive evaluation method for shrinkage and cracking performance of 3D printed UHPC according to claim 1 or 2, characterized in that, In S4, the preset path is a loop printing path, and the 3D printed crack test specimen (12) has a loop structure extending along the length direction, and the adjacent printed strips are in contact or slightly overlapping state.

4. The comprehensive evaluation method for shrinkage and cracking performance of 3D printed UHPC according to claim 1, characterized in that, S4 also includes an environmental simulation step: simulating a dry environment to accelerate the drying and shrinkage of the 3D printed crack test specimen (12).

5. The comprehensive evaluation method for shrinkage and cracking performance of 3D printed UHPC according to claim 1, characterized in that, The observation period for S4 is 1 to 3 months, and the observation content includes the number, length, width and location of cracks.

6. A comprehensive evaluation device for the shrinkage and cracking performance of 3D-printed UHPC, characterized in that, include: The molded shrinkage test module includes a mold (1) with a built-in strain monitoring component (2) and a data acquisition system (3) connected to the strain monitoring component (2). Print shrinkage test module, including displacement measuring device; The constraint cracking test module includes a base, a counterweight block disposed on the side of the base, and an anchoring bracket that is detachably mounted on the base by fasteners.

7. The comprehensive evaluation device for shrinkage and cracking performance of 3D printed UHPC according to claim 6, characterized in that, The strain monitoring component (2) in the molded shrinkage test module is an embedded vibrating wire strain gauge.

8. The comprehensive evaluation device for shrinkage and cracking performance of 3D printed UHPC according to claim 6, characterized in that, The displacement measuring device includes a dial indicator (6) and a clamp (5) for fixing the dial indicator (6); The clamp (5) includes at least two pairs of L-shaped metal components (7), which are used to circumferentially clamp the standard test piece (4) by means of connectors (9, 10, 11). The L-shaped metal components (7) are provided with mounting parts for fixing the dial of the dial indicator (6) and the dial indicator extension rod (8).

9. The comprehensive evaluation device for shrinkage and cracking performance of 3D printed UHPC according to claim 6, characterized in that, The anchoring bracket is an insert-type anchoring bracket (16) used to provide end constraints for the 3D printed crack test specimen (12); The insert-type anchor bracket (16) is provided with multiple exposed anchor bars at intervals along the height direction, and the anchor bars are threaded steel bars; The counterweight is a counterweight with a handle (15).

Images