Gradient temperature control self-repairing anti-crack concrete and preparation method thereof

Abstract

The invention relates to gradient temperature control self-repairing anti-crack concrete and a preparation method thereof, and relates to the technical field of civil engineering materials. The concrete is prepared from the following raw materials in parts by mass: conventional concrete components, three phase change microcapsules with different melting points, shape memory alloy fibers coated with thermosensitive interface layers on the surfaces, a composite expanding agent, a cement-based capillary crystalline material and basalt fibers. According to the invention, material distribution is controlled during pouring, so that the phase change microcapsules form gradient distribution from the surface layer to the core on the section of the concrete, stepped hydration heat absorption is realized, and the internal and external temperature difference is reduced. The thermosensitive interface layer reduces the interface bonding force during temperature rise, so that the shape memory alloy fibers can effectively retract to close cracks. Through the synergistic effect of the components, effective inhibition of early-stage temperature difference and middle and later-stage shrinkage cracks of concrete is realized, active crack closing and later-stage self-repairing capabilities are endowed to the concrete, and the durability of the concrete is improved.

Description

Technical Field

[0001] This application relates to the field of civil engineering materials technology, and in particular to a gradient temperature-controlled self-healing crack-resistant concrete and its preparation method. Background Technology

[0002] Concrete is one of the most widely used building materials. In large-volume concrete structures such as bridges, dams, and heavy foundation slabs, the formation of cracks is a persistent technical problem that directly affects the safety and long-term durability of the structure.

[0003] Cracks originate from a variety of factors. In the early stages after concrete pouring, the hydration of cement releases a significant amount of heat. Due to the uneven heat dissipation rate inside and outside large-volume structures, the core area has a significantly higher temperature than the surface layer. This high temperature gradient induces uneven volumetric deformation. When the resulting tensile stress exceeds the relatively low early tensile strength of the concrete, temperature cracks will form. Existing control measures, such as pre-embedding cooling water pipes inside the structure or using low-heat cement, are complex to implement and their effectiveness remains limited for ultra-large cross-section structures.

[0004] Throughout the service life of concrete, its volume continues to decrease due to autogenous shrinkage and drying shrinkage. When this volumetric deformation is constrained by internal reinforcement or external structures, tensile stress is generated, leading to shrinkage cracks. Although incorporating fibers or expansive agents into concrete is a common method to suppress shrinkage, conventional fibers only provide a passive bridging effect, while the expansion process of single-component expansive agents is often difficult to perfectly match the shrinkage patterns of concrete at different stages.

[0005] Whether temperature cracks or shrinkage cracks, once formed, they provide a pathway for harmful media such as moisture and chloride ions to invade. This intrusion leads to steel corrosion and deterioration of the concrete itself, shortening the structure's service life and resulting in costly repairs and reinforcement. Conventional concrete materials themselves do not possess the ability to repair cracks after they have formed. Therefore, how to systematically address cracking problems caused by various factors from a material perspective, and improve the damage tolerance and self-healing ability of concrete, is a pressing technical problem that needs to be solved in this field.

[0006] To address the aforementioned technologies, a gradient temperature-controlled self-healing crack-resistant concrete is provided. Summary of the Invention

[0007] The purpose of this application is to provide a gradient temperature-controlled self-healing crack-resistant concrete and its preparation method, aiming to improve the existing technology's difficulty in systematically solving the cracking problem of large-volume concrete caused by early temperature difference and mid-to-late stage shrinkage, and the lack of self-repair ability of the material itself after damage.

[0008] By adopting the above technical solution, a gradient temperature-controlled self-healing crack-resistant concrete, by weight, includes the following raw materials: 220-270 parts cement; 80-110 parts fly ash; 950-1150 parts coarse aggregate; 720-800 parts manufactured sand; 7-12 parts water-reducing agent; 130-160 parts water; 5-20 parts composite expansion agent; 10-25 parts cement-based penetrating crystallizing material; 5-20 parts basalt fiber; 20-50 parts including three phase change microcapsules with different melting points; and 1-15 parts including shape memory alloy fiber with a thermosensitive interface layer coated on the surface.

[0009] In one embodiment, the three phase change microcapsules with different melting points are microcapsules composed of a paraffin core and a silica shell, and their melting points are 35±2℃, 45±2℃ and 55±2℃, respectively.

[0010] In one embodiment, the shape memory alloy fiber with a surface coated with a thermosensitive interface layer has the following structural features:

[0011] The shape memory alloy fiber is a nickel-titanium alloy fiber that has undergone 3.5% to 4.5% pre-strain treatment.

[0012] The thermosensitive interface layer is a thermoplastic polymer coating with a glass transition temperature (Tg) of 60–65°C.

[0013] Furthermore, the glass transition temperature (Tg) of the thermosensitive interface layer is higher than the highest melting point among the three phase change microcapsules with different melting points.

[0014] This structure allows the interface layer to soften when the internal temperature of the concrete rises above the glass transition temperature Tg of the thermosensitive interface layer due to the heat of hydration, reducing the interfacial bond between the shape memory alloy fiber and the concrete matrix. When the temperature continues to rise to the point where the shape memory alloy fiber is activated to undergo phase transformation recovery, its shrinkage stress can be transferred to both ends of the crack. When the temperature falls back below Tg, the interface layer is re-cured, restoring the interfacial bond.

[0015] In one embodiment, the composite expansive agent is composed of calcium sulfoaluminate and lightly calcined magnesium oxide in a mass ratio of 3:1. Calcium sulfoaluminate undergoes a hydration reaction and expands within 3 to 7 days after concrete molding to compensate for the early shrinkage of the concrete; lightly calcined magnesium oxide undergoes a hydration reaction and expands within 14 to 28 days or longer to compensate for the later shrinkage of the concrete.

[0016] In one embodiment, the diameter of the shape memory alloy fiber is 0.1±0.02 mm, and the thickness of the thermosensitive interface layer is 1 to 5 μm.

[0017] In one embodiment, the basalt fiber is a short-cut fiber with a length of 12±1 mm.

[0018] A method for gradient temperature-controlled self-healing crack-resistant concrete includes the following steps:

[0019] S1. Provide pre-strained nickel-titanium alloy fibers and coat their surface with a thermoplastic polymer having a preset glass transition temperature Tg to form a thermosensitive interface layer, thereby obtaining shape memory alloy fibers with a thermosensitive interface layer coated on the surface.

[0020] S2. The cement, fly ash, coarse aggregate, manufactured sand, composite expansion agent, cement-based penetrating crystallizing material, and three phase change microcapsules with different melting points are dry-mixed for the first time; then the shape memory alloy fiber and basalt fiber with a heat-sensitive interface layer on the surface obtained in step S1 are added for the second dry mixing; finally, water and water-reducing agent are added for wet mixing to obtain concrete mixture.

[0021] In one embodiment, in step S1, the pre-strain treatment is a pre-strain of 3.5% to 4.5%, and the glass transition temperature Tg of the thermosensitive interface layer is 60 to 65°C.

[0022] In one embodiment, in step S2, the first dry mixing time is 60-90 seconds, the second dry mixing time is 60-90 seconds, and the total wet mixing time is 180-210 seconds.

[0023] In one embodiment, the method further includes the step of pouring the concrete mixture, wherein, during the pouring step, by layering the mixture or controlling the pouring sequence, the three phase change microcapsules with different melting points form a gradient distribution from the surface to the core within the concrete cross-section. This gradient distribution allows the phase change microcapsules with different melting points to undergo phase change and absorb heat sequentially from the surface to the core as hydration heat is generated inside the concrete, thereby reducing the temperature gradient within the concrete cross-section.

[0024] In summary, this application includes at least one of the following beneficial technical effects:

[0025] 1. This application incorporates phase change microcapsules with three different melting points (35℃, 45℃, and 55℃) into concrete, enabling phase change endothermic reactions in different temperature zones to be triggered sequentially during cement hydration based on the temperature gradient from the surface to the core of the concrete. This stepped heat absorption mechanism can reduce the maximum temperature difference between the core and the surface of the concrete, thereby suppressing the generation of early temperature cracks caused by excessive temperature differences.

[0026] 2. This application establishes a temperature-responsive interfacial bonding control mechanism by setting a thermosensitive interface layer with a glass transition temperature (Tg) of 60-65℃ on the surface of shape memory alloy fibers, and making this Tg higher than the highest melting point of phase change microcapsules. When the internal temperature of concrete rises, the interface layer softens, reducing the bond strength between the fiber and the matrix, so that the shrinkage force of the shape memory alloy fibers can act on the closed cracks rather than destroy the matrix. After the temperature drops, the interface layer re-cures, stabilizing the cracks in a closed state, thus achieving active control of the crack width.

[0027] 3. This application uses a composite expansion agent composed of calcium sulfoaluminate and lightly calcined magnesium oxide with different hydration rates. Calcium sulfoaluminate hydrates rapidly in the early stage of concrete hardening and expands to compensate for the plastic shrinkage in this stage; while lightly calcined magnesium oxide hydrates slowly over a longer period of time and expands to compensate for the drying shrinkage in the later stage of concrete. This phased expansion compensation effect matches the shrinkage pattern of concrete at different ages and reduces the shrinkage stress throughout the entire hardening process. Detailed Implementation

[0028] Example 1:

[0029] This embodiment provides a method for preparing gradient temperature-controlled self-healing crack-resistant concrete, including the following steps:

[0030] Preparation of key functional components:

[0031] Preparation of gradient phase change microcapsules:

[0032] Three types of paraffin@silica microcapsules were prepared using the sol-gel method, with industrial-grade paraffin wax (melting points of 35℃, 45℃, and 55℃) as the core material and tetraethyl orthosilicate as the shell precursor. The specific steps are as follows: 25 parts of paraffin wax were heated to 20℃ above its melting point, and 1.5 parts of Span 80 were added. The mixture was stirred at 800 rpm while 100 parts of deionized water at the same temperature was added dropwise to form an emulsion. The emulsion was transferred to a three-necked flask, and the mixture was kept at 65℃ in a water bath. 35 parts of tetraethyl orthosilicate were added dropwise, and the pH was adjusted to 9.5 with ammonia. After reacting for 4 hours, the mixture was filtered, washed, and vacuum dried at 65℃ for 18 hours to obtain three independent white powdery phase change microcapsules.

[0033] Preparation of shape memory alloy fibers with a surface coated with a thermosensitive interface layer:

[0034] Nickel-titanium alloy fibers with a diameter of 0.1 mm were selected and a pre-strain of 4.0% was applied on a universal testing machine. The fibers were ultrasonically cleaned in acetone and anhydrous ethanol and then dried. An 8% (w / w) dichloromethane solution of polymethyl methacrylate was prepared. Using the dip-coating method, the pre-strained fibers were immersed in and pulled out of the solution at a rate of 150 mm / min, and then suspended in a constant temperature drying oven at 45°C for 18 hours to obtain SMA fibers with a surface coating of a 3 μm thick thermosensitive interface layer.

[0035] Concrete preparation:

[0036] Mixing ratio:

[0037] The concrete mix design in this embodiment is as follows:

[0038] Cement: 245; Fly ash: 95; Coarse aggregate: 1050; Manufactured sand: 760; Water-reducing agent: 9.5; Water: 145; Composite expansion agent: 12; Cement-based penetrating crystallizing material: 18; Basalt fiber: 12; Three phase change microcapsules with different melting points: 35 parts in total; Shape memory alloy fiber with a thermosensitive interface layer on the surface: 8.

[0039] Mixing and shaping:

[0040] Using a forced-action single-shaft concrete mixer, proceed as follows:

[0041] The weighed coarse aggregate, manufactured sand, cement, fly ash, composite expansion agent, cement-based penetrating crystallizing material, and three phase change microcapsules are put into the mixer and dry-mixed for 75 seconds.

[0042] Evenly sprinkle basalt fibers and shape memory alloy fibers coated with a thermosensitive interface layer into the mixture, then dry mix for 75 seconds.

[0043] Mix 80% of the water with all the water-reducing agent and pour it in, then mix for 120 seconds; add the remaining 20% ​​of the water and continue mixing for 75 seconds.

[0044] The mixture is poured into a mold and cured at a temperature of 20±2℃ and a relative humidity of ≥95%.

[0045] Example 2:

[0046] This embodiment provides a method for preparing gradient temperature-controlled self-healing crack-resistant concrete, including the following steps:

[0047] Preparation of key functional components:

[0048] The preparation methods for gradient phase change microcapsules and shape memory alloy fibers coated with a thermosensitive interface layer are the same as in Example 1, but the process parameters are adjusted as follows:

[0049] A pre-strain of 3.5% was applied to the nickel-titanium alloy fiber; the thermosensitive interface layer was made of a thermoplastic polymer with a glass transition temperature (Tg) of 60℃; after coating, it was dried at 40℃ for 24 hours to obtain a coating with a thickness of 1μm.

[0050] Concrete preparation:

[0051] Mixing ratio:

[0052] The concrete mix design in this embodiment is as follows:

[0053] Cement: 220; Fly ash: 80; Coarse aggregate: 950; Manufactured sand: 720; Water-reducing agent: 7; Water: 130; Composite expansion agent: 5; Cement-based penetrating crystallizing material: 10; Basalt fiber: 5; Three phase change microcapsules with different melting points: 20 parts in total; Shape memory alloy fiber with a thermosensitive interface layer coated on the surface: 1.

[0054] Mixing and shaping:

[0055] Using a forced-action single-shaft concrete mixer, proceed as follows:

[0056] Add the weighed dry powder and aggregate to the mixer and dry mix for 60 seconds.

[0057] Sprinkle in both types of fiber and then dry mix for 60 seconds.

[0058] Mix 80% of the water with the water-reducing agent and pour it in, then mix for 120 seconds; add the remaining 20% ​​of the water and continue mixing for 60 seconds.

[0059] The mixture is poured into a mold and cured at a temperature of 20±2℃ and a relative humidity of ≥95%.

[0060] Example 3:

[0061] This embodiment provides a method for preparing gradient temperature-controlled self-healing crack-resistant concrete, including the following steps:

[0062] Preparation of key functional components:

[0063] The preparation methods for gradient phase change microcapsules and shape memory alloy fibers coated with a thermosensitive interface layer are the same as in Example 1, but the process parameters are adjusted as follows:

[0064] A pre-strain of 4.5% was applied to the nickel-titanium alloy fiber; the thermosensitive interface layer was made of a thermoplastic polymer with a glass transition temperature (Tg) of 65℃; after coating, it was dried at 50℃ for 12 hours to obtain a coating with a thickness of 5μm.

[0065] Concrete preparation:

[0066] Mixing ratio:

[0067] The concrete mix design in this embodiment is as follows:

[0068] Cement: 270; Fly ash: 110; Coarse aggregate: 1150; Manufactured sand: 800; Water-reducing agent: 12; Water: 160; Composite expansion agent: 20; Cement-based penetrating crystallizing material: 25; Basalt fiber: 20; Three phase change microcapsules with different melting points: 50 parts in total; Shape memory alloy fiber with a thermosensitive interface layer on the surface: 15.

[0069] Mixing and shaping:

[0070] Using a forced-action single-shaft concrete mixer, proceed as follows:

[0071] Add the weighed dry powder and aggregate to the mixer and dry mix for 90 seconds.

[0072] Sprinkle in both types of fiber and then dry mix for 90 seconds.

[0073] Mix 80% of the water with the water-reducing agent and pour it in, then mix for 120 seconds; add the remaining 20% ​​of the water and continue mixing for 90 seconds.

[0074] The mixture is poured into a mold and cured at a temperature of 20±2℃ and a relative humidity of ≥95%.

[0075] Comparative Example 1:

[0076] Compared with Example 1, the difference is that it does not contain composite expansion agent, cement-based penetrating crystallizing material, basalt fiber, three phase change microcapsules with different melting points, and shape memory alloy fiber with a thermosensitive interface layer on the surface. Its mix ratio is the same as ordinary concrete, and the rest of the preparation process is the same.

[0077] Comparative Example 2:

[0078] Compared with Example 1, the difference is that the total amount of 35 parts of the three phase change microcapsules with different melting points was replaced with an equal amount of phase change microcapsules with a single melting point of 45°C, and all other aspects were the same.

[0079] Comparative Example 3:

[0080] Compared with Example 1, the difference is that 8 portions of shape memory alloy fibers with a surface coated with a thermosensitive interface layer were replaced with an equal amount of uncoated nickel-titanium alloy fibers, while the rest were the same.

[0081] Comparative Example 4:

[0082] The difference from Example 1 is that it does not contain 8 parts of shape memory alloy fibers with a surface coated with a thermosensitive interface layer; otherwise, they are the same.

[0083] Comparative Example 5:

[0084] The difference from Example 1 is that it does not contain 35 parts of three different phase change microcapsules with different melting points; otherwise, they are the same.

[0085] Comparative Example 6:

[0086] The difference from Example 1 is that it does not contain 12 parts of composite expanding agent, but all other aspects are the same.

[0087] Comparative Example 7:

[0088] The difference from Example 1 is that it does not contain 18 parts of cement-based penetrating crystallizing material, but all other aspects are the same.

[0089] Test Example 1: Temperature Rise and Temperature Difference Test Inside Large-Volume Concrete

[0090] Experimental steps:

[0091] The test subjects in this test case are Examples 1-3, Comparative Examples 1, 2, and 5.

[0092] A test mold with an inner size of 1m×1m×1m was made using wooden templates, and the inner wall of the test mold was lined with 50mm thick extruded polystyrene insulation board.

[0093] Thermocouple temperature sensors are placed at the geometric center of the mold and at a point on the surface 50 mm vertically downward from the center of the top surface.

[0094] The concrete mixtures prepared in each group were poured into the mold at one time and vibrated to compact them. Then, the top surface of the mold was covered and sealed with insulation board to simulate the insulation conditions of large-volume concrete.

[0095] All thermocouples were connected to the data acquisition system, and temperature data at each measuring point was continuously collected at 30-minute intervals starting from the moment the pouring was completed, with a total monitoring time of 72 hours.

[0096] After the monitoring is completed, two key parameters are extracted from the data collected in each group: the highest temperature value recorded at the core area measuring point, and the maximum temperature difference between the core area measuring point and the surface measuring point at any time during the entire monitoring period.

[0097] Experimental data:

[0098] The temperature rise test results of large-volume concrete in each group are recorded in Table 1.

[0099] Table 1. Temperature rise test results of mass concrete in each group.

[0100] Group Core zone maximum temperature / °C Maximum temperature difference / °C Example 1 61.7 16.2 Example 2 63.1 18.4 Example 3 59.4 14.1 Comparative Example 1 78.2 28.5 Comparative Example 2 68.9 21.3 Comparative Example 5 76.5 26.9

[0101] Results analysis:

[0102] According to the test data in Table 1, the highest temperature and maximum temperature difference in the core area of ​​Examples 1-3 are lower than those of Comparative Examples 1, 2, and 5. Comparative Examples 1 and 5 have the highest values ​​for both the highest temperature and maximum temperature difference in their core areas. This result indicates that incorporating phase change microcapsules into concrete can intervene in the exothermic process of cement hydration, thereby reducing the temperature rise and temperature gradient within the concrete.

[0103] Comparing the data from Examples 1-3 and Comparative Example 2, the highest temperature and maximum temperature difference in the core area of ​​Examples 1-3 were both lower than those of Comparative Example 2. This is because in Examples 1-3, the three phase change microcapsules with different melting points formed a gradient temperature control structure in the concrete. As the heat of hydration is conducted and accumulated from the inside out, the microcapsules with a melting point of 35°C first undergo a phase change and absorb heat in the outer layer; as the internal temperature further increases, the microcapsules with melting points of 45°C and 55°C undergo phase changes sequentially in the middle and core regions.

[0104] This phased and regional heat absorption process creates multiple heat absorption platforms within different temperature ranges, preventing concentrated heat bursts in the core area. Compared to the single-melting-point phase change material in Comparative Example 2, which only produces heat absorption near a single temperature point, the gradient phase change structure in this invention can smooth and homogenize the heat of hydration over a wider temperature range, thus demonstrating effectiveness in both reducing the peak temperature in the core area and minimizing the cross-sectional temperature difference. This confirms that the combination of three phase change microcapsules with different melting points can effectively control the early temperature field of concrete.

[0105] Test Example 2: Cracking Performance Test under Restricted Shrinkage Conditions

[0106] Experimental steps:

[0107] The test subjects in this test case are Examples 1-3, Comparative Examples 1, 3, 4 and 6.

[0108] A concrete shrinkage cracking ring testing device was used, which consisted of a steel inner ring and a concentric outer mold. Four resistance strain gauges were uniformly attached circumferentially to the inner wall of the steel inner ring.

[0109] The concrete mixtures prepared in each group were poured into the annular space between the inner steel ring and the outer mold, compacted by vibration, and the surface was smoothed.

[0110] After the specimen is formed, the outer mold is removed, and the inner steel ring with the concrete ring is immediately transferred to a constant temperature and humidity environment with a temperature of 20±1℃ and a relative humidity of 50±4%.

[0111] Using a data acquisition system, strain data on the steel ring was continuously monitored and recorded from the moment the specimen was placed in a constant temperature and humidity environment. Simultaneously, a visual inspection of the concrete ring surface was conducted hourly, recording the time at which the first macroscopic crack penetrating the cross-section appeared, i.e., the initial crack time.

[0112] When the test age reached 28 days, the width of each crack was measured at three different locations using a reading microscope. The average value was taken as the width of the crack, and the maximum width value among all cracks was recorded.

[0113] Experimental data:

[0114] The results of the restricted shrinkage crack test for each group are recorded in Table 2.

[0115] Table 2 Results of restricted shrinkage cracking test for each group

[0116] Group Initial cracking time / h Maximum crack width at 28 days / mm Example 1 218 0.05 Example 2 195 0.07 Example 3 231 0.04 Comparative Example 1 39 0.46 Comparative Example 3 92 0.23 Comparative Example 4 74 0.31 Comparative Example 6 58 0.38

[0117] Results analysis:

[0118] According to the test data in Table 2, the initial cracking time of Examples 1-3 was significantly longer than that of all comparative examples, and their maximum crack width at 28 days was significantly smaller than that of all comparative examples. Comparative Example 1, as the reference concrete without any functional components, had the shortest initial cracking time and the largest final crack width. The component combination described in this invention can delay the cracking time of concrete under constrained conditions and reduce the final crack width.

[0119] By comparing the data of Example 1 with those of Comparative Examples 3 and 4, the role of the fiber system can be analyzed. Comparative Example 4 showed a certain degree of improvement in crack resistance compared to Example 1, which is attributed to the bridging effect of basalt fibers on microcrack propagation. Comparative Example 3 exhibited better initial crack time and crack width than Comparative Example 4, demonstrating that the introduction of SMA fibers can provide additional crack control capabilities. The data of Example 1 were significantly better than those of Comparative Example 3 because, when the hydration temperature rose to the range of 60–65°C, the thermosensitive interface layer softened, reducing the interfacial bond between the SMA fibers and the concrete matrix. At this point, the phase transformation restoring force of the pre-strained SMA fibers could act on the crack surface with lower interfacial shear stress, applying a closing force rather than causing new damage at the interface. After the temperature dropped, the interface layer re-cured, fixing the crack to a smaller width.

[0120] Comparing the data from Example 1 and Comparative Example 6, the initial cracking time in Example 1 was longer than that in Comparative Example 6, and the crack width was smaller in Example 1. This demonstrates the effect of the composite expansive agent. The composite expansive agent in this invention comprises calcium sulfoaluminate and lightly calcined magnesium oxide, both of which undergo hydration expansion reactions at different time scales. The early expansion of calcium sulfoaluminate offsets the early autogenous shrinkage of concrete, while the later expansion of lightly calcined magnesium oxide compensates for drying shrinkage. This staged volume compensation effect generates pre-compression stress within the concrete, reducing the tensile stress caused by shrinkage, thereby delaying the onset of cracking.

[0121] Test Example 3: Length Change Rate Test under Unconstrained Conditions

[0122] Experimental steps:

[0123] The test subjects in this test case are Examples 1-3, Comparative Example 1, and Comparative Example 6.

[0124] A standard prism mold with dimensions of 100mm×100mm×515mm was used. When pouring the concrete mixture for each group, copper measuring heads were pre-embedded at the center of both ends of the mold.

[0125] Demolding was carried out 24 hours after the specimens were cast. Immediately after demolding, the initial length of each specimen was measured and recorded using a horizontal length comparator. ).

[0126] All demolded specimens were transferred to a constant temperature and humidity chamber at a temperature of 20±1℃ and a relative humidity of 60±5% for curing.

[0127] At 7, 28, and 56 days of age, the specimens were removed from the constant temperature and humidity chamber, and their lengths were quickly measured using the same horizontal length comparator. ).

[0128] According to the formula: (in (where 500 mm is the effective length of the specimen). Calculate the length change rate at each age. Negative values ​​indicate shrinkage, and positive values ​​indicate expansion.

[0129] Experimental data:

[0130] The test results of the unconstrained length change rate for each group are recorded in Table 3.

[0131] Table 3. Results of Unconstrained Length Change Rate Test for Each Group

[0132] Group 7-day length change rate / % 28-day length change rate / % 56-day length change rate / % Example 1 0.004 -0.007 -0.016 Example 2 0.001 -0.011 -0.023 Example 3 0.009 -0.003 -0.011 Comparative Example 1 -0.016 -0.039 -0.054 Comparative Example 6 -0.014 -0.035 -0.051

[0133] Results analysis:

[0134] According to the test data in Table 3, the specimens of Comparative Example 1 and Comparative Example 6 exhibited continuous length shrinkage throughout the entire 56-day test period. In contrast, the specimens of Examples 1-3 all showed length expansion at 7 days of age, and their shrinkage values ​​at the subsequent 28 and 56 days of age were significantly lower than those of Comparative Example 1 and Comparative Example 6. This indicates that the components in Examples 1-3 can compensate for the shrinkage of concrete.

[0135] Comparing the data from Example 1 and Comparative Example 6 reveals the mechanism of action of the composite expansive agent. Example 1 showed an expansion of 0.004% at 7 days, while Comparative Example 6 showed a shrinkage of 0.014%. This is because the composite expansive agent in Example 1 contains calcium sulfoaluminate, which reacts rapidly with water in the first few days of concrete hardening to form ettringite crystals. The volume increase of these crystals offsets the early plastic and chemical shrinkage of the concrete, resulting in net expansion.

[0136] During the curing period of 28 to 56 days, the shrinkage of Comparative Example 6 continued to develop, while the shrinkage development rate of Example 1 was relatively slow. This phenomenon is attributed to another component in the composite expansive agent—lightly calcined magnesium oxide. Lightly calcined magnesium oxide has a slower hydration reaction rate, continuously and slowly generating magnesium hydroxide crystals over a longer curing period. This long-term micro-expansion effect compensates for the drying shrinkage of concrete caused by moisture evaporation during this stage. Therefore, through the combined effects of calcium sulfoaluminate and lightly calcined magnesium oxide at different time stages, full-cycle shrinkage compensation for concrete from early to late stages is achieved.

[0137] Test Example 4: Waterproofing performance test after crack self-healing

[0138] Experimental steps:

[0139] The test subjects in this test case are Examples 1-3, Comparative Example 1, and Comparative Example 7.

[0140] Cylindrical specimens with a diameter of 150 mm and a height of 150 mm were prepared using the mixing ratios of each group and cured in a standard curing room for 28 days.

[0141] The cured specimens were placed on a universal testing machine and subjected to splitting loading. Loading was controlled by placing a 5mm diameter steel bar between the specimen and the pressure plate to induce a through-crack. Crack formation was monitored using a displacement sensor, and loading was stopped when the crack width reached 0.15±0.02mm.

[0142] The specimens with pre-existing cracks were placed in an automated wet-dry cycle system. One cycle consisted of immersion in clean water at 20±2℃ for 23 hours, followed by air drying for 1 hour. This process was repeated 28 times.

[0143] After the repair and curing are completed, the specimens are removed from the wet-dry cycle equipment, wiped dry, and installed on the concrete permeability tester.

[0144] A constant water pressure of 0.5 MPa was applied to the specimen and maintained for 24 hours.

[0145] After the penetration test, the specimen was removed and split along the original crack surface. The maximum penetration depth of the water on the crack surface was measured using calipers. Three specimens were tested for each group, and the average value was taken.

[0146] Experimental data:

[0147] The results of the anti-permeability test after crack repair for each group are recorded in Table 4.

[0148] Table 4. Test results of impermeability of each group after crack repair.

[0149] Group Average water penetration height / mm Example 1 11 Example 2 15 Example 3 9 Comparative Example 1 141 Comparative Example 7 125

[0150] Results analysis:

[0151] According to the test data in Table 4, after 28 days of wet-dry cycle repair, the average water seepage height of the specimens in Examples 1-3 was significantly lower than that in Comparative Example 1 and Comparative Example 7. The water seepage height of Comparative Example 1 was close to the total height of the specimen, indicating that its cracks had basically not healed and its impermeability was lost. This set of comparative data directly reflects the ability of the concrete described in this invention to restore its impermeability after cracks have formed.

[0152] Comparing the data of Example 1 and Comparative Example 7, the average seepage height of Example 1 was only 11 mm, while that of Comparative Example 7 was as high as 125 mm. The concrete in Example 1 contained a cement-based penetrating crystalline material. During the wet-dry cycle, when water seeped into the concrete through cracks, the active chemical substances in the material were activated and underwent a secondary hydration reaction with cement hydration products and unhydrated cement particles.

[0153] The reaction produces water-insoluble dendritic crystals, primarily hydrated calcium silicate. These crystals grow and fill the pores and interfaces within the cracks, gradually blocking the crack channels and preventing further water intrusion. Comparative Example 7, lacking this functional component, showed only slight restoration of impermeability due to the limited effect of the self-hydration of the concrete; therefore, the cracks remained essentially permeable. This confirms that cement-based penetrating binders are key components for crack repair and restoration of impermeability.

Claims

1. A gradient temperature-controlled self-healing crack-resistant concrete, characterized in that, According to parts by weight, the following raw materials are included: Cement: 220-270 parts; Fly ash: 80-110 parts; Coarse aggregate: 950–1150 parts; Manufactured sand: 720-800 parts; Water-reducing agent: 7-12 parts; Water: 130-160 parts; Composite expanding agent: 5-20 parts; Cement-based penetrating crystallizing material: 10-25 parts; Basalt fiber: 5-20 parts; Includes three types of phase change microcapsules with different melting points: 20-50 parts; Including shape memory alloy fibers with a surface coated with a thermosensitive interface layer: 1 to 15 parts.

2. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 1, characterized in that, The three phase change microcapsules with different melting points are microcapsules composed of paraffin core material and silica shell material, and their melting points are 35±2℃, 45±2℃ and 55±2℃, respectively.

3. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 1, characterized in that, The shape memory alloy fiber with a surface coated with a thermosensitive interface layer has the following characteristics: The shape memory alloy fiber is a nickel-titanium alloy fiber that has undergone 3.5% to 4.5% pre-strain treatment; The thermal interface layer is a thermoplastic polymer coating with a glass transition temperature Tg of 60-65°C. Furthermore, the glass transition temperature (Tg) of the thermosensitive interface layer is higher than the highest melting point among the three phase change microcapsules with different melting points.

4. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 1, characterized in that, The composite expanding agent is composed of calcium sulfoaluminate and lightly calcined magnesium oxide in a mass ratio of 3:

1.

5. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 3, characterized in that, The shape memory alloy fiber has a diameter of 0.1±0.02mm, and the thickness of the thermosensitive interface layer is 1~5μm.

6. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 1, characterized in that, The basalt fibers are short-cut fibers with a length of 12±1mm.

7. A method for preparing gradient temperature-controlled self-healing crack-resistant concrete, as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Provide pre-strained nickel-titanium alloy fibers and coat their surface with a thermoplastic polymer having a preset glass transition temperature Tg to form a thermosensitive interface layer, thereby obtaining shape memory alloy fibers with a thermosensitive interface layer coated on the surface. S2. The cement, fly ash, coarse aggregate, manufactured sand, composite expansion agent, cement-based penetrating crystallizing material, and three phase change microcapsules with different melting points are dry-mixed for the first time; then the shape memory alloy fiber and basalt fiber with a heat-sensitive interface layer on the surface obtained in step S1 are added for the second dry mixing; finally, water and water-reducing agent are added for wet mixing to obtain concrete mixture.

8. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 7, characterized in that, In step S1, the pre-strain treatment is a pre-strain of 3.5% to 4.5%, and the glass transition temperature Tg of the thermosensitive interface layer is 60 to 65°C.

9. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 7, characterized in that, In step S2, the first dry mixing time is 60-90 seconds, the second dry mixing time is 60-90 seconds, and the total wet mixing time is 180-210 seconds.

10. The gradient temperature-controlled self-healing crack-resistant concrete according to claim 7, characterized in that, The method also includes the step of pouring the concrete mixture, and in the pouring step, by layering the material or controlling the pouring sequence, the three phase change microcapsules with different melting points form a gradient distribution from the surface to the core in the concrete cross section.