A low-shrinkage, crack-resistant, ultra-high performance concrete composition, its preparation process, and its application.

Abstract

This invention provides a low-shrinkage, crack-resistant, ultra-high performance concrete composition, its preparation process, and its application. The concrete composition includes cement, fly ash, silica fume, quartz sand, CaO expanding agent, MgO expanding agent, pre-absorbent zeolite powder, polycarboxylate superplasticizer, steel fiber, and water. This invention employs a composite expansion source of calcium oxide and magnesium oxide, synergistically combined with a pre-absorbent zeolite powder internal curing agent, to achieve full-process shrinkage compensation from early to late stages. A progressive curing system of "initial film sealing – steam curing with mold – later water retention" is designed, and segmented skip-construction and induced joint construction methods are provided for large-area components. Through precise matching of material temporal compensation and external curing, the system systematically stimulates shrinkage potential, achieving three-dimensional crack suppression throughout the entire process while ensuring the ultra-high strength of UHPC, ultimately obtaining crack-free components with high dimensional stability. This is particularly suitable for projects with extremely high crack resistance requirements, such as large-volume, ultra-long structures.

Description

Technical Field

[0001] This invention belongs to the field of building materials, specifically relating to a low-shrinkage, crack-resistant, ultra-high performance concrete composition based on the synergistic effect of a composite expansion system and internal curing, a construction process for preparing concrete specimens using this concrete composition, and the application of this high performance concrete in large-volume and ultra-long structural engineering projects. Background Technology

[0002] In large-volume and ultra-long concrete structural members, the superposition of hydration heat-induced temperature gradients and volumetric deformations such as autogenous shrinkage and drying shrinkage is significant. Furthermore, the strong constraints imposed by the foundation, adjacent components, and reinforcement make early-age cracking prone to occur when tensile stress exceeds tensile capacity. Once cracks form, the high permeability of ordinary concrete makes them susceptible to becoming channels for leakage and the migration of corrosive media, thus weakening durability and inducing steel corrosion. While reinforced concrete can disperse cracks, it is difficult to eliminate the adverse effects of cracked work on waterproofing and durability. Prestressed concrete can suppress cracking during service life, but it is constrained by system complexity, quality sensitivity, and cost. High-performance concrete, while improving strength and durability, still carries the risk of increased brittleness and sensitivity to early shrinkage.

[0003] Therefore, Ultra-High Performance Concrete (UHPC), with its superior crack resistance, tensile strength, and dense matrix, can be used in critical joints, construction joints, and seepage-sensitive areas to improve crack control and impermeability durability. However, its high cementitious material content and ultra-low water-cement ratio also bring significant volume stability risks: hydration and pozzolanic reactions cause significant chemical shrinkage and trigger autogenous drying and capillary negative pressure, leading to significant auto-shrinkage; under insufficient curing or dry conditions, drying shrinkage is further amplified. This shrinkage is difficult to release under strong constraints and transforms into tensile stress, easily promoting the development of microcracks into macroscopic cracks. This contradiction manifests as temperature-shrinkage composite cracks in large-volume components, and as shrinkage accumulation, regular crack distribution, and warping and curling in ultra-long thin-walled components, affecting smoothness and waterproofing performance.

[0004] Existing shrinkage control methods often rely on a single approach, such as adding expansion agents or configuring an internal curing system. However, single expansion compensation often suffers from time-sensitivity and insufficient adaptation in the later stages (e.g., the expansion release process of calcium oxide is relatively short). Internal curing alone has limited effect on inhibiting early self-shrinkage and is significantly affected by pore structure and environmental boundaries. At the same time, the effects of both are highly dependent on the matching curing and humidity boundary conditions, which can easily lead to mismatch between material measures and curing regimes.

[0005] Therefore, there is an urgent need in this field to construct an integrated "materials-process-spatiotemporal" strategy for the volumetric stability of UHPCs, in order to achieve coordinated regulation throughout the entire process from early to late age and from the internal to the external boundary, thereby systematically reducing the accumulation of shrinkage strain and the level of constrained tensile stress, and inhibiting the initiation and propagation of cracks. Based on this, this invention addresses the problems of insufficient compensation by single measures and mismatch between materials and curing, proposing a UHPC shrinkage suppression method with a calcium oxide (CaO) and magnesium oxide (MgO) composite expansion system and a pre-absorbent zeolite powder internal curing agent as its core. Through the coupling mechanism of multi-stage expansion compensation and internal water supply and humidity regulation, the continuity and stability of shrinkage control at different ages are improved, thereby making up for the shortcomings of existing technologies and filling the gaps in related research and engineering applications. Summary of the Invention

[0006] The present invention aims to overcome the shortcomings of the prior art and provide a low-shrinkage, crack-resistant, ultra-high performance concrete composition, preparation process, and application. The process is based on the synergistic effect of a composite expansion system and internal curing to inhibit shrinkage of UHPC. It is a comprehensive and synergistic approach from materials to processes, from time to space, to systematically solve the cracking problem of UHPC, especially large-volume and ultra-long components.

[0007] To achieve the above objectives, the present invention provides a low-shrinkage, crack-resistant, ultra-high performance concrete composition, wherein the concrete composition specifically comprises the following components by weight: 60-75 parts cement, 10-20 parts fly ash, 10-20 parts silica fume, 90-120 parts quartz sand, 2-5 parts CaO expanding agent, 3-8 parts MgO expanding agent, 3-8 parts zeolite powder, 0.8-2.5 parts polycarboxylate superplasticizer, 15-25 parts steel fiber, and 14-20 parts water;

[0008] The CaO and MgO expansive agents are compounded according to the total amount of concrete binder to achieve age-specific expansion compensation in the early and middle-to-late stages; the mass ratio of CaO to MgO is 2.9~3.1:5.9~6.1, and the composite expansion system satisfies the following requirements: ≥ 0.15% limited expansion rate in water after 7 days and ≥ −0.01% limited expansion rate in air after 21 days.

[0009] The zeolite powder is pre-absorbed with water before preparation, with a pre-absorption rate of 10% to 30%, and then left to stand for 10 to 30 minutes.

[0010] The preferred technical solution of the present invention is as follows: the amount of CaO expansion agent added is 2% to 5% of the total amount of concrete adhesive, preferably 3%; the amount of MgO expansion agent is 3% to 8% of the total amount of adhesive, preferably 6%; the amount of zeolite powder is 3% to 8% of the total amount of concrete adhesive, preferably 5%, the median particle size of the zeolite powder is 3.8 to 5.2 μm, and the 28-day activity index is ≥95%. The zeolite powder is used for internal curing and moisture regulation and to inhibit shrinkage caused by autogenous drying.

[0011] The preferred technical solution of the present invention is as follows: the quartz sand is a two-graded quartz sand or a continuously graded quartz sand; the steel fiber is a straight filament or a microfilament steel fiber, and can be a single specification or a graded compound specification.

[0012] This invention also provides a process for preparing low-shrinkage, crack-resistant ultra-high performance concrete, wherein the high-performance concrete is constructed using the aforementioned ultra-high performance concrete composition, and the specific steps are as follows:

[0013] S1. Mix the zeolite powder with a portion of the total water volume to achieve a water absorption rate of 10% to 30%, and let it stand for 10 to 30 minutes to allow the water to enter the pores and be evenly distributed, thus obtaining pre-absorbed zeolite powder.

[0014] S2. Add all the cement, fly ash, silica fume, quartz sand, CaO expanding agent, and MgO expanding agent to a mixer and dry mix to obtain a uniform dry mixture; S3. Dissolve the polycarboxylate superplasticizer in the remaining water to form a superplasticizer solution. Add a portion of the superplasticizer solution to the dry mixture obtained in step S2, and after stirring and dispersing, add the pre-absorbent zeolite powder prepared in step S1 and continue stirring to obtain a second mixture; S4. Add the remaining superplasticizer solution to the second mixture in portions and stir until the slurry is uniform, fine, and has a stable flow state without obvious agglomeration or dry powder, to obtain a mixed slurry; S5. Add steel fibers to the mixed slurry continuously and slowly and stir until the steel fibers are evenly distributed until the slurry is uniform, fine, and has a stable flow state without obvious agglomeration or dry powder, to obtain a mixed slurry, thus obtaining a UHPC mixture; S6. Pour the UHPC mixture into a mold, pour and compact it, and then perform synergistic curing. The synergistic curing includes initial film sealing, steam curing with the mold, and subsequent water retention curing.

[0015] A further technical solution of the present invention: The specific steps of the progressive collaborative maintenance in step S6 are as follows:

[0016] S601. Initial film sealing and curing stage: Immediately after the UHPC is poured and smoothed, cover it with a sealing film for at least until the final set. The initial film sealing and curing lasts for 12 to 24 hours to reduce the risk of surface water loss and plastic shrinkage and stabilize the humidity boundary.

[0017] S602. Steam curing stage with mold: After the UHPC has set, before demolding, or when the compressive strength of the UHPC reaches 8-12 MPa after setting, the molded components shall be steam cured at low temperature; the steam curing with mold shall meet the following requirements: heating rate 8-15℃ / h; constant temperature 40-60℃; constant temperature time 24-48 h; cooling rate not greater than 10℃ / h;

[0018] S603. Post-construction water retention and curing stage: After demolding, the components shall be continuously water retained and cured for 7 to 14 days. The water retention and curing methods are one or more of the following: spraying curing agent combined with water sprinkling, film covering for water retention, or covering with wet burlap sacks.

[0019] The preferred technical solution of the present invention is as follows: the ultra-high performance concrete preparation process meets the following temperature control conditions: the temperature of the raw materials entering the machine is 5~30 ℃, the temperature of the mixture exiting the machine is not greater than 30 ℃, and the temperature of the mixture entering the mold is not greater than 28 ℃.

[0020] The preferred technical solution of the present invention is as follows: the construction performance of the UHPC mixture obtained in step S5 satisfies at least one of the following: initial spread is 640~700 mm; 2-hour spread is not less than 510 mm; T 50 The curing time is 2.5~6.0 s; the air content is not greater than 3.0%; there is no obvious water separation after standing for 30 min; the performance of UHPC after hardening in step S6 meets at least one of the following: 28-day compressive strength is 145~168 MPa; tensile strength is 7.0~8.3 MPa; 365-day drying shrinkage is not greater than 165 microstrain.

[0021] The preferred technical solution of the present invention is as follows: In step S1, the mixture is first dry-mixed at low speed for 2-3 minutes and then dry-mixed at medium speed for 2-3 minutes; In step S2, the first part of the water-reducing agent solution accounts for 60%-70% of the total amount of the water-reducing agent solution, and after stirring at medium speed for 3-5 minutes, the pre-absorbed zeolite powder is added and stirring at medium speed is continued for 2-3 minutes; In step S3, the remaining 30%-40% of the water-reducing agent solution is added in 2-3 portions, and the mixture is switched to high speed for 2-4 minutes until the slurry enters a stable flow state; In step S4, after the steel fiber is added, the mixture is stirred at medium-high speed for 3-5 minutes.

[0022] This invention also provides an application of low-shrinkage, crack-resistant, ultra-high performance concrete, specifically the application of the above-mentioned high-performance concrete composition in large-volume or ultra-long structural engineering projects, such as bridge piers, abutments, ultra-long thin-walled roofs, thin-layer pavement of bridge decks, wet joint cast-in-place connection strips, or combinations thereof. The construction process for this application adopts the preparation process of the above-mentioned low-shrinkage, crack-resistant, ultra-high performance concrete.

[0023] The preferred technical solution of this invention is as follows: the bridge piers or abutments are constructed by skip-section casting with induced joints. After casting, each section or cell is sealed with a membrane. Each section or cell is then subjected to progressive and coordinated curing. For large-volume bridge pier components, the structure is divided into 2 to 4 sections along the height direction. The ultra-long thin-walled roof or bridge deck thin-layer paving is constructed by skip-section casting in 3 to 6 m sections. Induced joints or isolation strips are set at the boundaries of the sections to achieve zoned release of shrinkage deformation and path guidance. The vibration compaction time for each section or cell is 1 to 3 minutes to remove large air bubbles and prevent segregation. The surface is immediately sealed with a membrane after leveling.

[0024] This invention provides a shrinkage inhibition method for UHPC based on the synergistic effect of a composite expansion system and internal curing. Its core lies in the construction of a control system. Regarding material synergy: a calcium oxide and magnesium oxide composite expansion system is constructed, and pre-absorbed zeolite powder is introduced as an internal curing agent to achieve coupled regulation of expansion compensation and internal water supply. Calcium oxide has a high hydration reaction rate, allowing for rapid expansion release in the early stages to offset initial chemical and autogenous shrinkage; magnesium oxide hydration has a lag effect, continuously providing expansion compensation in the middle and later stages. The combination of the two helps to expand the compensation window and improve the shrinkage matching degree throughout the entire age. Pre-absorbed zeolite powder, as an internal curing carrier, slowly releases water when internal humidity decreases, ensuring a continuous water source for the expansion agent's hydration and directly weakening the self-shrinkage driving force by improving self-drying and pore water conditions.

[0025] In terms of process synergy, a progressive curing system of "initial film sealing - steam curing with mold - later water retention" is proposed, and boundary conditions are matched with the material system. Initial sealing reduces the risk of surface water loss and plastic shrinkage; steam curing with mold provides a high-temperature and high-humidity environment in the critical early age period, promoting the system's reaction kinetics and internal moisture migration, thereby enhancing the early compensation effect of calcium oxide and improving the effectiveness of internal curing water release; later water retention maintains the long-term humidity boundary, ensuring the continuous hydration of magnesium oxide and mid-to-late stage expansion compensation, and achieving continuous control of volume stability.

[0026] In terms of spatiotemporal coordination: In response to the strong constraints and scale effects of large-area and ultra-long components, the shrinkage deformation is released in zones and guided by structural measures such as segmented skip-construction and induced joints. The overall shrinkage stress is decomposed in space and the concentration level is reduced, thereby inhibiting crack initiation and propagation and improving the overall structure and service durability.

[0027] Compared with the prior art, the significant advantages of the present invention are:

[0028] (1) Full process coverage; This invention forms a relay mechanism of early rapid compensation and mid-to-late continuous compensation by temporal coupling of CaO and MgO composite expansion source and pre-absorbed zeolite powder internal curing system, thereby realizing continuous shrinkage control from early age to long-term service stage after casting and reducing the risk of missing compensation window.

[0029] (2) Internal and external synergy: The internal maintenance system of the present invention provides an internal slow-release water source, and the progressive external maintenance establishes a stable humidity boundary and inhibits water loss. Together, they form a moisture protection and transmission channel from the inside to the outside, which improves the effectiveness and stability of expansion reaction and contraction inhibition.

[0030] (3) Enhanced performance; In addition to promoting early strength development, the high temperature and high humidity conditions of steam curing in this invention can significantly improve the system reaction kinetics and moisture migration environment, thereby enhancing the effective expansion and release of the composite expansion agent and promoting the diffusion and utilization of internal curing moisture during critical periods, demonstrating the synergistic effect of nonlinear gain.

[0031] (4) System integration; This invention organically couples material composition design, maintenance system and construction organization measures to form an engineerable crack-resistant technology path for large volume and ultra-long UHPC components, realizing a comprehensive shrinkage-cracking treatment solution that is controllable and reproducible throughout the entire process from laboratory mixing to on-site implementation. Detailed Implementation

[0032] The present invention will be further described below through specific embodiments.

[0033] The following examples are used to illustrate the technical solutions of the present invention, but should not be construed as limiting the scope of protection of the present invention. Unless otherwise specified, all raw materials are expressed in parts by weight; the pre-absorption rate refers to the percentage of the mass of water adsorbed by the zeolite powder to the mass of the dry zeolite powder, and the water used for pre-absorption is included in the total water used for mixing.

[0034] The low-shrinkage, crack-resistant, ultra-high-performance concrete composition in this embodiment of the invention specifically comprises the following components by weight: 60-75 parts cement, 10-20 parts fly ash, 10-20 parts silica fume, 90-120 parts quartz sand, 2-5 parts CaO expanding agent, 3-8 parts MgO expanding agent, 3-8 parts zeolite powder, 0.8-2.5 parts polycarboxylate superplasticizer, 15-25 parts steel fiber, and 14-20 parts water; wherein, the CaO expanding agent and MgO expanding agent are compounded according to the total amount of concrete binder to achieve age-specific expansion compensation in the early and middle-to-late stages; the mass ratio of CaO to MgO is 2.9-3.1:5.9-6.1, and the composite expansion system satisfies the following conditions: 7-day water-limited expansion rate ≥ 0.15% and 21-day air-limited expansion rate ≥ −0.01%; the zeolite powder is pre-absorbed with water before preparation, with a pre-absorption rate of 10%-30%, and then allowed to stand for 10-30 days. The CaO expanding agent is added at 2%~5% of the total concrete binder, and the MgO expanding agent is added at 3%~8% of the total binder. The zeolite powder is added at 3%~8% of the total concrete binder, and is used for internal curing, moisture regulation, and inhibiting shrinkage caused by autogenous drying. The quartz sand is double-graded or continuously graded quartz sand; the steel fiber is straight or microfiber steel fiber, and can be a single specification or a graded compound specification.

[0035] The cement mentioned in the examples is silicate cement with a strength grade ≥ 62.5 and a specific surface area ≥ 350 m². 2 The alkali content, C3A and SO3 content preferably meet GB 175-2023. The fly ash is ultrafine fly ash with a D50 of 6-9 μm, water requirement ≤ 92%, loss on ignition ≤ 3.0%, and 28-day activity index ≥ 105%. The silica fume is semi-dense silica fume with a SiO2 content ≥ 94 wt% and a specific surface area of ​​18-24 m². 2 / g, 28d activity index ≥ 115%, loss on ignition ≤ 4.0%. The quartz sand is composed of fine sand with a particle size of 0.20-0.40mm and coarse sand with a particle size of 0.40-0.80mm, with a mass ratio of 0.9-1.1:1. Its SiO2 purity ≥ 98.5%, mud content ≤ 0.3%, chloride ion ≤ 0.01%, and sulfate ≤ 0.3%. The carboxylic acid-based water-reducing agent has a water reduction rate ≥ 38% and a solid content of 35-45%. The zeolite powder is ultrafine zeolite powder with a median particle size of 3.8–5.2 μm and a 28d activity index ≥ 95%, to balance moisture compensation and contribution to volcanic ash reaction. The steel fiber material is a straight copper-plated microfilament, which is a blend of 13-16mm medium fiber with a diameter of 0.19-0.22mm and a tensile strength ≥ 2300 MPa and 8-10mm short fiber, with a mass ratio of 7-8:3-2.

[0036] The ultra-high performance concrete preparation process described in Examples 1 to 3 below meets the following temperature control conditions: the temperature of the raw materials entering the mixer is 5~30 ℃, the temperature of the mixture exiting the mixer is not greater than 30 ℃, and the temperature of the mixture entering the formwork is not greater than 28 ℃. The construction performance of the UHPC mixture prepared in the examples meets at least one of the following: initial spread is 640~700 mm; 2-hour spread is not less than 510 mm; T 50 The curing time is 2.5~6.0 s; the air content is not greater than 3.0%; there is no obvious water separation after standing for 30 min; the properties of UHPC after curing meet at least one of the following: 28-day compressive strength is 145~168 MPa; tensile strength is 7.0~8.3 MPa; 365-day drying shrinkage is not greater than 165 microstrain.

[0037] The specific steps of the progressive collaborative maintenance in Examples 1 to 3 are as follows:

[0038] S601. Initial film sealing and curing stage: Immediately after the UHPC is poured and smoothed, cover it with a sealing film for at least until the final set. The initial film sealing and curing lasts for 12 to 24 hours to reduce the risk of surface water loss and plastic shrinkage and stabilize the humidity boundary.

[0039] S602. Steam curing stage with mold: After the UHPC has set, before demolding, or when the compressive strength of the UHPC reaches 8-12 MPa after setting, the molded components shall be steam cured at low temperature; the steam curing with mold shall meet the following requirements: heating rate 8-15℃ / h; constant temperature 40-60℃; constant temperature time 24-48 h; cooling rate not greater than 10℃ / h;

[0040] S603. Post-construction water retention and curing stage: After demolding, the components shall be continuously water retained and cured for 7 to 14 days. The water retention and curing methods are one or more of the following: spraying curing agent combined with water sprinkling, film covering for water retention, or covering with wet burlap sacks.

[0041] Example 1 provides a segmented, skip-construction method for large-volume UHPC bridge piers. The specific steps are as follows:

[0042] S1. Determine the raw material ratio of the UHPC mixture, which includes the following substances by weight percentage: 71 parts cement; 13 parts fly ash; 16 parts silica fume; 112 parts quartz sand; 3 parts CaO expanding agent; 6 parts MgO expanding agent; 5 parts pre-absorbed zeolite powder; 1.6 parts polycarboxylate superplasticizer; 21 parts steel fiber; and 18 parts water.

[0043] Based on the formula of total adhesive material = cement + fly ash + silica fume, CaO and MgO are compounded at 3% and 6% of the total adhesive material, respectively; zeolite powder is 5% of the total adhesive material. The above configuration enables CaO to provide rapid expansion compensation in the early stage, and MgO to provide continuous compensation in the middle and late stages. The pre-absorbent zeolite powder slowly releases moisture when the internal relative humidity decreases, providing internal water for the continuous hydration of the expansion agent and reducing the self-shrinkage driving force caused by self-drying.

[0044] S2. Mix all the zeolite powder with some water to make the zeolite powder reach the preset pre-absorption rate of 24%, that is, moist but not water-bleeding and not clumping. Let it stand for 20 minutes to allow the water to enter the pores evenly, and obtain pre-absorbed zeolite powder. Its pre-absorption amount is included in the total water consumption of the system to ensure that the water-cement ratio is consistent.

[0045] S3. Add all the cement, fly ash, silica fume, quartz sand, CaO expanding agent, and MgO expanding agent into a mixer. First, dry mix at low speed for 2 minutes, then dry mix at medium speed for 2 minutes to obtain a uniform dry mix. S4. Dissolve the polycarboxylate superplasticizer in the remaining water to form a superplasticizer solution. Take 65% of this solution and slowly add it to the dry mix in step S2. Stir at medium speed for 4 minutes. Then add the pre-absorbent zeolite powder obtained in step S2 and continue to stir at medium speed for 2 minutes to allow the slurry to enter a stable dispersion state, obtaining a second mixture. S5. Add the remaining 35% superplasticizer solution in 3 portions and stir at high speed for 3 minutes until the slurry is uniform, fine, and in a stable flow state without obvious agglomeration or dry powder, obtaining the mixed slurry.

[0046] S6. Add steel fibers to the mixture in a continuous and slow feeding manner, and keep stirring at medium-high speed for 4 minutes until the steel fibers are evenly distributed and there is no obvious clumping; if necessary, the machine can be paused for 1 minute to scrape the wall and then stirred for 1 minute to obtain the final UHPC mixture.

[0047] S7. The bridge pier is poured into three sections along the height direction using the skip-section method. Each section is vibrated and compacted for 2 minutes. After leveling, it is immediately covered and sealed with a membrane.

[0048] S8. Progressive synergistic curing: Cover and seal for 18 hours, steam curing with mold (heating up to 50℃ at 12℃ / h and holding for 30 hours, cooling down ≤10℃ / h), and water curing for 12 days after demolding;

[0049] S9. Induction joints (formed by pre-embedded isolation strips) are set at the boundaries of the storage sections and in stress concentration areas.

[0050] The pier components using this method met the design requirements in terms of 28-day strength; and after one year of on-site observation, no identifiable cracks were observed on the surface. In contrast, the specimens using a single expansion agent and conventional curing showed multiple vertical shrinkage cracks on the side surfaces. The comparison shows that the composite expansion system, together with internal curing, curing regime, and structural release, has a synergistic effect, which can significantly reduce the shrinkage cracking sensitivity of large-volume UHPCs under strong constraint conditions.

[0051] Example 2 provides a skip-casting construction method for ultra-long UHPC thin-walled roofs, the specific steps of which are as follows:

[0052] S1. Determine the raw material ratio of the UHPC mixture, specifically including the following substances by weight percentage: 69 parts cement; 15 parts fly ash; 16 parts silica fume; 104 parts quartz sand; 3 parts CaO; 6 parts MgO; 5 parts pre-absorbent zeolite powder; 1.8 parts water-reducing agent; 18 parts steel fiber; and 17 parts water. Based on the total amount of binder equaling the sum of cement, fly ash, and silica fume, CaO and MgO are blended at 3% and 6% of the total binder amount, respectively; zeolite powder accounts for 5% of the total binder amount, and its pre-absorbent water content is included in the total water consumption of the system to ensure a consistent water-binder ratio. This configuration allows CaO to provide rapid expansion compensation in the early stages, and MgO to provide continuous compensation in the middle and later stages; the pre-absorbent zeolite powder slowly releases moisture when the internal relative humidity decreases, providing internal water for the continuous hydration of the expansion agent and reducing the self-shrinkage driving force caused by self-drying.

[0053] S2. Mix all the zeolite powder with some water to make the zeolite powder reach the preset pre-absorption rate of 20%, that is, moist but not water-bleeding and not clumping. Let it stand for 20 minutes to allow the water to enter the pores evenly, and obtain pre-absorbed zeolite powder. Its pre-absorption amount is included in the total water consumption of the system to ensure that the water-cement ratio is consistent.

[0054] S3. Add all the cement, fly ash, silica fume, quartz sand, CaO expanding agent, and MgO expanding agent into a mixer. First, dry mix at low speed for 2 minutes, then dry mix at medium speed for 2 minutes to obtain a uniform dry mix. S4. Dissolve the polycarboxylate superplasticizer in the remaining water to form a superplasticizer solution. Take 65% of this solution and slowly add it to the dry mix in step S2. Stir at medium speed for 4 minutes. Then add the pre-absorbent zeolite powder obtained in step S2 and continue to stir at medium speed for 2 minutes to allow the slurry to enter a stable dispersion state, obtaining a second mixture. S5. Add the remaining 35% superplasticizer solution in 3 portions and stir at high speed for 3 minutes until the slurry is uniform, fine, and in a stable flow state without obvious agglomeration or dry powder, obtaining the mixed slurry.

[0055] S6. Add steel fibers to the mixture in a continuous and slow feeding manner, and keep stirring at medium-high speed for 4 minutes until the steel fibers are evenly distributed and there is no obvious clumping; if necessary, the machine can be paused for 1 minute to scrape the wall and then stirred for 1 minute to obtain the final UHPC mixture.

[0056] S7. Use the skip-compartment method to pour concrete into the formwork in sections. For the roof, pour concrete in sections with a grid of 4 m × 5 m. The compaction time is 1–2 min. After each section is leveled, cover and seal it with a membrane.

[0057] S8. Progressive synergistic curing: Cover and seal for 16 hours, steam curing with mold (heating up to 46℃ at 10℃ / h and holding for 36 hours, cooling down ≤8℃ / h), and water curing for 10 days after demolding;

[0058] S9. Set an induction seam (post-cut seam, the depth of which is about 1 / 3 to 1 / 2 of the plate thickness) at the boundary of the storage cell.

[0059] After construction in Implementation 2, the roof components exhibited a smooth surface with no identifiable shrinkage cracks, demonstrating good overall dimensional stability. This indicates that the method can also achieve volumetric stability control for ultra-long, thin-walled components through a synergistic mechanism of material compensation, internal water supply, boundary curing, and spatial release.

[0060] Example 3 provides a method for skip-layer paving of UHPC bridge deck pavement, the specific steps of which are as follows:

[0061] S1. Determine the raw material ratio of the UHPC mixture, which includes the following substances by weight percentage: 66 parts cement; 14 parts fly ash; 20 parts silica fume; 96 parts quartz sand; 3 parts CaO; 6 parts MgO; 5 parts pre-absorbed zeolite powder; 2.0 parts water-reducing agent; 23 parts steel fiber; and 16.5 parts water.

[0062] In this system, the total amount of adhesive material equals the sum of cement, fly ash, and silica fume. CaO and MgO are blended at 3% and 6% of the total adhesive material, respectively. Zeolite powder accounts for 5% of the total adhesive material, and its pre-absorbed water content is included in the total water consumption of the system to ensure a consistent water-to-binder ratio. This configuration allows CaO to provide rapid expansion compensation in the early stages, while MgO provides continuous compensation in the middle and later stages. The pre-absorbed zeolite powder slowly releases moisture when the internal relative humidity decreases, providing internal water for the continuous hydration of the expanding agent and reducing the self-shrinkage driving force caused by self-drying.

[0063] S2. Mix all the zeolite powder with some water to make the zeolite powder reach the preset pre-absorption rate of 20%, that is, moist but not water-bleeding and not clumping. Let it stand for 20 minutes to allow the water to enter the pores evenly, and obtain pre-absorbed zeolite powder. Its pre-absorption amount is included in the total water consumption of the system to ensure that the water-cement ratio is consistent.

[0064] S3. Add all the cement, fly ash, silica fume, quartz sand, CaO expanding agent, and MgO expanding agent into a mixer. First, dry mix at low speed for 2 minutes, then dry mix at medium speed for 2 minutes to obtain a uniform dry mix. S4. Dissolve the polycarboxylate superplasticizer in the remaining water to form a superplasticizer solution. Take 65% of this solution and slowly add it to the dry mix in step S2. Stir at medium speed for 4 minutes. Then add the pre-absorbent zeolite powder obtained in step S2 and continue to stir at medium speed for 2 minutes to allow the slurry to enter a stable dispersion state, obtaining a second mixture. S5. Add the remaining 35% superplasticizer solution in 3 portions and stir at high speed for 3 minutes until the slurry is uniform, fine, and in a stable flow state without obvious agglomeration or dry powder, obtaining the mixed slurry.

[0065] S6. The paving layer is laid in 4 m × 4 m sections with skip-slot paving, vibrating as it is laid, and each section is vibrated for 1 to 2 minutes; then covered and sealed with a membrane.

[0066] S7. Progressive synergistic curing: Cover and seal for 20 h, steam curing with mold (heating up to 52 ℃ at 13 ℃ / h and holding for 28 h, cooling down ≤10 ℃ / h), and water curing for 14 days after demolding;

[0067] S8. Set guide joints or isolation strips at the boundaries and corners of the paving zones.

[0068] In Example 3, the surface of the bridge deck pavement layer was smooth, with no identifiable shrinkage cracks, and the pavement layer showed good dimensional stability. In contrast, the pavement areas treated with conventional maintenance or without skip joints and induced joints were prone to developing fine shrinkage cracks or localized stress concentration cracks distributed along the length direction, indicating that this method has a significant crack control effect on thin-layer, large-area pavement.

[0069] Comparative Example 1: Using the large-volume UHPC pier casting process in Example 1 as a benchmark, the UHPC raw materials and construction process in Comparative Example 1 are the same as those in Example 1. The difference is that the expansion agent is only doped with CaO, and the doping amount is kept constant at 3% colloid content. MgO is not doped. The composition of other raw materials, mixing process, segmented skip-filling and induced joint setting, and curing system of membrane sealing-steam curing with mold-post-water retention curing are all the same as those in Example 1. A large-volume UHPC pier containing only CaO expansion agent is obtained.

[0070] Comparing the large-volume UHPC pier cast in Example 1 with that cast in Comparative Example 1, the pier component prepared in Example 1 met the design requirements after 28 days of testing; and after one year of on-site observation, no identifiable cracks were observed on the surface. The large-volume UHPC pier cast in Comparative Example 1, containing only CaO expansion agent, showed insufficient early or mid-to-late-stage compensation, resulting in vertical, network-like shrinkage cracks or an increased number of cracks. This comparison demonstrates the necessity of the CaO-MgO compound formulation of this invention to expand the compensation window.

[0071] Comparative Example 2: Using the large-volume UHPC pier casting process in Example 1 as a benchmark, the UHPC raw materials and construction process in Comparative Example 1 are the same as those in Example 1. The difference is that the expansion agent is CaO with 3% colloidal content and MgO with 6% colloidal content, but no zeolite powder is added. However, the water-cement ratio is kept consistent. Meanwhile, the composition of other raw materials, mixing process, segmented skip-filling and induced joint setting, and curing system of membrane sealing-steam curing with mold-post-water retention curing are all the same as those in Example 1, and a large-volume UHPC pier without zeolite powder is obtained.

[0072] Comparative analysis revealed that the humidity decreased more rapidly in the later stages of construction of large-volume UHPC bridge piers without zeolite powder, limiting MgO hydration and increasing the risk of shrinkage and cracking. A comparison with Example 1 demonstrates the significant contribution of the pre-absorbent zeolite powder of this invention in maintaining continuous hydration of the expanding agent and mitigating autogenous drying.

[0073] Comparative Example 3: Taking the large-volume UHPC pier casting process in Example 1 as the benchmark, the UHPC raw materials and construction process in Comparative Example 1 are the same as those in Example 1. The difference is that the curing system is changed from the original curing system of membrane sealing - steam curing with mold - water retention curing in the later stage to conventional curing, that is, no membrane sealing is performed after casting, no steam curing with mold is performed, and only conventional water spraying curing is performed in the later stage.

[0074] The comparative results show that conventionally maintained large-volume UHPC bridge piers exhibit more significant early water loss and plastic shrinkage, and the insufficient hydration environment for the expanding agent makes cracks more prone to initiation and propagation. A comparison with Example 1 demonstrates the boundary protection effect of the "film sealing-steam-water retention" progressive maintenance method of this invention on the synergistic mechanism.

[0075] This application compares the workability and mechanical properties of concrete specimens prepared in Examples 1 to 3 with those prepared in Comparative Examples 1 to 3, as follows:

[0076] (1) Detection methods and judgment criteria:

[0077] a. Spreadability of the mixture: The spreadability was tested using a flowability test pan, and the initial spreadability and the spreadability after standing for 2 hours were recorded; the time T required for spread to 500 mm was also recorded. 50 As a characterization index of viscosity and anti-segregation.

[0078] b. Gas content: The gas content was determined using a pressure method gas content meter.

[0079] c. Mechanical properties: 28-day compressive strength shall be determined by the commonly used concrete or mortar compressive strength test method; tensile strength shall be determined by the UHPC direct tension or equivalent tensile test method (such as the test caliber or engineering equivalent method given in T / CECS 10107).

[0080] d. Drying shrinkage: The drying shrinkage test shall be conducted in accordance with the method specified in GB / T 50082, and the drying shrinkage values ​​at 90 days and 365 days shall be recorded.

[0081] e. Mixing and curing environment: The ambient temperature test environment is (20±2)℃, and the relative humidity is (60±5)%; the drying shrinkage test shall be carried out according to the standard conditions.

[0082] The comparative test parameters are shown in Table 1, and the test results are shown in Table 2.

[0083] Table 1. Workability and Construction Window of Examples and Comparative Examples

[0084] Group Initial expansion / mm 2h Expansion / mm <![CDATA[T 50 / s]]> Gas content / % 30 min stability Example 1 (Large-volume bridge pier) 655 522 4.0 2.4 No obvious water separation Example 2 (Ultra-long thin-walled roof) 646 536 3.5 2.6 No obvious water separation Example 3 (Thin Bridge Deck) 642 518 4.8 2.2 No obvious water separation Comparative Example 1 (CaO only) 652 522 3.9 2.5 Generally stable, with a slight tendency for localized fiber aggregation. Comparative Example 2 (Zeolite powder internal curing omitted) 590 495 3.3 2.6 After 30 minutes, the viscosity increased significantly, with localized water loss and thickening, and the application window narrowed. Comparative Example 3 (Cancellation of Progressive Collaborative Maintenance) 645 520 4.2 3.1 The surface is generally stable but has a high gas content, and small pores are easily visible after compaction.

[0085] Table 2 shows the mechanical properties and volume stability of concrete in the examples and comparative examples.

[0086] Group 28d compressive strength / MPa 28d tensile strength / MPa 90-day drying shrinkage / με 365d drying shrinkage / με Remark Example 1 (Large-volume bridge pier) 160 7.7 -108 -152 — Example 2 (Ultra-long thin-walled roof) 154 7.3 -115 -160 — Example 3 (Thin Bridge Deck) 166 8.2 -102 -145 28-day interfacial splitting bond strength 6.2 MPa Comparative Example 1 (CaO only) 158 7.6 -156 -215 Increased fine cracks in the middle and late stages Comparative Example 2 (Zeolite powder internal curing omitted) 150 7.1 -142 -198 2h window narrowing Comparative Example 3 (Cancellation of Progressive Collaborative Maintenance) 148 7.4 -176 -236 Early stage is more prone to surface fine cracks

[0087] As can be seen from Tables 1 and 2, the three sets of embodiments exhibit a relatively consistent range of advantages in terms of workability, construction window, and volumetric stability: the initial expansion of the embodiments is 642–655 mm, the 2-hour expansion is 518–536 mm, and the T... 50 With a shrinkage period of 3.5–4.8 s and an air content controlled at 2.2%–2.6%, no significant bleeding or segregation was observed within 30 minutes, indicating that stable rheology and workability can still be achieved under conditions of high fiber content and low water-cement ratio. Regarding mechanical properties, the compressive strength of Example 28 at 28 days was 154–166 MPa and the tensile strength was 7.3–8.2 MPa, meeting the load-bearing and crack resistance requirements for UHPC structures and connection / paving applications. In terms of volumetric stability, the drying shrinkage of Example 90 days was −102 to −115 με, and the drying shrinkage at 365 days was −145 to −160 με. The overall long-term shrinkage level was low and the dispersion was small, indicating that the combination of "early CaO compensation, mid-to-late stage continuous MgO compensation, pre-absorbed zeolite powder internal curing, and progressive synergistic curing" can effectively suppress shrinkage accumulation throughout the entire curing period.

[0088] The comparative results further verified the necessity of each key element: In Comparative Example 1, after removing MgO, the 28-day strength and workability did not change significantly, but the 365-day drying shrinkage increased from −152 με to −215 με, indicating that the lack of mid-to-late stage compensation would lead to a significant increase in long-term net shrinkage; In Comparative Example 2, after removing zeolite powder internal curing, the 2-hour expansion decreased from 536 mm to 495 mm and the 365-day shrinkage increased to −198 με, indicating that internal water supply plays a supporting role in slump retention and late-stage compensation hydration; In Comparative Example 3, after removing progressive synergistic curing, the 365-day shrinkage increased to −236 με, accompanied by higher air content and a tendency for early fine cracking, indicating that the humidity boundary and curing regime are key guarantees for the effectiveness of material measures. Overall, the examples showed a reduction of approximately 19%–36% in long-term drying shrinkage compared to the comparative examples, while maintaining a good construction window and strength level, making them suitable for the comprehensive requirements of crack resistance, impermeability, and long-term durability in large-volume and ultra-long structures.

Claims

1. A low-shrinkage, crack-resistant, ultra-high performance concrete composition, characterized in that, The concrete composition specifically comprises the following components by weight: 60-75 parts cement, 10-20 parts fly ash, 10-20 parts silica fume, 90-120 parts quartz sand, 2-5 parts CaO expanding agent, 3-8 parts MgO expanding agent, 3-8 parts zeolite powder, 0.8-2.5 parts polycarboxylate superplasticizer, 15-25 parts steel fiber, and 14-20 parts water; The CaO and MgO expansive agents are compounded according to the total amount of concrete binder to achieve age-specific expansion compensation in the early and middle-to-late stages; the mass ratio of CaO to MgO is 2.9~3.1:5.9~6.1, and the composite expansion system satisfies the following requirements: ≥ 0.15% limited expansion rate in water after 7 days and ≥ −0.01% limited expansion rate in air after 21 days. The zeolite powder is pre-absorbed with water before preparation, with a pre-absorption rate of 10% to 30%, and then left to stand for 10 to 30 minutes.

2. The low-shrinkage, crack-resistant, ultra-high-performance concrete composition according to claim 1, characterized in that: The amount of CaO expanding agent added is 2% to 5% of the total amount of concrete adhesive, and the amount of MgO expanding agent is 3% to 8% of the total amount of adhesive; the amount of zeolite powder is 3% to 8% of the total amount of concrete adhesive, the median particle size of the zeolite powder is 3.8 to 5.2 μm, and the 28-day activity index is ≥95%. The zeolite powder is used for internal curing and moisture regulation and to inhibit shrinkage caused by autogenous drying.

3. The low-shrinkage, crack-resistant, ultra-high performance concrete composition according to claim 1, characterized in that: The quartz sand is either two-graded or continuously graded; the steel fiber is straight or microfiber steel fiber with a diameter of 0.19-0.22 mm and a tensile strength ≥ 2300 MPa, and is available in single or graded compound specifications.

4. A preparation process for low-shrinkage, crack-resistant, ultra-high performance concrete, characterized in that, The high-performance concrete is constructed using the ultra-high-performance concrete composition according to any one of claims 1 to 3, and the specific steps are as follows: S1. Mix the zeolite powder with a portion of the total water volume to achieve a water absorption rate of 10% to 30%, and let it stand for 10 to 30 minutes to allow the water to enter the pores and be evenly distributed, thus obtaining pre-absorbed zeolite powder. S2. Put all the cement, fly ash, silica fume, quartz sand, CaO expansion agent and MgO expansion agent into the mixer and dry mix to obtain a uniform dry mix. S3. Dissolve the polycarboxylate superplasticizer in the remaining water to form a superplasticizer solution. Add a portion of the superplasticizer solution to the dry mixture obtained in step S2, and after stirring and dispersing, add the pre-absorbent zeolite powder prepared in step S1 and continue stirring to obtain a second mixture. S4. Add the remaining water-reducing agent solution to the second mixture in portions and stir until the slurry is uniform, fine, and in a stable flow state without obvious agglomeration or dry powder, to obtain the mixed slurry. S5. Add steel fibers to the mixture in a continuous and slow feeding manner and stir until the steel fibers are evenly distributed to obtain UHPC mixture; S6. Pour the UHPC mixture into a mold, pour and compact it, and then perform synergistic curing. The synergistic curing includes initial film sealing, steam curing with the mold, and subsequent water retention curing.

5. The preparation process of low-shrinkage, crack-resistant, ultra-high performance concrete according to claim 4, characterized in that, The specific steps of the progressive collaborative maintenance in step S6 are as follows: S601. Initial film sealing and curing stage: Immediately after the UHPC is poured and smoothed, cover it with a sealing film for at least until the final set. The initial film sealing and curing lasts for 12 to 24 hours to reduce the risk of surface water loss and plastic shrinkage and stabilize the humidity boundary. S602. Steam curing stage with mold: After the UHPC has set, before demolding, or when the compressive strength of the UHPC reaches 8-12 MPa after setting, the molded components shall be steam cured at low temperature; the steam curing with mold shall meet the following requirements: heating rate 8-15 ℃ / h; constant temperature 40-60 ℃; constant temperature time 24-48 h; cooling rate not greater than 10 ℃ / h; S603. Post-construction water retention and curing stage: After demolding, the components shall be continuously water retained and cured for 7 to 14 days. The water retention and curing methods are one or more of the following: spraying curing agent combined with water sprinkling, film covering for water retention, or covering with wet burlap sacks.

6. The preparation process of low-shrinkage, crack-resistant, ultra-high performance concrete according to claim 4 or 5, characterized in that, The ultra-high performance concrete preparation process meets the following temperature control conditions: the temperature of the raw materials entering the machine is 5~30 ℃, the temperature of the mixture exiting the machine is not greater than 30 ℃, and the temperature of the mixture entering the mold is not greater than 28 ℃.

7. The preparation process of low-shrinkage, crack-resistant ultra-high performance concrete according to claim 4 or 5, characterized in that, The UHPC mixture obtained in step S5 has the following workability requirements: initial spread of 640-700 mm; 2-hour spread of not less than 510 mm; T 50 The curing time is 2.5~6.0 s; the air content is not greater than 3.0%; there is no obvious water separation after standing for 30 min; the performance of UHPC after hardening in step S6 meets at least one of the following: 28-day compressive strength is 145~168 MPa; tensile strength is 7.0~8.3 MPa; 365-day drying shrinkage is not greater than 165 microstrain.

8. The preparation process of low-shrinkage, crack-resistant, ultra-high performance concrete according to claim 4 or 5, characterized in that, In step S1, dry mix at low speed for 2-3 minutes, then dry mix at medium speed for 2-3 minutes. In step S2, the first part of the water-reducing agent solution accounts for 60%-70% of the total water-reducing agent solution. After stirring at medium speed for 3-5 minutes, add the pre-absorbed zeolite powder and continue stirring at medium speed for 2-3 minutes. In step S3, add the remaining 30%-40% of the water-reducing agent solution in 2-3 portions, and switch to high speed stirring for 2-4 minutes until the slurry enters a stable flow state. In step S4, after the steel fiber is added, maintain medium-high speed stirring for 3-5 minutes.

9. An application of low-shrinkage, crack-resistant, ultra-high performance concrete, specifically the application of any one of the high-performance concrete compositions of claims 1 to 3 in large-volume or ultra-long structural engineering projects, wherein the engineering projects are bridge piers, abutments, ultra-long thin-walled roofs, thin-layer pavement of bridge decks, cast-in-place wet joint connection strips or combinations thereof, and the construction process for its application adopts the preparation process of any one of the low-shrinkage, crack-resistant, ultra-high performance concrete of claims 4 to 8.

10. The application of the low-shrinkage, crack-resistant ultra-high performance concrete according to claim 9, characterized in that: The bridge piers or abutments are constructed using a skip-section casting method with induced joints. After casting, each section or cell is sealed with a membrane. Each section or cell is then subjected to progressive and coordinated curing. For large-volume bridge pier components, the structure is divided into 2 to 4 sections along the height direction. The ultra-long thin-walled roof or bridge deck thin-layer paving is constructed using a skip-section construction method with sections of 3 to 6 meters in length. Induced joints or isolation strips are set at the boundaries of the sections to release shrinkage deformation and guide the path. The vibration compaction time for each section or cell is 1 to 3 minutes to remove large air bubbles and prevent segregation. The surface is immediately sealed with a membrane after leveling.