A method for manufacturing an ultra-high performance concrete precast bridge slab

Abstract

The application discloses a preparation method of an ultra-high performance concrete prefabricated bridge slab. The preparation method comprises the following steps: weighing cement, silica fume, quartz sand, steel fiber, water reducing agent and water according to a preset proportion; feeding the weighed raw materials into a mixer, and performing forced stirring in a sequence of dry mixing and then wet mixing to obtain an ultra-high performance concrete mixture; pouring the ultra-high performance concrete mixture into a prefabricated bridge slab mold, and performing layered vibration and compaction treatment to obtain a wet bridge slab blank; and sequentially performing temperature changing steam curing of static stopping, temperature rising, constant temperature and temperature dropping on the wet bridge slab blank to complete a hydration reaction; and demolding after the curing is completed and the strength of the blank reaches a demolding standard. The preparation method provided by the application significantly improves the uniformity of fiber dispersion through a specific stirring process, promotes material hydration reaction and strength development through an optimized temperature changing steam curing system, and significantly improves the product quality and production efficiency of the ultra-high performance concrete prefabricated bridge slab.

Description

Technical Field

[0001] This application relates to the field of bridge engineering technology, and in particular to a method for preparing ultra-high performance concrete precast bridge slabs. Background Technology

[0002] With the rapid development of modern transportation, bridge engineering is constantly moving towards longer spans, higher durability, and lighter weight. Traditional ordinary concrete materials, due to their inherent defects such as heavy weight, low tensile strength, and poor durability, can no longer meet the high standards required for modern bridge construction. Ultra-high performance concrete, as a new type of fiber-reinforced cementitious composite material, has shown great application potential in the field of bridge structures due to its ultra-high strength, ultra-high durability, excellent impermeability, and corrosion resistance. Especially in the production of precast bridge slabs, it can significantly reduce the structural weight, improve load-bearing capacity, and extend service life.

[0003] In recent years, ultra-high performance concrete (UHPC) has been widely used in bridge engineering due to its excellent mechanical properties and durability. However, UHPC materials are characterized by a low water-cement ratio, high cementitious material content, and high fiber content, which poses a serious challenge to the production and preparation process.

[0004] First, during the mixing process, the extremely high viscosity of UHPC mixtures makes it easy for traditional mixing techniques and feeding methods to cause steel fibers to clump together, forming fiber balls. This severely affects the uniform dispersion of fibers in the matrix, weakening the reinforcing effect and becoming a source of defects within the material. Second, during the curing process, UHPC undergoes a vigorous hydration reaction and rapid early strength development. If the curing regime is not properly set, it can easily generate significant self-shrinkage and temperature stress, leading to cracks on the surface or inside the bridge slab, severely impacting the durability and appearance quality of the components. Furthermore, the existing manufacturing process suffers from poor coordination between different stages and a lack of systematic control over the overall process, resulting in low production efficiency, large quality fluctuations, and an inability to meet the production demands of high-quality precast components. Summary of the Invention

[0005] This application provides a method for preparing ultra-high performance concrete precast bridge slabs to solve the problems of unstable production quality and insufficient production efficiency of ultra-high performance concrete precast bridge slabs.

[0006] To solve the above-mentioned technical problems, one technical solution adopted in this application is: to provide a method for preparing ultra-high performance concrete precast bridge slabs. The method for preparing ultra-high performance concrete precast bridge slabs includes: Step S1: Weigh cement, silica fume, quartz sand, steel fiber, water-reducing agent and water according to the preset ratio; Step S2: Put the weighed raw materials into the mixer and force mix them in the order of dry mixing first and then wet mixing to obtain ultra-high performance concrete mixture. Step S3: The ultra-high performance concrete mixture is poured into the precast bridge slab mold and subjected to layered vibration compaction to obtain a wet bridge slab blank. Step S4: The wet bridge slab blank is subjected to variable temperature steam curing in sequence, including static stopping, heating, constant temperature and cooling, to complete the hydration reaction; Step S5: Demolding is carried out after curing is completed and the strength of the blank reaches the demolding standard.

[0007] In some embodiments, the preset proportions by weight are: 700-900 parts cement, 200-300 parts silica fume, 900-1100 parts quartz sand, 150-250 parts steel fiber, 15-30 parts water-reducing agent, and 160-200 parts water. The steel fiber is either a straight steel fiber or a hooked steel fiber, with a length of 12mm to 16mm and a diameter of 0.2mm to 0.3mm.

[0008] In some embodiments, in step S2, the forced stirring performed in the order of dry mixing followed by wet mixing includes: Add cement, silica fume and quartz sand into a mixer and dry mix at a speed of 60-80 rpm for 120-180 seconds to form a uniform dry powder mixture. Add all the water and water-reducing agent to the dry powder mixture, and wet mix at a speed of 80-100 rpm for 90-120 seconds to form a fluid slurry; Steel fibers are evenly sprinkled into the flowing slurry, and stirring is continued for 180-240 seconds until the steel fibers are completely dispersed to obtain ultra-high performance concrete mixture.

[0009] In some embodiments, when uniformly sprinkling steel fibers into the flowing slurry, a multi-point dispersing feeding device is used to uniformly sprinkle the steel fibers into the rotating mixer; the feeding rate of the multi-point dispersing feeding device is matched with the rotation speed of the mixer, and the feeding time is controlled within 60 to 90 seconds.

[0010] In some embodiments, in step S3, the pouring into the precast bridge slab mold adopts a layered pouring method, and the thickness of each layer does not exceed 200mm; The vibration compaction treatment uses a combination of an attached high-frequency vibrator and an immersion vibrator, with a vibration time of 30 to 60 seconds, until the concrete surface is covered with slurry and no more bubbles appear.

[0011] In some embodiments, during the vibration process, the frequency of the attached high-frequency vibrator is controlled to be 100Hz to 200Hz, and the vibration force is distributed in a gradient decreasing manner from the bottom surface to the top surface of the bridge slab. After vibration, the top surface of the wet bridge slab blank is smoothed and immediately covered with a plastic film to prevent moisture evaporation.

[0012] In some embodiments, the curing parameters for variable temperature steam curing in step S4 are as follows: Static setting stage: Allow the concrete to stand for 2 to 4 hours at an ambient temperature of 20℃~25℃ to allow it to complete its initial setting. Heating phase: Increase the curing temperature to 80℃~95℃ at a heating rate of 10℃ / hour~15℃ / hour; Constant temperature stage: Maintain the temperature between 80℃ and 95℃ for 24 to 48 hours, and keep the relative humidity above 95%. Cooling phase: The temperature is reduced to less than 10°C from the ambient temperature at a cooling rate of no more than 15°C / hour.

[0013] In some embodiments, during the constant temperature stage, saturated steam at a pressure of 0.05 MPa to 0.1 MPa is introduced into the curing hood, and the steam supply is stopped after the constant temperature is achieved. During the cooling phase, the curing cover uses natural ventilation to cool down the concrete and a curing agent is sprayed on the concrete surface to keep it moist.

[0014] In some embodiments, before cooling down after the constant temperature stage, it is necessary to confirm that the compressive strength of the concrete has reached more than 75% of the design strength grade. If the compressive strength does not meet the standard, extend the curing time in the constant temperature stage until the strength meets the standard, and in the subsequent cooling stage, reduce the cooling rate to 80% to 90% of the original rate.

[0015] In some embodiments, in step S5, the demolding adopts a synchronous lifting demolding process, which uses multiple hydraulic jacks to simultaneously apply an upward lifting force to the bottom of the bridge deck, and the lifting speed is controlled within 5 mm / s.

[0016] The beneficial effects of this application are as follows: Unlike existing technologies, this application discloses a method for preparing ultra-high performance concrete precast bridge slabs. The embodiments of this application effectively solve the problem of steel fiber agglomeration in high-viscosity systems through a dry-mixing followed by wet-mixing process, significantly improving mixing efficiency and fiber dispersion uniformity. An optimized variable-temperature steam curing regime promotes the hydration reaction of the cementitious materials, ensuring the strength development and durability of the bridge slab, and achieving high-quality, high-efficiency production of ultra-high performance concrete precast bridge slabs. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 This is a schematic flowchart of an embodiment of the method for preparing ultra-high performance concrete precast bridge slabs provided in this application; Figure 2 This is a schematic flowchart of an embodiment of step S2 in the preparation method provided in this application; Figure 3 This is a schematic flowchart of an embodiment of step S4 in the preparation method provided in this application. Detailed Implementation

[0018] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0019] The terms "first," "second," and "third" used in the embodiments of this application are for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first," "second," or "third" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or devices.

[0020] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0021] To address the technical bottlenecks in the production of precast bridge slabs using ultra-high performance concrete (UHPC), such as high energy consumption during mixing, difficulty in fiber dispersion, high risk of curing cracking, and easy damage to edges and corners during demolding, this application provides a systematic solution. The core concept of this application lies in: reconstructing the mixing kinetics process to solve the problem of microscopic uniformity in fiber dispersion; designing a variable-temperature steam curing regime based on a thermodynamic model to resolve the contradiction between microstructure development and macroscopic stress cracking; and introducing mechatronics-integrated synchronous lifting demolding technology to solve the problem of non-destructive separation of the high-strength matrix from the mold interface.

[0022] Specifically, this application provides a method for preparing precast bridge slabs of ultra-high performance concrete (UHPC), see reference. Figure 1 , Figure 1 This is a schematic flowchart of an embodiment of the method for preparing ultra-high performance concrete (UHPC) precast bridge slabs provided in this application. The preparation method includes: Step S1: Weigh cement, silica fume, quartz sand, steel fiber, water-reducing agent and water according to the preset ratio.

[0023] Ultra-high performance concrete (UHPC), as a multiphase composite material, has its mechanical properties and durability determined by the microscopic characteristics and macroscopic proportions of its components.

[0024] In this embodiment, grade 52.5 silicate cement conforming to GB 175 standard is preferably used. The reason for choosing high-grade cement is that UHPC has an extremely low water-cement ratio, typically between 0.14 and 0.20. Low-grade cement has a higher content of admixtures, and in a low water-cement ratio environment, the activity of these admixtures is difficult to activate, which would instead dilute the density of the cement gel. In the mineral composition of the cement, the content of tricalcium silicate (C3S) should be controlled above 55% to ensure rapid early strength development; the content of tricalcium aluminate (C3A) should be controlled below 8% to reduce the peak heat of hydration and decrease the risk of thermal cracking during the static settling stage.

[0025] Silica fume is a key active mineral admixture for achieving ultra-high performance in UHPC. In this embodiment, high-quality silica fume with an average particle size of 0.1-0.3 μm and a specific surface area greater than 15000 m² / kg is preferred. The addition of silica fume mainly exerts two major effects: First, the micro-aggregate filling effect. The silica fume particles are much smaller than cement particles, and can fill the voids in the cement particle packing, significantly reducing the initial porosity of the system. According to the Aim and Goff model, this can significantly increase the bulk density of solid particles. Second, the pozzolanic effect. The active silica in silica fume can undergo a secondary reaction with the cement hydration product calcium hydroxide (CH) to generate hydrated calcium silicate (CSH) gel with cementitious properties. This not only consumes the low-strength CH crystals and increases the content of CSH gel, but more importantly, it improves the microstructure of the interfacial transition zone (ITZ), densifying the originally loose and porous ITZ region and significantly improving the interfacial bonding strength between the matrix and aggregate, and between the matrix and fibers.

[0026] Ultra-high performance concrete (UHPC) typically does not use coarse aggregates, but instead employs fine aggregates with a maximum particle size of less than 2 mm. In this embodiment, quartz sand is used as the fine aggregate, and its silica content should be greater than 95% to ensure hardness and chemical stability. The gradation design of the quartz sand follows Andreassen's particle size distribution theory, minimizing the particle packing porosity by mixing sands of different particle size ranges (e.g., 0-0.3 mm, 0.3-0.6 mm, 0.6-1.2 mm) in a specific proportion. In this embodiment, the design of using 900-1100 parts of quartz sand aims to form a dense, rigid skeleton to support the cementitious matrix and reduce shrinkage.

[0027] Steel fibers are key to imparting high toughness to ultra-high performance concrete. In this embodiment, the preset mix proportion is 150-250 parts by weight of steel fibers. This dosage, with a volume fraction of approximately 2%-3.5%, is an optimized selection based on the critical volume fraction of fibers. If the dosage is too low, the fibers cannot form an effective three-dimensional randomly distributed network, resulting in weak bridging effect on cracks; if the dosage is too high, such as exceeding 3%, the ability of the slurry to encapsulate the fibers decreases, the mixing resistance increases sharply, and fiber agglomeration is easily generated, thus becoming a weakness in strength.

[0028] In step S1, the steel fibers are either straight or hooked, with a length of 12mm–16mm and a diameter of 0.2mm–0.3mm. For the UHPC matrix, the matrix has extremely high strength and strong fiber gripping force. Straight fibers mainly rely on physical friction to transfer stress, while hooked fibers increase mechanical interlocking force. The choice of a length of 12-16mm is based on a balance between fiber length effect and dispersion difficulty: if the length is too short (<8mm), the fibers are easily pulled out, resulting in poor toughening effect; if the length is too long (>20mm), it is difficult to disperse in viscous UHPC slurry and easily entangles and overlaps in three-dimensional space, forming fiber balls, leading to excessive load on the mixer or even jamming. The design of fine fibers with a diameter of 0.2-0.3mm is to ensure a sufficient aspect ratio, approximately 40-80, while also allowing the fine fibers to bend and deform more easily in the matrix, adapting to complex stress fields.

[0029] Furthermore, the steel fibers are coated with a copper or zinc layer for corrosion protection, and the tensile strength of the steel fibers is greater than 2000 MPa. UHPC is often used in harsh environments such as bridges, where chloride ion corrosion is the main cause of steel corrosion. The copper or zinc plating not only prevents the fibers from corroding during the mixing process, but more importantly, in the hardened matrix, the plating acts as a sacrificial anode, providing cathodic protection when microcracks extend to the fiber surface. The requirement of a tensile strength greater than 2000 MPa ensures that the failure mode, based on the principle of strong fibers and weak matrix, is fiber pull-out rather than fiber breakage, because the pull-out process absorbs a large amount of fracture energy, giving the material excellent toughness.

[0030] The water-reducing agent selected is a polycarboxylate-based high-performance water-reducing agent (PCE). Its molecular structure is comb-like, adsorbing onto the surface of cement particles via the main chain, while the side chains extend in water, providing a steric hindrance effect, thereby releasing the fluidity of the slurry with extremely low water consumption. In this embodiment, the dosage of the water-reducing agent (15-30 parts) needs to be fine-tuned according to the mineral composition of the cement and the loss on ignition of silica fume. Clean groundwater or tap water is used, and the water-cement ratio is controlled between 0.16 and 0.20.

[0031] Step S2: The weighed raw materials are put into the mixer and forced to mix in the order of dry mixing first and then wet mixing to obtain ultra-high performance concrete mixture.

[0032] Traditional single-feeding methods, which involve adding all materials simultaneously, almost inevitably lead to agglomeration failure in UHPC systems with low water-to-binder ratios and high viscosity. This application employs a staged forced mixing process, utilizing particle dynamics and rheological principles to construct a homogeneous system step-by-step.

[0033] See Figure 2 , Figure 2 This is a schematic flowchart of an embodiment of step S2 in the preparation method provided in this application.

[0034] Step S21: Add cement, silica fume and quartz sand into the mixer and dry mix at a speed of 60-80 rpm for 120-180 seconds to form a uniform dry powder mixture.

[0035] The physical significance of this step lies in pre-dispersion. Cement, silica fume, and quartz sand have significant density differences: cement approximately 3.1 g / cm³, silica fume approximately 2.2 g / cm³, and quartz sand approximately 2.65 g / cm³. If water is added directly, the lightweight silica fume easily floats on the surface, forming slurry clumps that cannot penetrate the interior to participate in hydration. Dry mixing utilizes mechanical shear force to forcibly mix powders of different densities in a dry state. The denser quartz sand particles act as grinding media, impacting and breaking up the silica fume agglomerates, ensuring their uniform distribution within the cement-sand skeleton.

[0036] The stirring speed of 60-80 rpm was optimized through kinetic simulation: if the speed is too low, the shear force is insufficient to break up the silica fume clumps; if the speed is too high, the powder is prone to stratification and accumulation under centrifugal force. The dry mixing time of 120-180 seconds is sufficient to ensure that the coefficient of variation of the uniformity of the dry powder mixture is less than 5%, providing a uniform kinetic prerequisite for the subsequent hydration reaction.

[0037] Step S22: Add all the water and water-reducing agent to the dry powder mixture, and wet mix at a speed of 80-100 rpm for 90-120 seconds to form a fluid slurry.

[0038] Water-reducing agent molecules rapidly adsorb onto the particle surface, releasing encapsulated water and wetting the solid particles. Steel fibers are not added at this stage to allow the cementitious material to fully hydrate and form a continuous liquid medium. If fibers are added at the initial stage of water addition, they will instantly adsorb a large amount of free water, causing localized dehydration and drying of the slurry, a sharp drop in fluidity, and fiber clumping together. Increasing the wet mixing speed to 80-100 rpm generates a sufficiently high shear rate to disrupt the flocculation structure of the slurry, allowing the water-reducing agent molecules to fully expand and ensuring the slurry reaches the yield stress and plastic viscosity required for self-compaction.

[0039] Step S23: Evenly sprinkle steel fibers into the flowing slurry and continue stirring for 180-240 seconds until the steel fibers are completely dispersed to obtain ultra-high performance concrete mixture.

[0040] At this point, the slurry already possesses good rheological properties, and its slump extension is usually over 500 mm. After the fibers are added, they are encapsulated by the lubricated slurry, and the coefficient of friction between the fibers is greatly reduced.

[0041] This embodiment introduces a multi-point dispersed feeding process. In the step of adding steel fibers to the flowing slurry, a multi-point dispersed feeding device is used to uniformly sprinkle the steel fibers into the rotating mixer; the feeding rate of the multi-point dispersed feeding device is matched with the rotation speed of the mixer, and the feeding time is controlled within 60 to 90 seconds.

[0042] Traditional whole-bag dumping methods result in a large accumulation of fibers in a localized area of ​​the mixing pot. The mixing arm needs to expend enormous energy to break up these fiber clumps, and often fails to disperse them completely. Residual fiber spikes can become a fatal defect in the finished product. This embodiment employs a multi-point dispersion feeding device, which can be a vibrating screen feeder positioned above the mixer. The fibers pass through the vibrating screen and, under the combined action of gravity and vibration, are evenly scattered in a rain-like pattern. The feeding time is controlled at 60-90 seconds to match the mixer's cycle time. For example, for a twin-shaft forced mixer, the blades rotate once in approximately 1.5 seconds. Slow and uniform feeding ensures that each batch of falling fibers is immediately mixed and dispersed, preventing accumulation. The total mixing time of 180-240 seconds is calculated based on the fiber dispersion kinetics equation, ensuring that after the last fiber is added, there is still sufficient shearing time for it to achieve statistically uniform distribution in three-dimensional space.

[0043] In addition, between steps S2 and S3, this embodiment also incorporates a closed-loop control mechanism for rheological properties. The mixed ultra-high performance concrete mixture is unloaded onto a transport vehicle, and a slump spread test is performed, controlling the slump spread between 600mm and 750mm. If the slump spread is below 600mm, a water-reducing agent at 0.1% to 0.3% of the cementitious material mass is added to the mixture, and it is stirred at high speed for 60-90 seconds.

[0044] This step addresses the rheological uncertainties caused by batch fluctuations in raw materials, such as variations in cement fineness and sand moisture content. 600mm is the critical spread for UHPC to achieve self-compacting filling. Insufficient spread means excessive slurry yield stress, preventing adequate filling of template corners and hindering air bubble removal. Adding a trace amount of water-reducing agent as a post-admixture adjustment effectively replenishes uncovered adsorption sites on particle surfaces, releases free water, and restores fluidity without altering the water-cement ratio. High-speed mixing for 60-90 seconds rapidly disperses the added water-reducing agent, preventing localized segregation.

[0045] Through the refined control of steps S1 and S2, this application successfully prepared an ultra-high performance concrete mixture with uniform fiber dispersion and excellent rheological properties, laying a solid foundation for subsequent molding and hardening.

[0046] Step S3: Pour the ultra-high performance concrete mixture into the precast bridge slab mold, and perform layered vibration compaction to obtain a wet bridge slab blank.

[0047] Although UHPC mixtures are designed as self-compacting concrete (SCC), in practical engineering applications, especially for precast bridge slabs with large surface areas, relying solely on weight-based flow filling often fails to achieve the desired compaction. This is mainly because the extremely high viscous yield stress of UHPC hinders the escape of air bubbles. This application constructs a physical field conducive to air bubble displacement by optimizing the pouring process and vibration method.

[0048] In step S3, the precast bridge slab is poured into the mold in layers, with each layer not exceeding 200mm in thickness.

[0049] This parameter setting is based on the trade-off between the bubble's rise distance and the slurry's yield stress. According to Stokes' law, the rise velocity of a bubble in a liquid is directly proportional to the square of its radius and inversely proportional to the liquid's viscosity. UHPC slurry has a much higher viscosity than ordinary concrete, typically exceeding 100 Pa·s in plasticity, making the spontaneous rise of microbubbles extremely slow. If the thickness of a single pour is too large, such as exceeding 500 mm, the bubbles at the bottom cannot rise to the surface within the initial setting time window and will be locked inside the concrete, forming pores. This is not only a weakness in strength but also a channel for harmful media to penetrate.

[0050] By setting the layer thickness to no more than 200mm and combining it with high-frequency vibration, the escape path of air bubbles is essentially shortened. Each layer is vibrated immediately after pouring, so that air bubbles in that layer only need to overcome 200mm of slurry resistance to reach the surface of that layer. Combined with the liquefaction effect of vibration, the degassing efficiency is greatly improved.

[0051] The compaction treatment uses a combination of an attached high-frequency vibrator and an immersion vibrator, with a vibration time of 30 to 60 seconds, until the concrete surface is covered with slurry and no more bubbles appear.

[0052] The attached high-frequency vibrator is fixed to the outside of the steel formwork by clamps. Its working principle is to transfer vibration energy directly to the concrete mixture through the formwork. This method avoids the problem of holes left when the immersion vibrator is pulled out, and is particularly suitable for thixotropic materials such as UHPC, which are prone to leaving memory holes at the vibrator tip.

[0053] During the vibration process, the frequency of the attached high-frequency vibrator is controlled at 100Hz to 200Hz, and the vibration force is distributed in a gradient decreasing manner from the bottom surface to the top surface of the bridge slab.

[0054] The vibration frequency for ordinary concrete is typically 50Hz-100Hz. However, for UHPC, the microbubbles in the slurry are even smaller, requiring higher frequency shock waves to disrupt the tension balance at the gas-liquid interface, causing small bubbles to merge into larger bubbles and rise to the surface. High-frequency vibration of 100Hz-200Hz can generate a strong shear dilution effect in the mixture, causing the originally high-viscosity slurry to liquefy instantly, significantly reducing the yield stress, and facilitating bubble overflow.

[0055] The gradient distribution of vibration force from the bottom to the top of the bridge slab is achieved through multi-point controlled vibration technology. Its physical significance lies in: the maximum vibration force at the bottom ensures the slurry is fully liquefied and fills the corners of the template; the moderate vibration force at the top avoids over-vibration that could cause coarse aggregate (quartz sand) to sink and slurry to rise and segregate. This gradient field creates a bubble pumping channel from bottom to top.

[0056] After vibration, the top surface of the wet bridge slab blank is smoothed and immediately covered with a plastic film to prevent moisture evaporation.

[0057] UHPC has an extremely low water-cement ratio and very little free water. If exposed to air after vibration, the evaporation rate of surface moisture far exceeds the migration rate of internal moisture to the surface, leading to a sharp increase in surface capillary pressure and triggering plastic shrinkage cracks. Once these cracks form, they often penetrate the surface protective layer, severely affecting the durability of the bridge deck. Covering with a plastic film creates a near-saturated microenvironment, effectively blocking moisture loss pathways and ensuring the continuous progress of the hydration reaction.

[0058] Step S4: The wet bridge slab blank is subjected to variable temperature steam curing in sequence, including static stopping, heating, constant temperature and cooling, to complete the hydration reaction.

[0059] Curing is a thermodynamic process in which the microstructure of UHPC transforms from granular to stony. Due to the large amount of cementitious materials used in UHPC, its high heat of hydration, and its dense structure, even minor deviations in the curing regime can lead to catastrophic temperature cracking or insufficient strength development. The variable-temperature steam curing regime designed in this application is based on precise control using cement chemical and thermoelastic mechanical models.

[0060] See Figure 3 , Figure 3 This is a schematic flowchart of an embodiment of step S4 in the preparation method provided in this application. Step S4 specifically includes: Step S41: Let the concrete stand for 2 to 4 hours at an ambient temperature of 20℃~25℃ to allow it to complete its initial setting.

[0061] The static curing stage, also known as the pre-curing period, is when the hydrophilic film on the surface of cement particles begins to hydrate, forming CSH gel nuclei, and ettringite (AFt) crystals begin to grow. At this time, the concrete has not yet established a strength framework and is in a plastic rheological state. Setting the ambient temperature to 20℃~25℃ is to provide a mild thermodynamic environment and avoid excessive heat of hydration in the early stages, which could lead to explosive agglomeration. If the static curing temperature is too low, such as below 10℃, the hydration rate is too slow, the initial structure formation is delayed, and the structure becomes loose and easily disintegrates when the temperature rises later. If the static curing temperature is too high, such as above 35℃, hydration is too fast, and early structural stress accumulates, easily causing microcracks.

[0062] The static curing time parameter of 2–4 hours is determined based on the initial structural strength threshold. Experiments show that the critical strength of UHPC at the end of static curing should reach 0.5–2.0 MPa, at which point the matrix has the ability to resist volume expansion during the heating phase.

[0063] Step S42: Raise the curing temperature to 80℃~95℃ at a heating rate of 10℃ / hour~15℃ / hour.

[0064] Concrete is a poor conductor of heat, with a low thermal conductivity. If the heating rate is too rapid, exceeding 20°C / hour, a significant temperature gradient will form between the surface and center of the component, and between the surface layer and the environment. According to thermoelasticity theory, this temperature gradient will generate thermal stress. When the thermal stress exceeds the tensile strength of the concrete at that time (when the tensile strength is still developing and relatively low), microcracks invisible to the naked eye will appear. These microcracks will propagate into macrocracks under later loads, severely affecting the fatigue life of the bridge deck.

[0065] This embodiment strictly limits the heating rate to 10℃~15℃ / hour to ensure a smooth dynamic process of heat conduction from the surface to the interior, uniform temperature field distribution, and control of temperature stress within a safe range.

[0066] Step S43: Maintain the temperature between 80℃ and 95℃ and continue curing for 24 to 48 hours, while keeping the relative humidity above 95%.

[0067] High temperatures significantly accelerate chemical reaction rates. According to the Arrheniuse equation, the chemical reaction rate increases by approximately 2-4 times for every 10°C increase in temperature. Under high temperature and pressure conditions of 80°C–95°C, the hydration reaction of cement minerals shifts from surface-controlled to diffusion-controlled, leading to the rapid decomposition of C3S and C2S and the formation of large quantities of CSH gel.

[0068] More importantly, this temperature range is the optimal temperature range for the pozzolanic reaction between silica fume (SiO2) and calcium hydroxide. At room temperature, silica fume has extremely low activity, but under high-temperature excitation, the Si-O bonds on the surface of silica fume particles break, reacting with CH to form a secondary CSH gel. This secondary gel fills the capillary pores, reducing the porosity of the interfacial transition zone (ITZ) to below 5%, thus achieving microstructural densification.

[0069] Furthermore, during the constant temperature stage, saturated steam at a pressure of 0.05 MPa to 0.1 MPa is introduced into the curing hood, and the steam supply is stopped after the constant temperature is achieved.

[0070] The introduction of low-pressure steam (0.05MPa~0.1MPa) is based on the principle of pressure permeation. UHPC has a dense structure, making it difficult for atmospheric pressure steam to penetrate its internal pores and participate in hydration. Low-pressure steam, with its higher energy and penetrating power, can reach deep into microcracks and capillaries, providing sufficient moisture to prevent internal self-drying shrinkage. Furthermore, the pressure promotes the dissolution and precipitation of the cementitious material particles, resulting in more complete strength development. Maintaining a relative humidity above 95% prevents dry burning and ensures that the hydration reaction does not cease due to water shortage.

[0071] Step S44: Reduce the temperature to less than 10°C below the ambient temperature at a cooling rate of no more than 15°C / hour.

[0072] The cooling process is essentially a contraction of the concrete volume. At this point, the concrete has hardened and has an extremely high modulus of elasticity. If the cooling is too rapid, the surface contracts sharply while the interior remains in a state of thermal expansion. This mismatch between internal and external deformation will generate enormous tensile stress. For high-modulus materials like UHPC, the stress relaxation ability is poor, making it extremely prone to surface cracking. Controlling the cooling rate and keeping the temperature difference with the environment below 10°C allows the component to release heat slowly, ensuring a smooth transition between the internal and external temperature fields and achieving non-destructive cooling.

[0073] During the cooling phase, the curing cover uses natural ventilation for cooling, and a curing agent is sprayed on the concrete surface to retain moisture. The purpose of spraying the curing agent is to form a water-retaining film on the surface to prevent surface dehydration and peeling caused by moisture condensation during the cooling process.

[0074] Furthermore, a surface micro-treatment step is included between the static resting stage and the heating stage: the surface of the wet bridge slab is inspected, and if there are tiny air bubbles, they are filled and smoothed using the same cementitious slurry as the ultra-high performance concrete.

[0075] This process involves refined management of the appearance quality control of UHPC. After the static curing period, although the concrete has set, it has not yet hardened. If there are air bubbles on the surface, they can be filled with neat cement paste at this time. The neat cement paste can perfectly integrate with the substrate through subsequent high-temperature curing, eliminating the problem of cold joints. If repairs are made after hardening, roughening and interface agent treatment are required, and the color and shrinkage rate of the repair material are difficult to match with the substrate, affecting aesthetics and durability.

[0076] The embodiments of this application also introduce a closed-loop feedback mechanism, including: before the constant temperature stage ends and cooling is prepared, it is necessary to confirm that the compressive strength of the concrete reaches more than 75% of the design strength grade; if the compressive strength does not meet the standard, the curing time of the constant temperature stage is extended until the strength meets the standard, and in the subsequent cooling stage, the cooling rate is reduced to 80% to 90% of the original rate.

[0077] This design takes into account the impact of raw material fluctuations and unforeseen environmental factors on the hydration process. For example, if the activity of a cement batch is slightly lower, its strength may be insufficient after the standard isothermal time. In this case, the system does not directly discard the product but automatically triggers a remedial procedure: extending the isothermal time to gain more reaction time. Simultaneously, given the potential for accumulated thermal damage from prolonged isothermal use, the system automatically reduces the subsequent cooling rate to 80%–90% of the original rate, employing a gentler cooling strategy to reduce thermal stress and offset the potential increased brittleness risk caused by the extended high-temperature time. This logic of trading time for strength and rate for safety ensures the reliability of every bridge deck that leaves the factory.

[0078] Through the aforementioned refined, model-based, and intelligent maintenance process, this application successfully solved the industry problem of UHPC precast bridge slabs having high strength but being prone to cracking, achieving a balance between microstructure optimization and macroscopic mechanical properties.

[0079] Step S5: Demolding is carried out after curing is completed and the strength of the blank reaches the demolding standard.

[0080] Demolding is the final critical step in the production process of UHPC precast bridge slabs, and also a difficult point in controlling appearance quality. Due to the high density and high bonding strength of UHPC material, its adhesion to the steel formwork is far greater than that of ordinary concrete. If the demolding method is not appropriate, it can easily lead to chipping of the corners of the components, surface cracking, or even damage to the overall structure.

[0081] Demolding is carried out after curing is completed and the strength of the green body reaches the demolding standard. The demolding standard here refers not only to strength, but also to the stability of the physical state.

[0082] As mentioned earlier, during the cooling phase, the component temperature needs to be reduced to less than 10°C from the ambient temperature. If the temperature difference is too large, the cold airflow impacting the component surface at the moment of demolding will generate an instantaneous thermal shock effect, leading to surface microcracks. These microcracks will become fatigue crack sources during subsequent service.

[0083] UHPC develops strength extremely rapidly. After high-temperature steam curing, its compressive strength typically reaches over 80% of the design value, such as over 120 MPa, and its flexural strength is also extremely high. At this point, demolding safety is extremely high.

[0084] In step S5, the demolding adopts a synchronous lifting demolding process, which uses multiple hydraulic jacks to simultaneously apply an upward lifting force to the bottom of the bridge deck, and the lifting speed is controlled within 5mm / s.

[0085] Traditional demolding methods mostly involve hoisting or prying with crowbars.

[0086] Lifting and hoisting: A crane is used to lift the components using a lifting device. Due to the extremely smooth surface of UHPC, a vacuum-like adsorption effect is formed between it and the mirror-finish steel mold. Furthermore, the hydration products of the cementitious material penetrate into the micro-texture of the mold surface, creating a physical interlocking effect. At the moment of lifting, the lifting device typically applies a concentrated load to the lifting point of the component. At this time, there is a huge adsorption and frictional force between the bottom of the component and the mold. This uneven stress can easily cause localized tensile cracking at the lifting point, or cause brittle failure due to impact vibration from the instantaneous release of the adsorption force.

[0087] Prying with a crowbar: This is a highly destructive method of operation. Relying solely on the operator's experience to apply force results in the force being concentrated in a localized area, making it extremely easy to break the edges and corners.

[0088] The synchronous lifting and demolding process used in this embodiment involves pre-setting lifting holes at specific locations on the bottom of the bridge deck mold, typically corresponding to the nodes of the reinforcing rib skeleton. During demolding, multiple hydraulic jacks operate simultaneously.

[0089] The demolding process adopts a synchronous lifting demolding process, which uses multiple hydraulic jacks to simultaneously apply an upward lifting force to the bottom of the bridge deck, with the lifting speed controlled within 5mm / s.

[0090] The PLC hydraulic synchronous control system ensures that the lifting displacement error of all jacks is controlled within millimeters. This means that the force is uniform at all points at the bottom of the component, and the component rises as a whole, avoiding torsional torque caused by uneven local force.

[0091] The lifting speed is controlled within 5 mm / s, which is an extremely low creep rate. At this speed, the adsorption force between the component and the template is slowly released, avoiding impact loads. Physically, this is equivalent to the critical state of converting dynamic friction into static friction, greatly reducing the peak force required for demolding and protecting the integrity of the component's internal microstructure.

[0092] After demolding, the edges and surface pores of the bridge deck are repaired. The repair material is UHPC repair mortar with the same ratio. After repair, the deck is left to cure for no less than 12 hours.

[0093] Because UHPC has a low water-cement ratio, even after high-frequency vibration, a small number of pinhole-like pores may still remain on the surface. The repair mortar is not ordinary building putty, but a pre-prepared special mortar with a completely identical mix ratio to the base material. This ensures that the coefficient of thermal expansion, modulus of elasticity, and color of the repaired area match the base material. The 12-hour curing period after repair allows the repair mortar to fully hydrate and harden, forming a strong molecular bond with the base material, rather than simply being a physical accumulation.

[0094] After step S5, the final finishing step is also included: the surface of the demolded bridge deck is roughened by using a high-pressure water gun to expose some of the steel fiber heads, so as to increase the anti-skid structure depth when the bridge deck is laid on the road surface.

[0095] Traditional anti-skid treatment of bridge decks often uses a grooving machine to groove the surface laterally. This method not only generates noise and dust pollution, but also has limited grooving depth, which can easily damage coarse aggregates (such as limestone aggregates, which are easily crushed), leading to a decrease in the wear resistance of the road surface.

[0096] This embodiment utilizes uniformly distributed steel fibers within the UHPC as anti-slip aggregate. A high-pressure water jet with a pressure greater than 50 MPa is used to forcefully roughen the surface of the UHPC. The high-pressure water jet can break through the surface mortar protective layer, washing away some fine sand, causing the ends of the steel fibers originally embedded in the matrix to be exposed by about 1-2 mm.

[0097] The exposed steel fiber ends have an extremely high hardness, exceeding 2000 MPa. Under the pressure of vehicle tires, they not only provide excellent mechanical engagement friction to prevent vehicle slippage, but also, due to the excellent thermal conductivity of the steel fibers, help melt snow and ice on the road surface in winter. This in-situ fiber-reinforced anti-skid technology eliminates the need for an additional anti-skid layer, saving costs and improving the functional durability of the road surface.

[0098] Example verification To more clearly illustrate the effects of the technical solution of this application, the following detailed description is provided in conjunction with specific embodiments.

[0099] Example 1 A precast bridge slab made of ultra-high performance concrete (UHPC) with dimensions of 2m×1m×0.2m.

[0100] Raw material proportions (parts by weight): 800 parts of 52.5 silicate cement, 250 parts of silica fume, 1000 parts of quartz sand (0-1.2mm gradation), 200 parts of copper-plated hook-shaped steel fiber (13mm in length and 0.22mm in diameter), 22 parts of polycarboxylate superplasticizer (30% solid content), and 170 parts of water.

[0101] Preparation steps: S1: Weigh the raw materials according to the proportions.

[0102] S2: Add cement, silica fume, and quartz sand, and dry mix for 150 seconds (70 rpm); add water and water-reducing agent, and wet mix for 100 seconds (90 rpm); use a vibrating screen feeder to evenly sprinkle steel fibers within 80 seconds, and continue mixing for 200 seconds.

[0103] S3: Layered pouring, each layer 200mm, with attached high-frequency vibrator (frequency 150Hz) for 40 seconds, and covered with plastic film.

[0104] S4: Stop for 3 hours (23℃); increase the temperature to 90℃ at 12℃ / h; keep the temperature constant for 36 hours (introduce 0.08MPa saturated steam); decrease the temperature to 50℃ at 15℃ / h.

[0105] S5: The strength test meets the standard, synchronously lift and demold (5mm / s), repair air holes, and roughen with a high-pressure water gun.

[0106] Comparative Example 1 The traditional one-time feeding and mixing process is adopted, and the other steps are the same as in Example 1.

[0107] Results Comparison: The peak stirring current exceeded that of Example 1 by 30%, and obvious fiber balls were found after discharge, requiring manual removal. Fiber clumps were visible on the hardened surface, and the compressive strength exhibited a large coefficient of variation.

[0108] Comparative Example 2 Standard curing at room temperature was used, without steam curing, and other proportions were the same as in Example 1.

[0109] Results comparison: The strength after 3 days was only 45MPa, which could not be demolded. It required 28 days of curing to reach the design strength, resulting in extremely low production efficiency.

[0110] Comparative Example 3 Traditional hoisting and demolding methods are used, with a hoisting speed of 0.2 m / s.

[0111] Results comparison: The component vibrated significantly at the moment of demolding, and a chip appeared at one corner of the bottom, which required repair and affected the appearance.

[0112] Performance test data table

[0113] Microstructure analysis The samples from Example 1 and Comparative Example 2 were analyzed by scanning electron microscopy (SEM).

[0114] Example 1 (High-temperature steam curing): The microstructure is extremely dense with very few unhydrated particles. The CSH gel exhibits regular layered stacking, and there is no clear boundary between the interface transition zone (ITZ) and the matrix. The steel fiber surface is densely wrapped with gel, resulting in strong adhesion.

[0115] Comparative Example 2 (cured at room temperature): The microstructure is relatively loose, with a large number of unhydrated cement particles and silica fume microspheres visible. There are many pores, obvious microcracks in the ITZ region, and pores at the fiber-matrix interface.

[0116] As can be seen from the detailed description of the above-described embodiments and the verification of the examples, the method for preparing ultra-high performance concrete (UHPC) precast bridge slabs provided in this application solves the global problem of fiber agglomeration in UHPC production through a process of dry mixing followed by wet mixing and multi-point dispersed feeding, significantly reducing mixing energy consumption and improving fiber utilization. The established variable-temperature steam curing model based on thermoelasticity ensures both the high hydration and density of the microstructure and effectively avoids the risk of thermal stress cracks, achieving a dual improvement in strength and durability. The synchronous lifting and demolding process achieves non-destructive separation, ensuring the integrity of the components. Utilizing the inherent characteristics of UHPC, anti-skid pavement is directly prepared through roughening treatment, realizing the integration of structure and function.

[0117] Unlike existing technologies, this application discloses a method for preparing ultra-high performance concrete precast bridge slabs. The embodiments of this application significantly improve fiber dispersion uniformity through a mixing process involving dry mixing followed by wet mixing and phased addition of materials. An optimized variable-temperature steam curing regime, particularly precise control of the high-temperature isothermal stage, promotes the hydration reaction of the cementitious materials, constructing a dense microstructure and significantly enhancing the early strength and durability of the bridge slab. By establishing a strength determination and parameter adjustment mechanism between the isothermal and cooling stages, dynamic closed-loop control of the curing process is achieved, avoiding quality risks caused by insufficient strength or over-curing. Through a synchronous lifting demolding process and subsequent repair and finishing treatments, non-destructive demolding and surface functionalization of the components are achieved, significantly improving the production yield and efficiency of ultra-high performance concrete precast bridge slabs.

[0118] The above description is merely an embodiment of this application and does not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for preparing ultra-high performance concrete precast bridge slabs, characterized in that, include: Step S1: Weigh cement, silica fume, quartz sand, steel fiber, water-reducing agent and water according to the preset ratio; Step S2: Put the weighed raw materials into the mixer and force mix them in the order of dry mixing first and then wet mixing to obtain ultra-high performance concrete mixture. Step S3: The ultra-high performance concrete mixture is poured into the precast bridge slab mold and subjected to layered vibration compaction to obtain a wet bridge slab blank. Step S4: The wet bridge slab blank is subjected to variable temperature steam curing in sequence, including static stopping, heating, constant temperature and cooling, to complete the hydration reaction; Step S5: Demolding is carried out after curing is completed and the strength of the blank reaches the demolding standard.

2. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 1, characterized in that, The preset mix proportions, by weight, are: 700-900 parts cement, 200-300 parts silica fume, 900-1100 parts quartz sand, 150-250 parts steel fiber, 15-30 parts water-reducing agent, and 160-200 parts water. The steel fiber is either a straight steel fiber or a hooked steel fiber, with a length of 12mm to 16mm and a diameter of 0.2mm to 0.3mm.

3. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 1, characterized in that, In step S2, the forced stirring in the order of dry mixing followed by wet mixing includes: Add cement, silica fume and quartz sand into a mixer and dry mix at a speed of 60-80 rpm for 120-180 seconds to form a uniform dry powder mixture. Add all the water and water-reducing agent to the dry powder mixture, and wet mix at a speed of 80-100 rpm for 90-120 seconds to form a fluid slurry; Steel fibers are evenly sprinkled into the flowing slurry, and stirring is continued for 180-240 seconds until the steel fibers are completely dispersed to obtain ultra-high performance concrete mixture.

4. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 3, characterized in that, When uniformly sprinkling steel fibers into the flowing slurry, a multi-point dispersing feeding device is used to uniformly sprinkle the steel fibers into the rotating mixer; the feeding rate of the multi-point dispersing feeding device is matched with the rotation speed of the mixer, and the feeding time is controlled within 60 to 90 seconds.

5. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 1, characterized in that, In step S3, the pouring into the precast bridge slab mold adopts a layered pouring method, and the thickness of each layer does not exceed 200mm; The vibration compaction treatment uses a combination of an attached high-frequency vibrator and an immersion vibrator, with a vibration time of 30 to 60 seconds, until the concrete surface is covered with slurry and no more bubbles appear.

6. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 5, characterized in that, During the vibration process, the frequency of the attached high-frequency vibrator is controlled at 100Hz to 200Hz, and the vibration force is distributed in a gradient decreasing distribution from the bottom surface to the top surface of the bridge slab. After vibration, the top surface of the wet bridge slab blank is smoothed and immediately covered with a plastic film to prevent moisture evaporation.

7. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 1, characterized in that, In step S4, the curing parameters for the variable temperature steam curing are as follows: Static setting stage: Allow the concrete to stand for 2 to 4 hours at an ambient temperature of 20℃~25℃ to allow it to complete its initial setting. Heating phase: Increase the curing temperature to 80℃~95℃ at a heating rate of 10℃ / hour~15℃ / hour; Constant temperature stage: Maintain the temperature between 80℃ and 95℃ for 24 to 48 hours, and keep the relative humidity above 95%. Cooling phase: The temperature is reduced to less than 10°C from the ambient temperature at a cooling rate of no more than 15°C / hour.

8. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 7, characterized in that, During the constant temperature stage, saturated steam at a pressure of 0.05 MPa to 0.1 MPa is introduced into the curing hood, and the steam supply is stopped after the constant temperature is achieved. During the cooling phase, the curing cover uses natural ventilation to cool down the concrete and a curing agent is sprayed on the concrete surface to keep it moist.

9. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 7, characterized in that, Before cooling down after the constant temperature stage, it is necessary to confirm that the compressive strength of the concrete has reached more than 75% of the design strength grade. If the compressive strength does not meet the standard, extend the curing time in the constant temperature stage until the strength meets the standard, and in the subsequent cooling stage, reduce the cooling rate to 80% to 90% of the original rate.

10. The method for preparing ultra-high performance concrete precast bridge slabs according to claim 1, characterized in that, In step S5, the demolding adopts a synchronous lifting demolding process, which uses multiple hydraulic jacks to simultaneously apply an upward lifting force to the bottom of the bridge deck, and the lifting speed is controlled within 5mm / s.

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