Abstract:Ultra high performance concrete (UHPC) has received increasing attention due to its excellent performance and broad application prospects. However, due to factors such as mix proportion, mixing process, and cost, the production and application of UHPC have always faced certain technical difficulties. Therefore, this article will explore the key technical control points of UHPC mixing, providing certain reference and guidance for the production and application of UHPC.

Keywords: ultra-high performance concrete; Mixing technology

Introduction:Ultra high performance concrete (UHPC) is a new type of high-strength and high durability concrete. Its physical and mechanical properties such as compressive strength, tensile strength, freeze-thaw resistance, and resistance to chloride ion erosion have extremely high levels, making it widely used in fields such as bridges, tunnels, and buildings. However, the key control points of UHPC mixing technology are whether the material can achieve excellent performance. This article will introduce the control points of UHPC mixing technology, hoping to provide some reference value for the production and application of UHPC.
1、 Ultra High Performance Concrete (UHPC)

1. High strength

UHPC is a type of concrete with extremely high strength, and its compressive strength is usually above 150MPa, which is more than 3 to 4 times higher than ordinary concrete. This high strength is mainly due to the finer proportion of raw materials such as cement, quartz powder, silica fume, titanium dioxide, and high-performance fine aggregates in UHPC compared to ordinary concrete, as well as the use of higher strength reinforcing materials such as steel fibers and glass fibers. Therefore, UHPC can withstand higher pressure and weight, making it widely used in engineering fields such as bridges, tunnels, and high-rise buildings.
2. High density

The density of UHPC is usually between 2.4-2.6g/cm ? Between them, the density is more than 50% higher than that of ordinary concrete. This is due to the use of high-density raw materials such as cement, quartz powder, and silica fume in UHPC, as well as the use of higher cement binder content in concrete formulation, resulting in higher compactness of the concrete. High density UHPC has better performance in resisting bending, tension, shear, and other aspects, making it widely used in construction projects such as high-rise buildings, large bridges, and tunnels.

3. Excellent durability

UHPC has excellent durability and can withstand extreme environmental conditions and harsh climates, such as high and low temperatures, freeze-thaw cycles, acid and alkali corrosion, etc. This is due to the finer particle size and more uniform mixing of raw materials in UHPC, as well as the addition of special additives, which give it better durability and crack resistance. In addition, the high-performance fiber materials used in UHPC can effectively control the development of microcracks, thereby improving their crack resistance and durability.
4. Good wear resistance

Due to its high density and strength, UHPC also exhibits excellent wear resistance. In the use of highways, airport runways, and other places, UHPC is more wear-resistant than ordinary concrete and has a longer service life. This is because the raw materials in UHPC contain high hardness fine aggregates such as hard quartz and quartz sand, as well as reinforced materials such as high-strength steel fibers, which make UHPC perform well in bearing heavy loads and wear.
5. Easy to construct

Although UHPC has high strength and density, due to its low viscosity, it can be easily vibrated and molded during the construction process. UHPC can also be constructed through spraying, pouring, prefabrication and other methods, making it suitable for various engineering needs.

6. No need for prestressing

To achieve sufficient strength, ordinary concrete often requires prestressing treatment, while UHPC can achieve extremely high strength without prestressing treatment. This is because UHPC has higher density and better crack resistance, which can meet engineering needs under the strength of concrete itself. In addition, the high strength and excellent durability of UHPC enable it to ensure the safety and durability of engineering structures while reducing structural thickness and component size.
7. Strong plasticity

Due to the addition of special additives in UHPC, its plasticity has been greatly improved, allowing for free plastic deformation during component processing. This plasticity can help designers create more complex and unique architectural forms, while also making UHPC more widely applicable in fields such as repairing and strengthening aging structures. In addition, the plasticity of UHPC can also improve the seismic performance of engineering structures, thereby better protecting people's lives and property safety.
2、 Key technical control points for mixing ultra-high performance concrete (UHPC)

1. Selection of raw materials for UHPC

Firstly, for the selection of raw materials for UHPC, precise control is required based on its performance indicators. Cement is one of the main raw materials in UHPC, and it is necessary to choose cement with high strength, high reactivity, and low alkalinity, and pay attention to its shelf life and storage conditions during the production process to ensure the stability of its quality. In addition, it is necessary to choose appropriate mineral admixtures and aggregates to ensure the strength and durability of concrete.
In terms of raw material ratio, it is necessary to make a reasonable proportion ratio based on the performance indicators of UHPC. Ultra fine mineral powder is an important raw material in UHPC, with a particle size generally less than 10 microns, which has a significant impact on the strength, density, and durability of concrete. Therefore, in the production of UHPC, it is necessary to finely control the selection and proportion of ultrafine mineral powder to ensure its performance indicators meet the requirements.
In terms of water cement ratio control, the water cement ratio should be controlled at 0.15 or lower. The specific control can be based on the actual situation by selecting the value of the water cement ratio and using precise weighing and proportioning tools for control. The water quality should use clean tap water or purified water.

In terms of aggregate control, fine aggregate should have an appropriate distribution, usually using a multi-level grading method, such as dividing the aggregate into 3-4 levels and ensuring that the content of fine aggregate accounts for about 20-30% of the total aggregate. The selection of aggregates should be circular or polygonal, and the specific selection can be determined based on experimental data and actual application situations.

In addition to the selection and proportion of raw materials, attention should also be paid to factors such as the quality, shelf life, and storage conditions of raw materials. Due to the production process of UHPC requiring stable and reliable quality of raw materials, it is necessary to strictly control the storage conditions of raw materials to ensure their quality stability and reliability. In addition, regular testing and inspection of raw materials are also necessary to identify and resolve issues in a timely manner.

2. Mixing method

The mixing method is one of the important factors affecting the performance of UHPC, as different mixing methods directly affect the quality and performance of concrete. Dry and wet mixing methods are commonly used mixing methods in UHPC. The dry mixing method is to finely mix various raw materials, use a high-speed mixer for dry mixing, and if necessary, add an appropriate amount of water for adjustment to form a uniform mixture. The dry mixing method is suitable for producing large quantities of UHPC, but it requires complex equipment and difficult operation. The dry mixing method has low viscosity and good workability of concrete, but there are also certain uniformity issues that need to be solved through appropriate water addition and mixing time.
Wet mixing method is the process of adding various raw materials to a mixer and conducting wet mixing while controlling parameters such as cement dosage, water cement ratio, and mixing time. The wet mixing method has a simple process and is suitable for small-scale production of UHPC. In the wet mixing method, it is necessary to control the amount of water and mixing time to ensure the uniformity and density of the concrete. Meanwhile, the wet mixing method also needs to pay attention to factors such as humidity and temperature of raw materials to ensure the quality and stability of concrete.

During the mixing process, it is necessary to strictly control parameters such as mixing method, time, and speed. The mixing time of concrete should not be too long or too short, otherwise it will affect the uniformity and density of concrete, leading to a decrease in concrete strength and durability. In actual production, it is necessary to determine the optimal mixing parameters based on different raw materials, mixing methods, and production scales to ensure that the performance indicators of UHPC meet the requirements.

3. Strengthen maintenance

The maintenance process of UHPC is generally divided into two stages: initial maintenance and later maintenance. Initial curing refers to the curing of concrete after it has just been poured, usually lasting 1-3 days. At this stage, it is necessary to wet cure the concrete and keep the surface moist to prevent the occurrence of dry cracking and cracking on the concrete surface. At the same time, it is necessary to control the temperature of the concrete to avoid temperature differences inside the concrete, which can lead to cracks inside the concrete.
Post curing refers to the curing of concrete after reaching the design strength. At this point, the concrete has formed a relatively complete structural system and requires long-term curing to improve its durability and service life. Post maintenance usually lasts about 28 days, and the maintenance conditions include maintenance temperature, maintenance humidity, and maintenance time. The curing temperature should be controlled between 15-25 ℃ to avoid surface cracking and internal temperature stress caused by high or low temperatures on the concrete. The curing humidity should be controlled above 90% to ensure that the concrete surface is moist and promote the hardening and strength development of the concrete. The curing time is generally around 28 days, during which regular inspections and maintenance of concrete are required to promptly identify and address potential issues.

Conclusion:

This article analyzes the impact of factors such as the mix proportion, mixing process, and cost of UHPC on the performance of UHPC from the perspective of key control points of mixing technology. It is hoped that this can provide some reference and guidance for the production and application of UHPC. I believe that with the continuous progress and improvement of science and technology, the performance and application prospects of UHPC will be more extensive and extensive.



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