Abstract: With the acceleration of urbanization in China, concrete is one of the most widely used building engineering materials in the current construction industry. Concrete materials, mix ratio, pouring process and maintenance directly affect the pouring effect of concrete. The discussion from these four aspects is expected to provide inspiration and help for concrete pouring work.
Key words: concrete; pouring; construction; Technology
Chinese Library Classification Number: TU37 Document Identification Code: A
Introduction
With the development of science and technology, concrete pouring is programmed and more scientific and reasonable. The material, mix ratio, pouring process and construction maintenance of concrete will affect the pouring effect of concrete. This paper discusses four aspects in order to provide help for concrete pouring construction.
I. Common Materials of Concrete
Concrete is a heterogeneous three-phase body, that is, solid, liquid and gas. Among them, water, cement, admixture, admixture, sand and stone are the six main raw materials of concrete. The quality of raw materials largely determines the performance of fresh concrete, the strength and durability of hardened concrete. The strength of cement is an important technical index for evaluating and selecting cement. The setting time of cement is also of great significance in construction. Therefore, it is very important to select suitable cement for concrete. Additionally, adding appropriate additives can improve the performance of concrete and improve the impermeability of concrete.
Concrete aggregates are usually divided into coarse aggregates and fine aggregates. The coarse aggregates are made of gravel with a particle size of 5mm-40mm and the fine aggregates are made of medium sand with an average particle size of more than 0.5mm. The concrete made of aggregates with large particle size and good gradation has better workability and higher compressive strength. At the same time, the amount of water and cement can be reduced accordingly, which is beneficial to reducing the hydration heat of cement and reducing the temperature rise of concrete. Fly ash is also a common concrete material. Fly ash is beneficial to reduce hydration heat, improve concrete and workability. Fly ash mixed into concrete can significantly improve workability and fluidity of concrete. It is often used to prepare mass concrete and pumping concrete. However, the early tensile strength and the early limit tensile value of concrete with fly ash decrease, which is unfavorable to the impermeability and crack resistance of concrete. The change of Fly Ash Fineness causes the change of coagulation slump and strength. The addition of fly ash with small fineness, high activity and small water requirement into concrete can save the amount of admixture used in cement, while the fly ash with large water requirement will introduce large amount into concrete. When water is used, the strength of water-cement ratio decreases too much. If used, the dosage of admixture should be increased, which is often not worth the loss. Therefore, the dosage of fly ash should be controlled within 10%. The maximum amount of fly ash replacing cement is 25% when slag Portland cement is used to mix large volume fly ash concrete.
II. Design Requirements of Concrete Mix Ratio
Concrete mix ratio refers to the proportion of each component material in concrete. The volume change of concrete mixing material is one of the main causes of concrete cracks. Because of the quality change of concrete raw materials, the mix proportion of concrete should be adjusted accordingly. There is no universal fixed mix proportion. When designing the mix proportion of concrete, the volume change of mix material should be minimized on the premise of meeting the relevant requirements. When preparing concrete, first of all, according to the requirements of workability, strength and durability of the project, the raw materials should be selected reasonably and the mixing proportion should be determined to achieve the purpose of economic application. The design of concrete mix proportion is usually carried out according to the requirement of water cement ratio rule. The design is carried out according to the relevant requirements of JGJ55-2011 Specification for Concrete Mix Ratio Design, Specification for Construction and Acceptance of Concrete Structures and Technical Specification for Application of Fly Ash Concrete. At the same time, the mix proportion of concrete should take into account the supply of cement of different plant grades to meet the construction requirements.
THINKING ABOUT THE CONCRETE MIXTURE RATIO DESIGN
In the 1980s, some experts in China put forward the view that "high strength may not be durable, low strength may not be durable", and high strength concrete is often accompanied by a series of factors that are not conducive to durability: such as high hydration heat, high shrinkage, easy cracking, brittleness and so on. However, the technical requirements for durability of concrete in China's existing standards are still imperfect, which makes our technology extensive. Technical managers do not know enough about the durability of concrete structures, which leads to poor durability of our buildings, often some structural damage occurs before the service life is reached. Therefore, the Chinese Academy of Architectural Sciences has put forward the idea of "life first, strength appropriate" concrete mix design, which guides the future development direction of concrete mix design, that is, to configure concrete with good workability, high durability and suitable strength as the goal.
(1) Choosing cement of suitable strength grade according to the design strength grade of concrete, minimizing the generation of hydration heat on the premise of satisfying the strength, using active materials such as fly ash or slag to partly replace cement, improving the working performance of concrete, improving the internal structure of concrete, reducing the hydration heat, and making the later strength of concrete increase continuously.
(2) Strictly control the quality of sand and stone raw materials, aggregate plays a skeleton role in concrete, and occupies the largest proportion. Therefore, aggregate particle size distribution, grain shape, strength, harmful substance content (mud content, mud content, organic matter, needle-like particles, etc.) all have an important impact on concrete quality. Therefore, qualified sand and stone raw materials are to optimize concrete mix ratio. To ensure the strength, workability and durability of concrete.
(3) Pay attention to and evaluate the workability of concrete in indoor mixing test to ensure that concrete mixtures with good cohesiveness, water retention and fluidity are produced. At least three water-binder ratios should be prepared in the same mix ratio, and the water-binder ratios that meet the requirements of configuration strength should be found after strength regression.
4. Casting Procedures of Concrete
Concrete pouring is an important link in the final forming of concrete construction and guaranteeing the construction quality. According to different construction units, units and sub-projects, different concrete pouring construction is carried out. Generally speaking, concrete pouring includes layered paving, placing, tamping and other processes.
(1) Layered concrete paving
Large-area concrete slab pavement should be laid sequentially, large-area concrete slab pavement should be layered sequentially, and the second layer should be laid before the initial setting of the first layer. The method of laying material generally adopts the layer-by-layer laying method. The concrete is laid layer by layer continuously according to the level until the specified height of the pouring block is reached. The thickness of the layered concrete paving material can not exceed 30 cm, so that the inner solid and outer light can be poured continuously. When laying material, the interval time between upper and lower layers should not exceed the initial setting time of concrete. If the interval time must be shortened as much as possible. If the warehouse surface is large, inclined layer casting method should be adopted, that is, according to the inclined layer, the gradient of the inclined layer should not generally exceed 10 degrees, and one continuous casting should be completed. The pouring thickness of surface vibration tamping is 120 MM-200 mm, the pouring thickness of large concrete is 200 mm-300 mm, and manual tamping is 150 mm-250 mm.
(2) Vibration of concrete flat warehouse
Concrete flat warehouse is to unload piles of concrete into the warehouse, quickly spread out the required thickness according to the requirements, manual use of iron braze to assist the average spread of concrete materials. Poor placing of silos will affect casting speed. Coarse aggregates in silos should be evenly distributed to more places of mortar when they are stacked. Cement mortar should not be used to cover honeycombs, otherwise, aggregate overhead will occur, seriously affecting the quality of concrete.
Vibration rammer is used to flatten the fluidized concrete by itself after the concrete is leveled. Vibration rammer is forbidden to replace the leveling of the concrete. The purpose of vibration tamping is to reduce the voids in concrete as much as possible, eliminate the holes and honeycombs in concrete, and make concrete closely combined with formwork, reinforcement and buried parts, so as to ensure the maximum compactness of concrete. Vibration is divided into manual vibration and mechanical vibration. In order to improve the effect of vibration, mechanical vibration should be selected as far as possible under conditions. Vibrator is used in concrete pouring to compact concrete to the maximum possible compactness. Vibration time at each position should be strictly carried out in accordance with relevant regulations when concrete no longer sinks significantly and no air bubbles appear, and concrete surface begins to flooding.
V. CONCRETE MAINTENANCE REQUIREMENTS
Concrete curing is to create necessary humidity, temperature and other conditions for concrete hardening, to control the temperature difference between inside and outside of concrete within a certain range, so that concrete can achieve the expected strength requirements. Concrete pouring process is greatly affected by the temperature change of the external environment, which is prone to produce temperature stress, which is not conducive to concrete condensation. Therefore, attention should be paid to the environmental temperature in construction and maintenance. The temperature of pouring environment affects the hydration of cement, and then affects the setting, hardening and strength of concrete mixtures. Concrete should be covered and moisturized within 12 hours after completion of pouring; concrete mixed with Portland cement, ordinary Portland cement or slag Portland cement should not be cured for less than 7 days; concrete mixed with retarding admixtures or impermeability requirements should not be cured for less than 14 days.
Concluding remarks
Concrete is one of the indispensable main building engineering materials in the contemporary era. It is necessary to strictly detect and control the raw materials of concrete for pouring high-quality concrete. Constant exploration is made in concrete mix ratio, pouring technology and construction maintenance, and construction quality control is carried out according to the characteristics and advantages of concrete. Modern construction workers should strengthen their knowledge structure of concrete, constantly improve relevant professional skills to control the quality of concrete from all aspects, so as to ensure the construction quality of the whole project.
Reference
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