Construction Engineering-Performance and Application of Concrete I. Technical Requirements for Concrete Composition Materials (1) Cement Cement strength grade: generally, cement strength grade is 1.5-2.0 times of concrete strength grade; for high strength grade concrete, 0.9-1.5 times is preferable. When using low strength grade cement to prepare high strength grade concrete, the amount of cement will be too large and uneconomical, and it will also affect the other technical properties of concrete.When low-strength concrete is prepared with high-strength grade cement, the amount of cement used will be less, which will affect the workability and compactness, and lead to poor durability of the concrete. Therefore, a certain amount of mixed materials must be added when doing so. (2) Fine aggregate Under the same quality condition, the total surface area of fine sand is larger than that of coarse sand.In concrete, the surface of sand needs to be wrapped by cement slurry, and the voids between sand grains need to be filled by cement slurry. In order to save cement and improve strength, the total surface area of sand and the voids between sand grains should be reduced as far as possible, i.e. coarse sand or medium sand with good gradation should be selected. The particle size distribution and fineness of sand are determined by sieving analysis.The fineness modulus is used to indicate the fineness of sand.The larger the fineness modulus, the thicker the sand.When choosing sand for concrete, the particle size distribution and the thickness of sand should be considered at the same time.The sand in zone II should be preferred in the preparation of concrete. (3) Coarse aggregate In reinforced concrete structural engineering, the maximum size of coarse aggregate should not exceed 1/4 of the minimum size of the structural section, and should not exceed 3/4 of the minimum net distance between reinforcing bars.For concrete solid slabs, aggregates with a maximum particle size of 1/3 thickness can be allowed, but the maximum particle size should not exceed 40 mm.For pumped concrete, the maximum particle size of gravel should not be greater than 1/3 of the diameter of the conveying pipe, and the maximum particle size of gravel should not be greater than 1/2.5 of the diameter of the conveying pipe. (four) water The water quality inspection items of concrete mixing water include pH value, insoluble matter, soluble matter, Cl-, alkali content (when using alkali active aggregate). Concrete mixing water should not float in obvious grease and foam, and should not have obvious color and odour; concrete equipment washing water is not suitable for prestressed concrete, decorative concrete, aerated concrete and concrete exposed to corrosive environment, and shall not be used for alkali activated or potentially alkali activated aggregate concrete.Untreated sea water is strictly forbidden to be used in reinforced concrete and prestressing concrete.Seawater can be used in plain concrete, but it is not suitable for decorative concrete. (5) Additives Admixture is a kind of material which can improve the performance of concrete according to the requirement and is added before or during the mixing of concrete. The amount of admixture is less than 5% of the cement quality (except in special cases). (6) Admixture Function: In order to improve the performance of concrete, save cement and adjust the strength grade of concrete, natural or artificial mineral materials are added in concrete mixing, which are collectively called concrete admixtures. 2. Technical Performance of Concrete (1) workability of concrete mixtures Convenience means that concrete mixtures are easy to construct and operate, and can obtain uniform and compact concrete performance. Convenience includes three aspects: fluidity, cohesion and water retention. On the construction site, the slump degree is used as the fluidity index, and the Vibro consistency is used as the fluidity index for hard concrete whose slump value is less than 10 mm. The factors affecting the workability of concrete are water consumption per unit volume, sand ratio, properties of materials, time and temperature, etc.Water consumption per unit volume determines the quantity and consistency of cement slurry, which is the main factor affecting the workability of concrete. (2) Strength of concrete 1.Standar

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