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13FACTORS AFFECTING THE QUALITY OF CONCRETE AGGREGATES
Bakhromjon Adhamovich Otakulov Khumoyun Sharifjon o'gli Sultonov
Phd. Docent of Fergana Polytechnic Assistant of Fergana Polytechnic Institute Institute
ABSTRACT
The strength of the filler also depends on the size of the grains. Deformation during the erosion or crushing of rocks occurs at weak points in the structure of the material, and as the size decreases, the weak points in the grains decrease, while the strength increases.
Keywords: lightweight aggregate, rocks, frost resistance, clay mixtures
The strength of the lightweight filler is determined by pressing on a 150 mm steel cylinder, as in the previous experiment. However, in contrast to hard rock, for lightweight concrete, the relative strength is determined by the sinking of the piston by 20 mm during the compression of a piece of material with a height of 100 mm. The relative strength of expanded clay is 3-5 times, and that of agloporite is 20-30 times less than the strength of natural material.
The strength of the filler also depends on the size of the grains. Deformation during the erosion or crushing of rocks occurs at weak points in the structure of the material, and as the size decreases, the weak points in the grains decrease, while the strength increases. Natural sands usually have a higher compressive and elongation strength than composite or concrete cement. Therefore, there are no special requirements for ordinary sand. The strength of light sand is determined by the brand of concrete and the appearance of the aggregate, such as a light coarse aggregate.
The frost resistance of gravel and pebbles depends on their structure. Frost resistance is determined by periodic variable freezing and thawing in water or by testing in sodium sulfate solution (accelerated method). The frost resistance of gravel and pebbles is standardized for use in structures that are not protected from the external atmosphere. In these cases, the frost resistance of the large aggregate must provide the concrete with the frost resistance mark required by the design.
The frost resistance of real stone varies from Sov 15 to Sov 300 and depends on the structure of the rock. As the porosity and water content of the filler increase, its cold resistance decreases.
The strength and cost-effectiveness of concrete are greatly influenced by the purity of the aggregate. Dusty and especially clayey mixtures form a crust on the surface of the grains that prevents them from contacting the cement stone. As a result, the strength of concrete is significantly reduced (sometimes by 30-40%). For this
reason, the filler regulations specify the maximum allowable levels of pollutants. Crushed rock admixtures are determined by water treatment and are allowed to be 1% for M300 concrete and 2% for lower grade (low strength) concrete. The total amount of impurities in the sedimentary rocks should not exceed 2 and 3%. The amount of contaminants in the gravel should not exceed 1%, and in natural sand - no more than 3%. The amount of organic matter in the sand is also limited and is controlled by special tests (colorimetric testing).
The effect of dirty and poor quality aggregate on concrete cannot be controlled by increasing cement consumption.
The choice of aggregate for concrete usually takes into account the properties of the concrete mix and its overall effect on the concrete. It is advisable to use gravel or crushed stone in the concrete as much as possible, in which case the conditions of concreting the structure are met, as the aggregate has a minimum specific surface area. In order to pour and compact the required level of concrete mix, the minimum dimensions of the structure may not be larger than gravel or crushed stone, which in turn must be smaller than the minimum dimensions between the reinforcing bars in the reinforced concrete structure. . When concreting slabs, floors and coverings, the maximum size of gravel or crushed stone should be ^ in relation to the slab thickness.
In order to reduce the void size of the coarse aggregate, mixtures of several fractions are used if the potential size limit is available, and a mutually acceptable ratio is chosen to keep the void to a minimum.
For high-strength concretes, strong crushed stone is used. This type of stone is reliably connected with cement stone. Gravel has a smooth surface, which makes it a more mobile concrete mix. However, it is poorly bonded to cement. Therefore, gravel is used in low-grade concrete. In addition, the gravel is contaminated with mud and other contaminants, which require washing.
The use of coarse-grained sand for concrete gives good results. However, the presence of large particles in the sand can increase the voids (up to 40%) and these voids will have to be filled with cement paste. This increases the cost of cement and the cost of concrete. Therefore, the best results are obtained by sand with large, medium and small particles in the optimal ratio, and sand of this ratio provides minimal voids. In high quality sand, the void content should not exceed 38%. With optimal grain content, this figure is reduced to 30%.
If the gaps between the grains of sand in the concrete or mix are filled only with cement paste, a low-mobility, hard-to-pour coarse mixture is formed.
It is necessary to separate the sand grains from each other and surround them with a cement shell, which will ensure the mobility of the mortar or concrete mix. The larger the sand, the lower the specific gravity of the grains and the more cement is used to
form the crust. However, as mentioned above, only coarse-grained sand has a large number of voids and should not be used.
For the preparation of concrete, it is recommended to choose coarse sand with small to medium particles. In this mixed state, the voids are reduced and the surface area of the grains is not large. Such a positive sand composition complies with the recommendations of the UzRST.It is advisable to use sand with an uneven surface, as this sand binds well to the cement stone and increases the strength of the concrete. Due to the complexity and cost of the sand washing process, it is recommended that the sand be as clean as possible. Usually river sand is known to be preferred.The density of sand depends on its true density, voids, and moisture, and is determined by whether it is dry or friable. Sand intended for concrete in structures that are likely to freeze when saturated with water, or for concrete of grades M200 and above, shall have a density of 1550 kg / m3. In other cases - not less than 1400 kg / m3. During the shaking process, the sand is compacted and its density can reach 1600-1700 kg / m3. The largest volume is occupied by sand with a moisture content of 5-7%; the volume of sand decreases with increasing or decreasing humidity. This property must be taken into account in the process of receiving and quantifying sand (by volume), as well as in the preparation of concrete.
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