In the construction and repair of individual residential buildings and in the improvement of personal plots, artificial stone material is widely used – concrete. Ordinary concrete is obtained from a mixture of cement with water and various aggregates (sand, gravel, gravel, pebbles, etc.) after it is molded and hardened. Before molding, this mixture is called concrete mix. Conventional concrete can withstand large compressive loads, but not withstand tensile stress; it is used in structures such as foundations, thick walls. To give concrete greater bending strength in structures that accept tensile forces (lintels, slabs, ceilings, etc.), the concrete mixture is reinforced, that is, steel or iron reinforcement is included in it. Reinforced steel or iron concrete is called reinforced concrete.
The prepared concrete mixes come in different consistencies (densities), which determines their mobility, i.e., the ability of the concrete mix to flow under its own weight or under the action of mechanical forces applied to it. According to the degree of mobility, concrete mixtures are rigid, plastic and cast. Rigid concrete mix during molding requires strong compaction and compaction; It is used in the manufacture of foundations, thick walls and floors without reinforcement or with rarely located reinforcement. Plastic concrete mix requires manual molding and compaction; It is used in the manufacture of beams, columns, lintels with reinforcement of medium density. Cast concrete mixture – a moving mass, easily filling the form, requiring almost no compaction; It is used for the manufacture of thin walls and beams of small sections and in structures highly saturated with reinforcement. The mobility of concrete mixtures is determined by the method of precipitation of a standard cone (Fig. 1), made of metal, 30 cm high, 20 cm wide in the lower base, 10 cm in the upper base. The cone is pressed to the site with your feet and filled with concrete mixture in three steps, bayonet (screwing) each serving with a steel rod (probe) at least 20 times. After filling the cone, the concrete mixture is cut flush with the edges, and the cone is slowly raised. The difference in height of the settled concrete mixture and the height of the cone in centimeters is the number of stiffness of the concrete mixture. In hard mixtures, the sediment is from 0 to 2 cm, in plastic mixtures from 4 to 13 cm, in cast mixtures, from 14 to 22 cm.
The strength of concrete is characterized by their brand. For ordinary concrete, the brand is determined by compression testing of a concrete sample of certain sizes with a curing time of 28 days at a temperature of 20 ° C and humidity not lower than 90%. Ordinary concretes used in individual construction have grades from 50 to 150. The axial tensile strength of ordinary concrete is about 10 times lower than the compressive strength of concrete (its grade).
The preparation of concrete mix begins with the selection of aggregates – gravel, gravel, pebbles and sand of different sizes. To obtain high-quality concrete, aggregates must be clean, without admixture of earth and clay. If necessary, the sand is washed in a stepped tray, with a bottom upholstered in iron. Sand is thrown from the low end of the tray through the net, onto the lower ledge of the tray, where it falls due to the presence of a breaker shield. Then, with a scraper (chopper), sand is moved along the ledges towards the flow of water and dumped onto a sheet of iron or into a container that allows water to pass through.
The quality of concrete with minimal cement consumption largely depends on the ratio of sand and gravel in the mixture; the best ratio is 30–45% of sand and 55–70% of gravel (by weight). The size of the gravel should be no more than 1/3 of the smallest size of the part. For reinforced concrete structures with dense reinforcement, the largest fineness of aggregate grains is 20–40 mm. Cement must be used with a grade that would exceed the specified concrete grade by 2-3 times.
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