Concrete is an artificial stone material for construction, which is obtained by molding and curing a properly selected mixture, including a binder, water, and fine and coarse aggregates. All this undergoes mandatory compaction. In some cases, special additives are used, and in the case of asph alt concrete, water is not used at all.
Components
At its core, concrete is a mixture of cement and water, as a result of the reaction between which a cement stone is formed, fastening the grains of the fillers used into a single monolith. The structure and properties of concrete depend on these substances. They change its degree of porosity, response to loads, hardening time, and also significantly reduce the deformation of concrete during its hardening. Concrete has become the main building material used in all areas due to the fact that it provides ample opportunities for obtaining mixtures with different properties withadding a variety of fillers. It is these characteristics that open up such a wide range of applications.
Concrete is a durable material with a high degree of fire resistance, its density, strength and other characteristics can be changed, giving it some specific properties. With proper processing, the mixture can be made into structures of the required shape from the standpoint of architecture and structural mechanics.
A bit of history
As an artificial building material consisting of water, fillers and a binder, concrete has been known since ancient times. More than seven thousand years ago, it was used by the inhabitants of Mesopotamia for the construction of outbuildings and dwellings. It was also used by the builders of the Great Pyramids. The ancient Romans took concrete construction to a new level - they left behind not only the foundations of buildings, but also entire blocks of concrete buildings. The design features of Roman roads, domes, vaults and floors made of this material have not lost their significance even now. However, in the Middle Ages, the technology for making Roman concrete was irretrievably lost.
Of course, ancient concrete is not the same as modern concrete. Its main difference lies in the composition, at that time there was no cement in it. Gypsum, lime or clay was used as a binder.
Features
The strength of concrete is its most important characteristic, which has a direct impact on the operational parameters of the material. By this concept it is customary to mean the ability of concrete to withstand the effects of aggressive media and external mechanical forces. This value is determined by control methods: ultrasonic and mechanical. GOST 18105-86 specifies the rules for testing the strength of concrete for bending, tension and compression. One of the characteristics is the coefficient of variation, which demonstrates the homogeneity of the mixture.
In accordance with GOST 10180-67, the tensile strength of concrete is determined by compressing a control cube with a rib size of 200 millimeters at the age of 28 days. This type is commonly called cubic strength. In addition to GOSTs, SNiPs are also used to determine strength. For example, the minimum stripping strength of concrete of horizontal unloaded structures with a span of up to 6 meters should be at least 70% of the design strength, and with a length of more than 6 meters - 80% of the design strength. In this case, it is strength that is the most important property. Like natural stone, this material resists compression better than tensile strength, which is why the tensile strength for this indicator was chosen as the main criterion.
Properties
Concrete is a material for which strength is a characteristic that grows as a result of physico-chemical processes of interaction between cement and water, which takes place properly in humid and warm conditions. Should it happen that the material freezes or dries out, this process will be completed. Early drying or freezing affects the finalmaterial characteristics.
Uniformity
Together with all other factors, uniformity in strength depends on the quality and content of aggregates used, especially if some properties of the latter do not allow obtaining concrete of the required strength. Therefore, this parameter is associated with the previous one, although experimental data show that such a relationship does not always take place. As concrete becomes more homogeneous, there are opportunities for more efficient use.
The homogeneity index is determined as a result of tests of control samples made from working concrete with certain specified properties. For example, in the process of calculating this indicator, the results of testing identical in size and storage conditions of equally aged material samples are taken into account. Uniformity in water resistance is determined by testing samples of the same thickness using the same methods.
Density
This characteristic of concrete is quite tricky, as it changes depending on which components are added to the mixture. To increase the density of concrete, you can use pozzolanic Portland cement, which expands, or alumina cement, which does not form voids when solidified. This parameter is also influenced by plasticizer additives, which often improve the characteristics of an already finished mixture. If the composition of the cement corresponds to GOST, then its density will be a known value.
Classes
At the moment there are several types. Lightweight concrete is a material whose density is 500-1800 kg/m3. This class includes: foam concrete, expanded clay concrete, aerated concrete, cellular, wood concrete, perlite and vermiculite concrete. In such a mixture, the bearing capacity after solidification is rather small. Ordinary, or heavy concrete, is characterized by a density of 1800-2500 kg/m3. As a filler, crushed stone, gravel is used here. This type is used in industrial construction, which is ensured by its increased resistance to wear. An especially heavy class of concrete is a material characterized by a density of more than 2500 kg/m3. Such mixtures are used for the construction of nuclear power plants, as they have the properties of protection against ionizing radiation.
Concrete grade
This is another important characteristic of this material. The compressive strength index shows the resistance to axial compression. The concrete grade in relation to tension shows the resistance to axial tension of control samples. The frost resistance index shows the number of cycles of alternating thawing and freezing. The concrete water resistance grade indicates at what one-sided hydraulic pressure the concrete will not let water through during standard testing.
Conclusions
When building an object for any purpose, the right decision would be to buy ready-made concrete, made in full accordance with GOSTs, since it is difficult to achievethe desired result when making it yourself and without special equipment.
