Concrete is an artificial stone material for construction, which is obtained as a result of molding and hardening of a correctly selected mixture, including a binder, water, as well as small and large fillers. All this undergoes compulsory compaction. In some cases, special additives are used, and when it comes to asphalt, 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, holding together the grains of the fillers used in a single monolith. The structure and properties of concrete depend on these substances. They change its degree of porosity, reaction to the effects of 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 great opportunities for mixtures with different properties with the addition of a variety of fillers. It is these characteristics of it that open up such wide opportunities for application.
Concrete is a durable material with a high degree of fire resistance, its density, strength and other characteristics can be changed, giving it any specific properties. With proper processing of the mixture, it is possible to produce structures of the necessary form from the standpoint of architecture and structural mechanics.
A bit of history
As an artificial building material, consisting of water, fillers and binders, 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 farm buildings and dwellings. It was also used by the builders of the Great Pyramids. The ancient Romans brought construction from concrete to a new level - after themselves they left not only the foundations of buildings, but also entire blocks of concrete buildings. The design features of Roman roads, domes, arches and floors made of this material have not lost their significance even now. However, in the Middle Ages, Roman concrete manufacturing technology 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. As a binder , gypsum, lime or clay was used.
Specifications
The strength of concrete is its most important characteristic, which has a direct impact on the operational parameters of the material. Under this concept, it is customary to mean the ability of concrete to withstand the effects of aggressive environments 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 in bending, tensile 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 having a rib size of 200 millimeters at the age of 28 days. This type is called cubic strength. In addition to GOSTs, SNiPs are also used to determine strength. For example, the minimum formwork 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 durability that is the most important property. Like natural stone, this material better resists compression rather than stretching, which is why it is the tensile strength in this indicator that is chosen as the main criterion.
The properties
Concrete is a material for which strength is a characteristic that increases as a result of physico-chemical processes of interaction between cement and water, which takes place properly in humid and warm conditions. If the material freezes or dries, this process will end. Previous drying or freezing adversely affects the final characteristics of the material.
Uniformity
Together with all other factors, uniformity in strength depends on the quality and content of the aggregates used, especially if some of the properties of the latter do not allow concrete to be obtained with the required strength. Therefore, this parameter is associated with the previous one, although experimental data show that such a relationship does not always occur. With increasing homogeneity of concrete, there are opportunities for its 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 test results of the same size and storage conditions of equal-aged material samples are taken into account. The uniformity in water resistance is determined by testing samples of the same thickness under the conditions of using the same methods.
Density
This characteristic of concrete is rather complicated, as it varies 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 during solidification. Plasticizing additives, which often improve the characteristics of an already prepared mixture, also have an effect on this parameter. If the composition of the cement corresponds to GOST, then its density will be a known value.
Classes
There are currently several types. Lightweight concrete is a material with a density of 500-1800 kg / m 3 . This class includes: foam concrete, expanded clay concrete, aerated concrete, cellular, arbolite, perlite and vermiculite concrete. In such a mixture, the bearing capacity after solidification is quite small. Normal, or heavy concrete, is characterized by a density of 1800-2500 kg / m 3 . Crushed stone and gravel are used here as a filler. This type is used in industrial construction, which is ensured by its increased resistance to wear. A particularly heavy class of concrete is a material characterized by a density of more than 2500 kg / m 3 . Such mixtures are used for the construction of nuclear power plants, since 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 axial compression resistance. The concrete grade relative to tensile strength indicates the axial tensile strength of the control samples. The frost resistance indicator shows the number of alternating thawing and freezing cycles. The brand of concrete for water resistance shows at which one-way hydraulic pressure the concrete will not pass water during standard tests.
conclusions
When constructing an object of any purpose, the right decision would be to buy ready-made concrete made in full accordance with GOSTs, since it is difficult to achieve the desired result when manufacturing it yourself and without special equipment.