Concrete is used today in absolutely all areas of construction. It may be characterized by completely different characteristics, which depend on the purpose for which it is made. The mortar is usually kneaded at the construction site, but for sufficient strength of the concrete structure, its structure is erected from the factory material, which is called ready-mixed concrete.
Basic properties
Among the most important technological properties of the concrete mixture should be highlighted:
- density;
- strength;
- water tightness;
- plastic;
- fire resistance.
Of these, the main indicator is the strength, which is expressed in the ability to resist destruction from loads. The increase in the latter is allowed only to a certain limit. The strength of the material is determined by its brand. It is understood as the ultimate compressive strength of cubes, the side of which is 20 cm. They harden after pouring for a month. The tensile strength is expressed in kg / cm 2 .
Having familiarized yourself with the concept of a brand, you can find out that the following values are set in the documentation. For ordinary concrete, the figure varies from 25 to 600. As for lightweight concrete, here the limits of the norm are from 10 to 300. Strength depends on how active the cement is. This parameter is affected by the quality of crushed stone, sand, water and gravel, as well as the conditions of transportation, mixing, age, laying and hardening of concrete.
Considering the properties of concrete mix, you should definitely pay attention to density. The described material is not completely dense, because it has air pores formed during the evaporation of water or penetration into the air. Density is the degree to which a solid is filled in a volume. So, if the density is 0.95, then 95% of the volume is solid materials, and 5% is pores.
In order to obtain a dense concrete, the amount of water should be reduced, and the grains of aggregates should be chosen of different sizes. This will help to reduce the number of voids. Among the properties of the concrete mixture, plasticity should also be highlighted. It is determined by the mobility of the composition, which when laying should fill all the bends, voids and sinks. Concrete can be:
The plasticity index of the mixture is evaluated by a device called a cone. This tool is a steel shape without a bottom in the form of a cone.
An important property is also water permeability. Its degree is characterized by the highest water pressure at which fluid seeps through the sample. Water resistance depends on the pressure of water, density and structure, as well as hardening conditions, density and age of concrete.
Getting acquainted with the properties of concrete mix, you will need to pay attention to fire resistance. This is the ability of the material, which is expressed in resistance to destructive factors associated with high temperature. Concrete must withstand normal use up to 250 ° C.
In addition, it is necessary to pay attention to shrinkage of the material. When hardening in air, concrete decreases in volume. Outside, this happens faster than inside; cracks become a consequence of this. Shrinkage, as a rule, is 0.15 mm per 1 m. If you choose the composition of the solution, you can reduce the amount of shrinkage or prevent it at all.
A feature of the fill is the heat generated during hardening of the mixture. Therefore, in buildings under construction, one can observe an increase in temperature even at low ambient temperatures. This property allows concreting in winter conditions without heating.
Concrete composition
In order to achieve the desired properties of the concrete mixture, it is necessary to adhere to a certain composition. The usual concrete solution is obtained by mixing in certain proportions the following ingredients:
- cement;
- sand;
- water;
- crushed stone.
The ratio of components depends on the desired result. Substances designed to improve the properties of the material are often added to the finished mixture. The mixture in this case will be different, but the main components will remain unchanged.
Cement and water are necessary to ensure the viscosity of the solution and the formation of its integrity. The amount of fluid and cement, as well as the moisture content of sand and aggregates, should be controlled.
The main types of mortar for outdoor use
The properties of the concrete mix may vary depending on the purpose of the material. Among the main varieties of concrete that will be used outside, it should be noted:
- reinforced concrete;
- silicate concrete;
- asphalt concrete;
- hydraulic concrete;
- expanded clay concrete;
- perlite concrete;
- tuff concrete.
Reinforced concrete is a combination of base material with reinforcement. The mixture is used in all climatic zones, because it does not lose properties up to -45 ˚. Operation is possible up to +60 ˚. Most are familiar with this type of material from reinforced concrete slabs used in ceilings.
Silicate concrete is a composition of silicon and lime. The mixture may include quartz and silica. Sand acts as a filler. This type of concrete is made by autoclaving. The material is steamed at a temperature of 174 to 198 ° C.
A rather dense mixture is asphalt. It consists of:
- mineral powder;
- sand;
- bitumen;
- crushed stone.
Each part is separately dried, and then heated to 150 ° C. According to the laying temperature, asphalt concrete can be hot or viscous, cold or liquid. The operating temperature of the former reaches 120 ° C, and the latter should not be lower than 10 ° C. Roofs and road surfaces are made from such concrete.
Among the main properties of the concrete mix, which is also called hydraulic concrete, it is worth highlighting the increased water resistance. From this material, buildings are erected that will be operated in marshy areas or in regions often exposed to floods.
Expanded clay is one of the types of lightweight concrete. Expanded clay is aggregate here. This reduces the cost of work, and the mass of structures is less.
Perlite is a placeholder in perlite concrete. This material belongs to the class of light, concrete fencing is made from it. The filler in tuff concrete is volcanic tuff. Walls and floor slabs are made from such material.
The main types of concrete for interior work
The properties of concrete and concrete mixture may be such that the material can only be used for internal work. This applies to gypsum concrete. Instead of cement, building gypsum is used here, to which stone aggregates are added. Additional components are straw and wood.
Instead of cement, an organic polymer is also used as a binder in plastic concrete. Any sand can become a filler. This material is used for pouring floors in public and industrial premises. Filler in concrete can also be pumice. We are talking about pumice concrete, which acts as a heat-insulating material.
Aerated concrete is divided into several subspecies. This should include gas and foam concrete. Both of these types are used as thermal insulation components in construction. The cellular material is inferior to the heat insulator. A separate type is heat-resistant concrete, which is used in the metallurgical industry in the form of the foundation of open-hearth furnaces.
Types of strength classes
Considering the basic properties of concrete mix, you should pay attention to strength. The material is divided into strength classes for concrete:
- lungs;
- heavy;
- especially heavy.
In the former, the strength does not exceed 1,800 kg / m 3 , in the latter it varies from 1,800 to 2,500, and in the third, the value exceeds 2,500 kg / m 3 .
Concrete grades and their application
The properties of the concrete mix and methods for evaluating them are to be studied if you are going to erect a building structure yourself. Assessment methods include placing the cone in the mixture and determining the water permeability of the material.
And the quality of concrete can be determined by paying attention to its brand. The most common are brands from M100 to M550.
Concrete B7.5 (M100) is a lightweight material that is used at the stage of foundation pouring. Using this solution, you can prepare the surface and lay the fittings. In road construction, such a composition is used in the arrangement of borders.
One of the varieties of lightweight concrete, which is also called skinny, is concrete B12.5 (M150). The area of use is the pouring of monolithic plates and foundations. These materials are used very widely, for example, for the formation of screeds, when floors, pedestrian walkways, and also when installing curbs are being filled. You can use the mixture in the construction of foundations for small structures.
A fairly wide area of construction work covers concrete B15 (M200). It has high compressive strength and is used in the construction of various retaining walls, foundations, walkways and in the arrangement of sites. You can use the material in the manufacture of stairs, as well as in the construction of concrete pillows for curbs and roads.
Concrete B20 (M250) in terms of use and characteristics can be compared with B15, however, the composition can be used in the manufacture of plates that will be subjected to a small load. This material is much stronger than previous samples. A fairly popular brand that is used in the construction of pile-grillage and other monolithic structures is concrete B22.5, better known under the M300 brand. It is customary to use it when casting the blind area, the manufacture of stairs and fences, as well as platforms.
In the construction of high-rise buildings for foundations, concrete B25 (M350) is usually used. The composition is characterized by high strength, it can be used in the manufacture of hollow core slabs and beams. The material has become widespread in monolithic housing construction, as well as in the manufacture of road slabs, bowls for pools, supporting columns and much more. This concrete withstands high loads, due to which it is widely used in the construction of commercial and public buildings.
The average grade of concrete is B30 (M400). The composition is distinguished by greater cost and quick setting, therefore, it is not so popular. It is characterized by high strength and reliability, therefore, it is an indispensable material in the construction of hydraulic structures and reinforced concrete products, as well as bank vaults, which are subject to special requirements. This brand of concrete is recommended for objects with high safety requirements. This should include:
- indoor pools;
- entertainment and shopping complexes;
- water parks.
A large percentage of cement in its composition has concrete B40 (M500) and B45 (M550), which has high strength and is used in reinforced concrete products for special purposes, as well as in hydraulic construction. For the construction of buildings, it is usually not used.
Characteristics of concrete M200 in accordance with GOST
Pouring concrete mixture, the main technological properties of which are important, must be controlled from start to finish. At large construction sites, the foreman does this. His experience allows us to judge the quality of concrete. For example, he knows that material of the M200 brand belongs to the category of lungs, and its bulk density and density depend on the type of filler. This parameter ranges from 500 to 1800 kg / m 3, the mobility of the material varies from P2 to P4, the frost resistance is F100, the water resistance is W4.
The composition complies with GOST 10181-2000, according to which among the ingredients must be:
- large aggregates in the form of crushed stone;
- sand;
- cement;
- water.
Sometimes a plasticizer is added. The most commonly used crushed stone fraction for this type of concrete is 10-20 mm. If crushed stone with finer or larger grain is present in the composition, its volume should not exceed 5%.
Considering the composition and properties of concrete mix, you should pay attention to the following. In the manufacture of materials M200, the best component will be crushed granite, which has high strength characteristics. The brand of stone in strength should not be less than M800. The quality of the final product is influenced not only by the properties of the materials for making the concrete mixture, but also by the purity of the coarse aggregate. If the presence of clay and dusty inclusions is noticed, this will become an obstacle to the adhesion of crushed stone to cement, as a result of which the strength of concrete can decrease by up to 30%.
Characteristics of concrete M450 and its compliance with state standards
Among the main properties of heavy concrete and concrete mix there is mobility. For the mentioned brand, this parameter is equal to the limit P3 - P5. Thermal insulation properties decrease when the concrete hardens; there is also an increase in frost resistance and water resistance. Therefore, it is possible to build hydraulic structures where the water level periodically changes.
This material has a high level of strength and is resistant to frost. Compliance with the class according to the latest specification - F300. As for water resistance, the mixture belongs to the class W8 - W12. This suggests that the use of additional sealing additives is not necessary.
The solution corresponds to GOST-7473. Most often, samples of category P2, P3, P4, P5 are used. It is possible to make concrete M450 from one share of cement of the M400 brand, 1.1 parts of sand and 2.5 parts of crushed stone. If cement of the M500 brand is used, then the proportions should be observed: 2.9 parts of crushed stone, part of cement, 1.4 parts of sand.
Property Retention
Assessment of the persistence of concrete mixture properties over time consists in obtaining data on changes in characteristics over a certain period. From the moment of selection to the moment of testing the concrete mixture, the correct storage conditions for the sample must be observed.
Determining the properties of the concrete mix will allow you to understand how well the material will remain in time. The product can belong to one of three classes, each of which determines the level of preservation. Separate low, medium and high.
If the solution was made on the basis of quick setting cement, then the finished material can be attributed to the class C-1, which is characterized by an incredibly low shelf life, equal to only 20 percent.
If the material is made of normally setting cement, then it can already be attributed to class C-2, which is characterized by an average persistence. It can vary from 20 to 60%.
If the composition contains slow-setting cements and components with a low temperature, then the finished material can be attributed to class C-3 with a high level of storage - more than 60%. Such material is more likely to not collapse over time.
Special properties of concrete
Under the influence of external factors, loosening of the structure occurs, after which the bonds between the elements are weakened. As a result, the ability to deformations increases, the mobility increases. The ability of systems to change their rheological properties of a concrete mixture under the influence of mechanical stresses and recover after cessation of exposure is called thixotropy.
The thixotropic properties of the concrete mixture are among the main ones. They determine the ability of the material to liquefy, namely: to acquire the properties of a liquid body. This occurs with periodically repeated mechanical stresses.
A special characteristic of the visco-plastic properties of concrete is workability. It is understood as the ability of the composition to fit into the form and compact under the action of receptions and mechanisms.
According to GOST 7473-2010, for workability concrete can be:
- super rigid;
- tough;
- movable.
The first hardness is more than 50 seconds, the second it varies from 5 to 50 seconds, the third hardness is less than 4 seconds.
To describe the behavior of concrete in different conditions, the rheological properties of the concrete mixture are used. Among them are the following:
- viscosity;
- ultimate shear stress;
- relaxation period.
Thus, knowledge of the composition and special properties of concrete mixtures will help in the future to use solutions without the risk of destruction of the erected structures.