Concrete mixture is a well-composed mass of concrete components, carefully mixed before hardening and setting. The composition is determined in accordance with the requirements of the building. Cement dough is the main structural element.
Regardless of the material used, the mixture should retain its original uniformity during transport and installation, and also have sufficient workability in accordance with the compaction method used.
Concrete, under the influence of increasing force, undergoes first elastic deformations, and after changing the structural strength, it acquires the form of a viscous fluid. The definition of thixotropy is used to describe the characteristics of dilution under mechanical influence and thickening in its absence.
Specifications
Convenience of laying is the most important property when concreting structures and creating products from reinforced concrete. It provides the necessary form filling while maintaining the previous structure.
The mobility is determined by the draft determined by the draft of the cone obtained from the material being tested. Workability has a high stiffness parameter with zero draft of the cone.
The stiffness is calculated by the period of vibration, which was required to seal and level the pre-prepared cone of the mixture in a special device.
The main properties of concrete and the uniformity of the object depend on the connectivity of the mixture. Of particular importance is the uniformity of the mass during transport, installation and compaction. In a concrete rolling mixture during compaction, the constituent grains begin to converge, leading to a rise in part of the water. Prevention of stratification of moving materials and increase the ability to retain water is achieved by using plasticizing compounds, reducing the total amount of water for kneading and careful selection of grain components.
Formability
The main workability factor is the amount of fluid used for mixing. Water is placed between aggregates and cement paste. The rheological properties of concrete, based on viscosity and maximum displacement stress, also depend on its quantity.
The aggregate acquires greater water demand with an increase in the total plane of grains, which is typical for fine sands.
It is necessary to maintain a constant level of water-cement ratio to ensure the strength of the material, since its over consumption is caused by an increase in water demand. The use of fine sand is rational after adding crushed or natural coarse sand with plasticizing qualities.
Deformation
The use of concrete under load differs from the use of metal and other materials with greater elasticity. The properties of concrete with increasing axial load depend on the conglomerate base. It is characterized by elastic deformation changes under load for a short time and at low voltage. An increase in strength increases the existing modulus of elasticity, which is also affected by the porosity of concrete. Regulation of the material module is possible by controlling its structure.
Creep is an increase in concrete deformation under the influence of a static constant load. Such properties of concrete depend on ambient humidity, application conditions, type, composition of the material and the duration of its manufacture, the presence of certain aggregates. Crushed stone from mountain igneous rocks, belonging to the category of dense aggregates, and high-quality material reduce the total creep of the mass. At the same time, its strengthening is noted when using porous aggregates, therefore, heavy concrete is less creep in comparison with light.
These mechanical properties of concrete increase with premature setting of the material, which also negatively affects the structure.
Swelling and shrinkage
Shrinkage of cement occurs during hardening in the open air, at this time, cement is compressed and the linear parameters of the elements are reduced. It depends on the structural and moisture content. Concrete and reinforced concrete objects during shrinkage of concrete acquire the corresponding stresses, therefore, cutting is used by shrink seams for structures with a large length, which helps to prevent the formation of cracks.
Massive concrete is characterized by quick external drying, with long-term preservation of humidity inside. Inhomogeneous shrinkage leads to the occurrence of hidden cracks in the cement stone and in the places of contact with the aggregate due to tensile external stress.
A reduction in concrete shrinkage is required to maintain the monolithic qualities of the objects and adjust the shrinkage stress. Thanks to the addition of filler in a unit of total volume, the amount of binder is reduced, and the formation of a kind of filling frame that prevents large shrinkage is also noted. That is why cement stone is more susceptible to it than concrete and mortars.
Concrete, the construction properties of which provide use for roads and hydraulic structures, is subjected to systematic wetting and drying. A change in the level of moisture contained contributes to alternate deformations, which accordingly leads to the occurrence of cracks and a reduction in the period of operation of the object.
Frost resistance
Resistance to frost is determined by alternately freezing and thawing in water. Samples that have undergone heat treatment are tested after a week or a month, provided that the standard solidification is maintained in the chamber. Resistance depends on the capillary porosity of the composition and the additives used. Frost resistance and moisture permeability are largely determined by the volume of capillary macropores. An increase in these characteristics at a porosity of up to 7% is noted.
Moisture resistant
Moisture-proof properties of concrete are reduced with a decrease in the volume of capillary pores, for this, hydrophobizing and sealing elements introduced during manufacture are used. The surface tension of oil products is less than water, and therefore they have a greater degree of penetration into concrete. The use of special additives is used to reduce the filtration of petroleum products. The use of expanding material instead of Portland cement causes a sharp decrease in the permeability of oil products and water.
Thermophysical basic properties of concrete
One of the most important characteristics is thermal conductivity, which is of particular importance for the material used in the building envelope.
Heavy concrete has a high level of thermal conductivity, which in some cases reduces the possibility of its use. In the manufacture of external wall panels, the use of internal insulation is required.
Such concrete components as mortar and coarse aggregates have different expansion coefficients and correspondingly different deformations during temperature fluctuations. With large changes, latent cracking may occur due to a different level of thermal expansion of the solution and aggregate. Cracks are located on the aggregate plane, and they can also appear in weak grains and in solution. Internal damage can be avoided with the right choice of components with similar expansion parameters.
Lightweight concrete
Lightweight concrete based on porous aggregate is becoming more widespread in construction due to the sufficient level of strength at low density and a list of such positive characteristics as low cost and thermal conductivity, increased resistance to fire, moisture, frost and durability. Such material is safe, environmentally friendly due to the use of harmless impurities and a mineral base for the manufacture of raw materials. The properties of lightweight concrete allow them to be used in monolithic and prefabricated load-bearing structures. The increase in the quality of aggregates, the expansion of raw materials, the improvement and development of technology contribute to great opportunities for use.
The greatest distribution is noted in the creation of structures for fencing and wall materials for masonry. But because of the relatively small bearing capacity and strength, lightweight concrete is used in capital construction only if reinforced belts and metal frames are created. Despite this, the existing disadvantages of concrete are minimized due to a systematic change in the type and shape of the material.
Heavy concrete
Heavy concrete is the most popular material with great strength and widespread use. It is from it that the monolithic parts of objects are formed. Distinctive properties of heavy concrete, ease of installation and supply, affordable cost allowed to achieve this prevalence. A decrease in efficiency is noted when creating light ceilings and wall structures, since here a reduction in heat loss is required.
Aerated concrete: properties, application
This type belongs to the category of economical and highly efficient materials for construction, allowing you to create objects for various purposes with a small number of floors for operation in any climate.
This is one of the types of lightweight concrete obtained after solidification of a mixture of siliceous, astringent components, swollen through the use of a blowing agent. Due to the latter, a “cellular” structure is formed having air pores uniformly located throughout the volume. A material of this type has sufficient strength, low heat-conducting properties and low bulk density. Such properties of cellular concrete, together with light technology and available raw materials, make it a convenient progressive option for covering objects created from lightweight reinforced concrete and wall structures. Concrete is based on familiar components that do not contain harmful substances.
Benefits
In the production process, quite easily adjusting the porosity and obtaining a material with various purposes and bulk density are possible.
When using materials having a low density, porous concrete forms a sufficient resistance to extraneous sounds and noise. It is also possible to cut into any kind of shape and at various angles. For work, fairly common tools, such as a planer or saw, can be used.
Reinforced cellular concrete, a composition whose properties can be actively used in regions characterized by high seismic hazard, can sometimes become an indispensable material. Residential and technical objects, for the creation of which it was used, have great stability during an earthquake. This is due to the small mass that reduces the overall load on the structure.