Concrete mixture is a well-composed mass of concrete components, thoroughly mixed before hardening and setting. The composition is determined in accordance with the requirements of the building. Cement dough is the main structure-forming element.
Regardless of the material used, the mixture must retain its original uniformity during transport and installation, and must have sufficient workability according to the compaction technique used.
Concrete, under the influence of increasing force, first transfers elastic deformations, and after a change in structural strength, it takes on the form of a viscous liquid. The definition of thixotropy is used to describe the characteristic of thinning under mechanical influence and thickening in the absence of it.
Specifications
Ease of laying is the most important property in concreting structures and creating reinforced concrete products. It provides the necessary form filling while maintaining the previous structure.
Characterized by the mobility determined by the draft of the cone obtained from the material being tested. Workability has a high stiffness parameter at zerodraft cone.
Hardness is calculated by the period of vibration that was required to compact and level a pre-prepared mixture cone in a special device.
The basic properties of concrete and the homogeneity of the object depend on the consistency of the mixture. Of particular importance is the homogeneity of the mass during transport, installation and compaction. In a concrete movable mixture during compaction, the components of the grains begin to converge, leading to the rise of part of the water. Preventing segregation of moving materials and increasing water retention is achieved by the use of plasticizing compounds, reducing the total amount of water for mixing and careful selection of grain components.
Formability
The main factor in workability is the amount of fluid used for mixing. Water is placed between the aggregates and the cement paste. Its amount also determines the rheological properties of concrete, based on viscosity and maximum displacement stress.
The aggregate acquires a greater water demand with an increase in the overall plane of the grains, which is typical for fine sands.
Water-cement ratio must be kept constant to ensure the strength of the material, as its overspending is caused by an increase in water demand. The use of fine sand is rational after the addition of crushed or natural coarse sand with plasticizing qualities.
Deformation
The application of concrete under load is different fromuse of metal and other materials with greater elasticity. The properties of concrete depend on the conglomerate base with an increase in axial load. It is characterized by elastic deformation changes under load for a short time and at low voltage. The increase in strength increases the available modulus of elasticity, which is also influenced by the porosity of the concrete. The regulation of the module of the material is possible by controlling its structure.
Creep is the increase in concrete deformation under the influence of a static constant load. Such properties of concrete depend on the ambient humidity, the conditions of use, the type, composition of the material and the prescription of its manufacture, the presence of certain aggregates. Crushed igneous rocks, which are in the category of dense aggregates, and high-quality material reduce the overall creep of the mass. At the same time, its strengthening is noted when porous aggregates are used, therefore, heavy concrete is characterized by less creep compared to light concrete.
These mechanical properties of concrete increase with premature setting of the material, which also negatively affects the structure.
Swelling and shrinking
Shrinkage of cement occurs during hardening in the open air, at this time there is compression of the cement and a decrease in the linear parameters of the elements. It depends on the structural and moisture content. Concrete and reinforced concrete objects, when concrete shrinks, acquire the appropriate stresses, therefore, cutting with shrinkage joints is used.for structures that have a large extent, which helps to prevent the formation of cracks.
Massive concrete is characterized by fast external drying, while maintaining moisture inside for a long time. Non-homogeneous shrinkage results in hidden cracks in the cement stone and in contact with the aggregate due to tensile external stress.
Reducing concrete shrinkage is required to maintain the monolithic qualities of objects and adjust shrinkage stress. Due to the addition of filler per unit of total volume, the amount of binder is reduced, and the formation of a kind of filling frame is also noted, preventing large shrinkage. That is why cement stone is more susceptible to it than concrete and mortars.
Concrete, whose construction properties ensure the use for roads and hydraulic structures, is subjected to systematic wetting and drying. Changes in the level of moisture content contribute to alternating deformations, which accordingly leads to the occurrence of cracks and a reduction in the period of operation of the object.
Frost resistance
Frost resistance is determined by alternate freezing and thawing in water. Samples that have undergone heat treatment are tested in a week or a month, subject to exposure to a standard solidification chamber. The stability 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 is noted with porosity up to 7%.
Moisture resistant
The moisture-proof properties of concrete are reduced with a decrease in the volume of capillary pores; for this, water-repellent and sealing elements introduced during manufacture are used. The surface tension of refined products is less than water, and therefore they have a greater degree of penetration into concrete. The addition of special additives is used to reduce the filtration of petroleum products. The use of an intumescent material instead of Portland cement causes a dramatic reduction in oil and water permeability.
Thermophysical basic properties of concrete
One of the most important characteristics is thermal conductivity, which is of particular importance for the material used in building envelopes.
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 from it, the use of internal insulation is required.
Concrete components such as mortar and large aggregates have different coefficients of expansion and, accordingly, different deformations with temperature fluctuations. With large changes, latent cracking may occur, caused by a different level of thermal expansion of the mortar and aggregate. Cracks are found on the plane of the filler; their appearance is also possible in weak grains and in solution. Can be avoidedinternal damage with the right choice of components with similar expansion parameters.
Lightweight concrete
In construction, lightweight concrete based on porous aggregate is becoming more widespread due to a 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 concretes allow them to be used in monolithic and prefabricated load-bearing structures. Increasing the quality of aggregates, expanding raw material sources, improving and developing technology contribute to greater opportunities for use.
The most widespread is in the creation of structures for fencing and wall materials for masonry. But due to the relatively low bearing capacity and strength, lightweight concrete is used in capital construction only if reinforced belts and metal frames are created. Despite this, the existing shortcomings of concrete are minimized by systematically changing the type and shape of the material.
Heavy concrete
Heavy concrete is the most popular material with great strength and ubiquity. It is from it that monolithic parts of objects are formed. Distinctive properties of heavy concrete, ease of installation and supply, affordable cost allowedachieve such prevalence. A decrease in efficiency is noted when creating light floors and wall structures, since a reduction in heat losses is required here.
Aerated concrete: properties, applications
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 hardening of a mixture of siliceous, binder components, expanded through the use of a blowing agent. Due to the latter, a "cellular" structure is formed, which has air pores evenly spaced throughout the volume. The material of this type has sufficient strength, low heat-conducting properties and low bulk density. Such properties of cellular concrete, combined with light technology and available raw materials, make it a convenient progressive option for covering objects created from lightweight reinforced concrete and wall structures. The basis of concrete is the usual components that do not contain harmful substances.
Benefits
During the production process, it is possible to easily adjust the porosity and obtain material with different purposes and bulk density.
When using materials with low density, porous concrete forms sufficient resistance to extraneous sounds and noise. It is also possible to cut into any kind of shape and under variouscorners. Quite common tools, such as a planer or saw, can be used for work.
Aerated reinforced concrete, a composition whose properties allow it to be actively used in regions with high seismic hazard, can sometimes become an indispensable material. Residential and technical facilities, for the creation of which it was used, have greater stability during an earthquake. This is due to the low weight, which reduces the overall load on the structure.
