The concept of hydrophobic cements and their use in specialized construction. Non-shrinking cement mortar for monolithic joints of reinforced concrete structures Maintenance of concrete structures

Non-shrink cement- used when necessary to obtain a concrete coating that does not allow moisture to pass through. This type of cement mixture is characterized by a fast setting process (the beginning of setting starts a few minutes after the connection, and ends no later than 5-10 minutes). At the same time, the mass quickly hardens, reaching about 60-80% of the total brand strength by the end of the third day. The resulting cement stone has high moisture resistance and is able to withstand water pressure of 0.7 MPa.

Initially, waterproof non-shrinking cement was created on the basis of a different mixture - alumina. The basic raw materials for cement are bauxite and limestone. The principle of operation of a waterproof non-shrinking mixture is that when the solution solidifies, the process of crystallization of calcium aluminates occurs, under conditions of counteracting the free expansion of the solution. This affects the significant compaction of the cement stone, as a result of which it becomes waterproof and acquires waterproofing qualities.

Non-shrinking cement is produced in factories, in the process of grinding aluminous type cement with calcined lime and gypsum. If the volumes of gypsum and lime can vary, then the amount of cement should be 85% of the total mass. Addition of asbestos is allowed (no more than 5%).

A well-prepared cement stone acquires moisture resistance after an hour, and full activation of all its properties after 28 days.

The material has the following advantages:

resistance to corrosion formations;

tightness;

reliability;

durability.

The disadvantages include:

• inability to use in an environment that does not have sufficient humidity;
• intolerance to temperatures exceeding 80 degrees Celsius.

Waterproof non-shrinking cement is used when pouring a foundation that is not subject to water filtration. It is indispensable for flooring in garages and basements, in cellars where isolation from contact with groundwater is required. I fill the walls of the cesspools with this cement so that the contents do not get into the groundwater.

Expanding PC

The shrinkage of cement stone causes tensile stresses that often exceed the strength of concrete and lead to cracking. When repairing building structures (monolyzing cracks), sealing areas of interface of two or more structural elements, it is not possible to achieve a high quality of work due to the fact that, as a rule, highly mobile repair compounds are used, the shrinkage of which is significant. On the contact area of ​​the "new" concrete with the "old" tensile stresses arise, the strength of the contact layer is significantly reduced.

Cements, solutions based on which give an increase in volume, are called expanding. All expanding cements are mixed: they consist of a binder and an expanding additive.

Extension mechanisms:

Oxide - as a result of hydration of MqO or CaO to the formation of Mq (OH) 2, Ca (OH) 2. The expansion is caused by a twofold increase in volume during hydration to hydroxide.

Sulfoaluminate in the formation of calcium hydrosulfoaluminates.

The expansion is caused by the presence in the composition of the cement of substances that form the gas phase

According to the indicators of free expansion of the cement paste during hardening in water, cements are classified:

Non-shrink, in which the expansion fully compensates for the shrinkage…….2-5 mm/m

Slightly expanding…………………………………………………………….5-6

Medium expanding…………………………………………………………….8-10

Strongly expanding………………………………………………………………12-15

Expansion of concrete (with a cement content of 250-300 kg/m3 is 10% of the expansion of the test, with a cement content of 400 kg/m3 - 20%, with a cement content of 600 kg/m3, the expansion will reach 45% of the possible expansion of the test.

The expansion rate depends on many factors: the mineral composition of the cement, the type of expanding additive, its quantity, and the conditions for cement hardening.

As expanding additives are used:

high-calcium aluminates 4CaO ∙Al 2 O 3 ∙13H 2 O, 4CaO ∙3Al 2 O 3 ∙CaSO 4

Minerals with a high amount of alumina (aluminous cement, aluminous slag),

Cement characteristics: Fineness of grinding T 02 not >1%, T 008 not >7%,

Beginning of setting no earlier than 30 minutes, end no later than 12 hours

Expansion rate 0.4%

Marks 400,500,600. Strength at the age of 28 days prevails over the strength of Portland cement by 7-8 MPa, there is no drop in strength by 28 days.

Cement can be steamed

Expanding cements have high water, sulfate, frost resistance. The stone has high marks for water resistance. Expanding cements are used to reinforce reinforced concrete structures, because in the absence of shrinkage, the adhesion strength of the new concrete to the old increases.

Concretes based on the previously considered hydraulic binders during hardening in air decrease in volume, i.e. their hardening causes shrinkage- an extremely negative phenomenon affecting the quality of finished structures.

Volumetric shrinkage deformations are one of the main reasons for the appearance of cracks in concrete, which reduce the durability of engineering structures. In this regard, new types of cements are currently being used, the hardening process of which in the initial period is accompanied by either an increase in the volume of the cement stone (the so-called expanding cements), or by compensating for cement shrinkage ( non-shrink cements).

The essence of these phenomena is as follows. With the hydration of all mineral binders, their absolute volume decreases due to chemical contraction. In the case of the use of expanding cement, its volume increases when mixed with water. Such an "unexpected" increase in volume can only occur if the following inequality is met:

where C is the mass of cement, g; p c - density of cement, g/cm 3 ; B is the mass of water, g; C x - mass of cement that has not reacted with water, g; B x is the mass of water that has not reacted with cement, g; p g - average density of cement hydration products, g/cm 3 ; a - the pore volume of the cement stone, cm 3.

It follows from the above inequality that the expansion of the cement stone should be accompanied by an increase in the pore volume due to the “pushing apart” of the hydrating cement grains, which is taken into account by the increasing pore volume of the cement stone (a). According to P.P. Budnikova and I.V. Kravchenko, such a separation is caused by a significant crystallization pressure of growing crystals of the "cement bacillus" - calcium hydrosulfoaluminate (3Ca0A1 2 0 3 3CaSO 31H 2 0).

It is known that the necessary component of the "bacillus" - calcium hydroaluminates (ZCa0A1 2 0 3 6H 2 0) - is formed during the hardening of aluminous cement. Therefore, expanding and non-shrinking cements necessarily contain aluminous cement in their composition. Another "standard" component is gypsum dihydrate. The remaining components of the expanding cement composition can be represented by Portland cement clinker or other active mineral additives. The name of the expanding cement depends on its composition (Table 4.7):

• ? gypsum alumina cement;
• ? fast-setting expanding Portland cement;
• ? waterproof expanding cement (WRC);
• ? tensile cement.

Types of expanding cement and their parameters

Table 4.7

Gypsum-aluminous expanding cement, expanding Portland cement and tensioning cement have found the greatest use.

Gypsum alumina expanding cement- a fast-acting hydraulic binder obtained by joint fine grinding of high-alumina blast-furnace slags (70%) and natural gypsum dihydrate (30%) or by thorough mixing of the same materials, ground separately.

The beginning of setting should occur no earlier than 20 minutes, the end - no later than 4 hours from the start of mixing.

Gypsum alumina cement expands only when hardened in water; when cured in air, it is non-shrinking.

Ultimate compressive strength after 1 day. hardening should be 35 MPa (grade 400) and 50 MPa (for grade 500). Grades of cement correspond to three days of age.

This cement is used to obtain non-shrinking and expanding waterproof concrete, for waterproofing plastering, for strengthening wells, etc.

Expanding Portland cement- fast-hardening hydraulic binder obtained by joint fine grinding of Portland cement clinker, high-alumina slag, gypsum dihydrate and granulated blast-furnace slag.

Cement stone on expanding Portland cement in the initial period of hardening increases in volume by 0.3 ... 1.2%, and therefore concretes and mortars on this binder have greater water permeability compared to concretes on ordinary cement.

Concrete based on such cement makes it possible to reduce the time of their steaming to obtain the design tempering strength.

Expanding Portland cement is used in the manufacture of concretes and mortars for sealing joints and monolithic reinforced concrete structures.

Prestressing cement (NC) is a fast-setting and fast-hardening hydraulic binder obtained by joint grinding of Portland cement clinker (65...70%), gypsum dihydrate (8...15%) and high alumina component (10...20%). The fineness of grinding is not less than 4000 cm 2 /g. The term for the beginning of setting is not earlier than 30 minutes, the end of setting is no later than 4 hours. It is characterized by increased rates of water and gas impermeability, frost resistance, tensile and bending strength. It has the ability to significantly expand (up to 3.5 ... 4%) during hardening. Cement grades 400 and 500.

In reinforced concrete, NC creates prestressing reinforcement after hardening, which is used in the manufacture of prestressed reinforced concrete structures. This type of cement is also used for waterproofing mines, basements, chasing seams, for the construction of road and airfield cement concrete pavements.

A number of concrete structures, in accordance with their special purpose, must have one degree or another. Among the "designs for special purposes": all types of hydraulic structures, basements, cellars, water storage tanks, etc., waterproof cement is used for this.

Using waterproof cement for the manufacture of concretes and mortars, it is possible to ensure guaranteed protection of structures from the penetration of moisture into the thickness of their structure. And also significantly reduce or completely eliminate costly waterproofing measures.

How to waterproof cement

The permeability of a concrete structure for moisture supplied under excess pressure is determined by the presence of pores and capillaries through which moisture actually penetrates. Thus, in order to effectively eliminate leaks, waterproof cement must have a minimum amount of air pores and air capillaries in its thickness.

In other words, to create waterproof cement means to localize air inclusions by a set of measures: expanding the material, filling pores with another substance, or eliminating them by other methods. Among them:

• Prepare concrete mortar based on waterproof cements.
• Vibration of a cast structure.
• Special care for concrete structures.

Let's consider these methods in more detail.

Types of waterproof cements

Depending on the mechanism for obtaining "watertightness", the following types of binder are distinguished: waterproof non-shrinking cement, waterproof expanding cement and cement with special additives.

• Waterproof VRC. Popular brands: M400 VRTs D0(D20), M500 VRTs D0(D20), M600 VRTs D0(D20). Waterproof fast-hardening cement is a type of expanding material. Popular brands: M400 VRTs B, M500 VRTs B, M600 VRTs B. Due to the special composition and special manufacturing technology during setting, this building material expands in volume and quickly hardens. On average, the setting of the mixture occurs within 10 minutes. At the same time, the expansion value depends on the degree of ambient humidity - with an increase in humidity, the expansion value increases. Therefore, it is very important to maintain a high humidity of the structure for 72 hours after pouring (cover with plastic wrap, spray with water). The scope of application is the construction of hydraulic structures (dams, dams, bridges), the repair of structures, the construction of underground facilities. It is also the most preferred waterproof cement for swimming pools.
• Waterproof non-shrink cement. Popular brands: M400 VBC D0 (D20), M500 VBC D0 (D20), M600 VBC D0 (D20). This type of special binder is produced on the basis of clinker with the addition of calcium aluminate. Thanks to the addition of calcium aluminate, after mixing the mixture, a very dense stone is formed with a minimum number of capillaries. As in the previous case, a prerequisite for the hardening of concrete or mortar based on non-shrink cement is high ambient humidity. If the ambient humidity is less than 70%, the "non-shrink" properties of the material are lost. Scope - construction of foundations, floors in garages and basements, cesspools, waterproofing of tunnels, sealing joints, construction of pool bowls and artificial reservoirs.
• hydrophobic cement. The material of this type is a general construction Portland cement with special water-repellent additives (oleic acid, acid residues of synthetic origin, asidol, soap naphtha, etc.). Popular brands: M400 D0 HZ, M500 D0 HZ, M600 D0 HZ. The mechanism for achieving “moisture impermeability” is the combination of cement particles with additive molecules, followed by the formation of a monomolecular air layer that protects the surface of the particles from contact with a humid environment. Scope: erection of bowls of artificial reservoirs, production of waterproofing plasters, laying of roads and runways of airfields.

Producers of waterproof cements: PJSC "ECOSYSTEM" (Moscow), Yekaterinburg Cement Plant, MAPEI (Italy). Due to the very high price of waterproof cement, limited use and short shelf life, most domestic cement manufacturers produce it in separate batches under a separate contract.

Vibration

Maintenance of concrete structures

Within 72 hours after pouring the structure, the concrete surface needs special care - regular, at intervals of several hours, moistening (sprinkling with water) and wrapping with plastic wrap. These measures help to avoid the appearance of cracks and reduce the amount of shrinkage.

Do-it-yourself waterproof cement

Considering the limited availability of moisture-resistant cements in free sale and their high cost, private developers can make a waterproof composition by adding the additive "Liquid waterproofing hyperconcentrate Dehydrol lux 10-2" manufactured by NPK Dehydrol (Russia, Krasnoyarsk) to the cement. The additive is introduced into the concrete mixer after the addition of the mixing agent or into the already prepared mixture in the amount shown in this table:

With the help of this relatively inexpensive additive (269-270 rubles / liter), you can make a structure on your own that has the following technical characteristics: water resistance up to W20 (throughout the entire “body” of the structure), frost resistance up to F600, increased compressive strength (up to 30%) and increased steel protection.

The use of waterproof materials for the preparation of concrete mix greatly simplifies construction work. If it is necessary to pour concrete or manufacture monolithic structures, it is advisable to use non-shrink cement. In addition to the lack of shrinkage properties, it is able to provide intensive setting and hardening in a short time.

In modern construction, the construction of buildings and structures is carried out not only in a temperate climate and with a normal level of groundwater, but also in conditions of an increased humid environment, swampy areas and flooded areas. For the installation of dams, dams, concrete channels, waterproof cement is used, which will meet all the requirements for water tightness and resistance to aggressive environments.

It is marked as VBC and incorporates additives that can improve technical characteristics. Also in the composition there is aluminous cement and aluminum oxide. They provide excellent binding properties and reduce the setting time. Additionally, the non-shrink mixture is based on bauxite and limestone, which are mined directly on the territory of the Russian Federation.

The main factors of the VBC include the following:

1. The amount of cement of the total volume should be 85%, the entry of asbestos does not exceed 5%, and the proportions of lime and gypsum are ambiguous (depending on the purpose of the solution).
2. In factory production, the manufacture of a waterproof material relies on the careful grinding of cement, as well as the addition of calcined lime and gypsum flour.
3. According to the principle of action, crystallization of calcium aluminates is formed during solidification. Further, the ability to resist expansion is taken into account, which contributes to a high degree of concrete compaction. Thanks to this, the solution becomes waterproof and obtains excellent waterproofing qualities.

All characteristics begin to appear 1-1.5 hours after the manufacture of the material. After 28-30 days, it completely hardens and acquires better performance and strength properties. Due to the fact that the fast-hardening waterproof cement is used in a sufficiently humid environment, it must meet anti-corrosion requirements.

It is these qualities that do not allow rust and collapse of reinforcing bars and embedded parts. To achieve them, aluminum powder, calcium nitrate and ferrosilicon as an astringent are added to the composition. The proportional amount of fillers may vary, and therefore the cost of production also changes.

Technical properties of the solution

1. Waterproof cement has tension and expansion properties with negligible shrinkage. This applies to both plastic and setting properties. Hardening time - 2-3 days.

2. Rheoplasticity is a characteristic of fluidity with a minimum amount of water. In a proportional ratio, about 20-30% of it is enough for 3 kg of dry mix. When kneading 25 kg, the volume of the finished composition at the exit is 16.5-17 liters. However, the consistency in this proportion is very liquid, so it is practical to use it for pouring concrete products or structures into the formwork.

3. Due to the high fluidity, properties such as good workability and an increased density coefficient in the early and final stages of application appear. It has excellent opeophobic characteristics, i.e. resistance to hydrocarbons and oil formulations.

4. The resulting viscous concrete in proportions coincides with the one with a reduced draft of the cone, while there is practically no water separation. It is also valued for its high resistance to the influence of sulfate compounds. Due to the small amount of water in the composition of the non-shrinking mortar, periodic treatment with special concrete care products is required during the drying process. If this is not done, the thin casting layer may begin to crack due to the quick setting.

Application area

The main purpose is the manufacture of insulating shells of elements in large-scale reinforced concrete structures that are used for water filtration. It also acts as a waterproofing material in underground tunnels or channels under water. Due to its high water resistance, it can be repaired or sealed in large-panel buildings.

VBC is selected in conditions of high humidity from 70% and above, since in fairly dry rooms up to 65% the mixture can shrink significantly. This type of cement is selected to protect concrete structures from the effects of moisture-containing chlorides, sulfites and caustic sulfates.

Among other things, quick-setting cement is allowed to be used for the following types of work:

1. Filling of monolithic systems at low humidity, as well as at a device at a sufficiently high height from ground level (more than 200 m).
2. Mixing solutions for embedding embedded parts, anchor elements, loops. Additionally, it is used for cementing thin seams and joints in masonry or brickwork.
3. Sealing of cracks and defects in concrete products after the influence of high mechanical stresses, as well as during operation.
4. Production of densely reinforced reinforced concrete products, filling of joints in prefabricated elements of buildings.
5. Construction of foundations or foundation pads for nuclear power plants, ports, piers and turbine generators.
6. Repair work in industrial plants where lubricating or fuel mixtures, mineral oils are used, as well as the restoration of prestressed structures that are subject to normal or eccentric forces.

Modern manufacturers produce materials according to individual technologies and produce under their own trademarks - HYDRO-SI, NTs 10, Master Emaco A 640 (MacFlow) and others.

Portland cement stone, when cured in air, dries out and undergoes shrinkage, which is often the cause of shrinkage cracks. In order to tightly seal the seam between the prefabricated structural elements and obtain a practically impermeable mortar, or concrete, it is necessary to use a binder that, after mixing in the initial period of hardening, can increase its volume without structural disturbances. Expanding cements have controlled expansion, which, manifesting itself in cramped conditions, causes self-compacting of cement stone (and concrete). Mortars and concretes on expanding cements are virtually impermeable to water and oil products (kerosene, gasoline, etc.), which, due to low surface tension, easily seep through the capillary pores of Portland cement stone.

Waterproof expanding cement(developed by V.V. Mikhailov) is a quick-setting and quick-hardening hydraulic binder. It is obtained by thorough mixing of aluminous cement (~70%), gypsum (~20%) and ground specially made high-basic calcium hydroaluminate (~10%).

Gypsum alumina expanding cement(developed by I.V. Kravchenko) - a fast-hardening hydraulic binder obtained by joint fine grinding of high-alumina clinker or slag and natural gypsum dihydrate (up to 30%) or by thorough mixing of the same materials, crushed separately. Gypsum alumina cement has the property of expansion when hardened in water; when cured in air, it exhibits non-shrink properties. It is used for monolithic joints of prefabricated structures, waterproofing plasters, dense concrete in reinforced concrete shipbuilding and in the construction of tanks for storing petroleum products.

Expanding Portland cement– hydraulic binder obtained by joint fine grinding of the following components (% by weight): Portland cement clinker 58-63; aluminous slag or clinker 5-7; gypsum 7-10; blast-furnace granulated slag or other active mineral additive 23-28. Expanding Portland cement is characterized by rapid hardening under conditions of short-term steaming, high density and water resistance of cement stone, as well as the ability to expand in water and in air with constant moisture during the first 3 days.

Prestressing cement(developed by V.V. Mmikhailov), consists of 65-75% Portland cement, 13-20% aluminous cement and 6-10% gypsum; its specific surface is not less than 3500 cm3/g. In the process of expansion under certain hardening conditions, this cement creates a prestress in the reinforcement, regardless of its location in the reinforced concrete structure. Consequently, the chemical energy of the binder is used to obtain prestressed structures without the use of mechanical or thermal methods that require special equipment.

Depending on the achieved self-stress energy, determined by a special method and expressed in MPa, there are: NTs=2, NTs=4 and NTs=6. the beginning of setting of NC should occur no earlier than 30 minutes and the end - no later than 4 hours after mixing. Stressing cement quickly hardens, the compressive strength of NC after 1 day should be at least 15 MPa, after 28 days of hardening - 50 MPa.

Self-stressed reinforced concrete structures at NC are characterized by increased crack resistance, therefore NC is used for gas-tight structures, gasoline storage facilities, underwater and underground pressure structures, and sports facilities.

5. Composition, properties and application of acid-resistant cement.

Compound:
This is a powder material obtained by joint grinding of pure quartz sand and sodium silicofluoride (it is possible to mix separately crushed components). Quartz sand can be replaced in acid-resistant cement with beshtaunit or andesite powder. Acid-resistant cement is closed with an aqueous solution of liquid glass, which is an astringent; the powder itself does not have astringent properties.

Properties:
The compressive strength of acid-resistant concrete reaches 50-60 MPa. Being resistant to acids (except for hydrofluoric, hydrofluorosilicic and phosphoric), acid-resistant concrete loses its strength in water, and collapses in caustic alkalis.

Application:
Acid-resistant cement is used for the manufacture of acid-resistant mortars and concretes, putties. At the same time, acid-resistant aggregates are taken: quartz sand, granite, andesite. Tanks, towers and other structures in chemical plants, baths in pickling shops are made from acid-resistant concrete. Acid-resistant solutions are used for lining with acid-resistant tiles (ceramic, glass, diabase) of reinforced concrete, concrete and brick structures at chemical industry enterprises.

Task number 1.

Determine the porosity of a hardened Portland cement paste if its water content is 48% and 20% is required for the curing reaction to proceed. The density of Portland cement is 3.1 g/cm.

Decision.

1. Absolute volume occupied by cement paste

2. Absolute volume occupied by cement stone

3. The volume of cement paste without pores


Answer: porosity of hardened cement paste = 34.9%

Task number 2.

The mass of a sample of oak wood 2x2x3 cm is 8.6 g, when compressed along the fibers, its tensile strength turned out to be 37.3 MPa. Find the moisture content, density and tensile strength of oak at a moisture content of 15%, if the mass of the dried same sample is 7 g.

Tensile strength

R 15 \u003d R 12 * (1 + (W - 12)) \u003d 37.3 * (1 + 0.04 (31.5 - 12)) \u003d 66.4 MPa.

Answer: humidity = 31.5%; density = 0.54 g/cm; ultimate strength = 66.4 MPa.

Questions

1. Areas of application of cast, mobile and rigid concrete mixtures.
Cast concrete mixes.

Thanks to the use of complex chemical additives, including a superplasticizer, non-separating self-compacting cast concrete mixtures can be obtained without increasing the consumption of cement. The use of such mixtures instead of standard vibro-compacted low-mobility mixtures laid with the use of small-scale mechanization in the areas of engineering arrangement of roads (exit ramps, crossings, stopping areas, etc.) in cramped urban conditions when constructing driveways, sidewalks, as well as when repairing road surfaces allows you to significantly reduce labor costs, increase its productivity and, on this basis, obtain an economic effect while improving the quality of construction and improving working conditions.
Cast self-compacting concrete mixtures include mixtures that do not have external signs of delamination, the mobility of which, measured immediately before laying in the structure, is characterized by a standard cone draft of 20 cm or more in accordance with GOST 10181.1-81.

The preparation of cast standard concrete mixtures is carried out in two stages using truck mixers.

Work on the use of cast concrete mixtures in the construction of coatings and bases should be carried out in accordance with SNiP 3.06.03-85. preparation and transportation of the initial low-slump concrete mix, arrangement of expansion joints, maintenance of freshly laid concrete, etc. Cast concrete mixes can be used in the construction of monolithic bases and coatings, both single-layer and two-layer. The design of the pavement and all pavement is determined by the project. The transverse and longitudinal slopes in the sections of the coating (base), where cast self-compacting concrete mixtures are used for concreting, should not exceed 3%.

Concretes obtained from cast mixtures are distributed and compacted mainly under the influence of their own weight, which determines the effectiveness of their use. They are characterized by the same or 3-7% less cement consumption compared to concretes from slow-moving mixtures and are not inferior to them in terms of strength, deformability and frost resistance.

The technical and economic efficiency of the use of concrete from cast mixes instead of standard ones is also ensured by a significant reduction in labor costs when constructing road bases and pavements, improved working conditions, and a reduction in energy intensity and construction costs.

Mobile concrete mixes.

The mobility of the concrete mixture is characterized by the measured slump (cm) of the cone (OC) molded from the concrete mixture to be tested. To determine mobility, i.e. the ability of the mixture to spread under the action of its own mass, and the cohesion of the concrete mixture is a standard cone. It is a truncated cone, open on both sides, made of sheet steel 1 mm thick. The height of the cone is 300 mm, the diameter of the lower base is 200 mm, the upper one is 100 mm. The inner surface of the cone mold and the pallet are moistened with water before testing. Then the form is placed on a pallet and filled with concrete mixture in three steps, compacting the mixture with a bayonet. After filling the form and removing excess mixture, the form is immediately removed, lifting it slowly and strictly vertically up by the handles. The mobile concrete mixture, freed from the form, gives a draft or even spreads. A measure of the mobility of the mixture is the value of the draft of the cone, which is measured immediately after removal of the form.

Depending on the draft of the cone, mobile (plastic) concrete mixtures are distinguished, the value of the draft of the cone for which is 1 ... 12 cm or more, and rigid, which practically do not give a draft of the cone. However, when subjected to vibration, the latter exhibit different molding properties depending on the composition and materials used. To assess the stiffness of these mixtures use their own methods. The mobility of the concrete mixture is calculated as the average of two determinations made from one sample of the mixture. If the draft of the cone is equal to zero, then the workability of the concrete mixture is characterized by rigidity.

Rigid concrete mixtures.

The stiffness of a concrete mix is ​​characterized by the time (s) of vibration required to level and compact a preformed concrete cone in a stiffness tester. The cylindrical ring of the device (its inner diameter is 240 mm, height is 200 mm) is installed and rigidly fixed on the laboratory vibration platform. A standard cone is inserted into the ring and fixed, which is filled with concrete mixture in the prescribed manner and then removed. The disk of the device is lowered onto the surface of the molded cone of the concrete mixture using a tripod. Then simultaneously turn on the vibration platform and the stopwatch; vibration is carried out until the release of cement paste from the holes of the disk with a diameter of 5 mm begins. Vibrocompaction time (s) characterizes the rigidity of the concrete mix. It is calculated as the average of two determinations made from one sample of the mixture. Laboratories sometimes use a simplified method for determining the stiffness of a concrete mix, proposed by B.G. Skramtaev. According to this method, the test is carried out as follows. A standard cone is inserted into an ordinary metal mold for making cubes measuring 20 x 20 x 20 cm. Previously, the stops are removed from it and the lower diameter is slightly reduced so that the cone enters the cube. The cone is also filled in three layers. After removing the metal cone, the concrete mixture is subjected to vibration in the laboratory area. A standard vibratory platform should have the following parameters: kinematic moment 0.1 N m; amplitude 0.5 mm; oscillation frequency 3000 min–1. Vibration lasts until the concrete mixture fills all the corners of the cube and its surface becomes horizontal. The duration of vibration (s) is taken as a measure of the stiffness (workability) of the concrete mix. The time required to level the surface of the concrete mixture in the form, multiplied by a factor of 1.5, characterizes the stiffness of the concrete mixture.

Cast and moving mixtures have a hardness of 0, slow moving 15...20, hard 30...200 and extra hard 200 s. Apply superrigid, rigid and mobile concrete mixes.

2. Methods of winter concreting.

Concrete laid in winter must, in winter, acquire strength sufficient for demoulding, partial load, or even full load of the structure.
With any method of concrete work, concrete should be protected from freezing until it acquires a minimum (critical) strength that provides the necessary resistance to ice pressure and, subsequently, at positive temperatures, the ability to harden without significant deterioration in the basic properties of concrete.

When concreting in winter, it is necessary to ensure the hardening of concrete in a warm and humid environment for a period determined depending on the desired strength. This is achieved in two ways: the first is by using the internal heat reserve of concrete; the second - additional heat supply to the concrete from the outside, if the internal one is not enough.
In the first method, it is necessary to use high-strength and fast-hardening Portland cement. In addition, it is recommended to use a cement hardening accelerator - calcium chloride, to reduce the amount of water in the concrete mixture by introducing plasticizing and air-entraining additives into it, and to compact it with high-frequency vibrators. All this makes it possible to accelerate the hardening of concrete during the construction of structures and ensure that the concrete gains sufficient strength before freezing.
The internal heat reserve in concrete is created by heating the materials that make up the concrete mixture; in addition, in hardening concrete, heat is released during a chemical reaction that occurs between cement and water (cement exotherm).
Depending on the massiveness of the structures and the outside temperature, only water for concrete or water and aggregates (sand, gravel, crushed stone) are heated. Water can be heated up to 90 C, aggregates - up to 40 C, cement is not heated. It is required that the temperature of the concrete mixture when leaving the concrete mixer be no higher than 30 ° C, since at a higher temperature it thickens quickly. Thickening, i.e. the loss of mobility of the concrete mix makes it difficult to lay, and water cannot be added, because water reduces the strength of concrete. The minimum temperature of the concrete mixture when laying in arrays should be at least 5 C, and when laying in thin structures - at least 20 C.

Recently, a new method has been used - electric heating of the mixture in a special bunker immediately before laying in the structure. In this case, an electric current is passed through the mixture and heated to 50 - 70 C. The heated mixture must be immediately laid and compacted, because. she's fading fast.
During the hardening of concrete, cement releases a significant amount of heat, which depends on the composition and fineness of cement grinding, concrete temperature and hardening time. Heat is released mainly during the first 3 to 7 days of curing. In order to keep it in concrete for a certain period, it is necessary to cover the formwork and all exposed parts of concrete with good insulation (mineral wool, shag, sawdust, etc.), the thickness of which is determined by thermal engineering.

The method of winter concreting described above is often called the thermos method, because. heated concrete mix hardens under thermal insulation conditions. The use of this method is rational if the heat necessary for its initial hardening is retained in concrete for at least 5–7 days.
Structures thin or with poor thermal insulation, as well as those erected in very severe frosts, must be concreted with heat supplied from outside. There are three variations of this method.

Heating of concrete with steam passed between the double formwork surrounding the concrete, either through tubes inside the concrete or through channels cut from the inside of the formwork. The usual steam temperature is 50 - 80 C. At the same time, concrete hardens quickly, reaching within 2 days the strength that it gains in 7 days of normal hardening.

Electrical heating of concrete, carried out using alternating current. To do this, steel plates-electrodes connected to electrical wires are laid on top or on the sides of the concrete structure at the beginning of its setting, or longitudinal electrodes are laid in the concrete, or short steel rods are driven in to connect the wires. After the concrete has hardened, the protruding ends of these rods are cut off. Plate electrodes are used mainly for heating plates and walls, longitudinal electrodes and transverse short rods - for beams and columns.
When concreting massive structures in winter, it is advisable to use electrical heating only of the surface layer of concrete and the corners of the structure (the so-called peripheral electrical heating) in order to protect it from premature freezing.
The heating of the air surrounding the concrete is carried out as follows: they arrange a plywood or tarpaulin greenhouse in which temporary furnaces, special gas burners (in this case, fire regulations must be strictly observed), air heating (heaters) or electric reverberatory furnaces are installed. In greenhouses, vessels with water are placed to create a moist environment for hardening, or concrete is poured. This method is more expensive than the previous one and is used at very low temperatures, with small volumes of concreting, as well as in finishing work.
In addition to the winter concreting methods described above, which require heating of the concrete components or the concrete itself, a cold winter concreting method is used, in which the materials are not heated, but a large amount of salts are dissolved in the water for concrete preparation: calcium chloride CaCl, sodium chloride NaCl, sodium nitrite NaNO, potash KCO. These salts lower the freezing point of water and allow concrete to harden in the cold (albeit very slowly). The amount of salt added to concrete depends on the expected average curing temperature of the concrete.

Potash-added concrete quickly thickens and sets, making it more difficult to place in the formwork. In order to maintain the workability of the concrete mixture with potash, sulphite-yeast mash or soap naft is added to it.
Winter concreting with the use of anti-frost additives is a simple and economical way. However, a large amount of salt introduced into concrete can degrade the structure, durability and some other properties. When the structure is used in wet conditions, corrosion of the reinforcement under the action of chloride salts is possible (sodium nitrite and potash do not cause corrosion). In addition, caustic alkalis formed during the curing of concrete with additives can react with the active silica contained in some aggregates and cause concrete to corrode.

Therefore, concrete with antifreeze additives is not recommended for use in critical structures, in concrete structures intended for operation in wet conditions in the presence of reactive silica in the aggregate grains, and concrete with chloride salts in reinforced concrete structures.

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