Features of concreting at negative temperatures. The effect of temperature on concrete Do-it-yourself concrete pouring in sub-zero temperatures

The specificity, and often the problem of construction, is seasonality - it is impossible to carry out many types of work in winter. Concreting is considered the most critical process for low temperatures. Given the responsibility of such work, especially when pouring foundations, the developer must have a good idea of how to pour concrete, how low temperatures affect the hardening process concrete mix and what problems may arise in connection with this. We will tell you at what temperature concrete can be poured, how to pour concrete correctly in frost, whether it is possible to pour concrete at sub-zero temperatures, and what needs to be done for normal concrete hardening in such conditions.

Of the components of the concrete mix in the process of hardening with the formation concrete stone involved cement and water. Aggregates (sand, gravel, crushed stone) are bound into a single complex due to the hardening of the cement milk between them - a mixture of water and cement. Crystallization occurs when cement particles are hydrated with water. Water molecules bind with cement to form a crystalline mass, i.e. gradual hardening occurs due to the loss of water.

The direction of the process is from the periphery to the center of the stone, and the process itself takes place under conditions with certain restrictions. Some characteristics of the process under different conditions:

If the hardening temperature of the laid concrete is +15 - +25ºС, then the concrete poured into the formwork will gain maximum design strength in 28 days. During the first week, under normal conditions, the concrete mixture gains up to 70% of the design strength. are most important for the normal curing of concrete during the first few days after pouring. At this time, for normal hydration, you need to retain moisture. To do this, the concrete surface is covered with a PVC film or wet burlap.

If the temperature of the medium drops to +5ºС, the process of stone formation slows down by half, i.e. it will be possible to count on the normative strength of concrete in about two months.

Hardening stops completely at 0ºС - the freezing point of water. With more low temperatures water in the concrete mix freezes. If by this moment the critical strength of concrete has already been gained, then hardening continues until the end after defrosting. The critical strength of concrete characterizes the moment when the hardening of the mixture is achieved to such an extent that a favorable result of the process is already provided. At normal temperature critical strength occurs one day after pouring concrete. Such a threshold also depends on the brand of cement - mixtures on cements of high grades have a critical strength that is only a quarter of the full standard.

If the strength of concrete before freezing is less than critical, then the freezing water inside the monolith expands and breaks the bonds, increasing the porosity of the stone. Such a monolith loses strength, passes water - its service life decreases, and often the concreting has to be done again.

Pouring concrete in winter conditions

The necessary conditions for concreting in winter are described above - the main thing is to ensure a sufficient temperature of the mixture during hardening. The choice of a specific method depends on the specifics of the work and the nature of the object. For example, if it is not possible to provide concrete heating, and the foundation is small, instead of belt type foundation to arrange bored.

All measures to ensure reliable concreting in winter conditions can be divided into:

Preheating of the concrete mix before laying.

External care of the concrete mixture poured into the formwork.
The rise in temperature of the filled concrete mass electrical heating.
Addition of modifiers to the concrete mix, accelerating hardening or shifting the crystallization temperature of water.

The final choice of winter concreting method depends on many factors. For example, if it is not possible to deliver a heated mix to an object due to the large concrete delivery arm, then it makes no sense to consider this option. In addition, often the building site is not electrified or the supplied power is limited. Great importance has a reliable weather forecast for the period of hardening of the mixture. If we compare the cost of each of the proposed methods, then the most budgetary is last option– adding to concrete mix special additives. Let's briefly review each of the methods.

Heating the mixture during cooking

The method consists in preheating the components of the mixture (except cement) so that at the time of laying the mixture its temperature is 35-40 o C. Sand and gravel are heated to about 60 o C, water - up to 90 o C. The cement is first placed in a heated room and warmed up to room temperature. It is impossible to warm up the cement - it quickly irretrievably loses its activity and becomes simply unusable!

For kneading, mixers with or without heating can be used. At large construction sites, aggregates can be heated in special drying ovens. A private developer can use the hot air of a heat gun or heat from a stove transferred through an air duct.

Winter concrete mixing has technological features. Water is poured into the mixer, crushed stone and sand are added to it, and only then cement. The mixture is kneaded especially carefully, in time a third longer than usual.

Formwork insulation after pouring

The method is especially relevant for maintaining heat after laying the heated mixture. Various methods can be used:

At large construction sites, electric heating mats are used, which are laid on poured concrete and connected to the network. The method is expensive - the cost of such a mat is 2500 rubles per square meter.

Pouring concrete at negative temperatures in individual construction can be accompanied by insulation with everything that is at hand: old clothes, straw bales, etc. Often satisfied with the so-called. "greenhouses" - a kind of greenhouses over poured concrete, where warm air from oven, heater, etc. At the same time, the concrete surface must be moistened. The surface of concrete can also be covered with film, sawdust, peat and even snow.

Laid concrete at sub-zero temperatures can be heated infrared emitters for directional heating of concrete. The flow is regulated by aluminum guides. The method is expensive due to the large need for lamps and power consumption.

Electric heating of laid concrete

Such methods are usually used in industrial construction. For their implementation, sufficient electrical power and an electrician with a serious vocational training. Electric heating can be:

Through - when the mass of concrete is heated from the inside by currents going inside the formwork along rod or string electrodes. The layout of the electrodes and the load must be accurately calculated. The mass of concrete acts in this case as a source of resistance.

Peripheral - when heat is transferred from sources to strip electrodes fixed to the formwork (steel strip or sheet). Warming up to a depth of about 0.2 m is intensively provided, but even deeper incoming heat affects the strength criteria.

Methods are used with a small reinforcing cage or without it at all. With a thick reinforcing mesh, the currents close to it and the system does not work or works unevenly. After the mixture hardens, the electrodes remain inside the monolith forever.

Additives in concrete mix

The most cost-effective method. Often used in conjunction with different methods heating, increasing their efficiency and reducing costs. At certain conditions can be used without warming up. Additives in laid concrete at sub-zero temperatures can be divided into:

Lowering the freezing point of water in solution.

Promote crystallization at low temperatures. Salts of calcium, sodium or potassium are used. The most common reagent is potash.

Increasing the speed of hardening.

Such reagents include potash, mixtures of calcium salts with urea, calcium nitrite-nitrate, etc.

Additives in liquid concrete at sub-zero temperatures, they are introduced at a dosage of 2 - 15% of the mass of cement in the mixture. Yes, at design temperature concrete - 10 o C - 15 o C requires potash in the amount of 10% by weight of cement, and at a temperature of - 21 o C - 25 o C - in the amount of 15% by weight of cement.

Potash is a popular additive for winter concreting.

Benefits of winter concreting

Concrete laying and curing at low temperatures for Russian conditions often a necessity. Residents in many regions can count on just a few months of the year when concrete can be laid without creating special conditions for hardening. Postponing work for 7-8 months is unreasonable, therefore, it is natural to know how to properly pour concrete in frost and achieve standard hardening for our conditions. There are clear advantages in winter concreting:

Frozen soil in winter can be used as the most durable formwork, since it is rigid, does not crumble and does not require strengthening.

In winter, the price usually drops sharply. building materials, therefore, it is possible to significantly reduce the cost of construction.

Most of the construction crews are out of work waiting for the season, so it is possible to agree on work on pasture conditions.

The negative factor is low speed process, but nothing can be changed here - the technology of laying concrete at low temperatures has its own laws: the acceptance of concrete on the foundation (or floor) must be continuous, i.e. "cold seams" are excluded. Otherwise, it is important to choose the right best method concreting and exactly fulfill the requirements of the technology for the care of the laid mixture.

Pouring concrete at low temperatures updated: December 13, 2016 by: Artyom

The minimum temperature for pouring concrete is approximately 5°C. That is, this indicator is considered critical - up to this figure, any work with concrete can be carried out as usual. However, it must be remembered that optimum temperature for pouring concrete - 17-25 ° C, since such conditions make it possible to obtain the maximum strength of the solution. If the temperature approaches 5°C, the grout will likely be less solid and less durable and will not reach the brand specifications. Even when carrying out work, it should be remembered that the hardening time can increase significantly.

However, sometimes it becomes necessary to carry out construction work in winter at low temperatures. There may be several reasons:

limited building time summer period due to unfavorable climatic conditions;
the need to pour concrete on loose soils;
seasonal decline in prices for materials, including cement;
reduction in the cost of work by specialized firms due to low demand for their services.

There are two main technologies with which you can carry out concreting in the cold season.

The first is that concrete is poured at low temperatures on heated fittings, which can be ordinary electrodes. A standard 12-volt cable is connected to the armature, through which current flows. This is quite enough to metal rods heated up and heated the solution, preventing it from freezing. The disadvantages of this method are obvious - in addition to the use of special materials, you will have to spend a hefty amount on electricity.

In addition, pouring concrete at low temperatures can be carried out using warm formwork. The essence of this technology is to insulate the solution with special heat-insulating materials, such as foam or mineral wool. But there are also disadvantages here - it is necessary to cover concrete from all sides, which leads to high consumption. auxiliary materials and increase the time of work.

Pouring concrete at low temperatures: general recommendations

Thus, we can conclude that pouring concrete at low temperatures should be carried out only if it is not possible to postpone this stage. construction works until more favorable conditions arrive.

In winter, when the temperature outside drops below zero, the construction of reinforced concrete, monolithic and concrete structures becomes more difficult. The liquid included cement mortar, begins to crystallize, as a result, the process of concrete hardening almost completely stops.

As soon as the warm season sets in, the hydration of the cement resumes, and the mortar that has frozen during the construction phases has a high thermal conductivity and low strength. This is due to the fact that the frozen water expands, making the concrete loose. In order to eliminate such consequences, it is customary to use antifreeze additives. They resist liquid crystallization at sub-zero temperatures.

Additives in concrete

Having visited hardware store, you can find concrete additives in large assortment. For example, sodium nitrite is in the form of a liquid or white crystals. Carbamide is known by its second name - urea - and is produced in the form of colorless crystals. You can find compounds of calcium nitrate and urea. The kind of white powder is sodium formate.

The dark brown liquid is a compound that suggests the presence of:

acetylacetone;
nitrochloractinide;
acetoacetic ether.

It is noteworthy that this composition is used at temperatures up to -25 ° C. Asol-K is made from special modifiers and potash solution. It is able to withstand temperatures down to -10 °C, while when the temperature changes above 0 °C, concrete hardening is accelerated. High-quality antifreeze for mixtures is Hydrozim, which provides hydration at temperatures below -15 ° C. From NH3 gas and ordinary water, a composition is made that can withstand frosts down to -30 ° C. Hydroconcrete has plasticizing properties, which can be used at temperatures not lower than -15 °C.

Additives for cement slurries

The antifreeze additive in the solution allows the latter to be used in the construction of monolithic structures, buildings and the manufacture of reinforced concrete elements. Today, such additives are presented in a wide range, but the most popular are electrolyte compositions and their compounds. They provide antifreeze effect and accelerate the hardening of the solution. Among the most common are:

HK, HH1.
NNHC.

sodium formate

Formate is an antifreeze additive that has the appearance of a crystallized white powder that dissolves well in water. An additive is made from sodium salts of formic acid. The main actions are aimed at reducing the temperature of the water in the solution and prolonging the hydration of the concrete.

The additive does not have plasticizing properties and is capable of creating tension in the structure due to salt crystallization. In order to avoid this, you should additionally use the C-3 plasticizer, which is available in liquid and powder form.

Consumption

The consumption of an antifreeze additive must be known in order to purchase right amount funds. In the manufacture of a solution with an antifreeze additive FN + S-3, warm water, into which sodium formate and a plasticizer should be poured. The amount of additive is determined depending on weather conditions. If the temperature varies from 0 to -5 ° C, then the consumption will be 3% of the total mass of cement.

When the temperature changes in the range from 0 to -10 ° C, an additive in the amount of 4% by weight of cement should be used. The amount of additive increases to 6% if the ambient temperature drops to -15 °C. The volume of C-3 plasticizer always remains at the level of 0.8-1% by weight of cement, which does not depend on air temperature. For an antifreeze substance, the consumption will be different. This parameter will be influenced by the properties of the additive and the air temperature.

Application of additives

Antifreeze additives must be added to the solution along with the liquid. It is desirable to do this with the last third of the water. Additives should not be added to the dry mix. As soon as the additive is added, you should wait a certain time, which is necessary for the distribution of the components. Antifreeze additives are used at sub-zero temperatures according to certain rules.

If you have to work in snowy conditions, then it is important to take care of shelter. When the solution leaves the mixer, it is important to keep its temperature in the range from +15 to +25 °C. The working mixture should be prepared using heated water. Aggregates are heated before direct use.

Application restrictions

Antifreeze additives of some types can be used subject to restrictions. They apply to prestressed structures and products that will be subjected to dynamic loads. Use when sealing joints reinforced concrete structures solutions of chloride salts are not allowed. This applies to products that have embedded parts and fittings. The rule is relevant if chemical protection has not been carried out.

Mixtures of antifreeze additives should be used after preliminary testing of samples for corrosive attack. The described means can cause efflorescence on the concrete surface, as well as reduce the strength characteristics of structures. Concrete with additives must have a positive temperature by the beginning of curing.

This parameter should be higher than the outside air temperature by 5 °C or more with a plus sign. For example, if the air temperature is -13 °C, then the concrete temperature should be +18 °C. Unburied surfaces of structures should be thermally insulated to prevent freezing of water from these areas.

The use of antifreeze additives requires compliance with the outside temperature limits. If chloride salts (sodium nitrate) are used, the temperature should be -15°C or higher. As for potash, this limit will be -25 ° C. The technology of using the described additives has one important virtues, which consists in the minimum material and physical costs for implementation. The disadvantage of technology is a long period acquisition of critical strength by the structure. If the rules for using additives are violated, some problems may arise, such as efflorescence on the surface and corrosion of the reinforcement.

When an antifreeze additive is used in concrete, complex methods of maintaining structures are often used. Thermos technology is used to reduce the hardening time. To do this, the mixture is prepared on the heated components of the liquid with the thermal insulation of concrete.

Why You Should Use a Chloride-Based Supplement and When You Should Avoid It

Antifreeze additives made on the basis of chlorides have many advantages, among them should be highlighted:

low cost;
no effect on the structure of the solution;
no effect on the rate of solidification, so that the solution can be prepared in advance;
an increase in the mobility of the particles, due to which it is possible to give the solution the desired shape.

The above additive has a minus, which is expressed in high level corrosive activity. As a result, such a tool can be used only for those structures that do not have reinforcement and metal in the structure. The latter begins to oxidize, exfoliate from the structure, thereby violating the integrity.

Precautions for use

Wear protective gloves when handling additives. If the product got on open areas skin, they should be washed with soap and water. The additive should not get into the eyes, but if this could not be avoided, then they should be rinsed large quantity water and see a doctor. Additives must be disposed of in accordance with local regulations, as these formulations contain harmful components. This indicates that the mixture must not be poured onto the soil, discharged into sewers or water bodies.

Conclusion

Using antifreeze additives, it is possible to increase the structural strength by almost 2 times. These ingredients allow you to save on cement, because it can be of a lower class, which means it will cost less. Structures can be made of less impressive thickness, while strength and other properties will not be lost.

Optimal temperature range for the device concrete foundations on traditional technologies– from 15 to 25 °С. Therefore, in the vast majority of cases, these works are carried out in the warm season. But what if you need to continue or start construction in the fall or in the middle of winter, when it's freezing outside? To answer this question, we propose to understand under what minimum temperature you can pour concrete and what it is fraught with for the developer.

Is it possible to concrete in frost

After filling with favorable conditions the hydration process begins in concrete, which takes place in 2 stages:

The water enters chemical reaction with cement, which causes bonds between the particles and the solution begins to thicken. The setting stage lasts 10-20 hours and is accompanied by heat release.
The second stage is the hardening (hardening) of the concrete mixture, which lasts up to 4 weeks. During this period, the monolithic structure gains maximum strength.

The lower the temperature environment the longer it takes to hydrate. For clarity, we give an example in the form of a table reflecting the behavior of standard M250 concrete based on Portland cement grade 400 at different temperatures.

Reference. According to building codes, the formwork is removed after reaching 50% strength, and further work- not earlier than at 70%.

At negative temperatures, water freezes and practically does not react with cement. If ice crystals are formed, then the monolith that has not had time to harden begins to collapse. From the above table it is clearly seen that even at zero temperature your construction will stretch for an indefinite period.

To avoid this, you need to prevent the water from freezing, and ideally, heat it to the required condition in order to accelerate hydration. There are several methods for organizing the concreting of the foundation for a house in the autumn and winter:

introduction of antifreeze additives into the concrete mixture;
warming up monolithic construction inside and outside;
covering and insulating concrete after pouring.

Construction experience shows that correct application of these methods, singly or in combination, allows you to safely build foundations down to -15 ° C on the street. If you live in major city, then they probably noted that in winter, work at large facilities does not stop. AT equally these technologies are also applicable to the construction of small bases for a house or a bath.

Use of additives

These chemical substances are divided into groups according to the method of impact on the concrete mixture:

Additives that prevent water from freezing at sub-zero temperatures and allow cement hydration to proceed. They are best used in combination with heating to shorten setting and curing times.
Additives - accelerators with antifreeze properties. Their function is to sharply increase the activity of the cement and thus force the concrete to gain design strength regardless of the ambient temperature. Successfully used without heating structures.
Substances that accelerate the setting of the solution and cause maximum heat release, which is why the monolith is heated independently at first.

Reference. From a financial point of view, the introduction of additives is considered the most profitable way, since electric or air heating of concrete is more expensive.

A feature of the use of any additives is the observance of the exact dosage in accordance with the conditions of concreting, especially when combining substances. The fact is that some of the compositions initiate the appearance of rust of the reinforcing cage, but give the water antifreeze properties. Others are corrosion inhibitors, so both groups are used together.

Hence the conclusion: do not try to make a solution for winter construction yourself by adding additives, such material must be ordered at the factory and delivered by a concrete truck (in jargon - a mixer). Contact the specialists of the enterprise and find out at what temperature it will be possible to use their products in order to pour the foundation without consequences. In case of emergency, correctly combined and accurately dosed admixtures can reduce the mortar laying threshold to minus 25 °C.

The following substances most often act as antifreeze and accelerating additives:

potassium carbonate (potash);
various calcium and sodium salts;
urea (otherwise - carbamide);
sodium formate.

You will learn more details about chemical impurities - modifiers from the following video:

Structure heating

This method is widely used for pouring reinforced concrete foundations small buildings and is implemented in two ways:

The entire area of the future house is covered with a film tent mounted on a frame made of wooden beams. Inside are electric or gas heat guns, maintaining a certain temperature regime during the entire period of concrete hardening.
Before pouring, the reinforcement cage of the foundation is wrapped with a heating cable, which is connected to the network after laying the mortar in the formwork. There are other types of electric heaters - heating elements, nichrome spirals and so on.

Note. Both options are usually used without the introduction of antifreeze additives into the concrete mixture. But at too low temperatures (-15 ° C and less), additives are indispensable.

Device concrete base followed by air heating - not too effective technique in severe frosts and frozen soils. The injection of heat into the space under the tent allows you to increase the air temperature by 15-20 ° C, which is not enough. In turn, the heating cable can burn out inside the monolith and then part of the foundation will remain without heating.

Unloading concrete from the machine into the heated shelter

The main disadvantage of these methods is high energy consumption and large initial investments for the construction of a canopy or the purchase of a cable (heat guns can be rented). Again, heaters or cable operation must be monitored and kept on site at all times. To understand the process, we suggest watching the video

Construction work is not always carried out under favorable conditions. weather conditions. Pouring a concrete floor or building a foundation - that is, all the processes associated with the preparation and placement of a concrete mixture, are limited by a rather narrow temperature range of the environment. In particular, reduced values to a large extent affect the processes of setting and hardening, as well as the time for concrete to gain grade strength. Is it possible to carry out such work at sub-zero temperatures and how justified is this? Let's try to answer this question.

Features of concrete mix

Physical and chemical properties of concrete determine optimal range temperatures when working with it, ranging from +17 to +25 degrees. Normal conditions provide the declared brand strength of the hardened solution after about 28 days.

Speed of the hydration process cement mixture greatly slows down in case of temperature decrease and practically stops at +5 degrees. Upon further reduction to negative values, the water contained in the solution freezes, and its volume increases significantly. The emerging forces of internal pressure lead to deconsolidation and loosening of the concrete structure, and its solidity is ensured only by frozen moisture.

With an increase temperature values to positive values, water thaws, cement hydration reaction resumes, and concrete gradually hardens. However, the consequences of structural damage during freezing significantly affect the strength of the monolith.

Experimentally and after carrying out various calculations, critical points were determined when various grades of concrete mortar could be subjected to freezing without tangible consequences. The critical strength of concrete, upon reaching which the negative process ceases to have a noticeable impact on the characteristics of the structure, was set at 50% of the branded strength index.

Thus, pouring concrete at sub-zero temperatures is reduced to a set of measures to prevent freezing of water until it reaches critical strength. There are several methods for this:

making a solution from preheated components;
formwork insulation;
heating the laid mixture;
cold concreting with various chemical additives that lower the freezing point.

The rational use of one of the methods is determined by the volume of the structure being built, the performance of the declared strength characteristics, the availability and availability of energy resources. But meteorological conditions, nevertheless, are a decisive factor when choosing a filling option.

All of the above methods work well both singly and in combination.

Concreting using the mortar's own heat

This method is used for daily temperature fluctuations with its transition through the zero mark, as well as for slight frosts. The bottom line is that a heated concrete mixture is placed in the prepared insulated formwork, for the preparation of which it is important to choose the right brand of cement. The higher it is, the faster the setting and hardening of the solution will occur, and the more heat will be released during hydration.

Mixing is carried out on water having a temperature of at least 90 degrees, and fillers preheated by blowing hot air. This changes the order of laying the components of the mixture: first, water is poured into the concrete mixer, then - building sand and rubble. Adding cement, which must have room temperature, is produced last, after several revolutions of the tank.

Under no circumstances should the cement be heated or poured into hot water!

For concrete to gain critical strength, it is necessary to maintain optimal thermal conditions for as long as possible, preventing rapid cooling. For this purpose, any materials are used - plastic film, tarpaulin, straw mats. The most effective is the use of insulated polystyrene formwork. The material has a low coefficient of thermal conductivity, which allows to extend the cooling time interval, contributing to the high-quality maturation of concrete. Being non-removable, such formwork further provides reliable thermal insulation of structures.

Concreting with heat supply from external sources

Pouring concrete in winter at sub-zero temperatures requires constant heating of the laid mortar, preventing it from freezing. There are several ways to provide conditions for a monolith to gain critical strength.

The device of greenhouses

Most reliable way maintaining positive temperatures of the laid concrete implies the erection of a temporary structure above the poured structure. Teplyak is a frame upholstered with plywood or covered plastic wrap like a garden greenhouse. The dimensions of the temporary shelter should be minimal. The heating of the internal air is carried out with the help of heaters, infrared heaters or portable gas burners.

The most significant point here is the control and maintenance of the humidity regime, since the circulating heated air intensively takes moisture from the solution, which is necessary for a complete cement hydration reaction. To prevent moisture from evaporating, the laid concrete is covered with plastic wrap and moistened periodically.

During winter concreting, heating of the laid mortar with steam is sometimes used. low pressure. For this, a special formwork with a steam jacket is constructed, covering the entire structure with it. However, this method has recently been replaced by electric heating.

Electrical heating of concrete

Creation of conditions for the maturation of concrete at sub-zero temperatures can be carried out using electric current connected to the electrodes. Special metal rods or plates are placed on the surface of the formwork or immersed inside the solution by connecting to different poles of the mains. Wet concrete closes the circuit and, having a certain resistance, converts electricity into heat, heating the solution. This technology significantly reduces the aging of the mixture, which by 28 days of age can acquire up to 80% of critical strength.

This method is possible only for non-reinforced and low-reinforced structures, which is a significant disadvantage of its use for heating the solution. In addition, high electricity consumption makes it economically unprofitable.

In individual construction, it would be better to use the laying of special heating cables along the reinforcing cage or along inside formwork. At the same time, it must be reliably thermally insulated, preventing heat leakage through the walls. The heating of the concrete solution should be properly monitored around the clock, taking measurements every few hours and not allowing it to heat up more than 30 degrees.

A relatively new method of thermal exposure in winter construction is the use of thermomats. In fact, this is a large electric heating pad, consisting of heating element and thermal insulation, hermetically mounted inside a waterproof shell. Heating mats provide a uniform distribution of the temperature field in the thickness of the concrete and along the perimeter at a distance of up to 20 cm from itself. Their use is possible at ambient temperatures up to -20 degrees.

Cold concreting

Antifreeze additives are widely used to enable concrete to gain critical strength under conditions of mortar pouring at low temperatures. They support the cement hydration reaction, prevent premature freezing of moisture in the mixture, normalize the concrete hardening process, and have the following positive qualities:

accelerate the set of the required critical strength;
lowering the crystallization point of the water contained in the solution;
increase the mobility of the concrete mass, making it easier to work with it;
protect metal fittings from corrosion.

Antifreeze additives should be used only at negative temperatures external environment and in strict accordance with the attached manufacturer's instructions, since their irregular use can change the properties of the concrete solution for the worse.

The most common antifreeze additives for the preparation of concrete are:

potash and other substances, which are based on salts of monocarboxylic acids, accelerate the action during concrete hardening. They do not cause corrosion of reinforcement, do not form on the surface frozen solution efflorescence and allow working with it at temperatures up to -30 degrees, while maintaining its properties.
sodium chloride - used in conjunction with Portland cements. It plasticizes the mixture, preventing its excessively rapid thickening. The main disadvantage is its corrosive effect on steel reinforcement.
sodium nitrite - must not be used in conjunction with aluminous cements. The additive allows work with concrete mortar at a temperature not lower than -15 degrees.
sodium formate - used only in combination with plasticizing additives. Otherwise, it can cause defects in the concrete mixture in the form of voids due to the formation and accumulation of salts.

The cold concreting method has some disadvantages:

its use is prohibited in prestressed structures;

the laid mortar has increased shrinkage;
concrete has a reduced rate of frost resistance and water permeability.

In addition, some additives impose their own additional restrictions on the use.

It is advisable to carry out work related to pouring concrete under the most favorable conditions. At the same time, it must be remembered that work should begin at a temperature of at least +10 degrees, and if it is not expected to decrease in the next 28 days. Modern technologies, of course, they allow concreting even at negative ambient temperatures, but this leads to considerable additional financial costs and it is possible to start them only in case of emergency. If it is not possible to reschedule the work for a more favorable period, it is worth considering some expert advice that helps to achieve acceptable quality when filling:

fillers - sand and gravel used to prepare the solution, in without fail are warmed up to prevent inclusions of ice or snow from getting into the batch;
the formwork must be cleared of ice and insulated in advance;
the bottom of the pit and the fittings must be warmed up, achieving at least the minimum positive temperature values;
pouring concrete should be done at one time with a continuous supply of the mixture;
the maximum temperature of the solution used is allowed no more than 35-40 degrees;
finished cast segments of the structure are covered with heat-insulating materials, preventing leakage own heat from concrete.

Throughout the entire period of concrete gaining critical strength, compliance with temperature regime. At the same time, one should not forget about controlling the uniformity of the distribution of the thermal field in the thickness of the structure, since the use of heating electric cables can lead to overdrying of its individual segments.

Pouring concrete at sub-zero temperatures is carried out mainly during large-scale capital construction, as it requires the use of special equipment, the availability of additional building materials and financial resources. The feasibility of carrying out such work on an individual basis largely depends on the availability of resources and is associated with certain risks.

Categories

Popular