At what temperature is pmd 10 added. Concreting at low temperatures. What affects the setting and hardening of concrete at low 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 of the concrete mixture and what problems may arise in connection with this. We will tell you at what temperature it is possible to pour concrete, how to pour concrete correctly in frost, whether it is possible to pour concrete at sub-zero temperature and what needs to be done for the normal hardening of concrete 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. For the first week in 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.
Adding to concrete mix modifiers that accelerate hardening or shift the crystallization temperature of water.

Final choice of method winter concreting 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” are a kind of greenhouses over poured concrete, where warm air is supplied from a furnace, 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 with infrared emitters for targeted 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 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

At temperatures below zero, solidification concrete mortar becomes problematic. This is often encountered when building foundations in autumn and winter. Experts assure that pouring concrete at sub-zero temperatures is possible without heating, but certain requirements are met for this to ensure proper hardening of the concrete mixture.

Concrete is a mixture of aggregates - sand and gravel, fastened together by a hardened cement milk. When it reacts with water, it hydrates, then it hardens with the simultaneous evaporation of water. Critical strength at normal temperature is gained within one or one and a half days, depending on the humidity of the surrounding air.

The optimal temperature for the reaction is about 20⁰С, the solution gains its design strength within 28 days. So that in the early days the water does not evaporate too quickly, the concrete is covered with waterproofing.

At 5⁰С, the solidification of the composition slows down by 2 times, and at zero temperature, hydration stops. If before that the critical strength of concrete is gained, nothing will happen to it, it will gain strength after warming. If, before freezing, a set of critical strength did not occur, the material will not gain the required indicators, and will crumble after defrosting. In this case, it is impossible to pour any brand of concrete at sub-zero temperatures.

Methods of concreting in winter conditions

The main condition correct filling concrete at low temperatures is the preservation of heat sufficient to ensure curing. Popular styling methods mortars in winter:

Pre-heating of the produced mixture;
The device of reliable thermal insulation and care of the solution;
Electric heating of concrete poured into the formwork;
The addition of special additives that reduce the freezing point of water and accelerate hardening.

Thus, it is possible to concrete on the street in winter without loss of strength indicators, but for this you need to adhere to the selected methods. In terms of costs, the use of heat guns is the most unprofitable option, the cheapest method is the addition of additives. Electric heating and thermal insulation device are intermediate options.

Temperature increase during kneading

To pour concrete in sub-zero temperatures, the components are heated. Fillers are heated to 55-60⁰С, and water is fed into the solution at 90⁰С. The cement is heated to room temperature otherwise it loses its binding properties. Before laying, the temperature of the solution should not be lower than 35⁰С.

When mixing, it is required to use a concrete mixer, into which heated water is first supplied, then fillers, and only then cement. When pouring such a mixture, the thermal energy of the monolith is enough to gain critical strength, taking into account the fact that additional heat is released during cement hydration.

Heating and insulation of the solution

At very low temperatures, the heated mixture requires additional insulation or heating. It is more economically feasible to insulate with the help of inexpensive heat-insulating materials that do not require additional sources energy. Hay or straw is lined on a concrete surface, old rags, peat, film or heat-insulating blankets are used. Sometimes the so-called "greenhouses" are arranged, similar to greenhouses.

If concreting at temperatures below -5⁰С, additional heating will be required. For this, the following technologies are used:

Heating with heat guns or stoves under greenhouses. This is a costly method that requires constant additional moisture. Suitable for sites that do not have electricity.
The use of thermomats powered by electricity. They are laid out on the surface of poured concrete and connected to a power source. Requires a large amount of electricity.
Infra-red radiators are installed above the flooded surface or around the formwork, the intensity and direction of heating is regulated by reflectors. Suitable for vertical and inaccessible structures.
Used to heat concreted area special cables or electrodes through which an electric current is passed. The technique is convenient to use, but requires large volumes electricity. Installation of the electrode system requires more costs, because when drying, the resistance of the solution, which itself is a conductor, increases.

Introduction of additives

Improving the characteristics of the solution with special additives is the most convenient and economical method of pouring the solution in winter. Using it in conjunction with heating, you can speed up the work and improve the quality of concrete. There are two main types of additives for pouring concrete mortar in winter:

Compounds that reduce the freezing point of water. The solution hardens for quite a long time, but the water does not crystallize, so the quality of the concrete does not suffer. To speed up the reaction, thermal insulation is required. In this capacity, calcium or sodium salts and potash are used, which prevent the crystallization of water.
Additives that increase the rate of hardening of the solution. They reduce the time required for concrete to gain critical strength, so the water in the heated mixture does not have time to crystallize. Calcium nitrite-nitrate, the same potash, calcium salts mixed with urea are used.

The amount of additives depends on the temperature range in which the filling will be carried out. concrete structure. From -5 to -10⁰С add up to 5-8% by weight of cement. With a decrease in temperature to -15⁰С, the concentration is increased to 10% by weight of the added cement, and up to -25⁰С, at least 15% of additives must be added.

To achieve maximum strength, you need to know at what temperature to pour concrete, and the best methods for ensuring curing. In addition, it is required proper preparation formwork. Before pouring the solution, it is necessary to thoroughly clean it from frost. The soil and fittings need to be warmed up, for which braziers are used, heat guns, infrared emitters and other devices. That's why to do slab foundations in the low temperature range is not recommended, since it is difficult to fully heat all the elements over a large area.

Working with a strip foundation in such weather is quite possible. To do this, you need to warm up the trench gradually, pouring concrete into it. After pouring, a mandatory step is high-quality thermal insulation. The process continues until the perimeter is closed. With the use of additives in concrete mortar and high-quality insulation strip foundation can be poured at temperatures up to -15⁰С.

When working on laying concrete, regardless of the type of structure, continuity of work is needed until the monolith is completely poured. For the successful completion of work, it is necessary to calculate to ensure the supply the right amount solution and the optimal number of workers.

Filling in parts can lead to uneven properties of the structure and reduce its quality.

Before pouring the solution into the formwork, you need to make sure that its temperature is optimal - in the region of 38 ° C. If it exceeds 40 degrees, then the hardening rate will decrease due to a decrease in the quality of the cement. As a result, it will take too long to reach critical strength, the liquid in the solution risks freezing, and the concrete will lose its properties.

Answering the question whether it is possible to pour concrete in winter, one can say - definitely yes. With the right technological approach, these works can be carried out at the lowest temperatures. Laying without additional heating can be carried out in slight frosts, this will require good thermal insulation and pre-heating of the concrete solution.

At low temperatures, additional It is carried out various methods, which you need to choose directly at the construction site. The cost of heating and thermal insulation pays off, since substandard concrete will reduce the quality of the entire structure.

Construction of prefabricated concrete and reinforced concrete structures, as well as the construction of monolithic structures, does not cease to increase its pace, but often the craftsmen have to face the rush due to the approaching end of the construction season. This is explained operational characteristics cement slurry, one of which is the presence of a liquid phase, which contributes to the continuous process of hydration and maturation of the composition. If the temperature drops below 5 degrees, the concrete maturation phase slows down, and if negative values it stops, which is due to the crystallization of water, which is part of the cement slurry. This leads to the destruction of the concrete structure, which becomes unusable. Despite this, most craftsmen with experience in the field monolithic construction, are faced with the need to continue the cycle of concrete work in winter time, in connection with which, they face the question: “How to prolong the liquid phase of concrete, and, consequently, its vital activity. To solve this problem, experts suggest using antifreeze additives in concrete, specifications and the main varieties of which will be discussed in this article.

Antifreeze additives in concrete: main varieties

Antifreeze additives in concrete are a chemical substance in the form of a dry mixture or solution, which, through involvement in the process of concrete crystallization maximum number water, accelerate the process of hydration of the concrete mixture, contributing to the hardening of concrete in conditions of negative temperatures. However, the main purpose of the antifreeze additive is to maintain liquid state concrete solution and the subsequent acceleration of its hydration, which significantly slows down at low temperatures.

Important! When using antifreeze additives in concrete, it is important to remember that the strength of concrete with antifreeze additives under conditions of negative temperatures does not exceed 30% of the maximum possible design strength, the remaining 70% of the strength of the concrete is gaining in the process of thawing. In this regard, the structures, the concreting of which took place in winter period time, should not be subjected to high loads.

In accordance with the chemical basis, the following types of antifreeze additives in concrete are distinguished:

Antifreeze;
sulfates;
Antifreeze additives-accelerators.

Let us consider in more detail the characteristics of each presented variety.

Antifreeze is an antifreeze additive in concrete, which helps to reduce the crystallization temperature of the liquid that is part of the solution, and also increases or slightly reduces the setting rate of the solution. However, it does not have any effect on the rate of formation of structures.
Sulfate based concrete additives are another popular antifreeze component that provides top speed formation of a dense solution. characteristic feature antifreeze additives based on sulfates is the active release of heat, which begins after they are added to the solution and is accompanied by the interaction of the concrete solution with hydration products. Due to the fact that additives based on sulfates are characterized by strong binding to sparingly soluble compounds, they cannot be used to lower the freezing point of the working mixture.
At the heart of the action antifreeze additives-accelerators lies in the increase in the degree of solubility of the silicate components of cement, which, reacting with the products of its hydration, form double and basic salts, which reduce the freezing point of the liquid component of the concrete mortar.

Important! Modern complex antifreeze additives for concrete not only regulate the kinetics of its strength gain, but also correct its rheological properties. By lowering the crystallization temperature of the liquid component of the solution, they reduce the time of its primary setting, affecting the hardening of the cement stone and increasing its grade strength.

There are several types of accelerator additives, each of which has a certain set of chemical and operational properties. Let's consider them in more detail.

Potash or calcium carbonate, which is a crystalline substance, is a strong antifreeze component, significantly accelerating the process of setting and subsequent hardening of concrete. Like any antifreeze additive, calcium carbonate reduces the strength of the concrete structure, and in order to minimize this Negative influence for construction, experts recommend combining potash with sodium tetraborate or sulfide-yeast mash, the concentration of which should not exceed 30%. Due to the fact that calcium carbonate is a potential dangerous substance, during its operation it is necessary to observe certain safety measures;

Sodium tetraborate, also called brown or sulfate-yeast mash, is a mixture of sodium, calcium, ammonium or lignosulfonic acid salts. Experts recommend adding this substance as an admixture when using calcium carbonate, which helps prevent the loss of strength characteristics of concrete structures after they have been thawed. Otherwise, one can observe not only the appearance of cracks in the structures, but also a decrease in their water resistance and frost resistance. Thus, the use of potash as an antifreeze additive without the addition of sodium tetraborate will reduce the strength characteristics of the structure by 20-30%;

sodium nitrite- crystalline powder used as an antifreeze additive to concrete mortar. Given that sodium nitrite is a flammable toxic substance, during its operation it is important to observe the maximum permissible concentration of the substance, which is determined empirically and usually does not go beyond 0.1 - 0.42 l / kg of cement mortar, provided that temperature environment will be from 0 to -25 degrees. At the enterprise, in the process of working with sodium nitrite, the maximum permissible concentration of the substance in the workplace should not exceed 0.005 mg/l. In accordance with the requirements of the Research Institute of Concrete and Reinforced Concrete, the container that was used for the transportation, storage and manufacture of sodium nitrite must be marked "POISON". The joint use of sodium nitrite and lignosulfonic acids is prohibited, since their interaction is accompanied by the formation of toxic gases;

sodium formate- white crystalline powder, which also acts as an antifreeze accelerator. In most cases, it is used in conjunction with naphthalene lignosulfonate to improve water-reducing and plasticizing characteristics. Sodium formate is an antifreeze additive in concrete, the consumption of which does not exceed 2-6% of the total mass of cement.

Important! In addition to the above substances, sodium formate on alcohol, calcium chloride, ammonia water and urea.

Advantages and disadvantages of antifreeze additives in concrete mortar

Advantages of antifreeze additives in concrete

Using antifreeze additives in concrete, you will be able to carry out concrete works on the construction site even in winter;
Due to the fact that antifreeze additives increase the degree of adhesion of the mortar components, they significantly increase the strength of the monolith;
Thanks to high strength products made using antifreeze additives in concrete, they can be used for industrial purposes;
They have a positive effect on the durability of the mixture, extending the life of the building;
Increases plasticizing and stabilizing characteristics cement mixture- the use of concrete with increased plasticity makes it possible to manufacture structures that will not crack after the working composition has solidified;
Increases the frost resistance of the concrete mix. This indicator is especially important for concrete intended for the construction of critical structures, for example, bridge supports. In most cases, it is directly dependent on the density of concrete. More dense grades of concrete are characterized by large quantity possible freeze and thaw cycles;
Unlike alternative methods increasing the frost resistance of concrete, the use of antifreeze additives is characterized by a relatively low cost;
Using antifreeze additives, you will significantly reduce the risk of shrinkage deformations of a concrete monolithic structure;
Improving the moisture resistance of concrete structures by filling the pores with plasticizers that prevent the penetration of water;
The acceleration of the process of hardening of the concrete mortar is the main point due to which the mortar can "not be afraid" of the cold;
By opting for an antifreeze additive in concrete, you will reliably protect the used reinforcement from corrosion processes that occur due to the water that is part of the concrete solution.

Disadvantages of antifreeze additives in concrete

The desire to increase the reliability of the strength characteristics of concrete, it is necessary to increase the consumption of cement;
The individual components that make up the additives are poisonous;
In some cases, the declared power of concrete is reduced;
In the case of using antifreeze additives in concrete, the speed of the set of strength characteristics of the concrete structure is reduced.

Experts advise introducing an antifreeze additive into the concrete solution along with water. It is important to note that it is desirable to do this with the last third of the liquid. It is not recommended to add additives to the dry mix. By adding to the solution antifreeze additive, wait a certain period of time, during which there will be a uniform distribution of the components.

When carrying out installation activities in conditions of negative temperatures, follow the instructions below:

If you are working in snowy conditions, take care to organize appropriate shelters;
The temperature of the solution that came out of the mixer should not go beyond the recommended range from +15 to +25 degrees;
To prepare the working mixture, experts recommend using heated water;
As for the heating of aggregates, it is recommended to do it before direct use.

Important! Specialists in the construction industry recommend paying attention to SNIP 3.03.01, according to which, in order to achieve the necessary strength characteristics of the concrete solution, it is necessary to comply with the requirements for the care of concrete in the winter. In the process of performing these measures, by the time the temperature for which the additive dosage was calculated was reached, it is not recommended to achieve a structural strength exceeding 20% of the declared design strength.

Dosage and consumption of antifreeze additive in concrete

The dosage of the antifreeze additive in concrete, the consumption of which is an extremely variable parameter, is selected taking into account each specific situation through tests in production and laboratory conditions.

The consumption of antifreeze additive depends on the following factors:

Ambient temperature under which installation activities will be carried out;
Declared grade strength of the cement used;
Chemical-mineralogical and material composition of the cement used in the process of work, as well as its estimated rate of curing;
The temperature of the solution, which it will reach at the outlet of the mixer;
Maintenance conditions for concrete structures.

Important! In the case of long-term use or storage of a solution in which additives were added, it is necessary to check its homogenization by periodically stirring. Calculation required amount antifreeze additive is produced with an error of 2%.

Do-it-yourself antifreeze additive in concrete

If the warm days have already passed, but you are suddenly faced with the need to pour a monolithic structure, you can not do without the use of an antifreeze additive in concrete. The most preferred option, in this case, would be to purchase an antifreeze additive in a specialized store, which is explained by their relative cheapness, low consumption and the ability to significantly improve the properties of the concrete solution, subject to minimal negative consequences. If the proposed scope of work is small, and you plan to carry out installation activities at a temperature of at least -10 degrees, this option is the most optimal.

However, if you do not have the opportunity to purchase a ready-made antifreeze additive in concrete, you can easily make it yourself, since the only material that you need in the process of work is chlorides (salts). Chloride salts reduce the freezing point of the solution, reduce the time of its initial setting and reduce the consumption of cement. However, experts are confident that a self-made chloride-based antifreeze additive can only be used for non-reinforced structures, which is due to corrosion processes developing under the action of chlorides.

Benefits of an antifreeze additive based on chlorides

Low cost;
No effect on the rate of hardening of concrete, due to which, the preparation of the solution can be carried out in advance;
No effect on the structure of the cement mortar;
Increased particle mobility, thanks to which you can give cement mortar desired shape.

Disadvantages of antifreeze additive based on chlorides

A high level of corrosive activity, as a result of which, an antifreeze additive based on chlorides cannot be used for the manufacture of structures in the structure of which metal and reinforcement are present. The latter will oxidize under the influence of chlorides and exfoliate from the concrete structure, violating its integrity.

How does ambient temperature affect chloride consumption?

Calculation of the proportion of chlorides in ready solution produced according to the following scheme:
If installation activities are carried out at an average daily temperature of at least -5 degrees, the optimal proportion of chlorides in the finished solution should not exceed 2%;
If work is carried out at lower temperatures (-6 to -15 degrees), the optimal proportion of chlorides should be 4% of the total mass of the solution.

Important! In this case, the scheme for gaining the expected strength of the structure during drying under conditions of negative temperatures will look like this:

For the first option, where the salt concentration is 2%:

30% after a week;

80% after a month;

The construction will reach 100% strength only after 3 months.

For the second option (salt concentration is 4%), these figures will be 15%, 35%, 50%, respectively.

Important! Despite the fact that salt is an independent antifreeze additive, experts recommend using it in conjunction with calcium chloride, mass fraction which, when used at temperatures up to - 5 degrees, is 0.5% by weight of the solution, and 2% - when used at temperatures from -6 to -15 degrees.

Precautions when working with antifreeze additives

In the process of working with antifreeze additives, it is necessary to use protective gloves;
In case of hitting open areas skin, wash with soap and water. Avoid contact of the antifreeze additive with the eyes, if this cannot be avoided, rinse the eyes with plenty of water and consult a doctor immediately.
Disposal of the additive is carried out in accordance with local regulations, which is explained by the presence of harmful components in the composition of antifreeze additives. As a result, it is forbidden to pour the mixture into the soil, waterways or sewers.

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 ambient temperature, the longer the hydration takes. 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 mix;
heating of a monolithic structure from the 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 chemicals are divided into groups according to the way they affect 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 placed electric or gas heat guns that support a certain temperature regime throughout the curing time of the concrete.
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

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Concrete work in winter conditions

Features of concreting at negative temperatures

In the production of concrete and reinforced concrete works in winter conditions, when the average daily outdoor temperature is expected to be below 5 °C and the minimum daily temperature is below 0 °C, as well as when concreting structures located in permafrost soils, concreting methods are used that make it possible to obtain concrete of the required quality.

If not applied special ways concreting, then when the concrete freezes, the free water contained in it turns into grandfather and the hardening of the concrete stops. If hardening did not begin before freezing, then it will not begin after it, but if it did, then it practically stops until the free water in the concrete is in a frozen state. Water frozen in concrete increases in volume by approximately 9%. The resulting internal ice pressure breaks the weak bonds in the unhardened concrete.

Water accumulating on the surface of grains of coarse aggregate, when frozen, forms a thin ice film that breaks the adhesion between aggregate and mortar and reduces the strength of concrete. An ice film forms on the reinforcement, which breaks the adhesion of the reinforcement to the concrete.

When concrete thaws, the ice in it melts and the hardening of concrete resumes, but the final strength of concrete, its density and adhesion to reinforcement are reduced. These losses are greater the more early age frozen concrete.

The most dangerous freezing of concrete during the setting of cement. Also harmful is repeated freezing and thawing of concrete at the beginning of hardening, which happens when thaws are replaced by frosts. The strength of concrete at the time of freezing or cooling below design temperatures, the so-called critical strength, at which the final strength does not decrease or decreases slightly, should be indicated in the work design or in the technological map.

For concrete without antifreeze additives of monolithic structures and a monolithic part prefabricated monolithic structures strength at the time of freezing should be at least 50% of the design for concrete grade 150, 40% for concrete grades 200-300, 30% for concrete grades 400-500, 70%, regardless of the brand of concrete for structures exposed at the end of curing freezing and thawing, 80% for concrete in prestressed structures, 100% for concrete of structures subjected immediately after the end of curing to the design water pressure, and structures to which special requirements frost resistance and water resistance.

For concrete with antifreeze additives, the strength at the time of its cooling to the temperature for which the amount of additives is calculated must be at least 30% of the design value for grades up to 200, 25% for grade 300 concrete and 20% for grade 400 concrete.

The conditions and period by which the freezing of concrete blocks of massive hydraulic structures is allowed are indicated in the project.

Concrete that has reached critical strength by the time of freezing acquires design strength only after thawing and keeping at a positive temperature for at least 28 days. In cases where structures concreted in winter (including concrete of prefabricated elements with conventional and prestressed reinforcement, which are part of prefabricated monolithic structures) are subject to full loading at a negative outside temperature, it is required to withstand the concrete at a positive temperature until until design strength is reached.

The value of the strength of concrete in the structure at the time of its freezing is determined by the minimum strength of the sample from the control series.

To obtain the necessary strength of concrete, special measures are taken to prepare the concrete components and prepare the concrete mixture. Particular attention is paid to the protection of concrete structures from direct exposure to negative temperatures and wind.

It is necessary that the concrete mixture placed in the formwork has a certain temperature set by the calculation.

Various methods are used to protect concreted structures from exposure to negative temperatures, to create an artificial heat and moisture environment for concrete prepared on heated materials, and to keep it under such conditions until the required (critical) strength is acquired.

Concrete laid in massive structures in winter is most often cured using the thermos method, based on the use of insulated formwork, the heat of the heated components of the concrete mixture and the heat released during setting and hardening of cement. Well-covered concrete cools so slowly that it has time to gain critical strength by the time it freezes.

To expand the scope of the thermos method, pre-heating of the concrete mixture is used before laying in the formwork, chemical additives-accelerators, cements with increased heat release and fast-hardening cements, and also combine the thermos method with various concrete heating methods, for example, with peripheral electric heating or heating structures.

When using preliminary electrical heating of the concrete mixture, the heating temperature for concretes based on Portland cements with a content of tricalcium aluminate up to 6% should not exceed 80 ° C; on Portland cements with a tricalcium aluminate content of more than 6%, it is established by a construction laboratory after experimental verification; for concretes based on Portland slag cements - should not exceed 90°G.

The concrete mixture is heated in specially equipped bunkers and tubs, which ensure its uniform heating, as well as in car bodies equipped for this purpose.

Often, when concreting foundations located in separate pits, the thermos method is combined with the use of heat transfer from thawed soil. In this case, the pits are well insulated from above, due to which a slight positive temperature is established in them.

Concrete in thin structures cools quickly, so they have to be heated electric shock, ferry or warm air. Sometimes, in order to save electricity, they combine the method of a thermos with heating.

Lightweight concrete on porous aggregates in winter conditions is kept according to the thermos method with preliminary electric heating of the concrete mixture.

In addition to the above methods of winter concreting, based on concrete hardening at a positive temperature, there is a method of concrete hardening at a negative temperature. In this case, the concrete mixture is prepared with the introduction of antifreeze additives. Antifreeze additives lower the freezing point of water to such an extent that they provide hardening of concrete at negative temperatures down to -25°C. When choosing a concrete curing method, first of all, consider the possibility of using the thermos method, the thermos method with additives - hardening accelerators.

If, using this method, it is impossible to obtain the required strength of concrete within the specified time, then the possibility of using concrete with antifreeze additives, methods of electrical heat treatment, heating with steam, warm air is consistently considered. If it is impossible to keep concrete in structures with the help of these measures, concrete work is carried out using greenhouses.

One or another method of producing concrete and reinforced concrete works in winter conditions is adopted on the basis of comparative technical and economic calculations.

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