Winter concreting: available methods and conditions for performing work

The use of concrete mixture is a common way to create high-strength structures. For hardening, special conditions are required: warm, moderately humid air. In this case, the solution sets faster.

Changing environmental conditions can negatively affect the structure and properties of the material. This is the main reason why the quality of the concrete mixture decreases in winter if standard construction technology is used.

When planning to build a foundation or other structures, it is important to understand at what temperature concrete can be poured and what should be done to preserve its characteristics.

Is it permissible to pour concrete when the air temperature drops?

In order not to violate construction deadlines, various options are being considered, including carrying out work in winter. This method is not prohibited, but it has its peculiarities. The problem lies in the disruption of the cement hydration process. Under normal conditions, crystalline hydrates are formed in the solution. This is the result of the interaction of cement with water.

The structure changes, the viscous solution becomes solid. When concrete work is carried out under normal environmental parameters, due to hydration, strong bonds are formed that prevent the destruction of the finished structure. The service life of such a building will be long.

When studying the question of whether it is possible to pour concrete in winter, take into account that during a sharp cold snap, the water contained in the solution expands. Porosity immediately increases, which leads to a decrease in resistance to mechanical damage.

Due to the freezing of the liquid after pouring the mixture in winter, microcracks gradually appear. They increase, which provokes complete destruction of the structure.

Concrete work carried out at subzero air temperatures is carried out on the basis of a different technology: they consider ways to maintain parameter values ​​at a sufficient level, and also change its composition.

What conditions are considered “winter” when concreting and masonry work?

Before you understand what features and limitations entail negative temperatures during “wet” processes, you need to understand what is meant by winter concreting conditions.

Many people believe that winter conditions are conditions in which the outside temperatures are consistently below freezing and it is snowing. Actually this is not true. In accordance with SP 70.13330.2012 “Load-bearing and enclosing structures”, winter conditions are when the average daily outside air temperature is below +5 °C, or the minimum daily temperature is below 0 °C .

That is, as you can see, the concept of “winter” conditions covers a wide range of temperatures. They can occur, depending on the region of residence, both in autumn and early spring, and without precipitation in the form of snow.

There is also a widespread belief among novice developers that at subzero temperatures, and even more so if the thermometer drops below -15...-25 °C, it is impossible to pour a foundation or lay wall cladding. Although, if you look at commercial construction, work on the construction of monolithic high-rise buildings and residential apartment buildings is carried out all year round, regardless of weather conditions.

This is due to the use of special concreting techniques and the use of special anti-frost additives - chemicals that modify the concrete mixture.

What is the difference between concreting in winter and summer?

The pouring process and methods of maintaining the properties of the solution in winter are more complicated. In the warm season, it is often enough to mix the components taking into account the proportions, prepare the formwork and reinforcing frame, and pour the mixture. Under normal conditions, strength gains occur in a shorter period of time:

• setting: this period lasts up to 24 hours, a thin layer of hydrosilicate is formed on the surface, at the same time the outer layers become denser;
• hardening: over the next 4 weeks, the mixture gains strength, after which further laying of building materials for the construction of walls becomes possible.

When it's frosty outside, the setting and hardening time increases. In addition, concreting in winter requires the use of special grades of cement:

• M200;
• M250;
• M300;
• M350;
• M400.

They are used under different conditions, but if the question of how to concrete in winter is being decided, then the indicated grades are the minimum acceptable for the construction of houses of a certain type.

For example, the foundation of a brick house in winter can be concreted with a mixture based on the M350 or M400 cement brand. More is possible, but not less.

In addition, in winter, before pouring the solution, auxiliary components are always added - special modifiers that improve the properties of the material. In case of mild cold weather, the covering method and insulation of the massif are sufficient. The stronger the frost, the more heating agents are used.

Monolithic slab before pouring

Methods to combat freezing of concrete during the period of gaining critical strength

For concreting in winter, warm or cold concrete is used. The cold mixture will begin to gain hardness without additional heating if an antifreeze agent is added to the material. This additive lowers the freezing temperature of water, and at the same time accelerates the hydration process.

Electrolytes, sodium and potassium salts are widely used as antifreeze additives. However, their use also has limitations:

Na salts are not used if the concrete is reinforced, as they cause metal corrosion;

certain types of Portland cement are not used together with electrolytes;

additives based on sodium or potassium salts are not used in mixtures with potentially reactive rock filler;

electrolyte salts are pre-tested for the formation of efflorescence.

Antifreeze additives of the new generation are complex; they do not have the disadvantages of electrolyte salts. These additives allow concrete to be cast in winter and provide a complex effect (anti-frost, plasticizing, etc.).

If the concrete mixture is heated after placement, the mixture is called warm concrete.

Features of winter concreting

The main parameters act as a catalyst for the solidification process of the massif. For this reason, you need to understand how to pour concrete so that it does not freeze, gains strength, and performs its functions in the future. Recommended temperature to ensure the mixture can set and harden in winter:

• within +5°C or higher – for cases when a monolithic base is being poured;
• not less than +20°С – if structures are made of thin concrete.

Permissible deviation is up to 2°C downwards. The mixture will harden at temperatures in the range of +18...+20° for 4 weeks. If you are wondering to what temperature concrete can be poured, you should focus on +5°C.

Moreover, in 28 days the material will have time to gain only 70% of its strength. You need to wait longer. The exact duration of this period is determined in each case separately.

It is also important to understand at what temperature concrete freezes: when the mixture is exposed to frost (below 0°C), the duration of the mass until it hardens can increase many times over. In this case, the setting process is disrupted. Concrete freezes, but it does not gain strength in harsh winter conditions.

Advice: To avoid the development of negative processes in the thickness of concrete, it is recommended to carry out construction work at temperatures of +10°C and above.

How to get quality concrete in winter

Ambient temperature is one of the main factors influencing the strength gain of concrete. It is believed that the optimal conditions for concrete to gain strength are the temperature range from +18 °C to + 20 °C. In this case, the concrete will gain 70% of its original strength in approximately 10 days, and 100% in 28 days. At a temperature of +5 °C, the strength of concrete increases approximately 3-4 times slower. It will gain strength of 70% of the brand name in about 15 days, and will gain 100% much longer than the required 28 days. When the temperature drops to 0 °C, the hardening of concrete and masonry mortar practically stops, and at lower temperatures it stops. The question arises how to circumvent the above problems.

Kirill Lebedev

There are several ways to carry out concrete and masonry work in winter. For example, related to the heating of building structures, the use of heat guns, electrical heating of freshly laid concrete, the construction of greenhouses, the use of the “thermos” method, the so-called. “warm” (heated) concrete mixtures. The use of antifreeze does not cancel the use of these methods, but it can significantly reduce the time it takes for concrete to gain strength, and, therefore, save time and money.

Antifreeze additives work in a comprehensive manner: they reduce the freezing point of water, and complex additives also reduce the required amount of mixing water; accelerate the hardening process of the mixture at low temperatures (down to -25 ° C); increase the workability of the mixture and the grade strength of concrete.

Important point : according to SP 70.13330, as part of the preparatory work associated with concreting, measures should be taken that prevent freezing of the concrete mixture when transporting it in a mixer to the place of laying and until critical strength is achieved.

At the same time, antifreeze additives should not have a negative effect on the reinforcement. Moreover, the “Code of Rules” prescribes the use of antifreeze both when using “warm” concrete and using heating systems, and when laying a “cold” mixture.

The main advantage of antifreeze additives is that they allow concrete to quickly reach critical strength at which it can be frozen without negative consequences.

The main increase in strength of the freshly laid mixture occurs in the first 3-5 days. Therefore, it is critical to prevent the concrete from freezing during this period, which is what antifreeze additives can do. In addition, complex antifreezes are economical to use and reduce cement consumption. This allows you to reduce construction costs without losing quality.

Winter concreting methods

The hydration process is accompanied by the release of a small amount of heat. This allows the concrete to harden even in cold weather.

In order to prevent freezing of the array, all available methods of preserving the structure of the material and properties are considered:

1. The use of additives that increase the resistance of concrete mixtures to low temperatures: from 0°C and below.
2. A method of creating formwork with thermal insulation, it also involves the use of a screen on top. This winter concreting technology is multi-stage. First, the formwork panels are installed in 4 rows. Each pair will form one wall on the side of the planned foundation. Thermal insulation material is laid between the panels. Additionally, a screen is placed on top to reduce heat loss. This method is used when pouring concrete for non-massive structures.
3. Introduction of electrodes into the thickness of the mixture. When current passes through the concrete structure, heat will be generated, which is necessary for hardening and strength gain.
4. Heating directly while preparing the mixture. In this case, improvised means are used. The result is heating of the mixer, which will transfer heat to the concrete. They also add hot water (up to +80°C) or heated additives.
5. The infrared method is based on exposure to infrared radiation. The method is based on the use of heaters. These can be ceramic rod emitters, as well as devices designed for voltage of 220 or 380 V. They are installed next to the formwork - on both sides. The mixture is protected with a tarpaulin. Concreting in winter conditions in this way is carried out provided that a sliding foundation is built. A large area cannot be heated using electrical appliances.
6. Exposure to steam. The formwork is covered with a tarpaulin so that there is a distance of up to 20 cm between this material and the shields. Jets of steam are directed there. The method is effective if it is frosty outside.
7. Application of the convection method. The formwork is covered with a tarpaulin, and hot air is supplied into the gap between it and the formwork (a heat gun is used).
8. Thermal insulation. In case of mild cold, prepare materials that can be used as coverings: straw, sand, OSB sheets.

Important: When winter concreting methods are considered, the disadvantages are taken into account, the main one being the loss of elasticity of the mixture. During construction, it is necessary to use compounds containing plasticizers.

Device for heating concrete with wires before concreting

Thermos method

After pouring, winter concrete can gain strength if heated correctly. One of the common methods is the “thermos” technology. It is based on artificially increasing the cooling period of the mixture. After concrete has been poured in winter, you need to pay attention to the air temperature. If it is not lower than -5°C, you can protect the material in this way.

To do this, you do not need to use sources of electric current; a structure made of wooden panels, heat and waterproofing is sufficient. The insulation is protected by film on both sides. It must be covered by the walls of the formwork. This design is created on all sides of the concrete structure.

Thermos method

Methods of artificial heating and heating of concrete

The essence of this method is to create and further maintain the temperature of the mixture at the maximum permissible value until the concrete gains the required strength.
This method is used in cases where the “thermos” method is not enough. There are several options to achieve the desired result:

• Electrode
  The physical meaning of electrode heating is similar to the above-described method of electrode heating of the mixture. In this case, the heat that is released by the mixture when an electric current is passed through it is used. To conduct electric current to concrete, several types of electrodes are used: plate, string, strip, rod. The most effective are plate electrodes made from roofing steel. The plates are sewn onto the surface of the formwork, which is in direct contact with the concrete, and connected to opposite phases of the network. Current exchange occurs between opposing electrodes, resulting in heating of the entire concrete structure.

• Conductive (contact)
  The essence of contact or conductive heating is the use of heat generated in a conductor during the passage of electric current through it. By contact method, heat is transferred to all surfaces of the concrete element. From the surfaces, heat spreads throughout the structure. For contact heating of concrete, thermoactive flexible coatings or thermoactive formwork are used.

• Infrared The
  infrared heating method is based on the ability of infrared rays, when absorbed by the body, to be transformed into thermal energy. Heat from the emitter to the heated body is carried out instantly without the use of a heat carrier. Quartz and tubular metal emitters are used as infrared wave generators. Infrared heating is used to warm reinforcement, frozen concrete surfaces, and thermal protection of laid concrete mixture.

• Induction
  Induction heating uses the heat that is released in steel formwork or reinforcement parts and products located in the electromagnetic field of an inductor coil. This method is used to warm up previously made concrete structures at any ambient temperature and in any formwork.

To speed up the process of stripping and further loading the structure during the cold period, it is advisable to use a concrete class that is an order of magnitude higher to quickly gain normalized strength.
Compliance with the recommendations for winter concreting will help avoid loss of strength characteristics of concrete and reinforced concrete structures made at low outside temperatures.

Requirements for winter concreting

The main conditions for preserving the structure of the material are: normal temperature, holding the mixture until it reaches sufficient strength.

Accordingly, winter concreting is carried out taking into account the requirements:

• air temperature not lower than +5°C: in this case, simpler methods of retaining heat in the concrete structure can be used;
• insignificant cooling (up to +15°C): thermal insulation is used;
• reducing the temperature below 0°C: the mixture is prepared with antifreeze additives, and forced heating methods are also considered.

With natural hydration, the temperature of concrete increases by only 2...3°C.

The parameters of the solution are also taken into account. Warm concrete is prepared for pouring, the temperature is +20°...+30°C.

Concrete works

This part of the construction support services includes all the procedures necessary to create a monolithic base or structure made of cement and slag, gravel, reinforcement, sand:

• marking the territory;
• leveling and development of land;
• soil strengthening;
• backfilling of the base;
• formwork work;
• creation of a reinforced base;
• pouring ready-mixed concrete.

Concrete work is an essential part of various construction stages. They are needed for pouring the foundation, creating screeds, walls and partitions. You can entrust these and other procedures to the production staff.

The most common methods of concreting in winter

Some of the heating methods above are used more often than others. This is due to the high speed of achieving the desired result under given conditions in the winter months:

• heating during the mixing stage;
• application of the “thermos” method;
• use of electrical appliances.

When different methods of winter concreting are considered, it is taken into account that almost all of them require the use of anti-frost additives. The exception is those options that are based on the natural process of preserving the properties of the solution.

Thermos method

The technological essence of the “thermos” method is that the concrete mixture, which has a positive temperature (usually within 15...30°C), is placed in insulated formwork. As a result, the concrete of the structure gains a given strength due to the initial heat content and exothermic heat release of the cement during cooling to 0°C.

During the hardening process of concrete, exothermic heat is released, which quantitatively depends on the type of cement used and the curing temperature.

High-quality and fast-hardening Portland cements have the greatest exothermic heat release. The exotherm of concrete provides a significant contribution to the heat content of the structure maintained by the “thermos” method.

Therefore, when using the “thermos” method, it is recommended to use a concrete mixture based on highly exothermic Portland and quick-hardening cements, lay them at a high initial temperature and carefully insulate them.

Concreting using the “Thermos with accelerator additives” method

Some chemicals (calcium chloride CaCl, potassium carbonate - potash K2CO3, sodium nitrate NaNO3, etc.), introduced into concrete in small quantities (up to 2% of the cement mass), have the following effect on the hardening process: these additives accelerate the hardening process in the initial curing period of concrete. Thus, concrete with the addition of 2% calcium chloride by weight of cement already on the third day reaches a strength 1.6 times greater than concrete of the same composition, but without the additive. The introduction of accelerator additives, which are also anti-freeze additives, into concrete in the specified quantities lowers the freezing temperature to -3°C, thereby increasing the cooling time of concrete, which also helps concrete acquire greater strength.

Concrete with accelerator additives is prepared using heated aggregates and hot water. In this case, the temperature of the concrete mixture at the outlet of the mixer fluctuates between 25...35°C, decreasing to 20°C by the time of laying. Such concretes are used at outdoor temperatures of -15... -20°C. They are placed in insulated formwork and covered with a layer of thermal insulation. Hardening of concrete occurs as a result of thermos curing in combination with the positive effects of chemical additives. This method is simple and quite economical; it allows the use of the “thermos” method for structures with MP

Concreting "Hot thermos"

It consists of short-term heating of the concrete mixture to a temperature of 60... 80°C, compacting it while hot and holding it in a thermos or with additional heating.

Under construction site conditions, the concrete mixture is heated, as a rule, by electric current. To do this, a portion of the concrete mixture is included in an alternating current electrical circuit using electrodes as a resistance.

Thus, both the power released and the amount of heat released over a period of time depend on the voltage supplied to the electrodes (direct proportionality) and the ohmic resistance of the heated concrete mixture (inverse proportionality).

In turn, the ohmic resistance is a function of the geometric parameters of the flat electrodes, the distance between the electrodes and the specific ohmic resistance of the concrete mixture.

Electro-razofev of the concrete mixture is carried out at a voltage of 380 and less often 220 V. To organize the electro-razofev at the construction site, a post with a transformer (voltage on the low side is 380 or 220 V), a control panel and a switchboard is equipped.

Electric heating of the concrete mixture is carried out mainly in buckets or in the bodies of dump trucks.

In the first case, the prepared mixture (at a concrete plant), having a temperature of 5...15°C, is delivered by dump trucks to the construction site, unloaded into electric buckets, heated to 70...80°C and placed in the structure. Most often, ordinary tubs (shoes) with three electrodes made of steel 5 mm thick are used, to which the wires (or cable cores) of the power supply network are connected using cable connectors. To ensure uniform distribution of the concrete mixture between the electrodes when loading the bucket and better unloading of the heated mixture into the structure, a vibrator is installed on the body of the bucket.

In the second case, the mixture prepared at the concrete plant is delivered to the construction site in the back of a dump truck. The dump truck enters the heating station and stops under the frame with electrodes. With the vibrator running, the electrodes are lowered into the concrete mixture and voltage is applied. Heating is carried out for 10... 15 minutes until the temperature of the mixture is 60°C for quick-hardening Portland cements, 70°C for Portland cements, 80°C for Portland slag cements.

To heat the mixture to such high temperatures in a short period of time requires large electrical powers. Thus, to heat 1 m of mixture to 60°C in 15 minutes, 240 kW is required, and in 10 minutes - 360 kW of installed power.

Application of antifreeze additives

The most effective options: sodium nitrate, potash. The latter variety is used when the frosts are the most severe (at temperatures from -15°C). Sodium nitrate can be added to prevent freezing of concrete in winter when the temperature is moderate. In this case, the air temperature can be -15°…0°С.

The answer to the question whether it is possible to concrete in winter will be positive if special anti-frost additives are used:

• Sementol.
• Betonsan.
• Lingopan.
• Hydrozim.

Concrete hardeners are also used:

• sodium formant;
• asol-K;
• formulations containing acetyacetone.

Increasing the effectiveness of using antifreeze additives

If concrete work is carried out in winter, you can consider methods that further improve the result of introducing modifiers into the solution:

• it should be moderately warm (heat while kneading);
• choose the appropriate cement: high grades are the best option;
• The finished massif is insulated in winter with insulating materials.

You need to understand how to pour concrete in the cold: first clear the formwork and reinforcing frame from snow, remove ice.

Adding salt to concrete in winter

The composition also includes components that are on hand. For example, salt is sometimes used. But at the same time, the finished solution may lose its properties. It is recommended to carry out concrete work in winter using special additives. They solve 2 problems at once: the array acquires the necessary properties; sufficient strength of the material is ensured, since it becomes less susceptible to the influence of low temperatures.

What are penetrating additives

To increase the water resistance grade of concrete, mineral materials of penetrating or penetrating action are often used. They are added to the mixture during preparation or applied to a prepared surface, that is, cleaned of dirt and cement laitance, as well as a water-saturated surface using a plaster sprayer or brush.

Active chemical additives in the material react with the components of the concrete mixture. As a result, insoluble compounds or crystals are formed. They create a continuous barrier that prevents the flow of water. Depending on the type of concrete being processed, the degree of its waterproofness can be increased by two or three levels.

Dependence of design strength on the degree of exposure to cold

When concreting is carried out in cold weather, attention is paid to the critical strength. Once the set value is reached, the cold has less effect on the material. This value is 50-70% of the design strength.

Peculiarities:

• if the concrete hardened in winter before the minimum critical strength was achieved (50% of the design strength), it cannot be used;
• when the massif has already crossed the threshold corresponding to the critical strength, a slight loss of properties by the material is noted (a decrease in the actual indicator to 10%);
• achieving 70% of the design strength allows you to obtain a material with excellent characteristics.

Dosage and consumption

The amount of antifreeze additive required depends on the following factors:

• strength of the cement used;
• ambient temperature indicator;
• chemical composition of the protective mixture.

How much additive is required to obtain a certain strength of concrete mortar is presented in the table:

Reasons for the decrease in strength

Factors contributing to the loss of concrete mix quality in winter:

• freezing of water contained in concrete;
• cold weather (temperature above 0°C) slows down the process of cement hydration;
• internal ice crystals interfere with the adhesion of concrete to reinforcement;
• reduction of liquid content: if a solution that has not hardened is exposed to low temperatures for a long time, the water is displaced (ice is formed), and without liquid the hydration process is impossible.

The appearance of cracks due to loss of concrete properties

The appearance of cracks due to loss of concrete properties

Increasing the grade of concrete for water resistance and frost resistance

Concrete mixture is a popular construction material for many reasons. First of all, due to reliability and strength, durability and ease of use, the ability to create a wide variety of projects based on the composition, as well as other qualities, including high levels of water resistance and frost resistance.

By improving these qualities, we increase the general characteristics of the material, making it absolutely problem-free in subsequent maintenance and increasing its service life.

It has long been known that special finishing or even waterproofing of concrete does not always cope with the task, and it quickly deteriorates due to exposure to the same atmospheric phenomena. We definitely don’t need this at our dacha, because if we are building a house, garage or even a small gazebo, we need the structure to last for a very long time.

Therefore, it is advisable to use special additives that have a positive effect on the quality and characteristics of concrete, especially when it comes to foundations and plinths, where protection is most needed. They can also include floors in the shower, on the area near the garage, where we often wash the car, in swimming pools, as well as in structures that are built on sites with a high groundwater level.

To obtain the desired characteristics, bituminous substances, surfactants, as well as components with the maximum inclusion of sodium aluminate are introduced into the composition. They work simply - they fill the pores in the finished concrete.

We have already said that often not one, but several additives are used at once, or several characteristics are improved with a certain composition. So, to improve water resistance qualities, you can use silicate glue, the same liquid glass, which increases the grade of concrete in terms of frost resistance and water resistance, and also increases the setting speed of the mixture and future strength.