Hardening of concrete at sub-zero temperatures. At what temperature is it possible to pour concrete in winter. Type of structures and conditions of their operation

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 period at negative temperatures. There may be several reasons:

• limited construction time in summer 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.

Let's try to answer this question, taking into account the characteristics of concrete and the peculiarities of its behavior during sub-zero temperature.

To begin with, water in concrete is needed not only for its fluidity, it also plays a very important role. important role during the hardening process.

At sub-zero ambient temperatures, water begins to turn into ice, as a result of which it not only stops the hardening process, but also contributes to the destruction of the bonds that formed before it freezes.

You should be aware that concrete can freeze at a temperature of about -4 0 C. At temperatures below +5 0 C, the process of its hardening slows down several times, and strength gain is delayed until it becomes warmer. So, the longer the stagnation during the hardening process, the less strength it will gain in the end.

This means only one thing - at sub-zero temperatures, we need to create conditions under which concrete will not only not freeze, but will continue to gain its strength.

Let's try to figure out how to do it.

Technology of pouring concrete at sub-zero temperatures

There are several technologies for pouring concrete when the ambient temperature is below zero:

• By adding antifreeze additives to concrete
• With the help of electrical heating of concrete from the inside
• With the help of insulation of the pouring object

The use of additives in pouring concrete

The use of antifreeze additives in prepared concrete is very common. This is due, first of all, to low costs and their availability. And if you use purchased concrete, then all the necessary antifreeze additives will already have to be in it, in accordance with the values ​​​​of sub-zero temperatures.

These additives contain various chemical impurities, like salts of monocarboxylic acids, sodium nitrite and formate, etc., which significantly accelerate the hardening process, increase the final strength of concrete, and most importantly, prevent the water contained in concrete from freezing.

The main disadvantages of using various kinds additives in concrete is that they do their job well only at low sub-zero temperatures - about -5 0 C.

In severe frosts, some of the additives will work, but all the same, hardening will slow down significantly and eventually there will be a shortage of concrete strength up to 30%. Also, some additives are aggressive to metal reinforcement, and act destructively on it.

The technology of heating concrete during its pouring

Heating of concrete with electricity is carried out in two ways:

• Passing current through concrete using electrodes
• By passing current through a pre-laid wire inside the concrete

If someone is interested in the technology of electric heating of concrete in winter, I described it in detail in one of my articles. Therefore, I will not focus on this technology much attention here.

The first method is much cheaper, since fittings can act as electrodes.

If you follow the rules, then additional special equipment is needed for these purposes, but some craftsmen, having inserted reinforcing electrodes into concrete, connect them to them welding machine, thereby passing current through the concrete and heating it.

It is worth noting that reinforced concrete they are heated with voltage up to 127 V, and concrete without metal reinforcement can be heated with voltage up to 380 V.

The second method involves additional costs for special heating cable, which, as a rule, must be laid along the reinforcing cage and connected to it by current through special substations.

This method is much more expensive, but at the same time - much more efficient than heating concrete with electrodes.

The main disadvantage of using this method is significant financial costs, and at the same time, when heating concrete, it is necessary to constantly monitor its temperature. Heat will cause the concrete to dry out and can cause significant damage to the future structure.

In severe frosts additional insulation formwork or concrete.

Insulation of concrete during curing

In addition to creating artificial heating of concrete, at a slight sub-zero temperature, standard methods of insulation can be applied.

As a rule, the formwork itself is insulated, if any, as well as all visible parts of the concrete. Warming is carried out various materials, from conventional sawdust to the use of Styrofoam.

One of the most popular methods of insulation is the creation of a so-called tent around the concreting object. Additionally, heating guns can be installed inside such a tent.

The disadvantage of this method is that the concrete must be supplied already heated, and conventional insulation, without additional heating, will be ineffective in severe frosts.

So we examined the main methods of pouring concrete at sub-zero temperatures, to all this I would like to add that often for this purpose they use not one specific method, but a set of measures is planned in advance, ranging from the addition of antifreeze additives to heating with subsequent insulation .

But despite this, the process of pouring concrete in sub-zero temperatures is not cheap, despite the fact that manufacturers make huge "winter discounts" on building materials.

In addition to this, it is very difficult with the help of artificial heating to maintain a constant comfortable temperature, and what is not unimportant - the temperature should be evenly distributed throughout the entire volume of the fill.

Attention! Pouring concrete at sub-zero temperatures is only possible if you have no other choice, because minimum temperature environment in the process of hardening, without additional interventions, in spring it should be +5 0 С, and in autumn - +10 0 С.

For autumn, the requirements are stricter, because the main hardening period is 28 days, and during this time the autumn air temperature can drop significantly.

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 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. 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.

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 heated 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 an 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.

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

Undoubtedly, to engage in construction in winter time harder than in summer. Most often, the weather conditions are not too conducive to the performance of high-quality work. Usually work in severe frosts continue either professional construction teams, or craftsmen who want to move into their own homes as soon as possible. invaluable help for those and others there will be specialized antifreeze additives. The use of the latter helps to prevent premature hardening of the cement.

What is an antifreeze additive in the solution?

As a rule, with a significant decrease in ambient temperature, builders begin to experience additional difficulties in working with concrete and all kinds of solutions. This is because cement-based compounds cannot stand frost. So, at a temperature of -5 o C, it would seem that high-quality solutions cease to gain strength.

AT last years craftsmen are increasingly using antifreeze additives that can withstand temperatures down to -35 ° C or more. Specialized formulations for contain chemicals, active ingredients which are reduced As a result, the concrete sets in the masonry even in extreme cold.

The problem of organizing brickwork in cold weather

When building in winter, it becomes most problematic to provide the structures with the necessary strength. When the temperature drops below zero, the liquid in the solution crystallizes. Thus, the process of cement hydration is stopped.

With an increase in air temperature, the ice formed inside the masonry and on the surface of the building material begins to melt, which necessarily causes a decrease in the adhesion of the solution. Another negative effect is the formation of small cavities in the concrete structure, which can subsequently lead to a rather rapid destruction of walls and ceilings.

Features of the preparation of the solution

In spite of difficult conditions work, can be no less qualitative than in the warm season. However, in order to obtain the expected result, it is necessary to prepare a special solution, which must contain antifreeze additives.

Getting Started Cooking building mix you need to take care of your personal safety. It should be borne in mind that almost all antifreeze additives differ in composition based on fairly aggressive chemical substances which may be harmful to health. Therefore, during the preparation of concrete, it is necessary to work in tight gloves, goggles, rubber boots and, if possible, overalls.

With a more serious cooling down to -30 ° C, it is advisable to use potash. The advantage of this solution is protection against corrosion manifestations in case of construction reinforced floors. The use of potash also prevents the appearance of efflorescence on the hardened mortar. As for chlorine-containing additives, the latter do not slow down the destruction of the reinforcing parts of building structures.

Antifreeze additive: consumption

In the manufacture of formulations for brickwork it is important to observe the necessary proportions. They depend primarily on the ambient temperature.

Let's look at the average norms typical for the use of the most popular "antifreeze" in the table. It reflects the consumption of antifreeze additives in percentage to the mass of cement.

plasticizers

Can be replaced with special plasticizers, which increase the elasticity of the mortar and reduce the need for liquid. Thanks to the use of the latter for several days, there are practically no changes in the laid composition. Thus, the solution has time to seize even with significant frosts.

In addition to masonry, such a specific anti-frost additive can be used for glue, preparation of mixtures for arranging self-leveling floors, when performing concrete works. Plasticizers are mixed with a minimum amount of water. Their content in the solution is about 5% by weight of the cement. Compliance with this proportion is sufficient to perform reliable masonry at low temperatures.

What you need to know about masonry work in the winter?

In order not to be disappointed in the results of construction activities that are carried out at negative air temperatures, it is enough to pay attention to the following recommendations:

1. For masonry, it is forbidden to use building materials covered with snow, frost or ice.
2. All components of the future solution, including antifreeze additives and plasticizers, should be stored in dry, ventilated areas at room temperature.
3. Regardless of the composition of the solution and temperature conditions laying in winter is recommended to be done as soon as possible. The lack of delay allows the substances to seize faster.
4. Going on a break, the rows of masonry must be insulated plastic wrap or other suitable material suitable for the role of an effective temporary insulator.
5. As a basis for preparing the solution, it is worth using cement of a grade not lower than M-50. Mixing the components, even in the case of the use of "antifreeze" should be carried out in a warm room.
6. If necessary, work with low temperatures it is better to give preference to the acquisition ready solutions, to which antifreeze agents are added in optimal proportions at the production stage.

Eventually

When performing construction work in winter, the main thing is not to forget to add special antifreeze additives to the solution. However, the introduction of "antifreeze" in cement composition looks appropriate only if the ambient temperature drops to -5 o C.

During the preparation of the solution, it is absolutely not recommended to use long-stored, old antifreeze additives with a questionable shelf life. Otherwise, you will have to pay with the formation of abundant smudges and efflorescence on the surface of the structures.

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

Antifreeze additive in the solution allows you to use the latter in the construction monolithic structures, buildings and manufacturing 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 waterways.

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.