What is the difference between concreting in winter and

Concrete mix is ​​an ideal building material for the construction of monolithic walls and supporting frames of all types of buildings and structures.

Thanks to the plastic and mobile structure, the freshly prepared mixture is able to fill voids and take any shape, following the contours of the installed formwork, which allows you to create the bearing elements of building structures of almost any complexity.

Monolithic concrete can harden without air access, and some brands of building mixtures can reach their strength even with full immersion in water, but there is one significant limitation in the use of this material.

Normal solidification of a liquid concrete mixture can occur at ambient air temperatures not lower than +5 ° C, and at negative temperatures structural changes occur in the mortar that lead to loss of strength of the entire structure, which makes it difficult to use it in winter.

Features of concreting in the cold season

In order to deal with the question of how to concrete in winter in conditions of negative temperatures, this article will discuss the main causes of the violation of the performance properties of liquid concrete, as well as describe various methods of dealing with such phenomena.

In addition, the reader will be presented with detailed instructions, which describe the most common methods of performing concrete work in the winter.

Reasons for reducing the strength

The process of solidification of the liquid concrete mixture occurs as a result of hydration, that is, the reaction of the binder, in this case cement, with water. In the process of hydration, cement particles are fastened together, thereby binding the aggregate particles (sand, gravel, sifting), as a result of which a monolithic concrete mass is formed.

Under the influence of low temperatures in the finished building mixture, which is poured into the formwork, depending on the specific conditions, the following processes may occur:

1. At temperatures below +5 ° C, the hydration reaction slows down significantly, thus significantly reducing the rate of solidification of the working mixture.
2. At negative ambient temperatures, all the water in the solution gradually turns into a solid state, as a result of which the hydration stops completely.
3. Turning into a solid state, the water increases in volume by about 9%, while breaking apart the particles of cement and aggregate, which only began to bond together, thereby disrupting the structure and reducing the strength of the concrete.

1. Frozen water, turning into ice, pushes the mortar dough from the elements of reinforcement, increasing in volume, creates foci of local stress, and after thawing it forms shells and voids inside the monolithic structural element.
2. With prolonged exposure to negative temperatures, water completely freezes out of the mortar, as a result of which subsequent hydration becomes impossible.

In the aggregate, all these processes lead not only to a deterioration in the quality of the finished concrete, but also to a decrease in strength, bearing capacity and durability of the entire building structure, which, moreover, will not comply with the norms in accordance with SNiP.

Dependence of design strength on the degree of exposure to cold

However, even taking into account all the above, the question: is it possible to concrete in the winter, you must answer in the affirmative, since it is in principle possible to perform concrete work in the cold season, subject to certain conditions.

The fact is that the influence of low temperatures on the decrease in the quality of the building mixture does not occur immediately. If the concrete solution has time to gain critical strength before the onset of the phenomena described above, its further solidification can take place in a normal, albeit slightly slower mode, without any unpleasant consequences.

Critical is considered to be such a value of strength at which the effect of low temperatures will not have a negative effect on the hardening solution. In general construction works, this value is 50% of the design strength, and during the construction of critical nodes it can be increased up to 70%.

There is a definite dependence of the design strength on the effects of low temperatures after reaching the critical strength.

1. At full freezing of the solution until it reaches its critical, i.e. 50% of the design strength, further use of structures is not allowed. Even after thawing, such a solution is considered unsuitable.
2. When the solution freezes after reaching 50% of the design strength, the internal structural changes manifest themselves to a minor extent, and after thawing, the hydration proceeds normally. Reducing the final strength can be no more than 10% of the design value.
3. The freezing of the solution after reaching 70% of the design strength does not cause any changes in its structure, but only increases the time for complete solidification. After thawing, hydration will be continued in the normal mode until the set design strength.

Thus, when concreting in the cold season, it is important immediately after pouring to prevent the mortar from freezing until it reaches a certain value of strength. Further, various methods of preventing the early freezing of a concrete mix will be considered.

Note! For the machining of concrete structures, which has reached its design strength, can only be used to cut reinforced concrete with diamond circles using a special power tool.

Antifreeze additives

One of the most common methods of counteracting low temperatures is the use of antifreeze reagents, which are added to the mortar during the preparation process in accordance with the instructions for use.

The main distinctive feature of this method is the relatively low cost, as well as ease of use, which allows you to perform concreting with your own hands without using special equipment.

All antifreeze additives on the principle of action can be divided into three groups.

1. Additives that increase the plasticity and slightly accelerate the hydration reaction, thereby contributing to the uniform distribution of the mixture and accelerate the setting of cement. They are mainly used at low positive temperatures.
2. Substances that significantly accelerate the process of hydration, and in addition have a strong antifreeze effect, thereby significantly reducing the setting time of cement and preventing water from freezing. Such additives can be used without restrictions in conditions of significant negative temperatures.
3. The third group includes modifiers, which have a mild antifreeze effect, but greatly accelerate the hydration process. They can be used at low negative temperatures for work in open areas.

Tip! Antifreeze additives of the third group, due to their very short setting time, are not recommended for use in hard-to-reach places and hidden cavities, for example, for concreting piles.

Thermos effect

This method is most often used in the manufacture of massive concrete structures, for example, when casting strip foundations, in conditions of low positive or negligible negative temperatures.

Its essence lies in the fact that hydration is an exothermic reaction, that is, the cement, when interacting with water, emits a certain amount of heat, which, under certain conditions, may be sufficient for the solution to gain the necessary strength until it freezes.

In order to fulfill these conditions resort to the following actions.

1. First of all, in order to reduce heat transfer, it is necessary to perform thermal insulation of the formwork, as well as to prepare the covering material for the subsequent thermal insulation of the filled solution.
2. The preparation of the working solution must be carried out directly at the place of work, using heated water for its preparation.
3. If possible, the concrete mixer bucket can be heated during operation using a gas burner.
4. To increase the cooling time of the solution, the metal elements of the rebar must also be heated to above zero temperature.
5. Preheat the concrete mix at one time pour into the formwork, evenly distribute throughout the volume, preventing the formation of voids and air bubbles, then cover with insulating material and leave until fully cured.

Tip! It must be remembered that when heating the mortar, its mobility and plasticity are significantly reduced, so a plasticizer must be added to the water to prepare the solution.

Warm formwork

More progressive, compared with the previous method, is pouring the building mixture into insulated formwork with electric heating. This method allows you to perform construction work regardless of the season and climatic conditions and does not require the use of special antifreeze additives.

However, the purchase price and operation of such formwork systems is quite high, so their use makes sense only in the case of mass serial construction.

Heated monolithic construction

The use of certain types of antifreeze additives for concrete can cause corrosion of metal elements of the reinforcement, therefore, to combat the harmful effects of low temperatures, artificial heating of the concrete solution, which has already been laid in the formwork, is often used.

For these purposes, depending on the specific conditions, various sources of heat are used.

1. Electric heating by means of electrodes, which are immersed in the thickness of the solution, occurs due to the electrical conductivity of water and mineral salts dissolved in it. Two electrodes are immersed in the solution at a certain depth in different parts of the formwork, and when electric current is supplied to them, it passes through the thickness of the solution, causing water to heat up.
2. Hot steam heating is performed using a steam generator installation, which supplies heated steam to the inside of a special heat-insulated casing installed directly around the formwork.
3. Sometimes laying of the heating cable is used, but it should be understood that this is an extreme measure, because it will no longer be possible to remove it from there, and it will remain there forever.

1. For warming up large areas above the place of pouring the formwork, a special tent made of canvas or awning fabric is erected. Diesel or electric heat guns, or alternatively, infrared emitters are installed inside the tent. They heat the concrete surface of the mortar and the air inside the tent, and heat is transferred into the mixture through the thermal conductivity of the concrete.

The latter method is convenient to use for pouring a monolithic slab under the foundation of a residential house during individual construction. It should be noted that all the necessary communications should be carried out in advance, but if it so happened that they did not, for drilling through the solid slab, you can use diamond drilling of the holes in the concrete after it is completely cured.

The choice of brand of concrete

Table 1 presents the most suitable brands of concrete for various objects of individual housing construction, suitable for use in the winter period.

Table 1 (compiled in accordance with

SNiP 2.03.01-84).

Conclusion

After reading this article, it becomes clear that even despite the adverse effects of negative temperatures, there are a lot of ways to help solve the problem of pouring concrete solution in the cold season.

For more information on this issue, it is recommended to watch the video in this article or read similar materials on our website.