Pouring concrete at sub-zero temperatures - features and technology. Pouring concrete at low temperatures Concreting at low temperatures

At the turn of the seasons, the question of at what temperature it is possible to pour concrete under the foundation is of particular relevance so as not to endanger the structural elements of the new building. To get an answer, it is necessary to understand the ongoing processes inside the cement slurry.

Hydration of concrete - thickening and hardening of cement mortar

Hydration is the chemical process of hardening a mixture of cement and water into stone. It begins after mixing the mortar, however, the concrete setting occurs only after the foundation is poured. While the mixture is mixed with a mixer or concrete mixer, it is subjected to vibrations, due to which hardening is delayed, and the quality of the concrete does not suffer at all. After the mortar is removed from the concrete mixer, the material begins to harden and goes through two stages:

1. 1. At the first stage, the mortar sets. This process, depending on the composition of the mixture and external conditions, takes from 1 to 20 hours.
2. 2. At the second stage, the mortar hardens and gains up to 90% of its final strength. This process takes place within 28 days. After that, the concrete continues to harden throughout the entire service life.

The course of hydration directly depends on external factors, in particular, on the ambient temperature. At +5 degrees Celsius, the setting process begins after 2 hours and lasts up to 10 hours. At +20 degrees, 3 hours after pouring the concrete, its hardening begins, while the setting has not yet had time to complete. The hardening of cement is due to the components that make up its composition: tricalcium aluminate, tricalcium silicate, dicalcium silicate, tetracalcium aluminoferrite.

Answering the question at what temperature can the foundation be poured, we can say that ideal conditions for hydration are as follows:

1. 1. The temperature of the mixture is +30 degrees Celsius in normal weather conditions, up to +70 degrees in the cold months due to the heating of the components (water, sand and gravel) warm air or ferry.
2. 2. Temperature environment(air) - from +5 to +30 degrees Celsius.

Due to the fact that the solidification (hardening) temperature of concrete has a framework, when pouring the foundation, it is necessary to take into account external conditions.

Rules for concreting the foundation in the summer months

It is believed that pouring the foundation in winter is the most difficult, because due to sub-zero temperatures, the solution may freeze before the completion of the hydration process. However, the heat is no less dangerous for a fresh foundation than winter frosts. Due to the high outside temperature during the chemical reaction of the cement, the mixture will increase in volume. After the formation of the cement stone, it will inevitably cool and decrease, but this will not happen due to the formed crystalline structure.

As a result, freezing high temperature leads to strong internal stress in the foundation before construction begins structural elements a new house, and the owner will receive a foundation in operation, capable of cracking at any time. Usually, their first signs appear already a few hours after laying the solution inside the formwork.

High summer temperatures force builders to use special fast-hardening Portland cements if foundation pouring is planned to be carried out at temperatures above +25 degrees and at air humidity less than 50%.

The brand of cement used should be one and a half times the design parameters of concrete, in addition, to increase operational qualities foundation, plasticizers and all kinds of modifying additives will have to be added to the mixture, due to which hydration will slow down. For concreting, we recommend using the least hot hours of the day (early morning or evening). Another danger to the foundation of a house in the summer is the possible dehydration of the solution. To protect concrete from water evaporation, you must:

• cover the surface with chips, sand or sawdust;
• moisten the foundation from the watering can;
• water wooden formwork from buckets.

Winter work – construction at low temperatures

At low temperatures hydration does not accelerate, but slows down, as a result of which the foundation does not have time to gain the strength it needs. At zero degrees, the process of solidification of the solution in the outer layer stops completely, and inside the structure it continues for several hours until all components have cooled down. In this case, water does not have time to react with cement, after which it freezes, increases in volume and literally breaks the structure of concrete.

Specialists are well aware at what temperature concrete can be poured for the foundation without fear of losing its performance. Frosts are a significant obstacle to concreting, therefore, in winter, specialists are forced to use special technologies and means:

• heating of concrete elements before pouring and formwork heating;
• laying a heated cable inside a slab or;
• feed to reinforcing cage elements electric current for heating;
• installation of heaters around the foundation for the first three days;
• creating a greenhouse by covering the entire perimeter of the foundation with a film;
• introduction to the mixture special reagents, which reduce the t of water crystallization or accelerate the hardening time of the foundation.

The use of any of the described technologies significantly increases the cost of building a foundation. Therefore, it is difficult for private owners to recommend these funds. There are other ways to pour concrete when sub-zero temperature. One of them is to change the amount of water and cement in the produced solution. It requires the most accurate and competent dosage of the used components of the mixture. Calculate at home required amount water and cement to improve the hardening of the foundation in winter is almost impossible.

• heating two-thirds of the water for the solution to t of the order of 70 degrees Celsius;
• adding sand and gravel to the water;
• adding the remaining third of the water and cement to the mixture.

Mixing the solution should be done twice as long as usual. The time of vibration compaction also increases by 1.5 times. Before pouring the solution, the formwork must be checked for ice, snow removed and the underlying layer warmed up. We cover the poured solution polymer film, which will keep the mixture warm for a while. If the above rules for concreting the foundation cannot be observed at home, it is recommended to wait for the air temperature to rise above +5 degrees. With a favorable forecast for the next three weeks, the owner of the house will be able to fill using standard technology.

The dependence of the cost of work on the weather - how to save on the foundation?

Almost all modern construction companies work on pouring concrete summer months, in late spring and early autumn. All this is explained very simply - at this time it is cheapest to carry out construction in general and build a foundation in particular.

Building a house in winter can lead to additional and extremely significant financial expenses:

• with the most optimistic forecasts, the construction budget in the cold months increases by 25-30% ;
• thaws in winter do not eliminate the need to use expensive reagents in the construction of the foundation;
• in winter it is impossible to prepare the solution on the spot, therefore the owner will have to additionally order the delivery of concrete by mixers;
• a transformer substation is required to heat the foundation, since the home electrical network cannot withstand such high loads.

The only advantage of building a foundation in winter months lies in the possibility in early spring to start erecting the structures of the facility and complete the construction, necessary finishing and other works by the end of autumn. In practice, the owner has to invest colossal funds in winter construction, which will definitely overshadow the joy of building a private house.

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

Features of concrete mix

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

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

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

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

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

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

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

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

Concreting using the mortar's own heat

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

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

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

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

Concreting with heat supply from external sources

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

The device of greenhouses

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

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

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

Electrical heating of concrete

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

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

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

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

Cold concreting

For the possibility of concrete gaining critical strength under the conditions of pouring the solution at negative temperatures, antifreeze additives are widely used. They support the cement hydration reaction, prevent premature freezing of moisture in the mixture, normalize the concrete hardening process, and have the following positive qualities:

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

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

The most common antifreeze additives for concrete preparation are:

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

The cold concreting method has some disadvantages:

• its use is prohibited in prestressed structures;

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

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

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

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

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

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

Industrial construction does not stop during the cold season, but hardening of concrete at low temperatures is problematic. Crystallization of water leads to a decrease in strength, which negatively affects the quality of structures. The use of antifreeze additives in concrete will help to solve the problem, wide selection offered by the industry today.

The use of antifreeze additives allows pouring mortars at temperatures up to -50ºС. They are liquid or powder formulations that are added to the mixture. It is important to know that concrete additives added during frost provide only 30% hardening. The final hardening occurs after the defrosting of the monolith.

There are several types of antifreeze additives that differ in the principle of action:

• Compositions that lower the freezing point of the liquid, as a result, the process of cement hydration continues, setting proceeds according to the standard mechanism;
• Additives based on sulfates accelerate the compaction of concrete, while generating excess heat, which increases the rate of hydration of the cement;
• Complex additives increase the solubility and activity of cement laitance, while the compounds that appear as a result of reaction with water reduce its crystallization temperature.

Varieties

There are various antifreeze additives, each of which has its own mechanism of action. A popular additive is sodium carbonate, otherwise known as potash. This is a powdery crystalline composition that accelerates the hardening of the concrete mixture. The use of this type of additive reduces specifications material, including strength. To reduce this effect, sodium tetraborate is added to potash.

Attention! Potash belongs to dangerous substances, therefore, when working with it, safety requirements are observed.

Sodium tetraborate is a complex substance consisting of sodium and calcium salts, with the addition of ammonium. It is an additional antifreeze additive used with sodium carbonate. Without it, the structure can lose up to 30% of its strength after thawing and complete curing.

Sodium nitrite is an effective antifreeze additive that reduces the crystallization temperature of water, accelerates the hardening of the composition, and has an anticorrosive effect. Its use is dangerous, since sodium nitrite powder is a flammable, explosive, toxic substance. It is used in frosts down to -25ºС. It cannot be miscible with lignosulfonic acids, as it releases toxic gas when interacting with them. Calcium nitrite-nitrate has similar properties.

Sodium formate is an antifreeze additive for concrete that reduces the crystallization temperature of water and accelerates the hydration of cement. It is added in a proportion of not more than 6% of the total mass of concrete. To improve plasticization, naphthalene lignosulfonate is added to the additive.

Urea - PMD, prolongs the liquid phase of water, practically does not affect the setting speed.

calcium and sodium chlorides, ammonia water reduce the freezing point, but have an increased corrosive effect. They strongly influence metal elements therefore not recommended for use in reinforced concrete products.

How to use

The choice of additive and the method of its application depends on the conditions and material where it will be introduced. Any concrete additives used at sub-zero temperatures are introduced into the solution with water, according to the manufacturer's instructions. After thorough mixing, it is recommended to wait a while for this component to diffuse in the composition.

According to SP 70.13330.2012, in order for the composition to achieve the desired strength, it is important that before the temperature of the composition reaches the mark for which the additive is designed, it gains no more than 20% of the planned strength.

Consumption of antifreeze additive per 1 cubic meter material depends on the average daily ambient temperature. Up to -5 degrees, it is recommended to add up to 2% of the additive by weight of the solution, up to -10 degrees, this figure increases to 3%, up to -15 degrees, no more than 4%. At severe frosts calculation is made individually for each type. The rate of hardening of the solution is reduced, and the maximum strength is achieved after the end of frost.

When adding plasticizers and PMD, it is necessary to observe certain rules work. The recommended range of the poured solution is from +15 to +25ºС. To dissolve the additives, a certain amount of water is required, which must be heated, this ensures complete dissolution of the substances. Sand and gravel used in the solution are also heated immediately before adding. Cement must not be heated, as it will lose its astringent properties. The poured solution must be covered, this is especially true during snowfall.

Advantages and disadvantages of additives

The main advantage of concrete with antifreeze additives is the ability to perform work all year round. Picked up in correct proportion, they improve the adhesion of the components, improving the quality of the solution. They also have other advantages:

• increasing the service life due to the compaction of concrete;
• increase the plasticity of mixtures, making it easier to work with them;
• the frost resistance of ready-made concrete increases, which is important for elements of load-bearing structures;
• application of PMD is the most cheap way pouring at negative temperatures;
• the use of additives reduces shrinkage during solidification, while maintaining the integrity of the structure;
• antifreeze additives fill the pores of concrete, thereby significantly increasing its water resistance;
• some compositions significantly increase the corrosion resistance of the monolith, extending the service life of structures, buildings and structures at times.

The use of antifreeze additives has its drawbacks. If used incorrectly, the strength characteristics of concrete are reduced, therefore, when working, it is necessary to strictly adhere to the instructions. Some additives are flammable and toxic, which must be taken into account when working with them. Even with additives, the hardening rate will be relatively slow in frost. To achieve the desired strength when laying in winter, it is required large quantity cement, which increases the cost of construction.

Do-it-yourself anti-frost additives

In private construction, if it was not possible to carry out all the work in the warm season, it is necessary to continue in the winter. Therefore, it is allowed to prepare anti-frost additives for concrete with your own hands at home. To do this, you only need table salt or sodium chloride. Such an additive reduces the freezing point of water and reduces the time of critical solidification of the mixture.

To make a do-it-yourself antifreeze additive in concrete, salt is dissolved in water and added to the solution. The concentration up to -5 degrees is 2% by weight of the solution, up to -15 degrees, mass fraction chloride reaches 4%.

The disadvantage of this additive is that it is corrosive to metal, so it is not suitable for iron concrete structures. It should be borne in mind that the rate of solidification of the solution at negative temperatures will be, on average, 3 times lower than under normal conditions.

Construction in modern world you can start not only in spring, summer and early autumn, but now late autumn and even in winter.

And this became possible with the invention of antifreeze additives. We will talk about this interesting additive later.

Application conditions

Such additives make it possible to use low-quality instead of expensive high-quality ones, and this makes it possible to reduce the cost of concrete products. Due to this, it is also possible to reduce the thickness of the product, for example, floor slabs, moreover, without compromising quality.

About the use of antifreeze additives

How to use

How to cook containing special additive?

An antifreeze additive is added to concrete along with other ingredients (sand, water, plasticizer). Better supplement add along with water, because the additive can replace water. Everything must be mixed well with a concrete mixer for an hour, until a homogeneous mass.

The additive is introduced into the mixture according to the instructions on the package. This amount is approximately 0.2% of the amount of cement at an air temperature not exceeding or decreasing the value indicated on the package.

In the event of a decrease in temperature, the dosage of the additive should be increased at the rate of 0.05% per degree. On the packaging, the dosage is indicated in grams.

In the event that the dosage of the additive is indicated in milliliters, then in order to determine the amount of the substance, its mass is divided by 1.45. The result obtained will be the volume in milliliters. Then we measure right amount and add to the mixture.

Antifreeze additives are used only at sub-zero ambient temperatures. They cannot be used under other conditions! After pouring concrete, structures made of this material are moistened for about two days.

Construction objects

The use of antifreeze additives makes it possible to build at any time of the year and under any weather conditions. Due to this, the construction time of facilities is reduced, as well as the cost is reduced due to rational operation. construction equipment and labor force.

Antifreeze additives are used:

• in the manufacture of heavy and light reinforced concrete structures,
• in the construction of monolithic reinforced concrete structures,
• it is added to
• when laying pavement and production of road elements,
• build bridges, oil and gas platforms, various buildings, dams, dams and much more.

Antifreeze additives are applicable at air temperatures from -15 to approximately -25 ° C, depending on the substance used.

Antifreeze additives are delivered to stores and retail chains in containers with a volume of 1 m3 or 5 liters each.

If you are thinking of building in the cold season, we recommend that you use antifreeze additives and be sure to carefully read the instructions on the packaging of additives.

We hope that this article was useful to you. We wish you good luck in your endeavors!

As early as the beginning of the last century, work with concrete mixtures were seasonal. In winter, concrete was not laid due to the loss of the strength characteristics of this building material. The builders tried different ways shift the schedule of concrete laying closer to the beginning of persistent frosts. To do this, the surface of the concrete monolith was insulated using various organic materials: sawdust, peat crumbs, reed mats woven for this purpose.

In parallel, scientists were trying to create concrete, the setting temperature of which would be below zero degrees. But since the builders were not satisfied with the gain in time, the search for alternative insulation (heating) of concrete at sub-zero temperatures continued.

Acceptable mixture temperature

During the course of the research, scientists determined what is the temperature of the concrete best for getting quality designs. Its values ​​are in the range between +5 and +15 degrees. The boundary indicators that were worked out by the researchers are minus 20 and plus 45 degrees. At outdoor air values ​​from +5 to -3 degrees. the temperature of the freshly prepared product is not allowed below +5 degrees. These indicators are suitable for a cement mass of 240 kg / cu. m (with brand M200 and more). If less cement is used, the temperature index of the mixture should correspond to +10 degrees. or higher.

Ways to increase the setting temperature of concrete

If it is necessary to pour concrete in winter, the temperature of the mixture can be increased in the following ways:

* through the use of heated water;

* by introducing frost-resistant additives into the mixture;

* with the help of electric heating;

* the method of steaming concrete structures in stationary conditions in special autoclaves up to a curing of 80-85%;

* using electric heating of a concrete monolith, which has reinforcement in its composition. In this case, the electrodes are switched over the entire area of ​​contact between the reinforcement and concrete when a low voltage current is connected;

* by using heat guns with concrete mix enclosure.

Dependence of concrete quality on outdoor air

Does the strength of concrete change with outside temperature? Certainly. When working with building materials winter season chemical reaction accompanying strength gain decays. Therefore, at negative temperatures, hardening will stop. "Save" the mixture of additives in the form of various salts that can stop the formation of ice.

There is a situation when the product began to set, but then froze. In this case, after thawing, it will harden only in the absence of internal damage by freezing water. Experts allow a one-time freeze-thaw cycle, subject to the condition: the temperature of the mixture for three days should not fall below +10 degrees.

If you know certain requirements, then in winter concreting can be done no worse than in the most favorable period. The first condition is the competent delivery of the material. Best Option- use . The second is to build an insulated formwork, it is even better to take care of heating the concreted area.

Speaking about at what temperature to pour concrete into summer period, it should be noted the fact of a decrease in the strength of the product at +30 degrees. A practical way out is to moisten the concrete surface with water. In summer, due to evaporating moisture, concrete is made more liquid. And of course, it should be reported at what temperature to pour concrete in winter - it is recommended to perform all work up to -15C.

Reacting to the effects of temperature, concrete sets more evenly in wet and cool weather in summer. And if the work is carried out during the rainy period, then the resistance of the material to moisture is increased special cement. To prevent the solution from being washed away, the site is covered with polyethylene. However, in heavy rains lead concrete works outdoors is not recommended. If construction begins in a new climate zone, then experts advise testing concrete for strength in laboratory conditions or at a construction site.

Air temperature affecting concrete is not the only factor influencing given material. The quality of the product depends on the humidity of the environment, solar radiation, wind speed and how the mixture is handled.

And now, briefly:

At what temperature can concrete be poured? (on the street / in the foundation / in winter and summer)?
Optimum temperature- 5 to 20 degrees C above zero. With the use of additives and heating of concrete in winter to minus 20 degrees C.