At what temperature can the foundation be poured: a step-by-step process of how to pour the foundation in winter. Pouring concrete at low temperatures: useful recommendations Concrete additives at low temperatures

In most of the territories of our country, cold or cool temperatures persist for more than half of the year. Considering that at concrete work ah "winter" begins with a temperature drop to +5 o C, then the "window" for work with concrete is very small. However, it can be extended, and significantly, through the use of various means. These are the so-called technologies of winter pouring of concrete.

During the normal course of the concrete curing process, moisture serves as a "gluing" element for cement particles. Upon her transition to solid state all processes are stopped.

But this is not the only problem. It is known that when freezing, the volume of water increases by about 9%. As a result, inside the mass of concrete is formed high blood pressure. If the grains of cement up to this point have not yet gained a certain level of strength, they are destroyed under pressure. After freezing, they will no longer fully acquire their properties and the concrete will not be strong enough.

There is another unfavorable moment in the winter pouring of reinforced foundations. Steel is an excellent conductor of heat, and it facilitates the removal of heat from the thickness of the concrete. Possessing good heat-conducting properties, the rods quickly cool down. Around them, the water freezes first. Ice pushes concrete particles aside, and in their place comes water that has not yet frozen from the still warm layers. It also freezes, pushing the concrete further back. As a result, the array is no longer a monolith: the frame is not connected to the concrete stone. The strength of such a base after thawing and final hardening will be several times lower.

Of all these processes it follows that than less water in an incoherent state will be at the time of freezing, the less will be the loss of strength. Through various experiments and calculations, the boundary strength values ​​were determined at which concrete can be frozen. They are called the point of critical strength. Depending on the class of concrete and the purpose of the building, the type of use of the structure, it is required to wait for the maturation of some compositions by 20%, for others, all 100% is required.

For reinforced concrete with non-tensile reinforcement (the type used in private housing construction), it is 50%, for foundations that will be subjected to alternate defrosting / freezing (baths and country houses without heating) - 70%. After reaching this point, the foundation can be frozen. After thawing, all processes in it will resume. The loss of strength in this case is no more than 6%.

Concreting methods in winter conditions

The speed of the hardening process depends on the temperature of the solution. With its increase, the activity of water increases significantly, the rate of curing increases. Therefore, when carrying out concrete work in winter or at temperatures below +5 o C, it is important to create and maintain the required level of heating. The optimum aging temperature of the solution is from +20 o C to +30 o C. There are several ways to do this:

• make the solution warm;
• insulate the formwork;
• use additives and additives that accelerate hardening and/or lower the freezing point of water;
• preheat the already poured concrete mass.

All of these methods work well. They are used singly or in combination.

First of all, it is necessary to choose the right cement for winter concreting foundation. It is known that during the hardening of concrete, reactions occur in which heat is released. For winter, it's a great feature. At the same time, fast-hardening Portland cements and compositions of high grades emit a greater amount of heat. Therefore, for kneading at low or sub-zero temperatures it makes sense to buy them.

Only this will allow you to fill the foundation with a strip or slab foundation at positive temperatures during the day, and slight frosts at night. But at the same time, it will be necessary to make the batch warm (read below), and after pouring the foundation, the formwork will need to be thermally insulated: covered with mats, straw, etc. If you have already purchased a heat insulator, you can use it, you just need to monitor its condition, cover it with a film or other moisture-proof materials.

Temperature increase during kneading

During the winter pouring of the foundation, the temperature of the solution is adjusted to 35-40 o C. To do this, heat the water and backfill. In no case should the cement be heated: it will “brew” and become practically useless.

It is good if it is possible to use an electrically heated concrete mixer: it is connected to the network and the drum is heated. In another case, it is advisable to preheat it by scrolling through the heated water.

When kneading, water is heated to 90 o C. Crushed stone and sand must be heated to 60 o C. This is done by blowing hot air, heating in special ovens. Furnaces are for a private builder from the realm of fantasy, but you can arrange hot air blowing. For example, from a stove or a fire, stretch several duct pipes into a pile of rubble or sand.

Once again, we pay attention: do not heat the cement. It can be brought into a warm room so that it takes room temperature but cannot be heated.

During the winter mixing of the solution, the order of laying the components changes: water is poured, crushed stone and sand are poured into it. After several revolutions cement is added.

In addition, it is necessary to increase the kneading time. It should be longer by 20-50%: due to better mixing, reactions are activated and the temperature rises during hardening.

Warming and heating solution

To extend the cooling time of concrete, it is required to retain heat to the maximum. Because using everything possible means and available materials, carry out the insulation of the walls of the formwork. You can use tarpaulins, mats, some old warm clothes, fill the gap between the walls of the formwork and the soil, straw. Anything, as long as the heat doesn't leak into the air.

One of the tasks is to keep the heat of the solution

During construction in industrial scale electrical heating is also used different kind electrodes. They can be located on the surface, fixed on the formwork or introduced into the concrete solution. The method is effective, but rarely implemented in private construction. This pleasure is very expensive: the consumption of electricity for heating a cubic meter of concrete is 60-80 kW / h. At the same time, it is necessary to strictly control the temperature: measure it every two hours (or more often) and turn it off when it reaches +30 o C. Then turn it on again after a while. Control must be around the clock.

When pouring the foundation with your own hands in winter, it is realistic to use only heating cables. They are attached from the inside to the formwork, and after it is removed, they are dismantled. There is a second option - to "drown" the wire in concrete. Both methods work well, but only if the walls are insulated from the cold.

There are also on sale special heating mats for heating concrete. They are laid out on the surface, included in the network. Its cost is 2.5 thousand rubles / m2.

To maintain the temperature, there are greenhouses above the object. These are structures very much reminiscent of greenhouses. And their task is similar: to keep warm. The frame is erected, it is covered with a film or other similar materials. Putting a stove inside heat gun etc., with their help maintain positive temperature. But at the same time, it is also necessary not to forget about moistening so that the moisture from the solution does not evaporate.

Another method of concrete heating is using infrared emitters. This method is good because, under the influence of waves, the solution itself is heated directly. The emitters are covered with aluminum casings, creating a directed flow. However, for effective heating, a large number of lamps will be needed.

Additives and Additives

Another way to pour concrete negative temperatures- the use of chemicals. Some of them accelerate curing by initial stage process. Mass fraction all additives - no more than 2% by weight of cement. Large quantities can adversely affect the quality of concrete, so stick to the recipes.

One of the methods of winter concreting is the addition of special antifreeze additives to the batch.

The most common additive that increases the “frost resistance” of concrete and accelerates its hardening is calcium chloride. Potash and sodium nitrate are also used. If you add them during normal mixing, the freezing point will drop to -3 o C.

One "BUT". Chlorides cannot be used for reinforced concrete - they provoke the rapid destruction of steel. So the most common concrete hardening accelerator - calcium chloride - is not suitable for pouring the foundation.

Pouring concrete at sub-zero temperatures is possible if the solution is heated with the same additives. In this case, you can work at -15 o C. But for the normal quality of the foundation, you will need to insulate the fill and follow simple but mandatory rules.

Rules for winter pouring concrete

The solution is poured into the prepared formwork. Preparation consists in removing ice and snow, heating the reinforcement and the bottom of the foundation. This is the most difficult stage. Scraping off ice is half the trouble, but warming up the reinforcement and the entire perimeter of the foundation is a problem. The temperature should not be high, but it is necessary to achieve its positive values.

As an option, you can consider the device of portable braziers, which are lowered into the pit and kindled there. It is possible to use heat guns powered by gas cylinders. The use of other means is difficult due to their high cost.

For this reason, concrete in winter slab foundations problematic: such areas cannot be heated. For this type of bases, "winter" will be limited to light frosts at night and positive daytime temperatures. Filling can be started after the reinforcement and the bottom have a positive temperature.

The strip foundation can also be poured during frosts: it is realistic to heat such a base and reinforcement in a limited volume. Not easy, but possible.

Everything can be done step by step. Break the entire tape into small sections, start heating at the same time or with a certain time interval on several of them (two or three, depending on the time required for kneading and heating the pit). Start pouring one area by moving the braziers further. While the first heated area is being poured, the next one will reach the required temperature. The flooded area is immediately covered with heat-insulating materials and proceed to the next, and moving along the entire perimeter.

It is imperative to knock down the ice and heat the reinforcement - this is the only way the foundation will be strong

The mechanism is clear. So it is possible to pour the foundation with concrete at -15 o C (but with appropriate additives, "hot" mixing and measures to preserve heat).

One more important condition- work must be carried out continuously. In winter, it is impossible to fill the foundation in parts. This is 100% correct. The gap between fillings should be such that a film does not have time to form on the surface of the previous part, and even more so that moisture does not freeze. Work must be carried out continuously until the end of the pour. The flooded parts must immediately be covered with heat-insulating mats. As you can see, several people are needed for this work. One cannot cope with all tasks.

note that Maximum temperature solution should be 35-40 o C. Its excess leads to a slowdown in the curing process. The situation will, of course, be better than when freezing, but not by much.

Results

Pouring the foundation in winter is not an easy task, but it is possible even with your own hands. Need helpers and careful preparation, but you can make a normal base at sub-zero temperatures. At what temperature can concrete be poured? Depends on its composition, but for private traders it’s real, albeit with at great expense, achieve normal quality at temperatures not lower than -10-5 o C. Filling at positive temperatures during the day and frost at night will turn out to be less expensive.

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 mixture, cement and water are involved in the hardening process with the formation of a concrete stone. 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.
• Addition of modifiers to the concrete mix, accelerating hardening or shifting 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 special additives to the concrete mixture. Let's briefly review each of the methods.

Heating the mixture during cooking

The method consists in preheating the components of the mixture (except cement) so that at the time of laying the mixture its temperature is 35-40 o C. Sand and gravel are heated to about 60 o C, water - up to 90 o C. The cement is first placed in a heated room and warmed up to room temperature. It is impossible to warm up the cement - it quickly irretrievably loses its activity and becomes simply unusable!

For kneading, mixers with or without heating can be used. At large construction sites, aggregates can be heated in special drying ovens. A private developer can use the hot air of a heat gun or heat from a stove transferred through an air duct.

Winter mixing of concrete 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 are introduced at a dosage of 2 - 15% of the mass of cement in the mixture. Yes, at design temperature concrete - 10 o C - 15 o C requires potash in the amount of 10% by weight of cement, and at a temperature of - 21 o C - 25 o C - in the amount of 15% by weight of cement.

Potash is a popular additive for winter concreting.

Benefits of winter concreting

Concrete laying and curing at low temperatures for Russian conditions often a necessity. Residents in many regions can count on just a few months of the year when concrete can be laid without creating special conditions for hardening. Postponing work for 7-8 months is unreasonable, therefore, it is natural to know how to properly pour concrete in frost and achieve standard hardening for our conditions. There are clear advantages in winter concreting:

• Frozen soil in winter can be used as the most durable formwork, since it is rigid, does not crumble and does not require strengthening.

• In winter, the price usually drops sharply. building materials, therefore, it is possible to significantly reduce the cost of construction.

• Most of the construction crews are out of work waiting for the season, so it is possible to agree on work on pasture conditions.

The negative factor is low speed process, but nothing can be changed here - the technology of laying concrete at low temperatures has its own laws: the acceptance of concrete on the foundation (or floor) must be continuous, i.e. "cold seams" are excluded. Otherwise, it is important to choose the right best method concreting and exactly fulfill the requirements of the technology for the care of the laid mixture.

Pouring concrete at low temperatures updated: December 13, 2016 by: Artyom

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 the optimum temperature for pouring concrete is 17-25 ° C, since such conditions allow obtaining 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 at low temperatures. There may be several reasons:

• limited building time summer period 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.

Construction work is not always carried out under favorable conditions. 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 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 grade strength of the hardened mortar 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.

With an increase temperature values to positive values, water thaws, cement hydration reaction resumes, and 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 to prevent freezing of water 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 temperature fluctuations 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 should be at room temperature, is done last, after a few rotations 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.

During winter concreting, heating of the laid mortar with steam is sometimes 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

Creation of conditions for the maturation of concrete at sub-zero temperatures can be carried out using electric current connected 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

Antifreeze additives are widely used to enable concrete to gain critical strength under conditions of mortar pouring at low temperatures. 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 the mobility of the concrete mass, making it easier to work with it;
• protect metal fittings from corrosion.

Antifreeze additives should be used only at negative 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 the preparation of concrete are:

• potash and other substances, which are based on salts of monocarboxylic acids, accelerate the action during concrete hardening. They do not cause reinforcement corrosion, do not form efflorescence on the surface of the solidified solution 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.

General provisions. The concept of "winter conditions" in the production of concrete works differs from the calendar one. "Winter conditions" for a specific construction site begin when the average daily outdoor temperature drops to + 5 ° C, and during the day it drops below zero.

At temperatures below 0°C, hydration processes in concrete stop, i.e. interaction of cement minerals with water. The hardening of concrete is suspended, as the concrete freezes, turning into a monolith, the strength of which is determined by the forces of freezing. Internal stresses appear in concrete, caused by an increase in the volume of free water by about 9% when it freezes. These stresses break the fragile adhesive bonds between the individual components of concrete, reducing its strength. Free water, freezing on the surface of aggregate grains in the form thin film, prevents the adhesion of the cement paste to the aggregate. It also worsens the strength properties of concrete.

After thawing of concrete, hardening at a positive temperature resumes, but the strength is lower than the design one, i.e. the one that would be achieved by curing under normal conditions. Other properties of concrete also decrease: density, durability, adhesion to reinforcement, etc. The properties of concrete deteriorate the more significantly, the earlier it freezes after laying. If concrete gains a certain strength by the time of freezing, then the negative effect of freezing on its properties is small: after thawing, the strength of concrete can reach the design value. In this case, the adhesive bond between the cement paste and the aggregate is much greater. internal stresses. Therefore, the probability of deformations in the contact zone is less.

The minimum strength of concrete at the time of its freezing, sufficient to achieve its design strength after thawing, is called critical. This strength for concrete in structures with non-stressed reinforcement should be at least 30 ... 50% of the design, depending on the class of concrete, and not less than 50 kg / cm2. In prestressed structures, it should be at least 70% of the design value. If the structures are supposed to be loaded in winter, then by the time of freezing, the concrete strength in them should reach 100% of the design value.

To obtain concrete of design quality in winter conditions, it is necessary to provide for it a temperature and humidity regime in which the physical and chemical processes of hardening are not disturbed and do not slow down. The duration of maintaining such a regime should ensure the achievement of critical or design strength.

The task of "winter" concreting: to obtain concrete of a given strength. For this, general activities and various technologies for ensuring the normal mode of concrete hardening.

General events:

a) Work is carried out on heated concrete mix. This mixture at the time of laying in the structure must have a positive temperature, the opposite of the ambient air temperature. This is achieved by heating water, crushed stone and sand (steam) during the preparation of the concrete mixture at the plant.

b) To prevent cooling on the way, the body of the dump truck is closed from above with shields, and from below it is heated by exhaust gases from the car engine through the arranged double bottom of the body.

c) Buckets and bunkers are covered with insulated wooden lids, and sheathed on the outside. In severe frosts, they are periodically heated with steam. Concrete pumps are installed in heated rooms. Before starting work, it is pumped through the concrete pipeline hot water. Pipe links of the main concrete pipeline at a temperature below minus 10 ° C are enclosed in thermal insulation together with a heating rough pipeline.

d) Before laying the concrete mixture, the formwork and reinforcement are cleaned of debris, snow, ice. To do this, if necessary, blowing with hot air from heaters or steam is used, as well as washing with hot steam, followed by blowing with hot air.

e) In frosts below minus 15°C, reinforcement made of rods with a diameter of more than 25 mm and rolled profiles is heated to plus 5°C to ensure good adhesion of concrete to reinforcement. For the same purpose, protruding beyond the insulated formwork metal elements after heating, they are insulated at a length of at least 1.5 m from the block.

f) The quality of concrete is strongly influenced by the condition of the base on which it is laid. It is important to exclude early freezing of concrete at the junction with the base and subsequent deformation of the heaving soils of the base.

Before foundation concreting heaving soils heated by steam, fire, or electricity. Non-heaving soils do not warm up. The temperature of the mixture to be laid must be at least 10°C higher than the temperature of the substrate. It is not allowed to lay the concrete mixture on frozen ground (“frozen” base).

If it is necessary to lay the concrete mixture on the previously laid and frozen concrete, it is heated to a depth of at least 400 mm and protected from freezing until fresh concrete acquires critical strength.

g) When concreting, to reduce heat losses, the concrete mixture is laid in small sections along the length and width, so that the previously laid layers overlap with new ones faster, and the concrete temperature does not have time to fall below the calculated one.

h) Concreting is carried out around the clock without interruptions, since the preparation of frozen working joints is very laborious and the required quality is not always ensured.

Technologies that ensure the normal mode of concrete hardening:

1. Application of chemical additives.

Chemical additives lower the freezing point of the liquid part of the concrete mixture, which ensures concrete hardening at temperatures below 0°C, which increases the curing time.

This method is relatively inexpensive (additional cost compared to normal conditions(price increase) about 16%) and is widely used in construction. As additives are used: sodium chloride, calcium chloride, potassium carbonate (potash), sodium nitrite, etc.

Additives are introduced into the concrete mixture during its preparation. Depending on their number, a given effect is obtained:

At 1-2% by weight of cement - acceleration of concrete hardening;
- at 3–5% by weight of cement – ​​lowering the freezing point by 5–10°С;
- at 10–15% of the weight of cement - complete elimination of freezing " cold concrete”, but at the same time, curing continues for 40–90 days.

2. Concrete heating.

a) Thermos method. The heat generated during chemical reactions concrete hardening. For this, the structure is additionally insulated.

The method is effective for massive structures of a simple shape, especially for buried structures and structures on the ground and in the ground (foundations, basement walls, foundations for equipment, floors on the ground, etc.). To enhance the effect in the preparation of the mixture, cements with increased heat release are used.

b) Steam heating. A "shirt" of roofing material, wooden or steel panels is arranged around the concreted structure, under which steam is supplied (Fig. 4.52). The “jacket” provides the necessary heating of the structure and humidity (does not dry out the concrete).

Low pressure steam 0.5–0.7 atm is used. with a temperature of 80–90°С. Approximate mode of steam heating: rate of temperature rise (gradient) no more than 5–10 deg/h; isothermal heating at a temperature of 80°C for concretes on ordinary Portland cement and 95°C on slag Portland cement and pozzolanic cement. The cooling rate (gradient) of concrete should be 10 deg/h. Steam heating of concrete can be carried out until it gains design strength, which is especially important for our eastern and northern regions, where the "winter period" is
8... 10 months.

The method is used to warm up various concrete structures, but only where there is steam in the required quantity.

in) Electrical heating. Internal - with the help of electrodes. Heat is released when an electric current is passed through a wet concrete mix. The electrodes may be embedded in freshly laid concrete or heating wires may be embedded in the structure prior to concreting. The number of electrodes, heating wires in each case is determined by calculation.

The advantage of the method is simplicity. Disadvantages - the complexity of control (round-the-clock surveillance) and high cost.

External - heat is released by "heating" formwork or heating flexible electrical cords.

3. Concreting in greenhouses. Above the concreted structure or part of it is arranged light frame fencing made of tarpaulin, film, etc. (tent) and warm air is supplied under it or heaters are placed inside the tent. Under the tent (temperature plus 5–10 °C), concreting is carried out under normal conditions.

Depending on the task, the greenhouse can “work” for 3–16 days, until concrete reaches 50% of the design (design) strength, or all of the design 28 days.

4. Concrete heating infrared rays(penetrating heating).

The peculiarity of the method is that the transfer of heat to concrete (warming up) occurs throughout the entire thickness of the structure simultaneously and with the same intensity (Fig. 4.53).

For heating monolithic concrete heating elements of the NVSZh type (heat-resistant air drying heater) or NVS (air drying heater) are used. The power of these heaters per 1 m of length ranges from 0.6 to 1.2 kW, the temperature of the radiating surfaces is from 300 to 600°C. Heating elements operate at a voltage of 127, 220 and 380 V.

Carborundum emitters have a power of up to 10 kW / h, and their working temperature reaches 1300–1500 °C.

The optimal distance between the infrared unit and the heated surface is 1–1.2 m.

It is possible to heat with infrared emitters both open surfaces of concrete and through the formwork. For better absorption of infrared radiation, the surface of the formwork is covered with black matte varnish. The temperature on the concrete surface should not exceed 80-90°C. To exclude intensive evaporation of moisture from concrete, open surfaces are covered with plastic wrap, glassine or roofing material.

Infrared installations are placed at such a distance from each other to warm up all areas concrete surface. The heating of concrete by infrared rays is conventionally divided into three periods: holding of concrete and its heating; isothermal heating; cooling down.

The method is used for heat treatment of concrete in thin-walled structures with a large surface modulus (for example, walls concreted in sliding formwork, slabs, beams). This method is also used to thaw frozen concrete in working joints, when laying concrete in grooves, as well as to thaw reinforcement, embedded parts and the “active” surface of the formwork-cladding before placing concrete into it.

Source: Technology construction processes. Snarsky V.I.