Deformation of concrete during freezing and thawing

Preparing to pour the foundation

Sometimes the construction of capital structures, especially private ones, occurs without taking into account the time of year. This may be a justified decision, but there will be many difficulties already at the stage of preparatory work. They consist of several stages:

1. The area intended for the foundation must be cleared of topsoil and marked in accordance with the project. In frosty weather this will be a rather labor-intensive task.
2. Next, you need to mark the width of the trench for the foundation. Its depth is determined by the depth of soil freezing, the number of storeys of the structure being built, and the material that will be used during construction. Earth-moving equipment is not always applicable, since the walls of the trench must be narrow, deep and even.
3. For waterproofing and strengthening, the bottom of the pit is compacted with sand (a layer of 90-150 mm), then with crushed stone. Usually in winter this building material is frozen. There is a possibility that with warming, the compacted layer will lose the required density, and this may affect the strength of the future building.
4. The next stage of work is the installation of formwork. Boards or wooden panels are used for this, and thick polyethylene film is used for waterproofing. Severe frost affects the elasticity of the film. It becomes brittle and holes may appear on it, which impairs the waterproofing of the foundation.
5. Next, a reinforced structure is made, which can be welded or twisted using steel wire. The thickness of the reinforcement used ranges from 8 to 18 mm. One of the properties of steel is to contract or expand with temperature changes, so a welded reinforcement cage in severe frost will change its dimensions as the temperature rises, which will affect the strength of the foundation.

Advantages of winter work

Sometimes situations arise when making a foundation in winter is the best option. There may be different reasons for this:

1. Features of the soil of the area. If the soil is loose, it is better to build the foundation in frozen soil to maintain the desired shape of the pit.
2. The climatic conditions of the region in the summer do not allow construction work.
3. They resort to construction in winter to save money. During this period, prices for building materials decrease.
4. Construction firms are reducing the cost of services as demand for their activities drops sharply in winter.

After the preparatory work, you can begin to calculate the composition of the concrete solution, be sure to take into account the temperature at which it will be poured into the formwork.

Hardening of concrete mass in winter

No matter what time of year the foundation is poured, the solution is prepared from cement and medium-sized crushed stone with the addition of plasticizers . With additives, concrete gains strength, its condition and moisture resistance improve. Plasticizers increase the resistance of the solution to frost, so they are often used when making foundations at low air temperatures.

The minimum concrete hardening temperature is not lower than +5 °C. This is the extreme indicator for quality ripening. But hot weather is not particularly suitable for construction work. The optimal temperature range is from +15 to +20 °C. By observing such conditions, it is possible to create a solid foundation for the construction of a building without additional costs and technologies.

You need to know at what temperature concrete hardens. Experts consider the normal air temperature for hardening to be from +15 to +20 °C. The foundation hardening period lasts about 30 days. If the temperature is below normal, concrete hardens more slowly - it will reach the required strength in about 60 days. When the temperature is below 0 °C, the process stops. At sub-zero temperatures, the solution poured into the formwork freezes. If the foundation has already gained the necessary strength, then in the spring after thawing the process of hardening will continue until the final result is complete.

If there is insufficient strength before freezing, the quality of the monolith will be unsatisfactory. When water in a concrete solution freezes, it will turn into ice and increase in volume, which will lead to porosity and cracks in the concrete. As a result, the operational life of the structure will be reduced.

There are methods by which the hardening of concrete at low temperatures can be brought to a state of critical strength by the time it freezes. Based on their action, they are divided into three types:

• external care is provided for the mortar poured into the formwork to the degree of critical strength;
• with the help of electrical heating, the temperature of the concrete mass increases until the moment of maximum hardening;
• introduction of modifiers into the solution that accelerate the hardening process.

The possibility of winter concreting depends on many factors: the availability of power sources at the construction site, weather conditions at the time of hardening, and the possibility of delivering heated concrete. The simplest and most cost-effective method is to add modifiers to the solution.

Winter concreting methods

So, concreting during frost is possible only if it is possible to prevent water crystallization. There are several methods to accomplish the task:

Antifreeze additives

Container with antifreeze for concrete

You may have heard something about various concrete modifying additives used to improve the quality of cement mortars. Among them there are those that cope with preventing premature freezing of water. There are three main groups of such modifiers:

The use of antifreeze additives for concrete has a number of advantages:

• Affordable price . The cost of their use is lower than the options for using electrical equipment.
• Easy to use with your own hands . By and large, you just need to add the purchased mixture to the solution.

Advice: it is recommended to increase the concrete mixing period when adding anti-frost mixtures by one and a half times. It is necessary that the modifier spreads throughout the solution as evenly as possible, and there are no areas left uncovered by it. Otherwise, it will have a detrimental effect on the entire structure.

Concrete should be mixed more thoroughly in cold weather

• Effective result . Using the described means, you can both protect the solution from freezing during the hardening period, and shorten the setting period so that the cement does not have time to deteriorate.

But there is also a negative side: using antifreeze additives, you can concrete in frosts of at least -10 degrees Celsius, which is why you sometimes have to combine them with other methods of protecting concrete from freezing.

Heating cable

Laying heating wire over reinforced rods

The instructions for implementing this method look like this:

1. Before pouring the solution, we lay a special single-core wire, fixing it to the elements of the reinforcement and formwork, so that it ends up inside the future structure.

Electric heating system installation process

Advice: do not allow sharp bends in the cable, as this may lead to a break in the internal core.

1. We bring the ends out into the unheated area.
2. Connect to the transformer.

Connection to transformers

1. Fill the solution and turn on the system.

Despite some difficulty in heating concrete with an electric wire and considerable cost, it is the most effective because:

• Calmly withstands frosts down to -30 degrees Celsius.
• It heats the entire structure, and not just its upper part, as happens with subsequent methods.

So in this case the water will not freeze, and in the end only cutting reinforced concrete with diamond wheels can cause any damage.

Diamond drilling of holes in concrete that has already gained its strength in winter

Construction of a tent

Tents at an industrial construction site

This method involves installing a tent over a concrete screed and heating it inside with heat guns.

But here it is immediately worth noting the difficulties associated with such a process:

• Great labor intensity.
• High price.
• Limited to application to horizontal surfaces only.
• Covering only the upper zone of the entire structure.

Insulation with various materials

Protecting concrete from cold using improvised methods

If you cover the screed with plastic film, boards or other available materials, you can protect it from cold down to -3 degrees Celsius. Of course, this option cannot be called universal, but it combines perfectly with other methods of warming up, enhancing them, and does not require significant investment of effort and money.

Additives to the solution

When carrying out the concreting method using additives, the mortar is poured in winter without heating. Additives are used during the cold season and are divided into two types:

1. Substances that lower the freezing point of water in concrete mortar: potash, calcium and sodium chlorides, sodium nitrite and their combinations. They provide fairly good hardening at low temperatures. The type of additive is selected according to the requirements for the solidification temperature of the solution.
2. Components that accelerate the hardening period. These are modifiers. These include potash and mixtures of calcium chloride with urea or calcium nitrite.

The volume of chemical compounds introduced into the solution ranges from 2 to 10% of the weight of the cement powder. Their quantity is determined according to the expected hardening temperature of the concrete. With the use of anti-frost additives, concreting can also be carried out at -25 °C. But such experiments are not recommended for private builders. Pouring at minimum temperatures is accompanied by a number of features and requirements for performing the work. The main point is to prevent freezing and defrosting of the solution.

The only advantage of building a foundation in the winter months is that in early spring it will be possible to begin construction and then finishing work and continue until the end of autumn.

Why does concrete freeze?

Factors that affect concrete freezing:

• Poorly filled seams/joints between concrete slabs. Through them, moisture gets inside, which destroys the structure and cracks appear.
• Cheap or additionally diluted concrete mortar. Due to poor quality materials, the material becomes frozen on both sides.
• Premises that are poorly heated due to improper design of the heating system. If you did not correct the error yourself, or did not contact the necessary special services, then you “froze” your premises yourself.
• Cracks and damage to metal elements. Air, moisture will seep inside, and corrosion will appear. This leads to an acceleration of the processes of destruction and freezing.
• Small wall depth.
• A poorly ventilated building will suffer from freezing.
• Poor quality waterproofing (depth and thermal protection material).
• Poor quality concrete compaction during repair work.
• The depth and installation of the finishing layer do not meet the requirements.

You should not skimp on materials and the construction process. In case of defects and poor-quality work, you will face destruction, lack of a guarantee of safety and limited functionality of the premises.

Frost layer size

The type of concrete material is directly related to its freezing. Please note that concrete with a high water-cement value has a lower freezing thickness. For types of concrete that have water-repellent properties, this value is higher (use such concrete).

A special thickness gauge device has been developed (the layer, depth and size of the wall are measured). The indicators are determined by the distribution of the electromagnetic field. The instrument takes accurate measurements, compared with similar ones (no deviations). The device is suitable for professional use (measurements accurate to the millimeter) and for household use.

How to make calculations?

For correctness and accuracy of measurements, pay attention to the following characteristics:

• technical indicators of the structure (these values ​​can be found in purchase documents, material quality certificates, agreement with the enterprise);
• take into account the degree-day value (for residential buildings, during the operation of the heating system);
• heat transfer resistance indicator (contained in documents, quality certificates).

What determines the thickness of the walls?

Before starting construction, depending on the purpose of the building, it is recommended to make calculations about the feasibility of the dimensions (in some cases the depth will be required) of the walls. How to choose the right wall density yourself? Consider the main parameters:

• terms of Use;
• frequency/level of mechanical loads;
• purpose of the walls.