If it is necessary to carry out concreting in winter conditions, then low temperatures, due to which building materials are freezing, become the main problem. According to SNiP 3.03.1, winter concreting conditions are temperatures below 5 degrees Celsius.
Features of work in the winter
All technologies used in concreting at low temperatures are designed to prevent this from freezing. You can point out 2 main features that make the process of laying concrete, at low temperatures, quite difficult.
- Water freezing в бетонных порах. Frozen water expands, leading to an increase in internal pressure. This makes the concrete less durable. In addition to all this, ice films may form around the aggregates, which in turn leads to disruption of the bond between the components of the mixture.
- Cement hydration slows down at low temperatures., and this means that the timing of the set of hardness of concrete is greatly increased.
Note! Concrete is gaining around 70% of the design strength per week at an ambient temperature of 20 degrees. In winter conditions, this period can be 3-4 weeks.
It is necessary to elaborate on such an important factor as the freezing of water. Of great importance for the strength of the whole structure is the time when the water is frozen. There is a direct correlation: the earlier the freezing of concrete occurred, the more fragile the concrete will be.
The period when the concrete mix is set is the most critical and decisive. Concreting technology in winter conditions says that if the concrete mix freezes immediately after laying in the formwork, its further strength will depend only on the strength of the frost.
With increasing temperature, the process of hydration will certainly continue. But the strength of such a structure will be significantly inferior to a similar structure, whose mixture was not frozen during the laying period.
If the concrete managed to gain some value of strength before freezing, then it can easily transfer further freezing without structural changes and internal defects. It is also necessary to try to avoid the so-called cold seams. For this, concrete must be laid continuously.
When working in conditions of low temperatures, it is important to remember about the critical value of concrete strength. This value is equal to 50% of the declared brand strength. This indicator is important to remember, because with modern winter concreting, the mixture is protected from freezing until it sets this same value to 50%.
If we are talking about an object of special importance, then protection against freezing is carried out up to the mixture with a mark of 70%.
Ways of winter concreting
At the moment there are 3 main ways of laying concrete in conditions of low temperatures. The use of anti-frost additives. This is the cheapest and most technologically sound method for protecting the mixture from frost. All supplements of this kind are divided into 3 main groups, depending on the method of their action.
|The first group of additives||Additives that can slightly accelerate or slow down (depending on the task) the hardening process. This includes some electrolytes, and some organic compounds, such as, for example, polyhydric alcohols.|
|The second group of additives||Additives that, unlike the previous ones, greatly accelerate the process of hardening and have strong anti-frost properties. Such additives are made on the basis of calcium chloride.|
|The third group of additives||Substances that do not have strong antifreeze actions, but that promote the quick setting of the mixture, followed by a powerful heat release immediately after pouring.|
Features of concreting in winter conditions are such that it is often impossible to do only antifreeze additives. It is necessary to take a series of measures that will enhance the effect, applied chemicals, and speed up the curing time.
These additional measures are:
- Pre-cleaning of formwork and fixtures from snow and ice. Iron fittings should be warmed to positive temperatures.
- All work must be carried out at the maximum possible pace.
- Direct transportation of the mixture should be carried out in a machine equipped with a double bottom, where exhaust gases must be supplied for heating.
- During unloading, it is necessary to protect the construction site from wind gusts, and the unloading facilities themselves should be as insulated as possible.
- After the installation has been completed, it is necessary to cover the mixture with mats in order to preserve heat for as long as possible.
- Ideally, all components of the mixture should be preheated.
Note! When preheating the components, it is necessary to apply a special order of loading into the mixer in order to avoid “brewing the mixture”. At low temperatures, water is first poured into the mixer, then coarse aggregate is fed, the drum is rotated several times, and only then sand and cement are filled. This instruction must be strictly followed.
The method of "thermos"
This method consists in laying the mixture having a positive temperature in insulated formwork. There is also, similar to him, the method of "hot thermos", the use of which, the mixture is preheated for a short period of time to marks 60-80 degrees.
Then it is compacted in such a heated state. Recommended additional heating. Heat the mixture most often with the help of electrodes.
Note! It is recommended to apply this method in combination with chemical additives. This will allow in a shorter time to achieve the desired effect.
Heating and heating of concrete with electricity and infrared radiation
It is used when the "thermos method" is insufficient. Its essence lies in warming up the concrete and maintaining heat until it reaches the necessary margin of safety, and such that it may be necessary to cut the reinforced concrete with diamond circles.
Most often, the solution is heated using electric current. Concrete becomes part of the electrical circuit and resists. As a result, it heats up, and the goal is achieved.
For electric heating of concrete use electrodes, which are of several types:
- String electrodes
On the best side, lamellar electrodes, which are made of roofing iron, have proven themselves. The technology is as follows: electrodes are sewn onto the surface of the formwork that will be in contact with the concrete. Then the electrodes are connected to the electrical network.
A potential difference arises between the electrodes, a current begins to flow through the concrete, which leads to its heating. The total price of the object on which the heating methods were applied will of course increase. But this is a feature of construction work in the winter.
And these costs are fully justified, as they allow us to prevent the subsequent destruction of the structure due to the fragility of the concrete.
Infrared heating method
Sometimes the method of infrared heating is used, which is based on the ability of infrared rays when penetrating into any object, or substance, to transform into thermal energy.
In order to generate infrared waves, quartz or metal tubular radiators are used. This method is used mainly when it is necessary to warm the frozen concrete structures, to warm the reinforcement, to carry out thermal protection of the already laid concrete mix.
Induction heating method can also be used. In this case, the effect of an induction coil is used, which generates heat in metal parts (such as steel formwork, fittings and other iron objects) in the field of its action.
This method is resorted to when it is necessary to warm ready-made concrete structures and, for example, conduct diamond drilling of holes in concrete. Heating by this method can be effective at any ambient temperature.
Without exception, working with concrete in the winter period is difficult. But thanks to modern technologies, it is possible to shorten the construction time as much as possible without compromising the quality of the object being built.
It is also important that, even in such difficult conditions, some types of work can be done by hand. This, for example, concerns the preparation of mortar with the use of antifreeze additives for concrete.
Do not be afraid to work with concrete, even in freezing temperatures. After all, with all the rules, it will be possible to maintain the strength characteristics of the materials at a high level, and the video in this article will help to understand many of the nuances