The problem of building in the winter time for our country has always been relevant. Concrete work in the cold time requires a special approach. In serious construction companies, engineers make a special project for the production of works or outage for winter concreting. We will try to tell about the intricacies of this process in a more accessible language.
What is the problem?
Initially, it should be noted that the calendar winter offensive to construction work is indirectly related. According to SNiP 3.03.01, the cold season occurs when the average daily temperature drops to +5? С and the likelihood of short-term frosts at night.
Now let's consider what is dangerous for fresh concrete casting low temperature.
As a guideline, the optimum temperature for solidification of the array is +20? C.
- At this temperature, the monolith gains a predetermined strength of 70% in 5–7 days, conventionally considered to be a week.. When the temperature drops to +5? С the processes of hardening in concrete slow down and the same strength is gained in 3 - 4 weeks.
- As you know, the catalyst for most chemical reactions is elevated temperature.. The concreting process is no exception.
- For example, in factories for the production of concrete products, in the process, steaming is necessarily used when the product is placed in a steam chamber with a temperature of 70-80? С and high humidity. As a result, the notorious 70% are recruited in 8 - 24 hours.
- But if at temperatures close to 0? С the process of hydration in the solution only slows down, then it freezes at all during freezing.. The reason is simple and known from the school curriculum, the water freezes and the reaction stops. Water, in its liquid state, is an indispensable condition under which a cement stone is capable of forming and, accordingly, maturing concrete.
- According to the existing construction standards, at +20? С, the regulation on the full strength of the monolith is 28 days. In winter, the pouring instruction can be very different from the traditional one. Currently there are several ways to solve this problem.
Important: it is considered that the critical strength of concrete during winter concreting is at least 50%. In other words, if the monolith gains a fortress of 50% or more and then freezes, then the thawing process of maturation in it will continue without loss of quality. Otherwise, the characteristics of concrete can significantly change for the worse.
Common solutions to problems
As is known, this problem exists from the moment of the appearance of the concrete itself, and they have always tried to solve it. Modern methods of winter concreting develop in several directions.
With a reasonable approach, you should start with the preparation of the solution, because the temperature is easier to maintain than later reheat the monolith.
- A common mistake of inexperienced builders is to use boiling water to prepare a solution. In this case, the composition is simply "brewed."
- The optimum water temperature for preparing a warm solution is 60 - 70? С. For some types of Portland cement and quick-hardening cement, water with a temperature of +80? C can be used. The remaining components should also be heated to about the same temperature.
- An important point here is the technology of mixing the solution. If in the warm season all the ingredients are poured into a concrete mixer filled with water at the same time. Then in the winter, when loading with your own hands, after you have filled in warm water, rubble or another large filler is poured into it and several turns are made. Only after that you can add cement, sand and bring the solution to the desired condition.
Tip: in the cold season, the time of mixing the solution in the concrete mixer is recommended to increase by at least a quarter.
- Preparing a warm composition correctly is of course important, but it is equally important to quickly deliver it to the construction site. Nowadays, modern machines equipped with electric or gas heating from the inside are used for this purpose. Some companies assemble mini concrete plants directly at the construction site.
- About 50 years ago, the brilliant Soviet scientist I.A. Kiriyenko developed a thermos method for winter concreting. Despite such advanced age, this technology has been successfully used to this day.
- The essence of the technology lies in the arrangement of special formwork of insulating materials. In the classic version, the solution is poured into the thermally insulated formwork and, if possible, sealed. The process of cement hydration is accompanied by active heat release and due to the heat released the monolith ripens.
- But for a long time, the technology has been improved and at the moment a pre-heated composition is poured into a special formwork for concrete. Plus, it adds special additives that activate the heat transfer process. It is noted that the highest heat release in fast-hardening compositions, for example in Portland cement.
- In addition, the so-called hot thermos method appeared. Its essence is that the solution is brought to a temperature of about 70? C for a short time, after which it is poured into a thermo-formwork equipped with electric heating and compacted. As a result, in a short time, up to 3 days, the concrete matures by 70%.
Electric heating methods
- At the moment, concreting in the winter time with heating of various kinds of electrical appliances has become widespread. This is facilitated by the relatively small energy intensity, as well as the availability and simplicity of the method.
- Although there is one significant disadvantage, not every owner can afford to acquire the appropriate power transformer and related equipment to it.
- Most often, voltage is applied to electrodes of various configurations, and the concrete monolith itself acts as a great resistance, due to which it heats up. The most effective for this are plate electrodes that are attached directly to the formwork.
- Also common is the method of applying voltage to the reinforcement cage, where it plays the role of an induction coil or pulling several heating filaments inside the monolith.
- In the past few years, heating of various kinds of structures, including concrete, with the help of infrared radiation has become widespread. The price of infrared lamps is small, plus the energy they consume is much less than traditional heaters. It is enough to protect the structure from the wind and it is desirable to paint it black.
Construction of warmths
In the past, this method was the most common.
But, despite the emergence of many new technologies, it is still very popular.
- The technology can be safely called the simplest, its essence is that a framework is constructed around the poured monolith and covered with technical polyethylene or tarpaulin.
- After that, an electric or gas heat gun is installed in such a tent and hot air is blown. From the point of view of energy intensity, the method is perhaps the most costly. Currently, it is more used for heating structures in closed, non-heated buildings, new buildings.
Important: in this way, it is easy to organize the steaming of the structure, which will speed up the ripening of concrete by an order of magnitude, but for this you will need a steam generator. Plus, there may be problems with the freezing of condensate emanating from the greenhouse.
Frost resistant additives in the solution
Among experts, this method is called cold concreting. As mentioned earlier, without water, cement hydration is impossible. But, besides the fact that water can be heated, you can also use additives for winter concreting, which will lower the freezing point of water and accelerate the ripening of the monolith.
On the market at the moment there are 3 directions for the creation of such additives.
We do not undertake to assert that some of them are better or worse, just that each direction was developed for narrowly defined purposes.
- This group is designed to slightly speed up or slow down the maturation of the solution. Most of all electrolytes are used in it, but polyhydric alcohols, carbamides and organic compounds are also found.
Important: electrolytic additives are prohibited to use when creating foundations for electrical appliances or electrically conductive structures. In view of their increased electrical conductivity and the presence of eddy currents.
- The next group is focused on enhancing the antifreeze qualities of the composition, it significantly accelerates the process of setting and maturation of the solution. Compounds and derivatives of calcium chloride are widely spread here.
- In this group, antifreeze properties are less pronounced, but it significantly speeds up the ripening processes. A distinctive feature here is that these additives contribute to an increase in the temperature of the solution, which has found its use when using a "thermos".
- Because of the acceptable cost and ease of use, the most common in this niche is considered "Potash". It is nothing like some types of salts of monocarboxylic acid. They are also good because with the right dosing it is possible to make compositions that can withstand temperatures up to -30? С.
- But in this case, you need to strictly observe the proportions and remember that it no longer means better, while enhancing some properties of the solution, you can lower others.
- Large construction organizations, in the construction of new buildings often use sodium nitrite. The price here is also quite affordable, but to apply it you need to have certain professional knowledge. The fact is that this composition is flammable, plus it may actively release toxic gases when in contact with plasticizers. He himself also has a pungent smell.
- Sodium nitrite shows the best results in fast-hardening solutions based on portland cement or slag portland cement.
Important: experts categorically do not recommend using sodium nitrite for alumina cement.
- Additives such as frost and frost, are compounds with a complex action. In addition to increasing the coefficient of frost resistance, they give the solution a good plasticity and other useful qualities.
What could be the consequences
Often, it sometimes happens that in the second half of autumn frosts come for a few days and then the weather is warm for another month. If you did not have time to warm the monolith and it, still grabbed, do not despair.
The concrete will not be frozen deep, the monolith will be heated naturally from the inside, and short-term freezing of the upper layers will not cause much harm.
- Naturally, during freezing, there will be a slight loss of strength compared with laboratory characteristics, but our solutions, as a rule, are designed for this.
- In a fresh solution, water is the lightest component and, according to all the laws of physics, rises, this is especially characteristic of compositions that have been further diluted with water. In this case, short-term freezing will even be useful. Subsequently, the monolith is peeling off like old paint, dust is swept over and that's it.
- In the case when time is still missed, strong frosts have hit and warming is foreseen only in the spring, try to save what is possible. We recommend wrapping concrete with polyethylene, it will save from snow and winds.
- In the spring, when the snow begins to conceal and the thaws begin to alternate again with the night frosts, the sheltered monolith will remain, and will not additionally sate in water and collapse. Of course, you won’t get a design fortress, but the losses may not be so painful.
Important: cutting of reinforced concrete with diamond circles, as well as diamond drilling of holes in concrete in a frost-bitten massif is not recommended, it is necessary to allow the concrete to mature completely and only after that all further work should be done.
The video in this article shows the nuances of winter concreting.
Often the features of winter concreting are in an integrated approach. We have listed you the most common measures to protect the array in the cold. But experts do not recommend, do not rely on only one way.
So, for example, antifreeze additives for concrete structures - a good thing, but with excessive use, they can damage. Therefore, it will be reasonable to combine them with the thermos method and any type of electrical heating.