Hardening accelerator for concrete: steam heating at

Any construction should be carried out in certain terms, which are laid in the project, but sometimes there are certain force majeure circumstances, and therefore the question may arise how to accelerate the hardening of concrete, as the main binder.

Such unforeseen circumstances may be inconsistencies with subcontractors, weather conditions and even banal negligence of workers, but the timing of this can not change, therefore, the process in some moments has to be accelerated.

To do this, there are several ways to allow solutions to quickly grab, and they will be discussed below, and besides this, we will be able to show you the video in this article on this topic.

Normal heavy concretes

Note. According to GOST 18105-86 (Rules for Strength Control), the brand strength of concrete is achieved 28 days after it is laid at the place of use. But the current pace of construction, as well as the various force majeure circumstances mentioned in the introduction, require the use of various methods to accelerate the hardening of concrete.

The quality of the product does not deteriorate, but the production time is reduced, and with it its cost, which is also very important.

What it is

• In the factory, the production of reinforced concrete is particularly in need of rapid strength building for a number of reasons.. First of all, it is an opportunity to wrap forms faster, therefore, it is more efficient to use equipment, which leads to an increase in productivity in general. Such intensification is applied by accelerating the hydration and hydrolysis of clinker materials.
• As a rule, in such cases, the water content in the cement paste is low enough and a rapid glut of hydration and hydrolysis products (cement minerals) of the aquatic environment is created here.. Such an increase in strength is most effective in rigid mixtures with low B / P, which are sufficiently well compacted, and all processes are performed in thin films of dough.
• Цемент более мелкого помола здесь наиболее эффективен, ведь реакция его частиц на различные процессы значительно возрастает, особенно если совершается так называемое «мокрое домалывание» в вибрационных мельницах до 5000-5500 см²/ g (specific surface). In those situations when the specific surface is greater than the specified values, the costs for non-energy and water consumption increase.
• Among the main methods of acceleration can be identified three main methods that are used in modern construction.. Firstly, it is steam heating of concrete at atmospheric pressure, secondly, it is steam heating of concrete in an autoclave at elevated pressure and, thirdly, it is electric heating of concrete. In addition to the methods, there are still various chemical components that help speed up the process.

Steam heating at atmospheric pressure

The most popular hardening accelerator for concrete on most ZZHBI, is its heat treatment of steaming chambers using saturated steam. The whole process can be divided into four main stages, the first of which can be called shaping the RC products (also learn what is the modulus of elasticity of concrete).

Forming or stage number 1 occurs at a temperature of 18? C-22? C, where the poured design is maintained for some time (depending on its size). This leads to the fact that the solution begins to seize, that is, it acquires initial strength.

After that, proceed to the stage number 2, when the molded design begin to doused with hot steam and that warms up from the upper layers - to the middle, being filled with water. Such saturation occurs due to the process of condensation - hot steam, coming into contact with the colder walls of concrete goods, penetrates its pores. Due to the increase in temperature increases the rate of hardening of concrete.

It is at this stage that the most significant destructive processes occur due to the thermal expansion of the components from the heating of the structure. Since the stove heats up unevenly (from the top — faster, from the inside — slower) and the water vapor creates a certain pressure, this also increases the destruction. This process develops most effectively when the temperature exceeds 50 ° C - the volume expansion coefficient of water and air increases.

When all RC products achieve uniform heating in all their layers, destructive processes are completed, then an intensive increase in strength begins, which can be called stage No. 3.

This is followed by step number 4, when the cooling of the concrete begins after isothermal cooling. It turns out that the whole structure is squeezed as it were, and at the same time its pores are shrunk, thereby squeezing moisture onto the surface, where it dries out quickly enough. But this process should be strictly controlled, since with a sharp cooling cracks can form in the structure, especially for low grades of concrete.

Due to this danger, the temperature in the chamber is lowered quite slowly, depending on the size of the structure - the greater its volume - the higher the risk of cracking.

Therefore, the instruction provides for smaller products to lower the temperature not faster than 30? C-40? C per hour, and for larger products - to 20? C-30? C per hour. Attention is also paid to removing the slab from the chamber - the difference in the temperature inside the chamber and outside should not exceed 40? C.

Note. It is noteworthy that such a method can be made more efficient by using various accelerators for setting and hardening concrete (chemical additives), which reduce destructive changes. This not only improves the quality, but also reduces the entire workflow.

Autoclave method

When steam is heated by concrete products at a temperature of from 160? C to 180? There are no significant differences in the process of cement hardening between the autoclave method and the atmospheric pressure.

Although here after four to six hours of intense heating, the strength of the product may even exceed the grade, which leads to greater crystallization of the cement stone and, as a result, its hardening after that occurs more slowly than after hot processing at atmospheric pressure.

But in autoclave processing, there is one significant distinguishing feature - the grains of acidic rock aggregates interact with the hydrolytic lime of Portland cement (at temperatures above 100? C). This improves the technical properties and structure of the concrete product.

But due to a certain lack of such equipment and the complexity of production technologies, the price of such processes increases - therefore, for ordinary concrete, this method has not been widely used and is more often used for making cellular concrete.

Electric heating

This method of heating is quite simple and is used to a greater extent in winter conditions on construction sites at temperatures below -5? C - this is the use of PNSV (Steel Heating Wire Vinyl sheath) and step-down transformer. Its simplicity is also confirmed by the fact that the same method can be carried out at home without much difficulty and mental expenses using a welding machine as a step-down transformer.

Before the pouring in the middle of the reinforcement cage begins, loops of PNSV are laid, which will subsequently be heated from the step-down transformer. It is noteworthy that when using the unit with a power of only 80kW, you can warm up 90m at a time³ concrete - it is quite small costs that ensure low cost of production. The disadvantages of heating the concrete with a welding machine include the inconvenience of laying the cable itself, which needs to be fixed to the frame so as not to rub the insulation - otherwise a short circuit across the ground will occur and the loop will fail.

The principle of heating the formwork with electrodes is inherently no different from the method using PNSV, only here a rebar or a thick rod (8-10 mm) acts as heating elements.

The difference is that PNSV loops are conveniently laid in a slab or slab foundation, while electrodes are more suitable for vertical structures, that is, for vertical formwork. And another difference is that the electrodes are usually stuck immediately after pouring, and not before it.

The distance between the electrodes is kept about 60-100 cm, but it depends on the temperature outside - the lower it is, the more often the reinforcement is inserted to increase the heating intensity.

Also, it is not the electrode itself that is heated here, as in the case of PNSV, but the water between the fittings (to whom the principle of the boiler of two blades is familiar, no need to explain this). It is noteworthy that when the column is heated, only one reinforcement is enough, which will serve as a phase, and the metal frame of the structure will be the ground.

Note. The disadvantage of electrode heating is high energy consumption. One electrode will consume about 45-50A.

In this case, the heating elements are mounted directly in the formwork panels, and they can always be replaced if they become unusable. The advantages of such heating lies in its profitability - the formwork can be used for almost any height of buildings and used even at 25 degrees of frost. But at the same time, it cannot be used for non-standard designs, and its cost is quite high.

Some chemical boosters

Explanation. To begin with, it should be clarified that there is a difference between the hardening and setting accelerator. So, on setting the drug works in the first hours and makes more intensive the set of plastic strength or molding after mixing cement with water. But the hardening chemical works not only for hours, but for days, making the fastest curing during the period of its impact.

«Форт УП-2» это комплексная добавка, которую часто используют для очень широкого спектра бетонов и железобетонов различных марок. Наибольший эффект ускоритель твердения для пенобетона и бетона показывает, когда его  применяют для ускорения твердения в строительстве монолитных конструкций, а также формовочных изделий - данный модификатор обычно используется для беспропарочного производства ЖБИ при температуре воздуха в помещении от 10?C и выше.

Количественная масса «Форт УП-2″составляет всего 0,5%-0,7% от общей массы используемого цемента и за одни сутки обеспечивает 70% набора прочности и это при полном отказе от ТВО.

Asilin-12 is the acceleration of the hardening of concrete of the liquid type, which is used at both low and high temperatures and is practically safe for the human body. Most often, such a modifier is used when the ambient temperature is lower than 10? C and higher than 25? C, which increases the entire cycle of work by about 1.5-2 times.

Compared to powder formulations, Asilin-12 has an undoubted advantage, since it is distributed evenly in the solution when mixed, which significantly improves the quality of production.

Conclusion

In conclusion, it should be noted that the technical properties of RC products, where any types of hardening accelerations have been applied, remain virtually unchanged. Consequently, cutting of reinforced concrete with diamond circles and diamond drilling of holes in concrete proceeds in the same way as natural hardening and does not require any additions (find out here how dust removal of concrete is done).