How Does Concrete is Made. What are some categories of Concrete?
Concrete Particles Defination and Categories
Concrete strength grades
The concrete grade is the main indicator of its capabilities. Competent selection of material in relation to this criterion becomes the key to the reliability and durability of the building. Let's mark the most popular solutions.
M100 - concrete used in the course of preparatory work. If you buy concrete of this brand in Moscow , you can solve many problems at once. The lower layer of material provides strong protection from groundwater, and the structure as a whole increases the reliability of the foundation.
M200 is the ideal solution when it is necessary to pour reinforced monolithic slabs. The material has become widespread due to the versatility of its characteristics. It is used as a basis for parking lots and areas, as well as in private construction.
M250 - concrete of such a plan is used when creating floors with a low load.
Concrete hardening Terms
Hydration is the process of interaction of cement with water, in which concrete gains its strength. If water dries out or freezes in young (a couple of weeks old) concrete, the hydration process stops, which negatively affects its characteristics. In the first week, if the concrete is in normal humidity and temperature, it gains 70% strength.
In general, the first days of concrete life are very important, it is necessary to combat the loss of moisture for normal hydration. Freshly laid concrete at high temperatures can be covered with wet burlap or PVC foil or sprinkled with water.
At low temperatures, the water in concrete, which is necessary for hydration, freezes - the process naturally stops. By the way, it can continue in the spring, when it thaws, if the structure is not washed out by that time, but at the same time, a decrease in durability strength is possible. Nevertheless, there are methods of the so-called early freezing of concrete, in which concrete with special antifreeze additives is laid at sub-zero temperatures, freezes, and in the spring, revived, the process of cement hydration begins.
There is such a thing as the critical strength of concrete - this is the edge, the time after which you can no longer worry about concrete. This threshold is different for different grades of concrete. High grades of concrete have a lower% threshold of critical strength (25-30% of the design strength), low grades have a higher%. Under normal conditions, critical strength is reached within the first day.
To prevent concrete from freezing, you can do the following:
- use antifreeze additives in concrete (PMD) - they prevent water from freezing and partly accelerate the hydration process. Now for these purposes, compounds and preparations are used that do not harm the reinforcement and other parts of the structures.
- electric heating of concrete. There are special transformers, electrodes, heated formwork. Ideal for winter concrete pouring, but problematic for a private developer due to high energy consumption.
- cover freshly poured concrete with foil. It is applicable, although it is not a fact that it will help if the daily temperature is not very low, within 1-2 degrees. The film will retain the heat that is released during hydration. You can install a gas or diesel cannon to blow warm air under the film.
It is necessary to carefully monitor the concrete in the first day of its life.
In precast concrete factories, there is no such problem with finished structures - piles, concrete foundation blocks FBS, floor slabs, road slabs - no. They are steamed in special warm and humid chambers, which allows you to achieve excellent results in gaining strength and be completely ready for use.
Concrete grade (M)
In all information materials, price lists, etc. concrete is indicated with a numerical and alphabetic index. The brand M-, class B-, mobility P-, water resistance W-, frost resistance F- must be indicated.
Your project determines the choice of a specific type and grade of concrete. If there is no project, then you can heed the advice of the builders or figure it out on your own.
The numbers of the concrete grade indicate the average compressive strength in kgf / cm2. Compliance with the declared concrete grade is checked by casting a mixture of cubes or cylinders from a sample and holding them for 28 days for hardening.
Currently, such a characteristic of concrete as a class is used - this is approximately the same as the brand, but with the only difference that the average strength value is indicated in the brands, and in the classes - the strength with guaranteed security with a coefficient of variation of 13%. In accordance with ST SEV 1460, all modern design requirements are indicated precisely in the classes, although most construction companies use the brand for ordering.
It is important for you that the concrete grade matches the one you ordered. How to check? To do this, you need to take a sample and cast several cubes from it measuring 10X10X10 cm or 15X15X15 cm.It is most convenient for this to use the necessary forms hammered from the boards, having previously moistened them so that they do not absorb the water that is part of the concrete and do not interfere with hydration. For the purity of the experiment, it is necessary to pierce the poured mixture so that there are no air voids left. You can also compact the mixture by hitting the sides of the mold with a hammer .. Samples should be taken directly from the concrete carrier, do not dilute the mixture in a concrete mixer truck, carefully compact the mixture in molds and store at medium temperature (about 20 C) and high humidity.
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