Most developers of individual houses classify concrete, using a concept such as concrete grade. However, this building material has another characteristic - class. What is the reason for such selectivity, and what is the difference between a brand and a class?
Concrete is the most widespread, unparalleled building material, and the second most consumed resource after water on Earth. They are artificially obtained stone, resulting from the hardening of a homogeneous mass of components mixed in certain proportions, such as cement, water, fillers and / or modifying additives.
There is a fairly large number of very different concretes, differing both in their purpose and in the type and type of the main binder, as well as fillers - both in terms of maturation and in its structure. Nevertheless, in spite of all this variety of concrete, there are only three main qualitative characteristics that are used in the calculations when designing future objects - strength, water resistance and frost resistance. To determine the numerical indicators of these important properties of concrete , the concepts of brand and class were introduced.
For example, concrete grade 200, widely used in individual construction for the construction of various foundations, belongs to class B 15 and has frost resistance - F75-F100 and water resistance - W2-W4.
Of course, the first thing that begs the question is what is a concrete brand and what is its difference from the class.
To begin with, the classes and grades of concrete are extremely strength characteristics, however, which have some nuance in practical application.
Strength grade is
standardized numerical value obtained from laboratory tests for compression and tension. In other words, this is the value that determines what maximum mechanical load a square centimeter of a surface can withstand. Since concrete is characterized by an increase in strength over time, reference samples (molded cubes with a side of 10 centimeters) at the age of maturity of at least 28 days are tested. Existing concrete grades range from M50 to M800 (with an increase in density by increasing numerical value). For individual construction, concrete is used no higher than grade 400.
However, the strength declared by the brand is just a laboratory quantity, since in practice it is affected by a number of destabilizing factors, such as violations in manufacturing technology, mismatch in the quality of sand and water, changes in laying and setting conditions. All this leads to a decrease in strength characteristics. And this error, or coefficient of variation, is the main difference between the class of concrete and its brand.
In essence, this is simply the actual strength with a small (5%) deviation. In practical applications, the class of concrete is an important calculated value used (unlike the brand) in the design of future structures. Measured in MPa, and regulated by GOST 26633-85. In total there are sixteen strength classes in the range from B 3.5 to B 60.
The following quality characteristics - water resistance and frost resistance - are classified only by brand.
The concrete grade characterizing frost resistance is also a laboratory quantity. It is the maximum numerical indicator of the number of tests during which the sample was subjected to alternate freezing and thawing. Frost resistance is determined by eight grades in the range from F 50 to F 500.
Another quality characteristic is the waterproof grade of concrete. For the classification of water resistance, six grades are used in the range from W2 to W 12, which represent the maximum value of the water pressure at which the reference samples are able to not pass water (subject to standard test conditions).