There are several ways of classifying concrete. We will present some of them below.
One of the most popular classifications of concrete is its exposure class. This parameter is the exposure of the composite and its reinforcement to environmental effects. Where no hazard is present, we are dealing with class X0. Carbonation (XC), and freezing/thawing (XF) each have 4 degrees of exposure, while chloride corrosion derived from sea water (XS), from outside seawater (XD), chemical aggression (XA) and mechanical aggression, e.g. grinding (XM) each have three. A description of each level of exposure class can be found in PN-EN 206:2014.
The next method of classification to be discussed is the class of consistency. It is tested with the slump cone method. The consistency class has five values (S1 to S5); the lower the value, the more dense concrete.
Another criterion is the maximum size of aggregate grains. In this case the size of individual aggregate grains is tested in their largest fraction, which is usually (in millimeters), 8, 16, 32, or 64. These are the values most often used in conventional mixtures.
However, the most important way of classification is compression strength. It is determined in reference to samples that had been settling for 28 days, which take two forms: a cylinder with a diameter of 15 and a height of 30 cm and a cube with 15 cm sides. Resistance is expressed in megapascals (MPa), and also means the class of concrete, where the two numbers (e.g. C 20/25, C 40/50) provide information about the minimum resistance of the cylinder and the cube, respectively.
The next type of classification relates to the density of the composite. Dense concrete carries at least 2600 kg/m3, standard concrete from 2000 kg/m3 to 2600 kg/m3, while the lightweight variant can hold from 800 kg/m3 to 2000 kg/m3. In addition, lightweight concrete is also classified from D 1.0 (maximum 1000 kg/m3) to D 1.5 (maximum 2000 kg/m3). Further decimals (1.0 to 1.5) mean an increase in the density by 200 kg/m3.
It is also worth mentioning the chloride content class for the maximum content of chloride ions in concrete, depending on its type. The fewer ions, the better, since they cause e.g. corrosion of reinforcement. Another parameter that describes concrete is the so-called strength development, which takes the form of the ratio of composite strength in the 2nd and 28th day of maturing. It’s easy to guess that the higher the ratio (i.e. 30 MPa/60 MPa = 0.5), the higher the strength development coefficient.
Also, be aware of such parameters as fire resistance, water resistance, porosity and the heat transfer coefficient, often used by concrete manufacturers to describe their products.
