Density of Concrete – Gross Density by Type of Concrete

Density of Concrete – Gross Density by Type of Concrete

Even if concrete always looks almost the same, the individual types differ considerably: density, compressive strength, and many other technical properties are highly individual for the individual types of concrete. With the exact knowledge of the different types of this artificial stone, you can precisely calculate your needs. Find out in this text everything you need to know about types of concrete and their areas of application.

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What is concrete

Concrete is a mixture of aggregate, water, and cement. In addition to these basic components, concrete can still be expanded in its technical properties with numerous additives: tightness against penetrating water, increased frost resistance, or colorants are only a small selection of enhancements that are possible with concrete.

Advantages of concrete

Components of almost any size and shape can be produced from concrete. This material is mixed at the construction site and poured into the prepared mold. The hardening process begins about an hour after mixing. The concrete solidifies in the form in which it was poured. These forms are called “formwork”. The particular advantage of concrete is that it forms a continuous, monolithic structure. These can be made as strip foundations, floor slabs, false ceilings, walls, beams, or pillars. The enormous compressive strength of concrete is particularly advantageous. 

For this reason, it is the material of choice for the most heavily loaded structures.

Double the power with steel

The concrete block has enormous compressive strength, but fails very quickly if it is subjected to tensile or shear stress. This disadvantage is compensated for with the addition of steel reinforcement. Both materials complement each other perfectly: They have almost the same factor in thermal expansion. This means that the concrete block and steel expand evenly when heated and contract just as evenly when they cool down. Internal tensions and cracks do not occur and the component retains its function and stability for decades.

Concrete types

Types of concrete by place of manufacture

Concrete mix that is produced on the construction site is called “construction site concrete”. Fresh concrete delivered is called “ready-mixed concrete”.

Types of concrete according to flow properties

It is very difficult for the layman to find their way around the jungle of names around the concrete block. There are basically only two variables to consider when ordering the concrete: the consistency and the concrete density. The consistency is also called the W / C value. This indicates the ratio between water and cement. The more fluid a fresh concrete, the easier it is to pour it into a mold. However, as the water-cement ratio rises, its compressive strength also suffers. Liquid concrete is difficult to control when processed without a board. In the case of a temporary slope stabilization, a stiff concrete is sometimes more advantageous.

A distinction is made between the following flow classes of fresh concrete:

“Very Stiff” (formerly “KS”): “KS”): Can only be compacted by tamping. The aggregate sticks to each other and hardly spreads when pouring out. This fresh concrete can hardly be made frost-proof.  This fresh concrete has the designation “C0”

“Stiff”Stiff fresh concrete is well suited for free deformation. Unboarded foundations of hillside stones in horticulture can be easily produced with this material. In the concrete plant, this fresh concrete has the designation “C1”

“Plastic” (formerly “KP”): This is the standard concrete for walls and columns. After hardening, it is very pressure-resistant and can be easily compacted with a concrete vibrator when concreting. In the concrete plant, this fresh concrete has the designation “C2”

“Soft” (formerly “KF”): Well suited for stairs. It combines good dispersibility with sufficient concrete density after hardening. In the concrete plant, this fresh concrete has the designation “C3”

“Very soft”: This fresh concrete is well suited for complicated formwork that cannot be shaken off well. It is well distributed in the furthest corners. Heavily reinforced concrete bodies should be filled with the softest possible flowing concrete so that the cement slurry reliably surrounds the entire reinforcement.

“Flowable” and “Very Flowable “: These types of concrete are used to concrete large ceilings and floor slabs. In these structures, most of the statics are created by steel reinforcement. The flowability of the fresh concrete is also very important when using a concrete pump.

Concrete types according to concrete density

The concrete density does not vary as much due to the W / C value as it does due to the choice of the aggregate. Basically, one can assume that the compressive strength increases with the concrete density. The bulk density of concrete is therefore primarily determined by the choice of aggregate. It is by far not just normal gravel that can be used as an aggregate for concrete. Since the aggregate is the component with the greatest gross density, the resulting concrete density is always slightly lower.

The most common types of aggregate are:

Normal surcharge: The normal surcharge is the aggregate consisting of gravel and sand, which is usually added to the fresh concrete. It is obtained from dredged river beds, quarries. With a normal surcharge, a concrete density of 2.2 to 3.2 tons per cubic meter is achieved. The correspondingly designated normal concrete has a density of 2.0 to 2.6 tons per cubic meter.

Light aggregate: If the concrete density only reaches a value of fewer than 2.2 tons per cubic meter, “light aggregate” was used. Accordingly, the material is also referred to as “lightweight concrete”. The lightweight concrete has a bulk density of up to 2.0 tons per cubic meter. Porous stones such as pumice, lava, or artificially produced expanded clay balls are used here. Lightweight concrete has certain thermal insulation properties and can improve the energy balance of a house. Most of the use of lightweight aggregate is in the manufacture of hollow blocks.

Heavy aggregate: Concrete aggregate with a gross density of over  3.2 tons is processed into “heavy concrete”. This has a gross density of 2.6 tons per cubic meter. It is extremely pressure-resistant. Its technical properties are further enhanced by the addition of particularly hard-setting cement. Heavy concrete is used for heavily loaded foundations and pillars as well as for the construction of vaults. Colloquially, it is therefore also referred to as “safe concrete”. Blast furnace slag, scrap, or other heavy and pressure-resistant material is used as an aggregate for the types of concrete with this high gross density.

Types of concrete according to class name

The names of the types of concrete have changed in recent years. The previously used B5 – B45 designations have given way to a complex system, which, however, makes the selection more accurate and targeted. The classification takes no account of the bulk density. This always depends on the aggregate used and can vary within the individual types of concrete. Knowing the compressive strength is therefore much more precise than the bulk density. This is permanently determined in the form of test cubes and pressure tests. Due to the many variable factors involved in concrete production, the raw density alone cannot be relied on: the water-cement ratio, aggregates, and cement type and quantity all have an impact on the compressive strength of concrete. 

The compressive strength of the concrete types can be read directly from the designation. The unit is “Newton per square millimeter”. A “C 25 type of concrete” thus has a compressive strength of 25 N / mm². Often a second unit is placed next to it, which is a little higher. The numbers refer to the compression tests: The first number for the types of concrete refers to the cylinder test, the second to the cube test. Due to their shape, cubes are a little more pressure-resistant than cylinders, so the second number is always a little higher. In practice, however, only the first number is given to determine the type of concrete.

The most common classes of concrete today are:

Normal concrete with medium bulk density (2.0 to 2.6 tons per cubic meter):

  • C8 / 10
  • C12 / 15
  • C16 / 20
  • C20 / 25
  • C25 / 30
  • C30 / 37
  • C35 / 45
  • C40 / 50
  • C45 / 55
  • C50 / 60

Heavy concrete with high bulk density (over 2.6 tons per cubic meter):

  • C55 / 67
  • C60 / 75
  • C70 / 85
  • C80 / 95
  • C90 / 105
  • C100 / 115

The compressive strength classes of lightweight concrete are supplemented by an “L”. Despite its low bulk density, lightweight concrete can achieve considerable compressive strength classes. The following types with low concrete density (less than 2.0 tons per cubic meter) are available:

  • LC8 / 9
  • LC12 / 13
  • LC16 / 18
  • LC20 / 22
  • LC25 / 28
  • LC30 / 33
  • LC35 / 38
  • LC40 / 44
  • LC45 / 50
  • LC50 / 55
  • LC55 / 60
  • LC60 / 66
  • LC70 / 77
  • LC80 / 88

However, these types of concrete with a low bulk density are quite sensitive to damp and wet environments. They suck up easily and can then suffer severe frost damage in the next winter. They should therefore be used indoors or protected against the effects of the weather by effective external cladding.

Lightweight concrete is usually processed into hollow blocks and prefabricated elements. As flowing concrete, it is relatively difficult to process. In addition, it is much more expensive than normal concrete, so its use should be carefully considered.

Use of the concrete

It’s all about exposure

As already mentioned with lightweight concrete, the choice of concrete depends largely on where it will be used. Even if the concrete density is high, that does not mean that this material is permanently waterproof. Only specially treated types of concrete can be used for areas with direct contact with water. These are made water-impermeable by adding blocking agents. Environmental conditions such as busy roads or proximity to the coast also play an important role in the choice of concrete.

The exposure classes are:

  • X0 No specific hazards – minimum strength C8 / 10
  • XC / D / S1 Very dry or constantly wet – minimum strength: C16 / 20
  • XC / D / S2 mostly wet – minimum strength: C16 / 20
  • XC / D / S3 average damp – minimum strength: C20 / 25
  • XC4 alternately wet and dry – minimum strength: C25 / 30
  • XF1 Dry frost without thawing agents (salt) – minimum strength: C25 / 30
  • XF2 Medium damp frost with thaw aid – minimum strength: C25 / 30
  • XF3 Wet frost without thawing agents – minimum strength: C25 / 30
  • XF4 Wet frost with thaw aid – minimum strength: C30 / 37
  • XA1 has a low chemical load – minimum strength: C25 / 30
  • XA2 medium chemical exposure – minimum strength: C35 / 45
  • XA3 subjected to heavy chemical loads – minimum strength: C35 / 451
  • XM1 moderate mechanical wear – minimum strength: C30 / 37
  • XM2 severe mechanical wear – minimum strength: C30 / 37
  • XM3 very strong mechanical wear – minimum strength: C35 / 45

Accidental loss of density

The concrete density of the hardened concrete block depends on the compaction, especially in the case of stiff material. A pore-free concrete block is extremely important because only it can guarantee the concrete cover and frost protection. If segregation and holes are found after stripping, these must be carefully sealed immediately. Rusting reinforcement within a concrete body remains undetected for a long time, but can then suddenly lead to failure of the component. If you don’t know exactly how to get rid of the concreting mistake, the advice of a concrete repairer is recommended. These specialists know exactly what to do in such cases and can usually save every component.

Trust in the specialist

The average home builder can hardly be expected to struggle through the jungle of concrete specifications. But it is also completely wrong to simply want to mix all flowing concrete yourself according to the classic 1: 4 recipe. The concrete works in the vicinity are happy to provide detailed information about which types of concrete are most useful in the region. They also have the greatest experience. It is therefore worthwhile to introduce yourself there before a construction project and to discuss your construction project. Many free and very valuable tips can be quickly obtained here, which can save a building owner from expensive long-term consequences.

Would you like to calculate the specific weight of concrete? Here you can find detailed instructions with examples:  Calculating the specific weight of concrete

Tips for fast readers

  • Be able to differentiate between types of concrete
  • Concrete density is not critical, but compressive strength and exposure
  • Know exactly the conditions on the construction site
  • Always ensure sufficient compaction and concrete cover
  • Do not always order flowing concrete as thin as possible, always pay attention to the WZ value.