When mixing fresh concrete, it is important to choose the right ratio. The specific weights (= specific gravity) of the concrete types are decisive for this. It can be a thin mix with a low density of concrete, or it can be heavy mixes. In order to find the ideal conditions for your project, you can use the corresponding information in the building instructions or tables. But first of all, you have to know how the concrete densities and weights are determined. Read our guide to find out what relationships there are with other variables and how they are calculated.
The type of concrete (lightweight concrete, normal concrete, heavy concrete) determines the specific weights. This information can be determined with simple means on the construction site. For this, you can fall back on rules of thumb that allow a very good approximation. Always observe the correct units when working with tables, as these can vary from version to version. You do not need to have great mathematical skills for the calculation, and you can use the rules of thumb without the aid of a pocket calculator. However, the result is all the more important as it determines the success of the project. Only if you produce the perfect concrete mix is strength guaranteed and the desired effect can be achieved.
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Influence specific weight
Fresh concrete is a frequently used material and is characterized by its high strength. It is a mixture of an aggregate and a binding agent. As a rule, cement is used as a binding agent. You can use sand and gravel as aggregates. Mixing and chemical reactions take place through the addition of water. With various additives, you can influence the properties of the building material again. Depending on the exact production, one speaks of different types of concrete. For example, glass fibers, steel fibers, or plastic fibers were added to fiber concrete. Textile concrete is created when you use carbon or AR glass, for example. The composition of the concrete types determines the weights and thus the specific weights.
What is the specific weight?
The specific weight is the so-called specific gravity. This is the ratio of weight to volume. Thus, the specific weights can be used to determine the density of the affected body, i.e. in this case the type of concrete. The greater the specific gravity, the greater the density. At the same time, the fabric is perceived as being heavier and is particularly stable. Knowledge of the density or specific weight required in each case is crucial to select the right mixture for the building material and thereby achieve optimal results at work. For example, different requirements are placed on foundations than small jobs in the garden area.
Formula for calculation
To calculate the weight, the weight is divided by the volume:
Weight = weight force (wf) / volume (V)
It is, therefore, necessary first of all to determine the weight force. This is the product of the mass and the gravitational acceleration:
Weight force (wf) = mass (m) * gravitational acceleration (g)
You have mixed fresh concrete and now want to determine the specific weight. Then proceed as follows:
take a jar. This could be a 10-liter bucket, for example. Weigh the weight of the empty bucket and write down the value.
Fill the 10-liter bucket up to the mark with fresh concrete. You now know that the concrete has a volume of 10 liters.
Tip: Since the literature and tables often require information in m³, you can convert the 10 liters into m³ for better comparability. The rule here is that 1,000 liters correspond to a volume of 1 m³. Thus 10 liters correspond to a volume of 0.01 m³.
Now you need to weigh the fresh concrete. Place the bucket on the scales, read the value and subtract the size previously measured for the empty bucket. For our example, we assume that the concrete weighs 50 kilograms.
Next, calculate the weight force. An average value of 9.81 m / s² is assumed for the acceleration due to gravity:
wf = mass * acceleration due to gravity
wf = 50 kg * 9.81 m / s²
wf = 490.5 Newtons
A rule of thumb is often used to simplify the calculation. To do this, multiply the 50 kilograms by the factor 10. In our example, this results in a value of 500 Newtons.
In this step, divide the weight calculated in step 4 by the volume. In our example, a weight of around 500 Newtons and a volume of 0.01 m³ were determined. The result of this task is the specific weight.
Weight = weight force / volume
Weight = 500 Newton / 0.01 m³
Weight = 500 * 100 Newton / m³
Weight = 50,000 N / m³
A simple way to calculate
To be able to determine the specific gravity quickly and effectively on the construction site, you make use of a special feature. If you use a 10-liter bucket and want to calculate the result in N / l, this is particularly easy.
The concrete is poured into a 10-liter bucket and weighs 50 kilograms. This results in a weight force of rounded 500 Newtons. This result is now divided by 10 l volume. The result is again 50, this time in unit N / l. If the concrete weighed 30 kilograms, the specific weight would be 30 N / l accordingly.
Would you like to convert the weight calculated using this method into N / m³? Then multiply the result by 1,000. This results in a value of 50,000 N / m³ in our example. If the 10-liter bucket of fresh concrete weighs 30 kilograms, you get a density of 30,000 N / m³.
The density of fabrics
In addition to the specific weights, there are also densities in tables. These can easily be converted into weights or you can calculate the densities from the weights.
What is density?
Density is defined as the ratio of mass to volume. The weight force is thus bypassed, which leads to a simplification.
- Step 1: fill the concrete in a 10-liter bucket.
- Step 2: Weigh the concrete, subtracting the empty weight of the bucket.
- Step 3: Divide the weight by the volume. In our case, this is 10 liters or 0.01 m³.
|lightweight concrete||<2.000 kg/m³||. good thermal insulation |
. good sound insulation light weight
|Normal concrete||2.000 kg/ m³ to 2600 kg/m||under construction as standard used|
often with steel reinforcement
|heavy concrete||> 2.600 kg/ m³||radiation protection|
If the 10 liter bucket of concrete weighs 50 kilograms, this results in a density of
50 kg / 10 liters = 5 kg / l
50 kg / 0.01 m³ = 5,000 kg / m³
Convert weight and density
If you know the density, then multiply this value by a factor of 10 and you will get the corresponding weight. On the other hand, if you know the weights, then you have to divide them by 10 to get the density.
Tip: Here you work with the rounded acceleration due to the gravity of 10 m / s² instead of 9.81 m / s².
The concrete density of concrete types
The concrete density depends on the mix chosen. By varying the materials, a smaller or a larger density can be achieved. This in turn influences the later behavior of the building material. Since every substance has its density, the properties are also determined from this. If you dilute the concrete with water, for example, it not only becomes thinner but also has a low density or weight. Sand, on the other hand, increases the density of the concrete.
Types of concrete by density
For example, if concrete weighs between 1,500 and 2,500 kg / m³, this means that one cubic meter of the building material weighs around 1,500 to 2,500 kilograms. What sounds high at first glance is put into perspective at second glance when you consider that the soil weighs between 1,300 and 2,000 kilograms in the same volume ratio.
If the density is below 2,000 kg / m³, then one speaks of lightweight concrete. It is often mixed with expanded clay or pumice stone.
Normal concrete has a density of around 2,000 to 2,600 kg / m³.
The specific gravity is accordingly 20,000 to 26,000 N / m³.
Here the concrete density is above 2,600 kg / m³, ie above 26,000 N / m³.
The density varies
On the one hand, you have the option of influencing the density by mixing the cement, the aggregate, and the water. On the other hand, supplementary additives are often used. These include, among others:
- Prestressing steel
- Fibers made of plastic, glass, or steel
Tip: Note that the density of the building material changes again as it hardens. The crystallization of the cement leads to the formation of crystal needles. This creates a gearing and the volume decreases. Since the weight remains the same, the concrete density and thus also the specific weight is higher.
Tips for fast readers:
- specific weights are also called weights
- Weight = weight / volume
- Density = mass / volume
- Work with a bucket as a measure
- for a 10 liter bucket:
- The weight corresponds to the weight in N / l
- always pay attention to the units
- Weight and density differ by a factor of 10