Concrete is a rock that you can compose by using the following construction / raw materials: cement, sand, gravel, and crushed stone. These have the property of hardening with water. This creates concrete. In this article I will also walk you through step by step how you can make concrete yourself and what you should pay attention to.
What is concrete
Concrete is a mixture of sand, gravel, cement, crushed stone and water. You can let these building materials harden by mixing them with the correct amount and the correct grain size of the sand and gravel. This creates a stony material. When the building materials have hardened (the concrete) you can no longer let it dissolve in water (unlike plaster).
Concrete is a commonly used building material. This is due to the following:
- It’s easy to use
- The many possibilities that you have with concrete
- The building materials have a low price
The annual consumption per inhabitant is approx 1 cubic meter.
The History of Concrete
Building with reinforced concrete was already used by the Romans in the building of bridges and aqueducts. When the Romans built the Colosseum and the Pantheon, the concrete was already used. Back then, lime was mostly used as a binding agent. This technique was never used again later. After the Englishman Joseph Aspdin invented Portland cement, the concrete was, as it were, rediscovered. Blast furnace cement is used more nowadays. This is because this technique has better properties.
Strength classes / Composition
There are many people who have questions about the exact strength classes of concrete. I will try with a clear example and 3 tables to give a layout to make a desired decision. I hope now it gets a little clearer to everyone with this problem.
Example:
Environmental classes (table 1)
The description of the environments shows that classes 1, 2 and 4 represent the climate customary in the Netherlands, while classes 3 and 5 describe a number of special circumstances. Classification in environmental class 5 can only be done on the basis of NEN 5996 Determination of the aggressiveness of aqueous solutions, soils and gases. The requirements regarding the water-cement factors to be used, air content and type of cement, depending on the environmental class, are summarized in table 2.
A closer study of this table shows that there is a relatively large difference in the maximum applicable water-cement factor between environment class 1 and 2. This is based on the fact that corrosion of reinforcement in a dry environment, even with a relatively large porosity of the concrete, hardly takes place. However, the conditions of the dry environment must then be expressly met. Furthermore, it appears that concrete that is exposed to our outside climate may have a maximum permissible water-cement factor of 0.55.
Comparison with previous requirements (table 2)
In this respect it is useful to compare this requirement with, for example, B17.5 class I in consistency area 3. This composition is now quite commonly used for outdoor concrete structures. The prescribed minimum cement content according to the VB is 320 kg / m3. The average water content for consistency area 3 is approximately 180 l / m³. It follows that the average water-cement factor for this composition is 0.56. From this it can be concluded that approximately 50% of this composition would have met the requirements for environmental class 2.
In summary, it can be said that a small quality increase is necessary for the concrete B17.5 consistency area 3 to meet the requirements of environmental class 2. Incidentally, comparing new with old is a temporary matter and not very useful, because with the VBT is clearly no longer thought of in recipe terms, but is rather designed according to own experience and insights.
The designer indicates the environmental class
The designer, but usually the concrete constructor, will now have to indicate per construction or construction part in which environmental class it must be performed. Of course, a number of practical aspects should not be forgotten. Suppose a floor to be poured in one go is partly inside and outside. Obviously carried out environmental class (2). If the columns are located ‘outside’ on the ground floor and ‘inside’ on the veripings, it will be possible to apply an underlay in environmental class.
Alignment of environmental class – strength class (table 3)
The constructor must realize that it sometimes makes sense to make use of the fact that for the design of a concrete composition, the environmental class can be decisive instead of the strong grid class. Suppose a construction or construction part falls in environmental class 5c. The maximum water-cement factor to be used is then 0.45. Initially, the constructor wants to calculate with strength class B25. The average compressive strength level that can be expected when using a class A cement and a water-cement factor of 0.45 is approximately 45 N / mm². This compressive strength level is high enough to comply with strength class B35. It can therefore be economical to calculate the construction with this strength class.
Table 1
environmental class: | environment: |
---|---|
1 | dry |
2 | moist |
3 | moist in combination with de-icing salts |
4 | seawater |
5 (a, b, c or d) | aggressive (weak, moderate, strong or very strong) |
Table 2
environmental class: | 1 | 2 | 3 | 4 | 5a | 5b | 5c, d |
---|---|---|---|---|---|---|---|
maximum water-cement factor: | |||||||
– reinforced concrete | 0.65 | 0.55 | 0.55 | 0.55 | 0.55 | 0.50 | 0.45 |
– prestressed concrete | 0.60 | 0.55 | 0.55 | 0.55 | 0.55 | 0.50 | 0.45 |
minimum air content (not applicable if wcf <0.45): | |||||||
– Dmax = 63 mm | – | – | 3.0 | 3.0 | – | – | – |
– Dmax = 31.5 mm | – | – | 3.5 | 3.5 | – | – | – |
– Dmax = 16 mm | – | – | 4.0 | 4.0 | – | – | – |
– Dmax = 8 mm | – | – | 5.0 | 5.0 | – | – | – |
cemetn type | – | – | – | recommended: sulfate resistant blast furnace cement | sulfate resistant cement |
* when exposed to solutions with> 400 mg SO4²- / liter or soil with> 3000 mg SO4²- / liter
Table 3
wcf: | avg. cube compressive strength (N / mm²): | strength class: |
---|---|---|
0.45 | 45 | B35 |
0.50 | 40 | B25 |
0.55 | 35 | B25 |
0.65 | 30 | B15 |
* when using class A cement.
This example shows the correct way of optimizing. A reverse order is not correct. In other words: prescribing a ‘higher’ environmental class in connection with the chosen strength level is fundamentally incorrect. The required strength can be influenced not only by the water-cement factor but also, for example, by the cement class. Some insight into the relationship between water-cement factor and compressive strength level is therefore desirable in order to make the above considerations. Table 3 can be helpful here.
This example above was posted on March 29, 2009 due to the confusion of the strength class.
Making concrete
Before you can make concrete, you need to know which ratio to use
Type of concrete: | Ratio: |
---|---|
For reinforced concrete: | 1 cement, 2 sand and 3 gravel |
For stamped concrete: | 1 cement, 3 sand and 5 gravel |
Usually the building materials are mixed with a concrete mixer, but you can also use a cement tub for small jobs and mix the building materials with a drill.
What do you have to do to make concrete?
- Start the cement mixer
- Add half of the gravel, sand, and some of the water
- Add all the cement
- Add the rest of the gravel and sand
- Add water until you have the right proportion
- Let it mix for a few minutes
Different types of admixtures for concrete
For concrete you also have additives, which, for example, make the concrete harden faster, or less quickly, such as the following below:
Plasticizers:
These ensure that the workability of the concrete increases without adding water. They actually reduce the strength of the concrete.
Binding accelerators:
These ensure that the concrete hardens faster.
Binding retarders:
These ensure that the concrete hardens less quickly so that you can use / process it longer.
Air Bubblers:
These create air bubbles in the concrete itself, this is an insulation against frost. So it causes to increase frost resistance. (This is often added to foundations because of the moisture that can become ice in winter, thereby increasing the building structure (water that becomes ice expands).
Water-retaining substances:
To be able to pour concrete underwater.
Color pigments:
To add color in the concrete.
Foaming agents:
To obtain good tightness and better insulation value.
Filler:
A kind of powder of crushed brick, this will increase the stability of the mixture. This powder is a replacement for the sand and gravel.
Reinforcement:
Here the concrete is locally reinforced by means of steel rods. This increases the pulling power of the concrete. These bars are called reinforcement.