Curing concrete - what does it mean?

After mixing, the fresh concrete must be processed relatively quickly, because the chemical setting of the concrete begins. As a result, the concrete hardens. As curing takes time, concrete only gradually solidifies. Depending on the project, different times are important for the concrete contractor.

The chemical process of curing concrete

The setting is, as already mentioned, a chemical process. The Portland clinker in the cement reacts chemically to water and crystallizes. These crystals grow together like pointed needles and get caught up. This achieves the high compressive strength of the concrete. But of course, this process of curing takes time. Depending on what stage the concrete is, it also has different names:

  • Fresh concrete (fresh, straight mixed or mixed concrete)
  • Green or green concrete (concrete that is in the hardening phase)
  • Hardened concrete (concrete that has hardened to reach its standard strength)

Completely cures concrete only after years

In particular, the explanation of "hardened concrete" has it all. Because as a definition there is not read "until the green concrete is completely cured", but rather "if he has reached its minimum compressive strength according to standard". This, in turn, suggests that even if the concrete has already been made into hardened concrete, it is not fully cured yet. Rather, it may even take years for the concrete to harden completely.

Minimum compressive strength within a defined time

Important when curing so are the factors "minimum pressure resistance reached" and "according to DIN". This required minimum compressive strength is reached after 28 days. This is especially important for construction projects. In addition, when curing but also often important from when the concrete has hardened so far that he is at least carefully walkable. This is an important aspect, especially when it comes to DIY home improvement projects - for example, when casting concrete foundations outdoors, but also indoors.

Depending on the project, not just the hardening

But also in house construction, it has some relevance for home improvement, because even the screed in a new building is poured concrete. Often, the continuing interior work that home improvement perform on their own. So it is also important from when such a screed concrete can be further processed. Here, in addition to curing, the moisture of the concrete plays an important role. Depending on the weather conditions, it can take several months for the moisture to escape completely.

Curing concrete at different temperatures

This brings us to the next important point, which concerns the general hardening of concrete: the weather. The colder it gets, the worse the concrete hardens. From minus 10 degrees Celsius, this chemical process even comes to a complete standstill. A comparison: Concrete at 20 degrees, halves the curing compared to the same work in only 5 degrees. However, as concrete is also processed in winter, it is also important as of when the building material can no longer be attacked by the frost. Concrete is frost-proof from a compressive strength of 5 N / mm².

Post-processing in winter all the more important

For this value to be achieved reliably, the concrete must have a temperature of at least 10 degrees when concreting at an air temperature below 5 degrees. If the temperature of 10 degrees can be kept for three days, the required frost protection is achieved. Thus, the concrete after-treatment in winter is a real challenge. Of course, this also varies the period from when the concrete is accessible. Depending on humidity and outside temperature this range is between 1 and 3 days.

Tips & Tricks

Do not confuse the mentioned frost resistance during hardening with the pouring of frost-proof concrete foundations. It is about the frost effect on the foundation from the outside. So on the one hand, whether appropriate measures have been taken that frost can not lift a foundation. On the other hand, whether the concrete is suitably protected against the ingress of water, so that frost can not blow it up.

The times given in the text refer to conventional Portland cement at normal mixing ratios. If the proportion of cement, water-cement value, type of cement etc. is changed, this also has an influence on the deviating time for hardening.

Video Board: CURING