Concrete and rain


Concrete and incipient rain - just for the private concrete contractor, who has no deep experience with concrete, quickly raises the question of whether this could not harm the newly cast or built-in component. However, this question can neither be answered in the affirmative nor denied. Therefore, we have summarized here, what is important in rain, when you concretes.

Concrete does not dry, it binds

Colloquially, the setting of the composite construction material is also referred to as "dry time from concrete". However, this term is contentwise wrong. Because concrete does not dry in the traditional sense. Rather, the mixing water is bound as crystal water in the concrete. But it is also interesting, where concrete can be installed everywhere and still binds:

  • in conventional civil engineering
  • in wet construction pits as a base plate
  • under water

The water-cement value

Cement requires water for the chemical reaction. The cement can chemically and physically absorb a certain amount of water. That's about 40 percent for conventional Portland cement. This value is given as water-cement value (w / c value) as 0.40. DIY enthusiasts in particular tend to increase the flow properties of the concrete by adding as much water as possible.

Too much water in the fresh concrete reduces the quality of the building material

However, this water can no longer be bound. While the concrete hardens, this water is almost forced out of the green concrete - the concrete is bleeding. This creates capillary pores. These not only reduce the density of the concrete. They also make the composite construction material much more unstable and thus less pressure-resistant.

But the cement must not be deprived of the required water

But if concrete is concreted in high summer temperatures and if the w / c value has been adjusted beforehand, water is removed from the concrete by evaporation. As a result, the cement can no longer set properly and also an increased cracking is observed. Now you wonder what all this could have to do with rain.

The aftertreatment of concrete

At the beginning we wrote that one can not assume that rain damages the concrete. Especially in summer, when it is very hot, water must be supplied to the concrete anyway. This aftertreatment of the concrete is extremely important, because only so will ensure that the concrete will have the required properties.

Water in the concrete must not evaporate

How much the water evaporates in the concrete, but also depends on the relative humidity. The higher this is, the less water can evaporate. Thus, it can be quite advantageous, especially in summer, when it rains during concreting. But even during the curing phase, the rain may well fall on the green concrete.

Rain can be very helpful during concreting and after-treatment

Only directly after the installation or pouring of concrete should be taken to ensure that heavy rain does not flush out the cement paste. However, light rain does not bother even fresh concrete. Otherwise you would not be able to process concrete under water.

Tips & Tricks

If you want to concretes, you should anyway protect the inserted concrete with a foil (in the summer against evaporation, in winter against cooling). However, if the concrete has been able to set for some time and has developed a corresponding compressive strength, it is often even an advantage if you remove the film so that it can rain directly on the concrete. When watering concrete during the aftertreatment finally nothing else is done. Ultimately, however, it also requires some experience with concrete.


Video Board: Pouring concrete in heavy rain!