How does rain affect freshly poured concrete?

Concrete and rain

Concrete doesn't dry, it sets

Colloquially, the setting of the composite building material is also referred to as the "concrete drying time". However, this designation is wrong in terms of content. Because concrete does not dry in the conventional sense. Rather, the mixing water is bound as crystal water in the concrete. But it is also interesting to see where concrete can be installed anywhere and still set:

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

The water-cement value

Cement needs water for chemical reaction. The cement can chemically and physically absorb a certain amount of water. With conventional Portland cement, that's around 40 percent. This value is given as the water-cement value (w / c value) as 0.40. Do-it-yourselfers in particular tend to increase the flow properties of concrete by adding more water to it.

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 practically pushed out of the young concrete - the concrete bleeds. This creates capillary pores. These not only reduce the density of the concrete. They also make the composite building material significantly more unstable and therefore less pressure-resistant.

However, the required water must not be withdrawn from the cement

If, however, concrete is now poured at high summer temperatures and the w / c value has been correct beforehand, water is removed from the concrete through evaporation. As a result, the cement can no longer set properly and increased cracking can also be observed. Now you are wondering what all this has to do with rain.

Post-treatment of concrete

At the beginning we wrote that you cannot generally assume that rain will damage the concrete. Especially in summer, when it is very hot, the concrete has to be supplied with water anyway. This post-treatment of the concrete is extremely important because this is the only way to ensure that the concrete will have the required properties.

Water in the concrete must not evaporate

How much the water evaporates in the concrete also depends on the relative humidity. The higher this is, the less water can evaporate. It can therefore be an advantage, especially in summer, if it rains while the concrete is being poured. But the rain can also fall on the young concrete during the hardening phase.

Rain can be very helpful during concreting and post-treatment

Only immediately after installing or pouring concrete should you ensure that heavy rain does not wash out the cement paste. However, even a slight rain does not affect fresh concrete that has just been poured. Otherwise it would not be possible to process concrete under water.

If you want to pour concrete, you should protect the poured concrete with a foil anyway (in summer against evaporation, in winter against cooling down). However, if the concrete has already set for a certain 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 concrete is watered during post-treatment, nothing else is done. Ultimately, however, this also includes a certain amount of experience with concrete.

frequently asked Questions

Can fresh concrete tolerate rain?

Basically yes. Since concrete does not dry but rather sets, water on the concrete surface does not damage it. Only freshly poured concrete should be protected from excessive rain (washing out) immediately after pouring.

Can concrete harden when it rains?

The setting process in concrete is independent of the surrounding moisture; only the mixing water stored in the cement in the form of crystal water is required for the chemical setting process. Concrete can even harden under water.

Why is concrete covered when it hardens?

This is not done to protect against rain, but on the contrary to protect against dehydration. If evaporation is too high, the mixing water would also evaporate from the concrete and the concrete would be weakened by the lack of crystal water.