Drying time of concrete

Concrete is probably the most used building material in this day and age. Even if concrete always looks the same, there are considerable differences. These properties also apply when drying concrete. Even though the drying time of concrete may be a priority for you, you should nevertheless know about the associated processes.

Drying time of concrete is a bit misleading

The process itself is very simple: cement is mixed with sand, gravel and water, the gray mass, the concrete, is processed. Then the concrete has to dry. However, this "drying time" rather colloquial to understand. Because the water absorbed by the concrete does not evaporate or does not evaporate completely. To a certain extent, it must not even evaporate. Here are a number of factors in each other, which also affect the immediate concrete properties.

Cement glue: Cement, which together with water sets to cement stone

However, it is important to understand what actually happens in concrete production. Sand and gravel are initially irrelevant. Rather, these materials serve as fillers to minimize cement usage and save costs. The focus is rather on the so-called cement paste, which arises when cement and water are mixed together. This water is absorbed differently by the cement:

  • physically
  • chemical

The water-cement value

The ratio of a defined amount of water for a likewise defined amount of cement is referred to as water cement value (also w / c value, ZWK for cement-water coefficient or W / B value for water binder value). Overall, cement can bind 40 percent of its volume with water. With an average cement, the physically bound proportion of water is 15 percent and the chemically bound proportion is 25 percent, ie a total of 40 percent. Based on this absolute saturation, the c / w value would be 0.40.

The water must remain bound

The 25 percent that are chemically absorbed do not dry. Instead, the water converts the cement into many crystal tips, which grow into one another and thus allow the concrete to be strong. The fillers sand and gravel are covered in principle only by the setting water-cement mixture. The physically bound water at this value remains bound. At the same time, however, this water, which is physically bound in the cement, also influences the compressive strength.

Excess water - capillary formation and concrete bleeding

If even more water is added, for example, to make the concrete more fluid, the excess water can not be bound. While the cement sets with the maximum amount of water, this water literally remains and is permanently displaced by the formation of crystals in the cement - so-called capillary pores are created, which run through the entire concrete part. These pores not only have an effect on the compressive strength, they can also absorb water again later, thus causing a higher absorbency.

Absorbency is a quality reduction

Especially with high performance concrete with very high compressive strength that is an absolutely undesirable feature. Outside, depending on the weather conditions, the concrete will regularly become saturated with water. In winter, when there is frost, the water changes to its solid state, expanding and bursting the concrete. If more water is added, the concrete bleeds and the pores become even more massive. In order to achieve the highest possible compressive strength, the w / c value, based on the maximum water binding (in the example mentioned 40 percent), must be less than 0.40. For this cement, it would be between 0.25 and 0.40.

Processes during the drying time depending on the concrete mixture

A drying time in the true sense does not have concrete. Nevertheless, the following properties may occur after processing:

  • Cement binds with water
  • Excess water forms capillary pores
  • the concrete is bleeding

The decisive period: the achievement of the desired compressive strength

In fact, what really matters in concrete is when the building material gets its desired compressive strength. Strictly speaking, crystallization lasts for months. Of course, nobody can wait that long. Thus, a reference value is set in Germany, which must be complied with in order to meet the requirements for the desired compressive strength. According to DIN 1164 it is assumed that concrete has reached the desired strength.

Curing time better than drying time

Thus, the drying time can be better defined as a curing time. In fact, in the mentioned DIN is also assumed by such a curing time. However, 28 days are also very long and sometimes work must be carried out in which this quite long curing time can not be taken into account. First, however, the concrete is divided into three different stages:

  • Fresh concrete (dressed, not yet hardened concrete)
  • young or green concrete (not completely hardened concrete)
  • Hardened concrete (already completely hardened concrete)

Not to be underestimated - the aftertreatment of concrete

However, the stage of green or young concrete is very long at 28 days. During this time, the concrete is constantly changing its consistency. A big problem arises with the weather. If it is too warm, the water evaporates in the concrete. Even the physically bound water would now be removed from the concrete. In winter, however, the not yet fully bound water can freeze and expand.

Possible post-processing depending on weather conditions

In both scenarios, the young concrete would be permanently damaged. So follows during the curing time the so-called "concrete aftertreatment". If it is too cold, the concrete must be kept warm (plus 10 degrees Celsius would be optimal). In summer, however, water must be added to the hardening concrete to compensate for the evaporated liquid.

Aftercare for a week or more

This process must last at least a few days to a week, but even longer is also recommended. However, the added amount of water must be precisely metered, because after all, the concrete should not be flushed out. On the surface, ie the topmost concrete layer, only a gossamer-like cement curtain is allowed to form. To talk about the after-treatment of concrete, ie of a drying time, would be confusing. Therefore, the colloquial drying time of concrete describes the curing.

Tips & Tricks

Depending on the project, DIY enthusiasts may not be able to wait 28 days. Depending on how thick a concrete layer is and whether fast or conventional setting cement is used, most concretes should be carefully walkable at least the next day. Nevertheless, the concrete is still far from its desired compressive strength at this time.

Video Board: How Fast Does Concrete Dry? The Million Dollar Question