What are the mixing ratios of concrete

The mixing ratio of concrete

Basic composition of concrete

Basically, concrete consists of the following substances:

  • cement
  • water
  • Aggregate (aggregate)

This mixture can be supplemented with:

  • Concrete additives
  • Concrete admixtures

In order to be able to correctly assess the mixing ratio of concrete, it is important to know the tasks of the individual constituents of concrete better.

Cement and water

The cement is the glue, so to speak, the binding agent that “holds everything together”. To do this, a chemical process, the setting, must be triggered. To do this, you have to mix cement with water and hyadration takes place. The main component, the Portland cement clinker, now sets and crystal needles grow. These interlock and solidify the cement paste (cement mixed with mixing water) into cement stone.

The aggregate in concrete

Using only cement would be very costly and therefore not economically viable. Therefore, the cement paste must be provided with a filler - the aggregate or the aggregate. In addition, the strength also suffers with a high proportion of cement paste. The shrinkage of concrete is also increased if the proportion of cement paste is too high.

A significant improvement is achieved through the use of an allowance. But specifications are also linked to this aggregate. The same disadvantageous properties of cement paste can also be observed if only a large-grain aggregate were used. The gaps between the aggregates, which have to be filled with cement paste, would be too large.

Different grain sizes and their mixing

Therefore, the aggregate must be made in different grain sizes, and their mixing ratio must also be correct, so that the smallest and smallest possible gaps are formed, which are filled with cement paste. For this reason, two grain sizes (x / y) are always specified for the aggregate, which cover the entire spectrum of the required grain size. A 0/16 or 0/32 grain size would be typical for concretes that do-it-yourselfers mix themselves.

The result is that an optimal mixing ratio always depends on the mixing of the aggregate and consequently even the amount of cement does not allow a lot of leeway, since the ratio to the aggregate must always be taken into account. Inexperienced do-it-yourselfers are now coming and trying to improve the consistency in their favor by adding more mixing water.

The water-cement value

This brings us to the next important mixing ratio for concrete, namely the mixing ratio of cement and water. Cement can bind water in the form of around 40 percent of its volume - chemically and physically. If the amount of water is higher, there is oversaturation and there is excess water in the concrete.

Excess water in the fresh concrete

This excess water has to be processed somehow. The water wants to escape from the fresh concrete. For this purpose, capillary pores are formed. The water also collects under larger aggregates and leads to the formation of duckweed. In the worst case, if these water-filled cavities become even larger, even water sacs arise. The excess water is now looking for its way to the side and upwards. At the side, however, it comes to the casing. This is where so-called rivulets arise.

It is essential to avoid excess water

The value that puts the proportion of water to cement in the appropriate relation is the water-cement value. Overall, conventional Portland can bind around 40 percent water. This means that the maximum w / c value of this cement is 0.4. This value must not be exceeded, because all of the damage that the excess water causes leads to a considerable reduction in the quality of the concrete.

Only very tight tolerances even with concrete mixing plants

Ultimately, the scope for the mixing ratio is very narrow. At the same time, the mixing ratio also depends on the aggregate and its mixing ratio from rock flour to the largest grain size used.

Even in the case of concrete mixing plants (construction site for site or in-situ concrete or concrete plant for ready-mixed concrete), the deviation from a recipe may amount to a maximum of three percent. Consequently, one can easily deduce from this that mixing by eye (for example 1 scoop of cement, 4 scoops of sand, water by feel) does not produce high-quality concrete.

The best concrete mix ratio is not applicable if all other areas are not also carried out absolutely correctly. The mixing time and the concrete mixer used to mix concrete also have an impact on the quality of the concrete. Also post-treat the compacting of the concrete and the concrete.

In our series on concrete you will learn everything you need to know about concrete, its processing and everything that can be constructed with it.