Water–cement ratio

The water-cement ratio (short: w / c ratio and cement - water coefficient CEC ) or the water binder ratio (short: W / B value ) is a characteristic parameter for building materials with a hydraulic binder. The value describes the ratio between the mass of the mixing water and the mass of the binder a compacted mixture. In particular, in the production of concrete it is of high importance. A too high or to a lower value to deteriorate the properties of the concrete. In particular, the compressive strength decreases when the mixture in the calculation underlying value is not respected.

Nowadays, the term water binder ratio is more appropriate than water-cement ratio under certain circumstances. The binder is namely generally not only cement but also concrete additives such as slag, pozzolan, fly ash, limestone, coal fly ash or silica fume will be added to the Portland cement because they are cheaper and sometimes even have better properties. These additives must be in contrast to concrete admixtures included in the mixture calculation. The required properties are, for example, in the manufacture of retaining walls slower hardening, in order to reduce produced in the chemical reaction heat. This is achieved by the addition of fly ash. In Austria, hence the term water binder ratio used. In Germany it is called the crediting of additives from the equivalent water-cement ratio (w / c) eq.

To determine the water -cement ratio of a ready-mixed concrete, the Darrversuch used.

• 4.1 Germany

Frischbetonerhärtung

A mixture of cement and water forms the so-called cement, which hardens during setting of the cement stone, while the aggregates ( the aggregate ) of the concrete bonds firmly together. In this hardening by hydration of fresh concrete, a certain part of the water added for the reaction is needed. A typical cement can chemically and physically bind an amount of water of about 40 % of its mass. This corresponds to a w / c ratio of 0.40. The proportion of chemically bound was approximately 25 %, approximately 15% are bound physically.

If the water content (and thus the w / c ratio ) of fresh concrete is higher, the added water can not be fully committed. The excess water leaves pinnate, absorbent ( capillary ) pores.

Influence of w/z- or W / B value

One calculation for a concrete mixture are always based on the requirements of the construction project. These consist firstly of the required compressive strength, which results from the static calculations and the design. Secondly, the environmental impact, which is exposed to the component of decision. For example, a component that is alternately wet and dry, a higher request to the concrete in order to get a same durability as a component that is always dry.

The concrete mixture is "tuned" to provide these requirements while also w/z- or W / B value is determined in order for the concrete having the desired properties.

It can therefore not be generally say that a w / c ratio of 0.65 is "bad." It all depends on the requirements of the concrete. " Good " is therefore the value that was set during the calculation. In general, however, the higher the stress, the lower ( closer to 0.40 ) the w / c ratio should be.

For high-strength concretes even w / z values ​​from 0.40 to 0.25 are required to minimize the amount of physically bound water.

If this value is, for example, subsequently changed on the site, this has - if it is not provided - in most cases a negative impact on the quality of the concrete and can cause damage to the components. The modification may, for example, occur when " make more " of the concrete when the concrete is too stiff and additional water is added to save the more expensive flow agent. However, you may not just any flow agents are added, as this can have a negative impact on the concrete.

The value is larger than specified

Consequences for the concrete component are:

• It creates more and larger pores than expected. The pores in the concrete put down the compressive strength.
• The capillarity of the hardened concrete increases. The concrete can absorb more water than expected through the capillary pores, or water and other chemical compounds such as chlorides can penetrate deeper into the concrete. Consequences are: The frost sensitivity increases. Flaking affect the durability.
• The reinforcing steel is no longer protected against corrosion (rust ) if the water can completely penetrate the concrete cover. The load capacity decreases with increasing corrosion.

The value is less than specified

Consequences for the concrete component are:

• The amount of water required for complete hydration of the cement is not provided. Characterized all of the binder is not hardened, and the expected compression strength is not achieved.
• The concrete is more difficult to process.

Maximum water-cement ratio

The following table shows the maximum permissible water cement ratios for selected environmental impacts ( exposure classes) are shown.

Standardization

Germany

In Germany regulates the standard DIN 1045-2 - In addition to general requirements, production requirements, etc. - the properties of concrete.

For concretes containing cement and concrete additives in addition, is spoken by the equivalent water-cement ratio. The so-called "value " approach enables complete the acquisition of fly ash and silica fume to the cement content to be counted.

Here, the mass of the water, the weight of the cement, and the values ​​, as well as each and the mass of fly ash and silica fume of. The masses are always relative to 1 m³ of compacted fresh concrete.