The w / c value is the ratio between the mass of the effective water and the mass of the binder. The proportion of binder is taken into account in kg / m³ of a compacted mixture.
The effective water of the water component is the sum of the added water , the inherent moisture content of the aggregate and the water of added aqueous additives, minus the amount of water absorbed by the aggregate through pores.
This is why the term water binding agent value is also used in Austria . Nowadays a binding agent often not only consists of cement (Portland cement clinker), but also concrete additives such as blast furnace slag , pozzolan , fly ash , limestone , hard coal fly ash or silica dust are added to the Portland cement in order to save possible costs or to promote special properties in the concrete. For example, in the manufacture of very massive structures, such as dams, a slower hardening is desirable in order to reduce the heat generated during the chemical reaction. This is achieved by adding fly ash.
In Germany, when adding additives, one speaks of the equivalent water-cement value (w / c) eq .
Additional materials of the type II must be accurate, such as liquid concrete additive medium from a total amount of 3 l / m³ in the calculation of the equivalent water cement ratio will be considered.
Setting of fresh concrete
The mixture of cement and water forms what is known as cement paste in the concrete, which hardens during the setting to form a cement stone and thereby firmly bonds the aggregates (the aggregates ) of the concrete. When hardening of the fresh concrete through hydration a certain portion of the added water is consumed. A typical cement can bind around 40% of its mass of water. This corresponds to a w / c value of 0.40. The proportion bound by hydration is approx. 25%, further approx. 15% are bound in so-called gel pores.
With a w / c value of <0.4, the hardened cement contains proportions of unhydrated cement clinker. If the water content (and thus the w / c value) of fresh concrete is greater than 0.4, the added water cannot be completely bound. The excess water leaves branched, absorbent ( capillary ) pores .
A continuous (capillary) pore system is present in the cement stone from a water-cement ratio of about 0.5.
Influence of the w / c or W / B value
The w / c or W / B value plays a decisive role in many concrete requirements.
A mix calculation for a concrete is based on the requirements of the building project. The required load properties are derived from the static calculations and dimensioning. The environmental impacts to which the component is exposed also play a role. They are defined in so-called exposure classes. A lower w / c value is required for a component that alternates between moisture and drying than for a completely dry component.
The higher the load and the stress on the concrete, the lower (closer to 0.40) the w / c value will generally be selected (with the same cement strength class).
In construction practice, (w / c) eq values are usually chosen somewhat higher for practical construction reasons (e.g. for better workability) or certain additives such as liquefiers are added. For high-strength and ultra-high-strength (UHPC) concretes, w / c values between 0.40 and 0.20 are usually required.
If concrete, which begins to stiffen after some time, is liquefied again by adding water, this has negative effects on the concrete quality and can lead to damage to the components .
Consequences of excessively high w / c values
Consequences for the concrete component can be:
- There are more and larger pores than expected. The pores in the concrete reduce the quality of the concrete. Increased porosity leads to less firm concrete with the same cement strength class.
- 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. The consequences are:
- The sensitivity to frost usually increases, with the possible consequence of flaking.
- The protection of the reinforcing steel against corrosion ( rust ) is reduced if, due to the increased capillarity, water or sufficient moist air can penetrate from the outside through the concrete cover . The load-bearing capacity decreases with increasing corrosion.
- The shrinkage of the concrete increases due to the evaporation of the excess water. When shrinking (decrease in volume), cracks and internal stresses can arise.
Consequences of w / c values that are too low
The amount of water required for complete hydration of the cement may be provided, but because of the inhomogeneous mixture, not all cement grains are often supplied with water. This means that not all of the binder hardens. If water later penetrates the concrete, this causes the concrete to swell, which leads to a loss of strength.
Sufficiently good processability can often not be ensured with marginal addition of water without the addition of additives. The consistency is also determined by the cement used and the water requirements of the aggregate used. The concrete is stiffer in the installed state and there is, among other things, the risk that reinforcing steel is not completely enclosed by concrete.
Different cement strength classes can lead to different concrete compressive strengths with the same w / c value.
A high cement content or insufficient re-moistening of the concrete promote the process of shrinkage, as does too high a water content.
Maximum water cement value
The following table shows the maximum permissible water-cement values for selected environmental impacts ( exposure classes ).
|Description of the
|Max. w / z|
|XC1||dry or constantly wet||0.75|
|XC2||wet, rarely dry|
|XC4||alternately wet and dry||0.60|
|XF1||moderate water saturation without de-icing agent|
|XA1||chemically weakly aggressive environment|
moderate humidity (influence of chloride )
(except sea water)
no direct contact with sea water
|XM1||moderate wear and tear|
wet, rarely dry (influence of chloride)
(except sea water)
|XS2||under water (sea water)|
|XF2, XF3, XF4||
Frost attack with moderate to high water saturation
with or without de-icing agent or sea water
Chemically moderately aggressive environment
and marine structures
Alternating wet and dry (influence of chloride)
(except sea water)
Tidal areas, splash water and spray mist areas,
chemically aggressive environment
|XM2, XM3||heavy to extreme wear and tear|
In Germany, the DIN 1045-2 standard regulates - in addition to general specifications, manufacturing requirements, etc. - the properties of concrete.
Concretes that contain type II concrete additives in addition to cement are referred to as the equivalent water-cement ratio. The so-called " value approach" enables the proportions of fly ash and silica dust to be added to the cement content.
It is the mass of water, the weight of the cement, and the values, as well and in each case the mass of the F lugasche or the S ilikastaubs. The masses are always based on 1 m³ of compacted fresh concrete. The k values depend on the additive and are usually kf = 0.4 and ks = 1.0. The standard specifies restrictions on maximum levels of additives depending on the type of cement.
If additives are added to the mixture in liquid form and in a total amount of more than 3 l / m³, they must also be added to the water-cement ratio in accordance with DIN EN 206-1.
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