Concrete admixtures

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Concrete admixtures are agents dissolved or slurried in water that are added to the concrete in order to improve the properties of the fresh or hardened concrete through physical and / or chemical effects, e.g. B. processability, setting behavior, hardening or durability to change. The funds are added either directly during manufacture in the concrete plant or after delivery to the construction site. In the case of site addition, the concrete is referred to as starting concrete before the agent is added.

Since concrete additive agent be added only in small quantities, they need, as opposed to concrete additive materials , when creating the recipe of the concrete not to be considered as a volume components.

Definition and impact groups

Concrete admixtures are defined in the European standard EN 934 “Admixtures for concrete, mortar and grout”, which is binding in all CEN member countries. Part 2 of EN 934 contains the definitions and requirements for concrete admixtures. Accordingly, a concrete admixture is:

"A substance that is added during the concrete mixing process in an amount that does not exceed 5% by mass of the cement content in the concrete in order to change the properties of the concrete mixture in the fresh and / or hardened state."

EN 934-2 contains definitions and requirements for the following individual impact groups:

... as well as combined impact groups:

  • Retarder / concrete liquefier
  • Retarder / superplasticizer
  • Setting accelerator / concrete liquefier

application

Concrete admixtures are in powder or liquid form and are used, for example, in:

In addition to their desired effects, concrete admixtures can also have a negative impact on the system of cement , aggregate and water , which can be determined by a corresponding suitability test. They must not enter into any disruptive interactions with other concrete components and must not negatively influence the corrosion protection of the reinforcement. The use of concrete according to DIN 1045 requires an approval (test mark) with information about the use.

dosage

Concrete admixtures are usually added in liquid form and in small quantities. The addition, based on the cement weight, is generally in the range of 0.2-2%. The dosage range recommended by the manufacturer should be observed, whereby the actual dosage must be determined by means of suitable suitability tests with the substances later used on the construction site. When dosing more than 3 l / m³ (i.e. 0.3% by volume) of concrete, the amount of water contained in it must be taken into account when calculating the w / c value . Underdosing usually significantly reduces the desired effect. Overdosing, on the other hand, can bring about undesirable effects such as setting delays, loss of compressive strength or segregation. For this reason, limit values ​​have been set for building practice. Additives from different action groups should not be premixed with one another, as this can lead to undesirable reactions. Additives with the same mode of action but from different manufacturers should not be combined with one another (exception: several superplasticizers at the same time).

literature

  • EN 934: Admixtures for concrete, mortar and grout - Part 2: Concrete admixtures - Definitions, requirements, conformity, marking and labeling

Web links

Individual evidence

  1. ↑ Entry on concrete admixtures. In: Römpp Online . Georg Thieme Verlag, accessed on July 11, 2014.