Concrete liquefier

As concrete plasticizers (short BV , or subsequent addition) flow agents (short: FM ), are concrete admixtures denotes that the consistency of fresh concrete change, so that the concrete in otherwise uniform mixture is flowable. The purpose of this change, called “plasticization” or “liquefaction”, is either to improve the processability or to improve the strength while maintaining the processability.

The main difference between concrete plasticizers and superplasticizers lies in the amount and the time of addition. Concrete liquefiers must be added to a mixture in the concrete plant. Superplasticizers, on the other hand, require a significantly lower dosage and can also be used subsequently, e.g. B. on arrival at the construction site to be added. Due to the stronger effect of FM, these agents were also called high-performance plasticizers or high-performance concrete plasticizers ( HBV ) in the past . In many cases, the modern, high-performance superplasticizers are used both in the concrete plant and on the construction site, so that the distinction between BV and FM only depends on the time of addition and no longer on the substance added.

Means used

Chemicals that liquefy concrete work either because they are dispersants or because they are surface-active substances. In general, the plasticizing effect of the agents is not permanent, which is why superplasticizers have to be topped up on the construction site in many cases.

Surface-active substances

Surface-active substances reduce the surface tension of the water added to a concrete. This means that less water is required to wet the cement grains and the aggregate, so that the “excess” water makes the mixture more flowable. On the other hand, the mixture can also be reduced by this amount of water, so that the w / c value is reduced and the strength of the concrete is increased. The workability would remain the same in this case.

Naphthalene sulfonates are used for concrete plasticizers and superplasticizers and have good plasticizing properties. These sulfonates are a by-product of the cellulose industry . However, they can also lead to an additional delay in the concrete and to the entry of air pores.
Synthetically produced lignosulfonates are only used for concrete liquefiers, since a higher dosage is necessary than with other substances. Depending on the quality of these sulfonates, an increased influx of air pores can also occur.

Dispersing substances

Dispersing substances are deposited on the cement and rock grains of the mixture and polarize them in such a way that the individual particles repel each other and a more fluid mixture is created. Here, too, the water content can be reduced in order to achieve a lower w / c value with the same consistency.

Melamine resins are used both as concrete liquefiers and superplasticizers and do not lead to an additional delay in the fresh concrete. Because the liquid concrete becomes “stickier”, air pores hold themselves well in the concrete despite the liquefaction, which is why these resins can be used well for air-entrained concrete .

Polycarboxylates or polycarboxylate ethers are used for concrete plasticizers and superplasticizers ("PCE superplasticizers") and are characterized by a great effect at low dosages. The liquefaction of a concrete with PCE lasts for a long time, but the mixture remains well cohesive. A disadvantage is a sometimes high sensitivity to the temperature and other environmental influences, so that the amount added must be changed significantly if the mixture remains the same in order to avoid adverse effects, such as B. to avoid an increased delaying effect or a strong tendency to segregate. PCE superplasticizers are the latest development in the field and are sometimes referred to as "third generation superplasticizers". As high-performance concrete liquefiers (HBV), they are also used in self-compacting concrete , from which the contained air bubbles rise largely automatically.

Individual evidence

↑ ^a ^b Cement leaflet concrete technology: Concrete additives - admixtures and additives. (PDF; 444 kB) Association of German Cement Works e. V., September 2005, accessed October 17, 2013 .
↑ ^a ^b ^c ^d ^e ^f Andrea Kustermann: Building Materials FHM - Concrete Additives - Concrete Additives ( Memento from October 17, 2013 in the Internet Archive ), Institute for Construction Materials - University of the Federal Armed Forces Munich, accessed on October 17, 2013 (PDF; 354 kB ).
↑ Concrete raw materials and technology, concrete construction technology. (PDF; 1.1 MB) Association of German Cement Works e. V., p. 104 , accessed on October 17, 2013 .

[betonorg-1] Cement leaflet concrete technology: Concrete additives - admixtures and additives. (PDF; 444 kB) Association of German Cement Works e. V., September 2005, accessed October 17, 2013 .

[unibw-2] ↑ ^a ^b ^c ^d ^e ^f Andrea Kustermann: Building Materials FHM - Concrete Additives - Concrete Additives ( Memento from October 17, 2013 in the Internet Archive ), Institute for Construction Materials - University of the Federal Armed Forces Munich, accessed on October 17, 2013 (PDF; 354 kB ).

[3] Concrete raw materials and technology, concrete construction technology. (PDF; 1.1 MB) Association of German Cement Works e. V., p. 104 , accessed on October 17, 2013 .