Air entrainer
Air entraining agents are chemical substances (mostly liquids) that, when used in concrete construction, create many small air pores in the cement paste that remain stable even during mixing and compression.
Procedure
The addition of air-entraining agents (LP) increases the frost resistance and / or the frost-thawing agent resistance of the later hardened concrete , since the air pores are not continuous and do not fill completely with water even after the concrete has been stored in water for a long time. If the component temperature falls below a certain value, the water contained in the large pores of the concrete freezes. This is accompanied by an increase in the volume of the freezing water. This creates a hydrostatic pressure in the not yet frozen water , which, when the tensile strength of the cement paste is exceeded, bursts the cement stone . The air pores of the concrete mixed with LP therefore offer the not yet frozen water an alternative. The decisive factor is the short distance between the air pore and the expanding water (0.1–0.2 mm ideally). A dense system of the smallest air pores is therefore favorable (pore diameter <0.3 mm and distance factor AF ≤ 0.2 mm).
In addition, air- entrained agents cause a ball-bearing-like effect in mixed concrete, which is still liquid, so that a liquefying effect, similar to that of concrete liquefiers and superplasticizers , can be recorded and thus added water can be saved or the consistency can be improved (softer concrete).
Chemical substances
- Root resins (tall and balsam resins)
- Lignosulfonates
- Salts of carboxyl compounds and
- Protein acids
Mode of action
Foam and air bubbles are stabilized in the aqueous solution through preferential adsorption on surfaces and reduction of the surface tension of the water.
application
"LP concrete" is used, among other things, for storage areas, concrete lanes (motorway construction / bridge caps), scraper tracks for ring-shaped clarification basins in sewage treatment plants and hydraulic structures / parts in non-frost-free outdoor areas. The concrete technological framework conditions for exposure classes XF2, XF3 and XF4 according to EN 206-1 and in Germany also DIN 1045-2 and e.g. B. to comply with or apply the ZTV-ING .
Alternative procedures
In the meantime, finished, elastic hollow spheres with a diameter of less than 0.08 mm are available, which can be mixed into the concrete instead of foam.
Similar procedures in other departments
Another interesting, technologically important and commercially highly interesting application of pore-forming substances is found in the semiconductor sector . Pore formers have recently been used in semiconductor production to create layers with a low dielectric constant , so-called low-k layers . With the help of suitable pore formers, it is possible to produce microporous thin layers with a dielectric constant <3.9, the dielectric constant of the traditionally used silicon dioxide (SiO 2 ). Low-K dielectrics serve to isolate conductor tracks and electronic components on the semiconductor chip and are an essential component of current high-performance memory chips and microprocessors .
swell
- Script Building Materials Science II. University of Karlsruhe, 1998.
Web links
Individual evidence
- ↑ bvfs.at: White tub - waterproof concrete
- ↑ Concrete: types, manufacture and properties . John Wiley & Sons, 2002, ISBN 978-3-433-01340-3 , pp. 102 ( books.google.de ).
- ↑ Chapter 3.16 Elastic hollow spheres for air-entrained concrete , In: Betontechnische-Daten.de of HeidelbergCement AG
- ↑ Silicon Nitride, Silicon Dioxide, and Emerging Dielectrics 9 . The Electrochemical Society, 2007, ISBN 978-1-56677-552-6 , pp. 591 ( books.google.de ).