Air entrainment

Air-entraining agents are chemical substances (usually liquids) which, if used in concrete in the cement paste many small distributed air pores, which remain stable during mixing and compacting.

Method

The addition of air-entraining agents (LP ) of the frost resistance and / or freeze- thaw and deicing salt resistance of the later hardened concrete is increased because the air voids are not continuous and does not fill completely even after long immersion in water of the concrete with water. The component temperature falls below a certain value, the water contained in the large pores of the concrete from freezing. Along with this is an increase in volume of the freezing water. In the not yet frozen water by hydrostatic pressure is generated which, when the tensile strength of the cement paste is exceeded, the hardened cement paste breaks. 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 void for expanding water ( 0.1-0.2 mm ideally). A dense system of tiny air pores is therefore low (pore diameter <0.3 mm and spacing factor AF ≤ 0.2 mm).

In addition, air-entraining agents cause a ball bearing -like effect in the mixed still liquid concrete, so that a plasticizing effect, similar to through concrete plasticizers and superplasticizers, can be observed, thus saving the addition of water or an improvement in the consistency ( softer concrete) can be achieved.

Chemical substances

  • Root resins ( Tall and balsam resins)
  • Lignosulfonates
  • Salts of carboxylic compounds and
  • Protein acids

Operation

Foam and air bubbles can be stabilized in aqueous solution by preferential adsorption on surfaces and reducing the surface tension of the water.

Application

Among other things in storage areas, concrete roadways ( highway construction / bridge caps), Räumerlaufbahnen of annular clarifier in wastewater treatment plants and water structures / parts in frost- free outdoor area is " air-entrained concrete " used. The concrete technological framework of the exposure classes XF2, XF3 and XF4 according to EN 206-1 and in Germany in addition are also DIN 1045-2, as well as, for example, the ZTV -ING to be followed in.

Similar processes in other departments

Another interesting and technologically important and commercially most interesting use of pore-forming substances can be found in the semiconductor industry. Pore ​​formers have been used recently in the semiconductor production to produce layers with low dielectric constant, so-called low-k layers. With the help of suitable pore former succeeds in microporous thin films with a dielectric constant < 3.9, the dielectric constant of the traditionally used silicon dioxide (SiO2) to produce. Low-k dielectrics are used for the isolation of conductive traces and electronic devices on the semiconductor chip and are an essential component of current high-performance memory chips and microprocessors.

Swell

  • Script Materials Science II University of Karlsruhe, 1998.
Semiconductor memory
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