Anhydrite screed
The anhydrite screed (AE) or calcium sulphate screed (CA) is a type of screed and consists of anhydrite binder , aggregate (up to a grain size of 8 mm) and added water . Appropriate additives may be included to improve processing. The anhydrite screed can be walked on about two days after installation and only lightly loaded after five days at the earliest. It must not be exposed to permanent moisture load, which is why this screed is particularly suitable for dry areas in a building. Compared to cement screed, anhydrite screed has a lower strength and can therefore be subjected to less stress.
The anhydrite floor screed (AFE) represents a further development. It levels itself due to its liquid consistency. In addition, due to the lower tension during the drying process, additional expansion joints can be dispensed with on surfaces up to 1,000 m² (if no underfloor heating has been installed) .
The main component is anhydrite as a binding agent , which hydrates relatively quickly with water to form gypsum (calcium sulfate dihydrate) and solidifies in the process. As a rule, synthetic anhydrite is used, which is obtained by burning gypsum (e.g. FGD gypsum from flue gas cleaning in power plants). Mineral (granular) natural anhydrite is also used less frequently. Limestone and quartz sand are used as aggregates .
processing
The AE self-leveling screed is either delivered dry to the construction site (as bagged goods or loose in a silo / container) and in a normal plastering machine, which works continuously, its consistency can be pumped by adding water and is pumped to the installation surface by means of hoses, or it is delivered ready-made in the truck mixer and quickly installed with a worm or piston pump provided on the vehicle or separately. Thanks to this liquid consistency, the leveling of the flowing screed can be done almost automatically. After pumping in, the screed is knocked through with a coarse street broom or a so-called squeegee (buffing bar) (in a cloister) in order to remove air bubbles trapped during the pumping in. The surface is made horizontal and even. The longer the buffing rod, the greater the accuracy and evenness of the finished screed surface. Since this factory-made mixture is only mixed with water, the corresponding machines are able to achieve high mixing and conveying capacities. When laying large areas, for example in industrial halls, cast areas of 1,300 m² to 1,500 m² are possible in one day.
AE self-leveling screed can be installed as a so-called composite screed, screed on a separating layer and screed on an additional insulating layer (also known as floating screed), as industrial and installation floors, and as heating screed for underfloor heating systems. When preparing the substrate, it should be noted that due to its liquid consistency, it can flow away through unsecured or uncovered openings and form sound bridges on insulation layers or in the case of insufficient decoupling from walls or pillars. The AE self-leveling screed is self-compacting, so that no compaction work is required by the processor. Due to its components, it has very high flexural and tensile strength values . AE self-leveling screed based on calcium sulphate has low shrinkage values during hardening and drying and can therefore save time in surfaces because it can be applied over a large area in one continuous operation. The usual bulging of the cement screed (raised edges caused by uneven shrinkage) in the corner and edge areas and the tearing of the edge joints (during later use and due to the end of the shrinkage process) are not observed with AE self-leveling screed. The joint designs for screeds are regulated in DIN 18560 , Part 2, and also apply to AE flowing screed.
Installation as heating screed
Expansion joints are necessary for heated screeds if the side length of the screed surface is more than 6 m. Movement joints are also required in doorways if there are several rooms arranged one behind the other within an apartment. The flowing screed offers its special advantages when laying on underfloor heating . A tight connection to the heating pipes and the high thermal conductivity, which are above the values for thermal conductivity in DIN 4108 for cement screed , ensure unhindered and even heat emission from the heating hose or the like. to ambient air. Due to its thinner thickness of only 40 to 45 mm above the heating hose, this screed has a short heat-up time.
When used as a heated screed, it is heated up seven days after installation in order to shorten the natural drying-out times, which then significantly accelerates construction progress.
The heated screed is also used more and more as a decorative floor. When the screed has dried out, it is sanded down to the grain and then sealed to be transparent.
The CAF can also be used in different colors and with different percentages of pigments, thus opening up a wide range of possibilities.
Construction technology and processing instructions
AE self-leveling screed is produced in accordance with the specifications of DIN 18560 and in strength classes AE 12, 20, 30 or 40. Depending on the expected traffic load in accordance with DIN 1055 and the type of floor construction, a nominal thickness between 35 and over 90 mm is installed. Due to its chemical composition, anhydrite screed is highly absorbent and therefore the surface to be covered (e.g. with tiles) must be fully primed. The sintered layer that forms on the surface must be removed before this primer (for example by grinding).
When processing AE self-leveling screed, precautionary measures (gloves, respiratory protection) are necessary due to the caustic properties of the dry product. However, the liquid material and the hardened screed are no longer a source of pollutants. If synthetic additives are used, however, subsequent outgassing of residual or auxiliary materials cannot be ruled out.
Outside areas and damp rooms
Due to its sensitivity to permanent exposure to moisture, use outdoors, but also in wet and damp rooms that are not further sealed, is fundamentally excluded. According to the ZDB leaflet " Instructions for the execution of liquid composite waterproofing with cladding and coverings made of tiles and slabs for indoor and outdoor areas " (January 2010), the general rule is that no substrates sensitive to moisture may be used for surfaces with floor drains.
In bathrooms or damp rooms in residential buildings, the screed surface must be sealed (for example, using a tile structure with a bonded seal) so that surface moisture cannot penetrate the screed. Screeds must also be sealed against soil moisture, non-pressing and pressing water (bitumen sealing membrane).
Anhydrites are typical quick binders, but this only applies under optimal climatic conditions (not too cold, dry air). A vapor barrier must be installed in areas where vapor diffusion or moisture is to be expected . Vapor diffusion or moisture then leads to damage if z. B. due to the short construction times that are common nowadays, the concrete ceiling is not sufficiently dry and the excess and escaping water reaches the screed in the absence of a vapor barrier. Damage of this kind occurs mainly in connection with vapor-proof coverings such as PVC , linoleum , rubber or carpet coverings with dense backing if these were laid before the component was completely dry.
Norms and standards
- DIN EN 13813 - Screed mortars, screed compounds and screeds - Screed mortars and screed compounds - Properties and requirements
- DIN 18353 - screed work
- DIN 18560 - Screeds in construction
Individual evidence
- ↑ a b Hansjörg Frey: Structural engineering - specialist knowledge of construction . Europa-Lehrmittel Verlag, Haan-Gruiten 2003, ISBN 3-8085-4460-0 , p. 496 .
- ↑ Martin Mittag: Building construction theory . Vieweg Verlag, Braunschweig 2000, ISBN 3-528-02555-7 , p. 298 .