As a screed ( Old High German esterih ; via Latin astracus, astricus "patch (from clay bricks)" from ancient Greek ὄστρακον Ostracon "Shard, earthen tablets") is called in Germany the construction of the floor as a flat surface for floor coverings . Depending on the type and design, screeds are also called ready-to-use floors .
The Swiss word for screed is underlay , the word "screed" refers to the attic .
In addition to its function as a "filler and leveling material", a screed is primarily to be viewed as a load distribution layer under which heating, heat and sound insulation can be located. It can also be the direct wear layer.
A special form is the so-called " utility screed " or " exposed screed ". The screed is also the "wear layer" without a top floor covering. Screed is made from screed mortar, which consists of an aggregate (usually sand) and a binding agent (e.g. cement, calcium sulfate, magnesium oxide, bitumen). Alternatively, there is also dry screed made of precast panels
DIN EN 13318 defines the term screed as follows: Layer or layers of screed mortar that are laid on the construction site directly on the subsurface, with or without a bond , or on an intermediate separating or insulating layer, in order to achieve one or more of the following functions to meet:
- distribute the pressure evenly on the underlying insulation
- even substrate for a floor covering
- immediate usability
- to reach a given altitude
Screeds by binder
Screeds can be differentiated according to their binding agents.
Cement screed (CT)
The best-known screed is the cement screed referred to as CT (from Cementitious Screed) according to DIN EN 13 813 . It is a mortar whose grain size and mixture have been optimized for its specific use. Usually grain sizes up to 8 mm are used. For screed thicknesses over 40 mm, the maximum grain size may not exceed 16 mm. The mixing ratio of cement to sand is around 1: 5 to 1: 3.
The cement screed (CT) has the advantage of being resistant to water after hardening. And cold and heat are no problem either. In addition, high strengths can be achieved with cement as a binding agent. Disadvantages are the susceptibility of the cement to chemical attack (e.g. by acids) and the behavior on insulation or separating layers. "Shrinkage processes", which are expressed in creep and shrinkage as the screed hardens as a result of the uneven hydration , usually limit the field size to 36 m², as otherwise uncontrolled cracks will form in the construction. Furthermore, the cement screed needs a relatively long time until it is ready for covering.
Cement screed requires immediate processing after the mixing process. And a minimum temperature of 5 ° C (also at night) during the introduction and during the first three days of solidification. During the solidification phase, this temperature must not be fallen below, as otherwise severe losses in strength can be expected. The screed must also be protected from drafts and water ingress (leaking roof, pouring out of water, etc.). The draft leads to increased hydration in the surface area through the capillary draft. This means that “above” is a smaller volume than “below” and that the screed has a strong bowl. Too much heat, for example through forced drying with heaters, leads to the termination of hydration or crystal growth. This results in damage if the screed gets moisture, e.g. B. by water from a laying mortar. Accessibility depends on the type of cement (CEM I, CEM II), the thickness and the ambient conditions. A floating cement screed should be walked on after 3 days at the earliest. The first moisture measurement can be carried out after 28 days.
If the cement screed is to be provided with a floor covering , the screed must be "sufficiently dry" (3.1.1 of DIN 18365 - floor covering work). According to a recommendation from two associations from 2007, the moisture measurement should be carried out using the calcium carbide method (CM) according to DIN EN 18560. The so-called readiness for covering should be reached when the screed has a maximum residual moisture of 2.0 CM% (unheated) or 1.8 CM% (heated). Both the measurement method and the recommended limit values are criticized; According to a study published in March 2012 by the Technical Commission for Building Adhesives (TKB) and the University of Siegen, the CM limit value of 2% does not reliably separate screeds that are ready for covering from screeds that are not suitable for covering. With this limit value, wet screeds are also rated as dry. The DIN EN 18560 also says that the assessment of the readiness for covering is part of the upper floor layer's obligation to test directly before laying
So far, however, the moisture measurement for screeds has been carried out using the CM method according to DIN 18560-1. The latest version of the DIN EN 18560 standard is from 2015. This test method also applies to calcium sulphate and magnesia screeds, but not to synthetic resin and not to mastic asphalt screeds.
Cement-based rapid screeds consist of cement with additives. Different conditions apply here for hardening and readiness for covering, which depend on the type and effect of the additive. These screeds are not subject to DIN 13813 and are considered a special construction . Leaflet 14 of the Technical Commission for Building Adhesives (TKB) states that, in the case of rapid screeds, no reliable statements can be made about the readiness for covering. Screeds with ternary binders are the exception. These are three-substance mixtures consisting of Portland / normal cement, high-alumina cement (high-alumina cement), calcium sulfate and other additives. The information provided by the manufacturer is decisive.
Chiselled out cement screed is considered to be normal building rubble , provided it does not contain any organic components> 5%. The basis for this is the regulation on the European Waste Catalog (AVV).
Mastic asphalt screed (AS)
The water-free mastic asphalt screed (AS) (from Mastic Asphalt screed ) according to DIN EN 12591 consists of a mixture of bitumen and aggregates (including filler). Depending on the load requirements, maximum grain sizes between five and eleven millimeters are normally used.
Since this mixture has to be heated to a temperature between 220 ° C and 250 ° C, the mastic asphalt screed can be poured and painted during installation and does not need to be compacted. It can be introduced without thresholds or joints. Its low thermal conductivity and its sound-absorbing property can mean that, depending on the physical requirements of the ceiling construction, no insulation has to be installed. It is water- and water-vapor-tight and, in conjunction with suitable bitumen welding sheeting or asphalt mastic, represents a seal in accordance with DIN 18195.
The installation thickness of mastic asphalt screed is at least 20 mm. If the installation thickness is more than 40 mm, the screed must be applied in two layers. Before cooling down, the surface is rubbed with fine sand.
The mastic asphalt screed can either be installed as a bonded screed with a bitumen welding sheet as an adhesive bridge or as a floating screed on a separating layer with an insulating layer. Mastic asphalt can also be used as heating screed, although only hardness class ICH 10 is permitted here. In contrast to screeds with other binders, mastic asphalt screed is classified on the basis of its stamp penetration depth (according to DIN EN 12697-20). There are hardness classes IC 10, IC 15, IC 40, IC 100. The higher the number, the softer the screed.
Before covering with mineral materials (natural stone, ceramics, artificial stone) i. d. Usually to create a decoupling or a barrier layer. Mortar water is highly alkaline and can cold saponify the surface of the AS and make adhesion more difficult. In addition, there is a risk of discoloration from migrating bituminous substances. Another disadvantage is the slow movement with heat and static and dynamic loads.
The greatest advantage of the mastic asphalt screed is that it is not ready for covering. Mastic asphalt screed can usually be walked on after a short cooling time of 2-3 hours and in the best case after about 4 hours. And the laying is independent of the outside temperature or the weather. In addition, mastic asphalt screed is resistant to most alkalis and acids and is therefore also of interest for industrial floors.
The main disadvantage is the high cost. In addition, the installation on upper floors is often problematic, since the screed is hardly pumpable.
Synthetic resin screed (SR)
With the international designation SR (of synthetic resin screed ) are synthetic resin screeds, usually epoxy resin screeds, respectively. But polyurethane , polymethyl methacrylate and other plastics are also possible. Color pigments are also often added. Synthetic resin screeds are usually installed in a single thin layer of approx. 8–15 mm on dry substrates. It must be used immediately after the mixing process and compaction is usually also necessary.
These very expensive substrates are only installed in special cases, for example when short drying times or high dynamic load capacity are required. The shrinkage during polyaddition is 1 to 5 percent, depending on the product. This must be taken into account when selecting the laying material.
Synthetic resin screed is water-resistant, it forms a non-dusty, liquid-tight layer that can be used for heavy mechanical loads. The screed is insensitive to most chemicals. In addition to the high price, there is also the disadvantage that the hardeners, such as B. Bisphenol A. These are suspected of causing infertility. It is also possible to change the fire class of the overall construction if necessary. The screed loses its durability at higher temperatures and can usually not withstand temperatures above 100 ° C. Polycondensates such as polyester are not suitable due to their high shrinkage rate.
The curing times depend on the synthetic resin binder selected and the temperatures during installation and curing. After 3 to 7 days, the screed can usually be loaded.
Synthetic resin screed is considered hazardous waste and must be declared accordingly to the disposal company.
Calcium sulphate screed (CA)
Calcium sulphate screeds (CA) are screeds whose binding agent consists of calcium sulphate hemihydrate or anhydrous natural or synthetic calcium sulphate (so-called anhydrite ). Calcium sulphate dihydrate ( gypsum ) is formed when it reacts with water . Calcium sulphate screeds are marked with CA (from the English "calcium sulphate screed") according to DIN EN 13813 and are often referred to colloquially as anhydrite screed .
Due to their low shrinkage behavior, CAs do not have the usual bowls for cement screed or later lowered edges and can be laid over large areas (up to 1000 m²) without expansion joints . However, expansion joints in the substructure must still be used and expansion joints must be provided when combined with underfloor heating. They are installed as conventional screed or as flowing screed and can be walked on after 2-3 days. Calcium sulphate screeds should be subjected to higher loads after 5 days at the earliest. As flowing screeds, CAs can also be labeled CAF in accordance with DIN 18560-2. CAF have the other advantages of quick, easy-to-use laying, the lower screed thickness and the good thermal conductivity of heated screeds.
Calcium sulphate screeds are ecologically and biologically harmless and do not require any post-treatment. However, the screed must be kept warm at at least 5 ° C for at least two days after it has been applied and protected from harmful effects such as driving rain, excessive heating or drafts
CA are not water-resistant and must not be exposed to permanent moisture. They are therefore not suitable for use in commercial wet rooms or for outdoor use. In damp rooms at home (e.g. bathroom) they are protected by a bonded seal.
With later moisture penetration, a higher risk of mold is to be expected than with cement or mastic asphalt screed.
Before laying the floor covering or priming, the CA must dry to a residual moisture of 0.5%, as a heated screed to 0.3%. The residual moisture is determined with a CM measuring device.
Calcium sulphate screed is considered to be normal building rubble if there are no organic components> 5%.
Magnesia screed (MA)
Magnesia screed MA (of magnesite screed is) also under the previous designation as xylolite known. After 1945, cement was rationed, magnesite was not. That is why it can be found in many old buildings. Magnesia is known to many of the gymnastics competitions as a "drying agent" for the hands. In 1867, Stanislas Sorel discovered that magnesia solidified with magnesium chloride to form a cement-like mass. MA is easy to color and has often been mixed with wood flour or bits of wood.
Today, magnesia screed is made from caustic magnesia (MgO) and an aqueous magnesium salt solution (MgCl 2 , MgSO 4 ) according to DIN 14016 . Inorganic or organic fillers are used as an additive. Color pigments are also sometimes added
Its particular advantage is its low weight and, due to its conductivity, it can be used as an antistatic prefabricated floor. It also has good heat and sound insulation values. Its major disadvantage is its sensitivity to moisture and corrosiveness to metals, since when water is added the chloride and magnesium hydroxide contained are "washed out" and the MA swells. It must never come into direct contact with watery mortar. A typical use today is as a utility screed for large dry areas.
Like most other screed mortars, magnesia screed must be installed immediately after the mixing process. The temperature must be kept above 5 ° C during installation and the following two days. In addition, the fresh mortar must be protected from heat, driving rain and drafts for at least two days. The screed can be walked on after two days at the earliest and should not be subjected to higher loads for at least five days. Furthermore, magnesia screed over prestressed concrete ceilings is not permitted due to the high risk of corrosion.
According to DIN 18560, reinforcement for screeds is generally not required. It is mainly useful for cement screeds on insulation layers to accommodate stone or ceramic coverings. In addition to the possibility of reinforcement with screed grids, there is also fiber reinforcement. The screed grids are difficult to install precisely on soft insulation layers and also make the clean installation of a screed layer more difficult, especially on insulation layers or with heating elements. Fiber reinforcement, on the other hand, is easy to install, the fibers (steel fibers, alkali-resistant glass fibers, plastic fibers) are added to the screed mortar. Fiber reinforcement is mainly used to reduce cracks. A complete avoidance of cracks cannot be achieved even with fiber reinforcement. Fibers can only take on the function of constructive reinforcement in larger quantities, which is unusual for screeds. The addition of fibers can reduce the formation of shrinkage and early shrinkage cracks in the screed. It should be noted, however, that adding fibers reduces the consistency of the screed mortar and makes processing more difficult. Fiber reinforcement is significantly cheaper than previously used steel reinforcement meshes.
Alkali-resistant (AR) glass fibers are recommended for all cement-bound screeds . These are also stable in the alkaline environment in the cement. It is particularly useful for heated screeds or substrates for ceramic or natural stone coverings.
The construction types of the screed are not subdivided into screed binders, but according to the construction methods or the type of construction.
The bonded screed is applied directly to the load-bearing sub-floor and is firmly connected to it. Since all forces are diverted directly into the subsoil, the load-bearing capacity through the subsoil, i. d. Usually a concrete ceiling or limited by the compressive strength of the screed.
The thickness of the screed does not therefore play a decisive role. In the case of single-layer cement, calcium sulfate, magnesia or synthetic resin screeds, the nominal thickness should be a maximum of 50 mm. For mastic asphalt screeds between 20 and 40 mm. The most important thing when producing a bonded screed is the correct preparation of the substrate so that there are no cavities and the bond between the screed and the substrate is guaranteed. To do this, the surface must be cleaned thoroughly. It should also be as free of cracks as possible. For a good bond, it can also be useful to apply an adhesive bridge, for example made from a plastic dispersion or emulsion, to the base layer. Partial blasting or milling and, if necessary, pre-wetting of the base course is also necessary. If pipes or cables are on the ground, they must be embedded in a leveling screed. Even if the supporting substrate is not level enough, a level leveling screed must be installed on which the bonded screed can then be built. A bonded screed should be selected, especially with high dynamic loads. DIN 18560-3 applies.
Screed on a separating layer or separating layer
Another possibility to construct a screed is as a screed on a separating layer. There is a thin layer between the supporting sub-floor and the screed that separates the components from one another. This layer usually consists of two layers, so that the screed is decoupled from the supporting sub-floor and tension-free movement is possible. In the case of calcium sulphate and mastic asphalt screed, the separating layer should only be applied in one layer. The separating layer and an additional separating strip are also laid on the adjacent walls to prevent clamping. The material used for the separating layer is, for example, polyethylene film, plastic-coated paper, bitumen-soaked paper or raw glass fleece.
The screed construction with separating layer is used, for example, in the case of high bending loads in the supporting structure or if the supporting concrete is water-repellent. To protect the floor from rising damp, a seal can be installed, which is also counted as a layer of the two-layer separating layer.
For a well-functioning construction it is important that the supporting base is a flat surface without irregular elevations or disturbing pipes. Creep and shrinkage and the associated deformation of the raw concrete can also influence the evenness. This can restrict the movement of the screed and cause cracks to form due to constrained stresses. In the case of an old building, the risk is usually no longer given, as there are virtually no shrinkage effects in the older subsurface.
For a screed on a separating layer (DIN 18560-4), the required strength and hardness classes are regulated in DIN EN 13813
Screed on an insulating layer ("floating screed" or "heated screed")
Another type of construction is screed on an insulation layer. The screed rests on an insulating layer and is laterally encased by insulating strips so that there is no direct connection to the adjacent sub-floor and the walls, the screed "floats", so to speak. The screed is laid on a water-impermeable film that protects the insulation layer from moisture and further dampens the sound transmission. If heating elements are built into the screed or the insulation layer, one speaks of a heated screed .
The insulation layer has the function of impact sound insulation or thermal insulation. It is also possible to install several layers of insulation. Mostly, insulating mats or panels are used as the insulating layer. Typical materials are e.g. B. Polystyrene rigid foam (EPS), extruded polystyrene rigid foam (XPS), mineral fibers (rock or glass wool) or soft wood fibers. When choosing the insulation material, deformation stability is a crucial property.
Due to the soft insulation layer, damage caused by subsidence in floating screeds occurs again and again. This can be due to excessive loads, which are particularly problematic in panel corners. An excessive load can create a restraining effect, which causes the screed to lose its floating character. In particular, the problem of so-called "breaking up" is a recurring problem.
There are different types of heating screed. The heating elements can be arranged inside (type A), below the screed (type B) or in a leveling screed (type C).
In terms of standards, DIN 18560-2 applies, along with various leaflets from the ZDB ( Central Association of the German Building Industry ) and the BEB ( Federal Association of Screed and Covering ). In addition, measuring points for the CM moisture measurement must be identified, at least 2 measuring points per room, and at least 3 in rooms over 50 m². Expansion joints must be installed for heated screeds with more than 8 m side length or more than 40 m².
Leveling (leveling) with a long leveling rod
Readiness for covering
A definition of readiness for covering reads: "Readiness for covering is the achieved state of a screed with regard to setting and drying reactions, in which it is suitable for permanent absorption of a covering free of damage and defects." Three essential time-dependent parameters are named:
• Sufficient drying
• Sufficient strength
• Sufficient shrinkage reduction
Usually, however, the readiness for covering is only linked to sufficient drying; the CM measurement is used for this. A screed must have reached the so-called equilibrium moisture content in order for it to be considered ready for covering. This means that its water content is in equilibrium with the surrounding room air. For natural stone and ceramics, the deformation stability is also crucial, while for parquet or soft floors such as PVC, linoleum or rubber, moisture is crucial.
For the natural stone area it means that the expected shrinkage of the screed must be completed as far as possible. If the room temperature is too high or the underfloor heating is switched on, the screed looks dry, but is far from ready for covering. Sufficiently long drying times (including hardening) must be observed for the screed mortar mixed with water.
Depending on the air change, room temperature, relative and absolute humidity, this time can be considerably longer. The values for the permissible residual moisture up to readiness for covering depend on the type of screed, on the unheated or heated construction and on the subsequent type of covering. Forced drying can lead to interrupted hydration and, if the moisture penetrates later (mortar of the surface covering), it can cause deformations with cracking. The guide values for the moisture content at readiness for covering according to the CM method are 1.8 CM-% for heated cement screeds (for unheated 2 CM-%). If the measured value falls below the guide value, the screed is ready for covering.
Increased dryness is necessary for calcium sulphate screeds. The guide value is 0.3 CM-% (with unheated 0.5 CM-%). In addition, calcium sulphate screeds must be protected from rising damp or water vapor diffusion with vapor barriers and seals.
The specified values correspond to CM-%. These values are determined with a calcium carbide measuring device (CM device). The CM measurement is prescribed by standards. A small amount of screed is removed from the existing screed, crushed and shaken in a steel pressure bottle with the addition of calcium carbide. When the pressure rises, the calcium carbide reacts with the residual water to form ethine (acetylene). The pressure is measured using a manometer and can be converted to CM% using a calibration table. The readiness for covering can be shortened with additives that contain so-called “quick-drying screeds”. These rapid screeds are not standardized screeds, but special constructions. They should therefore be used with caution, because in some cases no reliable statements can be made about the readiness for covering; the manufacturer's information must be relied on.
A dry screed is a screed made of prefabricated parts that are positively connected to one another on the construction site. That is why it is also known under the name “precast floor screed” or “dry sub-floor”. All dry screeds are not standardized. These are generally special constructions that have to be specially commissioned. Here the planner has a significantly higher responsibility or planning liability. The VOB / C ATV DIN 18340 "Dry construction work" applies and DIN 68771 must be observed for precast screeds made from chipboard.
The following materials are used for dry screeds:
• Chipboard (also cement or magnesite bound)
• OSB boards, hardwood fiber boards
• Gypsum fibreboard, plasterboard
• Concrete and cement screed slabs.
Leveling is necessary on uneven surfaces, e.g. B. by a bed. Depending on the system, this consists e.g. B. from clay balls, plastics or other materials. A leveling of the subsurface would be possible with smaller unevenness. Here, however, the different expansion behavior and a reaction to dynamic loads must be taken into account (building physics). In wet areas, the resistance to moisture must also be taken into account. It is necessary that the individual prefabricated panels are structurally connected to form a load-bearing screed panel. There are the following four connection types:
• Butt jointed and glued
• Glued connection system with tongue and groove
• Wide rebate, glued or screwed with glue
• Two-layer installation with staggered joints, layers glued / screwed over the entire surface
Advantages and disadvantages of dry screeds
- Advantages of dry screeds:
- no waiting time due to drying,
- no drying protocols,
- no CM measurement necessary,
- no moisture load on the building,
- partially lighter structure, similar to a magnesite screed,
- lower construction heights than conventional screeds are possible,
- larger differences in height can be compensated for by embankments, thus lower weight load.
- less mass affected with underfloor heating, therefore rooms can be heated up faster.
- Disadvantages of dry screeds:
- level ground is required (fill, leveling),
- the combination of construction and type of covering may have to be calculated by a building physicist,
- lower resilience with dynamic loads, such as wheelchairs,
- Standard tables for impact sound insulation are not applicable,
- upper temperature limits must be observed for underfloor heating,
- the moisture sensitivity depends on the screed material and the height compensation system,
- Standard tables for thermal conductivity in heating systems are not applicable,
- higher cost,
- Generally special constructions with a higher liability risk for the planner and the executing company.
The applicable standards for screeds within the EU are:
- DIN EN 13318 screed mortar and screed terms
- DIN EN 13813 Screed mortar and screed compounds - properties and requirements
- DIN EN 13892 test method for screed mortar and screed compounds, parts 1 to 8
In addition, the following applies in Germany:
- DIN 18560 screeds in construction, German application rules
The conformity control for factory-made screeds recorded according to standards includes the initial test and a factory production control or self-monitoring.
An initial test must be carried out at the start of production of the screed or before the manufacture of a new product or when reactants are changed. A change and a conversion of the manufacturing process also require a respective initial test. The tests required for the respective type of screed are regulated in DIN EN 13813.
In so-called construction screeds a test of the effected delivery notes as well as a visual inspection of the reactants . The manufacturing process as such must be checked at regular intervals. In exceptional cases, a hardening test may be necessary and in special cases, if there are significant doubts about the quality of the screed in the building, a confirmation test may also be necessary
- Federal Association of Screed and Covering e. V.
- Association of Austrian Screed Layers
- Infoline flooring - online lexicon
- bga - Advice center for mastic asphalt application e. V.
- DIN EN 13318: Screed mortars and screeds - terms .
- Brokamp / Trettin: Readiness for covering and moisture, TKB report 1