The fourth edition of the book has been enlarged to cover three times more pages than its predecessors.
For this reason, a new bigger format had to be chosen.

Data on the eight edition:

Format (cover): 245 mm breit x 310 mm hoch
Pages: 1120
Volumes: 2
Illustrations: 682
Cover: cardboard +bookmark
Print (cover): 4 colours
Print (contents): 2 colours
Language: German

Due to the new size and more colours the selling price now is 109.00 euros (incl. VAT).
Large parts of the eight edition have been fully revised and a range of new chapters has been added, of which a representative sample is given on the right.

  • Information on the new terminology regarding screeds and insulating materials
  • Information on the new screed strengths and the changed testing regulations
  • New chapter „Fire protection of floors“
  • New chapter „Slab flooring“
  • New chapter „Slip resistance of floors“
  • New chapter with technical data on various floor materials e.g. apparent density, thermal conductivity, water vapour impermeability, fire protection classes
    Enlargement of the chapter on reinforcements with regard to all kinds of fibres and to the way they work
  • Enlargement of the chapter on „Textile floor coverings“
  • Enlargement of the chapter „Floor coverings“ by the topic of deformation (i.e. expansion and contraction of parquet floors)

example page FUSSBODEN ATLAS®

example page FUSSBODEN ATLAS®

example page FUSSBODEN ATLAS®


Volume 1
 1 Preface
How to use this book
 1.2 Contents
 2 Index
 3. Legend and technical data on various floor materials
 4 Information on the author and purchase of the book
 5 Why a book on floor constructions?
 6 History of floors
 7 Points to observe before laying the screed
 8 Screed bases
 9 Moisture protection
 10 Thermal protection/fire protection
 11 Sound insulation
Volume 2
 12 Screed
 12.1 Differentiation according to the bonding agent
 12.2 Differentiation according to the laying method
 12.3 Heated screeds
 12.4 Finished screed surfaces
 12.5 Imposed load
 12.6 Reinforcement
 12.7 Joints
 12.8 Drying process
 12.9 Deformation
 12.10 Accelerated Screeds
 12.11 Cracks
 12.12 Lightweight walls
 12.13 Outdoor screeds
 12.14 Precast slab flooring
 12.15 Estrichtechnik außerhalb von Deutschland
 13 Points to observe after laying the screed
 14 Testing of screeds
 14.1 Strength class
 14.2 Screed thickness
 14.3 Dimensional tolerance
 14.4 Surface quality
 14.5 Moisture content
 15 Underfloor installations
 16 Examples of defects and damage – an expert’s practical experience
 17 Floor coverings
 17.1 Elastic floor coverings
 17.2 Textile floor coverings
 17.3 Wood coverings
 17.4 Stone and ceramic tiles
 17.5 Measures to improve surface quality
 17.5.1 Impregnation
 17.5.2 Sealing
 17.5.3 Silication
 17.5.4 Coating
 17.6 Slip resistance of floors
 17.7 Depreciation due to defective or damaged floor constructions
 18 Suggestions for invitations to tender
 19 And if things do not work out as they should one day …
 20 … we will be glad to help you!
 21 – search engine building / construction industry
 22 QUO-VADIS – which road will the building/construction industry take?
 23 Bibliography
 24 List of addresses
 25 Thanks to…
 26 Further reading

Sample text (translated to English) without illustrations from the chapter 12.1 Differentiation according to the bonding agent and chapter 12.2 Differentiation according to the laying method

12.1 Differentiation according to the bonding agent and

There is an entire range of ways of specifying screeds. Primarily, these are differentiated in terms of the respective bonding agents that they use. These are the components in the screed which, together with the mineral aggregate (normally sand and/or gravel), and possibly also the addition of further substances, form the screed matrix and provide strength. The following designations are used for screeds:

  • Cementitious screed CT
  • Calcium sulphate screed CA
  • Mastic asphalt screed AS
  • Magnesite screed MA
  • Synthetic resin screed SR

With cementitious screeds and calcium sulphate screeds, their installation, into a conventional design (semi-dry/plastic consistency) or as a flowing screed (free-flowing consistency), must also be taken into consideration.


All screed layers must be as uniform as possible in terms of their thickness (see DIN 18 353, 3.1.4), apparent density and their mechanical properties, and have an even surface in accordance with DIN 18 202. In addition, they require a surface stability suited to their purpose. Where additional abrasion resistance requirements are defined for MAs, SRs and CTs for special applications, the parameters for these requirements must correspond to the strength class. Material and production related colour and structural differences in the surface of the screed are permitted.

12.1.1 Cement (in accordance with DIN EN 197, DIN 1 164, DIN EN 14 216 or with general German construction oversight approval) l176l

One advantage of cement is that it is precisely standardised, which cannot be said for all bonding agents. Legal experts point out that in theory, even non-standard cements can be used for screeds, as floor structures are statically irrelevant, and therefore require no construction oversight approval. This is the case for a number of ‚rapid-cure cements‘. It is however generally recommended that standardised products be used in order to ensure an appropriate levels of quality.

The following types of cement are mainly used to manufacture cementitious screed:

  • CEM I Portland cement
  • CEM II/A-LL Portland limestone cement
  • CEM II/B-M (S-LL) Portland composite cement
  • CEM II/A-S Portland slag cement
  • CEM II/B-S Portland slag cement
  • CEM II/B-T Portland shale cement

The density of cements before adding water can be estimated as follows (individual products can differ):

Cement type Density [kg/dm³]
Portland pozzolana cement
Portland fly ash cement
Portland slag, blast furnace,
Portland shale,
Portland limestone cement
Portland cement 3.1
Portland cement-HS 3.2

This value is therefore decisive when calculating the mixture for a screed formulation. The apparent density of the cement brick is dependent on the respective applicable W/Z value. At a W/Z value of 0.40, the water which is present binds 100%, both chemically and physically. For W/Z values above 0.40, excess water remains in the mixture. As time passes, the excess water dries, leaving voids. It can be said that the higher the W/Z value, the lower the apparent density of the cement brick. When cement is loaded loose in sacks and/or silos, this is called ‚bulk density‘. Bulk density can range between 900 kg/m³ and 1200 kg/m³. Cementitious screed CT

This consists of the following components: Sand and/or gravel, cement, and possibly an admixture and water. The substances mentioned above can be brought to the work site either individually or in a processing unit (silo, agitating lorry). Taking into consideration the appropriate mix proportion, the materials for in-situ screed are mixed in a mixing vessel before being transported to the application site by a pump with compressed air. Of course, it is also possible to receive the material as ready-mix mortar from a concrete plant. Cementitious screed mortars should be applied and distributed immediately upon completion of the mixing process and/or upon delivery to the construction site, and, for non-free flowing consistencies, trowelled up and compacted. The cement content must be limited to the necessary quantity. As a hydraulic bonding agent, cement can also set under water and is impervious to water when dry. Subsequent water loading followed by rapid drying can however lead to system deformation and stress (see Figure … in Chapter

In the event of stress by de-icing chemicals in exterior areas, the screed should be produced in accordance with DIN EN 1 992-1-1. A cementitious screed can be walked on after three days, depending on the temperature and climate of the construction site, and fully load-bearing after 28 days (see also Chapter 13). It is suitable for indoor and outdoor use. If the screed is laid on separating layers open to diffusion (e.g. sarking membranes), the conglomerate can be referred to as mainly ‚open to diffusion‘ and is appropriate for use on wooden beam floors. Special admixtures can significantly reduce the drying time. The use of appropriate ‚rapid-cure cements‘ enables the installation of a floor covering 24 hours after applying the screed. Correct thickness and strength dimensioning can allow high live loads. CT is suitable for use as a base for all commercially available floor coverings and coatings. Maximum producible quality is achieved by combining with a hard aggregate into a CT – C75 – F9 (Please refer to Chapter 12.4.2 for instructions for heavy-duty CT). Cementitious screeds shrink as they dry, i.e. they shorten by approx. 0.4 to 1.0 mm per [m] of slab length.

A properly planned team of screed layers can lay approximately 120-150 m2 of floating screed in a conventional design daily. Before laying the surface, the layer must check the existing screed surface. In some cases, gentle light grinding or mechanical brushing is necessary in order to achieve a surface suitable for laying. Even where the technical regulations stipulate otherwise, many cementitious screeds are laid without first abrading, vacuuming, pre-painting and levelling the surfaces. This is however subject to the company’s warranty, as DIN 18 365 requires levelling, regardless of the flooring type. Direct laying of this kind is risky when working with relatively inappropriate sands or if thin, elastic floor coverings (linoleum, PVC, etc.) are used. Examples of names in accordance with Screed

Standard DIN 18 560 – CT – F4 – S 55

CT Cementitious screed
F4 Flexural tensile strength 4 N/mm2
S Floating application
55 Nominal thickness 55 mm

Although DIN 18 560 stipulates no compressive strength class for floating screeds, DIN EN 13 813 does however do so for mortar. For directly used CTs, a Böhme abrasion resistance class (A) can be provided when laid on a separating layer; for directly used bonded CTs, the provision of this category is compulsory. Mineral aggregates l35l & l151l

Mineral aggregates are the granular components of the screed mortar, such as e.g. sand, crushed rock, gravel and crushed anhydrite. Mineral aggregates with a grain size of less than 63 µm are known as ‚fines‘. In addition to cement and water, mineral aggregates (normally a natural supplement) make up the largest amount of space in screed mortar, with approx. 75 to 80%. It also has a high compressive strength of approx. 150 N/mm². Because of this, their properties influence the overall ‚cementitious screed‘ product to a significant extent.
The requirements for mineral aggregates are set forth in European Standards DIN EN 12 620 and DIN EN 13 139. Corresponding requirement categories are defined for each of the relevant properties. The manufacturer must indicate the relevant categories in their product varieties for the user. In addition, regulatory requirements are applicable to common applications. These are listed in Appendix U of the national application standard DIN 1 045-2. The mineral aggregate used to manufacture cementitious screed is referred to as ‘grain mixture, 0 to 8 mm‘. This grain mixture is generally manufactured by blending ‚fine mineral aggregate, 0 to 2 mm‘ (sand) with a ‚coarse mineral aggregate, 2 to 8 mm‘ (gravel). DIN EN 13 813 provides no information for recipes; the screed manufacturer and/or layer themselves are responsible for achieving the desired results.

For CT-C35-F5 quality and above, it is recommended that 50% of grading range 0 to 4 mm and 50% of grading range 4 to 8 mm are used. The coarse aggregate should, in terms of its form, be large and round, in order to achieve smallest possible surface which must be wetted with the cement paste. A convenient way of affecting the strength achieved is to replace 20% of the aggregate with chippings, grading range 2 to 5 mm.

In my experience, with a grain mixture of 0 to 4 mm, it is barely possible to achieve a screed quality higher than C-25-F4, even when a CEMI 42.5 R type cement and a high-quality admixture are used. This can be seen by performing a practical test on a properly compactible bonded screed with a thickness of 30 mm. The reason for this is, among other things, that the finer grading range introduces greater numbers of air pores.

The relevant quality properties with the applicable regulatory requirements for grain mixtures of 0 to 8 mm are listed below. In addition, higher requirement categories can also be recommended on a case-by-case basis. Comments on the respective properties as part of the recommended requirement categories l151l

On 1 + 2 In accordance with European standards, manufacturers of mineral aggregates must respect the absolute limit values for oversized particles
and for fines for the granulometric composition. In the mesh minuses, the 1 mm test mesh requires 40 +/- 20% and the 4 mm test mesh 70 +/-20 % (the so-called 1 mm and/or 4 mm intermediate meshes are applicable to a grain mixture of 0/8 mm).

On 3 The grain form is the decisive factor in determining workability and pumpability.
In order to make use of these advantages, the higher category
(less than 20% by mass of poorly formed grains SI20) is recommended.

On 4 A lower proportion of light-weight, organic contaminants (the new term for ’swellable components‘, such as e.g. wood, peat, charcoal, etc.) in the fluctuation range of the regulatory requirements can have considerable qualitative and visual influences on the screed surface.

On 5 Chloride ions in the aggregate promote corrosion in the reinforcing components.

On 6 Acid-soluble sulphate in mineral aggregates can cause further damage to the concrete during hydration as a result of ‚driving‘. Under certain conditions, other sulphur compounds present in the mineral aggregate can also oxidise in the concrete and create sulphates. These sulphur compounds can also lead to further destruction of the concrete as a result of ‚driving‘.

On 7 + 8 For screed products in outdoor areas on which de-icing salt is expected to be used (such as on road surfaces), a de-icing salt-resistant category MS18 mineral aggregate should always be chosen. Tests on the aggregate are performed in 1% sodium chloride solution. Water has the highest creeping capability at this concentration, and can thus optimally break open the stone capillaries in frost conditions. From past experience, mineral aggregates which comply with the requirements of MS18 can also be included in the requirement category Frost resistance (water frost) F1.

On 9 + 10 As industrial floors on which forklifts are used are prone to increased mechanical stress from live loads and abrasion, the mineral aggregate should be highly resistant to crushing (Los Angeles value LA or Slag crushing SZ). The Micro-Deval coefficient, MDE (in accordance with DIN EN 1 097-1) may become increasingly important in the near future for the testing and evaluation of resistance to wear. Examples of information in accordance with standard for grain mixture l151l

0/8 mm from natural, normal mineral aggregate

GA90 – f3 – SI20 — Q0,05 – Cl0,02 – AS0,2 – MS18 – F1 – SZ26

The aforementioned designation descries the following mineral aggregates with the values actually determined during the tests:

Material type: Quarternary gravel, 0 to 8 mm grain size
Oversized particles: 1.8% (results from GA90)
Grain form index SI: 8.1 (index value)
Flakiness index FI: 9.4 (index value, can be given as an alternative to the grain form index)
Dry density: 2.72 t/m3 (to be provided on request)
Light-weight, organic contaminants: 0.00 M. -%
Chloride content: < 0.0002 M. -%
Sulphate content: < 0.03%
Resistance to de-icing salt (1% NaCl solution): 4.4 M. -%
Frost resistance F1 reached, as weathering for de-icing salt resistance < 8 M. -%
Resistance to crushing: 23.3 M. -% Grain size distribution curves for mineral aggregates l151l

Grain size distribution curves define the passage of the aggregate material through a mesh. Grain size distribution curves A/B are ideal for screeds, as grain size distribution curve A contains a high proportion of large grains, while grain size distribution curve C contains a high proportion of fine grains. In terms of its grading range, grain size distribution curve B lies between A and C. The grain size distribution curve should be in Area 3 (= coarse to medium grain) or 4 (= medium to fine-grain) (approximately equivalent to grain size distribution curve A/B 8). A higher powder (< 0.125 mm) content in the grain mixture increases the probability of cracks in the mortar. DIN EN 13 813 provides no information for recipes; the screed manufacturer and/or layer themselves are responsible for achieving the desired results. For CT-C35-F5 quality and above, it is recommended that 50% of grading range 0 to -4 mm and 50% of grading range 4-8 mm are used. In practice, I often find that the mineral aggregate preferred by screed layers is very fine, since the mixture can then be smoothed more easily. This sand frequently consists of 60% of grain size 0 to 2 mm and 40% of grain size 2 to 8 mm, which approximately corresponds to grain size distribution curve B8/C8. Sometimes however, this size range is closer to a pure C8. Cement flowing screed CTF (F = free-flowing)

The industry now offers a range of cement flowing screeds. The advantage of a cement flowing screed is that the free-flowing nature of the material makes smoothing to the desired height much easier. In addition, the physical effort required for laying is reduced to a minimum. For further properties of free-flowing compositions, please refer to the description of CAFsin Chapter Ten potential causes of possible damage to cement flowing screeds and solutions l154l & l240l

  1. 1. The guideline recipes of system providers sometimes don’t work.
    Experience from recent years has made one thing very clear: An individual suitability test using the grain mixtures available on site must be performed at each location (the screed manufacturing plant, for both dry or wet mortar plants). A nationwide or regional guideline recipe is not normally the answer.
  2. Use alkali-rich cements, cements with high water requirements
    Depending on the screed system, it has been shown that only few cements are appropriate for the production of CTFs, mainly because of their composition and/or their chemical and mineralogical behaviour.
  3. High gel proportion in the matrix
    See point 2
  4. Addition of water at the construction site
    Off-site addition of water should generally be avoided. Depending on the system, 5 litres of water can increase the slump flow by up to 20 mm per cubic meter of screed mortar! Currently available super-plasticising admixtures store a certain proportion of the water in the combined mixture, but also significantly increase its fluidity.
  5. Inappropriate mineral aggregates (grain size distribution curve, water requirement)
    Mineral aggregates should be tested in accordance with DIN EN 12 620 and also fulfil the necessary criteria. Depending on the system, grain size distribution curves are normally between B8 and C8. See also Chapter
  6. Insufficient mortar homogenisation
    An extended, complete intermixing of the mixed screed must be ensured. This also increases the activation of the admixtures, such as super-plasticising admixtures. Agitating lorry systems can normally mix significantly longer than silo systems.
  7. Hydrating additives (filter ash) with changing properties
    Investigations have shown that additives such as filter ash are not advantageous in cement flowing screed systems. Lengthy rehydration enables the screed to achieve ever greater strength over weeks, i.e. it is hard and brittle and can thus still suffer stress cracks after weeks have passed.
  8. High proportions of ettringite and/or calcite in the matrix
    High proportions of ettringite and/or calcite in the CTF should generally be avoided. In the event of any subsequent moistening of the screed, these proportions can lead to an enhanced source of screed, which in turn can lead to its complete destruction.
  9. Severely densified texture with insufficient pore space, causing extreme water retention and delayed drying
    The texture of the CTF plays a major role. This must above all incorporate an appropriate proportion of air pores, as otherwise the risks of cracking, of breaking down, as well as of delayed drying increase sharply.
  10. Alkali-silica reaction (AKR) when inappropriate mineral aggregates are used
    See Point 5; untested and/or inappropriate mineral aggregates should not be used.
To sum up:

It should now be possible, by selecting appropriate cements and mineral aggregates, as well as by using high-performance super-plasticising admixtures, shrinkage-reducing admixtures and further additives, to produce a functional CTF.

The experiences of individual manufacturers have however shown that not every cement, mineral aggregate or admixture function in their respective systems. It is possible to obtain a CTF with the standard strengths by observing all of the specifications.

To sum up:

It should now be possible, by selecting appropriate cements and mineral aggregates, as well as by using high-performance super-plasticising admixtures, shrinkage-reducing admixtures and further additives, to produce a functional CTF.

The experiences of individual manufacturers have however shown that not every cement, mineral aggregate or admixture function in their respective systems. It is possible to obtain a CTF with the standard strengths by observing all of the specifications.

A number of manufacturers have also gained a grip on the product’s shrinkage, deformation and drying behaviour such that these are within the range of the usual conventional cementitious screeds. Individual CTF systems have been on the market since 2005.

My issue with CTF brands is that because of their pore structure, they dry very slowly and exhibit a very noticeable moisture gradient. In practice, this leads to cracks and deformation. This effect is further exacerbated by floor heating, as the upper side of the screed is properly dried, but the pore structure on the underside prevents any appropriate drying. When the screed surface is closed off with a seal, there is a risk that the locked-in humidity will be released by diffusion over time, which can lead to delayed shrinkage cracks and bi-material effects. Some CTF brands exhibit an increased equivalent moisture content over time, and must for his reason be sealed in in order to even allow the application of a floor covering.

Otherwise, the CTF (designation in accordance with DIN 18 560) exhibit the same material properties as conventional cementitious screeds. Cement flowing screeds comply with DIN 18 560, but they do not have the same thickness benefits relating to load transfer as CAFs. The specifications in DIN 18 560 relating to joint formation, etc. should consequently be used for CTFs. It is expected that the commercial use of CTF will increase. In 2000, it held a market share of approx. 1.5%.

12.1.2 Calcium sulphate

This screed bonding agent is an umbrella term for chemical anhydrite (also known as ’synthetic anhydrite‘), thermal anhydrite (also known as ‚REA anhydrite‘), natural anhydrite and hemi-hydrate as well as mixed forms. It can refer both to conventional synthetic anhydrite screeds as well as flowing anhydrite screeds. In this regard, it is also worth mentioning that despite the of bonding agents standardisation, no Europe-wide harmonised binder (and thus no EC mark) is expected to be issued for CAs. The CA mortar is however regulated in DIN EN 13 813, which is why obtaining an EC mark for a screed is possible. This once again shows that with European standards, what counts is the result rather than the route which leads you there. Conventional calcium sulphate screed CA l267l

Because of its manufacture and application, conventional calcium sulphate screed is processed in the same way as the cementitious screed mentioned above. This screed consists of the following components: a mineral aggregate, a calcium sulphate binder, a screed admixture and water. In 2010, it made up a market share of approximately 10%.

Calcium sulphate screed mortar should be applied and distributed immediately upon completion of the mixing process and/or upon delivery to the construction site, and, for non-free flowing consistencies, removed and compacted. The surface is then abraded at this consistency and, where necessary, smoothed. Powdering or oozing at the edges of the screed should be avoided. The screed mortar must be allowed to dry unhindered.

Conventional calcium sulphate screeds usually have a very low swelling/shrinkage tolerance of < 0.20 mm/m. Induced contraction joints are not therefore normally required, or only required to a limited degree – expansion joints in heated structures on the other hand must be provided in accordance with the provisions of Chapter Calcium sulphate screed has a relatively high early strength and can, depending on the temperature and climate at the construction site, be walked on after three days – it will reach approx. 70% of its final loading capacity after five days. When the relative air humidity is high, this curing process can take significantly longer. The same applies to temperatures above 30 degrees Celsius. Calcium sulphate screed is suitable for use in dry indoor areas, but not in humid indoor areas or outdoor areas. The maximum quality achievable is usually CA – C45 – F7. The daily output of a team of screed layers is the same as for conventional, floating cementitious screeds, i.e. approx. 120-150 m2. From a chemical point of view, calcium sulphate is nothing else but anhydrous gypsum. This screed is therefore sensitive to moisture. In calcium sulphate screeds, an increased moisture content can reduce the flexural tensile strength by up to 30%. This can lead to a reduction in structural strength and an increase in deflection. The application of screeds of this kind in humid spaces is not recommended. These include bathing areas with floor drains (as I see it, even when these are purely ‚emergency drains‘), cellars with no underside damp-proof membrane, saunas, swimming pools, garages, balconies, commercial kitchens, and workshops with water flows. Compliance with the limit values is particularly important for calcium sulphate screeds because when laid, these can become damaged (e.g. loss of strength) when exposed to high levels of humidity. The levels at which damage will actually occur is dependent on the manufacturer and product. In practice, physical damage to natural anhydrite can be seen from values ≥ 0.8 CM-% upwards. Metallic screed reinforcing components in calcium sulphate screeds must be protected from corrosion by appropriate means. When preparing the screed surface for the installation of floor coverings, it is advisable to prime and level the screed surface before installing the floor coverings. This is because today’s dispersion adhesives have a high water content, which in the worst case could damage the screed surface. The drying times for aqueous primers must be carefully complied with. System-compliant, low-viscosity reaction resin-based primers (e.g. EP) are used increasingly for CAs and CAFs, and preference is given to very low-emission products. Particularly when laying large stone and ceramic floor coverings (> 1.600 cm²), these screeds should be protected using appropriate primers, normally reaction resin-based such as EP, from extended exposure to moisture and subsequent cementitious products. With slab sizes of this kind, it makes particular sense to use a mortar coating with effective (crystalline) water-binding or resin-based characteristics. Particularly large-sized slabs only allow the moisture of the adhesion mortar to escape slowly due to the lower joint proportions, which can cause problems if these are not protected. The screed surface can then soften and loosen from the slabs.

Conventional calcium sulphate screeds can basically be described as largely open to diffusion when laid in separating layers open to diffusion, such as e.g. sarking membranes. With intermediate concrete slabs, it is particularly important to provide a vapour barrier with braking action at the contact point between the floor slabs and the insulation, in order to prevent damage to the calcium sulphate screed from moisture from the concrete base. Examples of names in accordance with Screed

Standard DIN 18 560 – CT – F4 – S 55

CA Calcium sulphate screed
F4 Flexural tensile strength 4 N/mm2
S Floating application
55 Nominal thickness 55 mm

Although DIN 18 560 stipulates no compressive strength class for floating screeds, DIN EN 13 813 does however do so for mortar. Calcium sulphate flowing screed CAF (F = free-flowing) l115l

Before we start, I would like to say that ‚the‘ CAF does not exist in this form. Depending on the manufacturer, there are differences between the individual products, relating their properties, e.g. between CAF when used as a pre-mixed dry mortar or from the agitating lorry. In terms of its materials, calcium sulphate flowing screed is comparable to conventional calcium sulphate screeds. The main difference is that appropriate substances are used to make the screed so free-flowing that it is to a certain degree self-smoothing. In addition to the final smoothing, the screed surface must be ‚buffed‘ using a customised bar (normally made from lightweight aircraft grade steel). This work step must be performed with particular care, so as not to generate ’short waves‘.

Because of the relatively lower shrinkage, fewer induced contraction joints are normally required than for cement-bonded screeds. Thanks to its effort-saving feed technology, calcium sulphate flowing screed had achieved a market share of approx. 27% by 2009. A further advantage of CAF is its use with floor heating, since its flow consistency enables optimal sheathing of the heating pipes and a dense material texture with good heat conductions.

Daily application output is higher than for conventionally applied screeds. This is however partially offset by the higher expense incurred in laying the insulation. The film used to cover the insulation must thermally or adhesively bonded in a trough form such that the very fluid screed mortar cannot seep in behind or penetrate the overlaps. Failure to observe this can cause water on the screed surface to penetrate the substructure shortly after laying, and cause humidity penetration as well as sound bridges to the base (see Figure … in Chapter 11.6). When leaks occur, the culprit is not normally not pure water, however, but a liquid with mortar components. It is therefore also possible to analyse the liquid to determine whether it is actually originates from the flowing screed mortar.

The building paper layer beneath the screed can, depending on its sd value, represent a vapour barrier with braking action. It is therefore also possible that the free vapour diffusion balance between the floors in the area of the floor structure is obstructed. Therefore in my opinion, the use of this screed type, in combination with the braking action of vapour barriers, is less recommended on wooden beam flooring. The moisture sensitivity of the material, such as for conventional calcium sulphate screeds, must be given (see Chapter The application of screeds of this kind in humid spaces is therefore not recommended. These include bathing areas with floor drains (as I see it, even when these are purely ‚emergency drains‘), cellars with no underside damp-proof membrane, saunas, swimming pools, garages, balconies, commercial kitchens, and workshops with water flows. Compared to cementitious screed, calcium sulphate flowing screeds tend to have a greater flexural tensile strength, as well as a relatively lower compressive strength. In floating structures, this enables a slight reduction in thickness for the same load-bearing capacity. The calcium sulphate flowing screed should be applied unheated to a maximum thickness of approx. 50 mm, as otherwise, this can lead to considerable drying delays. As higher loads (e.g. in workshops) normally require greater screed thicknesses, I believe that this screed is not really appropriate for this purpose. The highest achievable quality is equivalent to CA – C45 – F7.

This is critical for calcium sulphate bonding agents if humidity is lost too early as a result of drafts or increased sunlight or from the outset too little water, e.g. due to insufficient water pressure in the silo. This can lead to early cracks, as the mortar shrinks before it has reacted sufficiently. Natural anhydrite binders are somewhat more vulnerable in this respect, as they build up their strength more slowly than other CAFs. The achievable final strength for natural anhydrite is also usually lower than for other anhydrite binders. The manufacturer’s instructions for the prevention of humidity removal should be strictly observed. In this connection, it should be noted that cracks which appear during the first two days after the screed is laid (under appropriate construction site conditions) are often attributable to inappropriate mortar mixtures.

Furthermore, not all visually identifiable ‚cracks‘ CAFs necessarily affect the screed cross-section. Buffing can cause fines in the binding and super-plasticising admixtures to collect at the screed edge areas and to float on the rising liquid in the event of intense water discharges. This fines-enriched liquid then settles, and can lead to the appearance of crack-like veins in the screed surface during the drying process, which are often identifiable by means of their darker colour; also refer to ‚Transparent surface appearance (CAF)‘ in the glossary in Chapter 2. A wetting test can be used to test whether the water still stands out for an extended period in the veins after the areas between the veins have dried (which can then be an indication of real cracks in the texture). Timely scraping or brushing of the screed edge area can help to avoid and/or minimise the formation of purely surface-level veins. If these measures are not taken, the screed can be milled at a later time until the veins are no longer visible to the naked eye. In any case, it must be ensured that the ‚cracks‘ do not affect the screed cross-section. If in doubt, you can also consider taking a core sample for testing.

Since flowing screeds, as we have seen, tend to transport the chemical additives which enable their fluidity to the surface, a surface ’sintered layer‘ can appear on this screed type, which must be removed before floor coverings can be installed. This is usually done by ’sanding‘ the surface. If this measure is not sufficient, ‚abrading‘ will be required. Whereas ’sanding‘ is an ancillary service which must be paid for, and is normally performed by the layer, ‚abrading‘ is usually seen as an unpaid ‚remedial measure‘, to be paid for by the screed-layer. If the manufacturer requires the screed to be ground at a certain earlier point in time, this should normally be performed by the screed-layer. Ideally, the screed should be ground until the grain visible. It is often helpful at this point to moisten the screed slightly, as the grain will then be easier to see. Generally, it should be noted that the screed-layer is normally required to produce a ground surface and/or a surface with an equivalent structure. In this respect, the surface finish after sanding should not be coarser and/or the use of surfacer should not be higher than for a ground screed. Finally, the surfaces should be primed and smoothed in the same way as for conventional calcium sulphate screeds. The drying times for aqueous primers must be carefully complied with. System-compliant, low-viscosity reaction resin-based primers (e.g. EP) are used increasingly for CAs and CAFs, and preference is given to very low-emission products.

When laying large stone and ceramic floor coverings, these screeds should be protected using appropriate primers (normally reaction resin-based such as EP), from extended exposure to moisture and subsequent cementitious products. With slab sizes of this kind, it makes particular sense to use a mortar coating with effective (crystalline) water-binding characteristics. Large-sized slabs only allow the moisture of the adhesion mortar to escape slowly due to the lower joint proportions, which can cause problems if these are not protected. The screed surface can then soften and loosen from the slabs.


12.1.3 Magnesite screed MA l251l

Magnesium binding was discovered in 1867 by French chemist Stanislas Sorel (hence its name ‚Sorel cement‘). It comprises the bonding agent components magnesium oxide (obtained by the mild burning of magnesium carbonate) and magnesium chloride (obtained from potash mining and sea water). The binding is characterised by its particular ‚ductility‘, and the fact that it will bind with an extremely wide range of aggregates. It can therefore be used to produce things as different as grinding stones for working natural stone, and insulating material slabs.
Even before the 1900s, magnesite screeds were primarily used as bonded screeds on wooden beam floors in residential and commercial buildings. Most were installed in the form of finished screed in attractive artisanal designs (multi-coloured, mottled or veined like marble stucco). Wood was the main aggregate used, and this is the origin of its designation as ‚magnesite (’stone wood‘ screed‘. Magnesite screed played a major role in residential housing until the post-war period. After 1950, wooden beam floors phased out, and as a consequence, magnesite screeds almost completely disappeared from residential housing. Today, these screed types are used for the renovation of wooden beam flooring as well as for customers whose focus is on the structural physiology (see Chapter ‚16.3.7 Magnesite screeds as ’stone wood screeds‘).

Magnesite screeds are used in industrial and commercial buildings not exposed to moisture as high-strength industrial screeds in accordance with DIN 18 560 – 7. In this context, concrete bases beneath compound magnesia screeds must form a closed structure. MAs can be laid not only over concrete, but normally also over calcium sulphate screeds, bituminous and wooden bases. As these generate no shrinkage stress, they place fewer demands on the base and as a result, there is a lower risk of shrinkage cracks. Cracks are therefore rare and are normally due to major mixing errors or material failures. Further information on ‚cracks‘ can be found in Chapter 2 under ‚magnesite screed (crazing)‘ and ‚magnesite screeds (cracks)‘. If cracks occur in MAs, these can be sealed with epoxy resin. Depending on the cause, the cracks may however reoccur. The screed is processed using a 23 to 25-grade saline solution. When producing the magnesite screed mortar, the mix ratio of anhydrous magnesium chloride (MgCl2) to magnesium oxide (MgO) should be in a range between 1 : 2.0 and 1: 3.5. The magnesite screed mortar should be applied and distributed immediately upon completion of the mixing process, and its consistency trowelled up and compacted as appropriate. The surface must be sanded down and smoothed as required. It must be ensured that the magnesite screed mortar is able to dry without hindrance.

The screen builds up strength relatively fast and requires no further treatment. Depending on the temperature and weather at the construction site, the screed can be walked on after two days, and is load-bearing after five days. Magnesite screeds usually conduct electrostatic charges with no additional measures. They normally correspond to workplace regulation ASR 8/1 in terms of their insulation properties. They can be coloured without loss of strength.

Magnesite screeds are not resistant to prolonged exposure to water. For this reason, they must not be used in areas with permanent or regular exposure to water. Wet cleaning of surfaces is now a daily occurrence, but care must be taken to prevent any cracks from feeding chloride-enriched water into the concrete slab. The provisions of DIN 18 195 must be observed with regard to damp-proof membranes (see Chapter 9.1.2). I would not recommend laying soil-adjoining concrete slabs without sealing measures, but this has not proven to be particularly prone to damage in practice; nevertheless this should specifically be avoided if there is a risk of moisture building up. Damage of a greater magnitude may occur if elastic, diffusion-inhibiting floor coverings or coatings are applied on on soil-adjoining MA structures. Ccoatings relatively open to water vapour diffusion should have a sd value < 8 m (the lower the better). Limited-area coverings such as cabinet or machine feet do not usually cause problems. Surface coverings such as drip pans beneath machinery must be separated from the MA using roughly sanded boards or special bubble wrap so that water vapour can be removed.

Because of their chloride content, magnesite screeds have a corrosive effect on ferrous metals, particularly in humid environments. Steel components must therefore be completely separated from the screed, or at least effectively protected. When laying on ferro-concrete, waterproof screeds must be provided in accordance with DIN 1 045 and 18 560. Reinforcing mesh in concrete slabs should be provided with an appropriate concrete covering. In particular, aluminium components must be effectively separated from the magnesite screed. Here, the corrosion is not caused by the chloride, but by the alkaline nature of the material. During demolition or renovations, particularly in magnesite industrial creeds installed before 1985, it should be noted that screeds of this kind may contain asbestos.

If a magnesite screed was previously in place, a newly laid cement-bonded screed should only be laid on an epoxy resin bonding layer (after prior structural testing), as it is known that mineral bonds can be destroyed by the residual magnesite screed. This has nothing to do with the ‚magnesite driving‘ which is sometimes mentioned; the entire process has not however been researched in sufficient detail. I believe that it may be better in such cases to ’stick with the material‘ and apply a magnesite screed once again. Heavy duty MAs

Heavy duty magnesite screeds must normally be planned and installed as single-layer bonded screeds in accordance with DIN 18 560-7. Single-layer MAs should normally have a nominal thickness of 20 mm and an apparent density class of ≥ 1.9. The use of a bonding layer is compulsory for heavy duty bonded screeds. Dry density

The apparent density classes given in the Figure below are applicable to the dry density of magnesite screeds. Requirements regarding dry density should only be stipulated when this is necessary due to the dead load and/or thermal conductivity. Examples of names in accordance with Screed

Standard DIN 18 560 – MA – C50 – F10 – SH 150 – V 20

MA Magnesite screed
C50 Compressive strength 50 N/mm2
F10 Flexural tensile strength 10 N/mm2
SH150 Surface hardness 150 N/mm2
V Bonded application
20 Nominal thickness 20 mm

The surface hardness (SH) must be indicated for directly used and bonded MAs. Magnesite screeds need only rarely be provided in their composite version with floor coverings. These are usually with impregnated with linseed oil and used as they are, but the surface may exhibit certain colour variations.

12.1.4 Mastic asphalt screed AS (standardised in DIN 18 354) l175l & l270l

Mastic asphalt screed is often used as a ‚problem-solver‘ and together with magnesite screed has a market share of approx. 3%. It can generally be used in different combinations, but is particularly effective in renovations. Because of its high price (particularly in thick insulation structures), its use is limited to specialised areas.

Mastic asphalt is a void-free, dense mix of fillers such as e.g. stone meal, sand, chippings or gravel and bitumen. The latter is a not-easily volatilised, dark-coloured substance, consisting of different types of organic components. it is obtained through the distillation of crude oil and further processing can alter its appearance widely.

Mastic asphalt screeds have a granular structure, and are constituted as follows for use in indoor areas:

  • Approx. 30% by mass of limestone or quartz filler (grading range < 0.25 mm)
  • Approx. 40% by mass of natural sand (grading range 0-2 mm)
  • Approx. 30% by mass of chippings (grading range 2-5 mm)

The maximum grain diameter can be increased depending on the use and requirements. For indoor areas in residential housing, a maximum grain size of 5 mm is normally used; in industrial and parking deck screeds, this figure is normally 8 mm; and for particularly stringent requirements, it is 11 mm. For further information on this topic, please refer to DIN 18 560-3.

A different maximum grain diameter can be used depending on the use and requirements:

  • Indoor areas in residential housing: Maximum grain size normally 5 mm
  • Office and commercial buildings: Maximum grain size normally 8 mm
  • Industrial use and parking decks: Maximum grain size normally 8 mm
  • Industrial use and parking decks with particularly stringent requirements: Maximum grain size normally 11 mm


For further information on this topic, please refer to DIN 18 560-3.

Bitumen acts as a bonding agent and has a ratio of 7 to 10% by mass to the weight of the AS. High vacuum bitumen is used for screeds in indoor areas. Additionally modified road construction bitumen is used for industrial and parking deck screeds.

The properties of mastic asphalt are defined by its softening point (the ‚ring-and-ball test‘) and penetration depth (determined using a 1 cm² stamp over 5.0 h at 22 degrees Celsius). Thus, a AS-IC10 screed has an softening point of approx. 100 degrees Celsius and an penetration depth of < 1 mm. An AS-IC15 screed has an softening point of approx. 85 degrees Celsius and a penetration depth of < 1.5 mm. AS-IC10 is used in residential housing; AS-IC15 is a typical mastic asphalt for industrial and parking deck screeds. The even more significantly soft AS-IC40 is also used purely exterior screeds. When a IC10 screen is used in an outdoor area, this often leads to cracking due to the contraction at low temperatures.

As mastic asphalt screeds are now normally produced in stationary asphalt mixing plants, the supplied compositions usually only suffer very low shrinkage with appropriate laboratory checks. The respective formulation is adjusted to the relevant requirement profile, which must be defined by the building planner taking into consideration loading, room temperatures, thicknesses, etc.

The mastic asphalt screed mass is transported from the mixing plant to the construction site ready for use, in special insulated containers. The screed-laying itself requires relatively high physical effort, as the material could not previously be pumped. Today, the use of piston pumps with heated hoses is however only cost-effective for very large screed surfaces. The material is normally brought to the corresponding level using an inclined elevator and must then be transported to the processing point using barrows, yokes and wooden buckets. Therefore, whether the installation surface is on the ground floor or an upper floor also has an effect on the price. Once it reaches the processing point, the tough plastic mass is spread by hand, or mechanically for large surfaces. Normal daily outputs using conventional techniques for floating structures are approx. 150 to 200 m2, and a maximum of 250 to 400 m2 for structures with separating layers. In order to ensure the adhesion of subsequent floor coverings as well as to collect excess bonding agent, after application, the hot mastic asphalt surface is normally abraded with large amounts of sand. When the sand is correctly applied, the screed usually does not require grinding before applying the pavement; the surface should however be vacuumed. Finally, a primer is normally applied to bind any dust. The instructions in Chapter must be observed for any subsequent levelling. Properties of mastic asphalt screeds

The mastic asphalt screed obtains its thermoplastic properties from the bitumen, meaning that the former can be affected by other granulometric compositions as well as by different types of bitumen and paraffin additives.

The following properties should normally be noted:

  • Mastic asphalt screeds are sealed against water vapour diffusion; the sd value for a 30 mm thickness is normally approx. 1500 m, whereas DIN 52 615 generally requires a value > 1500 m. This must particularly be taken into account on wooden beam flooring (see Chapter 16.3).
  • Because of its plastic (also known as ‚visco-elastic‘) behaviour, mastic asphalt screeds provide relatively good sound insulation. This is often also referred to as ‚internal insulation‘. In this connection, improvements of 14 dB with the installation of 30 mm AS laid on a separating layer on a concrete slab can for example be achieved. The use of sound insulation provides further improvements.
  • At 0.90 [W/mxK], the thermal conductivity of a mastic asphalt screed is somewhat lower than for CT and CA, but higher than for MA.
  • Mastic asphalt screeds are viable at installation thicknesses from 25 mm upward (new: 35 mm for floating structures?). The minimum thickness for a layer is 25 mm, regulation thickness is 30 mm, and maximum thickness is approx. 40 mm. Applying higher thicknesses in a single layer can lead to more pronounced deformation.
  • Mastic asphalt screeds are permitted as screeds/coatings for the storage of various substances hazardous to water in accordance with the Water Resources Act. Enquiries must be made for individual cases.
  • Due to its thermoplastic properties, the AS may run slowly, causing a certain amount of changes in construction changes without necessarily leading to cracking.
  • Mastic asphalt screeds can be laid almost seamlessly.
  • All standard floor coverings can be applied, and the screed type is particularly suitable for coverings sensitive to moisture (e.g. parquet). Only PUR materials may normally be used as coatings.
  • It is also suitable for floor heating systems provided that copper pipes are used, and an appropriate hardness class is selected. Depending on the system, an overlap of 15 mm is usually sufficient. Dual layers are necessary for screed thicknesses > 40 mm.
  • AS are usually applied floating or on a separating layer. Composite layers are possible, primarily on asphalt or bituminous sheeting covered with linings.
  • AS are hard to mill, as the milling residue combines easily with bitumen. Conversely, they can normally be processed with coarse diamond abrasion units without problems, although higher bitumen contents will prolong the abrasion process. Abrading (keyword ‚Bituterrazzo‘) usually starts with dry coarse abrading, and then with several wet fine abrasion passes. Only minor amounts of heat are generated during coarse abrading.
  • As a thermoplastic, plastic asphalt can be recycled by re-heating and re-application, provided that it is a single variety, and unsoiled by contaminants. This requirement is sometimes difficult to fulfil in practice (see Figure below ….). For disposal, the following costs (as of 2008, in Southern Germany) will be incurred: Mastic asphalt screed (tar-free): approx. € 135 inc. VAT/ton
    Mastic asphalt screed (with tar): approx. € 200 inc. VAT/ton
    By comparison, the disposal of cementitious screed at the same point in time cost approx. 10 € inc. VAT/ton. l297l Areas of application for mastic asphalt screeds

In residential housing, as:

  • An appropriate base for wood coverings.
  • Screed with a low design height with and without sound insulation requirements.
  • A screed in areas in which higher sd values for the load-bearing layer have a favourable effect (e.g. in cellars).
  • Screed in buildings where rapid laying is required.
  • Screed on very uneven bases in combination with appropriate granular materials.
  • Screed in buildings with room temperatures between 0 and 5 degrees Celsius, where the laying schedule is crucial. Care must be taken to select the correct hardness class.
  • Screed in buildings in which attempts are being made not to introduce further additional humidity.
  • Ground mastic asphalt screeds with terrazzo appearance for representative areas. With an appropriate grain structure, it is possible to grind AS and finally complete it as a finished surface screed with wax.

As mastic asphalt screeds are installed with thicknesses between 25 and 40 mm, unevennesses in the base cannot be smoothed out by the screed mass. This smoothing is achieved using granular materials, and these must have a bonded form in their installed state. Layer thicknesses are usually between 10 and 30 mm. The thermal and/or sound insulation is laid over the granular material.

In commercial areas, as:

  • Screed in buildings where rapid laying is required.
  • Finished screeds with rolling loads; additional measures are however necessary for high resting point loads in order to prevent impressions.
  • Screeds/coverings which must fulfil the requirements of the Water Resources Act (see Chapter
  • Screeds which must provide a certain walking comfort even without insulation, e.g. for standing activities on production lines.
  • Bonded screeds on roller-compacted asphalt base courses.

On parking decks, underground garages, etc. as

  • parking deck screeds with requirements in accordance with DIN 18 195-5 (commonly used construction: plastic primer, single-sheet welded bitumen sheet, mastic asphalt protective layer, mastic asphalt cover layer, roughened with chipping infill).
  • Screeds for covered parking spaces.

I would not recommend the use of mastic asphalt screeds in calcium sulphate components and in areas where openness to diffusion is desirable. Health aspects and installation temperatures

Mastic asphalt screeds produce extremely intense odours during installation. Discussions over the carcinogenic substances in bitumen have in this respect been going on for a long time l17l. As the BITUMEN forum, which comprises industry, institutes and associations which are responsible for working with bitumen, either themselves or through their member companies, wrote in a publication issued in November 2001 l90l:

„(…) In the MAK and BAT values lists for 2001, bitumen (vapour and aerosol) is deemed to be dermally resorbable (they are absorbed through the skin) and carcinogenic, category 2. (…)“

A previous publication of the Bitumen Forum dated March 2001 l87l had already made it clear that in May 2000, the Committee on Hazardous Substances had set the air threshold for vapours and aerosols from bitumen during hot processing at 10 mg/m3. This also made clear that exposures of up to 50 mg/m3 occurred during the processing of mastic asphalt in indoor areas because of installation temperatures as high as 250 degrees Celsius (e.g. during transportation in wheelbarrows). The Committee also noted that while the value of 10 mg/m3 was not achievable, a threshold value should be set for mastic asphalt working. Since 2008, mastic asphalt may only still be worked at low temperatures. This requires the addition of viscosity-altering additives in order to drop the temperature of the material to below 230 degrees l229l. The aim of this measure is to reduce the aerosol loading to the same level as when installing conventional roller-compacted asphalt. In the meantime the material temperatures have been lowered to <= 230 degrees Celsius in order to solve this problem. The aspects mentioned above primarily affect the person laying the mastic asphalt screeds, as the aerosols are normally only released at temperatures > 100 degrees Celsius. There is no risk when the product is used after cooling. The release of odours during the removal of a floor covering from an AS is often associated with the solvent-based adhesives which were often used previously.

Depending on its hardness class, mastic asphalt screed from modified bitumen is currently delivered and installed at material temperature of approx. 210 degrees Celsius. This still high temperature means that intense heat stress occurs during installation (freshly laid floorboards beneath the AS can deform, for example). In order to avoid significant damage due to thermal stress, the area should be ventilated so that excessively high room temperatures are prevented. Installations in the base such as plastic coverings for fluorescent tubes in floor slabs must also be monitored. The planner responsible should be queried in this connection, as e.g. false ceilings made of plastic panels or fire dampers, which close once a certain room temperature is reached, cannot be verified by means of a visual check. Display window panes or terrace doors which reach to the floor should be protected with thermal insulation before installation in order to keep the risk of thermal stress cracks in the panes as low as possible.

One positive aspect of these high temperatures is however the fact that these can kill off fungi and pests which may be found in the bases (e.g. in wooden beam floors) of old buildings. It must be ensured that the mastic asphalt does not come into contact with substances with a flashpoint below 230 degrees Celsius. The majority of problems are with plastics, as these sometimes have very low softening points between 40 to 60 degrees Celsius.

The major advantage of mastic asphalt has always been the fact that once it has cooled completely, it is already usable as well as layable, and that it can be laid in thin layers. The cooling process in this connection must not be accelerated. After cooling for three hours, the AS can normally be walked on, and can normally be fully loaded after 12 hours; after 24 hours, the floor coverings can normally be laid.

The insulating material layer, which lies beneath the mastic asphalt, must be able to withstand a temperature of 230 degrees Celsius for a short time, and should not have hollow layers to the base. Because of its resistance to heat, coconut fibre insulation was in bygone days often installed beneath AS. Today, insulation slabs made from Perlite fibre, wood fibre and mineral wool with low compressibility and heat-resistant fibre bonding are used. The use of phenolic resin insulation is also possible, if these are for example covered with a Perlite fibre insulation panel at least 25 mm thick (see ‚Phenolic resin insulation beneath AS‘ in the glossary in Chapter 2). Non-heat resistant insulating materials (e.g. in the form of pipe insulating sleeves) must be protected from the effects of temperature in appropriate ways. Insulating layers must with be covered with a temperature-resistant material made from rubbed board, raw glass wool, or similar. Unevenness in the base floor must usually be levelled with an appropriate granular material. In order to ensure the necessary stability, DIN 18 560-2 requires that insulating layers and/or granular materials with poor stiffness be covered with a sufficiently thick, deformation-resistant insulation coat (e.g. Perlite Fesco panelling). The edge strips must be resistant for a short time to a temperature of 230 degrees Celsius. Deformation behaviour and hardness classes

Mastic asphalt should physically be considered as a liquid. When high single loads with small resting surfaces act over extended periods on this screed type, this can lead to deformation of the load-bearing layer. ‚Breakdowns‘ in the edge area are often caused by displacements within the AS by applications of force. IBF l314l investigations revealed that single loads (to the degree permitted in residential housing) in the corner areas of the floating mastic asphalt let to extreme deformation, and even to slab fractures. This can be counteracted with higher screed thicknesses, stiffer insulation and/or covering slabs, glass fibre coverings instead of ribbed board, load distribution slabs to distribute single loads and thick rigid floor coverings such as e.g. stone slabs, which also have a load-distributing effect.

When deformation occurs slowly, this often allows the visco-elastic behaviour of the AS to be relaxed without cracking. It is on the other hand not unusual for cracking to occur in rapid stressed constructions. This can for example occur if surfacers or mortar bed layers harden in a stressed state on the mastic asphalt screed. Rectilinear cracks often start from seams (construction joints) which emerge when the appropriate force is applied.

In contrast to other screed types such as CT or CA, when a load is applied, the pressure is not applied to the load-bearing layer in a cone form, but vertically. For AS, the hardness class of the screed as well as the stability of the base are crucial to its load-bearing capacity; the screed thickness is only of secondary importance. It should be noted that mastic asphalt is impervious to water, but not to extended exposure to solvents. It is mostly laid seamlessly, and construction joints must be overlapped because of the risk of uneven settling. Mastic asphalt screeds can be used for indoor and outdoor areas – a different material setting (hardness class) is all that is necessary. Generally, hardness class IC10 mastic asphalt screeds should not cool to temperatures below + 5 degrees Celsius, and hardness class IC15 mastic asphalt screeds should not cool to below 0 degrees Celsius.

  • IC10 or IC15 for heated rooms
  • IC15 or IC40 for non-heated rooms and in outdoor areas
  • IC40 or IC100 for cold-storage chambers Joints in mastic asphalt screeds

Mastic asphalt screeds draw together as they cool, so that a joint appears on the edges. Under normal installation conditions, the width of the joint is between 4 and 5 mm. Ribbed board and mineral fibres are often used as the materials for edge strips. When laying cured floor coverings, the edge strips must be thick enough to provide a 10 mm wide joint between the screed and the wall. When laying finished screeds on a separating layer, check on a case-by-case basis whether the edge strips can be eliminated. In outdoor areas, an appropriate welded sheet can be used for direct application to generate a composite effect and then possibly eliminate the expansion joint (case-by-case dimensioning necessary!).

Depending on the requirements (e.g. on the basis of the Water Resources Act), an existing edge joint can or must

  • be left open,
  • be sealed with a PUR joint mass (paid service),
  • be sealed with a bituminous joint mass (paid service).

When performing installations in winter, it is usually recommended to use a joint sealant. At low temperatures associated with drafts, the mastic asphalt in the edge area cools abruptly, and therefore draws together particularly strongly. Joint widths of 10 to 15 mm can be created in this way. Experience shows that the temperature-related shrinkage of the AS lasts approx. 24 hours, so that the joint should only be filled a day later.

When driving on mastic asphalt screeds, a dual-layer application of the AS should be performed, so that a staggered joint system (made up of the two layers) can be achieved. This has proven to be effective in practice, as joints which pass across the entire cross-section are often damaged by traffic loads.

When performing an installation with natural stone, the Deutsche Naturwerkstein-Verband e. V. (DNV) writes that with mastic asphalt screed, the screed field length should be ≤ 6 m (max. 40 m²) l304l. Joint formation in industrial screeds

Metal profiles are recommended for the transfer of construction joints. The appearance of visible construction joints in large halls cannot normally be prevented, as the sections, which are laid one after the other, cannot bind. Two variants are possible in this connection, and the construction specification should refer to these.

Visible joint with a bitumen melt tape

  • For this purpose, a self-adhesive bitumen melt tape with its front side slightly cambered is stuck to the surface which was created first, after which the next surface is processed. Once the second surface has cooled, the slight camber of the melt tape is ’singed‘. The construction joint remains visible during this procedure.

Barely visible joint with a plasticiser

  • The ’new surfaces‘ then shrink onto the ‚old surfaces‘; the edge areas of the latter should be pre-warmed for this. This should prevent the creation of a broad joint. In a second work step, both edge areas are treated with a plasticiser. The once-again soft screed compositions are gently ‚raked‘ so that a homogeneous mastic asphalt mix can appear on both sides. Finally, both edge areas are once again sprinkled with sand and abraded. Laying mastic asphalt screeds l304l

In indoor areas, the mastic asphalt surface is generally shrunken using sand. ‚Balding‘ because of faulty abrading should be avoided, as otherwise the adhesive will not be able to achieve effective anchorage. As sprinkling results in excess material, the removal of the non-binded sand should be stipulated to be a separate follow-up task. When the screed reaches an appropriate evenness, it is theoretically possible to directly fit parquet using resin adhesives with no water additives and tiles with suitable flexible adhesives. The technically achievable evenness (due to the reduced material temperature of the AS) is however normally not sufficient to enable the problem-free bonding of large-area elements (tiles, parquet) without an intermediate layer. For this reason, levelling including a primer should generally be provided. Levelling and mortar bed layers from mastic asphalt screed must always set free from stress and must not exhibit any changes in length while hardening. When dispersion adhesives are used, the underlayment for removing water from the adhesive should have a thickness of between two and a maximum of three millimetres. If larger ‚cavities‘ must be smoothed out or levelling of significant thickness is required, the stable compositions used should also be ’stretched‘ by adding sand. Calcium sulphate surfacers are also often applied on AS, as these only exhibit low levels of shrinkage. The disadvantage of these however is that they are not moisture-resistant. There have been cases where cleaning water has seeped under the PVC through unsealed joints, and the surfacer has lost its strength as a result.

When the top layer is parquet, the IBF l314l investigations suggest that this can cause problems if the wooden layer is exposed to very high or very low air humidity. Very high air humidity will cause the parquet to swell, which can result in a curvature of the overall structure with lowered edges, possibly in combination with fractures in the AS. Very low air humidity will cause shrinkage of the wood, which can lead to curling of the overall structure with raised edges.

The application of natural stone on mastic asphalt screeds can be problematic, as indicated by the experience of the Deutsche Naturwerkstein-Verband e. V. (DNV). They even refer to a „high-risk construction method“. This is because mastic asphalt screed, a thermoplastic, becomes softer when warmed (e.g. by sunlight in large window areas). This leads to deformation of the AS, which can damage the natural stone slabs. In addition, according to the DNV, it can lead to to staining of the natural stone in bituminous bases with highly alkaline mortar coatings. In addition, no appropriate adhesive bond is often created between the AS and mortar coating. To minimise risk, the DNV suggests using plastic-modified mortars in the thin and/or middle beds (proof of suitability from the manufacturer). Natural stone slabs should be installed in small sizes (e.g. 40 x 40 cm) and square, and also installed in ‚cross-joints‘.

Where mastic asphalt screeds must be coated or cracks within the load-bearing layer must be gummed, PUR materials are available for these purposes, as these (like the mastic asphalt itself) have a certain elasticity, and thermal length variations then cause less damage. If on the other hand the mastic asphalt screeds are rigidly coated (particularly in outdoor areas), this often causes cracks in the coating and screed.

The same mechanism can also cause damage in outdoor areas when combined with rigid floor coverings (e.g. tiles). In addition, depending on the date on which they were laid, old mastic asphalts can stain subsequently applied floor coverings because of unsuitable bonding agent types. Heavy duty AS

Heavy duty mastic asphalt screeds must normally be planned and installed as a single layer laid on a separating layer in accordance with DIN 18 560-7. Dual layers are necessary for nominal thicknesses exceeding 40 mm. With IC10, no pressure > 1 N/mm2 must be generated under single load action over time. The AS load-bearing capacity for other hardness classes is given below. Examples of names in accordance with Screed

Standard DIN 18 560 – AS – IC10 – S 25

AS Mastic asphalt screed
IC10 Hardness class 10
S Floating application
25 Nominal thickness 25 mm

12.1.5 Synthetic resin screeds SR l86l & l272l

This is a synthetic resin mortar in which the synthetic reaction resin is the bonding agent, and which forms a rigid layer through the chemical reaction of the synthetic reaction resin on a liquid or smoothable mixture at the construction site. Suitable bonding agent particularly include polymethyl methacrylate (PMMA), epoxy resins (EP), polyurethane resin (PUR) and unsaturated polyester resins (UP). These products are laid largely seamlessly, depending on the manufacturer’s instructions; however, the shrinkage behaviour of the respective brand must be borne in mind for floating as well as separating layers. This can differ widely by product, and is dependent on different factors such as degree of filling, grain size distribution curve, type of resin, etc. Normally however, the shrinkage mass is less than that for CAs and the respective shortening takes place corresponding more rapidly. The higher the degree of filling (the major determinant), the lower the shrinkage mass and the strength values. In composite applications, the release of stress is partially achieved through internal relaxation.

These are screed systems to which no water is added and which normally have short drying times as well as a relatively low apparent density in their cured state. The precise hardening times for synthetic resin screeds based on single- and/or multi-component resins are to a great degree dependent on the temperature during the application, the hardening temperature, the type of resin and the hardening system.

Certain SR-type insulating materials and insulation coverings can be attacked by bonding agent components and/or solvents, and corresponding protective measures should therefore be taken, and/or appropriate materials (for coverings) be selected. Very little research has so far been performed into the behaviour of PE heating pipes in synthetic resin screeds. It is not however completely impossible that this (also depending on the actual pipe type) can cause damage to the heating pipe due to contact with the resin. For more precise information, the pipe manufacturer’s chemical resistance chart should be requested. The majority of SRs are used in bonded applications. For composite applications, the supporting base must have a surface tensile strength of 1 N/mm2 for non-traffic areas, and 1.5 N/mm2 for traffic areas. Synthetic resin screed mortar should be distributed over the base immediately upon completion of the mixing process, and should be compacted and screeded depending on their consistency. The respective application temperature is dependent on the resin type. The air and floor contact temperatures (of the base) must be at least 3 K above the dew point temperature. Different systems

To create a synthetic resin screed, sand is mixed with a bonding agent, such as e.g. epoxy resin, an oligomer system or (less often) methyl methacrylate. Epoxy resins are normally triple-component systems whereby an uncured epoxy resin, hardener and granulate material are delivered separately to the construction site and processed. A commonly encountered weight-based mix ratio for the resin and granulate is 1:6, up to a maximum of 1:15, depending on the strength required. Epoxy resins have an irritating effect on the skin and cause allergic reactions to large numbers of workers, some of which are irreversible. Appropriate protective measures should therefore be taken (gloves, glasses, skin care). I personally believe that working with compressed air systems can be dangerous in terms of air pollution, particularly for the person doing the laying. In oligomer systems, sand is mixed with a single-component bonding agent, which is delivered packed in an airtight package, and which cures when exposed to air (oxidation curing). A pungent smell is often released shortly after their application,which can cause eye and mucous membrane irritation in workers. Because of the high amounts emitted, the use of breathing equipment is normally still required 45 hours after application. In my view, the use of oligomer systems of this kind is relatively inappropriate due to health considerations.

Generally, the workplace regulations, occupational safety, processing guidelines and safety requirements set by the material suppliers must be observed when working with synthetic resin screeds. Synthetic resin screeds must have an approval and/or test results to prove the harmlessness of the emissions to the worker. Heavy duty SRs

Heavy duty synthetic resin screeds must normally be planned and installed in accordance with DIN 18 560-7. A system-compliant epoxy resin bonding layer should normally be used. Where the composite application of industrial screeds is not possible due to unfavourable base conditions, a separating layer application with a minimum screed thickness of > 40 mm and a compressive strength > 60 N/mm2 is possible.

An optimised grain size distribution curve with dry silica and/or ‚hard grain‘ must be used, such that the maximum grain size comprises a maximum of 1/3 of the thickness of the screed.
The grain size distribution curve C2 provided by Dorfner (see Chapter 24.1) can be used up to a thickness of approx. 25 mm. Depending on the thickness of the layer (from approx. 30 mm), gravel (2-8 mm) may also be used. There is still insufficient experience about the test procedure for for abrasive wear in accordance with DIN EN 13 813, and it is not therefore possible to provide reference values for abrasive wear. Examples of names in accordance with Screed

Standard DIN 18 560 – SR – C40 – F10 – AR2 – IR20 – B1.5 – V3

SR Synthetic resin screed
C40 Compressive strength 40 N/mm2
F10 Flexural tensile strength 10 N/mm2
AR2 Abrasion resistance class in accordance with BCA AR2 (abrasion depth 200 micrometers)
IR20 Impact resistance IR20 (20 Nm)
B1.5 Bond strength class B1.5 (bond strength 1.5 N/mm2)
V Bonded application
3 Nominal thickness 3 mm

For synthetic resin screeds, which are laid bonded, the bond strength class must compulsorily be provided. The bond strength must be determined in accordance with DIN EN 13 892-8. For directly used and bonded SRs, the ‚rolling wheel‘ (RWA) or BCA (AR) abrasion resistance must be given.

12.2 Differentiation according to the laying method

There are different techniques for applying screeds on substrates (e.g. concrete slabs, insulation). A distinction is made here between:

  • Floating screeds
  • Screeds laid on a separating layer
  • Bonded screeds

12.2.1 Floating screeds

This term refers to screeds which are laid on insulating layers and which ‘float’ on this to a certain extent. Floating screed is applied to improve the thermal and sound insulation of the floor structure. The term ‘floating’ already implies that the screed will be subjected to a certain amount of movement. These movements can, for example, on the one hand include shortening of the rocking joints as part of the shrinkage process or on the other, expansion of the rocking joints (positive changes in length) e.g. due to increases in temperature. It is therefore crucial that the screed at no point comes into contact with the lateral construction components, and is separated from them by an edge strip. Otherwise, there is a risk that in the event of a change in length, the rocking joint ‘hangs’ or ‘stands’ there, which will generate loads, resulting in cracks. Floating screeds often develop deformation in the joint area, and the loads generated can then lead to damage. A seal must be positioned beneath the insulating layer of floating screeds adjacent to soil in order to protect the structure against underground humidity. The insulating materials are covered with a film in order to prevent mixing water from the freshly laid screed penetrating the former during application. DIN 18 560 stipulates that in multiple insulation layers, the insulating layer with lowest compressibility must be uppermost. This does not, however, apply if there are underground pipes. The abbreviation ‘S’ in the screed name already indicates its floating properties.

All screed types can be applied as floating screeds (cementitious screeds, calcium sulphate screeds, magnesite screeds, mastic asphalt screeds and synthetic resin screeds).

Instructions for minimum thicknesses for floating screeds can be found in Chapter Examples of names in accordance with Screed

Standard DIN 18 560 – CT – F4 – S 55

CT Cementitious screed
F4 Flexural tensile strength 4 N/mm2
S Floating application
55 Nominal thickness 55 mm

Although DIN 18 560 stipulates no compressive strength class for floating screeds, (regardless of whether pressure is produced on the upper side of the slab when under load), DIN EN 13 813 does, however, do so for mortar.

12.2.2 Screeds laid on a separating layer

Screeds laid on a separating layer are only ever used when there are no thermal or sound insulation requirements. This affects areas such as underlying cellar or storage areas. Where required, a damp-proof membrane to prevent rising damp can also be applied. Since screeds laid on a separating layer provide no fixed binding with the base, length variations in the screed are also possible. In order to facilitate this, two layers of film must normally be placed beneath a CT; for CA and AS, one layer is sufficient. The following are usually used as layering materials for this separating layer, although other products with comparable properties can also be used:

  • PE film with min. 0.1 mm thickness (preferably 0.2 mm)
  • Tarred paper with min. 100 g/m2 grammage
  • Plastic-coated paper with min. 0.15 mm thickness
  • Raw glass wool with min. 50 g/m2 grammage

It is important to ensure that the separating layer is laid flat, with no folding or warping. It should generally be applied unfolded, as unfolding the material usually leaves a frangible point in the area of the fold (primarily with free-flowing screeds).

When there are significant unevenness or ridges in the base, these should be smoothed out before laying the film layers. Otherwise, there is a risk that the screed will collect in these uneven areas, and cracks will appear in the screed structure during the course of shrinkage. As with floating screeds, screed laid on a separating layer must also have no contact with the lateral construction components, such as walls. To achieve this, an edge strip should also be installed in this case. The separating layer is normally hitched to the edge strips if the edge strips do not act as the separating layer.

Screeds laid on a separating layer often develop deformation in the joint area, and the loads generated can then lead to damage.
Joints laid in screed should be fitted with joint profiles or filled with joint sealing compounds which are suitable for the forecast loading and for preventing contamination. Nominal thicknesses and strength classes for screed laid on a separating layers

For screed laid on separating layers (CT and CA) nominal screed thicknesses estimated by the author depending on

  • Area load
  • Single load
  • Screed type

can be found in Chapter

The reader will note that the thickness requirements for screed laid on a separating layer do not vary to any major degree from those for floating screeds (see Chapter, regardless of whether the former is provided with soft, resilient insulation. This initially sounds like a contradiction. If screed laid on a separating layer is deformed due to shrinkage, it no longer lies completely on the base, and it therefore behaves under load in a similar way to floating screeds. Loads applied in the perimeter areas then lead relatively quickly to fractures in the screed. Therefore, the dimensions of the screed laid on a separating layer must be thick enough to ensure that such fractures are prevented when the forecast loads are applied. Examples of names in accordance with Screed

Standard DIN 18 560 – CT – C25 – F4 – T 55

CT Cementitious screed
C25 Compressive strength 25 N/mm2
F4 Flexural tensile strength 4 N/mm2
T Laid on a separating layer
55 Nominal thickness 55 mm

When a screed laid directly on a separating layer is used, a Böhme abrasion resistance class may additionally be required. The abrasion resistance must be set using the provided screed mortar and the planned, surface-coating measures (e.g. hard aggregate dry shake application) during initial tests.

Cementitious screeds or even mastic asphalt screeds are mainly used when applying screeds laid on a separating layer (in cellar areas).

12.2.3 Bonded screeds

This application method requires that the screed be firmly affixed to the base. This is, however, not as easy as it first appears. Beforehand, it must be ensured that the base is as rough as possible, so that the screed can anchor itself in the rough areas. Requirements for the base

No further base preparation work is required. Today, we know that a bonded screed requires careful preparation, and that extensive knowledge about this preparation is necessary. First, it is necessary to carefully examine the base. The swept concrete floor must be prepared using shotblasting, milling or another similar technology. Note that the surface structure of the base is normally highly stressed during milling. Rough-milling a concrete slab as a base for a bonded screed can lead to a structural breakdown. Loosening the surface layer normally reduces the surface tensile strength. It may therefore be almost impossible to apply any form of bonded screed to this base without further work. As a result, after milling, bases of this kind are now normally (depending on roughness) again finely milled and then shotblasted before a surface tensile strength test is performed under appropriate conditions.

The reasons for shotblasting and/or milling are two-fold: Today’s concrete slabs contain substances which are intended to improve the fluidity of the cement. These substances tend to accumulate on the surface and prevent effective bonding with the subsequent layers. In addition, the hydrated lime in the cement mortar reacts with carbon dioxide from the air to form chalk and/or lime, which produces the light colour of concrete surfaces. The final result of this reaction is calcium carbonate; this is also referred to as the ‘carbonation’ of the surface. The surface carbonate layer can also cause a failure to bond with the subsequent screeds.

When using impermeable concrete slabs as the base for bonded screeds, particular care must be taken to check the absorbency of the concrete. These slabs have
a particularly high apparent concrete density and are normally manufactured with admixtures so as to achieve the greatest possible impermeability to water. Today, superplasticizing concrete admixtures are mostly used instead of hydrophobic sealants, which reduce the W/Z value. Complete compaction and the appropriate formulation of the cement are the main deciding factors for achieving this water-impermeable characteristic. Most commercially available mineral bonding layers can be used in this technique. If pore-sealing and/or hydrophobic sealants are to be used, however, the application of further layers onto the non-absorbent base can lead to bonding problems. Further measures will be necessary here, such as the application of particular bonding agents.

Once the edge area of the concrete has been suitably treated, it is then necessary to clean these surfaces with pressurised water and finally to extract the contaminated water.

This is important, as any residual dust layers once the water has dried can once again prevent bonding. A film of dust generated by activities such as milling can cause adhesion problems. Rough milling work often contaminates the concrete surfaces with a slurry of fine dust and water. The residual contamination after cleaning is very difficult to detect, and has a deleterious effect on bonding unless the bonding layer or bonding slurry undergoes intensive mechanical working (e.g. using steel brushing).

A bonding layer is then laid, to which the mortar is then applied ‘fresh on fresh’. Old concrete must be sufficiently moistened on the evening before the screed is applied. Puddles during the application of the bonding layer can significantly degrade bonding, as the concrete pores must be open, and not sealed off with water. Excess water should also be extracted from uneven bases, which usually results in additional work. In this way, the screed will be firmly bonded to the base and specialist application will ensure that it is not then loosened (for further information on the subject of ‘Voids’, please refer to Chapter 12.11.8). In terms of possible changes in length, it is therefore not necessary to place edge strips on rising structural components. Chapter 12.7.5 ‘Perimeter joints’ provides further details of the conditions under which edge strips should still be used with bonded screeds.

The base should specifically not have:

  • Pipes or cables
  • Loose particles
  • Soiling of any kind (e.g. oil, fuel, mortar residues, paints, etc.)
  • Efflorescence
  • Frozen areas
  • Cracks

Open cracks in the base can lead to cracks in bonded screeds superimposed in this area. For concrete bases, fines-enriched content, curing compound and admixtures must not hinder screed bonding. No joints should be placed in the screed itself (and no expansion joints in particular) if no joints onto which this can be applied are present in the base. This would run the risk of creating voids in the adjoining joint area. If cracks or joints in the base are not covered by a joint in the screed, there is a risk that this will cause cracks in the screed, although these will not necessarily affect the usefulness of the screed. Metal profiles built into the screed must be firmly anchored into the supporting base in order to provide edge protection. Joints laid in screed should be fitted with joint profiles or filled with joint sealing compound which are suitable for the forecast loading and for preventing contamination. Intense vibration during or shortly after the installation of the floor structure can cause a failure of the bond of the bonded screed to the base.

When levelling screeds are laid (e.g. to provide sufficient coverage for pipes and cables), these must be built up in such a way that they bond firmly with the recipient surface and are suitable as a supporting base for bonded screeds.

This chapter is primarily applicable to cement-based screeds, which comprise a major bonded screed market segment. It is, however, also possible to lay magnesite screeds, mastic asphalt screeds or synthetic resin screeds. The application of calcium sulphate screeds in combination with a barrier (e.g. epoxy resin-based) on concrete slab is also feasible. No barriers should lead to secondary ettringite formation. The selection of an appropriate bonding agent is dependent on the respective base. Thicknesses and strength classes for bonded screeds

In accordance with DIN 18 560-3, ‘Screeds in the building industry/bonded screeds’ l20l, it must be ensured that the nominal thickness of a single-layer screed:

  • 40 mm at AS
  • 50 mm at CA, SR, MA and CT

is not exceeded. DIN 18 560-3 does not, however, apply to the application of rigid floor coverings such as tiles and slabs directly onto a thick-bed mortar. For technical reasons, the minimum screed thickness (at the highest point of the base) should be approx. three times the maximum grain size of the mineral aggregate. Thus, the theoretical minimum thickness of a grain mixture of 0-8 mm is 24 mm. For AS, the nominal screed thickness should be at least 20 mm. For the minimum strength classes, please refer to the respective screed type in the table below.

It is the responsibility of the planner to size the floor structure. My estimates, given below, can be used on a non-binding basis as a guide for the sizing of cement bonded screeds:

  • Where <= 20 kN/m² area load: CT-C40-F6
  • Where <= 50 kN/m² area load: CT-C50-F6 or CT-C50-F7

It is, however, also very important to take into consideration the respective single loads as well as tyres and/or tyre contact areas, which can lead to more stringent requirements. The figures mentioned above refer to pneumatic tyres. Generally, it makes sense to work with concrete in this area.

Note that DIN 18 560-3 does not apply for tiles laid in a mortar bed. Examples of names in accordance with Screed

Standard DIN 18 560 – CT – C35 – F5 – A 15 – V 30

CT Cementitious screed
C35 Compressive strength 35 N/mm2
F5 Flexural tensile strength 5 N/mm2
A15 Abrasion resistance (A) with a maximum wear rate of 15 cm3/50 cm2
V Bonded
30 Nominal thickness 30 mm

When a direct bonded screed is used, Böhme abrasion resistance class (A) will also be required. Appropriate dry shake applications can, for example, be used to increase abrasion resistance.