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Mastic asphalt screed
Mastic asphalt screed (AS)
Mastic asphalt screed consists of limestone or quartz sand filler, fine and coarse aggregate, bitumen and various additives. Bitumen, a substance obtained from the distillation of petroleum, is used here as a binder and is responsible for the thermoplastic properties and the black color of the screed. If required, a different color design is also possible; for this purpose, the screed must be given the appropriate pigmentation, sanding or coating.
A typical feature of mastic asphalt screed is its flowable consistency, which is obtained by heating the screed to around 250 °C. The elastic screed forms a dense structure with few cavities. Not only is it insensitive to moisture, it is impermeable to water and water vapour, but it also has relatively good heat and sound insulation properties.
Applications of mastic asphalt screed
Mastic asphalt screed is mainly used in building and industrial construction, but is also used to seal buildings. Use in residential construction is theoretically also possible, but is rather rare in practice. Nevertheless, it is suitable for all standard floor coverings and also for the combination with underfloor heating. Compared to other types of screed, the thermal conductivity of mastic asphalt screed is 0,90 (W/(m·K)) below that of cement screed and calcium sulphate screed, but higher than that of magnesia screed. Mastic asphalt screed can also be laid in damp rooms (e.g. kitchen, bathroom).
Depending on the requirement, there are different hardness classes (IC10 to IC 100). If the screed contains hard aggregates, it can be used without any problems in industrial areas subject to high loads and traffic. Mastic asphalt screed is probably familiar to most people from road construction, because it is often used as a top layer on motorways, but also in tunnels or on bridges.
Caution: Mastic asphalt screed cannot be pumped. For this reason, it can only be brought to higher floors with the help of construction or inclined elevators, which transport the screed to the destination without it cooling down too quickly.
Laying techniques of mastic asphalt screed
The screed, which has been heated to approx. 250 °C, is smoothed out to create an even surface and rubbed down with sand. The subsequent cooling hardens it.
Possible laying techniques for mastic asphalt screed are:
– bonded screed
– floating screed
– Screed on separating layer
– Screed on insulation layer
– heated screed
Important: If the mastic asphalt screed is laid on a separating or insulating layer, this must consist of a temperature-resistant material such as raw glass fleece or ribbed cardboard.
What to look out for when laying mastic asphalt screed
Laying mastic asphalt screed is done manually, but this involves a lot of effort and physical exertion. Alternatively - although very rarely - mechanical installation is also possible.
Depending on the laying technique, edge insulation strips of different thicknesses are required. With screeds on a separating or insulating layer, the edge insulation strips should be at least 8 mm thick, with heated screeds or hard surface coverings at least 10 mm.
Mastic asphalt screed is softer than other screeds. Therefore, in theory, elastic or textile floor coverings can deform under point loads. This can be avoided if the screed is leveled with a filler before the floor covering is applied.
Attention: Due to the high installation temperature, there is a certain risk for heat-sensitive substances that are nearby. These include certain synthetic, insulating and gypsum building materials.
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Questions and answers about mastic asphalt screed
The binder in mastic asphalt screed is bitumen, which is derived from petroleum and is black in color. Mastic asphalt screed is therefore sometimes also referred to as black screed.
Harmful emissions can occur when processing mastic asphalt screed. It is therefore essential to comply with occupational safety measures when working with this screed.
Yes, very well. The prerequisite for this is that it is of the same type and has no impurities. This means that it must not be covered with other materials and, above all, must not have had a layer of tar underneath it, which is the case with some older buildings.