A screed is a construction element laid in a range of thicknesses, and its purpose is to bring the installation surface for the flooring to the design height and to provide a surface suitable for installing the specified flooring. Screeds are usually made from anhydrite-based binders. Depending on whether it is laid directly on a supporting structure, on an isolating layer or on a layer of thermal insulation and/or soundproofing material, it is known as a “bonded“, “unbounded” or “floating” screed. A screed may also have an underfloor heating/cooling system incorporated within its structure and, in such cases, is known as a “heated” screed.
When a screeds laid, it must mainly guarantee the following:
-that it forms a substrate suitable for installing the specified flooring;
-that it is laid on schedule;
-that its durability under various service conditions is not compromised.
The durability of flooring, therefore, is influenced by the characteristics or its substrate, which means the type of product selected to make the substrate, the quality of preparation work, how it is laid and the compactness and curing of the mix.
The sump, when choosing which product to use to make the screed, be it a special binder, a pre-blended mortar or traditional site-prepared mortar, you must take into consideration the final use of the screed, site conditions, the type of flooring to be installed, the time to wait before installing the flooring and the time required before putting the flooring into service.
Technical and Performance Characteristics Of Screeds
SUFFICIENT THICKNESS: its thickness depends on the type of screed to be laid, and must be sufficient for the type of flooring to be installed and the type/intensity of traffic acting on it when in service.
MECHANICALLY RESISTANT: its mechanical strength, as with thickness, must be sufficient for the final use of the floor and for the type of flooring be installed. As a general rule, the mechanical strength of a screed suitable for installing any type of flooring for domestic use must be at least 20 MPa, while for industrial use it must be at least 30 MPa.
COMPACT: the screed must be compact and homogeneous on the surface and through the whole thickness. The presence of layers or areas of crumbling lower density materials is a sign of poor mechanical characteristics which could cause breakage of detachment of the flooring. These areas must be carefully assessed and, according to the seriousness and extent of the defect, they must be removed and repaired or consolidated with suitable products.
CURED AND DIMENSIONALLY STABLE: before installing and type of flooring, it is absolutely essential that the screed is cured and that most of the shrinkage is completed. In fact, during the curing cycle, screeds are prone to hygrometric shrinkage due to part of the mixing water evaporating or drying off. This may cause curling or cracking. If cracks developer after installing the flooring, the floor covering could be damaged and/or become detached. The curing time for a traditional sand-cement screed is around 7-10 days per centimetre of thickness in good weather. Turnaround times for a screed made from traditional cementitious mortar, therefore, can be particularly long. However, it is possible to reduce curing times considerably by using special binders or pre-blended, controlled-shrinkage, rapid-drying and setting mortar in the mix.
CRACK-FREE: there are a number of causes of cracks in screeds, such as hygrometric shrinkage, too much water in the mix, the use of aggregates which are too fine or too much cement. Before installing flooring, all cracks must be monolithically sealed by filling them with epoxy resin. If there are hairline cracks on the surface of the screed, or it ceramic or stone flooring is to be installed, it is possible to lay an anti-fracture membrane on the screed.
CLEAN: the surface of the screed must be perfectly clean. Dust, dirt, detached areas, rubble and any other material or substance on the surface of the screed must be removed before installing the flooring to prevent compromising adhesion between the flooring and the screed.
DRY: the level of residual humidity in the screed must be checked. It must conform to the maximum level for that type of floor covering and must be uniform through the whole thickness of the screed, especially when installing flooring sensitive to humidity. For cementitious-based screeds, levels of less than 2% for wooden floors and less than 2.5-3% for PVC, rubber and linoleum are considered acceptable. For anhydrite screeds, the level of residual humidity must be less than 0.5%, whatever type of floor covering is installed.
Types Of Screed
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