FEATURE
THE SCREED SCIENTIST’S GLOSSARY
Written by our experienced professionals, this up-to-date glossary defines the most commonly used screeding terms.
Admixture – material added to screeds at the mixing stage to improve their properties, for example workability, setting and drying times, strength, durability, permeability, and so on.
BRE Drop hammer test – a method of testing for soundness or strength of
screeds. The test consists of four consecutive impact blows of the hammer on the same spot; the depth of indentation is measured with a feeler gauge.
Calcium Carbide test – an accurate method used for measuring the water content
in screeds. Calcium carbide reacts with the water in the screed sample, producing acetylene gas. The pressure of the gas is determined with the help of a pressure gauge.
Curing – a process in which special materials, for example polythene sheets, are laid over freshly installed screeds to control moisture evaporation and minimise cracking and curling.
Datum level – a fixed level, plane, or point of reference used to measure
the elevation of a floor.
Damp Proof Membrane (DPM) – a thick, durable, moisture-resistant polyethylene layer approved by the British Builders Association (BBA) for protection against rising damp in the foundations of buildings.
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Day joints – discontinuities between screeded areas, caused by interruptions in screeding operations at the end of the day. The next layer of screed is joined with the previously screeded area the following day.
Drying times – the time needed for newly installed screed layers to dry completely and provide adequate support for final flooring.
Forced-action screed mixers – specialised mixers that distribute water, aggregates, and additives through cement homogenously; they keep the mixing time to a minimum to prevent over mixing.
Laser level – a specialised device consisting of a laser source that projects a fixed beam of light detectable by a sensor
mounted on a levelling rod. Laser levels are used to establish datum levels and to measure departure from datum of finished surfaces.
Reinforcement – special materials, for example fibres, steel mesh, wires, or bars, which added to improve the strength of
screeds, control shrinkage, and reduce cracking and curling.
Screed – a cementitious material made by mixing one part of Portland cement to three, four, or five parts of sand with water until a homogenous, workable mixture is obtained. It can be applied over insulation in underfloor
heating or directly onto the substrate to provide a level and flat surface for the final floor.
Self-levelling screeds – screed formulations that are not only quicker to lay and less labour intensive than traditional screeds but also deliver solid, durable, perfectly level and flat surfaces, with maximum deviations of up to 3mm from the straight edge (SR1).
Self-smoothing Screeds – compounds that are easy to install in large areas, providing high-strength, flat, level, and smooth surfaces, which dry within 24 hours.
Surface Regularity (SR) – SR gives an indication of the flatness of a screeded surface, with achievable tolerances divided into three categories: SR1 (<3mm), SR2 (<5mm), and SR3 (<10mm).
Screed laid to falls – a method of installing screeds to achieve a slope for drainage.
Soundness of screed or In Situ Crushing Resistance (ISCR) – resistance of screeds to the crushing effects of traffic and loads in service; it is assessed with the BRE drop hammer test.
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