Bonded And Unbounded Screed, Definition and types


From prior posts on this platform we have successfully explained the terms screed and screeding, we highlighted them as methods of smoothening out walls.

 There is the Screeding Cement:

Screeding cement is a type of concrete used to form a level surface. It is used either to receive floor finishes, to encase underfloor heating pipes, or to be left as the wearing surface. … Furthermore it can be laid either on top of an existing concrete base, a damp proof membrane (DPM), or insulation.

Full meaning of Screeding:

We may not achieve synergy if we just jump into explaining the screed types without stating the meaning of screeding. What does screeding mean: a leveled layer of material (e.g. cement) applied to a floor or other surface. Screeding paints are otherwise called  POP paints, it is water-based paints used for screeding surfaces to be painted to get a very fine and smooth substrate.

Screeding Bond:

This is the factor that ensures that when screed, cement, or otherwise is used on your walls that there will be expected bonding with the screed and the walls. We have basically two types of screeding paints in use.

  1. Ordinary or Light screeding or POP emulsion: This is used in combination with other added materials like black cement and top bond to strengthen its binding power and viscosity.
  2. Thick screeding Bond: This type of screeding paint is thicker and stronger In both viscosity and binding power. It needs no additive, it is applied, directly to the surface with a screeding trowel/ screed tool to get a very smooth surface ready for painting after sandpapering. (paper with sand or another abrasive stuck to it, used for smoothing or polishing woodwork or other surfaces.)                There are very many inclusive kinds of screeding,  There is the Cement-Based flowing screed, The calcium sulfate-based screed, and the sand and cement-based screed.

What is Cement and Sand Screed:

The term “Traditional Screed” refers to the method of laying by hand, but since almost all such screeds are composed of suitable sand with cement (OPC) and water, the terms “Traditional” and “Sand and Cement” have become almost interchangeable. Cement is used with sand at a ratio of 3 or 4 parts sand to one part cement. Water is added and thoroughly mixed such that the final consistency is “semi-dry”. This semi-dry material must then be leveled and physically compacted on the floor. Sand and cement screeds can be produced off-site and delivered ready to use or can be mixed on-site in mixers or combined mixer/pumps.


 What is Cement Based Flowing Screed:

Cement-based flowing screeds (often referred to as self-levelers, but not to be confused with self-leveling / smoothing compounds) are just what the name would suggest – a screed that flows like a calcium sulfate but is based on cement, thus addressing the issues posed by calcium sulfate screeds – these screeds combine ‘the best of both.

The latest generation of cement-based screeds is produced with a careful selection of admixtures, fillers, and aggregates which when used with the Cement produces a very fluid mix that can be laid in a very similar fashion to any other pumpable screeds but with very minimal shrinkage (which until recently has always been an issue with cement-based products). This “next-generation” technology has only recently become available in the UK but is already is gaining in reputation.


Calcium sulfate Screed:

When we say “calcium sulfate screed” (or Gypsum screed or Anhydrite screed) what we are referring to is a liquid applied or pumpable screed that is based on calcium sulfate. Calcium sulfate has one fundamental advantage over cement as a basis of a liquid screed. As it sets and “hardens”, which is a process of crystallization rather than hydration, it doesn’t exhibit the same shrinkage as would a normal cementitious material. Since shrinkage causes cracking and curling and is a major cause of problems with screeds this is a big advantage.

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