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A comprehensive guide to aid successful screed installation

LKAB Minerals has a guide entitled ‘Floor preparation: a comprehensive guide to aid successful screed installation’. Here, in the company’s words, is what it says about its guide:
Are you tired of hearing conflicting advice about floor preparation? This guide will explain what the British Standards and CFA recommend regarding floor preparation for screed, which is the same for both anhydrite and cement-based screeds.

It all starts with the right preparation. Don’t overlook the importance of adequately preparing for screed to receive floorcoverings, as the quality and durability depend on it.

We all know that woodwork requires thorough sanding and priming before painting, and hardcore requires flattening and compacting before concreting. In the same way, screeds require adequate preparation before applying finished bonded floorcoverings. Whether you wish to lay ceramic tiles, vinyl tiles, sheet vinyl, or any other type of bonded floorcovering, you need to ensure the screed is suitably dry, sound, clean, and free from debris and contamination likely to cause a failure of the bond.

Moisture measurement for screeds
This is simple and should be carried out using either a carbide bomb tester or a suitable flooring hygrometer in line with the requirements of BS8204:2003. The moisture level should be below 75% relative humidity (RH). Where the carbide bomb test is employed, this will equate to 0.5% moisture by weight for an anhydrite binder such as Gypsol. Liquid damp-proof membranes and moisture control systems may be used at higher moisture levels. Options are available for screeds containing Gypsol binder exhibiting moisture contents up to 95% RH or 2% moisture.

Cracking or fracturing
Cracking or fracturing of the screed generally requires repair, often using flowable resin repair systems. Cracking is typically associated with early age or plastic shrinkage, which is usually a result of the screed drying too quickly in its plastic state shortly after installation. This can often be avoided by installing the screed into a suitable building envelope. Other causes of cracks include long-term shrinkage, thermal movement, perhaps owing to underfloor heating, and impact damage, such as heavy machinery or early loading.

Cracks should be dealt with before applying bonded floorcoverings. A simple flowable resin repair will suffice in most instances, but proper diagnosis is required in all cases, and bonded floorcoverings shouldn’t be placed on cracked screeds.

Cracks are usually avoided by ensuring the appropriate installation safeguards are in place, such as protection from extremes of temperature and elements and splitting the screed into suitable bays with the placement of movement joints. Gypsol screeds can be laid to bay sizes as large as 1,000sq m, while a typical flowable cement-based screed would have a maximum bay size of about 100sq m. Underfloor heating will also impact bay sizes and the requirements for movement joints. Movement joints should reflect through bonded floorcoverings and be dealt with at the design stage.

The soundness of the screed surface means it should be suitably hard and free from fractures. There is no easily defined surface hardness measure, which can be quite subjective in many instances. Hardness will often be driven by screed mix design, so when there’s doubt, it may be necessary to discuss the matter with the screed manufacturer or installer to determine it correctly. Fortunately, hardness is rarely an issue in most screeds.

The screed must be clean and free of anything likely to cause adhesion failure. To achieve this, all screeds must be lightly mechanically abraded, typically by sanding using a rotary or STR-type floor sander. Sanding the screed is the best method to remove extraneous construction debris such as mortar snots, plasterboard adhesive, mud, and general site contamination. It’ll also remove any loose friable material or curing compounds left from the screed installation.

The ultimate responsibility for preparation should remain in the finished flooring contract, as the finished floorlayer will have the necessary expertise to determine the suitability and level of preparation required for any floorcovering type.

Finalising screed installation
Once the screed is suitably dry, sound, and clean, it should be protected from contamination. For this reason, moisture testing, crack repairs, and sanding are best carried out as the last operation before the physical installation of primers, liquid damp-proof systems, smoothing compounds, or final floorcoverings.

Gypsol anhydrite screed
LKAB Minerals manufacture Gypsol anhydrite screed binders from a by-product from the acid production industry.

Contractors and ready-mix companies use the binder to offer nine unique floor screed formats. This allows the Gypsol range to offer huge design flexibility, accommodating diverse and novel applications in various building types.

Gypsol Complete binder can be used to manufacture all screed formats, including fast-drying Rapide. Standard Gypsol binder can be used to manufacture all screed formats except Rapide. It can be placed, floating, with or without underfloor heating and can be used bonded or unbonded to a depth of just 15mm.

Get in contact with one of our specialist technical sales managers for more information, or visit our website: www.lkabminerals.com/product/gypsol-binder/
0800 6226023

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