Working With Subfloors – Pumpable Self-Smoothing Screeds Based on Portland Cement

The quality of the finished floor will only be as good as the subfloor over which it has been laid. All flooring finishes require a clean, sound, level surface that is sufficiently strong and capable of supporting any applied subfloor preparation materials and the chosen floor finish.

Solid screeds and concrete subfloors fall broadly into four categories:

• Concretes, either hand or power float finished
• Traditional hand or power trowelled cement:sand screeds
• Calcium sulphate pumpable self-smoothing screeds
• Pumpable self-smoothing screeds based on Portland cement

This article focuses on the mechanical preparation and key information required for working with pumpable self-smoothing screeds based on Portland cement.

Bonded levelling screeds

Liquid cement-based levelling screeds can be bonded to concrete bases providing the base has been suitably prepared, with a minimum thickness of 20mm.

Un-bonded levelling screeds

Liquid cement-based levelling screeds can be laid unbonded normally over a separating layer, typically no less than 30mm thick. For thinner depths, seek manufacturers’ guidance.

Floating levelling screeds

Floating levelling screeds are laid over separating membranes on thermal or acoustic insulation boards or quilts and should typically be no less than 40mm thick. Some manufacturers offer proprietary screed mix designs which can be thinner.

Curing

Some liquid cement-based screeds are self-curing and need no external curing agents or membranes. Others require an externally applied curing membrane, usually by spray. Spray-on curing membranes are designed to entrap the water within the screed, preventing premature drying and associated issues this can cause. These membranes should be removed by sanding after 7 days in order to prevent extended drying times.

The screed should be placed into a weather tight environment and not be subjected to severe draughts, direct sunlight or heating for the first 72 hours. Foot traffic should be restricted for at least 24 (preferably 48) hours to prevent damage to the screed surface.

There are different types of liquid cement-based screeds presently available. Some produce a dense shiny surface with bonded laitance where others do not. Laitance will usually be prepared during the process of mechanical abrasion carried out on all screeds to provide a surface key and remove surface contamination.

Typically, liquid cement-based screeds can be loaded after 7 days following installation.

Drying

Drying times will vary according to the product used and manufacturers’ guidance should be sought on appropriate drying times. Drying times are usually based on 40mm to 50mm depth in warm and well-ventilated drying conditions, typically <65%RH at 20ºC. Poor site condition and deeper screed sections will extend drying times so it important to ensure that site conditions are well controlled.

Good ventilation or the use of dehumidifiers can assist in reducing the ambient humidity. Forced drying of these screeds is sometimes possible if required. Seek manufacturer guidance if assisted drying is required.

Testing residual moisture content

Before floor finishes are laid, the moisture content of the screed should be checked. The British Standard for testing a base to receive a floor covering is to use a surface or probe hygrometer. This provides a non-destructive test and when tested strictly to the method defined will give reliable results on Portland cement for RH up to 75% (the required limit for floor finishes).

The installation code of practice for some product groups also accept the hygrometer sleeve method. If this method is being considered it is important to check the code of practice that applies to the product type being installed.

The prepared surface of the screed is likely to be absorbent and should generally be primed using a suitable primer to prevent.

For more advice about substrates and subfloors, visit the CFA Guide to Contract Flooring.