TECHNICAL ADVICE

Can Cement Be Flexible?

PUBLISHED BY: Damian Groarke - Wall and Floor Sales Manager, Bostik

Damian has been involved in the industry for over 20 years. He’s passionate about flooring in all its guises, with his particular expertise being in subfloor preparation products and adhesives. Damian is an active member of the NTAI committee, and he sits on the NTAI Board. He aims to keep abreast of all the industry changes and feed this knowledge back to installers to ensure that when floors are laid, they perform to their maximum potential.

As part of standardisation with our neighbours in Europe, there is a requirement on many products to either get them tested or test in-house and declare the various properties. The idea is that, by doing this, all manufacturers will be on a level playing field with their product claims, which will help the end-user evaluate what they need.

This is particularly prevalent in the tiling market, where there is legislation to get external testing carried out, and performance declared. The adhesives used for bonding or fixing tiles are then classified according to various characteristics, such as whether they are cementitious (C), dispersion (D) or resinous (R) products and whether they have any special performance criteria such as fast setting (F), thixotropic (T) etc. They also have a classification of deformability, often referred to as elasticity or flexibility. All the classifications are given based on strict test regimes and conditions and enable performance differences to be appreciated between products. A tiler can then know what adhesive he requires for a particular project based on the needs of the tiles and the type of substrate.

There are no specific criteria for either smoothing compounds or adhesives for professional flooring. This is because the scenarios and criteria for flooring are far more variable. When installing tiles, it is known that a robust and sound substrate is needed; otherwise, tiles will crack under movement. Furthermore, an adhesive bed can be used if the substrate isn’t sufficiently flat, so the adhesive classification is more relevant.

We generally need to improve or prepare the subfloor to receive the floor covering by using a smoothing compound in flooring. Although there are test methods for assessing smoothing compound performance, there is no standard to test them against. Consequently, the industry uses the testing of “screeds” to offer some classification. You may have seen on datasheets or bags of smoothing compounds classifications such as CT-C18-F7 to EN 13813:2002.

Have you ever stopped to think what these are actually saying? Most people think, correctly, that the C refers to compressive strength (measured in N/mm2), which is a test that assesses how much of a load can be applied to the sample before it cracks. The sample is supported underneath during the tests, so this is meant to imitate the ability to resist loads on the smoothing compound from above. Compressive strength can loosely be correlated to how hard-wearing a subfloor is. A cementitious screed is typically 25 to 30N/mm2, whilst concrete can be anything up to 50N/mm2 depending on the construction needs.

The figure quoted is based on the achievement at 28 days under lab conditions and does not imply anything more than this. Equally crucial for flooring is the speed of strength build-up, as this will determine when the floor can be subjected to loads. A typical 18N/mm2 smoothing compound may only be 5N/mm2 the day after installation, but this is not a problem for walking on or laying flooring. It may take seven days to achieve a strength of 12N/mm2, which may be required to support heavy point loadings such as hospital beds, forklift or pallet trucks, or heavy plant such as computer servers; a high strength product may achieve this after 24 hours. Not all products are the same, so always do your research.

The reference for the F aspect above is often misunderstood, with most people thinking it measures the flexibility of a product. The actual test is called a Flexural Test and involves an unsupported sample of the product effectively trying to bend until it breaks. High flexural strength means a product that needs a high load to break it. It doesn’t indicate whether a product is flexible in the proper terminology, i.e. able to bend and distort. In fact, I can’t really come to terms with cement being flexible… it is a hard material that will crack if you try and bend it.

When we refer to flexible products, we are looking at products that can stay bonded or have micro-cracking rather than visible cracks when under deflecting loads. Such situations may be on timber floors, metal panelled floors, and underfloor heated systems.

Time and experience have shown us that lower compressive strength (and typically, these are lower flexural strength also) products are the best option because they don’t “sheet off” the floor. They will have lots of micro-cracks but still retain the adhesion. Fibre-reinforced products work by ensuring that any cracking will be somewhat held together by the fibres, resulting in microcracking that prevents sheeting off the floor… they should not be considered ‘flexible’ in the true sense as they will not bend and distort. In the tiling world, these products are prevalent and very successful but remember that subfloors for tiling need to be stable and sound regardless, hence 12mm upwards of Plywood when sheeting over floors.

I believe the old adage of not putting hard over soft is still a very valid rule of thumb for flooring. In Ireland, we limit preparation opportunities, often overlaying previous installations, which are likely to consist of relatively low strength products or perhaps adhesive residues.

So the moral here is, please don’t think of cement as having high flexural strength and therefore being flexible. Continually assess what the product can do. Is it low strength offering excellent adhesion? Will it build strength quick enough for my application? Is priming necessary to give the desired bond? If in doubt, phone technical support armed with all the information on the project and they can offer you a resolution.

Click here to request your Bostik Academy training

or

Contact your trainer: 

brian.monaghan@bostik.com