Smooth operators

Martin Cummins of Bostik looks at why smoothing compounds are advisable for workplace floors, and the key specification considerations to achieve the right result.

The stage preceding the application of a floorcovering is an often overlooked but critical part of the process of creating a finished floor. When it comes to getting the substrate ready to receive the floorcovering, specific knowledge of the subfloor becomes much more important.

In addition to mechanical preparation to remove problematic materials and mitigation of moisture that can result in floor failures, smoothing compounds are applied to enable a smooth floor to be installed. But what are the key issues for installing such compounds in different settings?

To enable adhesives to dry into the substrate, and therefore to bond, it is necessary to provide a 3 mm absorptive medium. This is particularly important on non-porous substrates such as DPMs, power floated concrete and terrazzo. Generally, priming is required but most smoothing compounds are suitable for this.

An isolation layer is often required for office refurbishment projects where a floor has been uplifted and there are old adhesives and even previous smoothing compounds that are neither smooth enough nor absorbent enough for adhesives to be applied. The smoothing compound needs to have excellent adhesion and generally lower strength. The reason for choosing a lower strength product is to minimise tension on the unknown layers below.

Although the industry uses the term ‘flexible smoothing compound,’ this is pretty much a misnomer. It is very hard to get a cement or calcium sulphate product to be flexible. What we can do however is fibre-reinforce the compound to help minimise crack formation under extreme conditions. Such conditions would include when the smoothing compound is being used over underfloor heating mats, or screed boards where the subfloor is not stable.

In terms of finish, in some settings the smoothing compound being applied also has to act as the finished wear surface. Manufacturers can advise on these specialist products, which need to be both strong and also capable of resisting abrasion from whatever is occurring above. They may need to be sealed or painted.

Often on projects there may be areas where the subfloor needs to be raised a significant amount – ‘deep section’ applications. This can include where floor levels vary, partitions have been removed or mat wells need filling. Many smoothing compounds may achieve this but often the drying and curing rates are slow. In this instance, rapid drying products are recommended, curing in days rather than weeks when applied at thicknesses of up to 50 mm.

Some substrates are not suitable to directly receive standard smoothing compounds due to chemical interaction or surface effects. In these cases, a suitable barrier primer or a non-standard smoothing compound may be the answer. Again, check with manufacturers if you have any unknown or atypical substrates as there usually is a solution.

Smoothing compounds are the key to ensuring the designed and specified floor stays firmly fixed, but it’s important to be aware they do not all do the same thing. Some have niche uses, whereas others are generally high performing workhorses to cover an array of situations.

Martin Cummins is UK technical support manager at Bostik