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Blisters In Vinyl Will Get You Popping

John Alcock on osmosis
A COUPLE of months ago I visited a recently constructed college in the North West that featured a large central hall fitted with the usual DPM, smoothing compound and vinyl covering.
Problems had arisen when the contractors had come to fit furniture during the final stages of the build, and noticed that a series of blisters had begun to form on the surface of the floor.
There were a number of these blisters spread quite randomly across the floor and investigating further, I pierced one which resulted in a good amount of water coming out under some pressure. Clearly this was a case of osmosis, although to be quite honest it had been some years since I’d last seen something like this.
So, imagine my surprise when only a few weeks later I came across another similar case. This time it was a public building in the South, where a ground floor corridor had a similar series of blisters containing pressurised water.
There’s nothing to suggest this is anything more than coincidence, but it does raise the issue of how to deal with a problem that, whilst rare, can be expensive to put right.
Briefly, osmosis is defined as the movement of a fluid, such as water, through a semi-permeable membrane into a solution where the solvent concentration is higher.
It’s not known for sure what causes it, and in my experience it’s fairly rare. However, its presence can be a big problem for flooring contractors, and it makes sense to be aware of a few simple good practice pointers.
Although there’s some evidence to suggest selecting certain types of subfloor preparation products – such as a polymer-modified levelling compound, which may help to reduce permeability – can minimise the occurrence of osmosis, the fact that it is so difficult to predict when it will occur means this is very tough to judge. For example, I have seen osmotic blisters occur in just one small patch of a large floor area, four storeys up – not exactly predictable.
My main tip is to check that the base concrete has been allowed to properly dry out after curing before you start work – it’s often excess moisture in the concrete that can lead to osmotic blistering.
Some recommendations suggest letting the concrete dry for at least a couple of months, although the presence of a water vapour barrier beneath the slab can also help here.
It is also thought that higher amounts of soluble salts in the mix can affect the likelihood of osmosis occurring.
In any case, checking before you start on site is always a good idea to be sure you’re not letting yourself in for problems – if in doubt, use a hygrometer to check moisture levels.
Needless to say, rectifying problems caused by osmosis can be expensive and time consuming. In the cases I covered above, we recommended that the contractor fully abrade the floor back to the base, and consider a deeper than usual abrasion as this may help to remove surface material causing the problem.
In short, because we’re not sure exactly what causes osmotic blistering, there’s no exact answer to avoid it.
It’s not often that as an industry we’re stuck for definite answers to problems like this, and so it’s important to share experiences of osmosis problems so the industry can work towards a proper solution to prevent it happening.

John Alcock is technical specifications manager at Bostik
T: 01785 272727

This article has been reproduced from the Contract Flooring Journal website. You can find them at