Sweating Concrete Floors

As a consultant and design and build flooring contractor Conslab get to see a lot of industrial concrete floors every year. Over the past 18 months Conslab have been receiving a number of queries regarding damp concrete floors. The dampness appears in a “mottled” pattern and tends to be quite transient – it occurs and then often disappears just as quickly. We’ve noticed this occurring on a variety of industrial floors in Auckland.

Example of sweating concrete floor with the concrete looking damp and mottled

This appears to be a phenomena called “sweating slab syndrome”.We are currently working on a research project reviewing this in more detail in New Zealand conditions. http://www.concreteconstruction.net/interiors/sweating-slab-syndrome.aspx

What we do know for sure is that there isn’t a weather tightness issue with any of these floors. The moisture is condensation on the floors surface from moisture in the atmosphere and we believe it tends to only occur in some buildings and not others due to micro climate factors (such as which direction the doors face, and the level of mechanical ventilation). It occurs only on certain days, typically for short periods and is very intermittent. The issue seems to be most predominant with new floors, and anecdotally on a lot of sites the issue resolved itself over the first couple of years after the floor is constructed.

Other than condensation being the main driver we believe there are a couple of other things that contribute. Firstly it seems that with a very dense and impermeable surface the floor cannot absorb much condensation – so we think the very good dense surface that we aim to produce to reduce dusting/abrasion means that when we get condensation it is not absorbed into the very dense concrete surface. It sits on the surface until the environmental conditions change and it can evaporate. This has been tested across a range of slabs using “sorptivity testing” and correlates very well with what we see – the denser areas of surface are more likely to get dark and damp. You can get a quick visual check on this yourself by comparing the areas that have moisture on the surface with those that don’t – typically the edges of the slab and around columns won’t be as bad – these are the areas that aren’t finished as densely.

There is very little literature on the issue but it also suggests that salts that leech out of the surface of the floor can be hygroscopic (water absorbing). We haven’t been able to verify this in practice, and many of the new floors that exhibit this issue actually have a moisture repelling lithium silicate densifier applied. However anecdotally regular cleaning of the floors does seem to reduce the number of incidents – and this makes sense in light of the “water absorbing salts” theory, and also the concept that having a dirty floor can cause issues.

Either of these scenarios (density of the surface, or salt deposits) fit with the fact that the issue seems to reduce over time. Over time the surface will open up and become less dense, and there also won’t be as much salt movement through the slab.

In terms of addressing the issue the expensive approach would be to look at upgrading mechanical ventilation. Often this isn’t very practical, and given that the dampness issue occurs very intermittently there is a risk of spending a lot of money resolving a problem that occurs on a handful of days during the year.

The most practical approach we have found so far is to focus on whether the occasional dampness actually causes a problem for the tenant . In most of these situations there isn’t actually a tangible problem, the tenant flags the issue because it is unusual, but it isn’t impacting their business. The two situations where it can cause a real problem are either the safety aspect (increased breaking distances on a damp floor), or with moisture sensitive product being stored directly on the floor. Either of these issues can be addressed specifically – for instance if slipperiness is an issue we can create a texture on the floors surface.

In most cases we suggest trying to avoid taking any action to address the issue for at least 12 – 24 months after the floor is constructed and monitor the situation. Feel free to contact me if you have any questions about “sweating slab syndrome”, or if you know of any examples of this occurring – we are keen to extend our current research project to cover as many floors as possible.