Why Moisture Test at all?

Another case of job deadline pressure vs. doing it right…

Floor Test Australia’s Patrick Atherton attended a watershed (pardon the pun) seminar jointly presented by the Concrete Institute of Australia (CIA) and Floorcoverings Association of Victoria (FAV), discussing the impending standards revision for subfloor moisture testing for floorcovering (June 2012), and the emergence of Relative Humidity (RH) Moisture Testing for concrete slab subfloors.

Aaron Martin of the FAV attested that, in the last 5 years, there has been a noticeably massive increase in moisture-related failures. The reasons for this are varied, but the solutions represent a more narrow corridor of action.


Concrete starts wet, and has to dry. The water/cement ratio generally should be between 40-50% water, but even small increases in water can lead to disproportionately higher drying times. A 5% increase in water can DOUBLE the drying time of the concrete. 10% increase- TRIPLE the drying time. The same thing occurs when the concrete is thicker- drying times increase disproportionately. (CSIRO – Ron Denning Feb 2012). A dry, old slab which has been re-wet for some reason (poor drainage, flooding, no vapour retarder beneath the slab, plumbing failure etc) will take longer to dry than a wet slab.


In simple terms, moisture migrating to the surface of the concrete breaks the bond between the adhesive and the concrete. Some argue that the move away from solvent-based adhesives towards lower- VOC water based types has increased failures in recent years. It’s also interesting to note that the higher water content in adhesives can itself contribute to failures- that is, it’s not the water itself but the alkaline solutions in the concrete surface activated by the water, which attack the adhesive and cause it to delaminate. Such moisture from adhesives has also been known to cause the flooring (particularly timber) to swell, the problem being incorrectly (in some cases) blamed on the slab. That said, drying slabs are the most common culprit but again, it’s not the water itself, it’s the fact that the water carries alkaline solutions from the concrete, which are the main detriment to adhesion.(CCAA paper April 2007).

Flooring types such as vinyl, are highly susceptible to their adhesives being re-emulsified by the alkaline solutions carried by moisture vapour. It can get very ugly, in the case of “Sick Building Syndrome”- moisture migration causes the alkaline solutions to break down the adhesive, and the resulting toxic emissions (including nasty substances like butanol and ethylhexanol) can exceed safe levels. It is no small matter.


Here’s where it gets complicated. Standards applicable to Timber Flooring (mainly Resilient Flooring AS/NZ 1884) have been recently amended. Once people become accustomed to the normative procedures outlined in the standards, manufacturers will choose whether or not they will refer to these standards when providing a warranty.

Adhering to standards is therefore relevant to the contractor on two levels: 1) To demonstrate that they have undertaken best practice, useful if litigation arises from a flooring problem and 2) to be covered by warranty and support from a manufacturer who specifies details from those standards.

The standards will need to do two things- specify WHICH measurement criteria will be deemed as gospel. There are currently three or four different methods of testing for moisture in slabs, from measuring moisture CONTENT (as a percentage, using a “meter”) to MOISTURE VAPOUR EMISSION RATE (MVER, using calcium chloride) and RELATIVE HUMIDITY.

The review of standards has meant crunch time for some of these methods. Relative humidity (RH) is emerging as the winner in the eyes of many stakeholders, but there are still some grey areas.

So who’s liability is it to ensure testing is undertaken? We’ll deal with that in a later post.

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