Ask Denver: Moisture in Concrete & Subfloors

by Darren
Ask Denver

Is below surface moisture a growing and problem? If so, why?

Denver Coleman from Polyflor explains. With more than 50 years’ experience in the local and international flooring industry, Denver Coleman, Chairman of Polyflor SA, answers questions posed by installers, architects and readers. In this issue, he discusses the prevalence of below-surface moisture with solutions to circumvent this challenge.

“Is it true that there is increasing evidence of moisture in subfloors? What are some of the reasons and how can moisture problems be avoided?” – James Kennedy, Kimberley

Below-surface moisture affects most types of flooring, including ceramic or porcelain tiles. In recent years there has indeed been increasing evidence of moisture in subfloors. One of the reasons why increased moisture is found is due to the fact that up to 15% more water is added to current concrete mixes for improved flowability and strength. Chemical additives are also increasingly being used to strengthen and harden the concrete, but this retards the drying process, while power floated surfaces also tend to slow down the dehydration and trap moisture within the screed or slab.

All these factors, compounded by today’s fast track building requirements, often result in wet screeds. Below-surface moisture can cause installation failures due to the effect that it has on the bond of the adhesive, which can emulsify and lose its grip strength when mixed with water. It may also cause delamination of a self-leveling product from the subfloor.

Moisture testing, using correct testing methods, has therefore become of critical importance for all floors prior to installation. Subfloors need to be tested with a probe test, as surface testing alone will not provide an accurate indication of moisture. This is especially important if the plastic DPM is compromised. Results documented show very different readings between surface test methods and probe test equipment and international standards advise a minimum of 40% below surface test.

When testing for below-surface moisture, it is also important to document the date, equipment used and reading obtained (preferably photographically) in site minutes. The contractor, client and specifying professional need to be informed of these readings and photographic evidence will also ensure an accurate record should there be any queries later on.

Where old vinyl is replaced with new vinyl, screeds can still have residual moisture which did not affect the previous installation. Remember that building methods were slower 25 years ago, giving surfaces sufficient time to dry. Older adhesives used years ago also had more solvents in them, making them slightly more resistant to moisture. Slower building methods gave them a chance to become fully cured before coming into contact with rising moisture, whilst using good quality wood floated screeds ensured the screed dried efficiently. As a result, the flooring was installed on a drier surface, so a fully cured and better adhesive bond was achieved.

Given that the moisture trapped under the screed has become more common and problematic, precautions need to be taken. To this end, a surface-applied damp proof membrane is advisable.  If there are other influencing factors, such as marsh ground conditions, slopes on the upside of the project or perhaps paving or concrete parking areas where soak away is limited, it would be necessary to take additional precautions. These precautions will ensure the surface and/or underground water is diverted around the building and it avoids water traveling below the structure, which may cause problems in rainy seasons.

In many projects, clients might be reluctant to spend money or claim insufficient budget for products such as self-leveling screed applications and surface applied moisture or damp proof barriers. But, have they considered how much a delay in completing the project would cost due to a wet screed in terms of loss of turnover, rental, or delay in getting patients into a new hospital or care facility? For example, a delay in filling hospital beds can be anything between R5 000 – R10 000 per day. Multiply this over a period of 8-10 weeks, and it could cost the customer around half a million Rand, aside from any of the other costs incurred such as loss of theatre hours etc.

It is therefore clear that a delay and loss of income will far outweigh the cost of an effective liquid surface applied moisture barrier, and will be dramatically less than replacement costs in case of an installation failure.

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