Building your own home is one of the biggest undertakings a person may undertake in their lifetime. A self-builder faces a massive workload when sorting out various details, making decisions, coordinating the work of different contractors, sourcing materials, and, of course, doing the actual construction work themselves. Conducting moisture testing may seem like a pointless and time-consuming waste of money. After all, concrete dries a centimeter a week, right? In this blog post, I’ll debunk two persistent myths and explain why skipping moisture testing could come back to haunt you.
Myth 1: One cent per week
The so-called "rule of thumb" from the old school regarding concrete drying is that concrete dries at a rate of "one centimeter per week." In other words, an 8-centimeter-thick concrete floor, for example, would be dry enough for coating 8 weeks after pouring. Unfortunately, it’s not that simple.
Several factors affect the drying of concrete, such as the aggregate particle size, the water-cement ratio, the additives used, the ambient air conditions, the concrete temperature and thickness, and the cleanliness of the job site. I’ll discuss these in more detail on my blog Concrete curing at a single-family home construction site.
Consider two identical 100-square-meter single-family homes located side by side. An 8-centimeter-thick floor slab is poured in both: in House 1 using concrete with a high water-cement ratio, and in House 2 using concrete with a low water-cement ratio. Eight cubic meters of concrete are used in both, but the concrete in House 1 contains up to 800 liters more water to evaporate than in House 2. If the estimate of one centimeter per week were accurate, how on earth could the concrete in House 1 evaporate 800 liters more water than House 2 in the same amount of time?
The same applies to other factors, such as environmental conditions. Environmental conditions have a huge impact on how concrete dries. If the indoor air isn’t dry enough, drying won’t occur, even if months go by. Many construction sites have indeed been delayed precisely because of poor conditions, since the concrete hasn’t been able to dry.
So this myth is not true.
Myth 2: Surface meters
When discussing moisture measurements, you often hear people say, “At our construction site, Reiska measures moisture with a surface moisture meter. He’s been doing it for 20 years, and we’ve never had any moisture problems.”
A surface moisture meter is a measuring device that measures the electrical conductivity of a structure. According to the RT guideline governing this measurement, surface moisture meters must not be used to determine a structure’s suitability for coating.
We also use surface moisture meters, for example, in moisture damage surveys, where they can be used to determine the extent of the damage by comparing the reading taken from a dry area with the readings obtained from the damaged area. However, surface moisture meters do not provide any information about the actual amount of moisture in the structure.
Risks of wet paving
When concrete is covered, the evaporation of water from it into the surrounding air slows down significantly, and moisture begins to accumulate in the concrete beneath the surface material. Properly conducted moisture measurements ensure that the concrete is sufficiently dry so that the surface material to be installed can withstand the moisture stress that occurs after installation. Typically, the more vapor-tight the surface material used is, the drier the underlying concrete must be.
However, if the surface material is applied over concrete that is too wet, the excess moisture that accumulates beneath it can cause various problems. These problems include conditions conducive to microbial growth and the resulting indoor air quality issues, buckling or peeling of surface materials from the substrate, chemical reactions between moisture and the surface material or the adhesive used to bond it, and the release of toxic volatile organic compounds into the indoor air. However, all of these problems can be avoided by ensuring that the concrete is sufficiently dry before coating work begins.
Check that the concrete is dry by measuring its moisture content before paving
Moisture Measurement is a quick and cost-effective quality assurance measurement performed during construction that helps you avoid the risks of paving on wet concrete. In practice, virtually every professional client now requires their contractor to verify that the concrete is dry through moisture measurements before paving. This practice is also in use at virtually all prefabricated house factories. Therefore, I can only strongly recommend moisture measurement to private homebuilders as well. This way, you can avoid many of the problems caused by excessively moist concrete later on.
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