All concrete contains moisture and it always will. Concrete is composed of three main ingredients: water, cement and aggregate. Each component functions to form a slab. Concrete slab moisture problems occur when installers do not permit sufficient time for water vapor to migrate to the slab surface – the only way moisture can leave a standard concrete slab – in order to evaporate and allow more moisture to move to the surface. This is the process of drying concrete.
But “dryness” can be a range that depends on the finish treatment. The real target for any installer is to achieve the relative humidity (RH) level necessary, within the slab, for success with the specified flooring adhesive or finish that will be applied to the slab. Flooring failures occur when installers lay floor coverings over high RH slabs, thus trapping the excess moisture within them.
Historical Industry Practices
The industry has wrestled with concrete moisture measurement methodology for decades. In the mid-20th Century, the flooring industry deployed the anhydrous calcium chloride test to determine concrete slab readiness. It measured the moisture vapor emissions rate (MVER) at the slab surface; results were assessed in the number pounds of water that evaporated for every 1000 square feet of concrete slab over a 24 hour period. If MVER met specifications, contractors believed, the anhydrous calcium chloride test provided a rationale to proceed with floor covering. At the time, the anhydrous calcium chloride test was considered a qualitative measure of concrete moisture content.
As society arrived in this century, building industry experts questioned the integrity of the anhydrous calcium chloride test. That assessment measured moisture content, but only to a slab depth of 20 mm. Over decades, building trades discovered that the anhydrous calcium chloride test had often overestimated low-moisture emission levels and underestimated high emission levels. Not only that, but anhydrous calcium chloride test results gave no deeper indication of moisture content levels.
The 21st Century has brought experts to a new age of concrete moisture content assessment. Relative humidity (RH) testing is a method which recognizes the concrete moisture migration process, during which moisture content is absorbed and released from slabs at a rate that can vary due to the RH levels of its ambient environment. RH testing includes in situ moisture content measurement so that measurement occurs well below the surface (40% below, to be precise) for a scientifically proven reading that will correspond to the final equilibrated RH level once the slab is sealed. In essence, the industry has evolved to incorporate RH testing as a quantitative way for contractors to determine slab readiness.
How to Really “Dry” Concrete
It is a process that takes time, and time can be influenced by the ambient conditions around the slab as it dries. This makes the drying process not one of mere mathematical calculation (slab depth x # of days = dry slab) but a process that will require accurate monitoring of the slab’s relative humidity over time. Extensive testing has proven that RH readings taken at 40% of the slab’s depth (or 20% of a two-sided concrete installation) give the most accurate indication of what the slab’s final RH would be if it were sealed at that point and allowed to equilibrate.
The anhydrous calcium chloride test indicates a moisture vapor emissions rate which could easily fluctuate depending on how much more moisture needs to migrate and evaporate. Relative humidity (RH) testing places probes into holes inside concrete, so builders can assess RH at up to 40% depth in the slab. It’s a depth that is crucial for both business and scheduling reasons.
Two Crucial RH Specifications
Relative humidity testing contains two depths of great importance when attempting to avoid concrete slab moisture problems.
When constructing structural or cast in place slabs, installers can determine concrete readiness by conducting relative humidity tests to 20% of slab depth. However, this form of relative humidity only works if a slab is drying on two sides. If moisture content is interacting with air on two sides and evaporation is occurring from two sides, RH testing at 20% gives the most reliable results.
For slabs at or below grade (installed with vapor barriers over ground) or elevated slabs poured in pan decking, contractors must assess moisture content at 40% depth. Installers drill holes in the slab and insert relative humidity probes which assess MC. Certain RH products, such as the Wagner Meters Rapid RH, deploy fully installed sensors which remain in the slab at equilibration levels for fast and accurate testing. As long as depth is 40%, Wagner Meters Rapid RH provides moisture content results which give accurate readings and the information necessary to make installation decisions.
If concrete slabs are to pass moisture content muster, testers must assure moisture content levels with deep and thorough relative humidity testing.