Concrete users expect optimum strength and durability. Slabs which contain stable, healthy moisture content (MC) deliver just that. However, flooring installation sometimes proceeds over a concrete slab despite borderline concrete moisture assessments. If a MC test calls for more drying time, then a successful flooring installation rests on moisture evaporation and human patience.

Concrete Moisture Basics
The American Society for Testing and Materials standard ASTM F2170 is a moisture measurement benchmark when installing finished floor products and coatings on concrete. It recognizes MC test methodology with relative humidity (RH) as its guide. When ambient RH rises, concrete can actually absorb MC; when ambient RH drops, concrete emits MC. Therefore, ambient RH is one influencing factor in the drying time which is required to ensure concrete slab stability and performance. In addition, internal RH levels give the best possible indicator of slab readiness for the proposed flooring or finish. ASTM F2170 solidifies RH testing as a viable way of determining concrete slab moisture.

MC is central to concrete slabs and floor covering materials. Concrete itself is comprised of water, cement and aggregate. Once combined, installers pour the concrete and…then the wait is on. For what?

Construction-Worker-WaitingWater vapor migrates from the bottom of concrete slabs to the surface in order to evaporate while the slab hydrates (hardens). And while curing (the chemical combination of the mix that leads to hardening) progresses at a steady rate, drying (the process of letting MC leave the mixture) does not. As has already been mentioned, drying time is dependent on ambient conditions, among other factors. From inside the slab to its surrounding environment, MC exercises influence over concrete dryness.

Too often, contractors do not allow sufficient time for concrete slabs to dry, leaving excess MC in need of escape. When installers proceed with floor coverings before MC is stabilized, excess water vapor is trapped with no place to evaporate. It continues to migrate to the slab surface, where it is trapped and can cause trouble with adhesion through moisture-related or pH-related complications. Trapped excess moisture in a concrete slab is a major cause of persistent flooring problems, as well as complete flooring failure.

Compatible Flooring Products
As the building industry embraces more organic building materials, the MC imperative gains importance. Building product manufacturers are increasingly discarding volatile organic compounds (VOCs) in favor of lower chemical replacements. The chemical interactions which govern adhesive and floor covering processes increasingly involve water. In fact, flooring material producers are including MC specifications on product labels.

Builders who ignore ASTM F2170 protocol may apply eco-friendly finishes to wet slabs, where the excess MC in concrete mixes with enhanced MC in adhesives and floor covering materials. If ever a situation demanded a MC test, this scenario applies.

Moisture Meters
Luckily, RH testing answers the call for accurate modern slab moisture management.

MC test technology has quite a history. In the mid-20th century, builders used the anhydrous calcium chloride (CaCl) test for the moisture vapor emission rate (MVER) of concrete slabs. Calcium chloride was not a moisture meter, but a means of assessing concrete surface moisture evaporation which was used as an overall MC measurement. Over the ensuing decades, moisture meter products evolved to include more advanced methods of measuring slab surface conditions. However, persistent slab performance issues eventually prompted experts to question MC test methodology. At the turn of this century, RH was established as a credible way of assessing MC inside concrete slabs; thus, ASTM F2170 was created (although calcium chloride is still used today).

RRH-Starter-KitConcrete moisture testing evolution has produced several options for profiling concrete dryness. First, the surface moisture meter provides qualitative (comparative) readings, by which assessors can establish placement for a RH test. Installers can then utilize an RH test, such as the Rapid RH® from Wagner Meters, to assess MC at multiple slab locations at 40 percent depth. In this way, they emerge with a more comprehensive picture of concrete slab readiness (not to mention achieving ASTM F2170 certification).

If MC test measurement assures slab performance and durability, concrete moisture testing evolution is a crucial answer to the age old headache of too much water in all the wrong places.

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