Recurrent moisture content is a plague on the modern flooring industry, especially as it applies to shrinking building schedules and budgets. When builders apply flooring over improperly dried concrete slabs, high moisture content levels can trigger perpetual problems which can jeopardize a floor, including adhesive breakdown, debonding, osmotic blisters, staining, efflorescence, curling and expansion.

In recent years, the flooring products industry’s shift away from volatile organic compounds (VOCs) has created a new generation of adhesives that are more sensitive to concrete moisture content than ever. Most manufacturers provide product specifications for flooring professionals, but the issue is one of concrete slab moisture measurement. The end result of a strong and resilient floor will require precise and thorough concrete moisture assessment.

When industry warranties and reputation rest on integral flooring installation, builders refer to industry guidelines. The American Society for Testing and Materials (ASTM) has two such guidelines for moisture content assessment of concrete slabs: ASTM F1869-11 and ASTM F2170-11. Each guideline refers to a specific concrete slab testing methodology.

However, it is important to first understand the environmental factors that influence fluctuations in concrete moisture content levels.


Concrete Moisture 101

Moisture is present from the moment a concrete mixture is conceived and created. Once a slab is poured, moisture content dissipates through surface evaporation as the slab cures and dries. As that moisture is drawn to the slab surface, moisture content insidethe slab redistributes for the same purpose: To rise and evaporate. Thus, installers must allow sufficient time for moisture to do what it does through this natural redistribution and evaporation process. Under ideal conditions, the rule-of-thumb is to allow approximately 30 days of slab drying time for each inch of concrete slab thickness.

Slab moisture levels are influenced by the original concrete mixture, but also by the slab’s surrounding environment. Concrete slab moisture levels can change due to poor drainage from beneath the slab, capillary action (known as wicking) from subsurface moisture, or from moisture-rich additives applied to the concrete surface. Although these influences may be natural to some degree, they invariably influence concrete slab drying times.

Ambientrelative humidity(RH) and temperature are powerful variables. The relative humidity of air can cause a slab’s surface evaporation rate to increase or decrease. When RH is high, concrete absorbs water vapor from the air; conversely, when RH is low, concrete emits water vapor into the air. Changes in ambient relative humidity influence slab moisture content even before floor coverings are applied. Once covered, slab moisture continues to migrate from the bottom of the slab until moisture content is proportionately equilibrated. Even if initial surface MC was low, after the floor covering application relative humidity will continue to rise from the bottom of the slab. In fact, moisture levels do not fully settle until after floor coverings are installed.

Of course, the most proactive approach is constructed by design. Builders have numerous ways to ensure that concrete moisture, a natural presence in the concrete flooring process, has avenues by which it can exit a concrete slab:

· Make sure drainage runs away from a newly poured slab and that the grade is adequate for local conditions.

· Specify a true vapor retarder, with a minimum thickness of 10 mils, under the slab to prevent moisture seeping in through the ground.

· Lower the water-to-cement ratio in concrete mixtures. Various admixtures suffice in minimizing initial moisture content while ensuring a workable mix.

· Allow the slab to dry naturally at service conditions. Protect it from the elements, and avoid maintenance or cleaning processes that will wet the slab.

· Specify, conduct, and verify reliable, accurate concrete moisture content testing before proceeding with floor covering.


Testing and Solutions

For decades, builders and flooring installers have assessed concrete moisture using the anhydrous calcium chloride test of moisture vapor emissions. The moisture vapor emission rate (MVER) is the amount of moisture emitted into a salt compound sealed to the surface of concrete with an enclosed moisture dome. The anhydrous calcium chloride test assesses slab surface moisture content for 60-72 hours. The resulting MVER had been deemed as a reliable method for determining concrete slab readiness.

However, the turn of the 21th century brought renewed inquiry to the anhydrous calcium chloride test method. The construction industry began to recognize that slab performance, as assessed by calcium chloride (CaCl), was often less stable than the moisture vapor emissions rate (MVER) first indicated. Specifically, professionals focused on the fact that by nature, the anhydrous calcium chloride test did not predict moisture conditions inside the slab. It was time to assess the historic tool of concrete moisture assessment itself: The anhydrous calcium chloride test and its accompanying industry standard, ASTM F1869. How could a general contractor and specifier help the flooring installer prevent future moisture content problems?

Relative humidity (RH) testing has now been acknowledged as a legitimate means of assessing overall slab moisture content levels. In fact, the American Society for Testing and Materials (ASTM) created ASTM F2170-11 as an alternate to F1869. Ultimately, the industry has recognized that relative humidity is an accurate way to determine a concrete slab’s readiness.

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Jason Spangler has 17 years experience in sales and sales management in a spectrum of industries, including 10 years in building products. During that time, he successfully launched a variety of products to the market, including the original Rapid RH moisture measurement devices. In his most recent position with a national healthcare organization, Spangler was District Manager for Northern Oregon, with complete sales and P&L responsibilities for six branches in the Portland, OR market. Spangler rejoined Wagner Meters in 2010 as Product Sales Manager for the Rapid RH. Spangler holds a B.S. in Business Administration from Southern Oregon University.