More F The moisture vapor emission rate only reflects the condition of the concrete floor at the time of the test. All concrete subfloors emit some amount of moisture in vapor form. Concrete moisture emission is a natural process driven by environmental conditions. All floor coverings are susceptible to failure from excessive moisture vapor emissions.
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Over decades, CaCl gained industry acceptance as a practical standard. Although builders continue to use the CaCl moisture test, post-installation moisture problems eventually redefined it as a test method worthy of scrutiny.
Moisture Vapor Emissions The CaCl moisture test produced data based on moisture vapor emission rate MVER , the quantitative indication of how many pounds of slab moisture evaporated from square feet of concrete over a hour period. At that point, it was safe to install floor covering over a concrete substrate.
Sub-Surface Emissions Concrete moisture vapor migrates from the bottom of concrete slabs to their surfaces in order to evaporate. This is how concrete dries. Generally, high ambient RH slows concrete drying as the slab absorbs moisture vapor; low ambient RH causes concrete to emit moisture vapor. Therefore, the natural migration process of concrete moisture drying is influenced by environment.
Where the challenge became clear was in determining whether the surface emission rate determined by desiccant weight gain truly reflected the moisture conditions deeper in the slab. In fact, moisture-plagued concrete slabs certified by ASTM F protocol proved to be a patterned and persistent challenge to the concrete flooring industry — so much so that industry professionals began to investigate the scientific integrity of the CaCl moisture test. Problem 1: surface measurement.
CaCl measured only the top one-half to three-quarters inch of the slab. Problem 2: distorted readings. CaCl tended to underestimate the true moisture conditions at the high end of the moisture spectrum, while it tended to overestimate the moisture conditions of older slabs.
Builders, then, made threshold floor covering decisions based on errant readings. Problem 3: scope of assessment. This led to continued moisture problems when that moisture eventually reached a covered concrete surface where they could no longer evaporate. In light of such research, the American Society for Testing and Materials made two changes to moisture measurement industry standards.
The CaCl moisture test is a building practice with lineage. But their truth has entered a new era and the challenges of moisture emissions-based testing are becoming increasingly clear.
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