Calcium Chloride (MVER)'s Achilles Heel
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Technical evolution has influenced two guidelines practiced by the American Society for Testing and Materials (ASTM). Specifically, ASTM F2170 co-exists with ASTMF1869 as construction moves from one era of moisture content measurement to another.
Moisture Vapor Emissions Rate (MVER) Regulation:
ASTM F1869: Use this test method to obtain a quantitative value indicating the rate of moisture vapor emission from the surface of a concrete floor and whether or not that floor is acceptable to receive resilient floor covering. The moisture vapor emission rate only reflects the condition of the concrete floor at the time of the test.
The calcium chloride (MVER) test has been construction's traditional choice since the 1950s. Its central tenet is that moisture content can be effectively measured by the rate of moisture vapor emissions. Assessors place a specific weight of anhydrous calcium chloride crystals in a dish which is sealed to a concrete surface for 60 to 72 hours. The weight differential before and after testing reveals the amount of moisture vapor absorbed by the calcium chloride crystals; the rate is indicated in pounds (of water) per 1000 square feet for every 24 hours of drying time. If MVER is below "3," the concrete is deemed suitable for flooring application.
A pound reading of calcium chloride does not necessarily indicate the total concrete dryness. MVER focuses on weight gain of crystals based on water vapors that rise to the surface area during a timed test. In fact, moisture vapor is a continuous migratory process that is influence by relative humidity (RH) conditions. An MVER test may reflect moisture content in the top three-quarters-inch of the slab but critics suggest the MVER test does not address moisture content deeper inside the concrete.
Moisture Content Regulation:
ASTM F2170: Moisture permeating from concrete floor slabs affects the performance of flooring systems such as resilient and textile floor coverings and coatings. Manufacturers of such systems generally require moisture testing to be performed before installation on concrete. Internal relative humidity testing is one such method.
Relative humidity (RH) testing assesses moisture content at varying depths and locations in a concrete slab. The differences in MC readings indicate the slab's natural process of moisture migration. Water vapors rise from the bottom to the top in order to evaporate. However, high RH causes concrete to absorb moisture vapor, which slows down evaporation. Conversely, low RH causes concrete moisture vapor to evaporate more quickly.
Wagner Meters' RH test method quickly and accurately assesses total internal moisture content - where stubborn vapors are known to hide. Left untraced, residual moisture vapor can destroy the adhesive bond of prematurely-installed floor coverings. They can also lead to costly and perpetual moisture content problems in the slab itself. Luckily, relative humidity testing empowers contractors to pre-empt these issues by testing RH levels long before they install a floor covering.
Relative humidity or calcium chloride (MVER)? Your choice determines the efficiency and quality of concrete installation.