42 www.paintsquare.comJ P C L J a n u a r y 2 0 0 8

t is widely accepted that moisture vapor transmission through concrete floor slabs can adversely affect theperformance of coatings that are applied to the concrete slab. Before the early 1990s, it was common to checkfor moisture vapor transmission (MVT) through a concrete slab using either the plastic sheet method or any ofa number of contact probes that were available for testing moisture. The plastic sheet method was strictly aqualitative test. A plastic sheet was taped to the bare floor, and the underside of the plastic sheet was laterexamined to determine whether moisture had condensed on the sheet. If moisture was found on the back of thesheet, it was assumed that the MVT through the concrete was too great for the successful application ofcoatings to the concrete slab.

The moisture contact probes usually work by measuring the conductivity between two points in theconcrete. The probe correlated the conductivity of the concrete to the moisturecontent and typically had a range that was considered safe for coating.Another MVT test was commonly used in the resilient flooring industry. This

method was known as the calcium chloride test. The test, believed to haveoriginated in the 1950s, was designed to quantitatively determine the rateof MVT through a concrete floor slab. ASTM International officially approved acalcium chloride test method in 1998 under the designation ASTM F 1869,“Measuring Moisture Vapor Emissions Rate of Concrete Subfloor Using

IEditor’s Note: This article is based on apaper given at PACE 2007, the jointconference of SSPC: The Society forProtective Coatings and the Paintingand Decorating Contractors of America.The conference was in Dallas, TX,February 11-14. The article is publishedwith permission.

Variables AffectingCalcium Chloride Testingfor Moisture in Concrete

transparent plastic covering (minus thearea of the dish), the weight gain of thecalcium chloride, and the exposuretime. The MVT rate is expressed inpounds per 1,000 ft2 per 24 hours. Itis commonly held that the maximumMVT rate acceptable for application ofcoatings is 3 lb/1,000 sq ft/per 24hours, although permissible rates canvary, depending on the coatingmanufacturer’s specific requirements.

Calcium Chloride TestVariables

The calcium chloride test is based onthe assumption that moisture absorbedby anhydrous calcium chloride in adish on the surface of the concrete slaboriginated either under a concrete floor

Anhydrous CalciumChloride.”The calcium chloride

MVT test has steadilygained acceptance inthe floor coatingindustry. This articlediscusses variablesthat affect the resultsof MVT testing withcalcium chloride, andit presents the resultsof testing designed tomeasure the affects ofcertain environmentalparameters on theresults of calciumchloride MVT testing.

BackgroundIn the calcium chloridetest method, a dish of dried anhydrouscalcium chloride is weighed and placedon the floor where the moisture vaportransmission rate is to be measured.The method requires that if a floorcoating has already been applied to theconcrete, the coating must be removedfrom an area at least 20 in. x 20 in. Asealant is then applied to the flange ofa transparent plastic covering, and thecovering is placed over the calciumchloride dish. The dish is allowed toremain for 60 to 72 hours. The dish isthen removed and re-weighed. Theweight gain is assumed to be the resultof moisture absorbed by the calciumchloride.The MVT rate is then calculated

based on the area under the

slab and migrated through the slab tothe surface from directly below, orwithin the concrete slab itself. Inreality, moisture vapor will travelrandomly in any direction. As a result,moisture vapor in the air above aconcrete floor can travel into aconcrete slab. Once moisture is in theconcrete, it is reasonable to expect thatthe moisture can then travelhorizontally. Given this expectation,one would further expect that somemoisture absorbed by the calciumchloride originated in the airsurrounding the transparent plasticcovering the dish. The amount ofmoisture absorbed would increase astemperature and relative humidityincrease.Porosity of the concrete near the

surface would also affect theabsorption rate, because migrationthrough the concrete would increase asthe porosity increased. ASTMStandard Test Method F 1869 providesno limitations on the temperature andrelative humidity of the environmentsurrounding the calcium chloride testapparatus if the conditionsapproximate the normal operatingservice environment. The test methodsimply suggests that the test siteshould be at the same temperature andhumidity expected during normal use.If the temperature and humiditycannot be controlled to the rangeexpected during normal use, the testmethod requires that the temperaturebe 65 F to 85 F, and the relativehumidity should be 40% to 60%.

J P C L J a n u a r y 2 0 0 8 43www.paintsquare.com

By Rick A. Huntley, KTA-Tator, Inc., Pittsburgh, Pennsylvania

For one thing, contrary

to popular belief, moisture vapor

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Concrete panel used for the calcium chloride test.Photos courtesy of the author

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a 24 in. x 24 in. area, which issomewhat larger than the 20 in. x 20in. minimum area required by the testmethod.Approximately 24 hours after the

coating was removed, a pre-weigheddish of calciumchloride wasplaced in each testarea and coveredwith atransparentplastic covering.The transparentplastic coveringhad a sealantapplied aroundthe flange and thesealant was firmlypressed into theconcrete to forman airtight seal.Afterapproximately70.5 hours, theplastic coveringwas removed, andthe plastic dishwith the calciumchloride was reweighed.After completion of the calcium

chloride testing, all of the floor coatingwas removed from the facility due totechnical problems with the coating(not necessarily associated withexcessive moisture). The temperatureand humidity within the facility weremaintained at approximately the same

level by the HVAC system. One weekafter the floor coating was removed,the calcium chloride test was againperformed in the same areas at whichit had previously been performed. Thecalcium chloride dishes were allowed

to remain inplace for 69hours. Thedishes were thenremoved andreweighed, andthe MVT ratewas calculated inaccordance withthe requirementsof ASTM F1869. Table 1compares theresults of thecalcium chloridemoisture testingon the floor thathad contactcoating with a24 in. x 24 in.area removed,and the calciumchloride

moisture testing performed where nocoating remained on the floor.The results in Table 1 demonstrate

that there was a significant differencebetween the MVT rate measured inareas where a 24 in. x 24 in. square ofcoating was removed, and the samearea after all of the coating wasremoved.

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When an impermeable coating isapplied to a concrete floor, nosignificant amount of moisture canmigrate from the concrete to the airabove, nor from the air above to theconcrete. The moisture, though, canmigrate horizontally under the floorcoating. The calcium chloride testmethod allows testing to be conductedon previously coated floors in areaswhere a minimum 20 in. x 20 in. squareof coating has been removed. Becausethe moisture in the surrounding areasaround the 20 in. x 20 in. square ofremoved coating cannot migratevertically through the impermeablecoating, it is a reasonable assumptionthat some quantity of the moisture willmigrate horizontally into the areawhere the coating has been removed,creating a kind of “chimney effect”(a type of moisture migration, definedbelow) and possibly biasing the resultsof the calcium chloride test.

MVT TestingIn order to determine the effect of thevarious variables on the calciumchloride moisture test results, two testprotocols were developed. The firsttest protocol was developed to assessthe chimney effect, or moisturemigration from areas of the concreteslab covered by an impermeable floorcoating to areas where the floorcoating has been removed to performMVT testing.The testing was conducted in a large

facility that had been recentlyconstructed with concrete floors and aself-leveling 100% solids epoxy floorcoating system. The facility was nearcompletion, and the HVAC system wasoperational. Four test locations werechosen within the facility. In each ofthe four test areas, the self-levelingepoxy floor coating was removed from

Test Area Full Coating Removal MVT 24” X 24” Coating Removal MVT % Increase(lb/1000 sq ft/24 hours) (lb/1000 sq ft/24 hours)

Area 1 1.32 3.04 130%

Area 2 1.82 3.63 99%

Area 3 3.97 10.91 175%

Area 4 2.48 4.48 95%

Table 1: Moisture Testing Before and After Full Coating System Removal

Concrete test panel in an environmental chamber

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J P C L J a n u a r y 2 0 0 8 47www.paintsquare.com

The second test protocol involvedmeasuring the MVT rate using thecalcium chloride test on concrete testpanels. Two sets of concrete test panelswere used. The first set was 16 in. x 16in. x 2 in. commercially availableconcrete paving blocks. The second setwas 12 in. x 12 in. x 1.5 in. pouredconcrete panels of normal weight. Allof the panels were completely sealed onthe bottom side and on the edges withapproximately 15 mils of an amine-cured epoxy. The panels were sealed toprevent moisture migration throughthe bottoms and the sides. The tops ofthe panels were lightly abrasive blastcleaned to remove laitance, but leftuncoated. The design of the panelsallowed for moisture migration throughthe tops of the panels only, whilepreventing moisture migration fromthe bottom and the sides.The calcium chloride test was

performed on each panel while thepanels were exposed to variousconditions of temperature andhumidity. The panels were stored for atleast 24 hours at 70 F and 50%relative humidity before the calcium

chloride test was performed. Once thecalcium chloride test apparatus wasattached to the panels, the panels wereplaced in an environmental chamber.The temperature and humidity werevaried to determine their effect on theamount of moisture that could migratethrough the top of the test panels.The first testing was performed on

the commercially available concretepaving blocks. Although the blocksappeared to be normal weight forconcrete, there was some visibleporosity in the surface. Table 2presents the results of the MVT testingon the concrete paving blocks.As can be seen from Table 2, the

MVT rate through the top of therelatively porous concrete paving blockwas considerable, leading to MVT ratemeasurements much greater than 3.0lb/1000 sq ft/24 hours, the maximumlevel generally considered acceptablefor coating application to a concretefloor. These high MVT ratemeasurements were observed despitethe fact that the backs of the panelswere completely sealed, and nosignificant moisture vapor could have

migrated through the concrete from thebottom.The rest of the testing was performed

on 12 in. x 12 in. poured concretepanels of normal weight. All the panelswere coated on the top and sides, andthe top of the panels were lightlyabrasive blast cleaned to removelaitance. Table 3 presents the findingsfrom MVT rate testing on the pouredconcrete test panels of normal weight.As can be seen from Table 3, the

measured MVT varied considerablyand appeared to increase significantlywith increases in humidity andtemperature. It is interesting to notethat the measured values were allgreater than the maximum value of 3.0lb/1000 sq ft/24 hours consideredacceptable for application of floorcoatings.

ConclusionsThe calcium chloride MVT testingperformed in this research indicate thatthe results are significantly affected bythe humidity and temperature of thesurrounding air, and they varyconsiderably depending on whether thetesting is performed on a bare concretefloor, or on a coated floor where asmall area of the floor coating has beenremoved. The testing has shown thatmeasured moisture vapor transmissionrates obtained from testing on a largearea of bare concrete can be less thanhalf the measured moisture vaportransmission rate in the same areawhen only a 24 in. x 24 in. area ofcoating has been removed. Moisturefrom surrounding areas of the concreteslab likely migrates horizontallythrough the slab and to the calciumchloride moisture testing site.Based on these results, it is likely

that the measured MVT rate obtainedusing the calcium chloride test before

Test Panel Temperature (°F) Relative Humidity (%) MVT

(lb/1000 sq ft/24 hours)

Panel 3 75 50 3.22

Panel 4 90 95 5.12

Panel 5* 75 50 3.01

Panel 6 55 70 1.82

Panel 7 75 70 3.47

Table 3: Moisture Vapor Transmission Testing on Poured Concrete Panels

Test Panel Temperature (°F) Relative Humidity (%) MVT

(lb/1000 sq ft/24 hours)

Panel 1 90 95 19.39

Panel 2 75 50 14.86

Table 2: Moisture Vapor Transmission Testing on Concrete Paving Blocks

*Testing repeated at same temperature and humidity as Panel 3

Rick Huntley is themanager ofConsulting Servicesfor KTA-Tator, Inc.,where he has beenemployed for 18years. He is certifiedby SSPC as a

Protective Coatings Specialist, and is acertified NACE International CoatingsInspector. His work with KTA includescoating failure analysis, specificationpreparation, and coating project man-agement. He earned a Bachelor ofScience degree from WashingtonState University.

48 www.paintsquare.comJ P C L J a n u a r y 2 0 0 8

application of a floor coating will beconsiderably lower than the MVT rateobtained after the coating has beeninstalled and sections of the coating areremoved for testing. Thus, unfortunately,

when assessing a floor coating failure,you may not be able to determine whatthe MVT rate was—as measured bythe calcium test—before the coatingwas installed. So knowing whether the

coating was applied over “dry”concrete might not be possible.The research also suggests that much

of the moisture detected by thecalcium chloride test did not migratethrough the slab but instead migratedinto the concrete from the surroundingair and migrated around the flange ofthe plastic cover. The research furtherrevealed that the amount of moisturethat enters into the concrete from thesurrounding air increases withincreased temperature and humidity.The moisture migration through thetop of the slab is sufficient to produceMVT test results higher than themaximum of 3.0 lb/1000 sq ft/24hours recommended by many floorcoating manufacturers. Under higherhumidity and temperature conditions,MVT test results may unnecessarilydelay application of a floor coating,even when very little moisture vaporis migrating through the slab. Underthese conditions, the plastic sheetmethod or relative humidity testingmay be more appropriate.

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