o achieve the expected life for acoating on steel or concrete, theapplicator and inspector must

carefully control the coating applicationprocess. It is also generally understood thatfor a large structure, such as a bridge, 70%of the total cost of applying a protective coat-ing lies in the access equipment and set-uprequired to bring the crew close enough tothe structure to apply the coating. So gettingthe work right the first time is very impor-tant in controlling the cost as well as ensur-ing the quality of the completed work.

Many issues need to be addressed, from thequality of the coating material to the preparation of

the surface and the monitoring of the conditions underwhich the coatings are applied. With the increasingly

complex coating formulations created under the pressureto reduce the volatile organic compound (VOC) content, the

care with which the processes are controlled also increases insignificance for the longevity of the protective coating.

Monitoring and measuring techniques and instruments normal-ly associated with the laboratory are constantly developing, but some of

the techniques and instruments now available can be used on-site aswell as in the lab. In addition, the instruments do not all require

50 www.paintsquare.com

Quality Controlof Coatings:

Bringing the Labto the Field and Other Tips

By John F. Fletcher,Technical Support Manager,

Elcometer Instruments Ltd., Manchester, UK

T

Fig. 1: Frikmar ViscosityCup with handlePhotos courtesy of ElcometerInstruments Ltd.

JPCL • October 2003 • PCE

supported by the DIN, AFNOR, ISO, and ASTM (Ford)cups used with a stand in the laboratory.

Two more gauges found in the paint laboratory can beused in the field. The Hegman Gauge is used to deter-mine the particle size in paint, and the Meier Gauge isused to check drying, shrinkage, and cracking of coat-ings. Both of these simple gauges allow assessment of thecoating material supplied for a job against its specifica-tion.

The Hegman Gauge (Fig. 2) is a precision instrument.The gauge and scraper are made of hardened stainlesssteel. A quantity of the paint is placed at the end of thegauge and then drawn along the sloping channel until thescrapper starts to drag the particles as their size matches

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the expertise of a coating inspector. All this means thatquality control can play an even greater role in the appli-cation process and is not just an add-on specifically forthe coatings inspector.

This article describes a set of tests, some of which aremore frequently associated with laboratory work, butwhich contractors and inspectors can also use in the fieldto improve the quality of the coating application. It alsodescribes some field tests that contractors and inspectorstoo often omit but if used, would also improve the quali-ty of coating application.

Testing Paint as a MaterialMany of the paint formulations that were in common usefor many years have now been replaced by more environ-mentally friendly formulations. It appears that at leastsome of these formulations have to be applied undermore carefully controlled conditions. This makes the as-sessment of the paint as a material more significant inthe achievement of a high quality finished coating.

Some of the paint testing methods that have been usedin paint formulation can also be used on site for checkingthe paint in the cans before the work begins. For exam-ple, flow cups can be used with a stopwatch to establishthat the viscosity of the coating material is correct, thusconfirming that the chosen application process, e.g.spraying, rolling, brushing, etc., will work satisfactorilywith the material supplied.

Flow cups such as the Frikmar type (Fig. 1) and theZahn cups, when fitted with handles for dipping in paintcans and with differing ranges of viscosity measurement,can be used conveniently on site and will meet the re-quirements of national standards. The cup is simplydipped into the paint can and allowed to fill completely.As it is withdrawn from the can, the time taken (in sec-onds) for all the paint to drain from the hole in the bot-tom of the cup is measured with a stopwatch. The viscos-ity is recorded by quoting the time in seconds and thetype and cup size used. This can then be compared withthe viscosity quoted in the product data sheet to ensurethat the paints meets the manufacturer’s specification orthat dilution has been carried out accurately in accor-dance with the specification.

These cups are appropriate for site work but are also

Fig. 2: Hegman Gauge for measuring particle fineness

the depth of the channel. The point where the pigmentparticles start to show is called the “fineness of grind” andis a measure of how well a paint has been made. The val-ue obtained can be checked against that quoted in theproduct data sheet. This test, however, will also showwhether a paint has been mixed properly before applica-tion because agglomerates of particles will be easily seen.It can also detect gelled particles from a partly cross-linkedcoating because these again would stand out on the gauge.In both cases, it could indicate whether a paint was fit tobe applied.

The Meier Gauge is also made of hardened stainlesssteel and is used to test drying, shrinkage, or cracking ofcoatings or similar products. The Meier Gauge has a slop-ing groove of 60 mm wide and 200 mm long, with 1, 2, or3 mm maximum depth.The paint material isapplied to the wholearea of the groove andvisually inspected forthe coating defects asso-ciated with drying.

Of course, the testsFig. 3: Meier Gauge for testing

shrinkage or cracking of coatings

Conductivity probes usingneedles to just barely pene-trate the surface of the con-crete are useful because theresulting measurement ap-plies to the concrete surfacethat will become the inter-face between the coatingand the concrete (Fig. 6).

Inductive moisture probescan be used to measure themoisture down to depths of30 mm.

If the moisture level is toohigh (~>5%), the concreteneeds to be allowed to dryfurther before a conventional coating (e.g., 100% epoxy)is applied. Otherwise, excess moisture moving through theconcrete will cause a conventional coating to delaminate,blister, and fail altogether. Alternatively, special “breath-able” coatings could be used.

Chlorides in concrete increase the rate of corrosion ofrebar. If the level is high, concrete removal and repair maybe necessary before coatings can be applied. Chloride con-tent of concrete in existing structures can be determindwith great accuracy by taking a measured sample of thecrushed concrete and extracting the chlorides using a pre-measured test solution for testing with a titration tube(Fig. 7). The measure-ment with the titrationtube takes about 90 sec-onds, and the test isspecific to the chloridecontent in the sample.Again, this is a test de-signed for field use.

While this kind of testis generally done beforepainting and by an inspector, it can also be used byblasters on site to measure the chloride content of blastingmedia, particularly when the media are being recycledand there is a risk of a build up of contamination. If thechloride content is too high, then the abrasive should bereplaced with new material.

Humidity MonitoringÑSteelElectronic humidity gauges for the field are now very fastand accurate and generally outperform the more tradition-al whirling hygrometer (also known as a sling psychrome-ter) because they acquire the readings in seconds. In con-trast, a whirling hygrometer should be operated for atleast 20 minutes to obtain stable readings for the wet anddry bulb thermometers.

above do not preclude testing in the lab for other proper-ties of the paint before application begins. Such tests in-clude those that can be conducted on coated sampleplates for properties such as hardness, hiding power, im-pact resistance, bending, and cupping.

Surface PreparationÑConcreteThe basic test for concrete structures is that of the con-crete cover (thickness of concrete over the rebar or struc-tural member). There is no benefit in applying a coatingto a structure that hasinadequate cover. Thelocation of the rein-forcement bars andthe determination ofthe bar diameter canbe part of this assess-ment.

Covermeters basedon the pulsed induc-tion technique used inmetal detection cangenerate a signal inthe buried rebar, andthis signal can beanalysed to determinelocation, cover, andbar diameter (Fig. 4).This is a test that ap-plicators should learnto conduct.

Many coatings areapplied to concrete aspart of a repair or re-furbishment process.If this refurbishmentrequires the structureto be drilled, the loca-tion of the reinforcement is critical to avoid damage tothe existing reinforcement. Specialist solutions, such asthe Borehole Probe (Fig. 5), allow complex three-dimen-sional reinforcement structures to be mapped.

Surface CleanlinessÑConcreteBefore applying a coating to a concrete surface, the appli-cator and inspector should check the condition of theconcrete for its chloride and moisture content and takeany steps necessary to reduce these levels and optimisecoating performance.

Moisture can be measured by the conductivity or theinduction method. The gauges available can be applied toa variety of building materials such as wood, brick, plas-ter, and concrete.

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Fig. 4: Concrete-cover meter in use on site

Fig. 5: The borehole probe for mapping lay-er after layer of rebar using a cover meter

Fig. 6: Moisture meter with surfaceor pin electrodes

Fig. 7: Chloride test kit for abrasivesand concrete

On site, this time pe-riod for the whirlinghygrometer is rarelyobserved, and the ten-dency is for the relativehumidity (RH) to bereported as higher thanthe true value.

The modern elec-tronic dewpoint meter(Fig. 8) will displayRH, air temperature,surface temperature,and dewpoint tempera-ture, and it will calcu-late and display thedifference between thedewpoint and the sur-face temperature sothat the operator can

be sure that the conditions are correct for applying thecoating.

Some modern coatings require the RH to exceed a cer-tain minimum for full cure, so knowledge of the climaticconditions is a very important aspect of quality control.

Post Application TestingÑAdhesion and Defects

Other tests intended primarily for the field but frequentlyomitted can be used to measure coating adhesion and de-fects like holidays, pinholes, cracks, porosity, and otherflaws. These properties can be measured on coated con-crete and steel.

Adhesion of coatings applied on concrete is tested us-ing a 2-inch diameter (50 millimeter) dolly so a specialve rsion of the adhesion tester, calibrated for this larg e rd o l l y, has been developed. Using the same pull-off prin-ciple as the adhesion tester for paint on steel, the newscale for coatings on concrete has a pull-off range of0–3.5 MPa (500 psi).

The testing of coatings on steel for pinholes, cra c k s,and other flaws can either be carried out using low - vo l t-a g e, wet sponge testing or high-voltage spark testing(Fig. 9), depending on the thickness of the coating. For acoating up to 500 µm, a 90V wet sponge test will detectpinholes through the coating to the steel.

H oweve r, for porosity detection on all concrete sur-f a c e s, the conductivity of the concrete due to the mois-t u re content means that low - voltage testing is not suffi-ciently sensitive. In this case, high-voltage spark testingis the pre f e r red method. High voltages will detect anyelectrical weakness in the coating, which is accepted asevidence of a physical weakness likely to cause pre m a-

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t u re failure of the coating at that point.In certain situations, where the coating is used to pro-

tect the structure against attack by the materials con-tained in the structure, checking for pinholes will bespecified as part of the acceptance testing for the wo r krather than as part of a quality control regimen, e.g., liq-uids in pipeline and storage tanks.

ConclusionsNo matter whether the protective coating is applied to asteel or a concrete structure, the quality control of thecoating process and the conditions under which the coat-ing is applied are crucial to the success of the work.

It is clear that coatings applied to dirty or contaminatedsurfaces in humid conditions will not achieve their ex-pected life performance. What is not always clear is thatthe coating supplied for the job may not be formulated asexpected, and this must be considered when planning thequality control test regimen.

This article therefore described laboratory test methodsfor coating materials that can be adapted for the field tocheck the suitability of a material for application. The ar-ticle also discussed the necessity of tests for adhesion,porosity, moisture, and other factors in coating concreteand steel. Such tests are too often ignored, to the detri-ment of coating performance.

The painter’s and inspector’s re p e r t o i re of field testmethods is increasing. As painters and inspectors take ad-vantage of this re p e r t o i re, they can optimise coating appli-cation and performance through vigilant quality contro l .

Knowledge is power, and the understanding gainedfrom the testing of the materials, the conditions, and theoutcome of the coating processes as the job proceeds willlead to better quality in the finished coating and a highexpectation that the design life of the coating will beachieved.

Fig. 8: Electronic dewpoint meter withoptional liquid probe

Fig. 9: High-voltage pinhole detector