This is Part 2 of a 2-part series describing the instrumentation used to inspect the quality of cleaning and painting.  Part 1 described the instruments used for determining the quality of cleaning and paint.  Part 2 addresses moisture detection.  The moisture content of the concrete should be determined prior to painting. In the author’s experience, moisture within the substrate is a leading cause of coating failures on concrete.  If the moisture is elevated, the source(s) should be identified and corrected before paint is applied.  One the paint is applied, the continuity of the film should also be determined to confirm that flaws are not present in the applied coating that will allow air, and therefore moisture, to pass through the film, to subsequently dampen the substrate in the future.

A number of instruments and techniques are used to determine the moisture content of concrete substrates.  Some (calcium chloride and RH probes) are primarily used on floors, while others (radio frequency, conductivity, electrical impedance, and plastic sheet) are suitable for any concrete substrate.  An instrument used to determine that the coating is free of flaws that could lead to future wetting of the substrate is based on creating a pressure differential across the film to locate detects.  All of the aforementioned tests and methods are described in this article.

Radio Frequency

The instrument described below utilizes radio frequency to assess and monitor the relative moisture content in concrete to a depth of ~ 1 inch.  It provides readings on a relative scale between 0 – 999.  The instrument displays results using both a color and a number. The green zone is from 0 to 145 units and signifies “safe air-dry conditions.”  The yellow zone is between 146 and 230 units and signifies “moisture levels are higher than normal but not critical; further investigation is recommended.” The red zone is greater than 230 units and represents “excessive moisture levels.”  See Photo 1.

Step 1 – Press the top button to turn the gage on and set the instrument to the prevailing weather conditions by pressing the lower “arrow” button for 3 seconds until the word “nul” shows.  Nul will flash and when it disappears, the gage is ready for use under the current ambient conditions.  If the instrument is being used on the exterior of a building, but you move to the interior, repeat this step when inside the building.

Step 2 – Hold the instrument (gage) flush to the concrete substrate with your fingers on the black plastic perimeter of the gage body.  Do not allow your fingers to extend to the front of the gage beyond the black.  The gage requires a firm 2-point contact to take a reading (the front nose of the gage and the protruding rounded base).

Step 3 – The instrument will give an audible signal when the reading stabilizes.  Record the value from the digital display and note the color.

Electrical Resistance (Conductivity)

The instrument described below utilizes conductivity to determine moisture content.  Two contact pins on the end of the instrument are pushed against the surface to measure the conductivity (relative moisture content) of the material between the pins.  Masonry nails can also be driven into the surface about ¼ inch in depth to assess moisture content below the surface.  The pins of the probe are touched to the heads of the nails.

Depending on the model, the instrument uses either an analog or digital scale.  When using the analog instrument, readings can be taken from 2 scales.  The scale for concrete is a relative scale from 0 to 100.  See Photo 2.

Caution: When using the analog scale, many users inadvertently record readings from the “wood” scale which is a percentage.  The percentage on the wood scale has no relationship to the moisture in concrete.  The concrete scale can be interpreted as follows:

• Green <85 units         (<2% moisture content)
• Yellow 85 to 95 units (2% to 4% moisture content)
• Red >95 units         (>4% moisture content)

Step 1 – Turn the instrument on and check the calibration.  For the analog instrument, press the button with the “√.” The needle should read 20 on the wood scale.  For the digital model, press the Read Button (a moisture droplet insignia is printed on the button) and the Calibration (check) button simultaneously.  It should display 12% (+/- 0.2).   If the readings are not in the above ranges, change the battery.

Step 2 – For the digital instrument, press the scale button (*) and set the scale to “2” for concrete.  For the analog instrument, nothing needs to be set, but make sure you are using the “reference” scale for concrete.  A very common mistake is to read the “wood” scale, which will provide incorrect results.

Step 3 – Press the probe firmly against the surface, making certain that both pins are in intimate contact with the concrete.

Step 4 – For the analog instrument, press the button with the “moisture droplet” insignia and record the number from the “reference” scale.  For the digital instrument, push the “moisture droplet” button and a digital reading will be displayed.

Step 5 – To obtain readings below the surface, drive concrete nails into the surface and hold the pins of the instrument probe on the nail heads (1 pin on each nail head).

Electrical Impedance

The instrument described below utilizes electrical impedance to determine moisture content to a depth of ~1 inch. The electrical impedance is measured by creating a low frequency electrical field between the electrodes on the bottom of the unit. The moisture readings are displayed on a moving coil meter ranging from 0% to 6%.  See Photo 3.

Step 1 – Press the on/off button to power up the instrument.  The lower LED will flash.  If both lights flash, replace the battery.

Step 2 –Hold the instrument flush to the concrete substrate.  All of the spring loaded feet of the gage should be in full contact with the surface.

Step 3 – Read the percentage from the top 0 to 6% scale.

Plastic Sheet Test

The plastic sheet test is a qualitative method for determining the presence of moisture within the substrate.  The test is addressed in ASTM D4263-83 (2012), Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method.

Step 1 – Cut a sheet of clear plastic approximately 18 in x 18 in size.  Note that when used on concrete block (CMU), in order to get a good seal with tape in Step 2, it may be necessary to cut the plastic in the shape of the block(s) so that the outside perimeter falls onto mortar joints.

Step 2 – Firmly tape the perimeter of the plastic to the surface to create a continuous seal.

Step 3 – Allow the plastic to remain in place for a minimum of 16 hours.

Step 4 – At the end of the exposure time, examine the underside of the sheet and surface of the concrete for the presence of moisture. See Photo 4.

Calcium Chloride

This method is addressed in ASTM F1869-11, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride.  The test requires exposing the concrete slab to anhydrous calcium chloride for a given length of time (Photo 5).   The results are expressed as the moisture vapor emission rate (MVER), reported in pounds of moisture over a 1,000 square foot area during a 24-hour period.

The testing should be conducted at the same temperature and humidity that is expected during normal use.  If this is not possible, the ambient conditions should be controlled to 75°F ± 10°F and 50% ± 10 % relative humidity for 48 hours prior to testing and during the test.  An exception to testing at the expected service temperature/relative humidity involves floors that operate at temperature or humidity extremes (e.g., cold storage rooms).  In these cases, the temperature/humidity criteria listed above should be maintained.

ASTM F1869 recommends a test frequency of 3 locations for the first 1,000 square feet, with an additional test location for each 1,000 square feet of floor area, or fraction thereof.

Step 1 – Lightly abrade a 20 in x 20 section of the concrete surface by grinding to produce a slight profile equal to ICRI CSP-1 to CSP-2 and to remove the thin layer of finished concrete, but not exposing large aggregate.  If floor coverings or coatings were removed in the test area, the concrete must be exposed for 24 hours after grinding before initiating the test.  If the concrete was not covered, or the coverings have been removed for more than 30 days, testing can begin immediately after grinding and clean up.  The 24 hour waiting period is not required.

Step 2 – Remove all dust from the surface.

Step 3 – Weigh the sealed plastic container containing the anhydrous calcium chloride to the nearest 0.1 gram.

Step 4 – Place the container on the prepared concrete and carefully remove the tape and lid to expose the calcium chloride. Store the lid and tape for reuse when the test is complete.

Step 5 – Cover the container of calcium chloride with the transparent dome provided by the manufacturer.  Press firmly to complete seal the gasket material to the concrete around the perimeter of the dome.

Step 6 – Allow the container to remain in place for no less than 60 hours, nor longer than 72 hours.

Step 7 – At the completion of the test period, place the lid on the container and firmly tape it in place with the same tape that was originally on the container.

Step 8 – Reweigh the container using the same scale used for the pre-test weighing.

Step 9 – Insert the pre and post weights and exposure time into the formula supplied with the test kit to determine the MVER, reported as pounds/1000 sq ft/24 hours.

Relative Humidity

This method is addressed in ASTM F2170-11, Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.  This is a destructive test that requires drilling small holes in the slab, inserting hollow sleeves, and after a given waiting period, inserting probes into the sleeves to determine the relative humidity.  The results are directly displayed as relative humidity; no conversions are needed (see Photo 6).

The slab should be at service temperature and the occupied air space above the floor should be at the service temperature and relative humidity for at least 48 hours prior to testing.  The hole depth for the probes is based on a percentage of the slab thickness.  If the slab is drying from the top only (e.g., slab on grade, or slab on a metal deck), the hole is drilled to a depth of 40% of the total thickness of the slab.  For a 4 inch thick slab, the hole depth is approximately 1.5 inches.  If the slab dries from both the top and bottom (e.g., elevated reinforced slab not on a metal deck), the hole is drilled to a depth of 20% of the total thickness of the slab.  For a 4 inch thick slab, the hole depth is approximately 0.75 inches.

ASTM F2170 recommends a test frequency of 3 locations for the first 1,000 square feet, with an additional location for each 1,000 square feet of floor area, or fraction thereof.  For on-grade and below-grade slabs, one location is to be within 3 feet of each exterior wall.

Step 1 – Drill the hole using a hammer drill and drill bit (dry).  The diameter of the hole is established by the manufacturer.

Step 2 – Vacuum the dust from the hole, use a round wire brush sized to the hole diameter to thoroughly scour the hole to remove any loose material and vacuum again.

Step 3 – Some manufacturers require the insertion of a sleeve in the hole or a sensor.  In both cases, the hole containing the sleeve or sensor is capped and allowed to remain undisturbed for 72 hours prior to testing to achieve equilibrium.

Step 4 – Follow the manufacturer’s instructions to obtain a reading.  First allow the instrument to reach equilibrium in the test environment.  Depending on the instrument being used a probe attached to an RH gage is inserted into the sleeved hole to obtain a reading, or a reader is attached to the pre-installed sensor to obtain a reading.

Paint Film Continuity – Air Leak Detector

ASTM E1186, Air Leakage Site Detection in Building Envelopes and Air Barrier Systems, describes a number of methods that are used to determine whether air barriers installed on buildings are effective.  Some of the methods test the entire enclosure, while others test specific locations, such as coatings, joints, penetrations, and junctions.  The instrument described below (Photo 7) is used to test specific locations on a structure to determine if the surface is properly sealed.  While the instrument can be used during commissioning, it should be used during construction to confirm that the installation practices are creating an effective non-leaking air barrier.  Random areas are tested to confirm that the paint application techniques are suitable for creating a continuous film.

Step 1 – Adjust the leak detector to the specified pressure differential limit and rate of depressurization.  The common test parameters are 500Pa and 25 Pa/sec, respectively.

Step 2 – Clean the area around the detail to be tested.

Step 3 – Apply a specially formulated liquid test solution to the surface.

Step 4 – Place the test chamber over the test area.

Step 5 – Start the instrument and carefully observe the test area for the formation of bubbles.  Bubbles indicate the presence of a leak and poor film continuity.  If not bubbles are present, the test area is free of air leaks.

Step 6 – Clean the test area to remove the test solution.

Conclusion

While inspections to confirm the quality of surface preparation and paint application are the most visible and obvious part of the painting process, an equally important aspect of the process involves the detection of moisture within the substrate and continuity of the film.  These aspects are often “invisible” and therefore not fully appreciated.  Common methods for detecting moisture and film continuity have been discussed in this article. See Part 1 of this Series for a discussion of instruments used for cleaning and painting.

ABOUT THE AUTHOR

Kenneth Trimber is the president of KTA-Tator, Inc. He holds a Bachelor of Science degree from Indiana University of Pennsylvania, is an SSPC Protective Coatings Specialist, is certified at a Level III coating inspection capability in accordance with ANSI N45.2.6, is a NACE-certified Coating Inspector and an SSPC-C3 Competent Person.Trimber has more than 40 years of experience in the industrial painting field, is a past president of SSPC, chairman of the Committee on Surface Preparation, chairman of the Visual Standards Committee, chairman of the Task Group on Containment and chairman of the SSPC Commercial Coatings Committee. He is also past chairman of the ASTM D1 Committee on Paints and Related Coatings, Materials, and Applications.Trimber authored The Industrial Lead Paint Removal Handbook and co-authored Volume 2 of the handbook, Project Design. He was the recipient of the John D. Keane Award of Merit at the SSPC National Conference in 1990 and is a former technical editor of JPCL. In 2009 and 2012 he was named by JPCL as one of the 25 Top Thinkers in the coatings and linings industry and in 2015 was the recipient of the SSPC Honorary Life Member Award.

When you spend money on a product or service, you expect quality, regardless of the cost.  If you purchase the most inexpensive Chevrolet that is made, you still expect quality.  While the braking system in the Chevrolet may be less sophisticated than a Mercedes, the brakes better work and exhibit quality commensurate with its design.  You expect the parts and installation to meet all of the standards imposed by the manufacturer for that class of vehicle.

Expectations of quality for cleaning and painting commercial buildings are no different.  Owners expect the paint in the can to meet the quality standards established by the manufacturer, and the installation to meet the requirements of the specification, whether it involves a sophisticated fluorourethane on a highly visible entrance awning, or a low cost acrylic on a back wall that is hidden from view.

But how is the quality of cleaning and painting determined?  For many, it simply involves rubbing a hand across the surface when cleaning is finished and after the application of each coat.  It isn’t clear what rubbing the surface does, but the hand cannot identify if the levels of moisture within the substrate are acceptable, or whether the ambient conditions and surface temperature are suitable, or if each coat is applied to the proper thickness. When coatings are required to resist penetration from wind-driven rain or serve as an air barrier, verification of proper workmanship at each stage of the installation is critical.

Many standards and instruments are available for verifying the quality of cleaning and painting.  Not only must the appropriate instruments be selected, but they must be used properly.  This article describes the operation of some of the common instrumentsused to evaluate the quality of cleaning and painting. Part 2 of this series addresses instruments and methods used for the detection of moisture.

Surface Cleanliness – Steel

SSPC: The Society for Protective Coatings (SSPC) has published standards that describe different degrees of cleaning when using hand or power tools, dry and wet abrasive blast cleaning, and water jetting.  In addition to the written words, photographic guides are also available to depict the appearance of the different grades of cleaning.  Some of the SSPC work was done in cooperation with NACE International (NACE).

The visual guides that depict surface cleanliness are (Photo 1):

• SSPC-VIS 1, Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning
• SSPC-VIS 3, Guide and Reference Photographs for Steel Surfaces Prepared by Power and Hand Tool Cleaning
• SSPC-VIS 4/NACE VIS 7, Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting
• SSPC-VIS 5/NACE VIS 9, Guide and Reference Photographs for Steel Surfaces Prepared by Wet Abrasive Blast Cleaning

SSPC-VIS 3

SSPC-VIS 3 is described below as the example for using the guides.  All four are used in the same manner.

Step 1 – Identify the initial condition of the steel so that the correct series of photographs is selected for the assessment of the quality of cleaning.  The initial conditions in SSPC-VIS 3 are:

• Condition A – not painted – adherent mill scale
• Condition B – not painted – mill scale and rust
• Condition C – not painted – 100% rusted
• Condition D – not painted – 100% rusted with pits
• Condition E – painted – light colored paint, spots or rust over blasted steel
• Condition F – painted – zinc rich paint over blasted steel
• Condition G – painted – heavy paint over mill scale

Step 2 – Determine the degree of cleaning required by the project specification.  The degrees of cleaning depicted in SSPC-VIS 3 are:

• SSPC-SP2, Hand Tool Cleaning (hand wire brush cleaning depicted)
• SSPC-SP3, Power Tool Cleaning (both power wire brush and sanding disc cleaning depicted)
• SSPC-SP15, Commercial Grade Power Tool Cleaning (needle gun/rotary peening cleaning depicted)
• SSPC-SP11, Power Tool Cleaning to Bare Metal (needle gun/rotary peening cleaning depicted)

Step 3 – Locate the reference photograph for the degree of cleaning over the initial substrate condition.  For example, the photograph of power tool cleaning (sanding disc) of a coating that exhibits light rust before cleaning is photo E SP3/SD (E represents the initial condition; SP3/SD represents power tool cleaning with a sanding disc).  See Photo 2.

Step 4 – Compare the prepared surface with the photograph to determine if the degree of cleaning has been met.

Surface Profile – Steel

The surface profile (roughening) of the steel is commonly determined using a depth micrometer or replica tape.  The methods for measuring surface profile are described in ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel.  Method B describes the use of a depth micrometer and Method C describes the use of replica tape.

Surface Profile Depth Micrometer (Method B of ASTM D4417)

The depth micrometer described in the ASTM standard contains spring loaded, 60° cone-shaped pin that projects from the base of the instrument.  The base of the instrument rests on the peaks of the surface profile and the pin projects into the valleys.  The distance that the cone projects into the valleys is displayed in 0.1 mil increments; readings can also be displayed in micrometers (µm).

Step 1 – Zero the instrument on the piece of plate glass supplied with the gage (the plate glass has been ground smooth to remove waviness), then place a horseshoe-shaped shim (also supplied with the gage) on the plate glass.  Measure the thickness of the shim to verify the accuracy of the gage.

Step 2 – Hold the gage just above the probe and firmly push it against the surface to be measured.   Record the reading.  Readings can also be stored in memory and uploaded or printed later.

Step 3 – Pick the gage up and reposition it on the surface to take another reading.  Do not drag it across the surface as dragging can blunt the tip.

Step 4 – Take a minimum of 10 readings at each test location. The maximum value of 10 readings (removing obvious outliers) represents the profile at that location.

Surface Profile Replica Tape (Method C of ASTM D4417)

The tape is used to create a replicate of the surface profile that is measured using a light spring-loaded micrometer.  One instrument manufacturer has also developed an attachment for a digital gage to read the replica tape and store the results electronically.  The directions below apply to the use of the spring micrometer to measure the replica tape.

Step 1 – Select the replica tape that covers the expected profile range. The tape is most accurate mid-range:

• Coarse – 0.8 to 2.5 mils
• X-Coarse – 1.5 to 4.5 mils
• X-Coarse Plus – 4.0 to 5.0 mils

Step 2 – Prepare the area to be tested by removing surface dust or contamination.  This can be done by brushing.

Step 3 – Remove the paper backing from the tape.  The measuring area consists of the 2.0 mil thick film of Mylar® (a polyester film) that holds a thin layer of compressible foam.  The foam conforms to the depth and shape of the surface profile.

Step 4 – Attach the replica tape to the surface and burnish the back of the white Mylar circle (3/8” diameter) with a burnishing tool. See Photo 3.

Step 5 – Remove the tape and place it in the anvils of the micrometer.  The surface profile is the total reading less 2.0 mils (2.0 mils is the thickness of the Mylar that holds the compressible foam).  Alternatively if the micrometer is set to -2.0 mils prior to inserting the tape into the anvils, the displayed reading is a direct indication of surface profile. Two readings are taken at each location and averaged to determine the surface profile.

Note – If the surface profile measured with the Coarse tape is 1.5 to 2.5 mils, the same area must be measured with the X-Coarse tape.  If that reading is also between1.5 to 2.5 mils, average the two values to determine the surface profile depth.  If the second reading with the X-Coarse tape is >2.5 mils, record that value as the surface profile.

Surface Profile – Concrete (ICRI  310.2R-2013)

ICRI Guideline No. 310.2R-2013, Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, and Polymer Overlays, and Concrete Repair describes methods of surface preparation used on concrete in both written text and through the use of tactile concrete surface profile (CSP) coupons that are replicas of the type of profile (surface roughness) created by the various methods of surface preparation.  While much of the standard addresses the roughness of floor surfaces, some of the methods apply to surfaces other than floors. The coupons range in texture from very smooth, typical of pressure washing (CSP1) to very rough, typical of jack-hammering (CSP 10):

• Detergent scrubbing – CSP1
• Low-pressure water cleaning – CSP1
• Grinding – CSP1-CSP2
• Acid etching – CSP1-CSP3
• Needle scaling – CSP2-CSP4
• Abrasive Blasting – CSP2-CSP7
• Shotblasting – CSP2-CSP9.
• High/ultra-high pressure water jetting – CSP3-CSP10.
• Scarifying – CSP4-CSP7
• Rotomilling – CSP6-CSP9.
• Scabbling – CSP7-CSP9.
• Handheld Concrete Breaker – CSP7-CSP10

Step 1 – Identify the method of surface preparation required by the specification or manufacturer’s requirements.

Step 2 – Select the concrete surface profile coupon(s) that represents the texture or range of textures that can be expected to be created based on the 310.2R-2013 guideline. See Photo 4.

Step 3 – Compare the prepared surface with the coupon(s) to determine if the degree of roughening is acceptable.

Ambient Conditions

For our purposes, the term “ambient conditions” encompasses air and surface temperatures, relative humidity, and the dew point temperature.  See Photo 5.  If the ambient conditions are outside of the limits of the specification or the coating manufacturer’s requirements, coating adhesion and film formation can be compromised, leading to reduced performance or failure.   The measurements must be obtained where the work is being performed because conditions can vary at different parts of a building (e.g., in the direct sun versus the shade). The least expensive way to measure ambient conditions is through the use of a sling or whirling psychrometer and contact surface temperature thermometer.  More expensive methods involve the use of digital or electronic psychrometers that contain a sensor that is exposed to the environment to determine air temperature, dew point temperature, and relative humidity.  A separate probe is touched to the surface, or a non-contact infrared sensor is used to measure the surface temperature.  Many different electronic models are available and the operating instructions are straight forward.

The instructions below apply to the most inexpensive method – the sling psychrometer and surface contact thermometer.

Sling Psychrometer and Surface Temperature Thermometer

Step 1 – The sling psychrometer contains two identical tube thermometers.  The end of one is covered with a wick or sock (called the “wet bulb”).  The other is uncovered (called the “dry bulb”).  Saturate the wick of the wet bulb with clean water.

Step 2 – Whirl the instrument through the air for 20 to 30 seconds and take a reading of the wet bulb temperature.

Step 3 – Whirl the instrument again (without re-wetting) for another 20 seconds and take a reading of the wet bulb.

Step 4 – Continue whirling and reading until the wet bulb remains unchanged (or within 0.5°F) for 3 consecutive readings.  Record the stabilized wet bulb temperature and the dry bulb temperature.

Step 5 – Plot the dry bulb temperature and the difference between the dry and wet bulb temperatures (delta) in the Psychrometric Tables or charts to determine the relative humidity and dew point temperature.

Step 6 – Attach a contact thermometer to the surface and allow it to stabilize for a minimum of 2 minutes to determine the surface temperature.

Step 7 – Compare the results with the specification requirements for air and surface temperature, relative humidity and the spread between the surface temperature and dew point temperature (typically the surface temperature must be at least 5°F above the dew point temperature before painting proceeds).

Wet Film Thickness (ASTM D4414)

Measurement of the wet film thickness of the coating during application provides assurance that the proper amount of coating is being applied.  The coating manufacturer can stipulate the range of wet film thickness to be applied to achieve the desired dry film, or the required wet film thickness can be calculated as follows:

Wet film thickness = Specified dry film thickness ÷ Volume solids content of the paint

The volume solids content will be shown on the can label or on the product data sheet.  If the solids by volume is 60% and the specified dry film thickness is 3 mils, the target wet film thickness is 5 mils (3 mils ÷ 60% = 5 mils), as 40% of the applied wet film will evaporate into the air, while 60% of the applied wet film will remain on the surface.

Wet film thickness is measured in accordance with ASTM D4414, Standard Practice for Measurement of Wet Film Thickness by Notch Gages.

Step 1 – Make sure the tips of the numbered notches (or teeth) of the wet film thickness gage are clean and free of any paint.

Step 2 – Immediately after the paint is applied, push the gage into the paint, making certain the end points of the gage make firm contact with the underlying surface (substrate or previously applied coating layer).  See Photo 6.

Step 3 – Withdraw the gage and examine the numbered “teeth” that are wetted with paint.  If none of the teeth are wetted, use a different face of the gage that displays lesser thickness.  If all of the teeth are wetted, use a different face that displays greater thickness.

Step 4 – Determine the wet film thickness by looking at the numbers on the gage (in mils or micrometers).  The wet film thickness is indicated by the highest wetted tooth or step.

Step 5 – Wipe all paint off the gage before it dries.

NOTE: The surface being measured has to be smooth in order to avoid irregular wetting of the teeth.  For example, the gage cannot be used on split-faced block, but it could be used on the adjacent mortar joints.

Dry Film Thickness – Ferrous and Non-Ferrous Metallic Substrates (ASTM D7091 and SSPC-PA2)

There are a number of instruments available for the measurement of coating thickness on metallic substrates that are based on both electromagnetic induction and eddy current principles.  The use of the instruments is covered in two standards:

• ASTM D7091, Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals
• SSPC-PA2, Procedure for Determining Conformance to Dry Coating Thickness Requirements

All of the instruments are calibrated by the gage manufacturer or an accredited calibration laboratory, but the accuracy of the instrument must be verified each time it is used, and the instrument may require an adjustment to compensate for substrate roughness.  The specific instructions of the manufacturer need to be followed, but the following steps apply to all of the Type 2 (electronic) instruments:

Step 1 – Use certified coated standards in the intended range of use to verify that the instrument is operating properly (known as verification of accuracy).

Step 2 – Place a certified or measured shim (in the intended range of use) onto the prepared, uncoated metal surface and adjust the instrument (as necessary) to closely match the value indicated on the shim. This step effectively removes any influence of the base metal (metallurgy, roughness, curvature, etc.) on the coating thickness readings (Step 3).

Step 3 – After verifying the accuracy of the instrument and adjusting it for substrate properties, take a minimum of 3 measurements within a 1.5” diameter circle and average them together to determine the thickness at the specific location.  See Photo 7. This is known as a spot measurement.  Multiple clusters of spot measurements are taken in 100 square foot areas to determine the thickness of the applied coating.

NOTE:  When measuring the thickness of a coating over existing paint or galvanize, the thickness of the existing paint or galvanize must be measured and subtracted from the total reading (i.e., the gages measure the cumulative thickness of all coats that are present on the substrate). One instrument manufacturer provides a gage that will measure the cumulative thickness of the galvanize-coating layers, then display the thickness of each layer separately.

Dry Film Thickness – Cementitious Substrates, Plaster, and Drywall (ASTM D6132 and SSPC-PA 9)

The dry film thickness of coatings applied to cementitious substrates is often estimated by measuring the wet film thickness at the time of application, calculating coverage rates, using a Tooke Gage (destructive in-situ technique described later) or removing chips of the dry coating for microscopic measurement of cross-sections.   If a sample of the coating can be removed with none of the substrate attached (although being able to remove such a sample could be an indication of problems), a micrometer can be used.  There is also one relatively new technique available for the non-destructive measurement of dry film thickness.  It involves a special instrument that measures thickness using ultrasound principles.  See Photo 8.  The technique is addressed in ASTM D 6132, Standard Test Method for Nondestructive Measurement of Dry Film Thickness of Applied Organic Coatings Using an Ultrasonic Gage; the frequency of measurement and the acceptability of the measurements is addressed in SSPC-PA 9, Measurement of Dry Coating Thickness on Cementitious Substrates Using Ultrasonic Gages.

The specific methods for using the instrument should be followed according to the manufacturer’s instructions, but the following basic steps apply:

Step 1 – Allow the probe to reach ambient temperature in the same environment where the readings will be taken by holding the probe in the air and pressing “zero” in the main menu.  This helps the gage to compensate for temperature extremes and the effects of wear on the probe.

Step 2 – Verify the accuracy of the gage using known reference standards.  For polymer coatings, place a plastic shim (reference standard) on the bare substrate, apply a drop of couplant on the surface of the shim, and place the probe on shim through the couplant to measure the thickness of the shim.  The couplant carries the ultrasound signal from the probe to the coated surface (the shim in this case). Adjust the gage to register the thickness of the shim.

Step 3 – Set the “gates,” which are the minimum and maximum range of thickness expected to be measured.

Step 4 – To measure the thickness of the coating, apply a drop of couplant to the surface of the coating and firmly place the probe on the coating through the couplant.  A second reading in the same area can be taken without reapplying the couplant.  But when moving to a new location, couplant must be reapplied to take a reading.

Dry Film Thickness (Destructive) – Any Substrate (ASTM D4138)

The Tooke Gage is a destructive microscope technique (50x ocular) for the measurement of coatings applied to essentially any substrate (all metals, cementitious substrates, wood, plaster, drywall, plastics).  See Photo 9.

The Tooke Gage is used in accordance with ASTM D4138, Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross Sectioning Means. In addition to total thickness, the Tooke Gage allows for the measurement of the thickness of each coat in multi-coat systems.  The gage requires the use of special cutting tips to make a precision incision through the coating layers(s) at a known angle and the thickness is determined using basic trigonometry.  By measuring the width of the scribe, the depth or thickness of the coating can be determined because the angle of the cut is known.  However, knowledge of mathematics is not required to use the instrument.  All of the conversions are established by the instrument.

Step 1 – Select the cutting tip that is in the range of coating thickness to be measured.  Three cutting tips are available.  The differences between them are the cutting angle provided by the tip:

• 10x tip – for thickness <3mils
• 2x tip – 3 to 20 mils
• 1x tip – 20 to 50 mils

Step 2 – Create a benchmark on the coating using a marker. Use the selected cutting tip to make an incision approximately 1” in length through the coating in the area of the benchmark.  The instrument requires 3-point contact when making the cut (two legs and the cutting tip).  Pull the cutting tip toward you to make an incision with the legs leading the tip.

NOTE: For the readings to be accurate, the incision must be made perpendicular to the surface.  Because of this, the area being measured must be smooth.  If the surface is irregular, the cutting angle will not be consistent and the results invalid.

Step 3 – View the incision through the 50x ocular with the reticule perpendicular to the incision.  See Photo 10.

Step 4 – Count the number of divisions of the reticule that line up with each coat to be measured.  The correlation between the number of divisions and thickness depends on the model of microscope supplied with the gage because 2 different oculars with reticules have been available.  Verify the conversion with the instructions supplied with the instrument, but it will be one of the following:

	Microscope A(typically purchased before 2011)	Microscope B (Universal ocular)(typically purchased after 2011)
1x Tip	1 division = 1 mil	1 division = 2 mil
2x Tip	1 division = 0.5 mil	1 division = 1 mil
10x Tip	1 division = 0.1 mil	1 division = 0.2 mil

Conclusion

There are a variety of standards and instruments available for verifying the quality of cleaning and painting.  The tests are easy to conduct, but specific steps are required to make certain that the instruments are used properly and that the results are valid.  A few tests and inspections during the work can make the difference between successful coating performance and premature coating failure.

See part 2 in this Series for a discussion of instruments and methods used for the detection of moisture in cementitious substrates.

ABOUT THE AUTHOR

Kenneth Trimber is the president of KTA-Tator, Inc. He holds a Bachelor of Science degree from Indiana University of Pennsylvania, is an SSPC Protective Coatings Specialist, is certified at a Level III coating inspection capability in accordance with ANSI N45.2.6, is a NACE-certified Coating Inspector and an SSPC-C3 Competent Person. Trimber has more than 40 years of experience in the industrial painting field, is a past president of SSPC, chairman of the Committee on Surface Preparation, chairman of the Visual Standards Committee, chairman of the Task Group on Containment and chairman of the SSPC Commercial Coatings Committee. He is also past chairman of the ASTM D1 Committee on Paints and Related Coatings, Materials, and Applications. Trimber authored The Industrial Lead Paint Removal Handbook and co-authored Volume 2 of the handbook, Project Design. He was the recipient of the John D. Keane Award of Merit at the SSPC National Conference in 1990 and is a former technical editor of JPCL. In 2009 and 2012 he was named by JPCL as one of the 25 Top Thinkers in the coatings and linings industry and in 2015 was the recipient of the SSPC Honorary Life Member Award.

Introduction

Most industrial and marine protective coatings rely on a mechanical bond to the substrate to remain attached while in service. This bond is generally provided by a surface profile or anchor pattern that is imparted into the surface prior to application of the coating system and effectively increases the surface area of the substrate (e.g., steel). A surface profile is typically generated by abrasive blast cleaning; although some types of rotary impact-type power tools can also create a surface texture. Without this mechanical “tooth” the coating system may become detached as the substrate and coating system expand and contract (e.g., due to temperature fluctuations and/or service loading/unloading) while in service, Coating specifications frequently invoke a minimum and maximum surface profile depth (e.g., 2-4 mils), but rarely invoke a minimum peak count or peak density.

The Significance of Peak Density

While surface profile depth is important, the number of peaks per unit area is also a significant factor that can improve long term coating system performance. According to a study conducted in the early 2000’s[1] a high peak count characteristic of surface profile helps resist undercutting corrosion when the coating system becomes damaged while in service, and provides the coating system with better adhesion to the prepared substrate. More recent research conducted by the DeFelsko Corporation[2] confirmed that a greater peak density (pd) promotes coating system adhesion. So, while there is typically a maximum peak height invoked by a specification (to prevent pinpoint rusting resulting from unprotected rogue peaks), there is little concern over too many peaks. The more peaks there are within a given area, the greater the surface area; the greater the surface area, the better the adhesion. Note that this is the primary reason why thermal spray coatings (metallizing) cannot be applied to steel surfaces prepared with steel shot. While the surface profile depth may be adequate (i.e., 3-4 mils), the peak density of a peened surface will not provide the necessary surface area for proper adhesion, and disbonding will likely occur.

Peak Density Verses Peak Count

Peak density and peak count are similar, but slightly different in how they are reported. According to ASTM, relative peak count or rpc is defined as, “the number of peak/valley pairs, per unit of length, extending outside a “deadband” centered on the mean line,” and is typically reported in peaks/cm. Peak density (pd) is the number of peaks present within a given surface area, and is typically reported in peaks/mm2.

Governing Industry Standards

Surface profile or anchor pattern is quantified/semi-quantified according to one of the three methods described (comparator, depth micrometer, replica tape) in ASTM D4417, Standard Test Methods for Field Measurement of Surface Profile of Blast Cleaned Steel, and peak count is quantified according to the method described in ASTM D7127, Standard Test Method for Measurement of Surface Roughness of Abrasive Blast Cleaned Metal Surfaces Using a Portable Stylus Instrument. The frequency and acceptability of the acquired measurements is described in SSPC-PA 17, Procedure for Determining Conformance to Steel Profile/Surface Roughness/Peak Count Requirements.

Quantifying Peak Count and Peak Density

Peak count is quantified using a portable stylus-type instrument. According to ASTM D7127, the apparatus consists of a portable skidded or non-skidded electronic surface roughness measurement instrument capable of measuring Rpc in compliance with ASME B46.1. The apparatus should have a vertical range of at least 300 μm and permit a sampling length of 2.5 mm and an evaluation length of 12.5 mm. The apparatus should include a stylus with a tip radius of 5 μm, and permit recording of Rpc up to 180/cm. Surface deviations are sensed by the stylus and converted to electrical signals within the device. Internal processing converts these signals into standard surface characterization parameters, which are then displayed or printed. ASTM D7127 recommends obtaining a minimum of five (5) traces per area to characterize the surface. Many of the stylus-type instruments that will measure peak count were designed for manufacturing and/or the machine finishing industry rather than for field use.  When used in the field, extreme care is necessary as the tip of the stylus can easily be damaged.

Peak density can be quantified using replica tape; however, this procedure requires the use of a slightly different version of the tape (called Optical Grade) than is traditionally used to measure surface profile depth per ASTM D4417, Method C (Coarse, X-Coarse and X-Coarse Plus). While the burnishing procedures are the same, the type of tape and the way that the tape is read differs. Both peak height and peak density are measured and reported using the Optical Grade replica tape and a Replica Tape Reader (RTR). ASTM recommends obtaining two measurements per area to characterize the surface.  

Use of Optical Grade Replica Tape to Determine Peak Density

The model RTR-P incorporates a digital camera and light source. Light is passed through the replica tape and imaged by the camera. Peak counts are revealed as bright spots on the photograph as taken by the PosiTector RTR’s digital image sensor (camera). The intensity of light that passes through the replica tape is inversely proportional to the thickness of the compressed foam. The below photographs of a back-lit piece of replica tape reveals light areas of higher compression (peaks) and dark areas of lower compression (valleys). An illustration using an image from a US coin is also provided below that demonstrates how the camera distinguishes higher and lower compression areas. All images are courtesy of DeFelsko Corporation.  

Since peak density can now be readily measured in the field (and measured simultaneously with peak height using the same replica tape), it is likely that specifications will start requiring measurements of peak density, especially for materials such as metallizing that rely on mechanical bonding. Not so fast… simply requiring the measurement of peak density will accomplish little without establishing a minimum acceptance criteria, just as specifying the measurement of coating thickness without an acceptable range is of little value. The minimum required peak density for proper bonding of the coating system will need to be established, and will likely vary depending on the coating system. In addition, the steps required to increase peak density without adversely affecting peak height will also need to be investigated.

[1] The Effect of Peak Count of Surface Roughness on Coating Performance; Hugh J. Roper, Raymond E.F. Weaver, Joseph H. Brandon; Journal of Protective Coatings & Linings, Volume 21, No. 6; June 2005

[2] Replica Tape – Unlocking Hidden Information; David Beamish; Journal of Protective Coatings & Linings, Volume 31, No. 7; July 2015