Coating Thickness Measurement on Masonry Substrates
DeFelsko manufactures a series of hand-held, non-destructive coating thickness gauges that are ideal for measuring the dry film thickness of coatings on masonry substrates.
Measurement Challenges
Traditionally a destructive test method is used to determine coating thickness on masonry substrates such as concrete. Coatings used on concrete range from hard to soft, smooth to textured, and cover a wide thickness range. The surface of concrete can be quite rough, which can create significant variations in thickness measurements.
Dry Film Thickness Measurement Solution
The ultrasonic PosiTector 100 C gage is ideal for measuring most coating on concrete applications. With a coating thickness measurement range of 50 to 4500 μm (2 to 175 mils), this gage is recommended for customers desiring a single non-destructive measurement solution for most applications. The PosiTector 100 C features concrete as a pre programmed substrate, so dependent upon the coating, little or no calibration adjustment is typically required. Figure 1 provides a cross-sectional view of a representative 125 to 375 μm (5 to 15 mil) coating on large aggregate concrete that is ideal for the PosiTector 100 C.
Figure 1 - Coating on rough, large aggregate concrete
The ultrasonic PosiTector 100 B gage is ideal for measuring coating thickness up to 500 μm (20 mils). While not specifically designed for measuring on concrete, after a simple calibration adjustment the B probe may demonstrate improved repeatability when measuring hard, thin coatings over smooth surfaces. Figure 2 represents an application where the PosiTector 100 B may be preferred.
The ultrasonic PosiTector 200 is a cost effective alternative for total thickness measurement of coatings on masonry substrates. The measurement range of the gage is 25 and 1000 microns (1 and 40 mils).
Figure 2 - Thin coating on smooth, cement surface
The ultrasonic PosiTector 100 D gage is ideal for measuring thicker coatings ranging from 1 to 15 mm (48 to 650 mils). While not designed for measuring thin coatings on concrete, after a simple calibration adjustment the D probe may demonstrate improved repeatability when measuring soft, thick coatings. Figure 3 depicts a thick coating application that is ideal for the PosiTector 100 D.
Figure 3 - Thick soft coating on concrete
If the concrete surface is smooth but uneven as shown in Figure 3, or rough and porous as shown in Figure 4, one would expect the measurement values to vary accordingly. When examined under magnification, locations where the coating has flowed into pores or crevices may appear as outliers when measurements are taken. For example in Figure 3, the bump in the substrate at "A" results in a significantly different thickness than nearby point "B". It is up to the user to compare results to the specification or expected thickness when analyzing results. Some results may need to be omitted when making calculations.
For an application such as Figure 4, that has both a rough substrate and a rough coating surface, it may be necessary to take several readings within individual small areas, calculate average readings, and then compare these values. These averages can then be compared against the specification. Destructive testing on the sample shown in Figure 4 identified a 1 to 3 mm (40 to 115 mil) variation within a ¼” diameter area.
Figure 4 - Coating on rough lightweight concrete
In the Screen Capture (Figure 5) shown below note that the graphical display clearly identifies the total coating thickness by showing the strongest return echo from the ultrasonic pulse. In some applications the instrument’s graphical display may provide additional information corresponding to the numeric reading on the left side of the display. In this example, a smaller spike indicates an entry echo at the surface of the coating. By adjusting the entry gate past this return echo, the user is able to to instruct the gage to treat such echos as false readings.
Add Figure 5 - Screen Capture
Background on Coatings on Masonry
What is the Application?
The focus of this application is the ability to non-destructively measure coating thickness on masonry. Masonry substrates include stone, brick, tile, cast concrete, pre-cast concrete, concrete block, asbestos-cement board, renderings and plasterboard. Masonry substrates are often porous with varying degrees of surface roughness. While useful in promoting adhesion, such porosity and roughness increases the difficulty in attaining repeatable thickness measurements.
Masonry coatings are used for a multitude of purposes including cosmetic appearance, durability, abrasion resistance, as well as protection from elements such as moisture, salt, chemical and ultra-violet light. Common coatings for concrete include latex paints, acrylics, lacquers, urethanes, epoxies, and polyester resins.
Primer layers are used as adhesion promoters, while overcoat layers serve as protection from surrounding elements. The ability of the PosiTector 100 to discern individual layers of coating is dependent upon the acoustic properties of adjacent layers, individual layer thickness, as well as the coating process itself.
Why measure?
The primary purpose for measuring coating thickness on concrete is to control coating costs while ensuring adequate protective coverage. Commercial contracts often require an independent inspection of the work upon completion.
A non destructive test method eliminates the need to repair the coating after inspection, saving time for both the inspector and the contractor.
Where is the market?
Customers requirements for non destructive measurements of coatings applied to masonry are virtually limitless. Markets range from government departments such as Transportation and Public Works to private sectors such as General Construction and Pipelines. The PosiTector 100 family of instruments are ideal for most coating applications on masonry substrates.