DeFelsko manufactures a hand-held, non-destructive coating thickness gage that is ideal for measuring the dry film thickness of thermal spray (zinc or zinc-indium-aluminum) applied to reinforced concrete structures.
The measurement principles of eddy-current and magnetic non-destructive gages do not typically allow them to measure on non-conductive substrates.
Measurements need to be periodically taken of the remaining anodic protection capacity of coatings throughout its life cycle, so a non-destructive measurement technique is preferred.
DeFelsko has developed a technology that uses our conventional eddy-current PosiTector 6000 NHS3 to non-destructively measure conductive thermal spray coatings such as zinc/aluminum/indium applied to non-conductive substrates such as concrete. This gage utilizes a magnetic field that induces eddy currents in the coating material. The instrument then correlates the resultant eddy current effects to a coating thickness.
Each instrument is factory calibrated to the properties of either sprayed zinc or zinc/aluminum/indium (on concrete) and is capable of measuring right out-of-the-box. The operator can select the preferred calibration setting by downloading it from the PosiSoft software included with the gage.
If a thermal spray application is encountered that is different than the two factory settings provided, the operator can develop a “user defined” calibration alternative. Simple instructions for calibrating the PosiTector 6000 for different coatings are available in the PosiSoft “Help” file. The instrument is adjusted to different thermal spray coatings by measuring a set of known thermal spray coating thickness samples spanning the thickness range of the application.
Two primary destructive techniques also exist for measuring thermal spray. Such methods are undesirable since they potentially reduce the effectiveness of the protective thermal spray layer by creating voids in the material. It also becomes necessary to repair or recoat the tested area.
One destructive method is to place masking tape over the uncoated concrete, apply the thermal spray coating, and then remove the coated tape. The total thickness can be measured using calipers or a micrometer. By subtracting the thickness of the tape, the thickness of the thermal spray can be calculated. Alternatively, a steel coupon can be adhered to the concrete prior to spraying the zinc and then removed for measurement. The thermal spray coating can be measured using an electronic or magnetic gauge, or a total thickness can be measured from which the coupon thickness may be subtracted. This destructive method is only applicable during the initial coating application and thus not practical for subsequent tests throughout the coating life cycle.
A second destructive method is to use a 4-conductor probe configured to measure surface resistance. By correlating with known resistivity of the coating, the thickness of a conductive coating applied to a nonconductive substrate can be determined. This technique requires that electrical contact be made with the surface of the coating, which may prove difficult when measuring. Also it is difficult to determine the remaining anodic capacity of partially corroded coatings that are covered with non-conductive corrosion.
For current pricing or to order these instruments, please contact us by telephone (315) 393-4450, fax (315) 393-8471, or e-mail techsale@defelsko.com. If you require additional technical information or have questions relating to your particular application, we encourage you to take advantage of our years of experience in recommending the best gage for your application.
In the presence of chlorides, moisture and oxygen, corrosion of reinforced steel used in concrete structures takes place at appreciable rates. The resulting corrosion products, which are more than twice the volume of the original uncorroded steel, create tensile stresses in the surrounding concrete. When these internal stresses are excessive, the concrete in the vicinity of the reinforcement cracks and eventually spalls or delaminates. Repairs then have to be done promptly before irreparable damage to the reinforcement occurs.
Cathodic Protection is a commonly used method of corrosion control that has been proven to eliminate (or arrest) corrosion of steel rebar in existing reinforced concrete structures. A cathodic protection system works by applying thermally sprayed zinc onto the steel rebar reinforced concrete structure. The zinc acts as a sacrificial anode that is intended to corrode in order to protect the embedded steel rebar, degrading the concrete structure.
Typical conductive coatings in use today include zinc and a zinc alloy (zinc/indium/aluminum). These coatings are especially suited for use in structures where an electrolyte (such as salt water) can leach into the concrete (e.g.: underground and underwater structures). The coatings are typically applied to the structure using a thermal spray system in which the coating is melted under intense heat and propelled onto the target structure to solidify.
The service life or "anodic capacity" of a
coating can be roughly approximated by the thickness of the applied coating.
During the service life of a coated structure, the remaining anodic
protection can be estimated by checking the coating thickness at regular
intervals.
As many reinforced concrete structures benefit from anodic protection there is
an increasing need to accurately measure its remaining capacity at various
points during the life cycle of the anodic layer. Conventionally the measurement of applied anodic capacity and
remaining anodic capacity typically involved measuring anode thickness using
destructive techniques in which the protective coatings are damaged and
subsequently repaired.
The primary market would involve contractors and transportation agencies (e.g. DOTs) responsible for performing scheduled maintenance and repair of concrete structures. Examples of concrete structures that use thermal spray zinc coatings include bridge structures, parking garages, retaining walls, dams, piers, arches, waste and water treatment facilities, pulp and paper mills, petrochemical plants, and other concrete structures in rivers and marine environments.