Field testing equipment for coatings applied to concrete
When coatings are applied to concrete structures, the initial survey to establish the scope of the remedial work is as important to the success of the project as the quality control of the coatings and coating process.
These inspections need to he carried out on site to ensure that the coating achieves the intended results. This means that testing equipment needs to be quick to operate, easy to interpret and, above all, rugged and portable.
Check the coating material
The first step is to check the condition of the paint, as no amount of inspection or monitoring of the process will overcome a problem with the coating material. The viscosity can be checked on site using the Elcometer Frikmar Dip Cup method (see Figure 1). Simply dip the cup in the fluid and record the time until the first break is seen in the flow. Calculated in centistokes (cSt ). it can be compared with your specification to ensure it is up to standard.
Prepare the surface
There are three factors to consider for successful surface preparation tor painting:
* surface hardness
* surface cleanliness
* surface profile.
Test hammers, such as the Elcometer 182 Digital Test Hammer (see Figure 2), are widely used for site assessment of surface hardness of concrete. The spring-loaded plunger strikes the surface at fixed and constant impact energy. The rebound value indicates if the concrete is at the desired harness.
As concrete is porous, the presence of soluble salts on, or in the surface, prior to coating will lead to premature breakdown of both the coating and the concrete structure. Testing for chloride salts has always been necessary, but it is now possible to also test for sulfates and nitrates. The Elcometer CSN (chloride, sulfates, nitrates) Test Kit (see Figure 3) uses a titration tube for chlorides, a colorimeter for sulfates and test strips for nitrates. All three tests come in a portable case and the calibration is directly in parts per million (ppm) so there is no need for conversion of the reading.
In the case of surface profile, the coverage of the surface area by the paint is affected by the profile, as is the adhesion of the paint to the concrete. The rougher the surface, the more paint is used to cover to the required thickness, resulting in better adhesion. Profile can he assessed using the same method of peak-to-valley height measurement used tor blastcleaned steel, where a sharp anvil locates the bottom of the valley and the travel of the anvil relative to a large-area foot that sits on the peaks is measured in microns (µm).
Surface replication using profile putty makes it possible to obtain a permanent replica of the surface topography that can form part of the project record.
Monitor the climate
All painting processes require temperature to be monitored to ensure conditions are not too extreme for satisfactory curing. In addition, it is also useful to monitor the relative humidity and dewpoint temperatures. When the surface is within VC of the dewpoint temperature, moisture will be condensing on the surface and the conditions are not suitable for painting. The Elcometer 319 Dewpoint Meter (see Figure 4) incorporates all the needs for climate condition monitoring in a single unit.
Moisture in the concrete
The moisture content of a concrete surface is critical for the performance of a coating, particularly when coating a recently poured surface. Excessive moisture in the structure can cause failures due to condensation, blistering, delamination and movement, leading to general deterioration and peeling. Non-invasive moisture meters calibrated specifically for measuring the moisture in concrete, such as the Elcometer 7410 (see Figure 5), can be used to assess the surface prior to coating application.
Coating thickness and covermeters
The measurement of the thickness of a coating on concrete can be carried out non-destructively on concrete substrates using specialist ultrasonic gauges. However, calibration against a destructive test is often required due to the uncertainty associated with the ultrasonic method because of differences in the response of different coating materials. Alternatively, metal coupons coated at the same time as the concrete can be used to monitor coating thickness using electronic coating thickness gauges, for example the Elcometer 456, familiar in the steel protection field.
Covermeters are widely used for the detection, location, sizing and depth of reinforcement bars. The use of Elcometer 331 Covermeters (see Figure 6) as a quality assurance tool has many benefits as the bars can move during the pouring process and inspection of the structure at this early stage can reduce the cost of remedial action. The measurement of the concrete cover can be considered as a thickness measurement and the determination of the cover over the reinforcement bar on both sides of a column, for example, would determine if the cage had moved during pouring.
The pull-off adhesion test is used to determine the fracture strength of the bond between paint layers and the concrete substrate. The basic princ-iple is that a test piece, or ‘dolly’, of known area is glued to the surface of the coating and then pulled until the coating or the substrate fails and the dolly is pulled off the surface. For testing concrete, dollies 50mm in diameter are used. There are two pull systems available. The mechanical system, used in the Elcometer 106/6 (see Figure 7), generates a force by compressing a spring assembly. The hydraulic system, such as the Elcometer 1920 PAT Adhesion Tester, generates a force by pressurising hydraulic fluid to transmit the force to the dolly.
To locate flaws in insulating coatings, the high-voltage spark test can be used to test coatings up to a thickness of at least 7mm. As the moisture, normally present in concrete, allows an electrical current to flow, this method of detection of pinholes and other coating defects can be used on concrete structures. The key to successful porosity detection is selecting the correct test voltage. The voltage needs to be sufficient to break down the air in the pinhole so that current will How to trigger the alarm, but not so high that it causes breakdown in the coating.
The surveying of concrete structures, either immediately after construction or during the life of the structure, needs to be simple, portable and provide unambiguous results. The test equipment also needs to be reliable and rugged for use in the hostile conditions experienced on construction sites. Elcometer’s complete range of concrete testing equipment has been specifically designed lor these conditions.
VICTORIA MELDRUM, ELCOMETER
Copyright The Concrete Society Apr 2006
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