Pull-off testing is used in QA and R&D to measure the adhesion of coatings, repairs and screeds to the concrete or steel substrates. To perform the test, a partial core is drilled through the covering into the substrate. A metal disc, commonly known as a dolly (or test disc), is then glued to the surface. The Pull-Off tester is then connected to the dolly, and a tensile force is applied. The peak load at failure or machine capacity is then recorded. The stress is then calculated from the test disc's contact area and the peak force. Care must be taken to also record the failure mode (interface of break, proportion where more than one interface is involved).
In Australia, AS 1580.408.5 is generally used for pull-off testing of coatings on both steel and concrete substrates. However, it does not provide guidance for assessing the adhesion of concrete repairs. For that, international ASTM standards are commonly adopted: ASTM C1583 covers tensile bond testing of concrete substrates and repair materials, while ASTM D7234 applies to coatings on concrete. ASTM D4541 is the equivalent standard for coatings on steel. Notably, the most common dolly sizes are 50 mm for concrete and 20 mm for steel, both referenced in the ASTM standards but not specified within the Australian Standard.
No, pull-off testing can be used not only for coatings, but also for concrete repairs and other surface coverings, and it can even be used to determine the tensile strength of the concrete itself. However, AS 1580.408.5 applies only to coating adhesion and does not cover repair materials or substrate strength. For concrete repairs, the correct standard is AS1012.24 a simmilar standard to ASTM C1583, which includes specific requirements for the dolly, including that it must be a 10–20 mm thick steel disc or
The preparations required for pull-off testing include:
How does the Load Rate affect Pull-Off Testing?
Ensuring the correct loading rate is essential for consistent, accurate tests with the correct failure mode. Loading to quick promotes brittle failures with random surface failures. Loading too slowly can create peeling-type failures. Uncontrolled, fluctuating loading rates increase variability.
How does the location of the failure in Pull Off Adhesion Testing affect the outcome?
The failure interface is critical for interpreting pull-off test results. Failures occurring in the glue or at the glue–surface interface are considered invalid, as they do not reflect the true bond strength of the system. A failure at the coating or repair interface provides a valid indication of adhesion performance. Failures within the substrate, coating, or repair material may also be acceptable, depending on the measured MPa value and the performance requirements of the system being tested.
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