Pull-Off Adhesion Strength ASTM D7234

Scope:

Pull-Off Adhesion Strength test by ASTM D7234 calculates the pull-off strength of a coating on concrete materials and also determines the ultimate perpendicular force (in tension) that the surface area can hold prior to a plug material being detached. The significant performance properties from a concrete substrate that is useful in specifications are mode of failure and pull-off adhesion strength of a coating. 

Test Procedure:

Pull-Off Adhesion Strength test by ASTM D7234 requires an adhesion-tester. The tester is selected which has a force calibration spanning and also the range of desired values along with its loading fixture. A comparable device or bearing ring is placed concentrically around the loading fixture, which is placed on the coating surface (in case it is used). Also, when a comparable device is employed, shims are necessary that are placed in between the comparable device and the tester base rather than on the coating surface. Then, the central grip of the detaching assembly is connected to the loading fixture without bending and bumping. If non-horizontal surfaces are used, the weight of the detaching assembly may be contributing to the force exerted in the test, so support is provided to avoid the weight. The device is aligned as per the manufacturer’s guidelines and also the force indicator is set to zero. Then, the load to the fixture is increased in a smooth and continuous manner at a constant rate of 0.2 MPs/s (30 psi/s). This helps to achieve the failure or maximum stress prior to 30 s. The force attained at failure is noted. The plug of material is labeled and the fixture is stored for qualification of the failed surface when it is detached as specified by ASTM D7234. Any types of shortcomings are reported from the procedure that might be hesitations in the force application, possible misalignment, etc. 

Sample size:

The samples for the test are provided as stated by ASTM D7234 guidelines. 

Data:

The calibrations chart is used provided by the manufacturer or the relative stress applied is computed to each coating sample as below:

X=4F/πd²

where: 

X = pull-off adhesion strength achieved at failure in MPa (psi)

F = Maximum force applied to the test surface at failure in N(lbf)

d = diameter of the loading fixture in mm (in.)

Other such as the percent of adhesive and cohesive failures, average % failure for each mode of failure, and average pull-off adhesion strength are also reported.