Concrete Pull-Off Testing
How direct-tension pull-off tests measure the surface strength of concrete and the bond of coatings and overlays to it — equipment, procedure, failure modes and acceptance criteria.
Key takeaways
- Concrete pull-off tests measure the tensile strength of the weakest plane in the system — often the concrete itself, not the coating bond.
- ASTM D7234 covers coatings on concrete; ASTM C1583 and EN 1542 cover concrete surface strength and the bond of repair materials and overlays.
- Always record the failure mode with the value. A failure deep in sound concrete at a modest value can be a better result than a high value at the bond line.
- Coring through the coating, adhesive cure and loading rate all affect results, so the specification should fix the procedure.
Concrete is weak in tension, variable from pour to pour and often carries a weak surface layer called laitance. A coating bonded to that surface is only as strong as the top few millimetres of concrete beneath it. Pull-off testing measures that strength directly, either before coating — to confirm the surface is sound and properly prepared — or after coating, to verify the bond.
The principle is the same as for pull-off adhesion testing on steel, but concrete brings its own procedures, failure modes and interpretation.
Why pull-off test concrete?
Pull-off tests on concrete answer three practical questions:
- Is the substrate sound? A test on the bare, prepared surface shows whether laitance, carbonation, freeze-thaw damage or contamination has left a weak layer that needs more removal.
- Was the preparation adequate? Comparing results before and after shot blasting or grinding confirms that the method has reached sound concrete without bruising it.
- Is the coating bonded? Tests on cured coatings or overlays verify adhesion on new work, qualify mock-ups, and help decide whether an existing floor can be overcoated.
Standards and equipment
ASTM D7234 is the standard test method for pull-off adhesion strength of coatings on concrete using portable testers. ASTM C1583 covers the tensile strength of concrete surfaces and the bond strength of concrete repair and overlay materials by direct tension. In Europe, EN 1542 covers bond strength by pull-off for concrete protection and repair products. ICRI 210.3R gives practical guidance on planning in-situ pull-off tests and interpreting the results.
Typical equipment includes:
- A portable pull-off tester — hydraulic or mechanical — with a calibrated gauge or digital readout and a controlled loading rate.
- Steel or aluminium loading fixtures (dollies), commonly 50 mm (2 in) in diameter for concrete; larger dollies average out aggregate effects.
- A rotary core drill with a diamond core bit to isolate the test area.
- A rapid-curing, high-strength adhesive, usually a two-component epoxy or methacrylate.
| Standard | Main use | Typical setting |
|---|---|---|
| ASTM D7234 | Coating adhesion to concrete | QC on floors, linings, walls |
| ASTM C1583 | Surface tensile strength; overlay and repair bond | Surface prep acceptance, repair mortars |
| EN 1542 | Bond strength of protection and repair products | European projects, product testing |
| ICRI 210.3R | Guidance on planning and interpreting tests | Test programs, specifications |
Test procedure
Always follow the standard and the instrument manufacturer’s instructions. A typical sequence looks like this:
- Select locations. Choose representative areas plus suspected problem areas, away from joints, edges and cracks unless those are being investigated.
- Core the test area. Drill through the coating and a short distance into the concrete so the load is confined to the dolly footprint. Use water cooling carefully and let the area dry.
- Prepare the surfaces. Lightly abrade and clean the coating face and the dolly so the adhesive bonds well.
- Bond the dolly. Apply adhesive, seat the dolly squarely and remove squeeze-out. Let the adhesive reach full strength for the conditions.
- Load the dolly. Attach the tester and apply a steady, perpendicular load at the rate the standard specifies until fracture.
- Record. Note the peak load or stress, the failure mode with estimated percentages, coating thickness, location and conditions.
- Repair. Fill and recoat the test site per the coating manufacturer’s instructions.
Core drilling generates heat and vibration. On thin or brittle coatings, a dull bit or excessive pressure can crack the bond line before the test even starts. Use a sharp bit, light pressure and steady speed — and discard results from cores that show visible damage.
Reading the failure mode
The fracture surface tells you where the system is weakest. Inspectors usually describe it by location and estimate the percentage of each mode:
- Cohesive in concrete — the dolly lifts a layer of concrete. The bond is stronger than the substrate; the value reflects surface strength. Deep failure into sound concrete is generally the desired result.
- Shallow concrete failure — a thin skin of paste or fines comes away, suggesting laitance or a weak surface that preparation did not remove.
- Adhesive at the coating/concrete interface — the coating lifts cleanly, pointing to contamination, inadequate profile, moisture or a primer problem.
- Cohesive in the coating or between coats — a weakness within the coating system, such as poor cure or intercoat contamination.
- Glue failure — the adhesive let go first. The result only shows that the system is at least as strong as the recorded value.
Setting acceptance criteria
Acceptance criteria should come from the project specification and the coating manufacturer. Many specifications combine a minimum tensile value with a requirement that failure occur predominantly in the concrete. Minimums for coatings and toppings on concrete are commonly in the range of about 200–300 psi (1.4–2.1 MPa), with higher values sometimes specified for heavy-duty overlays; check the governing document rather than assuming a number.
Because concrete strength varies, specifications typically set the number of tests per area and how to handle an individual low result — for example, by allowing retests nearby or averaging a set. A result well below the concrete’s expected surface strength with failure at the bond line is a stronger signal of a problem than a low value with deep substrate failure.
Values from different dolly sizes, tester types, core depths and loading rates are not directly comparable. If a specification states a minimum, it should also state the method and equipment so results are consistent from test to test.
Factors that affect results
- Surface preparation. The concrete surface profile (CSP) and removal of laitance strongly influence bond.
- Moisture. High moisture during coating can weaken the bond; concrete moisture testing before application helps explain low results.
- Concrete quality. Low-strength, carbonated or freeze-thaw damaged concrete limits achievable values regardless of coating.
- Coating stiffness and thickness. Thick, rigid systems can concentrate stress at the core edge; flexible coatings may stretch before breaking.
- Temperature and cure. Testing before the coating or adhesive has fully cured gives low, misleading results.
For more on designing systems that bond well in service, see coating concrete and concrete floor coatings.
Frequently asked questions
Do I need to core before a concrete pull-off test?
Coring is standard practice for concrete pull-off tests because it confines the load to the dolly area. Follow the scoring or coring requirements of the standard named in the specification.
Is a failure in the concrete a pass?
Usually it is considered acceptable if the value also meets the specified minimum, because it shows the coating bond is stronger than the substrate. Shallow failure in a weak surface skin, however, may indicate inadequate preparation.
How soon after coating can I test?
Wait until the coating has reached the cure state the product data sheet gives for service or testing, and until the dolly adhesive has fully cured. Cold conditions extend both.
Can I test the bare concrete before coating?
Yes. Surface tensile tests on prepared, uncoated concrete are a common way to confirm the surface is sound before any coating is applied.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
