A concrete repair, screed, overlay, or protective coating is only as good as its grip on the concrete underneath. If the bond is weak, the new layer can crack, delaminate, and eventually spall away, often taking the investment in the repair with it. The pull-off test measures that grip directly on site by pulling a small area of the surface away and recording the force it takes.
The Standards
The two methods most often referred to are:
- ASTM C1583, Standard Test Method for Tensile Strength of Concrete Surfaces and the Bond Strength or Tensile Strength of Concrete Repair and Overlay Materials by Direct Tension (Pull-off Method).
- BS EN 1542, Products and systems for the protection and repair of concrete structures: Measurement of bond strength by pull-off.
ASTM C1583 describes three uses for the test: checking the near-surface tensile strength of the concrete as an indicator of whether surface preparation is adequate before a repair or overlay goes on, measuring the bond strength of a repair or overlay to the concrete, and measuring the tensile strength of the repair material itself.
How the Test Works
- Isolate a test area. A partial core is cut into the surface with a core drill, isolating a small cylinder of material without removing it. For a repair or overlay, the core is cut through the new layer and into the concrete beneath.
- Bond the disc. A steel disc (often called a dolly) is bonded to the test area with a strong adhesive and left to cure. Depending on the method and the material, the disc may be bonded before or after the partial core is cut.
- Pull. A portable tensile loading device is attached to the disc and loads it steadily in direct tension until something breaks.
- Calculate. The failure load divided by the area of the core gives the pull-off strength.
Why the Failure Mode Matters
Where the break happens is often more informative than the number itself:
- In the concrete substrate: the bond is stronger than the concrete. The result is a measure of the concrete's own near-surface tensile strength.
- At the bond line: the interface between the repair or overlay and the concrete is the weakest point. The result is the bond strength.
- Within the repair or overlay: the new material itself is the weak link.
- In the adhesive holding the disc: the test has not measured the material at all and should be repeated.
Each result is reported with its failure mode, and a set of tests across the area gives a far better picture than any single one.
Where It Is Used
- Before a repair: to confirm that surface preparation has removed weak, contaminated, or carbonated material and left a sound surface.
- After a repair: to confirm that patch repairs, overlays, and toppings have bonded to the substrate.
- Coatings and waterproofing: to check the adhesion of protective and waterproofing systems.
- Condition surveys: to assess the surface strength of existing concrete before deciding on a repair strategy.
Limitations
- Partly destructive: each test leaves a small cored and damaged area that needs to be made good.
- Alignment: the load must be applied square to the surface. An eccentric pull gives a low result.
- Curing time: the adhesive, and any fresh repair material, must be fully cured before testing.
- Local result: each test reflects a small area, so the number and spread of test locations matter.
Note: The required pull-off strength and the number of tests are set by the repair specification or the product manufacturer. Results should be assessed by the responsible engineer.
