According to ACI 440.2R-17 (Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures), the Pull-off Test for carbon fiber reinforced polymer (CFRP) plates is a critical method for evaluating the bond performance between CFRP plates and substrates (e.g., concrete). Below are the criteria for success and failure:

1. Successful Pull-off Test Result: Cohesive Failure

• Definition: During the pull-off test, failure occurs within the concrete substrate (i.e., the concrete itself fractures), rather than at the bond interface between the CFRP plate and the concrete.

• Significance:

• Indicates that the bond strength is higher than the tensile strength of the concrete itself, which is the ideal outcome.

• Demonstrates excellent bond performance between the CFRP plate and the concrete, ensuring the CFRP plate can fully deliver its strengthening effect.

• ACI Standard: Cohesive failure is the preferred criterion for judging bond performance as acceptable.

2. Failed Pull-off Test Result: Adhesive Failure

• Definition: During the pull-off test, failure occurs at the bond interface between the CFRP plate and the concrete, meaning the CFRP plate detaches from the concrete surface.

• Significance:

• Indicates insufficient bond strength, which may be due to improper surface preparation, unsuitable adhesive selection, or construction issues.

• This failure mode suggests that the CFRP plate cannot fully utilize its strengthening potential, potentially compromising the structure's safety and durability.

• ACI Standard: Adhesive failure is generally considered unacceptable, requiring reevaluation of construction techniques or material selection.

3. Other Possible Failure Modes

• Mixed Failure: Partial failure occurs within the concrete, and partial failure occurs at the bond interface.

• ACI considers this failure mode as intermediate, requiring further analysis to determine the root cause.

• FRP Rupture: If the CFRP plate fractures during the pull-off test, it indicates that the bond strength exceeds the tensile strength of the CFRP plate itself.

• Although rare, this scenario also demonstrates excellent bond performance.

4. ACI Requirements for Pull-off Tests

• Testing Frequency: ACI recommends random sampling during construction to ensure consistent bond quality.

• Minimum Strength Requirement: The bond strength must meet the design value, and the failure mode should be cohesive failure.

• Surface Preparation: ACI emphasizes the importance of concrete surface cleanliness, roughness, and moisture control for optimal bond performance.