Influence of the substrate characteristics on the bond behavior of PBO FRCM-concrete joints

• FRCM-concrete joints were tested using single-lap direct-shear test set-up.
• Different surface preparations and concrete strengths were studied.
• Failure modes were matrix–fiber and concrete–composite debonding; bare fiber rupture.
• Substrate surface treatment and control of matrix shrinkage is recommended.

Fiber Reinforced Cementitious Matrix (FRCM) composites can be employed as a valid alternative to Fiber Reinforced Polymers (FRP) composites for strengthening existing reinforced concrete (RC) structures. The weakness of FRCM-concrete joints has been reported to be at the matrix–fiber interface rather than within the concrete substrate, as is usually reported in the case of FRP-concrete joints. The experimental investigations regarding FRCM strengthening available in the literature have been performed by applying the FRCM composite onto a surface that was treated before the application, usually by grinding or sandblasting. However, the role of the substrate characteristics on the failure mechanism of the FRCM composite has not been properly investigated. This paper presents the experimental results obtained from FRCM-concrete joints tested using single-lap direct-shear tests. FRCM composites with different bonded lengths and bonded widths were applied onto concrete specimens with different surface preparations and different concrete strengths. The influence of shrinkage cracks and load distribution along the composite width on the failure modes observed is discussed. In order to avoid the possibility of sudden composite detachment failure, the authors recommend that the substrate surface should be treated, and the matrix shrinkage should be controlled.