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Investigation of the bond between fiber reinforced composites and the substrates onto which they are applied is of critical importance to understand their failure mechanisms. The bond behavior can be studied using different experimental test set-ups; the most commonly used are the single-lap and double-lap direct-shear tests. Although single-lap shear tests are simpler to carry out than double-lap shear tests, the presence of an eccentricity between the pulling and restraining forces leads to a mixed mode fracture process at the interface, which may influence the results. This study investigates the eccentricity effect on the bond behavior of fiber reinforced cementitious matrix (FRCM) composite–concrete joints. FRCM composite strips with the same bonded length were applied to concrete blocks of different lengths and tested using the single-lap shear test. The use of digital image correlation (DIC) allowed for studying the strain field on the surface of the bonded composite. Results were compared with those from double-lap shear tests of the same composite. The results obtained confirm that the eccentricity effect is negligible for bonded lengths longer than the effective bond length.

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.