Mixed-mode interfacial fracture behavior of ultra-thin overlay system with high-strength ECC: Kinking-and-trapping mechanism

This study investigates interfacial fracture and kinking-and-trapping behavior of concrete beams repaired with High-Strength Engineered Cementitious Composites (HS-ECC) overlay system. The effects of fiber volume fraction (1.0%, 1.5%, and 2.0%) and overlay thickness (10 mm, 30 mm, and 50 mm) of HS-ECC were studied. Uniaxial tensile tests of HS-ECC revealed that increasing the fiber volume fraction enhanced the ultimate tensile strain by 62% (from 2.1% to 3.4%) and the peak tensile strength by 30% (from 4.4 MPa to 5.7 MPa), while the cracking strength remained unchanged. The number of cracks at ultimate strain increased from 13 to 27, and the average crack width decreased from 95.9 μm to 67.1 μm. The evolutions of crack width distributions were well-characterized by a probabilistic Weibull model. From four-point bending tests of composite beams, increasing overlay thickness from 10 mm to 50 mm enhanced the peak load from 0.45 kN to 5.99 kN but reduced deflection and interfacial crack extension length prior to kinking from 2.4 mm to 0.5 mm. Finite element analysis revealed that thicker overlays increased the phase angle, improving interface toughness and promoting earlier crack kinking. By contrast, increasing fiber volume fraction had negligible effect on the initial kinking condition but significantly enhanced post-kinking multiple cracking. The findings in this study provide a fundamental basis for enabling the rational design of durable HS-ECC repair systems.