Parameters affecting the debonding risk of bonded overlays used on reinforced concrete slab subjected to flexural loading

This research aims at better understanding the mechanisms involved in the cracking behaviour of bonded overlays used on reinforced concrete beams and slabs. The project involves the testing of reinforced concrete beams (1.8 m x 0.2 m x 0.2 m) repaired with concrete overlays using four different types of surface preparation and subjected to cyclic flexural loading. The concrete beam specimens were simply supported with a point load at midspan. Structural capacity (evolution of the apparent rigidity, maximum deflection, failure mode) and cracking behaviour (flexural cracking and interface debonding) were monitored. Technical data indicate that good adhesion alone is insufficient to guarantee bond durability. A total of 20 beam interfaces were prepared by scarification, sandblasting, chipping with a light jackhammer, and water jetting. A number of cores were tested to evaluate interface strength by direct shear and direct tension. Roughness was characterized quantitatively using a newly developed optometric method. The results indicate that cracking behaviour depends on the bond strength and on the surface roughness produced by a specific surface treatment. The relationships between adhesion, structural behaviour, and roughness were evaluated, and an updated debonding mechanism is proposed to take into account the influence of roughness. To achieve a monolithic behaviour, the surface treatment must generate critical adhesion and roughness levels. In addition to this roughness, the debonding risk decreases rapidly and monolithic behaviour is reached.