Effects of corrosion on the capacity of the nib of reinforced concrete dapped-end beams

Many studies have been conducted in the past decades to determine the impact of corrosion on public infrastructures such as bridges, due to the deterioration that has been witnessed since their construction, sometimes very early in their working life. This degradation has caused collapses, not only leading to economic losses but also provoking human casualties. The layout of certain structural elements complicates the inspection and maintenance processes, by creating tight spaces or blocking the access to certain surfaces. This is the case for dapped-end (half-joint) beams, whose height at the extremities has been shortened to create a support, called a nib, reducing the floor height and allowing the placement of an expansion joint. However, water can seep into this joint and stagnate on the lower nib if the drainage system and waterproofing are not adequate. During winter, when de-icing salts are spread on roads, this water becomes charged with chloride ions, which are the leading cause of corrosion in reinforced concrete structures. This support is placed at the location of highest shear stress and thus tends to crack rather early at its re-entrant corner. This crack, which can grow quite wide, becomes a direct path for the ingress of aggressive substances ; predominantly chloride ions and carbon dioxide. As corrosion progresses, other cracks form and accelerate the process further. This deterioration reduces the ultimate load-carrying capacity of the element over time by diminishing the cross-sectional area of the steel reinforcement. The purpose of this work is to numerically evaluate this reduction, first using a strut-and-tie method and then a non-linear finite-element based analysis that is able to represent an evolving crack pattern taking into account both the loading and the degradation. In this study, carbonation and the penetration of chloride ions are assumed to be the factors inducing corrosion ; their penetration mechanisms and the ensuing damage are described. Dapped-end beams and their detailing are presented, with the strut-and-tie model which was used to determine their reinforcement. The concepts behind the non-linear analysis are cited, such as the Modified Compression Field Theory, before describing the process of evaluation of the nibs by the finite-element based computer program. Finally, the results are presented, indicating a durability issue even for elements satisfying the minimum cover thickness requirements of EN1992-1-1, before suggesting several improvements to reduce this deterioration.