Robustness to corrosion of post-tensioned concrete slab bridges

Abstract

This master thesis aims to quantify the robustness of post tensioned bridge and to study the effect of corrosion on tendons. A post tensioned bridge has been first design. Concrete with medium compressive strength and steel strands are combined to form the post-tensioned model which provides a very strong and good work structure. The robustness is defined as the structural property that measures the degree of remaining structural performance after the structure has been damaged by corrosion. Corrosion is an electrochemical process which occurs under oxygen and moisture exposure. Many researches have been oriented toward the effect of corrosion on steel bars and toward the consequence on the robustness of the structure. As corrosion leads to reduction of steel area, the robustness of the structure is affected. Indeed, a loss of steel leads to a reduction of strength of the structure. Many definitions of robustness are available but very few practical proposals of a target of threshold value that defines the limit between what is robustness or not do exist. In the past 30 years, different researchers attempted to quantify the robustness. A new approach adequate to structures under deterioration has been used for this case study of a post tensioned bridge affected by corrosion. This approach consists of a robustness indicator IR,D which can be expressed by the area below the curve that defines the normalised structural performance f(D) as a function of the normalised damage D. The higher the robustness index, the more robust the structure is.Through grillage analysis aided by software SAP2000, three damage scenarios have been compared. First, a loss of prestressed area due to corrosion has been applied on the first three most external tendons, secondly on the three middle tendons and finally an equivalent loss of steel area on all the tendons. The index of robustness IR,D have been calculated in function of the damage D for each cases. It has been noticed that the index of robustness is the highest when the corrosion affects uniformly the structure whereas the lowest index of robustness appears when the corrosion affects the first three tendons. This is due to the distribution of loads on the structure that has a clear two dimensional behaviour: the highest loads are applied at the edge of the structure therefore a loss of prestress area at this location has more effect on the robustness of the structure. However, the robustness indicator of each case is closed to each other _less than 1.5% difference. It can be deduced that the robustness aspect of this type of bridge is not really affected by the corrosion and its reaction is little. The corrosion is an environmental factor that affects the most the robustness aspect of a structure under high loadings rather than an uniform corrosion on the entire structure; but corrosion does not reduce considerably the robustness of a post tensioned slab bridge with massive cross-section.