Time-dependent reliability analyses of prestressed concrete girders strengthened with CFRP laminates

Abstract

This paper presents a time-dependent reliability analysis of prestressed concrete girders subjected to degradation caused by pitting corrosion. The procedure presented includes the effects of both spatial and temporal pitting corrosions on prestressing steel, as well as the degradation of the strengthening CFRP laminate used for the rehabilitation of the member. Results indicate that the correlation of corrosion in different segments of the prestressing tendons impacts on the computed safety index for the deteriorated structure. Ditlevsen bounds are proposed for a better approximation of the correlation between failure modes in the spatial discretisation. Results show that such approach produces adequate estimates of the reliability index over the full range of analysis in comparison with other tested models. It is also observed that the degradation of the CFRP laminates does not affect the reliability as significantly as corrosion, and that traffic loads, models uncertainties, corrosion error and corrosion rate are the most relevant variables in the analysis, followed by prestressing strength and concrete cover. The significance of the variables changes with time: the corrosion rate, corrosion model error and concrete cover increase in importance with the development of corrosion, whereas traffic loads become gradually less important.

Assessment of corroded prestressed and posttensioned concrete structures: A review

“…Dias‐da‐Costa et al 2 also account for the fact that pitting can vary in space. To model the spatial variability, they divide the reinforcement into different segments, with different pit depths for each segment.…”

“…The resistance capacity of the strands is then governed by the tensile capacity of each segment, depending on the reduced prestressing area. In contrast to Darmawan and Stewart 21 who define the discretization length based on the transfer length of the prestressing force to the concrete, Dias‐da‐Costa et al 2 define the discretization length as the distance at which pitting corrosion influences the structural safety, that is, in the range of 0.1–1 m. The latter means that the reliability index cannot suddenly drop at the onset of corrosion. Taking this constraint into consideration, Dias‐da‐Costa et al 2 selected an element length of 0.45 m for the structure under consideration, which is indeed between the suggested element lengths of 0.1 and 1 m.…”

“…For example, when an accurate estimate of the reliability index is required, the element lengths assumed by Darmawan and Stewart 21 might be too large and can result in an underestimation or overestimation of the reliability index because of the mentioned possibility of non‐central strand ruptures and the assumed statistical independence of the segments. The criterion suggested by Dias‐da‐Costa et al 2 will lead to more accurate reliability estimates but at the cost of requiring small element lengths leading to higher computational costs. Hence, the engineer should consider all of these aspects when making decisions on the element length.…”

“…This differs from the approaches by Refs. 2,21 who assume that each strand is exposed to pitting, while the pitting factors for the wires are considered as independent random variables. As such, a different sample for the pitting factor is generated for each wire of each strand in each element.…”

“…For example, Podroužek et al 1 model the degradation due to corrosion by applying a reduction of prestressing tendons to critical elements in a finite element model, without modeling the actual corrosion process. Nevertheless, corrosion is a major durability concern since it might result in rupture of strands and might lead to a reduction of strength and serviceability up to an unacceptable level 2 . As such, the structure might fail at an early stage due to the occurrence of brittle fracture 3 .…”