Assessment of Mechanical Properties of Corroded Prestressing Strands

1. Introduction

The tendon is the most important structural element in a prestressed concrete (PSC) girder, and its damage can significantly influence the girder’s global behavior. The most critical type of damage in tendons is corrosion, which is difficult to detect through inspection. Corrosion-induced failure is not considered in the design of PSC structures and may cause the sudden collapse of a bridge. Once corrosion is found in a tendon, it is a difficult task to quantify the level of corrosion and its location through the entire length of the tendon. Some of the multiple strands in a tendon are invisible and cannot be observed. Therefore, a method to evaluate the level of corrosion as well as the corresponding mechanical properties of such corroded strands is needed.

In the bridges mentioned in a recent American report [ 1 2 ], such as the Niles Channel Bridge, Mid-Bay Bridge, Bob Graham Sunshine Skyway Bridge, and Varina-Enon Bridge, external tendon corrosion occurred within the last 20 years of a relatively short service period. Its causes were mainly infiltration of chloride-contaminated water, insufficient grout filling, and uncertain anchorage closure. When these factors occur together, severe corrosion may result. Uncertain anchorage closure could allow the entrance of water, air, and chloride, and imperfect grout filling may permit the exposure of internal strands to pollutants, causing corrosion, as shown in Figure 1 . According to the case study of Carsana et al. [ 3 ], one bridge with tendon fractures caused by corrosion had enough grout filling but, owing to the segregation of the grout, severe tendon corrosion occurred within 2 years. Chloride was not detected; however, a high sulfate ion content was found on the surface of the grout, and it was assumed to be the cause of the weakening of the grout segregation and strength.

Inspection of corroded strands is mostly been done for external tendons, as it is easier to observe and replace these. Corrosion occurring in inner tendons is difficult to repair even when detected. Therefore, most reports mentioned above and the inspection methods suggested therein focused on the corrosion of external tendons. Several studies [ 4 6 ] have been conducted to evaluate the section loss caused by the corrosion of a strand and the subsequent change in mechanical properties. In all of these research studies, the level of corrosion was evaluated using one of two methods.

The first method is to use the loss of mass to evaluate the section loss and apply it to the mechanical property evaluation. This approach uses electrochemical methods to induce the corrosion of a strand. The mass loss based on Faraday’s law for the evaluation of the chemical reaction of the metal is used as the specimen’s damage index in relation to the corrosion. This approach was introduced in ASTM G1-03 from American Society for Testing and Materials (ASTM) [ 7 ]. These studies obtained similar results for the decreases in strength and ductility caused by the progression of corrosion. However, there is no viable method to evaluate the corrosion on the strands of an existing PSC bridge. Evaluating the level of corrosion using the mass is not suitable.

The second approach is to use a pit-depth gauge to measure the depth of the corrosion pit. The section loss is evaluated using the methods to evaluate pitting given in ASTM G49-94 [ 8 ] regarding corrosion. As this approach measures the depth of the corrosion pit, an idealization of the section loss model is needed to quantify the section loss. Many researchers [ 9 14 ] have suggested section loss models such as hemispherical and planar pit configurations. Previous research [ 15 ] suggested three types of corrosion and their associated section losses through the observation of corroded strands from PSC bridges. With the help of tests and analyses, the mechanical properties of the corroded strands were determined.

In this study, inspections and tensile experiments were conducted on 100 corroded strand specimens obtained from bridges in service, which were also investigated in previous studies [ 15 16 ]. The difference from the previous research [ 15 ] is that the mechanical properties of the corroded strands were analytically obtained in the previous study; however, this study performed statistical analysis based on the experiment for practical use, and applied the results to predict flexural strength. Based on the experimental results, a reduction in the ultimate strength and fracture strain is suggested in the form of an empirical formula. Flexural tests were also conducted on PSC beams with corroded strands. The behavior of the corroded beams was evaluated, and the suggested equations were verified.