Mapping the chloride load around two Swedish reinforced concrete bridges - second season

Report, 2004

In this report a study of the road environment around two motorway bridges situated outside Göteborg is presented. The road environment has, in this study, been described in terms of exposure to chlorides on the examined bridge structures. The study is a follow-up of two earlier studies, where both the exposure to chlorides on structures and the chloride ingress into concrete have been studied. The results from these earlier studies show on large variations in both the exposure to chlorides on the structures and chloride ingress into the concrete. Therefore this new study has been initiated to further investigate the exposure conditions for the examined motorway bridges. The exposure to chlorides on a specific structure is influenced by several factors, e.g. height over and distance to the roadway, surface orientation towards traffic, weather conditions and characteristics of road, spread of de-icing salt and traffic etc. In this study, however, the focus has been on the significance of the height over the roadway and the orientation towards traffic together with the characteristics of the traffic and spread of de-icing salt. The study has been carried out by exposing mortar specimens, with similar mortar composition, on selected positions on bridge columns. The examined bridges have been bridge O 951, situated along the motorway E6 some 20 km south of Göteborg, and bridge O 978, situated along the motorway Rv40 some 15 km east of Göteborg. The mortar specimens have been exposed on different heights above the roadway and orientations towards the traffic. The exposure started in the beginning of December 2001 and ended in the middle of April 2002. After finalised exposure the mortar specimens have been analysed for chloride ingress in depth intervals. The results from study show that the exposure to chlorides on a structure depends both on the height over the roadway and orientation towards the traffic. Fairly large variations in exposure to chlorides on a structure were, however, observed both between the bridges but also within one single bridge. The exposure to chlorides on structures generally decreases with increasing height over the roadway. This effect, however, was only observed on one of the examined bridges, while no effect was observed on the other. The exposure to chlorides has also been found to depend on the orientation towards the traffic, where surfaces facing towards the traffic have more severe exposure than surfaces facing from the traffic. The effect from orientation towards the traffic has been observed up to 3 meters above the roadway. The exposure to chlorides on structures, measured in this study, has been compared with similar measurements made on the same bridges but during another winter season. This comparison shows that it seems not possible to directly compare severities in exposure to chlorides, measured during different winter seasons. The exposure to chlorides on structures has also been compared with chloride ingress measured in the concrete. This comparison shows large deviations between the exposure to chlorides on structures and the actual chloride ingress into concrete, e.g. that spots with severe exposure to chlorides have small chloride ingress and vice versa. The reasons for these observations are not known, but anyhow they show that it is difficult to correlate data on the exposure to chlorides, measured after exposure during only one winter season, with data on the actual chloride penetration into the concrete, following from exposure during several years. The comparison indicates that there are also other factors, which have significant influence on the exposure to chlorides on the structure, e.g. direction of commuter traffic and airstreams around the structures. Thus, it is not trivial to describe and quantify the exposure conditions for bridges situated along thaw-salted roads. The overall conclusion from the study is therefore that preferably each bridge should be treated separately when the exposure conditions are described and quantified.