Mapping of the chloride load around two Swedish reinforced concrete bridges

Rapport, 2002

In this report a study of the road environment around two motorway-bridges around Göteborg is presented. The study is a completion of an earlier study, made during the autumns of 1998 and 1999, where seven concrete motorway-bridges around Göteborg have been studied regarding moisture conditions, chloride penetration, frost damage and reinforcement corrosion. The results from this study show on large variations, especially in chloride penetration, both between different bridges, but also on one single bridge. The new study has been initiated to confirm and further investigate the results from this earlier study. The study has been carried out by exposing specimens made from mortar on bridgecolumns. The specimens are manufactured at one occasion with the same mortar composition to minimise variations in the results due to the execution and the material composition. The specimens have been exposed on two of the bridges investigated in the earlier study. Two bridges are bridge O 951, which is situated along the motorway E6 some 20 km south of Göteborg, and bridge O 978, which is situated along the motorway Rv40 some 15 km east of Göteborg. The exposure, made at different heights over the roadways and orientations towards the traffic, was started in the middle of February 2001, and was finished in the middle of May 2001. After finalising the exposure, the chloride penetration into each specimen has been determined by profile grinding in depth intervals and wet chemical analysis of the powder samples to determine the chloride and calcium content. The chloride penetration is presented as chloride penetration profiles where the quotient between the chloride and calcium content at different depths is presented. The results from the study show that the exposure for chlorides varies a lot depending on the height over the roadway and the orientation towards the traffic. The exposure for chloride was found to be largest for surfaces orientated towards the traffic on the same level as the roadway. Since de-icing salt normally is spread during the night or early in the morning, this means that surfaces that faces towards the morning traffic has the highest exposure to chlorides. This effect can be observed on bridge O 951, with marked commuter traffic to and from Göteborg, where the surface that faces towards the traffic towards Göteborg has significantly higher exposure for chlorides compared to the other surfaces. On bridge O 978, with less commuter traffic, this variation in exposure to chlorides cannot be observed. Furthermore, large chloride penetration was observed up to two meters above the roadway on bridge O 978. A possible explanation for this is a deposition of airborne chlorides at certain wind-directions on the lee-side of the column. This surface is not exposed to rain, that will wash the chlorides away, which means that the chlorides will be accumulated in the concrete. These results confirm the results from the study made during the autumns of 1998 and 1999.