Chloride ingress of carbonated blast furnace slag cement mortars

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Abstract

In the Netherlands infrastructural works, such as bridges and overpasses, are generally constructed using blast furnace slag cement (BFSC) concretes, because of the relative high resistance against chloride penetration from de-icing salts. However, sheltered elements constructed in BFSC concrete are prone to carbonation, for example abutments and support beams. From the field investigation at the Hattemerbroek overpass, concrete damages were found at the abutments and attributed to chloride induced corrosion enhanced by carbonation. This research focuses on the influence of carbonation on the chloride penetration resistance of BFSC mortars and the effect of slag content. For the influence of carbonation on chloride penetration resistance it is assumed that there is a sheltered carbonation period before exposure to chlorides, like an abutment exposed to chlorides after a joint starts to leak. This research was needed since there are up to 2000 concrete structures like Hattemerbroek, executed in BFSC and prone to carbonation enhanced chloride induced corrosion. In order to identify the influence of different slag contents (0-70%) on carbonation, chloride penetration and their coupled effect, twelve slag cement blends were tested. Chloride penetration resistance was expressed by means of electrical resistivity and chloride diffusion coefficient determined from the RCM-test in accordance with NT Built 492. Research shows that carbonation of slag cement mortars increases the porosity, consequently decreasing the chloride penetration resistance. A 70% slag sample with a relative high chloride penetration resistance before carbonation was found to have a significantly lower resistance after carbonation. The chloride penetration resistance is strongly dependent on the carbonation duration and its corresponding carbonation depth. Mortar specimens with a slag content between 35-55% carbonated before chloride exposure showed less influence of carbonation on the chloride penetration resistance.