Weathering of Pyritic Clay in the Vicinity of Concrete Undergoing Thaumasite Sulphate Attack

The possibility of attack of buried concrete due to the chemical aggressivity of the ground is of concern in the construction industry. Following the discovery in the late 1990s of seriously damaged foundations due to the formation of thaumasite in bridge structures on the M5 motorway in the UK, changes were made to the standards for buried concrete. Recommendations place limits on the amounts of sulphur and other aggressive species that different types of concrete can be expected to withstand. These values were based mainly on the results of experimental exposure of concrete samples in tanks containing potentially aggressive solutions. The paper presents the results of a long-term lab-based study in which commonly used concrete types, including sulphate resisting Portland cement, buried in pyrite bearing Lower Lias Clay for a period of 9 years. The conditions were conducive for the formation of thaumasite. All types of concrete tested suffered significant attack and that this attack was more severe than the expected given the aggressivity classification of the clay used. The changes to the clay over the duration of the experiment are compared with the effects of natural weathering of the clay, and also the clay adjacent to the damaged M5 bridge structures. The results suggest that the interaction with the clay is much more complex than is simulated by the standard exposure tests.