The presence of sulphur compounds in soils and rocks can lead to chemical attack on construction materials such as steel and concrete when these materials are used as backfill to structures. Sulphate attack on concrete is a well-known phenomenon and BRE guidance to some of the causes and prevention measures has been available for some time. However, recent cases of the corrosion of corrugated steel buried structures and thaumasite attack on concrete bridge foundations have highlighted the role played by sulphides such as pyrite. The sulphides can oxidise, producing high sulphate concentrations and acid conditions, which can then attack construction materials. Present methods of assessing corrosion potential tend to concentrate on sulphate and pH, and do not fully address the problem of sulphides.

This paper describes research carried out by TRL Limited and the University of Sheffield for the Highways Agency that aimed to develop better test methods for sulphur compounds in structural backfills. Advances in chemical test methods such as direct determination of sulphur by ICP-AES and novel methods of extraction have been investigated and compared with traditional methods for a range of materials with widely varying sulphur contents and mineralogy. The research led to recommendations for a revised suite of tests for sulphur compounds in structural backfills, appropriate methods of sampling and storage and limiting values for species such as reduced sulphur which are not covered by existing guidance. The research will be implemented by means of revisions to the relevant sections of the Specification for Highway Works and Design Manual for Roads and Bridges.

The paper also aims to highlight the distribution of naturally occurring sulphur species within the UK geological materials. As a result of inadequate sampling, storage and assessment procedures, these sulphur species may cause problems to buried steel and concrete structures.

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