The development of a new testing protocol for sulphur compounds in structural backfills

Abstract

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.

Atomic spectrometry update. Environmental analysis

“…NIST SRM 981 (certified common lead isotope standard) was used to perform mass bias correction on their QICP-MS measurements. As expected with this sort of instrument, typical within-run precisions were around 0.2% (1s) for both 207 Pb/ 206 Pb and 208 Pb/ 206 Pb isotope ratio measurements but are adequate for certain environmental applications.…”

“…Quadrupole ICP-MS was used to determine lead isotope ratios in eighteen commercially available CRMs 30 with the objective of validating a data set that could be used for quality assurance purposes in environmental analysis. Precisions of 0.19% and 0.14% RSD for 207 Pb/ 206 Pb and 208 Pb/ 206 Pb ratios, respectively, were achieved for triplicate analyses. Although the benefits include a simple and robust sample digestion procedure, its routine use has been hampered by the relatively poor precision (AE10-15%) and detection limits reported in previous studies.…”

“…They evaluated the need for detector dead-time correction and mass discrimination for this particular TOF instrument. The precisions of the isotope ratios for Pb at a concentration of 1 mg l À1 were 1.87, 0.70 and 0.28% for 204 Pb/ 206 Pb, 207 Pb/ 206 Pb and 208 Pb/ 206 Pb, respectively. The methodology was validated using the certified lead isotope ratio values for NIST SRM 981 and the total lead value for the NIST fresh water SRM 1643d.…”

“…25,28,29,154 Krachler et al 154 have developed an analytical protocol for determining Pb isotopes in peat by ICP-MS. Individual mass discrimination correction was applied to 207 Pb/ 206 Pb and 208 Pb/ 206 Pb ratios, resulting in increased accuracy and precise determinations of the Pb isotopes. The value of accurate data for Pb isotopes in peat samples was demonstrated in a study by Le Roux et al, 29 which utilised multiple ion collector ICP-MS and sector field ICP-MS and TIMS to measure Pb isotope ratios in peat cores from northern England.…”

Implications of Pyrite Oxidation for Engineering Works

“…Mitchell (1986), appreciating the importance of recording total sulphur and acid soluble sulphate values representative of the conditions when sampled, recommended storing the material at temperatures as low as 4°C. BRE (2005), referring to the chemical testing of concrete, states The samples must then be stored in a Table 1) cool dark place, at a temperature between 2 and 4°C, and be tested as soon as possible, the maximum delay being three weeks,(see also Czerewko et al 2003b).…”