Basic Oxygen Steel (BOS) slag is a by-product from the further refining of liquid iron to produce steel. At first, its use as an aggregate in road surface courses was typically confined to roads in or near steel works. Its performance in the Polished Stone Value (PSV) test, the standard laboratory test used to assess suitability of aggregates for use in surface courses, was very variable and sometimes low. Consequently, BOS slag was rarely used on public roads. However, in situations where it was used, its performance was thought to be better than the lower values of PSV suggested.
Tarmac Ltd., responsible for the supply of BOS slag produced in the UK, commissioned TRL to carry out a programme of research to investigate the issue. The work confirmed the variability of PSV results but also demonstrated that in-service performance of BOS slag was broadly consistent with that of an aggregate with a PSV of 60, making it suitable for more widespread use in spite of its performance in the PSV test.
This report describes a programme of skid resistance monitoring which has continued for some ten years. Over that time, many of the trial sites reached the end of their working lives but none of them showed deterioration in skid resistance performance.
The report concludes that, subject to limitations on its chemical composition, the use of BOS slag as coarse aggregate in thin surfacings should be allowed in non-event locations carrying up to 4,000 commercial vehicles per day, where a 60 PSV natural aggregate would otherwise be used (based on HA policy documentation at the time of writing). Furthermore, there is no longer a need for additional monitoring beyond that carried out routinely in support of skid resistance policy.
However, the work also confirms that BOS slag should not be used as coarse aggregate in locations where traffic is expected to perform heavy braking or cornering manoeuvres.

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