In August 1999, work was begun to investigate the more general use of Basic Oxygen Steel Slag (BOS) as a road surface course aggregate. Based on the evidence from initial on-road studies over a period of three years, the Highways Agency permitted the use of BOS slag in surface courses without the need for laboratory aggregate polishing tests, with the proviso that its performance continued to be monitored. Tarmac Group, responsible for the supply of steel slag, therefore commissioned TRL to continue the test and inspection programme. This paper provides an update on that ongoing work. Results from the extended monitoring confirm that BOS slag performs well in comparison with natural aggregates in a wide range of situations. There are indications that the skid resistance of BOS slag may improve over time but more detailed analysis of seasonal variation is required to confirm this. (A)

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