Steel slag aggregate has been used in premium surface course hot-mix asphalt for Ontario highways since the early 1970s. Asphalt pavement performance problems led to the Ministry of Transportation imposing a moratorium on the use on steel slag aggregate in late 1991. The problem has been addressed from the steelmakers producing the slag to hotmix design and production. Steel slag can serve as good aggregate for use in hot-mix asphalt if the volume expansion of the slag is controlled. Performance-based testing of steel slag aggregate, particularly volume stability, is recommended as a measure of the aggregate quality. Practices at the steelmaker and the slag processor were examined. The production of acceptable quality steel slag aggregate requires total quality management of all steps of production.

Steel slag has been used in Ontario as an aggregate in hot-mix asphalt since the early 1970s. The primary use of steel slag aggregate (SSA) has been in the premium surface, or wearing course, of asphalt pavement. The inherent physical properties of steel slag produced a hot-mix asphalt with high stability, good stripping resistance, and excellent skid resistance. A considerable amount of steel slag aggregate was produced in Ontario with some 488,000 tonnes used in hot-mix asphalt in 1990 (J). The Ministry of Transportation of Ontario (MTO) expressed increasing concern in 1985 about pavement performance where steel slag aggregate was used after about 1980. The initial concerns of MTO were random map cracking, grey veining around cracks," flushing" of asphalt cement to the pavement surface, and the general lack of SSA quality compared with natural aggregates. The Ontario steel industry, aggregate producers, and paving industry worked on these and related concerns in 1985 and 1986, but did not resolve the problem to MTO's satisfaction. Continuing performance problems, particularly map cracking of dense friction course (DFC) mixes, led to a moratorium in December 1991 on the use of all steelmaking and blast furnace slags in hot-mix asphalt. The moratorium has prompted the steel industry to better understand steel slag as an aggregate and be able to produce that aggregate to an acceptable level of quality for use in hot-mix asphalt.