Laboratory investigations suggest that the engineering properties of RAP-blended soils are comparable with those of virgin aggregates, they say.

“Gradation analyses indicate that the addition of RAP to virgin materials does not significantly change the particle size distribution,” Mokwa and Peebles say. “The outlook for the continued implementation of RAP as an additive to granular base and subbase materials for use in highway construction looks promising. Results from the extensive suite of laboratory tests indicate that blending asphalt millings with granular cohesionless material, like crushed aggregate or pit-run cohesionless soil, results in only minor changes to the engineering properties of the virgin material.”

Also, pit-run steel slag is extensively used for subbase construction in some areas, especially where weak subgrade conditions exist.

Steel slag is a crushed product having hard, dense, angular and roughly cubical particles. “Steel slag meets the requirements of ASTM D 694 and D 1241, of national agencies, and of local highway departments for macadam and crushed aggregate bases,” reports supplier Phoenix Services, Uniontown, Pa. “Local highway department standards or the producer’s recommendations are applicable for both base and subbase courses.”

Steel slag for use in bases and structural fills — where very high stabilities are required — may require proper selection, processing and aging (weathering) before use, Phoenix says. “Steel slag may contain free lime (CaO or MgO) that may cause the slag to be expansive or cause differential movement when used as a base,” Phoenix reports. “Steel slag is not recommended for use in rigid, confined applications such as concrete aggregate, base or fill under structures or floor slabs, or backfill against structures or bridge abutments.”