Is Slag the Future of Sustainable Pavements?

The asphalt paving industry in the United States has matured and improved due to the foresight of agencies, road builders/contractors, academic researchers, material suppliers, and consultants in our business of road building.

The past decade has seen the increased and successful use of reclaimed asphalt pavement (RAP), warm mix asphalt (WMA), recycled asphalt shingles (RAS), and ferrous; iron and steel slag (slag). Cooperation and careful planning for sustainability have been key to our successes.

According to the National Asphalt Pavement Association (NAPA), “Cost savings have driven some of these mix-design changes, but the adoption of sustainable practices by the asphalt industry are also a source of pride.”

Ensuring the excellent performance of asphalt pavements while increasing the recycled content (sustainability), and maintaining safety, is the successful measure of these Green Initiatives.

First use of slag

As a virgin product, the initial use of iron and steel slag meets the requirements of Green Initiatives by eliminating the use of additional mineral resources in construction applications.

Slag is a sustainable product that can be recycled and reused multiple times in various construction projects. Using iron and steel furnace slag helps preserve our natural resources and reduce green house gases. Based on numerous environmental tests, studies, and reviews by governmental agencies, the iron and steel industry, and slag processors we know that iron and steel slag is a safe and valuable resource. We encourage its continued use as a viable, environmentally friendly product for building and maintaining the United States infrastructure.

Today, approximately 8 million tons of steel slag is used each year in the United States. The primary applications for steel slag in the United States are as a granular base or as an aggregate material in construction applications. Origin, current management options, market sources, highway uses, processing requirements, and material properties are explained in a Federal Highway Administration publication (FHWA-RD-97-148).

Additionally, steel furnace slag has been used as a raw ingredient in Portland cement manufacture, fill, and soil amendment to name but a few applications. Its primary use is as premium asphalt aggregate. Thanks to the size of the iron and steel industry in the United States, there are thousands of people employed in the slag industry.

Second and subsequent uses of slag

RAP of any nature (limestone, dolomite, crushed gravel, and slag) comes from the crushing and screening of excavated or cold milled asphalt pavements. This processed RAP contains high quality, well-graded aggregates that have been coated with asphalt. Several years ago, the State of Illinois (Illinois Department of Transportation) and the Illinois Center for Transportation (ICT) at the University of Illinois researched the expansive characteristics of RAP used as base materials.

Their conclusions led to the acceptance of the use of steel slag RAP as pavement base course aggregate and the allowable use of steel slag RAP in all asphalt mixtures; surface, intermediate, and/or base. (Their findings were published as Research Report ICT 09-055 in August 2009.)

Get your materials

When agencies engage contractors to build hot mix asphalt (HMA) projects, quite often they are being built as Pay Within Limits (PWL) projects. With this change to payment schedules, the aggregate supplier has become even more actively engaged in the bidding and building processes.

Consistency of the aggregate materials is paramount to success. Material suppliers will not be paid for an inconsistent product because, as the FHWA states for a Quality Assurance Program, “confidence that a product or facility will perform satisfactorily in service” is the basis of payment. Material found to be non-compliant will not be allowed for use in the roadway, the agency will not pay the contractor, and the contractor will not pay the aggregate producer.