Highway infrastructure lends itself to meeting sustainability goals through the use of recycled material from highway components removed from service and from other sources. In the U.S., state specifications stipulate requirements for materials that can be used in all state and federal-aid highway construction projects. These specifications vary from state to state, depending on availability of local materials and the perception of their suitability for engineering application.

Indiana Department of Transportation’s (INDOT’s) standard specifications permit considerable use of recycled materials. Indiana has a number of steel manufacturers and heavy industries and, hence, an abundance of byproducts, such as slag aggregates and fly ash, that are suitable for highway/bridge construction.

Indiana’s Material Requirements

The amount of recycled material INDOT allows in each major construction components is described below. The percentages cited are by weight.

Concrete Structures. As much as 77 percent recycled materials is allowed for most concrete structures, and 50 percent for concrete bridge decks. This can be achieved by partial replacement of cement with fly ash and ground granulated blast furnace slag, the use of air cooled (ACBF) or granulated (GBF) blast furnace slag as coarse aggregate, and fine aggregate from foundry sand in decks and ACBF in other components.

Steel. The steel industry currently utilizes steel scrap at the rate of 95 percent to make structural shapes and plates and at the rate of 47.5 percent to make steel reinforcing bars. INDOT uses these rates.and does not have additional requirements.

Pavement. INDOT permits a maximum of approximately 50 percent recycled materials for Portland cement concrete pavement. Bituminous pavement can comprise as much as 93 percent recycled materials through the use of reclaimed asphalt and asphalt roof shingles, ACBF or GBF coarse aggregates and ACBF fine aggregate. Steel furnace slag may be used in HMA surface mixtures. Compacted aggregate base may be 100 percent ACBF. INDOT specifications also permit rubblization of the existing concrete pavement for use as the base for a new Portland cement concrete pavement.

Earthwork. INDOT requires that the removed and excavated materials, including soil and existing structures that have suitable engineering properties, be used within the project. If the materials are not suitable, the balance has to be borrowed from sources outside the project and may consist of air-cooled or granulated blast furnace slag, or crushed concrete, masonry, or stone removed from an old structure or pavement.

Case Study

The project selected for the case study is a bridge replacement and road realignment in a rural-suburban setting. The new bridge is a 63-m (208-foot) -long prestressed concrete structure and the road is a 1.45-km (0.87-mile) –long segment of a two-lane collector roadway. They were designed to serve approximately 9,000 vehicles per day.

The client was interested in the use of recycled materials for the project due to a potential unique funding opportunity. I took the opportunity to develop a project-specific specification that required the use of recycled materials based on the existing INDOT framework. At the end of this brief research, I and everyone who learned of the findings were surprised by how much recycled material could be used potentially in highway construction.

Table 1: Construction Materials for the Case Study Project.

The breakdown of major construction materials for this project is shown in Table 1. The maximum utilization of recycled materials as sources of the construction materials as permitted by INDOT specifications is also tabulated. It can be seen that as much as 97 percent of the materials by weight could potentially be recovered materials. Also, the optimal use of recycled materials in this project would result in the net reduction of materials that would otherwise enter the waste stream. This case study demonstrates that a highway construction project can be an environmentally friendly endeavor.

Hurdles Remain

The funding opportunity, unfortunately, never materialized and the project was constructed with typical federal-aid funds. As with most highway projects, the lowest bidder used recycled material in this project only when it is the most economically advantage.

The construction specifications and material availability already existed for the potential significant use of recycled materials in highway construction. The remaining hurdle for the use of recycled materials is a thorough understanding of their short- and long-term benefits and costs (environmental, societal, and economics).