Using Recycled Concrete Aggregates (RCA) in Unbound Aggregate Shoulders, written by Mark B. Snyder, Past President-ISCP and Pavement and Paving Materials Engineering Consultant, and sponsored by the Federal Highway Administration (FHWA), is the newest Technical Brief in the National Concrete Pavement Technology Center (CP Tech Center)’s Concrete Pavement Recycling Series.
Recycled concrete aggregate (RCA) can be—and has been—successfully used in unbound aggregate shoulder surface applications.
Nine of 13 states responding to a 2017 survey conducted by the CP Tech Center stated that RCA was allowed for use as shoulder surfacing by their agency. In 2004, the FHWA reported that the Michigan DOT (MDOT) also allows the use of dense-graded RCA in shoulder surfacing applications. While the use of RCA in unbound shouldering is allowed in many states—at least 10, its use is not common. This is likely because roadways with recyclable concrete mainline pavements typically have asphalt- or concrete-surfaced shoulders and little or no need for aggregate shoulder surfacing. It has been used successfully in many instances—although at least one state requires blending with 50% to 70% natural aggregate to ensure adequate stability immediately after construction. Additionally, it is far more common and broadly accepted to use RCA in unbound base applications beneath travel lanes and shoulders.
This brief describes qualification requirements, design techniques, and construction considerations for the unbound RCA shouldering materials that are being used by some states. The brief also outlines the economic and environmental factors that have made RCA attractive for use in unbound shoulder construction:
Qualification Requirements:
Gradation: Many highway agencies require only gradation control when recycling concrete pavements from known sources (i.e., their own networks) and require more extensive testing only for the processing of materials from other sources. The gradation of unbound aggregate shoulder surface materials is critical to the stability of the material under service.
Durability: When additional testing is required, RCA materials are generally required to meet the same quality requirements as conventional aggregate shouldering materials.
Other Qualification Tests: RCA materials may be subject to some qualification tests not generally applied to natural aggregates (e.g., limits on certain potentially deleterious substances, such as asphalt concrete, brick, plaster, gypsum board, and hazardous materials).
Blending with Natural Aggregate: RCA can be blended with natural aggregate for use in unbound shoulder surface applications.
Guidelines for Producing RCA Shoulder:
Material: Guidelines specific to the use of crushed concrete from existing pavements in unbound bases are available in Appendix B of the American Concrete Pavement Association’s (ACPA’s) Recycling Concrete Pavements (2009). This guidance is also generally applicable to the use of RCA in unbound shoulder surfaces.
RCA Shoulder Design and Construction:
Considerations: Design of unbound RCA shoulders should be performed using the same tools used for conventional unbound aggregate shoulders and should result in shoulders with similar thickness.
Placement and Compaction Equipment: RCA shoulders can be placed using standard equipment and techniques. Standard equipment and techniques can be used to construct RCA shoulders, although steps should be taken to minimize the potential for producing additional fines through abrasion and other mechanisms.
Moisture and Density Control: A key step is to place and compact the RCA at optimum moisture content (and blends of RCA and natural aggregate) to ensure that compaction efforts are efficient. This is typically higher than for natural aggregate materials.
Potential Economic Benefits: The potential economic benefits of using RCA in shoulder surfaces are often large, but vary among projects, mainly due to the cost and proximity of suitable natural aggregate sources— the difference in cost between using virgin material and using recycled concrete aggregate.
Environmental Considerations: The potential for negative environmental impacts with RCA shoulder surfaces is relatively small and is associated with diffuse high pH surface runoff and noise/ dust from production operations. These negative impacts are generally offset by reductions in impacts that would result from the use of natural aggregates.
For the full 6-page PDF of the Tech Brief, please go to: http://www.cptechcenter.org/concrete-recycling/docs/RCA_in_unbound_aggregate_shoulders_TB5.pdf