Summary of Minnesota Recycled Concrete Aggregate Review
September 15-18, 2003
The Minnesota Department of Transportation's (Mn/DOT) transportation goals and mission is "to improve access to the markets, jobs, goods, and services and improve mobility by focusing on priority transportation improvements and investments that help Minnesotans travel safer, smarter, and more efficiently." The Mn/DOT views recycling, and in particular the use of recycled concrete aggregate (RCA), as resulting in more efficient and effective use of highways, from the environmental and economics perspectives. Statewide use of RCA is permitted in the Mn/DOT Standard Specifications for Construction. The specifications establish that RCA can be used as coarse aggregate in portland cement concrete (PCC) in section 3137.2 B, as aggregate for surface and base courses in section 3138.2 A, and as granular material in section 3149.2.
Minnesota currently uses almost 100% of the concrete removed from its pavements as dense graded aggregate base. This material must meet the 3138.2 section of Mn/DOT specification and can include a maximum of 3% by mass of asphalt binder from recycled asphalt pavement.
From the late 1970s through the 1990s, RCA was used as coarse aggregate for PCC pavements on over 20 projects. Today, Mn/DOT uses a 60-year pavement design life on its high-volume freeways and a 35-year design life on all others. The associated requirements have limited the economic use of RCA in concrete pavements. Currently, Mn/DOT is incorporating RCA primarily as aggregate base in highways projects.
The Mn/DOT, Minnesota Office of Environmental Assistance, Minnesota Pollution Control Agency, and industry representatives that participates in this review helped identify the following benefits of using RCA in:
- Observations suggest that RCA used in the base and sub-base material performs similarly to new aggregate where recycled material is allowed. Research is underway to establish laboratory performance parameters for RCA used in aggregate base and sub-base.
- Rubblization, crack & seat, and unbonded concrete overlay have been used as reconstruction strategies. All these processes have shown to be provide good performance, being the unbonded concrete overlay the most used technique of pavement rehabilitation in the state.
- Substitution of new aggregate by RCA can provide savings in the final cost of the project.
- It is a common practice in Minnesota to crush the material on site. This lowers the transportation costs and has less effect on traffic.
- Reduction of landfill disposal material.
- Concrete from the reconstruction project is crushed and the entire aggregate product is used as a base material according to the specifications, generating no residual waste.
- Reductions of fuel consumption by eliminating haul distances when the concrete is crushed at the construction site.
- Preservation of natural aggregate resources, which were identified by Mn/DOT in a 10-year aggregate availability study as potentially in short supply. Stockpiled material in rural areas may become a viable source for the metro area.
Mn/DOT and the industry have overcome some barriers. A summary of Mn/DOT and industry's experiences are provided below.
- Recommendations provided by Mn/DOT for using RCA in state highways are:
- Washing of RCA is required if used in PCC pavements in order to eliminate excess fines.
- Quality requirements for new aggregate do not specifically apply to RCA when the pavement comes from a known source.
- In presence of drainage layers and/or perforated drainage pipes:
- A blend of RCA with new aggregate may be used as subgrade when at least 95% of the RCA is retained on the 4.75 mm sieve.
- RCA may be used up to 100% in construction of the filter/separation layer under a permeable aggregate base drainage layer in accordance with the applicable drainage specifications.
- Experiences shared by the industry are:
- There is no need to remove fines when RCA is used in absence of drainage layers and/or perforated drainage pipes, making the use of RCA more efficient.
- Recycled material coming typically from Mn/DOT's reconstruction projects may assure a consistent source of aggregate.
- Steel removal has become easier through years, generating a cleaner recycled aggregate.
- Recommendations given by the Mn/DOT Research Record of March 1995,"Uses of crushed Concrete Products in Minnesota Pavement Foundations", indicate ways for mitigating precipitate and drainage problems. These recommendations are:
- Eliminate the intentional inclusion of RCA fines (#4-minus) in drained, unstabilized pavement foundation layers.
- Design the drainage systems to accommodate the limited quantity of crusher fines, and insoluble residue that are produced by pavement bases, both natural and recycled.
- Blend open-graded RCA products with new aggregates to produce gradatings required to improved stability and density and to further reduce precipitate potential in drained pavement foundation layers.
- Use drain pipes that are either unwrapped or wrapped in filter fabrics with high initial permittivities. The report also offers a more detailed analysis of drain pipe and pavement drain specifications.
- The use of unstabilized RCA fines (#4- minus materials) should be restricted to areas that are below any drainage layers or structures.
This report also mentions that "while the effluent from RCA foundation layers is initially extremely alkaline, it has rarely been documented as being sufficiently alkaline to be considered an environmental hazard. With the effluent usually effectively diluted at a short distance from the drain outlet, it seems likely that environmental concerns are probably restricted to a very small region in the vicinity of the drain outlet."
- Establishment of Environmental Policy:
- RCA is being included in a permanent rule relating to Beneficial Use of Solid Waste, where RCA will be considered a standing beneficial use and not subject to review or permitting by Pollution Control Agency.
- Beneficial Use of Solid Waste rule will be instrumental in establishing a database of information on other non-RCA recycled source materials, conditional uses, evaluation process, and stockpiling requirements.
- Lack of data and base line information on effluent leachate and particulate quality was considered a potential barrier in light of new NPDES and TMDL rule or other local regulations.
Mn/DOT and industry suppliers are still looking to gain additional experience or information in the following areas:
- Development of performance curves for concrete made with recycled aggregate.
- Development of database for RCA final product performance.
- Research related to:
- Minimize reflective cracking in pavements built over thick RCA base due to increased base stiffness.
- Understand of recycled aggregate products; how they are affected in terms of strength, constructability, and long-term performance.
- Incorporate RCA affected by D-cracking and ASR and in what proportions.
People in Attendance at Discussion Groups
Environmental and Planning. September 17, 2003.
||Planner Principal State. Office of Environmental Assistance. MN.|
||Environmental Planner/ Rules Coordinator. Major and Remediation Division. Minnesota Pollution Control Agency.|
||Unit Supervisor. Environmental Compliance and Investigation Unit Office of Environmental Service. Mn/DOT.|
||Section Director. Environmental Analysis and Compliance Section. Office of Environmental Service. Mn/DOT.|
The following is a list of the materials that have been provided to the review team:
- Physical and Mechanical Properties of Recycled PCC Aggregate Concrete. Interim Report - Task A. Prepared by Mark B. Snyder, Julie M. Vandenbossche. Department of Civil Engineering at University of Minnesota, and Kurt D. Smith, Monty Wade, ERES Consultants, Inc., Champaign, Illinois. August 1994.
- "Uses of Recycled Portland Cement Concrete State-of-the-Art". Final Report-Volume II. An Annotated bibliography of publications by American and Selected Foreign Authors. Prepared by Lisa A. Tunison, Julie M. Vandenbossche, and Dr. Mark B. Snyder. Department of Civil end Environmental Engineering at Michigan State University. Prepared for Michigan Concrete Paving Association and Michigan Promotion Fund, Inc. October 1993.
- A Scoping Study of Recycled Concrete as an Aggregate Base. Internal Report. Minnesota Department of Transportation. Office of Material and Road Research. Updated September 11, 2002.
- List of Recycled Concrete Projects in Minnesota.
- Mn/DOT Standard Specifications for Construction Books. On-line Reference. www.dot.state.mn.us/tecsup/spec/index.html
- Mn/DOT On line Reports. www.mrr.dot.state.mn.us/research/MnROAD_Project/MnRoadOnlineReports.asp
- "Uses of crushed Concrete Products in Minnesota Pavement Foundations". Report Number 96-12. Mark B. Snyder. Minnesota Department of Transportation. March 1995.
- Proposed Permanent Rules Relating to Beneficial use of Solid Waste. Draft. Pollution Control Agency, Minnesota.
- "Values, Goals, and Principles". Pawlenty-Molnau Administration 2003-2007.
- "Helping Minnesotans Travel Safer, Smarter, and more Efficiently." Strategic Plan 2003. Minnesota Department of Transportation.
The following is a list of sites visited:
- I-694 Reconstruction Project in Brooklyn Center, MN. It was observed the recycled concrete aggregate material placed in the base.
This summary will be posted at FHWA web site: www.fhwa.dot.gov/pavement/recycling