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The focus on environmental issues and “green construction” techniques is readily apparent in today’s construction marketplace. The U.S. Green Building Council and their Leadership in Energy and Environmental Design (LEED) Green Building Rating System for New Construction program places the emphasis on use of environmentally friendly construction materials and techniques. The relative economics associated with the cost and availability of recycled materials often make it attractive for engineers and developers to specify such materials for use on their construction projects. 

Along those lines, the question of using recycled concrete as select backfill material for flexible pipes often is raised. A brief overview of the relevant factors and concerns is presented here. 

Generally, select backfill for flexible corrugated metal and plastic pipes should be a well-graded, granular, angular, durable, free-draining fill material. Locally available fill materials usually meet these criteria. However, there may be an incentive to consider the use of recycled concrete as select backfill in lieu of those more typical backfill materials. Prior to choosing to use recycled concrete as backfill, the following should be considered: 

  • The recycled concrete must be processed to ensure it conforms to particle size limits and is consistent in particle size and gradation – for all such material supplied to the jobsite.
  • It should be well-graded and angular. Ideally, it should meet the gradation of an AASHTO M145, Classification A-1-a gradation. 
  • Maximum particle sizes should be less than 3”.
  • It should be a durable material, capable of withstanding the rigors of placement, compaction and the anticipated environmental exposure at the site – generally meeting an LA Abrasion Test value of 50% maximum loss. 
  • This fill must be free of contaminants and materials that could prove corrosive to the pipe – such as road salts, some fly ashes or black rebar fragments (in the case of aluminum pipe). 
  • The fill must be properly placed, adequately compacted and tested for density. 
  • A suitable filter fabric may be required to separate this material from adjacent soil or fill that is finer grained to prevent soil migration. 

Assuming attention is given to these key factors, and pending approval of the project engineer and the site geotechnical engineer, recycled concrete may be considered for use as select fill – thus providing an economical source of backfill for the contractor while offering environmental advantages and possibly contributing to LEED’s credits for the project.

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