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Bridges & Special Structures Design

Underbridge inspection units (UBIUs) that reach from above deck to below.
Department of Transportation

Bridges & Special Structures Design

Reduction of pH Levels from Roadway Underdrain Outlets

Project Number: SPR-1696

Contract Number: 2016-0065 Z4

Status: Complete

Start Date: 05/01/2018

End Date 12/31/2020


Many government and private highway agencies have identified the benefits of using recycled aggregate in highway construction. The use of recycled aggregates in highway construction, not only decreases the demand for virgin aggregates, it offers a sustainable solution. However, previous research has identified some environmental concerns of using recycled aggregate in highway construction. This research study aimed to identify and quantify the concerns and offer mitigating or alternative solutions. A comprehensive literature review, a survey of state-of-the-practice and review of MDOT collected field data identified the key environmental concerns as high pH levels (> 10-11) in the pavement drainage discharge as well as high amounts of solids deposited at the drain outlets with the potential for clogging the pavement drainage layer. A field and laboratory investigation was conducted in this study to extend an existing MDOT field study. Results of this study show that Recycled Crushed Concrete Aggregate (RCCA) bases produce leachate with high calcium ion concentrations and high pH levels. Limestone (LS) and Slag Aggregate (SA) bases produce leachate with lower levels of soluble particles and pH levels. The leachate from RCCA bases can produce calcite deposits on filter fabrics and drainage pipes and outlets. At the same time, highly alkaline leachate is known to be harmful to vegetation and aquatic life. This research shows the alkalinity of the leachate quickly dissipates within 100 feet of the drainage outlet as it likely becomes diluted by rainwater runoff. Several recommendations were developed based on the accumulative findings from the field and laboratory investigation as well as the documented practices by other states. These include recommendations for RCCA base thickness, blending of RCCA with LS and SA, washing of RCCA before use, and other suggestions for planning and construction. Some of these recommendations can be readily implemented by MDOT and other recommendations can be considered for inclusion in future specifications and special provisions.



Research Manager Project Manager Performing Organization
 Staff photo of Michael Townley    
 Michael Townley Hal Zweng  Lawrence Technological University