Chemistry of RCA Leachate in Base Course Applications

Uncertainty regarding the high pH, high alkalinity leachate from recycled concrete aggregate (RCA) leachate limits the use of RCA as a substitute for virgin aggregate in pavement base course. The purpose of this work is to understand the time-dependent behavior of leachate chemistry from RCA in pavement base course applications and the persistence of high pH leachate in the environment. A state-of-the-art review of the existing laboratory and field investigations of RCA leachate chemistry identifies discrepancies in field and laboratory measurements of RCA leachate pH. Critical evaluation of the existing investigations indicates that conventional laboratory methodology, which employs abrasive, closed system batch reactors, is not representative of field conditions. In order to understand the physicochemical factors that control RCA leachate pH and alkalinity the physical properties, solid phase chemistry, and time-dependent leachate chemistry were integrated into a geochemical model. The model illustrates that RCA leachate chemistry can be described by two parameters: portlandite content of RCA available for dissolution, and the availability of carbon dioxide. The fundamental understanding of time-dependent behavior of RCA leachate chemistry was extended to applications in pavement base course using RCA-leachate contact times according to AASHTO base course drainage quality standards. Contact time experiments indicate that longer contact times do not increase peak pH associated with RCA leachate pH, such that using RCA in base course applications poses no additional concern regarding drainage quality, and that RCA leachate pH will equilibrate to a near-neutral value, pH 7.7 and pH 8.5 given sufficient exposure to atmospheric carbon dioxide or soil acidity.  The findings of this study can be used to provide guidelines for practice to ensure safe and wise use of RCA base course.


The full thesis may be found in the following file:

Sanger_M.S. Thesis

Recycled Materials Web Map

Recycled Materials Web Map

The Recycled Materials Web Map (RMWM) is an on-line Geographic Information System (GIS) web application that connects producers and consumers of recyclable material was developed to assist engineers and contractors in the beneficial reuse of nonhazardous recycled materials in transportation projects. The RMWM is comprised of four core layers: providers, stockpiles, specifications, and case studies. Providers of recycled material can locate their facility and enter contact information. The stockpile layer, connected to the provider layer, allows facility managers to add or update information about their recycled material stockpiles including material type(s), application(s), availability, and cost information. Multiple stockpiles can be associated with each provider. The specification layer includes both Department of Transportation (DOT) specifications and environmental regulations pertaining to the beneficial reuse of recycled material based on specific location, material type, and application. The case study layer locates projects that successfully utilized recycled materials and includes information regarding the material type, application, volume data, and any additional documentation. The web map utilizes search capabilities to locate nearby stockpiles to minimize transportation costs that typically dictate the use of large volumes of materials. The RMWM provides key information which engineers and contractors need to successfully utilize recycled materials, thereby preserving limited natural resources and benefiting the project and society as a whole.

The RMWM is hosted at the Center for Advanced Public Safety (CAPS) at The University of Alabama and was funded through a pooled fund supporting the Recycled Materials Resource Center (RMRC) at the University of Wisconsin-Madison. During the first phase of this work, the RMWM was designed, developed, and tested with preliminary data. A beta version of the web application has been demonstrated to be a useful tool, but there are several areas of the application that need to be addressed for widespread use of this web tool. To address these areas, The University of Alabama has been contracted for a second phase of the project which will wrap up in 2019.



System Wide Life Cycle Benefits of Fly Ash

The use of recycled materials in highway construction has the potential to achieve significant benefits affecting the triple-bottom line (environment, prosperity, society). Such benefits include reducing the need for mining virgin materials and transportation (in-situ applications), reducing environmental impacts of processing and transportation, and reducing life cycle costs. Although state departments of transportation (DOTs) have been in the forefront of introducing recycled materials, they have not been able to clearly convey the benefits in a quantitative and transparent manner using easily understood metrics. The main reason for this is the difficulty in tracking the quantities of recycled materials used in state DOT projects. To better define the benefits of using recycled materials, the RMRC undertook a project with two objectives. The first objective was to develop a means of tracking and reporting quantities used in state DOT projects annually. The second objective is to provide a tool to quantitatively analyze and report the environmental and life cycle assessment of using recycled materials in highway construction. A suitable method was recommended after studying how RMRC member states currently track their recycled materials quantities. Subsequently, an LCA analysis of three environmental parameters, energy use, water consumption and CO2 emissions, showed significant environmental benefits when states used recycled industrial byproducts such as fly ash.

2015 World of Fly Ash Conference

Assessing the Life Cycle Benefits of Recycled Material in Road Construction

Life cycle assessments of recycled material use in roadways are currently not well understood or well documented. The Recycled Materials Resource Center’s (RMRC) research is aimed at quantitatively determining the environmental and economic benefits of using recycled material in road construction. Two case studies were performed to analyze the impacts of incorporating recycled material in the reconstruction of two major roadways using life cycle assessment (LCA) and life cycle cost analysis (LCCA) tools. Results from both roads show that the use of recycled materials reduces energy and water consumption, greenhouse gas emissions, and cost. Because typical roadway construction projects do not separately track the extensive use of recycled materials, the RMRC was unable to utilize the LCA and LCCA technology in the first roadway’s analysis without making significant assumptions for the inputs. To clarify and verify some of these assumptions, the second roadway project was undertaken. This second case study is being studied to determine a better methodology for data collection with fewer assumptions, in addition to assessing the benefits of recycled material use. The methodology for data collection and analysis developed through the second project can be used to conduct LCAs and LCCA for future highway construction projects with greater confidence.

Geo-Chicago 2016 


Recycled Materials as Backfill for Mechanically Stabilized Earth (MSE) Walls

Granular materials are often the product of construction operations, industrial operations, or dredging operations in rivers, ports, and harbors. Traditional sources of reinforced granular backfill in Mechanically Stabilized Earth (MSE) wall construction (e.g., from crushed rock quarries and gravel pits) can be costly and environmentally not desirable. The use of recycled materials sourced from construction, industrial, or dredging operations could be a potentially more economical and environmentally beneficial source of backfill material than traditional sources. The overall goal of this project was to facilitate use of RAP and RCA in reinforced backfills for MSE retaining wall construction.

In this study mechanical and hydraulic properties of RCA and RAP for use in Mechanically Stabilized Earth (MSE) walls were evaluated. Results show that compacted RAP and RCA provide competitive pull-out resistance for woven geotextiles and uniaxial geogrids compared to compacted natural granular materials. Construction of a structural fill containing RAP is recommended to be undertaken during summer to reduce the creep strain and creep rupture potential and improve performance of the fill.