Assessing the Chemical and Physical Characteristics of Aged Recycled Concrete Aggregate in Road Construction

Abstract

EXECUTIVE SUMMARY The use of recycled concrete aggregates (RCA) in pavement construction applications is well established and successful, particularly its use as a granular and/or stabilized base course. However, leaching of harmful substances, such as heavy metals, has been identified. The leaching mechanisms are strongly dependent on pore water pH conditions, and therefore the carbonation level (aging) of the concrete material. As a result, developing an understanding of the chemical and physical processes involved in the aging of RCA is crucial in assessing its environmental impacts. This thesis examines the chemical and physical properties of aged RCA recovered from the MnROAD highway construction study site, in central Minnesota. In Chapter 1, a review of the existing body of academic and professional literature is provided. This chapter describes fundamental misconceptions of concrete and leachate chemistry in the existing literature. Furthermore, Chapter 1 is intended to provide the reader with sufficient knowledge of the concepts important to understand the physical and chemical processes involved in aged RCA use. Chapter 2 is a forensic examination of the physical and chemical characteristics of an aged RCA material, which was initially characterized by Chen at al. (2012, 2013) prior to an eight-year field deployment at a research facility in Minnesota. Chapter 2 provides a unique opportunity to determine the changes in RCA, used as subbase material, over a significant period of field deployment. Chapter 3 of this thesis is a study of the impact of contact time on RCA leachate pH and alkalinity in a laboratory setting. This chapter aims to address some oversights in conventional laboratory methodologies traditionally used to determine leachate chemistry, by more effectively simulating field conditions.

The literature review conducted in Chapter 1 identified that the existing body of work, while extensive, is generally lacking on several fronts. Particularly, previous laboratory-based studies conducted on the topic have used either contact times, or liquid to solid-ratios that are not representative of real-world RCA applications. Furthermore, the existing academic and professional literature characterizing leachate produced from contact with RCA uses the terms “high pH” and “alkaline/alkalinity” interchangeably. This subsequently leads to a great deal of confusion regarding the chemistry of RCA leachate, which has both a high pH and is high in alkalinity The forensic examination of the aged MnROAD RCA, conducted in Chapter 2, showed an increase in the acid neutralization capacity (ANC) of the material over the eight-year field deployment. From the ANC curves, the finer particles continued to show a higher acid neutralization capacity than the coarser particles. Furthermore, the ANC curves indicated significant carbonation in the fine fractions of the aged MnROAD RCA. The increase in carbonation was supported by X-ray diffraction and thermogravimetric analyses. The assessment of the impact on leachate chemistry of contact time with aged MnROAD RCA, conducted in Chapter 3, showed a slight increasing trend of effluent pH over time. Furthermore, a comparison of the effluent pH over a six-hour test duration showed significantly lower results, for both base course RCA and subbase soils, than material pH values from conventional batch test methods adapted in the study in Chapter 2. The results from this assessment supported the hypothesis that contact time and particle abrasion play a significant role in determining leachate pH in field conditions.