This session will provide an overview of recent research supporting mitigation of environmental concerns associated with use of recycled concrete aggregates in new infrastructure. Both bound and unbound uses will be included in the presentations, with a focus on potential impacts to water and air quality, as well as other community impacts. Case studies of the successful specification, use, and monitoring of recycled concrete aggregates will be presented, with a focus on strategies to address and mitigate potential environmental concerns.
Learning Objectives:
(1) Understand the impacts of novel recycled materials on concrete mechanical properties and durability performance;
(2) Understand the ability of RCA to serve as a construction material that can sequester carbon;
(3) Understand techniques that can be used to protect the environment when using RCA in unbound applications;
(4) Understand techniques that can be used to characterize RCA and concrete containing RCA, including mechanical properties and fresh concrete performance.
Eco-mechanical Performances of Precast Concrete Slabs Containing Recycled Concrete Aggregate and Steel Fibers
Presented By: Alessandro Fantilli
Affiliation: Polytechnic University of Turin
Description: To build environmentally friendly public constructions, authorities impose to tailor concrete mixtures with a minimum content of recycled materials. To reduce the environmental impact of frame structures, whose mass is mainly distributed on horizontal diaphragms, it is necessary to draw attention to the slabs of floors. As ready-mixed concrete with recycled materials is not easily available on the market, partially prefabricated one-way slabs, composed by both cast-in-situ concrete and precast plates (generally called predalles) were investigated. Only the precast concrete of the predalles contained recycled materials, and in particular recycled concrete aggregate (RCA) and recycled steel fibers (RSF). These were used to cast full-scale one-way slabs, subsequently tested in three-point bending. Test results and numerical analyses revealed an equivalence between the traditional rebar and RSF. The latter effectively compensated for the loss of flexural strength that the substitution of virgin materials produced. In this way, both GPP and the mechanical performances can be satisfied, though the cast-in-situ concrete does not contain any recycled material.
Carbonation-Treated RCA: Advancing Sustainability and Performance in Concrete
Presented By: Jiong Hu
Affiliation: University of Nebraska-Lincoln
Description: This presentation explores the dual environmental benefits of utilizing recycled concrete aggregate (RCA) through carbonation treatment. Drawing on recent laboratory and large-scale studies, we examine how varying CO2 pressures impact the carbon sequestration capacity of RCA and its influence on the performance of concrete. Our findings show that treating RCA under elevated CO2 pressures (20–60 psi) significantly enhances carbonation efficiency, resulting in higher CO2 uptake and improved RCA characteristics such as reduced water absorption and increased aggregate strength. When used in concrete, CO2-treated RCA demonstrates comparable mechanical and durability properties to those of concrete made with natural aggregate. Additionally, a life-cycle CO2 footprint analysis confirms that CO2-treated RCA can lead to a net reduction in carbon emissions. This research highlights carbonation treatment as a viable pathway for improving RCA quality while contributing to more sustainable and climate-conscious concrete construction.
Evaluation of Environmental Impacts of Recycled Concrete Aggregate in Highway Applications
Presented By: Qingli Dai
Affiliation: Michigan Technological University
Description: This study evaluated the short- and long-term environmental impacts of using recycled concrete aggregate (RCA) blends in highway applications, including pavement base layers, pipe bedding, and edge drain outfalls. The objectives were to determine leachate pH values, total dissolved solids (TDS), and how these parameters vary with precipitation or flow rates. The study also examined the influence of RCA type, blend ratios, and soil buffering capacity under various field conditions. Laboratory-based modified column leaching tests were conducted on RCA and virgin aggregate blends with different RCA percentages. Leachate samples were analyzed over time for pH, TDS, and elemental concentrations. Additional column tests used effluent solutions with controlled pH levels to assess the neutralization capacity of various soil types and layer thicknesses. Flow rates were adjusted based on the permeability of test samples, and pH changes between inflow and outflow were tracked. The buffering performance of sands and natural aggregates in the subbase was also evaluated. Complementing the column tests, lab-simulated field leaching experiments were performed to assess leachate behavior under more realistic environmental exposures. These included non-continuous precipitation tests to replicate field carbonation effects and seasonal wetting patterns. The results from simulated field tests aligned well with
column test findings, confirming that both RCA blend composition and subgrade material properties significantly affect leachate chemistry. The influence depth and buffering effect of various soils were quantified to estimate safe separation distances between RCA layers and groundwater.
Advanced Characterization of Surface-Treated Recycled Concrete Aggregate Using Microstructural and Mechanical Methods
Presented By: Aline Cristiane Kumpfer Nascimento
Affiliation: North Carolina State University
Description: Construction and demolition (C&D) waste represents a significant environmental challenge, with concrete waste comprising a major portion. Recycling this material into recycled concrete aggregate (RCA) offers a sustainable alternative to landfill disposal and reduces the demand for virgin aggregates. However, RCA typically exhibits higher porosity, lower density, and increased water absorption due to the presence of adhered mortar, which can compromise the interfacial transition zone (ITZ) in new concrete. This study investigates the effectiveness of surface treatment using a pozzolanic slurry made from residual rice husk ash (RHA), an agro-industrial byproduct rich in amorphous silica. RCA samples were treated with the RHA slurry and evaluated through a comprehensive set of novel and advanced characterization techniques. These included water absorption, porosity, X-ray diffraction (XRD), and scanning electron microscopy (SEM), with particular focus on microstructural analysis. Mechanical performance was assessed through abrasion resistance testing, and the ITZ was evaluated using Vickers microhardness measurements. Results showed that RHA treatment significantly improved the physical and mechanical properties of RCA and enhanced the hardness and quality of the ITZ. A Life Cycle Assessment (LCA) was also performed to quantify the environmental benefits of using RHA, reinforcing the potential of this method to support sustainable, performance-based concrete design using treated RCA.
Fresh-State Characterization of Low-Carbon Concrete Mixes Incorporating Recycled Concrete Aggregate
Presented By: Leandro Sanchez
Affiliation: University of Ottawa
Description: The growing demand for sustainable construction practices has driven interest in low-carbon concrete and the use of recycled aggregates. Recycled concrete aggregate (RCA), a multiphase material composed of original virgin aggregate (OVA) and residual mortar (RM), presents unique challenges in concrete design due to the influence of RM on both fresh and hardened properties. To quantify the RM content, a combined method involving chemical attack (26% sodium sulfate solution) and freeze-thaw cycles is employed to detach the RM. This study evaluates four concrete mix designs: conventional concrete (CC), direct RCA replacement (DRM), equivalent mortar volume (EMV), and equivalent volume (EV). While EMV and EV mixes account for the RM phase and exhibit mechanical properties comparable to CC, their fresh-state behavior can be adversely affected. To better understand these effects, the traditional slump test is compared against rheological measurements (yield stress and viscosity), providing a more comprehensive assessment of flowability. The findings contribute to optimizing RCA-based mix designs for sustainable concrete.