Charles W. Schwartz, professor and chair of the Department of Civil and Environmental Engineering at the University of Maryland–College Park, presented this year’s Kent Distinguished Lecture. The lecture took place April 16 at the College of Engineering’s Micro and Nanotechnology Lab on the University of Illinois at Urbana-Champaign campus. In his talk, Schwartz discussed the structural characteristics and environmental benefits of asphalt-stabilized, cold-recycled pavement materials.
Cold recycling processes include cold in-place recycling (CIR), cold central plant recycling (CCPR), and full-depth reclamation (FDR). Foamed asphalt or asphalt emulsions are typically used as the stabilizing agent in these processes.
The use of cold-recycled materials in the rehabilitation of asphalt pavements provides a significant environmental benefit by reducing greenhouse gas emissions. These reductions accrue from the use of recycled aggregates and the need for less added asphalt content. The materials also provide energy savings by eliminating the need to heat aggregates. With in-place recycling processes, the transport of materials from plant to jobsite is reduced, which further decreases emissions.
Schwartz told the audience the greatest impediment to more widespread use of these cost-effective, sustainable pavement rehabilitation strategies is the lack of quantitative values for the engineering properties of cold-recycled materials that can be used with confidence for pavement structural design. The problem, he explained, is measuring structural properties such as dynamic modulus and permanent deformation characteristics in the laboratory. Stiffness and other structural characteristics of cold-recycled materials increase during curing in the field after placement—and realistically simulating those conditions in the lab is not easy.
Researchers have begun to address those challenges by evaluating the structural properties of cold-recycled materials under field mixed, compacted, and cured conditions. They obtained cores of materials after 6 to 12 months of field curing from more than 25 locations throughout the United States and Canada. The critical objective, said Schwartz, was to compare the performances and environmental benefits of cold-recycling processes with those of hot-mix asphalt (HMA). Dynamic modulus and permanent deformation properties were measured using a novel subcoring technique to determine stiffness and susceptibility to rutting.
The performance results are promising. The dynamic modulus values of field-cured cold-recycled materials were close to those of conventional HMA. The measured permanent deformation properties were similar to those of HMA as well, except for CIR, which showed slightly greater susceptibility to rutting. The fact that CIR/CCPR/FDR layers are usually deeper within the pavement structure, however, meant they were less critically stressed than the surface layer.
What about the environmental benefits? Schwartz says those findings are encouraging, too. Comprehensive analyses show a 46% average reduction in GHG emissions for CCPR and a 78% average reduction for CIR compared with emissions related to conventional HMA overlay rehabilitation. Even adjusting for the lower structural characteristics of cold-recycled materials, the net emissions reductions are still significant: 26% and 53% average reduction for CCPR and CIR, respectively, compared with HMA overlays.
A video of Schwartz’s presentation is available online.