Evaluation of Laboratory and Field Produced Cold Recycled and Full Depth Reclaimed Asphalt Pavement Materials

Abstract

Cold recycling (CR) and full depth reclamation (FDR) are sustainable processes of recycling existing pavement materials to construct stronger stabilized base layers. The recycled asphalt pavement’s materials vary in composition, and research is necessary to study performance differences with respect to cracking resistance, rutting, stability, and indirect tensile strength considering the material’s physical and chemical properties. Hot mix asphalt (HMA) has had decades of research into national and worldwide standards for designing a mixture for all types of pavement performance requirements. The performance tests designed for HMA can identify potential deficiencies in the mixture during the design phase, sparing time and resources in constructing a pavement that would fail before its design life is achieved. Cold recycling and FDR currently do not have widely implemented national standards for mixture designs, and there are many agencies and associations that have their own recommend practices. Additionally, performance testing of CR and FDR mixtures in a laboratory setting are not as widely researched and developed as they are for HMA mixtures. Many CR and FDR field projects are constructed to find empirical evidence from field performance as it relates to different variables and outcomes recorded. A mechanistic-empirical approach is needed to describe the physical performance with empirically derived equations from laboratory performance tests. In 2019, the Minnesota Department of Transportation (DOT) Road Research Project (MnROAD) constructed 16 test sections comprised of various CR and FDR mixtures, as well as with multiple HMA mill-and-fill control sections. Two CR mixture processes, Cold in-place recycling (CIR) and cold central plant recycling (CCPR), as well as FDR were recycled and stabilized using foamed or emulsified asphalt. The new mixture was then paved as a strengthened iii recycled or stabilized base layer and overlaid with an HMA surface. The intention of the MnROAD project is to quantify the benefits of these pavement preservation techniques. The sections’ recycled and reclaimed mixtures were sampled during the construction phase. Some samples were compacted in a mobile laboratory using equipment from the National Center for Asphalt Technology (NCAT), to be tested for dynamic modulus. Multiple specimens were mixed and compacted at the main NCAT laboratory for the testing of indirect tensile strength, Marshall stability, cracking resistance, and rutting potential. Additional samples were mixed and compacted at NCAT and cut to large and small specimen sizes for additional dynamic modulus testing. Field cores were also recovered from the 70th Street CR and FDR test sections used to determine their dynamic modulus. Investigating the types of specimens used and their dynamic moduli allows for validation into whether difference in mixing and compaction methods affect the validity the test section laboratory results as compared to the field. A statistical analysis on the similarities of the mixing and compacting methods was performed. From the results it was found that some of the 70th Street CR test sections’ dynamic moduli correlated well to laboratory mixed and compacted sample’s types. At certain temperatures, typically ambient to high temperatures, the dynamic moduli were similar for all mixing and compacting methods. The small-scale laboratory mixed and compacted specimen did not equate to the field core dynamic moduli for most CR and FDR sections. It was found that at cold pavement temperatures, the dry density may be a good predictor for the dynamic modulus of CR mixtures.