Cold Recycling Study: Testing to Develop Standard Mix Design and Quality Control/Quality Assurance Testing Procedures for Cold Recycling Project Specifications (Metric Units)

This report summarizes the research to evaluate cold recycling mix design procedures and to standardize specifications for cold recycling. The study focused on a review of different test methods for specimen compaction and strength testing and different specification limits for emulsified and foamed asphalt. Important findings from the study include the following: (1) specimen densities from Marshall compaction are higher than gyratory compaction and nuclear gauge field densities, (2) gyratory compaction using 30 gyrations is typically marginally higher than field density, (3) indirect tensile strength (ITS) and stability are strongly correlated to density, (4) mix designs done with Marshall compaction may underestimate binder and cement content requirements, (5) using Marshall compaction to compact specimens for quality control testing can result in strengths that are not representative of those on the road (i.e., higher), thereby potentially meeting the passing requirement in the test, but not on the road, (6) Marshall density decreases with increasing binder and cement contents, (7) ITS results increase with increasing binder and cement content, (8) Marshall stability does not provide a meaningful measure of the material properties. Values can decrease with increasing binder and cement content, (9) ITS correlates with stiffness while Marshall stability does not, (10) ITS and Marshall stability test methods do not distinguish between different recycling agents, and (11) there was no discernible difference between emulsified asphalt and foamed asphalt in terms of strength, stability, and stiffness. The following recommendations are made based on the test results and conclusions: (1) remove Marshall compaction and stability as an option for mix design, allowing only gyratory compaction and ITS. Using 30 gyrations is considered appropriate, but agencies are encouraged to determine an optimal number of gyrations for local materials), (2) removal Marshall compaction for quality control specimen preparation; however, it is acknowledged that using gyratory compaction in the field can be problematic, and current research is investigating using vibrating hammer compaction equipment, and (3) standardize the mix design and quality control procedures for all cold recycled materials that are treated with asphalt recycling agents; differentiating between partial-depth, full-depth, and cold central plant recycling is not required.