Suggested Citation:
Louw, Stephanus, David Jones, Joseph Hammack, Heather Tom, Miriam Brichta (2024)

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

. Institute of Transportation Studies, University of California, Davis, Research Report UCD-ITS-RR-25-38

Abstract:

This report summarizes the research to evaluate cold recycling mix design procedures and to standardize the specifications for cold recycling in California. Questions were developed based on a review of relevant test methods surrounding the options for specimen compaction and the difference in strength testing method and specification limits for emulsified asphalt and foamed asphalt. Important findings from the study include the following:
• Specimen densities from Marshall compaction are higher than gyratory compaction and field densities measured with a nuclear gauge and corrected for gravimetric moisture content.
• Gyratory compaction using 30 gyrations is typically marginally higher than field density.
• Indirect tensile strength (ITS) and stability are strongly correlated to density.
• Mix designs done with Marshall compaction may underestimate binder and cement content requirements.
• 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.
• Marshall density decreases with increasing binder and cement contents.
• Indirect tensile strength results increase with increasing binder and cement content.
• Marshall stability does not provide a meaningful measure of the material properties. Values can decrease with increasing binder and cement content.
• Indirect tensile strength correlates with stiffness, Marshall stability does not.
• Indirect tensile strength and Marshall stability test methods do not distinguish between different recycling agents.
• There was no discernable 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:
• Remove Marshall compaction as an option for mix design, allowing only gyratory compaction. Using 30 gyrations is considered appropriate, but agencies are encouraged to determine an optimal number of gyrations for local materials.
• The removal of Marshall compaction for quality control specimen preparation is also recommended; however, it is acknowledged that using gyratory compaction in field testing trailers can be problematic in terms of calibration requirements. Current research is investigating alternative procedures using vibrating hammer compaction equipment.
• Remove Marshall stability as a test for mix design and quality control, allowing only indirect tensile strength.
• 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.