Recommendations for Standardizing Mix Design and Quality Control Testing Procedures for Asphalt-Treated Cold Recycled Materials

This paper summarizes a multiphase research project to standardize cold recycling specifications in California. The specifications for partial depth recycling (PDR), or cold in-place recycling, and cold central plant recycling (CCPR), using emulsified asphalt (EA) or foamed asphalt (FA), were developed independently, resulting in different mix design and quality control (QC) procedures for the two recycling agents. Most specifications require the use of Marshall stability for EA materials and indirect tensile strength (ITS) for FA materials. The specifications allow the use of gyratory compaction with 30 gyrations or Marshall compaction with 75 blows per face. This paper showed that, based on a review of 92 PDR and CCPR projects, the specification for EA produces mixes with lower binder and active filler contents. The two compaction methods were compared using field projects and laboratory testing, with the results showing that Marshall compaction overestimates density and, therefore, strength and stability, resulting in mixes with lower equivalent binder and active filler contents compared with gyratory, and facilitating passing minimum strength/stability criteria for QC. The comparison of the testing methods, using two different recycled asphalt pavement sources, and comparing the results with dynamic modulus results, indicated that Marshall stability does not produce a meaningful measure of the material properties. This paper recommends standardizing specifications for the EA and FA by removing Marshall compaction and Marshall stability, and adopting gyratory compaction and the ITS for mix design and QC testing. Attempting correlations between the different compaction and test methods is not considered appropriate.