Bituminous mixtures utilizing aggregates falling outside current gradation specifications were used as wearing courses on several low volume county roads in three counties in Minnesota. Evaluation consisted of visual observations, including photographic records, and frictional resistance testing. After five years of service, it appears that the use of finer gradation aggregates will result in satisfactory bituminous wearing courses on low-volume roads.
The main purpose of this investigation was to determine the amount of degradation that occurs during construction of granular bases and the amount which takes place while in service under a bituminous surface. A second objective was to determine which laboratory tests could be used to identify the materials which would degrade the most.
Six granular base projects were selected for testing. Six different aggregates were tested and results analyzed for degradation. Assessment of degradation was based on changes in the percent passing the No. 200 sieve of samples taken as produced, after compaction and after being placed in service.
Some of the more important findings and conclusions of this study are:
1. Some degradation was noted on all six projects, occurring both during compaction and under traffic.
2. None of the laboratory tests conducted were found to be suitable for indicating aggregates which degrade the most.
