Performance Based Measurement of Optimum Moisture for Soil Compaction

Project summary:

Subgrade materials provide the foundation for both asphalt and concrete pavements, and the compaction of these materials to optimum density is critical for the longevity of the overlying pavement. Part of the challenge of achieving maximum field density in subgrade materials is transferring the optimal compaction and moisture content data from laboratory testing to the field. This research investigated the proficiency of four different instruments for accurately predicting moisture contents of three subgrade soils (loam, silt, silty/clay) commonly used in Minnesota roadway construction projects.

The four instruments were the DOT600 (moisture content), WP4C dewpoint potentiometer (matric suction), the Button Heat Pulse Sensor (BHPS) (temperature rise vs. moisture content), and an exudation pressure test device. The DOT600 showed a strong correlation between the output period (measured in microseconds) and volumetric water content. The WP4C did not accurately measure matric suction for any of the loam, silt, or silt/clay soils at suctions below 250 kPa. Published data show that the matric suction of soils compacted at optimum moisture content is usually in the range of 200-300 kPa. The BHPS showed a strong correlation between measured temperature rise and water content, but in its current configuration, it is not rigorous enough to withstand field conditions. The exudation pressure device was applied to soils compacted in an AASHTO T99 mold at various moisture contents. Water was exuded from the packed samples at pressures between 100 and 500 psi corresponding to AASHTO-T99 moisture contents of 10 to 25 percent. Accurate moisture content readings from any of these instruments may not be as important as a more precise and simple calibration between the measurement units of the instrument and the optimum moisture content determined from the AASHTO T99 test.