Using Electrical Properties to Classify the Strength Properties of Base Course Aggregates (1341-2)

Eight different types of Texas aggregates and three Finnish aggregates were tested in order to relate their dielectric value and electrical conductivity, measured at different moisture contents and densities, to their strength and deformation properties. The dielectric value and electrical conductivity were measured in the laboratory using a surface probe and dielectric meter. The real and imaginary part of the dielectric value of aggregate fines were measured with a Surface Network Analyzer over a frequency range from 30 MHz to 3.0 GHz. Aggregate strength properties were obtained using a Dynamic Cone Penetrometer (DCP) and Resilient Modulus Tests. Aggregate suction properties and their behavior during the freeze/thaw cycles were monitored with a special Tube Suction Test and Freeze/Thaw Test developed by the authors.The purpose of this study was to evaluate what range of dielectric values and electrical conductivities can be expected when processing and interpreting Ground Penetrating Radar (GPR) survey results. Dielectric values affect the radar signal velocity and the time/thickness scale of radar profile. It was also hoped to determine if these dielectric properties of aggregates correlate with their strength and deformation properties of these materials.The result of the study showed that the dielectric value and electrical conductivity related to both the strength and deformation properties and frost susceptibility of base course aggregates. The dielectric value correlates well with the California Bearing Ratio (CBR)-value of compacted base materials. Low dielectric values (5.5 - 6.5) in compacted samples indicate the presence of thin and well-arranged adsorption water and optimum strength properties. Higher values indicate that material is sensitive to moisture, and dielectric values over 9 - 10 are "alarm values" because they can have unfrozen water in their structure when the material freezes. If the dielectric value is greater than 16 the base material will become plastic and deformation will occur in the structure. High electrical conductivity values indicate high amounts of ions dissociated to the free water, and this can cause positive pore pressure in base materials. Saturation hysteresis was also found to have a substantial effect on base strength.