Using Suction and Dielectric Measurements as Performance Indicators for Aggregate Base Materials

Abstract

A simple laboratory test for evaluating unstabilized granular base material is introduced. The test consists of monitoring the capillary rise of moisture within a 300-mm-high cylinder of compacted aggregate. The moisture conditions at the aggregate surface are monitored with a dielectric probe. A graph of surface dielectric versus time is used as the basis for performance classification. The poorest-performing materials are those that rapidly reach saturation and exhibit high surface dielectric values. The dielectric is a measure of the “free” or unbound water within the aggregate sample. It is not a simple measure of the moisture content of the material but an assessment of the state of bonding of the water within the fine aggregates. It is this unbound water that is thought to be directly related to the strength of the material and to its ability to withstand repeated freeze-thaw cycling. This test was developed by the Finnish National Road Administration and successfully used to investigate a major flexible base failure. The test is now under evaluation in Texas, where several Panhandle districts have reported cold-weather pavement cracking problems that they have attributed to the flexible bases. The test setup and equipment used are described, and test results from several base materials are presented. Also discussed are laboratory results from a recently completed project comparing eight Texas and four Finnish aggregates. One of the Finnish aggregates was classified as a poor performer; the remaining three were good performers. All of the Texas aggregates were rated as inferior to the high-quality Finnish aggregates. If this test is found to be successful in discriminating good- from bad-performing materials, it could be used to flag potentially poor performers and to evaluate the effectiveness of different base improvement techniques such as chemical stabilization or fines replacement.