Effectiveness of open-graded base layer on subgrade moisture regime and overall pavement performance

Rockcap (open-graded crushed aggregate with a nominal maximum size of 2.5 in.) base layer is commonly used in the construction of asphalt pavements in Idaho. The effectiveness of the rockcap layer on the subgrade moisture regime and the overall pavement performance were evaluated. Two pavement sites were selected in northern and southern regions of the state. At each site, two pavement sections that are identical and adjacent to each other were instrumented by temperature, frost, and moisture sensors using time domain reflectometry (TDR) probes. One was constructed by a well-graded 4/3-in. aggregate base, whereas the other was constructed by the rockcap base. Data collected over approximately 4 years included moisture, temperature, frost condition, climatic information, groundwater level, and deflections using a failing weight deflectometer for structural support evaluation. These data were analyzed to assess effects of the rockcap layer on the variation of moisture in the subgrade and on the overall structural capacity of the pavement system. Results showed that the rockcap layer helped reduce the subgrade moisture content in pavements with daylight ditch drains, and it showed an increase in subgrade moisture for sections where the rockcap was confined by native soil and did not continue to side drains. However, performance analysis showed that sections with rockcap layer were always stronger than sections with aggregate base, even when the subgrade moisture content under rockcap was greater. The predicted rutting life for pavement sections with rockcap base layers was approximately five times greater than for sections with aggregate base.