Determining Timing of Overlay on Cold In-Place Recycling Layer: Development of New Tool Based on Moisture Loss Index and in Situ Stiffness

Cold in-place recycling (CIR) is one of the most popular rehabilitation techniques in Iowa. The Iowa Department of Transportation has increased the minimum moisture criterion from 1.5% to 2.0% because many overlays have been successfully applied on the CIR layer with moisture contents above 2.0%. However, a contractor must make daily measurements of the moisture content with a nuclear gauge. It is difficult to achieve 2.0% moisture content during a rainy summer in Iowa. For the curing issue to be addressed systematically, the moisture contents and temperatures of one project of CIR with emulsified asphalt and four projects of CIR with foamed asphalt were monitored. Capacitance moisture and temperature sensors were embedded at the midpoint of the CIR layer to measure moisture contents and pavement temperature. A GeoGauge was used to measure the stiffness. The field data obtained from the embedded moisture sensors and from a weather station were used to develop a moisture prediction model as a function of the initial moisture content, CIR layer temperature, wind speed, and humidity. With the developed moisture prediction model, a contractor can predict optimal timing for placement of the wearing surface without continually measuring moisture contents of the CIR layer with a nuclear gauge. The stiffness of the CIR layer measured with the GeoGauge was found to continue to increase as the curing period increased, despite an increase in moisture content caused by rainfall. The stiffness measured by the GeoGauge steadily increased during the curing period until it reached a value of 30 MN/m. For the optimal timing of an overlay to be determined, the stiffness, rather than the moisture, of the CIR layer should be measured.