Hosin "David" Lee
About the research
Cold in-place recycling (CIR) involves cold milling 3 to 4 in. of a deteriorated asphalt pavement, processing and stabilizing the millings, and relaying, compacting, curing, and covering with a hot-mix asphalt (HMA) overlay or (rarely in Iowa) a bituminous surface treatment. This report documents an investigation of CIR projects in Iowa with the objective of understanding CIR pavement deterioration processes so that improvements can be made in project selection, design, construction, and maintenance. Multiple research methods were used, including a literature review, a meta-analysis of the literature, database analysis, recovery of field pavement cores and milling samples, laboratory testing, and development of conclusions and recommendations.
Analysis of 44 CIR pavement projects from the Iowa Pavement Management Information System database showed that rutting was the distress that triggered performance concerns first, followed by longitudinal wheel path cracking and transverse cracking. Pavement distress indices typically dropped below trigger values 10 to 15 years after CIR rehabilitation, and thicker CIR and HMA layers were associated with better performance outcomes.
Laboratory testing of field cores recovered from an Iowa CIR pavement with performance issues confirmed that the flexibility of the CIR pavement layers was greater than that of the overlaid HMA layers. Since CIR layers have a relatively high air void ratio, a possible deterioration process was postulated where heavy wheel loads cause compaction rutting in the CIR layer, which in turn causes the less flexible HMA layer to longitudinally crack in the wheel path after it is forced to conform to the rutted CIR cross section. Resulting recommendations are to specify a more flexible HMA overlay, use rut filling treatments after ruts appear, and avoid selecting CIR for roads that experience many heavy wheel loads.
This report also documents a forensic case study of two low-volume roads that did not meet performance expectations and a comparison of asphalt milling gradations from CIR versus hot in-place recycling (HIR) projects. The forensic study showed that pavement thicknesses were less than planned, the pavement structure was insufficient for typical local subgrade strengths, and gradations were finer than expected. In the gradation comparison, HIR millings were confirmed to be coarser than CIR millings.