University of California Pavement Research Center

The California Department of Transportation (Caltrans) has been using full-depth reclamation (FDR) as a rehabilitation strategy since 2001. Most projects to date have used a combination of foamed asphalt and portland cement as the stabilizing agent. Recently, the increasing cost of asphalt binder coupled with the relatively complex mix-design procedure for foamed asphalt has generated interest in the use of portland cement alone as an alternative stabilizing agent, where appropriate. However, shrinkage cracking associated with the hydration and curing of the cement-stabilized layers remains a concern, especially with regard to crack reflection through asphalt concrete surfacings and the related problems caused by water ingress. Considerable research has been undertaken on crack mitigation, and a range of measures related to improved mix designs and construction practices have been implemented by road agencies. One of the most promising measures, used in conjunction with appropriate mix designs, is that of microcracking the cement-treated layer between 24 and 72 hours after construction. In theory, this action creates a fine network of cracks in the layer that limit or prevent the wider and more severe block cracks typical of cement-treated layers. Limited research to assess microcracking as a crack mitigation measure has been completed on a number of projects in Texas, Utah, and New Hampshire. Recommendations from these studies have been implemented by the Texas Department of Transportation and other state departments of transportation. Longer-term monitoring on a range of projects in Texas and other states has revealed that microcracking has not always been successful in preventing cracking, with some projects showing reflected transverse and block cracks in a relatively short time period, attributed to a number of factors including but not limited to cement spreading, method of curing, and interval between base construction and placement of surfacing. Discussions with researchers in Texas indicated that additional research is necessary to better understand the microcracking mechanism, and to identify the key factors influencing performance, including but not limited to aggregate properties, cement content, the time period before microcracking starts, layer moisture contents, roller weights and vibration settings, the number of roller passes, the field test methods and criteria to assess the degree of microcracking, and the effects of early opening to traffic. Early research into the use of “hybrid” stabilizers (cement with small amounts of asphalt emulsion, foamed asphalt, or synthetic polymer emulsions) indicates that these, in conjunction with appropriate mix designs, may further limit the severity of shrinkage cracks on projects that include cement-treated layers.