University of California Pavement Research Center

The California Department of Transportation (Caltrans) has been using full-depth recycling (FDR) as a pavement rehabilitation strategy since 2001. Early projects were recycled with foamed asphalt and cement. Cement-only treatments were permitted from 2015 to improve the properties of more marginal materials. However, shrinkage cracking associated with the hydration and curing of the cement-treated layers remains a concern, especially with regard to crack reflection through asphalt concrete surfacings and the related problems caused by water ingress.Crack mitigation has been studied for decades, 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 48 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 were first implemented by the Texas Department of Transportation and then later by other state departments of transportation. However, longer-term monitoring on a range of projects in Texas, California, and other states had revealed that microcracking has not always been successful in preventing cracking. Some projects showed reflected transverse and block cracks in a relatively short time period, attributable to a number of factors including but not limited to cement content, cement spreading, the method of curing, and the interval between base construction and placement of surfacing.Discussions with researchers in Texas indicated that additional research was 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 used to assess the degree of microcracking, and the effects of early opening to traffic. A multiphase project was therefore initiated at the University of California Pavement Research Center (UCPRC) to investigate these outstanding issues. The second phase of this study is discussed in this report. This phase covered the design, construction, monitoring, and associated laboratory testing of a 37-cell test road to evaluate shrinkage crack mitigation procedures. The study found that microcracking is an effective mitigation measure, provided that design strengths do not exceed 600 psi (4.1 MPa) and that microcracking is done between 48 and 56 hours after compacting the layer.