Abstract:The California Department of Transportation has been using full-depth recycling (FDR) as a rehabilitation strategy since 2001. Early projects were recycled with foamed asphalt and cement, but 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.
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 had been completed on a number of projects in the United States. 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 of a range of projects 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, 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.
The research discussed in this report builds on earlier work with a focus on gaining a better understanding of microcracking mechanisms and identifying 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. In this phase of the study, a 36-cell test road was designed, constructed, and monitored to evaluate shrinkage crack mitigation procedures. The main findings from the study included revised mix design procedures based on the initial consumption of stabilizer and target unconfined compressive strengths of 450 psi (3.1 MPa) with a maximum permissible limit of 600 psi (4.1 MPa), the importance of appropriate curing, and a revised microcracking window of between 48 and 56 hours after compacting the layer. It is further recommended that a stiffness reduction target of 40% of the stiffness prior to microcracking, measured with the soil stiffness gauge, be considered instead of the current method specification.