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.

This first-level report describes the first two phases of a study comparing the performance of four different full-depth pavement reclamation strategies, namely pulverization with no stabilization (FDR-NS), stabilization with foamed asphalt and portland cement (FDR-FA), stabilization with portland cement only (FDR-PC), and stabilization with engineered asphalt emulsion (FDR-EE). A literature review, the test track layout and design, stabilization and asphalt concrete mix designs, and test track construction are discussed, as well as the results of Heavy Vehicle Simulator (HVS) and preliminary laboratory testing, and a forensic investigation of the HVS test sections. A number of problems were experienced during construction of the FDR-PC and FDR-EE lanes on the test track and consequently only the FDR-NS and FDR-FA lanes and one section of the FDR-PC lane (5 percent measured cement content) were considered satisfactorily uniform for the purposes of HVS testing. The FDR-FA and FDR-PC sections performed very well under both dry and wet conditions, with testing under dry conditions terminated long before the terminal rut of 0.5 in. (12.5 mm) or average crack density of 0.75 ft/ft2 (2.5 m/m2 ) were reached (cracks were observed on the wet tests only). The two FDR-NS sections tested performed acceptably, with a section with thicker asphalt surfacing (0.4 ft [120 mm]) outperforming a section with thinner asphalt surfacing (0.2 ft [60 mm]) under both dry and wet conditions, as expected. Terminal rut was reached on both sections. No cracks were observed during dry tests, but cracking was severe after completion of the wet tests. Observations during a forensic investigation confirmed the measurements taken during HVS testing. The performance advantages of full-depth reclamation strategies that either use foamed asphalt with cement or cement only over those with no stabilization are clearly evident from the results. Additional laboratory and field testing is in progress to collect sufficient data for the development of mechanisticempirical design criteria (and revised gravel factors) for full-depth reclaimed pavements. However, there is sufficient evidence to show that pavements that are rehabilitated using full-depth reclamation strategies will satisfactorily withstand design traffic levels common in California. Rehabilitation using this approach offers additional advantages of speed of construction, minimal disruption to traffic, reuse of all materials, and does not require removal of material from the site. FDR with these stabilization approaches also effectively replaces extensively cracked existing asphalt layers, providing a new base and thus preventing the reflective cracking that is common in more traditional overlay projects. Future research should include life cycle cost analysis (LCCA) and environmental life cycle assessment (LCA) to compare FDR with overlay strategies for the range of pavement, climate and traffic conditions where the two strategies might be used. Future research on partial- and full-depth reclamation should be coordinated to facilitate consistent design and specification documentation, and to facilitate the preparation of a comprehensive guide covering all forms of pavement recycling.

This first-level report describes the first phase of a study comparing the performance of four different full-depth pavement reclamation strategies, namely pulverization with no stabilization (FDR-NS), stabilization with foamed asphalt and portland cement (FDR-FA), stabilization with portland cement only (FDR-PC), and stabilization with engineered asphalt emulsion (FDR-EE). A literature review, the test track layout and design, stabilization and asphalt concrete mix designs, and test track construction are discussed, as well as results of Heavy Vehicle Simulator (HVS) and laboratory testing. A number of problems were experienced during construction of the FDR-PC and FDR-EE lanes on the test track and consequently only the FDR-NS and FDR-FA lanes and one section of the FDR-PC lane (5 percent measured cement content) were considered satisfactorily uniform for the purposes of accelerated pavement testing. The FDR-FA and FDR-PC sections performed very well and testing on both was terminated long before the terminal rut of 0.5 in. (12.5 mm) or average crack density of 0.8 ft/ft2 (2.5 m/m2 ) were reached (no cracks were observed on either section). The two FDR-NS sections performed acceptably, with the section with the thicker asphalt surfacing (120 mm) outperforming the section with the thinner asphalt surfacing (60 mm), as expected. Terminal rut was reached on both sections, but no cracking was observed. The FDR-EE sections performed poorly, with terminal rut and terminal cracking both reached after a limited number of load repetitions. This poor performance was attributed to problems associated with construction, and consequently no conclusions can be drawn from the test results regarding this stabilization strategy. The advantages of using foamed asphalt with cement and cement only recycling strategies over recycling strategies with no stabilization are clearly evident from the results. A second phase of accelerated pavement testing, full-scale field testing, and additional laboratory testing are planned to collect sufficient data for the development of mechanistic-empirical design criteria (and revised gravel factors) for full-depth reclaimed pavements. However, there is sufficient evidence to show that pavements that are rehabilitated using full-depth reclamation strategies will satisfactorily withstand design traffic levels common in California. Rehabilitation using this approach is quick, has minimal disruption to traffic, reuses all materials, does not require removal of material from the site, and effectively replaces weak base layers, thus preventing reflective cracking that is common in more traditional overlay projects. Based on these conclusions, it is recommended that full-depth reclamation be promoted as an appropriate rehabilitation strategy in California. Future research on partial- and full-depth reclamation should be coordinated to facilitate consistent design and specification documentation, and to facilitate the preparation of a comprehensive guide covering all forms of pavement recycling.