This synthesis will be of interest to pavement designers, construction engineers, and others interested in economical methods for reconstructing or rehabilitating bituminous pavements. Information is provided on the processes and procedures used by a number of states to recycle asphalt pavements in place without application of heat. Since 1975 a growing number of state highway agencies have reconstructed or rehabilitated asphalt pavements by recycling the old pavement in place. This report of the Transportation Research Board describes the processes used for cold in-place recycling, including construction procedures, mix designs, mixture properties, performance, and specifications.

Cold in-place asphalt recycling (CIR) has proven to be an effective technique for the rehabilitation of asphaltic concrete pavements. Though the process provides economic and performance benefits, engineers and contractors may be unfamiliar with project selection, design, and construction procedures. Some projects designed as CIR projects have failed during the construction process. After such failures occur, alternate rehabilitation methods must be used. This is expensive and inconvenient for both the contractor and the contracting agency. Subgrade stability problems can prevent a CIR project from being successful. After the existing asphalt is milled for recycling, construction equipment is only supported by the subgrade materialand the little asphalt that remains.

Cold in-place recycling (CIR) has become an attractive method for rehabilitating asphalt roads that have good subgrade support and are suffering distress related to non-structural aging and cracking of the pavement layer. Although CIR is widely used, its use could be expanded if its performance were more predictable. Transportation officials have observed roads that were recycled under similar circumstances perform very differently for no clear reason. Moreover, a rational mix design has not yet been developed, design assumptions regarding the structural support of the CIR layer remain empirical and conservative, and there is no clear understanding of the cause-effect relationships between the choices made during the design/construction process and the resulting performance. The objective of this project is to investigate these relationships, especially concerning the age of the recycled pavement, cumulative traffic volume, support conditions, aged engineering properties of the CIR materials, and road performance. Twenty-four CIR asphalt roads constructed in Iowa from 1986 to 2004 were studied: 18 were selected from a sample of roads studied in a previous research project (HR-392), and 6 were selected from newer CIR projects constructed after 1999. This report describes the results of comprehensive field and laboratory testing for these CIR asphalt roads. The results indicate that the modulus of the CIR layer and the air voids of the CIR asphalt binder were the most important factors affecting CIR pavement performance for high-traffic roads. For low-traffic roads, the wet indirect tensile strength significantly affected pavement performance. The results of this research can help identify changes that should be made with regard to design, material selection, and construction in order to improve the performance and cost-effectiveness of future recycled roads.