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Developing a Design Procedure for Pavement Millings for New and Rehabilitated Unpaved Gravel Roads
AdvisorHand, Adam J. T.
Civil and Environmental Engineering
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Douglas County has successfully been using reclaimed asphalt pavement (RAP) millings to surface low volume roads for more than two decades. The design of millings surfaced roads has been done based on field experience. The primary objective of this study was to provide design guidelines for construction and rehabilitation of millings surfaced roads consistent with the 1993 AASHTO Guide for Design of Pavement Structures. The project started with a field distress survey through which typical pavement distresses in both conventional asphalt pavement roads and millings surfaced roads were identified in Douglas County. Historically, when asphalt concrete pavements in Douglas County have required rehabilitation or reconstruction the asphalt pavement removed (commonly RAP millings) has been stockpiled by the county and used to surface low volume roads. In order to develop the design guidelines, laboratory tests were conducted on subgrade and RAP millings materials to obtain mechanical properties. Two different soil types were identified in the Douglas County area. Resilient modulus (MR) for the subgrade sources were obtained based on AASHTO T-307. The milling surfaced roads are made of 100% RAP millings without any binder or recycling agent. It is a specialized case to investigate the behavior of RAP material for this purpose. Initially, the RAP material was considered as an unbound material and resilient modulus was obtained as it is used for determination of layer coefficients in the AASHTO pavement design method. Layer coefficient is a critical parameter in determining the structural design thickness required. Another approach considered was to treat the RAP material as a bound material, measure dynamic modulus and use it for layer coefficient determination. Dynamic modulus tests were conducted on specimens with 15% and 22% air voids. The density variation significantly affected dynamic modulus value. Structural pavement designs were carried out based on both RAP milling layer coefficients separately using the PaveXpress software. Douglas County uses two traffic design levels for low volume roads, specifically 10,000 and 50,000 ESALs. As a result, the structural design recommendations were made based on these traffic levels. Life Cycle Cost Analysis (LCCA) was used to compare millings surfaced roads with other alternatives available for low volume roads. As a result, it was found that the millings surfaced roads are significantly more economically feasible for low volume roads.