Application of Shakedown Theory in Characterizing Traditional and Recycled Pavement Base Materials Mingjiang Tao 1 ; Louay N. Mohammad 2 ; Munir D. Nazzal 3 ; Zhongjie Zhang 4 ; and Zhong Wu 5 Abstract: To facilitate the use of recycled materials in pavement construction, a mechanistic-based approach is required to better characterize pavement base materials. This paper demonstrates the application of the shakedown theory to characterize the behavior of traditional and recycled base materials based on laboratory repeated load triaxial RLT tests and full-scaled accelerated loading tests. A new approach that is based on dissipated energy is proposed to explain different shakedown responses of tested materials under repeated loading. The dissipated energy approach illustrated that there are two responses during RLT tests, namely, stable and unstable responses, which are dependent on the loading levels and type of tested material. It was also observed that the transition from the stable to unstable response represents gradual microstructural adjustments of tested materials to accommodate the applied loading. This gradual transition explains the difficulties in identifying some material responses within the shakedown theory based only on the permanent strain rate criteria. Based on the results of this study, a mechanistic-based design procedure to incorporate various recycled materials into pavement bases is recommended. DOI: 10.1061/ASCE0733-947X2010136:3214 CE Database subject headings: Energy dissipation; Deformation; Pavements; Base courses; Recycling; Triaxial tests; Resilient modulus. Author keywords: Dissipated energy; Permanent deformation; Pavement base courses; Recycled materials; Repeated load triaxial tests; Resilient modulus; Shakedown theory. Introduction As a consequence of increasing demands on highway systems, high quality aggregates are being depleted at a rapid pace. To achieve a sustainable development in pavement engineering, there is a global interest in using recycled and marginal materials in pavement construction. Many recycled materials, such as recycled hot mix asphalt, recycled portland cement concrete, and glass cullet, among others, have proven to perform well at different pavement layers, although their properties may be different from those of traditional unbound aggregates. However, the current specification of unbound aggregates is not yet ready to character- ize recycled and marginal materials due to its “recipe-based” na- ture. Unbound aggregates are generally characterized on the basis of physical properties such as gradation, plasticity, hardness, and durability, and stiffness and strength determined from monotonic triaxial tests. Many of these properties are determined either em- pirically or from testing procedures without properly considering the relevance to the actual performance of pavement materials under traffic loading. The new mechanistic-empirical pavement design guide requires the resilient modulus obtained from re- peated load triaxial RLT tests to characterize base course mate- rials, however, recent studies have shown that the resilient modulus alone does not properly characterize the behavior of pavement materials National Cooperative Highway Research Program NCHRP 2004; Khogali and Mohamed 2004; Puppala et al. 2005; Mohammad et al. 2006. Pavement performance vir- tually depends on both resilient and permanent deformations, which are affected by other factors, such as environmental and traffic conditions. Due to the limitations in current specifications of unbound aggregates and the limited understanding of recycled materials’ properties strength, stiffness, and resistance to permanent defor- mation under repeated loading, a performance-based character- ization procedure along with a proper performance indicator is highly desirable in using recycled materials in pavements. Several studies have been devoted to the development of a performance- based testing procedure for better characterization of unbound aggregates. Under NCHRP Project 4-23, “Performance-related tests of aggregates for use in unbound pavement layers,” Saeed et al. 2001 recommended that shear strength of unbound aggre- gates under repeated loading is the parameter that has the most significant influence on pavement performance. AUSTROADS The Association of Australian and New Zealand Road Transport and Traffic Authorities developed a performance-based testing procedure to characterize unbound aggregates as well as marginal materials, in which resilient modulus and permanent deformation are determined by conducting RLT tests AP-T29/03 AUS- 1 Assistant Professor, Dept. of Civil and Environmental Engineering, Worcester Polytechnic Institute, Worcester, MA01609 corresponding au- thor. E-mail: taomj@wpi.edu 2 Professor, Louisiana Transportation Research Center, Louisiana State Univ., Baton Rouge, LA 70808. 3 Assistant Professor, Dept. of Civil Engineering, Ohio Univ., Athens, OH 45701. 4 Administrator of Pavement/Geotechnical Research, Louisiana Trans- portation Research Center, Baton Rouge, LA 70808. 5 Research Assistant Professor, Louisiana Transportation Research Center, Baton Rouge, LA 70808. Note. This manuscript was submitted on September 14, 2007; ap- proved on October 20, 2009; published online on February 12, 2010. Discussion period open until August 1, 2010; separate discussions must be submitted for individual papers. This paper is part of the Journal of Transportation Engineering, Vol. 136, No. 3, March 1, 2010. ©ASCE, ISSN 0733-947X/2010/3-214–222/$25.00. 214 / JOURNAL OF TRANSPORTATION ENGINEERING © ASCE / MARCH 2010 Downloaded 28 Jan 2011 to 118.97.186.66. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org