Fatigue and Healing Characteristics of RAP Mixtures Ramya Sri Mullapudi 1 ; Satya Lakshmi Aparna Noojilla 2 ; and Kusam Sudhakar Reddy, Aff.M.ASCE 3 Abstract: The healing properties of bituminous mixes prepared using reclaimed asphalt pavement (RAP) material are different from those of virgin mixes because the chemical characteristics of RAP binder blends are different (depending on the extent of aging) compared to virgin binders. Very limited research work has been reported in the literature on the healing potential of RAP mixtures. The current study aims to examine the fatigue and healing characteristics of RAP mixtures with varying rest periods between loads. To this end, different blends of RAP and virgin binders (VG30) were prepared at varying proportions (0%, 15%, 25%, 35%, and 45%) of RAP material in mix. The fatigue lives of different RAP mixes were evaluated by varying the rest period (400, 650, 900, and 1,400 ms) between load pulses. The healing potential of the mixes was evaluated in terms of the slopes of the lines drawn between fatigue life and rest period. The fatigue life of a mix increased with increases in the proportion of RAP in the mix. The degree of healing indicated by the slope of log (fatigue life) versus log (rest period) line was found to depend on RAP content and rest period. Mix time lag (between the peaks of stress and strain), a viscoelastic property of the mixes, was determined from the stiffness modulus test. Different rheological and chemical characteristics of the binder blends and the time lag values of the mixes were correlated with the fatigue lives and healing indexes of the mixes. The Superpave binder fatigue parameter (G Ã sin δ) and the mix time lag were observed to have good correlation with the fatigue life of RAP mixes. Surface free energy and different aging indexes of the binders (ICO and ICH) showed good correlation with the healing index of the mixes. Flow (consistency) parameters, such as softening point and viscosity at 60°C, correlated well with the healing index. These correlations are expected to improve understanding of the fatigue and healing behavior of RAP mixes. DOI: 10.1061/(ASCE)MT.1943-5533.0003484. © 2020 American Society of Civil Engineers. Author keywords: Reclaimed asphalt pavement (RAP) material; Surface free energy; Fourier transform infrared spectroscopy (FTIR); Aging index; Mix time lag; Healing index. Introduction Recycling has been an important pavement rehabilitation option for many decades. The materials already used once to construct asphalt pavements can be reused to repair, reconstruct, and maintain them (Taylor 1978; Kim et al. 2007; Copeland 2011). Mixes prepared with reclaimed asphalt pavement (RAP) material were reported to have similar, if not better, performance compared to conventional mixes. With an increase in the proportion of RAP in a mix, the stiffness and rutting resistance of the mix generally increase if no rejuvenators are used to soften the aged binder (Kim et al. 2007; Xiao et al. 2007; Zhou et al. 2011; Bernier et al. 2012; Boriack et al. 2014). However, contradictory results have been reported regarding the effect of RAP content on the fatigue cracking resistance of RAP mixes. Some researchers reported that fatigue cracking resistance increased with increases in RAP content (Widyatmoko 2008; Hajj et al. 2009; Tabakovic et al. 2010; Pasetto and Baldo 2017), while others (Boriack 2014) noted that an increase in RAP content led to a decrease in fatigue life and attributed this primarily to the increased brittleness of the mix due to the addition of aged binder from RAP material. Repeated application of traffic loads on pavement produces mi- crocracks, which coalesce to form macrocracks. The influence of various factors, such as mixture parameters (volumetric proportions of aggregate, filler and effective binder contents, binder age and quality, mixture stiffness), temperature, frequency of loading, du- rations of loading, and rest periods, on the fatigue behavior of as- phalt mixes have been studied in detail by a number of researchers (Epps and Monismith 1972; Raithby and Sterling 1972; Gibb 1996; Cooper and Pell 1974; Barksdale 1978; Bolk et al. 1982; Moutier et al. 1988). Pavements are subject to traffic load pulses of different durations and varying rest periods. The rest periods available be- tween successive load pulses give bituminous mixes the opportu- nity to heal and recover from damage. The significance of rest periods in enhancing the fatigue life of asphalt mix by the process of healing has been established in several studies (Raithby and Sterling 1972; Bonnaure et al. 1982; Shen et al. 2010; Shan et al. 2010; Santagata et al. 2009; MansourKhaki et al. 2015). Several attempts have been made to quantify the healing poten- tial of mixes under varying conditions. Raithby and Sterling, (1972) and Van Dijk and Visser (1977) examined the effect of rest period on the fatigue resistance of bituminous mixes and concluded that the factor by which fatigue life increases may range from 1 to 10 depending on the rest period provided. Francken (1979) developed an empirical relationship between the fatigue lives of mixtures tested in continuous mode and those obtained by introducing rest periods. This relationship was verified by Awanti et al. (2007). Knowledge of the degree of healing occurring in the mix during periods of rest is useful for predicting the fracture damage and fatigue life of asphalt pavement. 1 Research Scholar, Dept. of Civil Engineering, Indian Institute of Tech- nology, Kharagpur, Kharagpur, West Bengal 721302, India (corresponding author). Email: ramyamullapudi.nitw@gmail.com 2 Research Scholar, Dept. of Civil Engineering, Indian Institute of Tech- nology, Kharagpur, Kharagpur, West Bengal 721302, India. 3 Professor, Dept. of Civil Engineering, Indian Institute of Technology, Kharagpur, Kharagpur, West Bengal 721302, India. Note. This manuscript was submitted on December 30, 2019; approved on June 11, 2020; published online on September 28, 2020. Discussion period open until February 28, 2021; separate discussions must be sub- mitted for individual papers. This paper is part of the Journal of Materials in Civil Engineering, © ASCE, ISSN 0899-1561. © ASCE 04020390-1 J. Mater. Civ. Eng. J. Mater. Civ. 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