Investigating impacts of warm-mix asphalt technologies and high reclaimed asphalt pavement binder content on rutting and fatigue performance of asphalt binder through MSCR and LAS tests
Introduction
Warm-mix asphalt (WMA) technologies, initially introduced in Europe in the late 1990s, have been extensively adopted by the asphalt paving industry due to the pursuit of the sustainable pavement systems with prominent health, environmental and economic benefits (Kheradmand et al., 2013, Rubio et al., 2012, Shiva Kumar and Suresha, 2018, Yu et al., 2017). As fast emerging processes and products, WMA technologies allow asphalt mixtures to be mixed and compacted at temperatures approximately 20–40 °C lower than those typically applied to conventional hot-mix asphalt (HMA), which effectively reduces the emissions of greenhouse gas and asphalt fumes as well as the fuel consumption during construction. In addition, the reduction in production temperature helps extend the hauling distance and paving season, enabling the pavement compaction in cooler months. Currently, the technologies available for manufacturing WMA include the applications of foaming processes, organic additives, and chemical additives, which operate by reducing the viscosity of asphalt binder or decreasing the friction between aggregate particles, thus leading to desirable workability and proper aggregate coating at lower temperatures.
In recent years, there has been a growing interest in incorporating reclaimed asphalt pavement (RAP) into WMA mixtures (Arshadi et al., 2017, Cao et al., 2017, Guo et al., 2016, Hill et al., 2013, Song et al., 2018, Zhao et al., 2016). As another sustainable material, RAP consisting of crushed aggregates and aged binder is the primary product of the milled asphalt concrete pavements. Its utilization contributes to the reduced amount of waste and reduced demand for virgin aggregate and asphalt, bringing significant environmental and economic advantages (Gong et al., 2018, Huang et al., 2011, Shu et al., 2012, Zaumanis and Mallick, 2014). The combined use of RAP and WMA stems from the considerations of applying greener technologies and balancing the disadvantages of both materials. Because of the decrease in production and construction temperatures, the asphalt binder in WMA mixes tends to be less aged, consequently resulting in mixtures with superior fatigue resistance to HMA and concerns of increased rutting potential in hot weather. Several studies (Hurley and Prowell, 2005, 2006; Vargas-Nordcbeck and Timm, 2012) reported that WMA displayed higher rutting susceptibility, whereas other research attempts (Prowell et al., 2007, Xiao et al., 2010) indicated that WMA exhibited comparable rutting performance to traditional HMA. In this regard, RAP with stiffer oxidative binder affords better resistance to permanent deformation; however, its high stiffness may raise concerns of fatigue cracking distress and workability issues. To avoid these drawbacks, the addition of RAP to surface courses is generally controlled within 25% by many highway agencies (West et al., 2013).
Fortunately, through WMA technologies, higher percentages of RAP can be applied to recycled asphalt mixtures due to the decreased RAP binder viscosity. Some research efforts have been conducted to evaluate the performance of WMA-high RAP mixtures, particularly the rutting and fatigue resistance. Mallick et al. (2008) successfully recycled base course HMA with 75% RAP at lower temperatures using the Sasobit H8 additive. Zhao et al. (2012) investigated the performance of the foaming process based WMA mixtures containing up to 50% RAP through a variety of laboratory tests. The results showed that the addition of RAP had a more considerable effect on enhancing the rutting resistance of WMA than HMA and could provide WMA with a longer fatigue life. Zhao et al. (2013) also evaluated the combined impacts of high RAP percentages (up to 40%) and different WMA technologies (Evotherm additive and foaming process) on the rutting and fatigue resistance of the mixtures, and found that rutting remained a concern for WMA-high RAP mixes. Moreover, WMA-high RAP mixtures performed equally well or even better than HMA-low RAP mixtures in cracking and fatigue performance. Mogawer et al. (2013) reported that the high RAP content up to 40% had an adverse influence on the fatigue performance of the wax based WMA rubber mixture. Lu and Saleh (2016) compared the performance of high RAP (up to 70%) mixtures containing the rejuvenator Sylvaroad and WMA additive Evotherm, and observed that all the high RAP mixtures with Sylvaroad and Evotherm exhibited higher rutting performance than HMA.
Besides, the rheological assessment of the binders containing WMA additives and high percentages of RAP binder plays a critical role in understanding the performance mechanisms and improving the design quality of the corresponding mixtures. Lee et al. (2009) characterized the effects of Aspha-min and Sasobit WMA additives on the rheological behavior of the binders with artificially long-term aged binders through the BBR and DSR tests. Xiao et al. (2015) explored the rheological properties of high RAP content binders coupled with WMA technology at various aging states by means of the Superpave binder tests. These studies revealed the performance of asphalt binders to a certain extent; nevertheless, some researchers claimed that the traditional linear viscoelastic (LVE) theory based Superpave parameters, e.g., |G*|/sinδ and |G*|∙sinδ, might not effectively capture the realistic permanent deformation and damage properties of asphalt binders since the parameters employed are obtained merely within the LVE range of the material (Delgadillo et al., 2006, Golalipour et al., 2017, Johnson et al., 2009).
Instead, two recently developed rheological tests, i.e., the multiple stress creep and recovery (MSCR) and linear amplitude sweep (LAS) tests, could better reflect and provide an insight into the rutting and fatigue resistance of binders due to the adoption of viscoplastic and viscoelastic continuum damage (VECD) theories (Golalipour et al., 2017, Hintz and Bahia, 2013, Johnson et al., 2009, Sadeq et al., 2016). To date, very little research has been carried out on evaluation of the rutting and fatigue susceptibility of binders containing high percentages of RAP binder and various WMA additives using the MSCR and LAS tests. This study was focused on this topic and the impacts of two WMA additives, i.e., the wax based R and surfactant based M, and various high proportions of RAP binder were investigated.
Section snippets
Materials
A virgin asphalt PG 58-22, commonly used in the north area of China, was selected as the base binder in the present study. Considering that the laboratory aged binder always possesses relatively more stable and controllable properties (e.g., degree of aging) and is reproducible for further analysis, this study adopted an artificial RAP binder, which was obtained by subjecting the virgin binder to successive short-term rolling thin-film oven (RTFO) (ASTM, 2012) and long-term pressurized aging
Strain response analysis from MSCR test
Fig. 1(a) presents the shear strain response at 3.2 kPa and different temperatures from the MSCR test for 70%-C. As can be seen, the strain response increases significantly with the increasing temperature. The same observations were obtained for all the other binders. Fig. 1(b) shows the shear strain curves for 70%-C, 70%-R and 70%-M at 3.2 kPa and 46 °C. It is found that the addition of R remarkably reduces the deformation resistance of the control binder, whereas the use of M slightly
Conclusions
Based on the observations and analyses from this study, the main conclusions can be made as follows:
- (1)
From a viewpoint of mechanics, the MSCR and LAS tests provided an effective and efficient approach for quantifying, comparing, and ranking the rutting and fatigue performance of the binders with different WMA additives and RAP binder contents.
- (2)
Among the ten binders tested, the virgin binder, PG 58-22, always yielded the poorest rutting and fatigue performance at all the test temperatures.
- (3)
The
Acknowledgements
This study was sponsored by the National Natural Science Foundation of China (51808098 and 51878122) and Fundamental Research Funds for the Central Universities [DUT17RC(3)034 and DUT17ZD213]. The supports are gratefully acknowledged.
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