Kinetics-based aging evaluation of in-service recycled asphalt pavement

Abstract

Reclaimed asphalt pavement (RAP) is a type of material that already suffers long-term aging in the field, so its aging characteristics become prominent since they are closely related to premature distresses and longevity of recycled pavements. While most of investigations of RAP mixtures are carried out in the laboratory, this study focuses on in situ aging of asphalt pavements with RAP overlays. A kinetics-based aging modeling approach is proposed to analyze and quantify long-term field aging of RAP overlays using the Falling Weight Deflectometer (FWD) data and climate data. The kinetics-based approach contains a modulus aging model with kinetic parameters (e.g. aging activation energy) for asphalt mixtures. Eight asphalt overlays are selected with different mixtures (RAP and virgin), thickness (50 mm and 125 mm), and surface preparation (milling and no milling). An asphalt pavement with an overlay has a composite aging process since the aging speeds of different asphalt layers are different. Thus an approach to separate the FWD modulus is developed in order to obtain the actual aging behaviors and properties of the overlay. By applying the kinetics-based modeling to the separated FWD moduli, the aging activation energies of both the overlays and old asphalt layers are determined. It is found that the RAP overlay has the highest aging activation energies and slowest aging rates among the RAP overlay, virgin overlay, and old asphalt layer for the selected pavements. It also reveals through the aging activation energy that the thick overlays age slower than thin ones, and the overlays on milled pavements age slower than those placed without milling. The findings in terms of the aging activation energy can be used to explain the difference in the field performance of overlay pavement sections.

Introduction

The use of reclaimed asphalt pavement (RAP) has become a common practice in pavement structural rehabilitations due to its symbiotic environmental and economic benefits. RAP refers to pavement materials generated when an asphalt pavement is removed for reconstruction or resurfacing. The general benefits of using RAP include reduction in energy consumption and emission generation, conservation of natural resources, and savings in rehabilitation cost (Robinette and Epps, 2010; Ding et al., 2018). However, concerns have also arisen about premature distresses and the longevity of recycled pavements. This is because RAP is a type of material that already suffers long-term aging in the field. Aging is known to have a significant effect on pavement performance and has been investigated with numerous efforts. An asphalt pavement is inevitably aged when loss of volatile oils and oxidation occur in the field. The aged asphalt pavement becomes more brittle and is prone to cracking, such as fatigue cracking, reflection cracking, longitudinal cracking, etc.

The most common way to evaluate aging of RAP is to examine the results of binder or mixture testing in the laboratory. For example, Mohammad et al. (2003) used the extraction and recovery technique to obtain asphalt binder from an eight-year old polymer modified pavement. The field aged binder was characterized by binder tests. It was found that extensive age hardening occurred and changes in both chemical and rheological properties were noted. Hamzah and Shahadan (2011) performed a physico-chemical analysis on RAP. They obtained recovered asphalt binder from RAP and incorporated into virgin asphalt with the percentages of 15% and 30%. The RAP modified binders were subjected to different aging conditions and tests to measure the physical and chemical properties. They concluded that the RAP modified binders were further aged as the measured properties distinctly changed. Tarbox and Daniel (2012) prepared four plant-produced mixtures containing different percentages of RAP (0%, 20%, 30%, and 40%), which were aged long-term in the laboratory oven. The dynamic modulus test was conducted on each mixture. The results showed that the oven aging had less effect on dynamic modulus when the content of RAP increased. Compared to the virgin mixtures (0% RAP), the mixtures with RAP stiffened at slower rates in terms of the change of the dynamic modulus. Besides, Colbert and You (2012) and Poulikakos et al. (2014) also utilized laboratory aging procedures and testing methods to quantify the effects of short-term and long-term aging on chemical, physical, and mechanical properties of RAP materials. Clear evidence of aging was observed in terms of the increase of the oxidized products, viscosity, and modulus. In the laboratory tests to evaluate performance like low temperature behavior, fatigue or rutting, the test results showed that a well-designed mixture with 40% RAP produced similar trend with the mixture with virgin materials (Poulikakos et al., 2014).

As recycling techniques mature, pavement analysts and engineers become more and more interested in using high percentages of RAP. An asphalt mixture containing RAP above 25 percent is considered to be a high RAP content mixture. To design such a mixture, an appropriate procedure is needed to address the key issues like selection of rejuvenators and determination of heating methods. Knowing aging characteristics of high RAP content mixtures plays an important role in making these decisions. Rejuvenation of aged asphalt means restoring the performance of the binder. Ali and Mohammadafzali (2015) identified five different recycling agents to rejuvenate recycled asphalt binders, and studied their long-term aging behaviors through extended Pressure Aging Vessel (PAV) aging and the Superpave Performance Grade (PG) test. The level of aging was represented by the high temperature PG. A significant difference was observed in the aging rates of the samples recycled with different rejuvenators.

Another example about high RAP content asphalt mixtures is the process of heating, which prepares RAP samples for mix design and characterization tests. A recent study on incorporating RAP in the Superpave mix design recommended that heating RAP should be minimum to avoid changes in material properties due to aging (McDaniel and Anderson, 2001). In another study about improved mix design of hot mix asphalt (HMA) with high RAP content (West et al., 2013), the heating experiment was designed to evaluate how different heating times and temperatures affected the properties of RAP samples. After heating and mixing, the RAP binder samples were extracted, recovered, and graded. The true grade reflected the extent of aging induced by heating. They found that additional aging of the RAP binder occurred when heating more than three hours.