Aging: Avoiding the Inevitable

The chemical composition of asphalt depends primarily on its crude oil source and processing methodology. Differences in the asphalt composition can strongly affect its mechanical properties and chemical reactivity. Asphalt binder is a complex mixture of high molecular weight hydrocarbon molecules and naturally occurring heteroatoms (nitrogen, oxygen, and sulfur) and trace metals (e.g., vanadium and nickel) that contribute to the polarity within the asphalt molecules. Asphalt binder is often described as a colloid that consists of a dispersion of large molecular weight asphaltenes in an oily matrix constituted by saturates, aromatics, and resins. It has been established that asphaltenes are stabilized in crude oils by natural resins that are surfactant-like agents (Figure 1). The polarity among asphalt molecules varies widely and the physical properties of the asphalt binder are governed by the balance of polar and non-polar components. In an asphalt molecule, the polar matrix is responsible for the elastic behavior of the material and the continuous non-polar phase controls the viscous behavior. During the oxidative aging process, the concentration of polar functional groups in an asphalt binder increases, resulting in an immobilization of molecules through intermolecular association. Hence, the molecules or molecular agglomerates lose sufficient mobility to flow past one another under thermal or mechanical stress. This results in embrittlement of the asphalt binder, making it more susceptible to cracking and more resistant to healing.

Figure 1 Schematic of Asphalt Components

The properties of asphalt mixtures change as the asphalt binder ages. The aging of asphalt binders is caused by volatilization (i.e., evaporation of the light fractions of asphalt), thermal and ultraviolet oxidation, and other chemical processes. Volatilization and oxidation occur rapidly during production and construction of asphalt mixtures when the asphalt is spread in thin films to coat the aggregate at a high temperature. This is sometimes referred to as short-term aging. The oxidative aging processes continue throughout the pavement service life (i.e., long-term aging).

Factors affecting the rate of aging include the chemical composition and physicochemical state of the asphalt binder, air permeability of the mix, depth in the pavement structure, asphalt binder content, aggregate mineralogy, mix production-related factors, and in-service temperature and time. Surface layers age at a much faster rate than lower layers in the pavement due to exposure to air, solar radiation, and higher temperatures.

Asphalt binder oxidation has a significant impact on age-related pavement damage since it changes the time-temperature dependence of the viscoelastic asphalt binder. The chemical and/or physicochemical changes that occur in asphalt due to oxidative aging increase both the viscous and elastic properties of the binder, leading to a global hardening (i.e., stiffening) of the material. Aged asphalt binder can typically sustain high shear stress due to its increased elastic stiffness, but it has reduced stress relaxation properties through viscous flow. As a result, asphalt pavements become more susceptible to cracking and other durability-related distresses.

Simulating Aging in the Laboratory

Over the last few decades, several laboratory aging protocols have been developed to condition/age asphalt mixtures for mix design and performance testing. The standard short-term aging protocol in AASHTO R30 is to condition loose mix for two hours at the compaction temperature for volumetric mix design or for four hours at 135°C for mechanical property testing. This protocol is designed to simulate the aging of asphalt binders that occurs during plant production and construction. National Cooperative Highway Research Program (NCHRP) Project 9-52 evaluated over 40 asphalt mixtures and concluded that two hours of loose-mix aging at 135°C for hot mix asphalt (HMA) and 116°C for warm mix asphalt (WMA) was appropriate for simulating the effects of plant mixing and storage to the point of loading in haul trucks. Similar findings have also been reported by NCHRP 9-43 and a study by the Colorado Department of Transportation.

For long-term aging, AASHTO R 30 recommends aging compacted specimens for five days at 85°C. However, this protocol has been criticized over the years for lack of correlation with the actual field aging of asphalt pavements. NCHRP 9-52 indicated that five days of compacted specimen aging at 85°C could only simulate approximately two to three years of field aging. Research by the University of New Mexico and Mississippi State University also reported that this standard aging protocol was representative of no more than one year of field aging.

Recently, alternative methods have been proposed for long-term aging using loose mix prior to compaction. As compared to compacted specimens, aging of loose mixes increases the rate of oxidative aging due to increased exposure of the thin asphalt coating to heat and oxygen. The ongoing NCHRP 9-54 project has recommended loose mix aging at 95°C for a period of time based on climate, depth, and years of service. For surface layers with four years of service, the recommended aging time ranges from three to five days for most of the continental U.S. A longer time is required in order to simulate surface layers with a longer in-service time. Although this protocol is promising to simulate the field aging of asphalt pavements, the lengthy time span makes it difficult to implement into routine mix design and production practices. To address this shortcoming, research studies by the University of Illinois Urbana-Champaign, MTE Services, Inc., and NCAT have recommended loose mix aging at 135°C for much shorter time periods ranging from 8 to 24 hours. Besides loose mix aging, accelerated pavement weathering systems that provide simultaneous cyclic actions of thermal oxidation, ultraviolet radiation, and moisture infiltration and diffusion can also be used to simulate the long-term field aging of asphalt pavements. The limitation of this approach is special equipment that is not widely available.

Extending Service Life with Pavement Preservation Techniques

The FHWA’s Pavement Preservation Expert Task Group defines pavement preservation as "a program employing a network level, long-term strategy that enhances pavement performance by using an integrated, cost-effective set of practices that extend pavement life, improve safety and meet motorist expectations.” A pavement preservation program consists primarily of three components: minor rehabilitation (non-structural), preventive maintenance, and routine maintenance.

Pavement preservation has gained popularity among highway agencies as a proactive approach to maintain the functional and structural integrity of pavements and delay the need for more costly rehabilitation treatments. It refers to the application of correctly selected surface treatments at the optimum time to extend the service life of a pavement. There are many recognized techniques for preserving asphalt pavements, including crack sealing and crack filling, fog seals and asphalt rejuvenators, seal treatments (scrub, sand, chip, slurry, and cape), micro surfacing, and thin overlays. The purpose of these treatments is to preserve pavement functional and structural integrity and to retard deterioration in order to avoid more costly rehabilitation treatments in the near future.

In order to improve pavement preservation techniques and to spread their appropriate use among highway agencies, research is needed to advance a fundamental understanding of the effectiveness of pavement preservation treatments. It is known that pavement preservation treatments can temporarily seal or fill the cracks on the pavement surface, preventing water from penetrating into the underlying layers, which weakens the materials and ultimately reduces the structural capacity of the pavement. There is also a hypothesis that these treatments provide a protective “coating” delaying the oxidative aging and physical hardening of the asphalt binder located underneath the surface treatment, which consequently could reduce the susceptibility of the pavement to cracking and other durability-related distresses. However, the validity of this hypothesis remains unknown since field performance data still showed cracking as the primary type of distress on pavements that were treated. Thus, future research is needed to evaluate the optimum timing of treatments and specification criteria for the treatment materials and their application.

Slowing the Aging Process

Another aging-related topic for future research is the exploration of antioxidants as anti-aging additives for asphalt mixtures. Currently, many state highway agencies allow the use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures. The inclusion of these recycled materials provides economic and environmental benefits, but in some cases it may result in asphalt mixtures with less resistance to cracking. To address this, some agencies permit the use of recycling agents (a.k.a. rejuvenators) to improve the durability of asphalt mixtures containing RAS or high RAP contents. Studies have shown that recycling agents can help mitigate the stiffening effect of RAP and RAS materials through uniform dispersion within the mix and diffusion into heavily aged recycled binders. However, the effectiveness of recycling agents was found to diminish with aging. This raises the question whether these agents improve the long-term cracking performance of asphalt mixtures with RAP and RAS.

Another potential solution to reduce the detrimental effects of aging is to use antioxidants to decelerate the oxidative aging of asphalt binders. Although antioxidants have been successfully used to enhance the resistance of polymers to oxidation and embrittlement, research studies on their use in asphalt binders are limited. Some of the antioxidants that have been found successful in literature include lignin, hindered phenols, lead and zinc dithiocarbamates, sodium hydroxide, and carbon black. Nevertheless, more research is warranted to establish a comprehensive understanding of the mechanism of antioxidants to slow the aging of asphalt binders and to evaluate their effect on the engineering properties of asphalt mixtures.