Mechanisms of Cracking and Debonding in Asphalt and Composite Pavements

This chapter provides an introduction to cracking in asphalt and composite pavements. This is accomplished by reviewing common types of pavement cracking as evidenced in field and accelerated pavement testing studies, reviewing existing pavement cracking models, and summarizing contemporary thinking on the mechanisms behind the various forms of pavement cracking identified. As asphalt binder or mixture is cooled, it transitions from a brittle-ductile to a quasi-brittle physical state. To complicate matters, asphalt concrete tends to become oxidized, leading to material embrittlement with time. In addition, exponentially more aging occurs near the surface of the pavement, leading to a steeply graded material system after several years in service. The amount of aging, or the aging rate, has been found to vary significantly depending upon crude source, refining techniques, additives, type and amount of recycled materials used, climate, and volumetric characteristics of the mixture. Loading time and sequence, damage, healing, complexity of interface morphology and associated mechanics, and bulk material nonlinearity are additional complicating factors that have kept asphalt pavement cracking as a vibrant, open area of research. Interface behavior such as debonding, sliding and variable contact between surfaces further complicate the behavior of pavement systems under environmental and traffic loads. This chapter is not intended to be a comprehensive review of the subject of asphalt pavement cracking; rather it provides initial motivation for the subject along with a compilation of previous works published by members of RILEM TC241-MCD and the technical committees that have preceded this committee.

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.

The performance and durability of multi-layered pavements strongly depend on interlayer bonding between layers, especially for pavements with a thin or ultra-thin surface course. These pavements, comprised of several differing material layers, are often subjected to premature distresses (corrugation, peeling, slippage or fatigue cracking, etc.) caused by poor interface bonding. This chapter summarizes the different bond characterization tests available around the world (mostly in the laboratory) available to characterize the bond between pavement layers. Many of the tests can be performed on specimens prepared in the laboratory or on cores or slabs obtained from the pavement. Mostly, “pure” fracture mode test methods (opening mode I or in-plane, shear mode II or out-of-plane, shear mode III) are currently used worldwide for determining the interlayer bond of pavement layers. Most of the mixed-mode test methods (mainly for the combination of Modes I and II) were developed by a few research teams and there are therefore no standard tests. Although tack coat type and content are the main parameters studied by researchers and engineers, surface roughness, moisture, freezing, and presence of dust or debris on the interface are additional parameters that may decrease bonding performance.

The previous chapters describe numerical models and testing methods designed to simulate, to repeat and to understand the cracking phenomena in asphalt materials and asphalt pavement (or pavements with at least one asphalt layer). This chapter shows some of the most advanced systems to measure the parameters related to cracking. They are classified using a classification grid that considers the most relevant characteristics of the measurement system. The proposed classification method was designed to provide a quick understanding of what data the systems are able to provide and what they can analyze.

This chapter presents a summary of the key findings of this state-of-the-art report on the mechanisms of cracking and debonding in asphalt and composite pavements. Some of these key findings have been presented during the 8th Rilem International Conference on Mechanisms of cracking and Debonding in Pavements in Nantes (MCD 2016). It also contains the key recommendations developed by the TG2 group, which produced a RILEM recommendation document as a culminating activity in the final year of the RILEM Technical Committee TC241-MCD. Remaining challenges in the area of asphalt and composite pavement cracking and debonding and recommendations for further study are then summarized.