Hot mix asphalt (HMA) is used as the primary overlying material of concrete pavements during rehabilitation because of its inexpensive nature when compared to most Portland cement concrete (PCC) rehabilitation/reconstruction alternatives. However, due to the majority of the PCC pavements being in average to poor condition, many HMA overlays are exposed to extreme movements (both vertical and horizontal). The combination of associated load and environmentally induced movements creates complex stresses and strains in the vicinity of expansion joints and cracks in the PCC, thus dramatically reducing the life of the HMA overlay, typically in the form of reflective cracking. Reflective cracking is a fatigue cracking distress, which is initiated at the bottom of the HMA overlay and propagates to the surface. When the crack reaches the HMA overlay surface, not only does it affect the ride quality and overall integrity of the pavement surface, but it also creates a path for which water can migrate down into and below the PCC layer. This can ultimately reduce the overall structural support of the composite (HMA and PCC) pavement and result in a complete pavement failure. Medium to high severity reflective cracking results in poor surface conditions that could lead to poor driving conditions and higher accident rates. Therefore, this research is timely in that it not only addresses the structural integrity of the pavement system, but also the safety of the driving public, which is one of the main objectives of the administration at state agencies. To better understand the mechanisms associated with the development of reflective cracking, an extensive literature review was conducted. Analysis of the literature review indicated significant gaps in the current state of the practice in using bituminous overlays on PCC pavements. To fill in these gaps, a survey was developed, distributed to the state transportation agencies of all fifty states, and compiled to better define the scope of the research. The survey clearly identified that a major gap in the current state of the practice is linking the field conditions (climate, deflections, traffic levels) to appropriate laboratory testing protocols. Therefore, field test sections were selected with appropriate field forensic testing and traffic collection. During construction of the bituminous overlays, loose mix was collected and brought back to the laboratory for material characterization testing that would simulate the loading conditions associated with the respective test section. The research conducted during the development of this thesis has led to a rational approach in the prediction of reflective cracking potential in HMA overlays placed on PCC pavements. This methodology utilizes field forensic information that would normally be collected during the evaluation of the PCC/composite pavement prior to rehabilitation and laboratory fatigue and stiffness characterization of the HMA mixture(s), to predict the potential for reflective cracking in the bituminous overlay mixture(s). The extensive laboratory testing and field calibration/verification information utilized in the research has also led to "decision tree" methodology that would allow state agencies to properly select asphalt mixtures for overlaying PCC pavements.

Crack reflection through a road structure is one of the main causes of premature pavement deterioration. This is a widespread problem in many countries and highway maintenance authorities are having to find economic means of repairing and upgrading their pavements. This book is the eagerly awaited state-of-the-art report which considers all different aspects of the subject including assessment and use of overlay systems.

TRB's National Cooperative Highway Research Program (NCHRP) Report 669: Models for Predicting Reflection Cracking of Hot-Mix Asphalt Overlays explores mechanistic-based models for predicting the extent and severity of reflection cracking in hot-mix asphalt overlays. Appendices A through T to NCHRP Report 669 are available online--

Reflection cracking is one of the main distresses in hot-mix asphalt (HMA) overlays. It has been a serious concern since early in the 20th century. Since then, several models have been developed to predict the extent and severity of reflection cracking in HMA overlays. However, only limited research has been performed to evaluate and calibrate these models. In this dissertation, mechanistic-based models are calibrated to field data of over 400 overlay test sections to produce a design process for predicting reflection cracks. Three cracking mechanisms: bending, shearing traffic stresses, and thermal stress are taken into account to evaluate the rate of growth of the three increasing levels of distress severity: low, medium, and high. The cumulative damage done by all three cracking mechanisms is used to predict the number of days for the reflection crack to reach the surface of the overlay. The result of this calculation is calibrated to the observed field data (severity and extent) which has been fitted with an S-shaped curve. In the mechanistic computations, material properties and fracture-related stress intensity factors are generated using efficient Artificial Neural Network (ANN) algorithms. In the bending and shearing traffic stress models, the traffic was represented by axle load spectra. In the thermal stress model, a recently developed temperature model was used to predict the temperature at the crack tips. This process was developed to analyze various overlay structures. HMA overlays over either asphalt pavement or jointed concrete pavement in all four major climatic zones are discussed in this dissertation. The results of this calculated mechanistic approach showed its ability to efficiently reproduce field observations of the growth, extent, and severity of reflection cracking. The most important contribution to crack growth was found to be thermal stress. The computer running time for a twenty-year prediction of a typical overlay was between one and four minutes.