Low temperature cracking is the most prevalent distress found in asphalt pavements built in cold weather climates. As the temperature drops the restrained pavement tries to shrink. The tensile stresses build up to a critical point at which a crack is formed. The current Superpave specification attempts to address this issue by specifying a limiting low temperature for the asphalt binder. The specification does a reasonable job predicting performance of conventional asphalt cements, but this does not hold true for polymer-modified asphalt binders that are manufactured to reach very cold temperature grades needed in cold climates. Currently the low temperature specification considers only the asphalt binder. Specifications must be developed for the asphalt mixture as well. It is very important to understand the mechanism of crack initiation and propagation. Thermal cracks can be initiated by traffic loading or cycles of temperature changes and then propagated by a large drop in temperature. In addition, the significant effects of aging and moisture on crack formation and propagation are not fully understood and need investigation. A comprehensive research effort is in progress by a team led by Dr. Mihai Marasteanu at the University of Minnesota. This project is a unique partnership between Mn/DOT and four universities: University of Minnesota, Michigan Technological University, University of Illinois at Urbana-Champagne, and University of Wisconsin at Madison. The goal of the current project is to evaluate different laboratory procedures, material properties, and pavement features in order to develop an optimal system for selecting low temperature crack resistant materials. There is a critical need to validate the findings of this experimental work with field data. To this end, two sections will be constructed at MnROAD. MnROAD provides a unique combination of instrumentation, regular performance monitoring, controlled traffic applications, researcher expertise, and a safe work environment that makes it an ideal location for this study. There is enormous potential to integrate this study with the AASHTO 2002 Design Guide. It is recognized that the current version of TCMODEL in the software package could be upgraded. Several of its limitations and deficiencies have been noted in recent literature, and recent developments in modeling techniques are able to overcome these problems. Related studies include: . TPF-5(080): Investigation of Low Temperature Cracking in Asphalt Pavements (University of Minnesota) . LRRB 739 - Report 2004-23: Low Temperature Cracking of Asphalt Concrete Pavements (University of Minnesota) . LRRB 816: Low Temperature Cracking of Flexible Pavement Due to Thermal Fatigue and Combined Effects of Loading and Temperature (University of Minnesota) . LRRB 804: Investigation of the Low-Temperature Fracture Properties of Three MnROAD Asphalt Mixtures (University of Minnesota) . TPF 914: Evaluation of Modified Performance Grade Binders in Thin Lift Maintenance Mixes, Surface Mix, and a Reflective Crack Relief Layer Mix (Massachusetts) . NCHRP 1-41: Models for Predicting Reflection Cracking of Hot-Mix Asphalt Overlays (Texas A&M) . NCHRP 1-42: Top-Down Fatigue Cracking of Hot-Mix Asphalt Layers - Phase I (Don Christensen) The research proposed in this field study will build on all the previous research in the area of low temperature cracking performed in Minnesota and around the country. The next step is to validate the new models and laboratory test methods with field performance tests at MnROAD. The models being developed for top-down cracking and reflective cracking may be of some use for modeling thermal cracking. New asphalt materials, including modified PG binders, can be tested according to the principles developed in past research. Finally, upgrades to the AASHTO 2002 Design Guide could be proposed based on new innovations in modeling.

The main objective of this study is to validate the laboratory test procedures, models, and pavement design procedures that come out of Phase I of this study. This will be accomplished by monitoring two new test sections at the Minnesota Road Research Facility (MnROAD). Phase I was aimed at developing a fracture mechanics-based specification for a better selection of asphalt binders and mixtures with respect to their resistance to crack formation and propagation. This fracture mechanics approach will also be used to investigate the detrimental effects of aging and moisture on the fracture resistance of asphalt materials. This pooled fund study is strictly to perform the low temperature cracking research on newly built test sections at MnROAD, and its funding will come from Mn/DOT and other participating states. The funding for the construction of new test sections will be obtained separately from Mn/DOT and other partners.

This project is expected to consist of the following activities: . Work Plan: The work plan for this pooled fund study will be developed by the participating organizations. It will include selecting materials and mix designs for the construction of two cells at MnROAD. . Instrumentation Design: thermocouples, strain gages, acoustic emission, etc. . General Testing & Monitoring: Monitor the pavement performance over time on each test section with standard Mn/DOT practices. Monitoring activities will include FWD tests, rutting measurements, distress surveys, ride measurements, and analysis of pavement sensor data. . Special Testing & Monitoring: Laboratory testing using fracture mechanics concepts on asphalt binders and mixtures according to developments from Phase I study. . Forensics: If the pavement cracks, forensic activities will determine why. . Thermal Cracking Modeling & Validation: Validate the models developed in the Phase I study and for the M-E Design Guide. . Pooled Fund Travel: Money for each state to travel to discuss the progress of the study. . Data Analysis & Reports: Work done under a research contract will develop interim and final reports that document the findings of this study.

Mn/DOT along with other participating states are asked to contribute $20,000 per year for 5 years to fund the research proposed in this pooled fund study. The Minnesota Department of Transportation (as the lead agency) along with other partners will provide approximately $200,000 in construction funding outside of this pooled fund study to construct the pavement test sections for the validation of the results from the Phase I study.