Thin (TCOAP) and ultra-thin (UTCOAP) concrete overlays of existing asphalt pavement (also known as whitetoppings) are a pavement rehabilitation option that has been increasing in popularity in the U.S. over the past 15 years. The recent International Conference on Best Practices for Ultra-thin and Thin Whitetoppings (April 2005) demonstrated that many states now have experience with at least a couple of completed COAP projects. The recently completed NCHRP Synthesis 338 on Thin and Ultra-thin Whitetopping also shows the use of this product is currently of great interest to the transportation community. The one area of deficiency in the application of TCOAP and UTCOAP (TWT and UTW whitetoppings) is the lack of a unified national design guide. While several local and industry design guides have been formulated, few are based on mechanistic-empirical research born out of actual field performance. To address this issue, many research studies on the accelerated and field performance of TCOAP and UTCOAP are either in progress, or have been recently completed. The Minnesota Road Research (MnROAD) Project has contributed significantly to the understanding of the field performance of TCOAP and UTCOAP. The results from MnROAD and other accelerated loading facilities (ex. FHWA) can now be utilized to develop a unified comprehensive mechanistic-empirical based design method for thin and ultra-thin concrete overlays of existing asphalt pavement. Most of the existing TCOAP and UTCOAP test sections have been built on top of structurally sound asphalt pavements. Researchers and pavement designers still question the service life of these overlays when placed on older, distressed asphalt pavements. This study will have an opportunity to utilize early performance data from a thin concrete overlay of a distressed full-depth asphalt pavement to be constructed at MnROAD in 2008 (funding for the construction and monitoring of the test cell is not part of this study). Experience and knowledge from other related case histories will also be utilized.

The primary purpose of this project is to create a unified national design guide for thin and ultrathin concrete overlays of existing asphalt pavements. This consists of the following distinct objectives: 1. Study and understand the field performance history of TCOAP and UTCOAP as demonstrated by various research test sections. These include current (and future) test sections at the MnROAD facility, accelerated loading facility test sections (FHWA), and other test sections installed and monitored by various local, national and international agencies. 2. Develop a design guide for concrete overlays of existing asphalt pavements utilizing existing validated performance models, as well as new analytical models derived to address design aspects not currently considered in existing methods. The design guide will be based on mechanistic-empirical principles, including the effects of various concrete overlay and existing asphalt materials, panel thickness and geometry, joint opening and stiffness, traffic loads, and climates under which they must perform. 3. Create a user-friendly design guide software program and user¿s manual. The program format should be such that it could become a module in the future national Mechanistic-Empirical Pavement Design Guide (MEPDG) for highway pavements.

This research project would be carried out in 5 tasks: 1. Literature review of existing TCOAP and UTCOAP (TWT and UTW) design methods and field experiments. 2. Development of layer bonding degradation model(s) based on field testing results (and limited laboratory testing if beneficial). 3. Development of new, or improvement of existing, TCOAP and UTCOAP pavement response and performance prediction models that incorporate slab thickness and geometry, joint opening and stiffness, axle load configuration, condition of the existing asphalt pavement, and degradation of layer bonding with time (if any). 4. Development of a unified national mechanistic-empirical design guide for thin and ultrathin concrete overlays of existing asphalt pavements. The design guide, in the form of a user-friendly software program and user¿s manual, will accommodate a variety of climate conditions, axle load configurations, existing asphalt material conditions, and time dependent bonding conditions between layers. 5. Provide guidance on how the design guide can be implemented into future versions of the national MEPDG for highway pavements.

Formally titled "Analysis Of MnROAD Whitetopping Performance data For A Module In The Design Guide" This study will provide a unified national design process that could eventually be incorporated into the national MEPDG for highway pavements. States and Agencies will benefit from this process of developing a user-friendly design process through the synergistic machinery of a multi-state pooled fund.